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Nanotechnology in eco-efficient construction : materials, processes and applications [2 ed.]
 9780081026427, 0081026420, 9780081026410

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Woodhead Publishing Series in Civil and Structural Engineering

Nanotechnology in Eco-efficient Construction Materials, Processes and Applications Second Edition Edited by

Fernando Pacheco-Torgal Maria Vittoria Diamanti Ali Nazari Claes Goran Granqvist Alina Pruna Serji Amirkhanian

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2019 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-102641-0 For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Gwen Jones Editorial Project Manager: Charlotte Rowley Production Project Manager: Joy Christel Neumarin Honest Thangiah Cover Designer: Miles Hitchen Typeset by TNQ Technologies

List of contributors

Stefano Accoroni Dipartimento di Scienze della Vita e dell’Ambiente, Universita Politecnica delle Marche, Ancona, Italy Nihad T.K. Al-Saadi Swinburne University of Technology, Hawthorn, Melbourne, Victoria, Australia Serji Amirkhanian State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, China M.B. Anoop CSIR-Structural Engineering Research Centre, CSIR Campus, Chennai, India P. Arezes Ergonomics & Human Factors Group, Centre Algoritmi, University of Minho, Guimar~aes, Portugal Ana Bahamonde Institute of Catalysis and PetrochemistrydCSIC, Madrid, Spain U. Berardi Department of Architectural Science, Faculty of Engineering and Architectural Science, Ryerson University, Toronto, Canada Fabio Boccuni Italian Workers’ Compensation AuthoritydDepartment of Occupational and Environmental Medicine, Epidemiology and Hygiene, Rome, Italy Dragan Bojovic Institute for Materials TestingdIMS, Belgrade, Serbia Cristina Buzea

IIPB Medicine Corporation, Owen Sound, ON, Canada

Alessandro Cannavale Politecnico di Bari, Dipartimento di Scienze dell’Ingegneria Civile e dell’ Architettura (DICAR), Bari, Italy P. Carballosa Spain

Institute for Construction Sciences Eduardo Torroja, CSIC, Madrid,

Xun Cao State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai institute of Ceramics, Chinese Academy of Sciences, Shanghai, China J.L. García Calvo Madrid, Spain

Institute for Construction Sciences Eduardo Torroja, CSIC,

Ant onio Augusto Veloso de Costa

University of Minho, Braga, Portugal

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List of contributors

Stefano Cucurachi Leiden University, Faculty of Science, Institute of Environmental Sciences (CML), Leiden, The Netherlands Qingli Dai Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan, United States Maria Vittoria Diamanti Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, Milan, Italy Yining Ding E. Erkizia

Dalian University of Technology, Dalian, China Tecnalia, Sustainable Construction Division, Derio, Spain

Marisol Faraldos

Institute of Catalysis and PetrochemistrydCSIC, Madrid, Spain

Riccardo Ferrante Italian Workers’ Compensation AuthoritydDepartment of Occupational and Environmental Medicine, Epidemiology and Hygiene, Rome, Italy J.J. Gaitero

Tecnalia, Sustainable Construction Division, Derio, Spain

Giovanni Battista Goffredo DICEA - Dipartimento di Ingegneria Civile Edile e Architettura, Universita Politecnica delle Marche, Ancona, Italy Claes Goran Granqvist A. Guerrero Spain

Uppsala University, Sweden

Institute for Construction Sciences Eduardo Torroja, CSIC, Madrid,

Shuaicheng Guo Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan, United States Baoguo Han China Zhibo Han

School of Civil Engineering, Dalian University of Technology, Dalian, Dalian University of Technology, Dalian, China

Sergio Iavicoli Italian Workers’ Compensation AuthoritydDepartment of Occupational and Environmental Medicine, Epidemiology and Hygiene, Rome, Italy Zaid Ayaz Janjua Faculty of Engineering, University of Nottingham, Nottinghamshire, United Kingdom Ksenija Jankovic Institute for Materials TestingdIMS, Belgrade, Serbia Magdalena Janus Faculty of Civil Engineering and Architecture, West Pomeranian University of Technology, Szczecin, Poland Ping Jin State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai institute of Ceramics, Chinese Academy of Sciences, Shanghai, China Romana Cerc Korosec Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Slovenia

List of contributors

xvii

Yuanyuan Li State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, China Genjin Liu

Dalian University of Technology, Dalian, China

Hongjie Luo State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai institute of Ceramics, Chinese Academy of Sciences, Shanghai, China Francesco Martellotta Politecnico di Bari, Dipartimento di Scienze dell’Ingegneria Civile e dell’ Architettura (DICAR), Bari, Italy Lev Matoh Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Slovenia Alyaa Mohammed Victoria, Australia

Swinburne University of Technology, Hawthorn, Melbourne,

Muhammad Abdul Mujeebu Department of Building Engineering, College of Architecture and Planning, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia Ali Nazari Australia

Swinburne University of Technology, Hawthorn, Melbourne, Victoria,

Jinping Ou School of Civil Engineering, Dalian University of Technology, Dalian, China; School of Civil Engineering, Harbin Institute of Technology, Harbin, China F. Pacheco-Torgal C-TAC Research Centre, University of Minho, Guimar~aes, Portugal; University of Minho, Braga, Portugal Ivan Pacheco IIPB Medicine Corporation, Owen Sound, ON, Canada; Department of Pathology, Grey Bruce Health Services, Owen Sound, ON, Canada; Department of Pathology and Laboratory Medicine, Sch€ ulich School of Medicine & Dentistry, Western University, London, Ontario, ON, Canada MariaPia Pedeferri Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, Milan, Italy G. Pérez

Institute for Construction Sciences Eduardo Torroja, CSIC, Madrid, Spain

A. Pruna Center for Surface Science and Nanotechnology, Polytechnic University of Bucharest, Bucharest, Romania Akm Samsur Rahman Mechanical Engineering Technology, New York City College of Technology, Brooklyn, NY, United States Carlos Felipe Blanco Rocha Leiden University, Faculty of Science, Institute of Environmental Sciences (CML), Leiden, The Netherlands Jay G. Sanjayan Victoria, Australia

Swinburne University of Technology, Hawthorn, Melbourne,

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List of contributors

Saptarshi Sasmal Chennai, India

CSIR-Structural Engineering Research Centre, CSIR Campus,

F. Silva Technological Centre for Ceramic and Glass, Coimbra, Portugal; Ergonomics & Human Factors Group, Centre Algoritmi, University of Minho, Guimar~aes, Portugal  Urska Lavrencic Stangar Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Slovenia Alicja Stankiewicz School of Engineering and the Built Environment, Edinburgh Napier University, Edinburgh, United Kingdom Marko Stojanovic Institute for Materials TestingdIMS, Belgrade, Serbia P. Swuste Safety Science Group, Delft University of Technology, Delft, The Netherlands Hideki Takebayashi

Department of Architecture, Kobe University, Kobe, Japan

Francesca Tombolini Italian Workers’ Compensation AuthoritydDepartment of Occupational and Environmental Medicine, Epidemiology and Hygiene, Rome, Italy Cecilia Totti Dipartimento di Scienze della Vita e dell’Ambiente, Universita Politecnica delle Marche, Ancona, Italy L.N. Vakhitova L. M. Litvinenko Institute of Physical-Organic Chemistry and Coal Chemistry, NAS of Ukraine, Kiev, Ukraine Zigeng Wang College of Architecture and Civil Engineering, Beijing University of Technology, Beijing, China Shaopeng Wu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, China Xu Yang Australia

Department of Civil Engineering, Monash University, Clayton, Victoria,

Lingyun You Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan, United States Zhanping You Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan, United States Q.L. Yu Department of the Built Environment, Eindhoven University of Technology, Eindhoven, The Netherlands Kamila Zając Faculty of Civil Engineering and Architecture, West Pomeranian University of Technology, Szczecin, Poland

List of contributors

xix

Shaban Ali Zangena Civil Engineering Department, Faculty of Civil and Environmental Engineering, Near East University, Turkey  Bostjan Zener Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Slovenia Qiaofeng Zheng Dalian, China

School of Civil Engineering, Dalian University of Technology,

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F. Pacheco-Torgal C-TAC Research Centre, University of Minho, Guimar~aes, Portugal

1.1

Recent nanotechnology advancements and limitations

In the past decades, nanotechnology which is not considered a sector by itself but a highly dispersed multidisciplinary area has been on a rise not only in terms of papers and patents but also in terms of applications. A review on paper and patent production for the period 2000e16 shows that United States is the dominant player both in publications in top journals and also in patents although China has shown a relevant growth (Zhu et al., 2017). A recent survey has tracked the patent production in the field of nanotechnology (Ozcan and Islam, 2017), using the patent provider Thomson Innovation. The study shows that there are around 50,000 patent inventions, of which around 30,000 are owned by corporations, around 14,000 by inventors, around 11,000 by academia, and almost 2,000 by government. The shared patents explain the difference in the total. Of course, as Fig. 1 in the paper of Ozcan and Islam (2017) shows, different countries show different proportion of owners in the patent production. Inventors are the majority of the owners in United States. With regard to corporates, the United States and Japan have similar levels of patent ownership. France, for instance, has a higher proportion of government ownership, while in China its academia has the largest proportion. Of course it is important to take into account the method used for patent retrieval because the authors point out that there are some patents on the nanotechnology class that are not related to this field. Sabatier and Chollet (2017) using bibliometric data and a survey of French nanotechnology scientists showed that promoting ground-breaking, innovative research provides an important advantage for future scientific production. However, it is important not to overemphasize the importance of patents because van Raan (2017) showed that only a small amount of patents represent important, “radical,” technological breakthroughs. Also in an important essay, Archibugi (2017) mentioned that intellectual property rights may delay the diffusion of knowledge and that disruption by itself does not necessarily lead to progress or to greater economic efficiency, and if it is not properly managed it can lead not only to company losses, but to societal damages as well. In fact, Shapira and Youtie (2015) mentioned that many nanotech sales forecasts were adjusted downwards because some of the promised scale benefits are unlikely to be realized. On one hand complex nanomaterials may not be very environmental friendly and life cycle assessment (LCA) may require further

Nanotechnology in Eco-efficient Construction. https://doi.org/10.1016/B978-0-08-102641-0.00001-3 Copyright © 2019 Elsevier Ltd. All rights reserved.

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investigations. Up to this date the recyclability of nanomaterials is not being adequately tackled, as well as their environmental impacts in the end-of-life stage (Pacheco-Blandino et al., 2012). On the other hand the toxicity of various nanomaterials for human health and for the environment is still under debate (Kim et al., 2016). Safety management of nanoparticles and nanomaterials is also a critical issue (Spitzmiller et al., 2013). Still the potentially revolutionary technologies may have limited impact upon macroeconomic performance, if they do not give rise to a new wave in terms of capital accumulation and public investment in infrastructure (Lundvall (2017). According to this author, countries and organizations promoting “experience based” knowledge and combining it with science-based knowledge are more innovative than those that only give attention to codified knowledge. He also states that learning from experience may feed wisdom and that learning societies where men and women are expected to contribute to the production and use of knowledge are to be preferred to societies where only small intellectual elites produce knowledge. An interesting case in this regard is that of Russia, a country that shows a declining share of nano papers in spite of an increase in research funding (Terekhov, 2017) and a rigid academic structure that does not allow newcomers nor does engage in collaborations with the private sector (Karaulova et al., 2017). Also important in the field of nanotechnology is the performance of Asian countries that Ludvall (2017) deems crucial for the world economic growth. China being the new world scientific powerhouse (Tollefson, 2018) also identified nanotechnology as a priority area in its national agenda of science and technology development (2006e20), and has increased R&D investment in the field. In fact, China has consequently emerged as one of the key global players in nanotechnology, producing the second largest number of nanotechnology papers after United States (Wang and Guan, 2010). China has made significant advances and currently has the fastest growing nanotechnology publications. However it still lags behind in publication in leading nanotechnology journals. An analysis of papers published in nano-related journals with an impact factor above 20 shows that USA published 1068 papers, Germany (221), UK (193), France (149), Japan (121), and China only produced 76 papers (Dong et al., 2016). Of course, this may change in the coming future because China’s one-thousand-talents plan (www.1000plan.org) to attract its overseas researchers has already recruited more than 2000 researchers, most of them trained in the USA (Gao et al., 2016). Still in China, the pathways from laboratory research to successful commercialization remain problematic. The Chinese nanotech industry is relatively weak in commercializing basic research and in its production of nanotechnology devices (Shapira and Wang, 2009; Zhang et al., 2017). As to India, the other Asian giant, although and according to the World Bank is now growing more than China and will be fifth largest world economy in 2018, the fact is that concerning nanotechnology India is still lagging behind several other countries (Momaya and Lalwani, 2017). It is a relevant fact that these authors are aware of the fact that technological innovations can have negative externalities thus confirming the position of Archibugi (2017) previously mentioned.

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1.2

3

Nanotech-based materials for eco-efficient construction

Since 2013 when the first edition of this book was published, the number of publications in the field of nanotech-based eco-efficient construction materials saw a huge rise. Back then, a search on the Scopus database showed only a few publications. Now the same search returns several hundred. Of course let us not forget that in practical terms there are several and confusing definitions of what constitutes a nanomaterial and about the requirements to identify nano-enabled products. Some products are advertised as having nanoenabled features, something that is simply not true, while others fail to disclose the fact that they have nanoparticles or were obtained by nanomanipulation (Jones, 2016). Also, in the introduction chapter of the first edition it was argued that too little nanotech efforts were put in important construction materials like concrete, the material most consumed by the construction industry. Scopus now shows that nanotech concreteerelated publications have risen around 500%. Of course, some publications have exaggerated the promises of nanotechnology in the field of construction industry, failing to produce evidence that support such claims. Hanus and Harris (2013) wrote that “Nanotechnology has the potential to reduce the environmental impact and energy intensity of structures, as well as improve safety and decrease costs associated with civil infrastructure,” but no reference is given to back this statement. Also no cost data is given concerning the use of nanoparticles in concrete and in Section 2.2 the authors wrote that “The cost of CNTs is currently prohibitively high to allow for the use of CNT/cement composites” thus contradicting their initial claim. They also mention that the future implementation of advanced structural health monitoring systems will increase as the technology matures and associated costs decrease. However, this is just an expectation, which is very far from the cost decrease of current infrastructure mentioned in the introduction. Also they confirm that high cost is reported to be a main drawback for the use of carbon nanotubes (CNT) sensors in concrete. They also mention that self-cleaning hydrophobic paints are potentially valuable in the construction industry for the reduction of costs associated with maintaining building walls and façades, but again no data is given concerning any possible life cycle cost comparison. Instead, the authors prefer to “focus on up-front build costs over long-term cost, performance, sustainability and safety.” However, instead of blaming the construction industry because of the so-called “focus on up-front build costs over long-term cost, performance, sustainability and safety,” it would make more sense to highlight the fact that so far nanotechnology research has given very little importance to the factors that are important for the construction industry and that show a gap between what researchers consider important and what the construction industry needs. Taalbi (2017) showed that solving real-life problems was a source of innovation for several industries, meaning that it is not understandable that those engaged in nanotech for the construction industry have given so little attention to cost, because on the five criteria that identify the emerging technologies of great impact (Archibugi, 2017) the first one is precisely “drastic reduction in costs.” The contribution of nanotechnology for sustainability and the 2030 agenda for sustainable development that are related to

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the construction industry are also very important. Infrastructure resilience is the ninth goal of 2030 United Nations Agenda for Sustainable Development. It is also one of the 14 Grand Challenges of Engineering. Overpopulation will not only require new infrastructure, but it will put increased pressure on the existent infrastructures. That is why almost 50% of the chapters of this book concern infrastructure materials including health monitoringerelated materials. Concrete infrastructure encompasses bridges, piers, pipelines, dams, pavements, or buildings that are crucial to services and economic activities of modern civilization. European design codes now require a service lifetime of more than 75 years for concrete structures in large public works. But, experience has shown that infrastructures begin to deteriorate after only 20 or 30 years. Concrete deteriorates due to several causes including, for instance, mechanical deterioration like impact or excessive loading or deterioration due to physical causes like erosion or shrinkage, but also due to chemical detrimental reactions when it is exposed to environmental conditions containing chlorides from seawater or from deicing salts, atmospheric carbon dioxide, or other aggressive media (Glasser et al., 2008). In United States alone, costs related to wasted fuel and time loss due to traffic congestion is estimated to be between 50 and 100 billion dollars (Schlangen and Sangadji, 2013). Only in the city of Hong Kong, more than 580,000 vehicles cross its 900 highway bridges on a daily basis (Pei et al., 2015). This traffic volume is expected to duplicate in the next decades and as a consequence by 2035 it is expected that there will be 2000 million vehicles on the road (Pacheco-Torgal, 2017). This means that concrete highways bridges will be subject to increased use and will reach the end of its service life sooner than expected, and repair and rehabilitation costs will increase even further. The acting decision however is related to the assessment of the infrastructure performance, but monitoring activities are also costly and not all countries can afford it; in the United States bridges are inspected every 2 years (Rehman et al., 2016). The lack of regular inspections worsens the problem and contributes to premature infrastructure deterioration, thus leading to possible bridge failure with the inevitable loss of human lives. This shows the importance of the development of materials with self-healing ability. The development of materials that can provide real-time monitoring is extremely important in this context. Energy efficiency is also a very important issue under the 2030 United Nations Agenda for Sustainable Development. That is why seven chapters of this book are related to this theme as a way to minimize climate change impacts (goal 13) because energy production is the main aspect that is responsible for global greenhouse-gas (GHG) emissions. As the source of two-thirds of global GHG emissions, the energy sector is therefore pivotal in determining whether or not climate change goals are achieved. Photocatalytic water treatment is also covered in this book because it is a very important issue. For instance, Bhati and Rai (2017) show a worrying water scarcity scenario in India in the next decades. Also limited freshwater availability is identified as one of nine planetary boundaries and 4 billion people face severe water stress during at least 1 month per year, and 1.8 billion at least 6 months per year (Vanham et al., 2018). And that is why availability and sustainable management of water is also a goal of the UN agenda for sustainable development. This book’s structure gives an added value because other books concerning

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nanotechnologies for the construction sector have preferred to focus on technologic advancements and much less on societal challenges.

1.3

Outline of the Book

This book provides an updated state-of-the-art review on nanotechnologies for ecoefficient construction materials. Part I encompasses mortars and concrete-related applications (Chapters 2e7). Chapter 2 concerns the influence of nanoparticles on the mechanical strength of ultra-high performance concrete (UHPC). The influence of various supplementary cementitious materials such as metakaolin and fine ground fly ash and curing regimes (in water, steam, and autoclave curing) on the properties of UHPC are analyzed. The influence of the nanosilica as the most frequently applied nanomaterial and the curing regimes on the strength of UHPC are studied. Nanomaterial dispersion and distribution have significant influence on the properties of cement-based materials. Techniques for better homogenization of nanoparticles are listed. Chapter 3 discusses the effect of nanoparticles on the self-healing capacity of high performance concrete. Additionally, modifications promoted in certain properties by self-healing systems not based on nanoparticles in HPC are also given. Chapter 4 overviews the findings of inclusion of graphene oxide (GO) in the cementitious materials. This chapter describes in one way the improvements in the performance of the cementitious composites and in the other way the new properties that can be shaped. Chapter 5 deals with applications of nanomaterials in alkali-activated binders. The advantages and disadvantages of applying nanomaterials are assessed, and the healthrelated issues on applying nanomaterials are discussed. The effects of different types of nanomaterials, including nanosilica, nano-TiO2, nanoclay, and nanocarbon tube on the performance of alkali-activated materials and their optimal dosage are summarized. Chapter 6 covers the case of geopolymer composites reinforced with nanofibers. The effects of reinforcements on fracture toughness of geopolymers are addressed. A comparative analysis between the nanofibers with low and high interfacial bonding with geopolymer is presented. Chapter 7 provides a review of the use of nanoindentation for evaluation of properties of cement hydration products. The effect of fly ash addition on the mechanical properties of hydration products is also discussed based on the results of experimental nanoindentation studies. Applications for pavements and other infrastructure materials are the subject of Part II (Chapters 8e17). Chapter 8 reviews the performance of asphalt mixture with nanoparticles. It includes the types of nanoparticles, laboratory techniques of preparation of nanoparticles modified by asphalt mixture; properties like resilient modulus, dynamic creep, rutting distress, moisture susceptibility, and aging.

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Chapter 9 summarizes the current research and findings on performance enhancement of the nanomodification on asphalt mixture. It reviews the main nanomaterials applied for the modification, and it also addresses the dispersion protocol for these nanomaterials into the asphalt binder. Chapter 10 addresses the case of graphene oxide-GOemodified asphalt. The performance of this material is assessed through Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), gas chromatography-mass spectrometer (GC-MS), and thermogravimetric (TG) analysis. Chapter 11 reviews the recent progress of classification, fabrication and signal measurement, sensing mechanism, and application of nanocomposites for structural health monitoring. Chapter 12 presents a case study chapter on concrete with nanomaterials and fibers for self-monitoring of strain and cracking subjected to flexure. The effect of nanocarbon black (NCB) and conductive fibers on the workability of fresh concrete is analyzed, the relationships between the FCR and the strain of initial geometrical neutral axis (IGNA) are established, and the self-sensing ability to the loaddeflection process and cracking behavior of triphasic conductive concrete beam subjected to bending are investigated. Chapter 13 is related to icephobic nanocoatings for infrastructure protection. This review focuses on organic, inorganic, and hybrid nanocoatings, especially from the past decade, that have shown promise for future infrastructure applications. The chemical components of different coatings along with the application technique, anti/deicing tests and results have been summarized. Chapter 14 presents a review on nanocoatings for protection against corrosion. A range of coating materials, including polymers, metals, and ceramics, is described. The focus is on self-repairing nanocoatings. Various self-healing mechanisms and attitudes for achieving self-healing anti-corrosion coatings are examined. Developments and trends in coating technology are also discussed. The final section emphasizes particularly on functional nanocoatings in commercial applications. Taking into account the toxicity of the conventional coatings based on heavy metals, Chapter 15 covers the protection against corrosion by using nanofiller-based coatings. Various aspects of nanocoating performance against steel corrosion are focused in this chapter including the nanofiller type, their incorporation into varying corrosion coatings, and efficiency towards corrosion protection of steel substrate. Chapter 16 concerns fire retardant nanocoating for wood protection. The prospects and multifunctionality of nanocoatings for wood were demonstrated using as example the influence of layered aluminosilicates, nanooxides, nanosilica sol, and nanostructured carbon materials on the fire resistance of polymers, as well as on the flame retardant effectiveness of intumescent coatings and impregnations. Part III encompasses applications for building energy efficiency (Chapters 18e23). Chapter 17 reviews the current state-of-the-art of the aerogel-enhanced opaque systems. First cement-based products are reviewed. Then, the chapter describes aerogel-enhanced renders and plasters, proposed by different companies worldwide or developed by the author. Later, the focus moves on to aerogel-enhanced blankets.

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Finally, future research challenges for making aerogel-enhanced products more common in buildings are presented. Chapter 18 presents an overview of advanced glazing technologies, various aspects of aerogel windows and glazing units including: application in buildings, research progress, commercial status, case studies, and challenges ahead. Chapter 19 reports a review of current research trends aiming at the design of multifunctional architectural glazings. Challenges, opportunities, and technological evolutions are reported, with special reference to highly performing materials, like perovskites. Chapter 20 introduces the basics of electrochromic technology with a view towards its nanotechnology aspects, presents a case study on electrochromic foil for glass lamination, and ventures into some forward-looking aspects. Special consideration is given to the energy savings and other assets that are possible with electrochromic glazing. Chapter 21 addresses the case of VO2-based thermochromic materials (flexible foils and coated glass) for energy building efficiency. Future issues about the direction of these materials are discussed. Part IV is concerned with photocatalytic applications (Chapters 24e29). Chapter 22 provides a short overview of these mechanisms, focusing then on construction materials modified through the addition of titanium dioxide nanoparticles. Examples will be given of laboratory experiments carried out in the recent years, and of current applications in the built environment; attention will also be paid to the International standards that have been, and are still being, developed for this technology. Chapter 23 addresses the formulation of new photocatalysts with wider range of radiation harvesting by doping with metals or nonmetals, composites, or mixtures to increase interaction with substrate, and promoting pollutant adsorption has been discussed. Different techniques for coating application; main characterization methodologies to determine properties of substrate, photocatalyst, and coated thin film; and demonstration of photodegradation activity are examined. Chapter 24 presents an overview about the self-cleaning efficiency of nanoparticles applied on façade bricks. It covers the degradation of building façade s by natural and artificial agents. It also includes a short description of superhydrophobic additives. Photocatalytic and superhydrophilic additives are also discussed. The methods for façade preparation, the determination of self-cleaning properties, and the assessment of durability are also covered. Chapter 25 covers nanotreatments to inhibit microalgal fouling on building stone surfaces. Ananalysis of the biocidal ability of photocatalytic TiO2-based nanocompounds (in combination with Ag and Cu nanoparticles) sprayed on travertine surfaces to limit or inhibit algal colonization (using accelerated test in laboratory conditions) is presented. Chapter 26 describes the features of self-cleaning coating system. The chapter includes case studies concerning cooling load reduction for sites in Japan, Malaysia, and Thailand. Chapter 27 addresses the case of photocatalytic water treatment. Oxidation processes and photocatalysis are reviewed. Synthesis and characterization of titanium

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dioxide thin films are addressesd. The chapter also covers the degradation of different contaminants. Finally, Part V concerns toxicity, safety handling, and environmental impacts (Chapter 31; Chapters 28e31). Chapter 28 reviews nanoparticle toxicity and their adverse health effects. The following aspects are covered: nanoparticle size, shape and aggregation, and composition. Inhalation and ingestion of nanoparticles are addressed. Chapter 29 is concerned with managing risks related to the use of nanomaterials in the construction industry, including management of emission and exposure scenarios. Chapter 30 covers measurement techniques of exposure to nanomaterials in workplaces. Definitions as well as techniques and parameters for exposure measurements are reviewed. Devices and measurement techniques for workplaces are analyzed. Chapter 31 closes Part V with a chapter on the life cycle of engineered nanoparticles. In this chapter, we analyze potential releases across the life cycle of engineered nanomaterials (ENMs), and then focus on the application of the methodology of LCA to quantify the environmental impacts of ENMs throughout their life cycle. We use a review of available studies to highlight the benefits of applying LCA to ENMs, and describe data and methodological gaps that still require increased research efforts.

References Archibugi, D., 2017. Blade runner economics: will innovation lead the economic recovery? Research Policy 46 (3), 535e543. Bhati, M., Rai, R., 2017. Nanotechnology and water purification: Indian know-how and challenges. Environmental Science and Pollution Research 24 (30), 23423e23435. Dong, H., Gao, Y., Sinko, P., Wu, Z., Xu, J., Jia, L., 2016. The nanotechnology race between China and the United States. Nano Today 11, 7e12. Glasser, F., Marchand, J., Samson, E., 2008. Durability of concrete. Degradation phenomena involving detrimental chemical reactions. Cement and Concrete Research 38, 226e246. Hanus, M.J., Harris, A.T., 2013. Nanotechnology innovations for the construction industry. Progress in Materials Science 58 (7), 1056e1102. Jones, W., Gibb, A.G., Goodier, C.I., Bust, P.D., Song, M., Jin, J., 2016. Nanomaterials in construction-what is being used, and where? ICE-Construction Materials In Press. Karaulova, M., Shackleton, O., Liu, W., Goek, A., Shapira, P., 2017. Institutional change and innovation system transformation: a tale of two academies. Technological Forecasting and Social Change 116, 196e207. Kim, J., Rivera, J., Meng, T., Laratte, B., Chen, S., 2016. Review of life cycle assessment of nanomaterials in photovoltaics. Solar Energy 133, 249e258. Lundvall, B.Å., 2017. Is there a technological fix for the current global stagnation?: A response to Daniele Archibugi, Blade Runner economics: will innovation lead the economic recovery? Research Policy 46 (3), 544e549. Momaya, K., Lalwani, L., 2017. Systems of technological innovation: a review of research activities taking the case of nanotechnology and India. Technology Analysis and Strategic Management 29 (6), 626e641.

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Ozcan, S., Islam, N., 2017. Patent information retrieval: approaching a method and analysing nanotechnology patent collaborations. Scientometrics 111 (2), 941e970. Pacheco-Blandino, I., Vanner, R., Buzea, C., 2012. Toxicity of nanoparticles. In: PachecoTorgal, F., Jalali, S., Fucic, A. (Eds.), Toxicity of Building Materials. Woodhead Publishing Limited- Elsevier Science and Technology, Abington Hall, Cambridge, UK, pp. 427e475. Pacheco-Torgal, F., 2017. Introduction to Eco-efficient repair and rehabilitation of concrete infrastructures. In: Pacheco-Torgal, F., Melchers, R., de Belie, N., Xi, S., Tittelbom, N., Saez, A. (Eds.), Eco-efficient Repair and Rehabilitation of Concrete Infrastructures, vol. 1. Elsevier Science and Technology, Abington Hall, Cambridge, UK, pp. 1e12. Pei, H., Li, Z., Zhang, J., Wang, Q., 2015. Performance investigations of reinforced magnesium phosphate concrete beams under accelerated corrosion conditions by multi techniques. Construction and Building Materials 93, 989e994. Rehman, S., Ibrahim, Z., Memon, S., Jameel, M., 2016. Nondestructive test methods for concrete bridges: a review. Construction and Building Materials 107, 58e86. Spitzmiller, Mahendra, S., Damoiseaux, R., 2013. Safety issues relating to nanomaterials for construction applications. In: Pacheco-Torgal, D., Nazari, G. (Eds.), Nanotechnology in Eco-efficient Construction. Woodhead Publishing Limited- Elsevier Science and Technology, Abington Hall, Cambridge, UK, pp. 127e158. Sabatier, M., Chollet, B., 2017. Is there a first mover advantage in science? Pioneering behavior and scientific production in nanotechnology. Research Policy 46 (2), 522e533. Schlangen, E., Sangadji, S., 2013. Adressing infrastructure durability and sustainability by self healing mechanisms-Recent advances in self healing concrete and asphalt. Procedia Engineering 54, 39e57. Shapira, P., Youtie, J., 2015. The economic contributions of nanotechnology to green and sustainable growth. In: Basiuk, V., Basiuk, E. (Eds.), Green Processes for Nanotechnology. Springer International Publishing, pp. 409e434. Shapira, P., Wang, J., 2009. From lab to market? Strategies and issues in the commercialization of nanotechnology in China. Asian Business & Management 8 (4), 461e489. Taalbi, J., 2017. What drives innovation? Evidence from economic history. Research Policy 46 (8), 1437e1453. Terekhov, A.I., 2017. Bibliometric spectroscopy of Russia’s nanotechnology: 2000e2014. Scientometrics 110 (3), 1217e1242. Tollefsson, J., 2018. China declared world’s largest producer of scientific articles. Nature 553, 390. van Raan, A.F., 2017. Patent citations analysis and its value in research evaluation: a review and a new approach to map technology-relevant research. Journal of Data and Information Science 2 (1), 13e50. Vanham, D., Hoekstra, A.Y., Wada, Y., Bouraoui, F., de Roo, A., Mekonnen, M.M., et al., 2018. Physical water scarcity metrics for monitoring progress towards SDG target 6.4: an evaluation of indicator 6.4. 2 “Level of water stress”. The Science of the Total Environment 613, 218e232. Wang, G., Guan, J., 2010. The role of patenting activity for scientific research: a study of academic inventors from China’s nanotechnology. Journal of Informetrics 4, 338e350. Zhang, Z., Jin, J., Guo, M., 2017. Catch-up in nanotechnology industry in China from the aspect of process-based innovation. Asian Journal of Technology Innovation 25 (1), 5e22. Zhu, H., Jiang, S., Chen, H., Roco, M.C., 2017. International perspective on nanotechnology papers, patents, and NSF awards (2000e2016). Journal of Nanoparticle Research 19 (11), 370.

Influence of nanoparticles on the strength of ultra-high performance concrete

2

Ksenija Jankovic , Dragan Bojovic , Marko Stojanovic Institute for Materials TestingdIMS, Belgrade, Serbia

2.1

Introduction

Modern researchers in concrete technology pay attention to the development of materials with improved mechanical properties and increased durability (Cwirzen et al., 2008a). During the last 20 years, ultra-high performance concrete (UHPC) or cementitious composites have become an introduction of the most favorable ingenious high technology types of concrete (Acker and Behloul, 2004; Rebentrost, 2008; Fehling and St€ urwald, 2012). The world’s first engineering structures designed with UHPC were the Sherbrooke footbridge in Sherbrooke, Quebec built in 1997, Seonyu pedestrian bridge in South Korea built in 2002, and many constructions for highway bridges in the United States by the Federal High-Way Administration (Resplendino, 2008). UHPC is a modern composite material which was the outcome of the techniques of improving the microstructure. UHPC has extremely good mechanical properties with compressive strength greater than 150 MPa and a compact microstructure due to the large content of cement and silica fume as a binder, fine sand of 150e600 mm sizes, and crushed quartz of about 10 mm size (Lehmann et al., 2009). Very low water to binder ratio is also typically used in UHPC mixes resulting in reduced workability that may be managed by adding an effective superplasticizer (Graybeal, 2005; Tue et al., 2008). In order to obtain the desired mechanical properties of UHPC, enhancing the stiffness and strength of the interfacial transition zone (ITZ) between aggregates and the binding paste matrix to a level comparable to that of a bulk paste aggregate is vital (Smith et al., 2014). A large amount of Ca(OH)2 crystal is produced due to the hydration reaction between water and cement. Ca(OH)2 crystal is arrayed in the interfacial transition zone (ITZ). Silica materials such as silica fume and/or nanosilica can react with Ca(OH)2. The product of this reaction is the CeSeH gel and in this way the Ca(OH)2 crystals are absorbed and reduced. The CeSeH gel fills the voids, which improves the density of the interfacial transition zone (Ji, 2005). In cementitious systems, silica fume (SF) is the most commonly used amorphous silica which possesses an average particle size of about 10 times smaller than cement. It has been used in the ranges of 10%e25% by weight of cement since the 1950s, thus its pozzolanic and filling effects on the concrete properties have been widely Nanotechnology in Eco-efficient Construction. https://doi.org/10.1016/B978-0-08-102641-0.00002-5 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Nanotechnology in Eco-efficient Construction

known (Schr€ ofl et al., 2012). The pozzolanic reaction of silica with calcium hydroxide is more efficient with nanosilica (NS) which forms more CeSeH gel and at final stages increases the density of concrete (Ji, 2005; Quercia et al., 2012). Moreover, some researchers like Dunster (2009) agreed that contribution of SF and NS with concrete constituents would save the cement that accounts for sustainability of economic and environmental development. Nanotechnology has attracted considerable interest due to the new potential uses of particles in nanometer scale associated with high specific surface area, high purities, and small primary particles (Sanchez and Sobolev, 2010). When designing UHPC it is very important to follow the basic principles: reduce porosity, improve the microstructure, enhance homogeneity, and increase ductility. Apart from adhering to the basic principles for designing the UHPC composition, raw materials, preparation techniques, and curing regime can also influence on its characteristics. The binder content in UHPC is sometimes up to 5 times higher when compared to normal concrete. Therefore it is necessary to always consider a partial replacement of binder (cement and silica fume) and reduce the material cost of UHPC (Jankovic et al., 2011). At the same time UHPC with appropriate amounts of supplementary materials such as fly ash, slag, nanosilica, etc. could achieve compressive strengths of 150e200 MPa after a normal curing regime. In order to ensure the required fresh properties of UHPC, it is essential to choose the right amount and type of superplasticizer (Jankovic et al., 2011). The length of the side chain of polycarboxylates has an important influence on the performance of fresh and hardened UHPC. Superplasticizers with a long side chain length gave the highest early strength. An integral part of the UHPC matrix is steel fibers. Well selected dosage (Jankovic et al., 2010) could enhance the mechanical properties especially ductility, which is most important for very brittle materials, and decrease autogenous shrinkage of UHPC. Moreover, a combination of macro and microsteel fibers could produce tensile strain hardening behavior. With the increase in the amount of steel fiber in the cement composite, the tensile properties increase significantly. By increasing the mechanical characteristics, especially the tensile strength, the need for the reinforcement is reduced partly or completely. Increased amounts of fibers could decrease the workability of the mixture of fresh UHPC. Steam curing of UHPC at high temperatures has from the beginning been the subject of much research. The effect of hydrothermal treatment of UHPC is seen in the formation of an improved microstructure with the crystalline calcium silicate phase (CeSeH phase). The microstructure is improved due to the pozzolanic reactions between CH from the hydration of cement and supplementary materials such as silica fume and nanosilica. High temperature exposure affects the reaction of the pores in composite texture and results in the increase in compressive strength, compared to the samples cured under ambient conditions. Most current applications of UHPC are accomplished by factory prefabrication and on-site assembling. In consideration of the high cost and complexity of the

Influence of nanoparticles on the strength of ultra-high performance concrete

15

curing process, the use of conventional materials and common technology, such as conventional casting and room temperature curing, is the trend for UHPC (Shi et al., 2015).

2.2

Types of nanomaterials in cement-based composites

Modern infrastructures require possessing the specific functions in various aspects which provides a good opportunity for the research and development of new functional cementitious composites (Hang et al., 2003; Luo, 2009; Han et al., 2015). Nanotechnology developments will definitely affect the field of construction and construction materials (Cui et al., 2017). Nanomaterials can be defined as the particles with the particle size between 1 and 100 hm (Zhang et al., 2017). Based on the dimensions of the nanomaterials, they can be divided into four groups: zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and threedimensional (3D) nanoparticles (Pokropivnya and Skorokhoda, 2007, 2008). A 0D nanoparticle is defined as a particle with all its dimensions subjected to nanoscale. This group includes nanoparticles, nanoclusters, and nanocrystals. Nano-SiO2, TiO2, ZnO, and CaCO3 nanoparticles are categorized as 0D. A 1D nanoparticle can be described as having two dimensions in nanoscale, with the other dimension reaching above the nanoscale. Nanotubes, nanofibers, nanorods, and nanowires belong to this category. A 2D nanoparticle is originally a sheet with its thickness in the nanoscale and its sides spreading beyond the “nano” criterion. This group covers nanofilms, nanolayers, and nanocoatings. Three-dimensional nanomaterials include powders, fibrous, multilayer, and polycrystalline materials in which the 0D, 1D, and 2D structural elements are in close contact (Korayem  et al., 2017; Stefan cic, 2017). Effects of nanoparticles on the cement binder: 1. With their incorporation the grain composition of the particles changes, which affects the rheological properties of the cement binder in the fresh state. 2. They provide an additional surface area for the growth of hydration products. 3. They accelerate the development of hydration products. 4. Theyact as bridging particlesdreduce the distance between the hydration products. 5. With the application of reactive nanoparticles, a certain proportion of new hydration product  is obtained (Stefan cic, 2017).

Nanoconcrete is a concrete that utilizes nanomaterials or a concrete with nanomaterials added in which the size of the nanoparticles is less than 500 hm (Maholtra and Mehta, 1996; Aïtcin, 2000; Hui et al., 2004; Pellenq et al., 2008; Hou et al., 2015). Testing of the application of nanomaterials to improve the properties of concrete is very important (Li et al., 2004; Nazari and Riahi, 2010a; Nazari and Riahi, 2010b; Sanchez and Sobolev, 2010; Jalal et al., 2013; Rupasinghe et al., 2017). Nanoparticles such as nano-SiO2 (NS), nano-Al2O3 (NA), nano-TiO2 (NT), nano-Fe2O3 (NF), nano-ZnO2, nanoclay (NC), carbon nanotubes (CNT), and carbon nanofibers (CNF) can be used in cementitious materials (Sanchez and Sobolev, 2010).

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Nanotechnology in Eco-efficient Construction

Concretes with the addition of nanoparticles had lower porosity in relation to pore size of up to 0.015 mm (Niewiadomski et al., 2017). Nanomaterial in concrete accelerates the hydration of cement due to the seeding effect (Hou et al., 2012; Kong et al., 2015; Zhang et al., 2015), improves strength due to the seeding effect/high reactivity and the filler effect (Nazari and Riahi, 2011a; Du et al., 2014; Du et al., 2015; Heikal et al., 2015; Nochaiya et al., 2015), and thus contributes to an improved durability of the concrete (Moradpour et al., 2013; Saloma et al., 2015; Mohseni et al., 2016a). The fresh and hardened behavior of cement-based materials with NS could be enhanced by the mechanisms of filler effect and acceleration of cement hydration through the nucleation effect and pozzolanic effect (Garcia-Taengua et al., 2015). Nano-SiO2 is most widely used due to its ultra-high pozzolanic reactivity (Hou et al., 2013a; Hou et al., 2015). The addition of nanosilica especially improves the ITZ (Xu et al., 2017). Nano-SiO2 can make the hardened cement mortar less water-absorbable (Hou et al., 2015). The type, dosage, and size of SiO2 nanoparticles influence the compressive strength of HPC (Zhang et al., 2017). NS of particle size 40 hm produced a higher compressive strength compared with NS of 12 and 20 hm particle sizes due to its poor dispersion which results in agglomeration (Haruehansapong et al., 2014). The incorporation of pyrogenic nanosilica has more influence on increasing the compressive strength than on the splitting tensile and flexural strengths (Mobini et al., 2015). By adding 2% of nanosilica and 8% of microzeolite a more durable composite is obtained (Eskandari et al., 2015). Nanoalumina was identified as a setting retarder (Land and Stephan, 2015). Nanoalumina fills up the pores of inert materials and improves compactness of samples (Mohseni et al., 2016b). Hardened samples of mortar and concrete with binders incorporating nanoparticles of a-Al2O3 were characterized by lower strengths, increased overall porosity, and reduced resistance to degradation processes, as compared to the reference mortars and concretes. However, due to the increased proportion of entrapped pores, the modi  fied samples showed improved frost resistance (Stefan cic, 2017; Stefan cic et al., 2017). There is a direct relationship between the TiO2 content and the color of mortar (Pozo-Antonio and Dionísio, 2017). Compared with nano-TiO2, the core/shell TiO2@SiO2 nanoparticles exhibited better hydration properties in terms of accelerated cement hydration, higher degree of hydration, and lower porosity (Sun et al., 2017). Application of silicaetitania (mSiO2/TiO2) core/shell nanostructures exhibited self-cleaning and bactericidal properties when exposed to UV light (Sikora et al., 2017). With the increased use of Nano-MgO the compressive strength increased, while autogenous shrinkage and setting time decreased. Nano-MgO can be used as an expanding additive (Polat et al., 2017). The Fe3O4@SiO2 NPs showed as an ideal wave absorption surface-treatment agent for cement-based composites (Wang et al., 2017). Cementitious composites with carbon nanofibers, carbon nanotubes, and graphene oxide (CNFs, CNTs, and GO) showed significant improved compressive strength and

Influence of nanoparticles on the strength of ultra-high performance concrete

17

open porosity from the plain one (Tzilerogloua et al., 2017). Hydrated degree of cement paste increased with the addition of graphene oxide (GO) (Yang et al., 2017). The addition of nanographite platelets (NGPs) can effectively modify the mechanical, thermal, and electromagnetic properties of cementitious composites (Cui et al., 2017). Carbon nanotubes improve flexural strengths, fracture toughness, and other engineering properties of cementitious matrices because they have the role of reinforcement (Singh et al., 2017). With the incorporation of carbon nanotubes electromagnetic properties of the cement composites are improved (Mendoza Reales and Toledo Filho, 2017). A concrete column with nanofiber failed at higher loads with larger deflections that the columns made up of steel confined reinforced concrete (Venkat Raoa et al., 2015).

2.3

Role of nanoparticles in ultra-high performance concrete (UHPC)

The efficiency of UHPC depends, in particular, on its density. Actually, optimizing particle packing, ultra-high consolidation concrete matrix can be achieved (Ghafari et al., 2015). Nanoparticles, such as nanosilica (nano-SiO2), nano-CaCO3, nanotitanium oxide (nano-TiO2), and nanoiron (nano-Fe2O3), etc., can be added to UHPC. They can act as nuclei for cement phases, further promoting cement hydration due to their high reactivity, and as nanoreinforcement and/or as filler, densifying the microstructure and the ITZ, thereby, leading to a reduced porosity (Sanchez and Sobolev, 2010). By using nanosilica concrete better mechanical (Taheri-Behrooz et al., 2015), durability (Du et al., 2014), physical (Aly et al., 2012), and microstructure (Oltulu and Sahin, 2014) properties can be obtained. Large capillary pores were refined by nanosilica, due to the combined contribution of the nanofiller effect and the pozzolanic reaction (Du et al., 2014). With the addition of nanosilica there is a change in pore volume, pore-size distribution becomes finer, and microvoids in concrete are densified and refined to provide a neat concrete microstructure (Lindgreen et al., 2008; Belkowitz and Armentrout, 2009). NS could efficiently improve the microstructure of ITZ between the aggregates and the cement paste (Ghafari et al., 2012). Increasing the nanosilica content influences the improvement of the mechanical properties of UHPC especially at early ages due to the nucleation of CeSeH phases on the silica surface (Ghafari et al., 2015). Nanosilica in UHPC has similar effects to silica fume or microsilica in terms of performance strength and durability enhancement (Qing et al., 2007; Indhumathi et al., 2011; Bai et al., 2014). The results show that UHPC containing 2% NS exhibited the best results of compressive strength, splitting tensile strength, modulus of elasticity, flexural strengths, loade displacement behavior, and fracture energy at 90 days. The effect of 1% NS is almost equal to that of 10% SF (Gesoglu et al., 2016). The influence of NS on the properties

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Nanotechnology in Eco-efficient Construction

of UHPC depends on its type. Pyrogenic and precipitated NS and NS gels are in the form of relatively large agglomerates of spherical particles and their dispersion in water is difficult, while NS hydrosols are monodispersed particles in water (Madani et al., 2012). Determination of the reactivity of NS is most recommended as an essential step prior to its usage in UHPC (Oertel et al., 2014). The addition of nano-CaCO3 has influence on the increase of UHPC compressive and flexural strengths after hot water curing (Huang and Cao, 2012). TiO2 nanoparticles could introduce self-healing and smog-eating capabilities through photocatalysis (Falikman et al., 2012). The addition of TiO2 in UHPC has a great effect on self-cleaning ability (Maravelaki-Kalaitzaki et al., 2013). TiO2 accelerates the strength of concrete at early age. Also, abrasion resistance in concrete increases (Pacheco-Torgal and Jalali, 2011). TiO2 acts as a glass layer or pigment outside the concrete particles and also in the microstructure of UHPC and concrete. TiO2 in concrete forms a fiber reinforced system which contributes to the strength enhancement and more durable concrete (Chen and Poon, 2009). Incorporating nanometakaolin refines the microstructure of UHPC due to its work as an ultra-filler. Nanometakaolin also produces a secondary hydration product by optimizing the remaining calcium hydroxide (Fadzil et al., 2013). The application of nanoclay improves chloride penetration resistance of UHPC (Mohd Faizal et al., 2015). The addition of nanoalumina in cement speeds up the initial setting time for UHPC and reduces segregation. Also, nanoalumina acts as a nanofiller and refines the microstructure. Nanoalumina in UHPC acts as a dispersion agent in cement particles (Rosenqvist, 2002; Jo et al., 2007; Nazari and Riahi, 2011b; Hosseini et al., 2014). The cement content in UHPC is high, so nanoalumina is very important in dispersion of cement grains in UHPC. Without nanoalumina refining the hydration product, the hydration process would be slower because the internal structure of the hydration gel cannot be penetrated by silica component (Richardson, 1999; Rosenqvist, 2002; Hui et al., 2004). The advantage of using carbon nanotubes (CNT) in UHPC is improving its flexibility. Also, CNT has influence on increasing the UHPC strength (Morsy et al., 2011). With the addition of CNT, improvement in tension and compression abilities of UHPC can be obtained (Rostamiyan et al., 2015). Steel reinforcement can be replaced by CNT in UHPC. Carbon nanotubes could introduce piezoresistivity and make UHPC a self-sensing material (Shah et al., 2012). Meng and Khayat (2018) found that the increase in compressive strength can be attributed to the “bridging effect” of the carbon nanofibers and graphite nanoplatelets for microcracks and the “filler effect” for increasing the cement degree of hydration and reduced total porosity of the UHPC.

2.4

UHPC curing regimes

Properties of UHPC depend on its curing regimes (Soliman and Nehdi, 2011; Dils et al., 2012, 2014; Wille et al., 2012). Investigation results of the influence of different

Influence of nanoparticles on the strength of ultra-high performance concrete

19

curing regimes such as normal curing, curing in hot water, and steam curing on the mechanical properties of UHPC with blast-furnace slag (GGBS) or fly ash show that hot water and steam curing significantly improved compressive and flexural strengths compared to standard curing with the same curing duration. With extended curing in water after 28 days comparable flexural properties as those under hot water and steam curing can be obtained (Wu et al., 2017). High temperature in combination with pressure curing can improve the properties of UHPC. Applying a one day steam curing regime leads to an increase in the compressive strength of reactive powder concrete up to 30 MPa compared to the samples which were in water for 28 days. If 8 h autoclaving is used, a strength over 200 MPa is achieved (Zhang et al., 2008). Investigation of the influence of curing regimes and different supplementary cementitious materials on the strength of reactive powder concrete has shown that the compressive strength increased considerably after steam and autoclave curing, while the flexural strength and toughness decreased compared to those after room temperature curing (Yazici et al., 2009). It was explained in the increase in pozzolanic activity of silica fume, crushed quartz, and the chain length of CeSeH after high temperature curing (Masse et al., 1993). Also, under high temperature, a-calcium silicate hydrate (a-C2SH) can be converted to tobermorite in the presence of silica fume which can result in the decrease in porosity, improvement in strength, as well as a bond between matrix and aggregate/fiber (Aldea et al., 2000; Yang et al., 2000).

2.4.1

Curing in water

Standard room temperature curing is the most common, economical, and environmentally friendly process, which can be applied to practice. Researches on hydration and pozzolanic reaction of RPC showed that the average CeSeH chain length was short and the pozzolanic activity was weak when the curing temperature was 20 C (Zhang et al., 2008). If the curing age was reasonably prolonged, the compressive strength of UHPC could also reach 200 MPa (Zhang et al., 2008). During the curing of UHPC at standard temperatures, hundreds of millions of fine particles of supplementary cementitious materials such as silica fume, fly ash, etc. which chemically do not react fill the space between the cement particles. In literature this phenomenon is called particle packing model or microfiller effect and is similar to a small aggregate packing model in the pores of a large aggregate. In this way the particles of supplementary cementitious materials mechanically contribute to increasing the strength characteristics as they form a compact structure of the cement matrix (Nikolic, 2012; Bojovic, 2017). In the chemical sense after curing UHPC at a standard temperature the structure of supplementary cementitious materials remains the same until only a part enters in chemical reaction. For example, the silica fume which does not crystallize is in an amorphous state, remains in the same state, and therefore does not contribute significantly in the chemical sense. Considering the research in the field of influence of the amount of silica fume on the concrete strength, an experiment was made (Nikolic, 2012). The influence of the

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Nanotechnology in Eco-efficient Construction

amount of silica fume on the strength of the cement composite was observed. The diagrams in Figs. 2.1 and 2.2 show the effect of increasing the compressive strength on UHPC which is cured at a standard temperature with 2% fibers and without fibers. The same diagrams were done for flexural strength shown in Figs. 2.3 and 2.4. An increase in strength of about 10% with the increase in silica fume from 135 to 270 kg/m3 was achieved. Also, for one composite with 270 kg/m3 of silica fume and 2% of fibers a replacement of up to 2% of cement with nanosilica was done (Jankovic et al., 2019). In Fig. 2.5 it can be seen that there is no considerable influence on compressive strength of the cementitious composite.

120

Compressive strength, N/mm2

Steel fiber 0% 100 80 60 40 Silica fume 135 kg/m3

20

Silica fume 200 kg/m3 Silica fume 270 kg/m3

0 0

NC20°C/2d

NC20°C/7d

NC20°C/28d

Curing regime / days

Figure 2.1 Compressive strength of normal curing UHPC with silica fume without steel fiber (Nikolic, 2012).

Compressive strength, N/mm2

160 Steel fiber 2%

140 120 100 80 60 Silica fume 135 kg/m3

40

Silica fume 200 kg/m3

20

Silica fume 270 kg/m3

0 0

NC20°C/2d

NC20°C/7d

NC20°C/28d

Curing regime / days

Figure 2.2 Compressive strength of normal curing UHPC with silica fume and 2% steel fiber (Nikolic, 2012).

Influence of nanoparticles on the strength of ultra-high performance concrete

21

Flexural strength, N/mm2

10

Steel fiber 0%

9 8 7 6 5 4 3

Silica fume 135 kg/m3

2

Silica fume 200 kg/m3

1

Silica fume 270 kg/m3

0 0

NC20°C/2d

NC20°C/7d

NC20°C/28d

Curing regime / days

Figure 2.3 Flexural strength of normal curing UHPC with silica fume without steel fiber (Nikolic, 2012). 20

Flexual strength, N/mm2

Steel fiber 2% 15

10

Silica fume 135 kg/m3

5

Silica fume 200 kg/m3 Silica fume 270 kg/m3

0 0

NC20°C/2d

NC20°C/7d

NC20°C/28d

Curing regime / days

Figure 2.4 Flexural strength of normal curing UHPC with silica fume and 2% steel fiber (Nikolic, 2012).

2.4.2

Steam-curing regime

Through 24 h steam curing, about 15e30 MPa compressive strength could be further gained when compared to that after 28 days of standard room temperature curing (Zhang et al., 2008). Steam curing at 90 C for 12 days improved the compressive strengths of all mixtures with respect to standard room temperature curing. The effect on flexural strength showed a similar trend to that on compressive strength, but the influence degree on flexural strength was greater (Yazici, 2007). As the temperature increased to 90 C, it was important to prolong the heat curing duration time which influenced not only the pozzolanic activity of both silica fume and crushed quartz, but also increased the chain length of CeSeH (Masse et al., 1993).

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Nanotechnology in Eco-efficient Construction 200

Compressive strength, N/mm2

UnS0f2 UnS1f2

150

UnS2f2

100

50

0 NC20°C/2d

NC20°C/7d

NC20°C/28d

Normal curing regimes of concrete

Figure 2.5 Compressive strength of UHPC with 270 kg/m3 silica fume, 2% fiber content, and cement replacement by nanosilica 0%, 1%, and 2%dnormal curing (Jankovic et al., 2019).

High temperature curing accelerates the hydration of cement and improves the secondary hydration between supplementary cementitious materials (like slag, silica fume, and fly ash) and Ca(OH)2 (Wang et al., 2015). High temperature curing also has a positive effect on the pozzolanic reactions between CH from the hydration of cement and supplementary cementitious materials such as silica fume. Lee et al. (2018) found that the presence of MgO in silica fume after curing at high temperature increased the compressive strength up to 18.6%. It also increased the chain length of CeSeH (Shi et al., 2015). The effect of steam-curing regime of UHPC is seen in the formation of an improved microstructure with crystalline calcium silicate phases (CeSeH phases). High temperature exposure affects the reduction of the pores in composite texture and results in the increase of compressive strength, compared to the samples cured under standard temperature conditions (Jankovic et al., 2010, 2011). Nikolic (2012) investigated the impact of steam curing regime on the strength of cementitious composites with 0% and 2% fibers and silica fume in amounts of 135, 200, and 270 kg/m3. For the largest amount of silica fume 270 kg/m3 and 2% of fibers a replacement of cement was made in the amount of 1% and 2% with nanosilica (Jankovic et al., 2019). After 6 h, all samples were placed on steam curing at 90 C and humidity of 95% for 48 h. After 24 h of cooling, curing was set in standard temperature conditions until 28 days of age. Fig. 2.6 shows the results of compressive strength depending on the amount of silica fume in the cement composites. Fig. 2.7 shows the same mixture with the results for flexural strength. Such curing substantially increased the strength of concrete compared to standard curing in water. This increase was about 10%. At such concrete curing in concrete without fibers compressive strength increased up to 20% of the silica fume amount. Flexural strength was practically the same regardless of the amount of silica fume in the fiber-free cementitious composites. Cementitious composite

Influence of nanoparticles on the strength of ultra-high performance concrete

23

Compressive strength, N/mm2

180 160 140 120 100 80 60 SC95°C/48 h steel fiber 0%

40

SC95°C/48 h steel fiber 2%

20 0 135

200 Silica fume, kg/m3

270

Figure 2.6 Compressive strength of steam curing UHPC with silica fume and steel fiber 0% and 2% (Nikolic, 2012).

Flexural strength, N/mm2

20 18 16 14 12 10 8 6 NC20°C/28d steel fiber 0%

4

NC20°C/28d steel fiber 2%

2 0 135

200 Silica fume, kg/m3

270

Figure 2.7 Flexural strength of steam curing UHPC with silica fume and steel fiber 0% and 2% (Nikolic, 2012).

with 2% of fibers had significant increase in flexural strength for the largest amount of silica fume. Cementitious composite with nanosilica did not have significant increases in compressive and flexural strengths (Fig. 2.8). Obtained results are in line according to the allegations in the literature that a minimum quantity of nanosilica with significant effect on the strength of cementitious composites is 3.74% (Rong et al., 2015).

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Nanotechnology in Eco-efficient Construction

Compressive strength, N/mm2

180 UnS0f2

160 140

UnS1f2

120

UnS2f2

100 80 60 40 20 0

SC95°C/48 h Steam curing regimes of concrete

Figure 2.8 Compressive strength of UHPC with 270 kg/m3 silica fume, 2% fiber content, and cement replacement by nanosilica 0%, 1%, and 2%dsteam curing (Jankovic et al., 2019).

2.4.3

Autoclaving

The compressive strength of UHPC after autoclave curing was higher than those after standard room temperature curing and heat curing (90  C). It was only through 8 h autoclave curing that over 200 MPa compressive strength could be achieved for UHPC with 3% or 4% of fibers (Yang et al., 2009). Yazici (2007) found that compressive strengths of UHPC mixtures with high volume mineral admixtures after 8 h of high pressure steam curing at 210  C were higher than the specimens cured in water at room temperature. Massidda et al. (2001) found that the flexural strength of specimens reached 30 MPa after 3 h of high pressure steam curing. However, Yazici et al. (2009) found that autoclave curing greatly reduced the flexural strength compared with those after 28 days of standard room temperature curing. The interfacial bond strength after autoclave curing achieved 14.2 MPa, higher than that after steam curing or standard room temperature curing (Zhang et al., 2008). Curing regimes also affect shrinkage and creep of UHPC. Approximately 87% of the total autogenous shrinkage occurred during the thermal treatment due to accelerated rate of early hydration at higher temperature (Garas, 2009). However, its complicated operation and high energy consumption limit its application in practice. Development of UHPC production at room temperature is the key for its wide applications (Shi et al., 2015). Results of the influence of autoclave curing at 180 C on the physical and mechanical properties of reactive-powder concrete reinforced with brass-coated steel fibers have shown that this type of curing is beneficial both in terms of flexural and compressive strengths. Specimens reached flexural and compressive strengths of 30 and 200 MPa, respectively (Zhang et al., 2008). The research (Nikolic, 2012) clearly shows the contribution of the autoclave to the compressive strength of concrete. The influence of the increase in the amount of silica fume can be seen from the diagram in Fig. 2.9. In the cement composite with 2% of fibers, the increase in strength was 25%e30% depending on the applied amount of

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Compressive strength, N/mm2

250

200

150

100 AC20bar/4 h steel fiber 0%

50

AC20bar/4 h steel fiber 2% 0 135

200 Silica fume, kg/m3

270

Figure 2.9 Compressive strength of autoclave curing UHPC with silica fume and steel fibers 0% and 2% (Nikolic, 2012).

silica fume. Also, the obtained flexural strength was similar and the results are shown in Fig. 2.10. In another study where the effects of other materials such as metakaolin and fly ash were studied, this curing method significantly contributed to the mechanical properties of the cement composite (Jankovic et al., 2011; Nikolic, 2012). Figs. 2.11 and 2.12 show the obtained results. Using this curing method, mechanical properties of 30 MPa and 200 MPa flexural strength and compressive strength are obtained, respectively.

Flexural strength, N/mm2

25

20

15

10 AC20bar/4 h steel fiber 0%

5

AC20bar/4 h steel fiber 2%

0 135

200 Silica fume, kg/m3

270

Figure 2.10 Flexural strength of autoclave curing UHPC with silica fume and steel fibers 0% and 2% (Nikolic, 2012).

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Nanotechnology in Eco-efficient Construction 250 AC20bar/4 h

Compressive strength, N/mm2

Steel fiber 4% 200

150

100

50

0 Silica fume 270 Fine blended Fly metakaolin 10% Fine blended Fly metakaolin 20% kg/m3 Ash 10% Ash 20%

Mineral powder admixtures

Figure 2.11 Compressive strength of autoclave curing UHPC with mineral powder admixture and 4% steel fiber (Jankovic et al., 2011; Nikolic, 2012). 35 AC20bar/4 h

Steel fiber 4% Flexural strength, N/mm2

30 25 20 15 10 5 0 Silica fume 270 Fine blended Fly metakaolin 10% Fine blended Fly metakaolin 20% kg/m3 Ash 10% Ash 20%

Mineral powder admixtures

Figure 2.12 Flexural strength of autoclave curing UHPC with mineral powder admixture and 4% steel fiber (Jankovic et al., 2011; Nikolic, 2012).

In a study (Jankovic et al., 2019) of the impact of autoclaving UHPC with nanosilica with the experiment described in Section 2.4.2, the samples were cured at a pressure of 20 bar for 4 h. Concrete with 270 kg/m3 silica fume and 2% steel fiber content was considered and the amount of nanosilica of 1% and 2% was varied as a cement replacement. The obtained results are shown in Fig. 2.13. The influence of nanosilica on compressive strength is practically negligible as in the case of steam curing of UHPC with nanosilica.

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Compressive strength, N/mm2

200 180

UnS0f2

160

UnS1f2

140 UnS2f2

120 100 80 60 40 20 0 AC20bar/4 h Autoclav curing regimes of concrete

Figure 2.13 Compressive strength of UHPC concrete with 270 kg/m3 silica fume, 2% fiber content, and cement replacement by nanosilica 0%, 1%, and 2%dautoclave curing (Jankovic et al., 2019).

2.5

Strength of UHPC

In Section 2.4, the impact of the curing regimes on the structure of the cement composite is examined from the aspect of strength. The chosen curing regime is significantly influenced and cannot be ignored when considering the strength of UHPC. Besides curing regimes, the used component materials have an influence on mechanical characteristics of UHPC. Component materials differently affect each type of UHPC strengths. Thus, fibers have an impact on the tensile and bending characteristics of the cement composite, while micro and nanomaterials significantly increase the compressive strength. Because of this, it is necessary to separately observe each strength type of cement composite.

2.5.1

Compressive strength

Since its inception, concrete has been considered as a material in construction carrying on pressure. Consequently, it was found that the basic property of concrete is compressive strength. For cement composites such as UHPC the compressive strength is significantly higher than for conventional concrete. In order to achieve extremely high strength, it is necessary to reduce the waterecement ratio and to improve the structure of the cement matrix. According to the instances of many authors, the structure of the cement matrix can be improved by the application of nanomaterials. The application of nanosilica affects the increase in concrete strength. Nanosilica (NS) influences the system in two ways: chemical and physical. The chemical effects of NS are in the increase of the amount of pozzolanic CeSeH due to its inherent pozzolanic reactivity and its role as a nucleation site aiding rapid formation of CeSeH by cement hydration. The physical effect of nanoparticles is that they act as a filler

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material on partially hydrated cement paste which results in a compact and dense microstructure (Aly et al., 2012). The addition of NS acts similar to silica fume, but using 1% of NS has the same effect as 10% of silica fume at 90 days. With increasing NS content up to 3% better results can be obtained than for UHPC without NS. UHPC with NS and silica fume had better properties than concrete where only NS or silica fume was used. When 2% NS was added, compressive strength increased by about 8% at 90 days (Su et al., 2017). According to Rong et al. (2015) with the addition of 3% NS UHPC compressive and flexural strengths significantly increased, while Yu et al. (2014a) found that mixes with 4% of NS had only 3.6 MPa higher compressive and 2.7 MPa higher flexural strengths compared to the reference. On the contrary, with the addition of 1% NS compressive strength of UHPC decreased from 200 to 150 MPa (Wille and Naaman, 2013). In one study (Jankovic et al., 2016) the influence of the quantity of NS on the mechanical properties of cement composites was examined. Silica fume in quantity of 270 kg/m3 and 3% of the volume fraction of the fibers was used. The amount of NS varies from 0%, 2%, and 5% to the weight of the cement. Curing of samples was carried out by a standard procedure in water at the temperature 20 C for a period of 28 days. Fig. 2.14 shows the obtained compressive strength of cement composites. For specimens where 2% of cement is replaced by NS, there is a possibility of change in the structure of the cement matrix, which affects the compressive strength. An increase in strength can be caused by the reaction of nanomaterials with calcium hydroxide Ca(OH)2 crystals, which are arrayed in the interfacial transition zone (ITZ) between hardened cement paste and aggregates, producing CeSeH gel and the filling action of nanoparticles which causes a more densified microstructure. The impact on the compressive strength decreased with increasing the amounts of NS, which is clearly seen in the diagram of Fig. 2.14. One of the conclusions

Compressive strength, N/mm2

160 155 150 145 140 135

Silike fume 270 kg/m3 and steel fiber 3% NC20°C/28d

130 0

2 Nano silica, %

5

Figure 2.14 Compressive strength of normal curing UHPC with 2% and 5% NS and 3% steel fiber (Jankovic et al., 2016).

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29

of the study was that the critical amount of NS application is between 2% and 5% relative to the weight of the cement. The large specific surface area of the nanoparticles leads to their agglomeration. Besides this, great absorbency of nanomaterials contributes to the variations in compressive strength (Kong et al., 2012; Ghafari et al., 2014; Haruehansapong et al., 2014; Yu et al., 2014a,b; Bolhassani and Samani, 2015). At higher doses, due to the large amount of nanomaterials, weak zones are formed at the agglomeration sites which can produce a decrease in the mechanical properties (Kong et al., 2012). Agglomeration of nano-CaCO3 can be identified in the matrix with optical microscopy. Agglomeration occurs near the embedded fiber and can lead to a great reduction in fiberematrix bond strength which certainly affects the mechanical properties of UHPC (Wu et al., 2018). Because of this it is necessary to be very careful while using nanomaterials. Besides nanomaterials, which are being used more and more, in research on cement composites other materials can be used very successfully. Materials such as metakaolin (MK) and fine blended fly ash (FBFA) are good supplementary cementitious materials. Their fineness is not at the level of nanomaterials, but they have very fine particles. Their impact on compressive strength is also explored in the field of UHPC cement composites. In some studies (Jankovic et al., 2011; Nikolic, 2012), the effect of replacing silica fume with metakaolin and fine blended fly ash was compared. As the starting mixture 270 kg/m3 of the silica fume and 4% fibers by volume of the cement composite were used. Then 10% and 20% of silica fume substitutes with metakaolin and fine blended fly ash were made. Curing of concrete was carried out in three different ways: standard water treatment at 20 C, steam curing at 95 C for 48 h, and autoclaving for 4 h at a pressure of 20 bars. The obtained strength values at pressure are shown in Fig. 2.15.

Compressive strength, N/mm2

250 NC20°C/28d SC95°C/48 h

200

AC20bar/4 h 150 100 50 0 US2Sf3

U10MK

U10FBFA U20MK Mixture mark

U20FBFA

Figure 2.15 Compressive strength of UHPC with supplementary materials and 4% steel fiber (Jankovic et al., 2011; Nikolic, 2012).

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The obtained compressive strength greatly depends on the applied method of curing cement composite, which is in line with the conclusions of Section 2.4. Replacing a part of the silica fume with metakaolin and fine blended fly ash did not adversely affect the compressive strength. All results, depending on the application of concrete curing, are uniform regardless of the replacement of silica fume with metakaolin or fine blended fly ash.

2.5.2

Flexural strength

The strength of the cementitious composite, besides the component materials, is influenced by its connection. This is especially important when considering flexural and tensile strengths of cementitious composites. Thus, cement paste and aggregate have a higher tensile strength when tested separately than the cement matrix itself. This is caused by a negative influence of ITZ. The improvement of it, as the worst part of concrete, can be affected by the addition of nanoparticles. Nanosilica in UHPC affects the reduction of voids in ITZ, so denser and stronger ITZ could be obtained (Pacheco-Torgal et al., 2013). The improvement in ITZ by using NS contributes to the increase in UHPC tensile strength (Sanchez and Sobolev, 2010; Said et al., 2012). Su et al. (2017) have studied the combined impact of nanomaterials and fibers on flexural strength of UHPC. Different nanomaterials, nano-CaCO3, nano-SiO2, nano-TiO2, and nano-Al2O3 in the content of 3.0% by weight of cement, and fibers, twisted steel fiber (TF), waved steel fiber (WF), and microsteel fiber (MF), were used in the investigation. The results show that using an appropriate combination of these materials it is possible to obtain extraordinary UHPC properties, especially in terms of ductility and blast resistance capacity. For UHPC with twisted steel fibers (TF) the highest increase in all types of strength was found with the application of nano-Al2O3 addition. It was found that nanomaterials have a greater impact on the fracture energy of fiber reinforced UHPC beams than nonfiber reinforced UHPC beams (Su et al., 2017). In the study of the influence of NS on the mechanical properties of the cement composite (UHPC), which is detailed in Section 2.5.1, in addition to the compressive strength, flexural strength was also studied (Jankovic et al., 2016). For all applied quantities of NS of 0%, 2%, and 5%, the obtained results are shown in Fig. 2.16. The same conclusions were obtained as in the case of compressive strength testing. The best results were obtained with the application of 2% of the NS while increasing the amount of NS up to 5% led to a reduction in flexural strength. Probably the optimum amount of NS is between 2% and 5% of the applied NS. When it comes to flexural strength, the uses of materials that are not on the nanoscale have long been the subject of research. Thus, they can often be found in the literature on the application of micromaterials such as metakaolin and fine blended fly ash. The experiment is described in Section 2.5.1 in which the influence of metakaolin and fine blended fly ash on compressive strength was studied for flexural strength (Jankovic et al., 2011; Nikolic, 2012). The results of flexural strength tests are shown in Fig. 2.17.

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Flexural strength, N/mm2

30 25 20 15 10 5

Silike fume 270 kg/m3 and steel fiber 3%...

0 0

2 Nano silica, %

5

Figure 2.16 Flexural strength of normal curing UHPC with 2% and 5% NS and 3% steel fiber (Jankovic et al., 2016).

Flexural strength, N/mm2

40,0

NC20°C/28d

35,0

SC95°C/48 h

30,0

AC20bar/4 h

25,0 20,0 15,0 10,0 5,0 0,0 US2Sf4

U10MK

U10FBFA

U20MK

U20FBFA

Mixture mark

Figure 2.17 Flexural strength of UHPC with supplementary materials and 4% steel fiber (Jankovic et al., 2011; Nikolic, 2012).

The obtained flexural strength greatly depended on the applied method of curing cement composite, which is in line with the conclusions of Sections 2.4 and 2.5.1. Replacing a part of the silica fume with metakaolin and fine blended fly ash did not adversely affect the flexural strength as well as when it comes to compressive strength. All results, depending on the application of concrete curing, are uniform regardless of the replacement of silica fume with metakaolin or fine blended fly ash. In the case of flexural strength, there is a significant difference between the curing methods of cementitious composites. Flexural strength is greatest if the curing method of concrete autoclaving is adopted. The increase is much higher than for compressive strength.

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Cetin and Carrasquillo (1998) have observed that no single equation seems to represent the modulus of elasticity or flexural tensile strength of UHPCs with sufficient accuracy, and measured values should be used instead of the predicted ones. Analyzing test results, the relationship between the 28-day flexural and compressive strengths has been obtained as f fl ¼ 0:275ðf c Þ0:81 MPa where ffl and fc denote the flexural and compressive strengths of a cementitious composite expressed in MPa, respectively.

2.5.3

Tensile splitting strength

In concrete exposed to tension microcracks appear first and then macrocracks. The increase in the load encourages critical crack progress at the tip of macrocracks, which ultimately leads to concrete failure (Banthia, 1994). Many methods have been developed to determine the strength of the concrete under tension. Officially there is no standardized method for measuring the postcrack behavior of cement composites. Direct tension measurement is very complicated and therefore indirect methods are used. One of the methods of determining the tensile strength is indirectly by splitting tensile test or modified Brazilian test. Results of the investigation show that the splitting tensile strength increased with curing time and the addition of NS up to 2%. It can be explained by much denser ITZ of UHPC caused by the addition of silica nanoparticles. The effects of both SF and NS are better. By applying a higher amount of 2% NS, there is a decrease in UHPC splitting tensile strength (Gesoglu et al., 2016). For instance, depending on the surface area, 2% of NS in regular mixture covered nearly an area of 4.5 km2, which is almost twice greater than totally covered area by silica fume and Portland cement together. Thus, excess silica will leach out and cause dispersion of nanoparticles and week zones formed within the system as a consequence of a lack in strength. For one known investigation, the average ratio between the flexural and split tensile strengths of UHPC concrete with silica fume has been obtained as 1.65. The ratio between flexural and split tensile strength was obtained by Zhou et al. (1995) as 1.5. As per ACI, the ratio should range from 1.4 to 1.6. Zheng et al. (2001) reviewing works of previous researchers have reported that flexural tensile strength is generally 35% higher than split tensile strength.

2.6

Production problems and recommendations for practical application

Nanomaterial dispersion into mixture has significant influence on the properties of cement-based materials. One of the negative effects of using nanomaterials in concrete is a decrease in the workability due to its adsorption of free water (Hou et al., 2013b; Zhang et al., 2015).

Influence of nanoparticles on the strength of ultra-high performance concrete

33

Nanoparticles in the form of dust are particularly problematic in the production of cement composites (Taurozzi et al., 2012; Parveen et al., 2013). Particles with bigger primary particle sizes such as 40 nm are better dispersed (Haruehansapong et al., 2014). Poor dispersion of nanoparticles could create weak zones and form voids (Li et al., 2004). Incomplete homogenization negatively affects the quality of the final product (Li et al., 2004; Ozyildirim and Zegetosky, 2010; Nazari and Riahi, 2011b). The nanoparticles added in powder form agglomerate into a slightly interconnected fractal structure (Hiemenz and Rajagopalan, 1997). Spatial homogenization of the particle allocation between cement grains can be enhanced by mixing a previously prepared aqueous suspension of nanoparticles with cement (Gaitero et al., 2010). Justs et al. (2012) applied a hydrodynamic cavitation method for nanoparticles agglomerate disaggregation and surface activation. In addition to good homogenization, for the effectiveness of nanoparticles in cement composites, it is also important that they are actually incorporated as nanosized particles, so they need to be stabilized before installation. Conventional methods of mixing powder materials allow dispersing of agglomerates of nanoparticles in a powder form only up to a size of a few microns, which is not enough to separate nanoparticle agglomerates into individual particles. For this purpose, other techniques were introduced, which are divided into two groups: 1. mechanicaldgrinding in a ball mill, ultrasonic treatment (Sato et al., 2008; Taurozzi et al., 2012), hybridization, magnetically assisted impaction mixing (MAIM) (Scicolone et al., 2011), Rapid Expansion of Supercritical Suspension (RESS) (To et al., 2009), etc. and 2. chemicaldcovalent functionalization of the surface (Ntim et al., 2011), additive dispersing agent, or noncovalent functionalization (Kawashima et al., 2013).

The problem with mechanical techniques is that when the application of external energy is stopped, the particles are reaggregated, which means that the efficiency of these methods is limited. The system needs to be stabilized in the long run. A combination of mechanical and chemical techniques (Cwirzen et al., 2008b; Kawashima et al., 2013) proved to be the most effective. In practice, the stabilization of nanoparticles takes place in the liquid medium, which is most usually water, with the addition of the dispersing agent. It has been suggested to use a superplasticizer as a dispersing agent to facilitate the distribution of nano-SiO2 particles (Sobolev et al., 2009). Polycarboxylate superplasticizers proved to be the best dispersing agents and have shown far better results than any other surfactant for 0D, 1D, and 2D nanoparticles (Korayem et al., 2017).

Acknowledgments The work presented in this paper is a part of the investigation conducted within the research projects TR 36017, supported by the Ministry of Education, Science, and Technological Development, Republic of Serbia. This support is gratefully acknowledged.

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Hou, P., Wang, K., Qian, J., Kawashima, S., Kong, D., Shah, S.P., 2012. Effects of colloidal nanoSiO2 on fly ash hydration. Cement and Concrete Composites 34 (10), 1095e1103. Hou, P., Kawashima, S., Kong, D., Corr, D.J., Qian, J., Shah, S.P., 2013a. Modification effects of colloidal nano SiO2 on cement hydration and its gel property. Composites Part B: Engineering 45 (1), 440e448. Hou, P-k, Kawashima, S., Wang, K-j, Corr, D.J., Qian, J-s, Shah, S.P., 2013b. Effects of colloidal nanosilica on rheological and mechanical properties of fly ashcement mortar. Cement and Concrete Composites 35 (1), 12e22. Hou, P., Qian, J., Cheng, X., Shah, S.P., 2015. Effects of the pozzolanic reactivity of nanoSiO2 on cement-based materials. Cement and Concrete Composites 55, 250e258. Huang, Z.Y., Cao, F.L., 2012. Effects of nano-materials on the performance of UHPC. Materials Review 26 (9), 136e141 (in Chinese). Hui, L., et al., 2004. Microstructure of cement mortar with nano-particles. Composites Part B: Engineering 35 (2), 185e189. Indhumathi, P., Syed Shabhudeen, S.P., Saraswathy, C.P., 2011. Synthesis and characterization of nano silica from the pods of delonix regia ash. International Journal of Advanced Engineering Technology II (IV), 421e426. Jalal, M., Fathi, M., Farzad, M., 2013. Effects of fly ash and TiO2 nanoparticles on rheological, mechanical, microstructural and thermal properties of high strength self compacting concrete. Mechanics of Materials 61, 11e27. Jankovic, K., Bojovic, D., Nikolic, D., Lj, L., 2010. Some properties of ultra high strength concrete. Materials and Structures 53 (1), 43e51. ISSN 0543e0798.  Jankovic, K., Cirovi c, G., Nikolic, D., Bojovic, D., 2011. Mechanical properties of ultra high properties self compacting concrete with different mineral admixtures. Romanian Journal of Materials 41 (3), 211e218. Jankovic, K., Stankovic, S., Bojovic, D., Stojanovic, M., Antic, L., 2016. The influence of nanosilica and barite aggregate on properties of ultra high performance concrete. Construction and Building Materials 126, 147e156. Jankovic, K., Stojanovic, M., Bojovic, D., Loncar, L.J., 2019. The influence of nano sio2 and curing regimes on mechanical properties of UHPC. In: RILEM Conference on Sustainable Materials, Systems and Structures, Rovinj, Croatia, in Progress. Ji, T., 2005. Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2. Cement and Concrete Research 35, 1943e1947. Jo, B.-W., Kim, C.-H., Tae, G-h, Park, J.-B., 2007. Characteristics of cement mortar with nanoSiO2 particles. Construction and Building Materials 21 (6), 1351e1355. Justs, J., Shakhmenko, G., Mironovs, V., Kara, P., 2012. Cavitation treatment of nano and micro filler and its effect on the properties of UHPC. In: Proceedings of Hipermat 2012e3rd International Symposium on UHPC and Nanotechnology for Construction Materials, Kassel University Press, Kassel, Germany. Kawashima, S., Seo, J.-W.S., Corr, D., Hersam, M.C., Shah, S.P., 2013. Dispersion of CaCO3 nanoparticles by sonication and surfactant treatment for application in fly ashecement systems. Materials and Structures 47 (6), 1011e1023. Kong, D., Du, X., Wei, S., Zhang, H., Yang, Y., Shah, S.P., 2012. Influence of nano-silica agglomeration on microstructure and properties of the hardened cement-based materials. Construction and Building Materials 37, 707e715. Kong, D., Corr, D.J., Hou, P., Yang, Y., Shah, S.P., 2015. Influence of colloidal silica sol on fresh properties of cement paste as compared to nano-silica powder with agglomerates in micron-scale. Cement and Concrete Composites 63, 30e41.

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The effect of nanoparticles on the self-healing capacity of high performance concrete

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J.L. García Calvo 1 , G. Pérez 1 , P. Carballosa 1 , E. Erkizia 2 , J.J. Gaitero 2 , A. Guerrero 1 1 Institute for Construction Sciences Eduardo Torroja, CSIC, Madrid, Spain; 2Tecnalia, Sustainable Construction Division, Derio, Spain

3.1

Introduction

The formation of cracks in concrete structures during their service life is a common phenomenon related to different causes. The formation of these cracks can reduce the mechanical performance of the concrete structure thus limiting its durability. Moreover, in reinforced concretes, cracks act as preferential paths for the entrance of water and aggressive agents, which could reach the level of the rebar and promote their corrosion in shorter times than expected. Although crack repair is possible, it usually implies a high cost and it becomes difficult when cracks cannot be seen or are unreachable. For this reason, the development of self-healing concretes has been researched during the last 15 years. The final objective is to prevent the development of cracks at the macroscopic scale, thus improving the long-term durability and mitigating the aging effects on the materials. This would translate into an increase of their service life and a reduction of the costs associated to maintenance and repair (Mihashi and Nishiwaki, 2012; Wu et al., 2012; Li and Herbert, 2012; Van Tittelboom and De Belie, 2013). White et al. (2001) revolutionized this field with their study published in 2001. They reported the development of a structural polymeric material with the ability to heal cracks in its structure. In the specific field of construction materials, selfhealing may be accomplished by components already present in the cementitious matrices or caused by engineered additions specific for this action. Thus, depending on its origin/nature, self-healing in concrete can be classified as autogenous or autonomous (Joseph et al., 2010; Mihashi and Nishiwaki, 2012). Autogenous healing is an intrinsic characteristic of concrete that heals small cracks mainly by further hydration of cement and/or precipitation of calcium carbonate, although swelling of the matrix and blocking of the crack due to debris present in the ingress water or loose concrete particles can also play a healing role (Granger et al., 2007; Van Tittelboom et al., 2013). On the contrary, in the autonomous approach the healing is caused by engineered additions specific for this action added to the cementitious matrix, e.g., by introducing encapsulated polymers, or minerals, or bacteria (Van Tittelboom

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et al., 2011; Wu et al., 2012; Mihashi and Nishiwaki, 2012; Wang et al., 2014; Huang et al., 2014; Pérez et al., 2015a; Qureshi et al., 2016). Although the maximum crack width that can be healed was observed to differ substantially among different studies, the one healed by autonomous strategies is higher than the one obtained by autogenous healing. For the specific case of high performance concretes, an autogenous self-healing mechanism is usually observed. Mehta and Aïtcin (1990) proposed the designation of high performance concretes (HPC) to refer to concretes that had three different characteristics: high workability, high strength, and high durability. Considering the concrete composition, one of the main differences between conventional concrete and HPC is the higher binder content and the higher use of supplementary cementitious materials in the latter. Both aspects, and the low water/binder ratio commonly used, promote the remaining of a significant amount of anhydrous phases even after aging. Moreover, the use of fibers is also common in HPC. In this sense, a discontinuous and randomly dispersed fiber reinforcement in a concrete matrix is able to effectively control the opening of the cracks and hence provide a reliable support to any kind of selfhealing mechanism, since narrower cracks are expected to be better healed (Snoeck and De Belie, 2015). Additionally, materials in which instead of the formation of a single propagating unstable crack, a stable multicracking process takes place are being increasingly developed. The opening of each single crack in these materials is effectively controlled and restrained thanks to the bridging effect provided by the fiber. One example of these special concretes is those named as High Performance Fiber Reinforced Cementitious Composites (HPFRCCs). The autogenous self-healing capacity of HPFRCCs depends on the type of fiber used and the exposure conditions, but certain levels of both strength and stiffness recovery have been published (Hannant and Keer, 1983; Gray, 1984; Mihashi and Nishiwaki, 2012; Ferrara et al., 2014). Selfhealing Engineered Cementitious Composite (ECC) is also a special type of HPFRCC designed on a micromechanics base to obtain a high tensile ductility and a tight crack width under mechanical loading. The typical main components of ECC are type I Portland cement, fine silica sand, class F fly ash, and poly-vinyl-alcohol fibers. A complete recovery of strength and ductility of ECC has been reported in different wet and dry conditioning regimes due to autogenous self-healing, even in aggressive environments with chlorides (Yang et al., 2009; Li and Li, 2011; Yang et al., 2011a). Regarding ultrahigh performance concretes (UHPC), namely composed of very low w/c ratios, high cement contents, high supplementary cementitious materials high supplementary cementitious materials contents (mainly silica fume), fiber reinforcements, etc., that present decrease in porosity, improvement in microstructure and homogeneity, and increase in toughness, also certain autogenous-healing capacity was detected (De Larrard and Sedran, 1994; Richard and Cheyrezy, 1995; Habel and Gauvreau, 2008; Buck et al., 2013; Shi et al., 2015). As well as in HPFRCCs and ECC, this self-healing capacity is related to the combination of unhydrated cement particles with latent reactivity that get activated in the presence of moisture upon cracking and the use of fibers helping to control crack growth (Granger et al., 2007; Hillouin et al., 2014). Therefore, and taking into account the autogenous healing capacity of HPC, their self-healing potential can be improved by the incorporation of an autonomous

The effect of nanoparticles on the self-healing capacity of high performance concrete

45

self-healing system that allows the healing of wider cracks. The combination of these two innovative concrete technologies can be especially valuable to assure durable concrete structures under severe environmental and operating conditions, such as in underground applications (high pressures with high-temperature gradients), in marine environments (with high chloride concentrations), or arctic areas (with low temperatures and the action of ice). In these highly demanding scenarios, the initial cost increase associated with higher cost of HPC with respect to conventional ones, and the implementation of a self-healing system, is compensated within the context of a life-cycle analysis by the reduction of maintenance cost and the extension of the service life obtained. In this regard, not many works have been published where a self-healing HPC has been developed. Self-healing systems based on the combination of fibers and crystalline admixtures in HPFRCCs (Ferrara, 2014) and of fibers and superabsorbent Polymers in ECCs (Snoeck et al., 2014) have been tested with promising results. However, although the use of nanoparticles is significantly increasing in the construction industry, only few works can be found in the literature focused on the exploitation of nanoparticles for improving the self-healing performance of HPC. In the present chapter, the main publications related to the use of nanoparticles to improve the self-healing capacity of HPC are described, as well as the modifications that the inclusion of these nanoparticles causes in the microstructural and physicomechanical performance of the corresponding HPC. The durability of self-healing HPC is also mentioned.

3.2 3.2.1

Self-healing systems based on nanoparticles used in cementitious materials Nonencapsulated self-healing systems

It has already been a long time since nanomaterials stopped being a promise to become a reality and their production volumes and the range of applications keep growing year after year (Lazaro et al., 2016). However, 97% of nanomaterials usage arises from just two substances: carbon black (9.6 million tons globally) and synthetic amorphous silica (1.5 million tons), being the main reasons that they are relatively cheap and available in large volumes. Currently, there are almost no industrial applications of carbon black related to cement composites, although its effect on the conductivity of mortars and concretes has been studied in several research works (Xie and Beaudoin, 1995; Dai et al., 2010; Monteiro et al., 2017). On the contrary, amorphous silica nanoparticles are widely used and studied as addition for concrete, at least assuming that silica fume (SF) can be considered as a type of nanoparticle in spite of its relatively large particle size (Khayat et al., 1992; Aldred et al., 2006). SF is used for the production of high and ultrahigh performance fiber reinforced concrete because it increases the fluidity of the fresh concrete, reduces the porosity, and enhances the strength rate development and durability of the hardened material (Richard and Cheyrezy, 1995; Granger et al., 2007; Ferrara et al., 2017). As previously mentioned in the Introduction, these types of concretes are characterized by an important self-healing capacity; however, this cannot be attributed to the presence of SF but it is mainly a consequence of

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the low water to cement ratio and high clinker content. In fact, silica fume might even be detrimental because its pozzolanic effect reduces the amount of calcium hydroxide available to carbonate in the cracks. However, there are two examples where silica nanoparticles play an active role in the self-healing process. In the first one (Van Tittelboom et al., 2010), bacteria suspended in a salt solution were added to a commercial silica nanoparticle dispersion in order to protect them from the high pH of concrete. Although the obtained suspension was proven effective sealing 0.3 mm width cracks, this cannot be considered an example of self-healing because it required external action. In particular, the suspension was placed into the crack by means of a syringe and then the samples were immersed into an equimolar ureaecalcium solution to promote calcium carbonate precipitation. In the second one (Pérez et al., 2015b), silica nanoparticles functionalized with amine groups and silica microcapsules containing an epoxy resin were used as addition for cement composites. During the hydration process the nanoparticles reacted with calcium hydroxide to give place to amine functionalized CeSeH gel (Monasterio et al., 2015). The objective of the functional group was to serve as the curing agent for the epoxy contained in the microcapsules, which should be liberated in the event of the formation of a crack. This method was effective on sealing cracks of UHPC under different environmental conditions (García Calvo et al., 2017; Pérez et al., 2017). A completely different approach consists on the use of purified Na-montmorillonite nanoclay particles (500 nm diameter) as water reservoirs to induce internal curing of unhydrated cement in ECC (Qian et al., 2010). However, in spite of the presence of nanoclay, air cured samples showed a smaller degree of self-healing than CO2, wet/dry, and water cured specimens.

3.2.2

Encapsulated self-healing systems

Capsule based self-healing is one of the main healing approaches currently under study. Most of the current systems are based on the encapsulation of an adhesive as healing agent which is added to the cementitious matrix. When a crack occurs the (micro)capsules are broken releasing the healing agent which fills the crack and hardens sealing it. Many of the developed capsules are spherical containers and micrometric in size (Van Tittelboom and De Belie, 2013; He and Shi, 2017; Wang et al., 2013; Pérez et al., 2015b; Dong et al., 2016; Lv et al., 2016; Kanellopoulos et al., 2017; Beglarigale et al., 2018). Cylindrical shaped capsules, mostly in the millimeter range, have also been researched (Dry, 1994, 2000; Li et al., 1998; Sun et al., 2011). As healing agents, organic as well as inorganic curing compounds have been used in different works (Van Tittelboom and De Belie, 2013; He and Shi, 2017). In addition, some researchers have developed self-healing systems based on addition of microencapsulated bacteria or spores of bacteria that can survive the high alkalinity of the cement matrix and form CO2 which can react with the Ca2þ present in the matrix and give rise to CaCO3 precipitating and sealing the crack (Jonkers, 2007; Jonkers et al., 2010; Wang et al., 2014; Wiktor and Jonkers, 2016). It has to be underlined however that there is almost no development of self-healing systems based on nanocapsules for cement-based materials. A work was found where the authors describe the encapsulation of methyl methacrylate (MMA) healing agent in polyurethane capsules synthesized by in situ

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interfacial polymerization and the capsules obtained were 300 nm in size (Litina et al., 2014). Nevertheless, no cement based specimens with the polyurethane nanocapsules and their self-healing effect were reported in the work. Most of the self-healing systems found in the literature have been introduced in cement paste, mortar, or conventional concrete. However, studies where the microencapsulated self-healing system has been added to HPC are very few (Li et al., 1998; Huang et al., 2011; García Calvo et al., 2017) and none where nanometer size capsules have been used. However, in the latter work, they have developed UHPC with an epoxyeamine system as the healing agent. The epoxy was introduced encapsulated in silica microcapsules and the amine group is added to the matrix by mixing the cement and water with amine functionalized silica nanoparticles. Another approach found in the literature regarding the use of nanotechnology to improve an autonomous healing system has been published by Khaliq and Ehsan (2016). The crack healing phenomenon developed is based on microbial activity of bacteria and they were introduced in concrete by direct incorporation or immobilization in graphite nanoplatelets. Authors reported that graphene nanoplatelet can offer protection to certain bacteria spores. These nanoplatelets also could distribute the spores uniformly for self-healing of concrete cracks. Moreover, bacteria immobilized in graphene nanoplatelet showed high self-healing efficiency when samples were precracked at early stages (3 and 7 day).

3.3

Influence of self-healing systems in the characteristics of HPC

Once the different autonomous self-healing systems have been described, in the present section the modifications promoted in the properties of HPC due to the inclusion of these self-healing systems with nanoparticles are evaluated. However, although much research has been done concerning the self-healing capacity of cementitious materials, the use of nanoparticles in high performance concretes to enhance the aforesaid property has not been widely evaluated yet. Thus, an overall idea concerning the modifications promoted in certain properties by self-healing systems without nanoparticles is also given.

3.3.1

Modifications in the microstructural properties

The inclusion of self-healing systems may significantly modify microstructural properties of cementitious materials or it can produce physical effects, especially on fresh concrete, that affect microstructure development under curing. For example, reactions between the self-healing system and the cementitious matrix may occur affecting hydration reactions. In this sense, Wang et al. (2014) have added melamine-based microcapsules as bacterial carriers, the food for bacteria (yeast extract) and deposition agents (urea and Ca-nitrate) as self-healing system and this Ca-nitrate could accelerate cement hydration and increase hydration degree, while high dosages of yeast extract (higher than 0.85 wt.%) had the opposite effect. Moreover, when using microcapsules,

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changes in workability associated to dispersion of these microcapsules have been reported to give rise to differences in porosity (He and Shi, 2017). In this sense, capsules with a wide range of size distribution enhanced filling of voids between the cementitious paste and the aggregate, thus leading to a lower workability. In the case of self-healing systems based on nanoparticles, interesting microstructural effects have been identified by Pérez et al. (2015a). Cement paste specimens with addition of a self-healing system composed by amine-functionalized silica nanoparticles and epoxy encapsulated in silica microcapsules have been studied. The blends considered included 24 g of distilled water, 3.2 g of superplastizicer (Structuro 351 from Fosroc) and 20 g of additions (microcapsules, functionalized nanosilica, and silica fume) per 80 g of ordinary Portland cement type CEM I 52.5N. Substitution quantities of 5% and 10% by weight of cement for the microcapsules were tested, with a constant value of 0.75 for the microcapsules to nanosilica content ratio and both capsules and nanoparticles were added in substitution to SF. The first microstructural effect of the use of nanoparticles as part of self-healing system was a good dispersion and chemical reaction with the cement matrix, as nanosilica could not be identified by SEM-EDX in the cement paste samples with this selfhealing system. On the contrary, microcapsules with a well-defined shape and protecting the epoxy compound inside were clearly identified and the images confirmed their stability upon mixing and hydration. Furthermore, a smooth interface with the cement paste was observed with no sign of separation or damage of the microcapsules. The composition of this tight interface measured by EDX spectra suggested a chemical reaction of the silica capsules’ shell with the cement matrix (see Fig. 3.1). Another microstructural effect was identified by XRD analysis of the pastes at different hydration ages up to 28 days (Fig. 3.2), which indicated that the addition of the self-healing system led to an increase of crystalline order with preferential orientation of the portlandite crystals in the (0 0 1) planes. At the same time, a clear decrease in Ca(OH)2 concentration with increasing amount of healing additions was measured by thermal analysis, coherent with the high pozzolanic activity of the two components of the self-healing system (Pérez et al., 2015a).

Figure 3.1 BSEM image and EDX microanalysis of the cement paste samples with high content of self-healing system at 28 days of hydration (Pérez et al., 2015a).

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1d

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Figure 3.2 X-Ray diffraction patterns of the cement paste samples with high content of selfhealing system at 1, 7, and 28 days of hydration (Pérez et al., 2015a). A, alite; Bebelite; Alealuminate; Cecalcite; Peportlandite; Eeettringite; Qequartz.

Both results were also confirmed in the SEM-EDX images of Fig. 3.3. In the case of reference cement paste with no self-healing addition (REF in Fig. 3.3(a)) portlandite crystals randomly oriented and located within the pores can be seen. In the case of a sample with lower content of self-healing system (MIX 5 in Fig. 3.3(b)), portlandite crystals with low degree of orientation and smaller than in the reference sample were abundant in the surface. Finally, the portlandite crystals were very scarce in the sample with a higher content of self-healing system (called MIX 10) and always found to be developed as a set of overlapping crystals of a particular orientation at the boundary of a quasi-round structure. The samples with these self-healing additions also showed different pore network properties as compared to the reference cement paste. In fact, in specimens with the two system components hydrated for 24 h, a peak at 0.017e0.012 mm was observed in the pore size distribution curve that was broader and shifted to lower mean pore diameters than in reference specimens. This development of a more compact microstructure may be associated to a seeding effect of the nanoparticles and a more efficient gel development by pozzolanic reaction induced by the additions, as suggested by the results from XRD and thermal analysis. Nevertheless, an additional minor contribution was observed for both samples in the macropores range at a pore diameter equal to 0.07 mm. The results showed a clearly different evolution of these two contributions. The mesopores contribution clearly decreased with increasing curing age, following the expected tendency of refinement of the capillary pores network during hydration of the cement paste. On the contrary, the macropores contribution was present with a similar intensity up to 28 days of hydration. A similar qualitative effect was found on the porosity of UHPC specimens upon addition of this self-healing system involving nanoparticles. In fact, results of mercury

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(a)

(b)

(c)

Figure 3.3 SEM images showing the different morphologies of the portlandite phase in cement pastes at 28 days of hydration. (a) REF (without self-healing system); (b) MIX 5 (with low content of self-healing system); and (c) MIX 10 (with high content of self-healing system) (Pérez et al., 2015a).

intrusion porosimetry (Fig. 3.4) evidenced that the self-healing concretes showed a clear refinement of microporosity with respect to the reference one and a small peak appeared at about 10 mm. Moreover, the total porosity of the concrete was small (around 7.5%) with no significant effect of the self-healing additions (García Calvo et al., 2017). According to the works reported by other authors, the presence of micropores in the self-healing concrete specimens might be attributed to the microcapsules (Wang et al., 2014; Dong et al., 2017).

3.3.2

Modifications in the physico-mechanical performance

The mechanical properties of the concretes with self-healing capacity must be evaluated taking into account two phases: the mechanical performance on precracking phase, evaluating the influence of introducing an autonomous self-healing mechanism in concrete strength, and mechanical performance on postcracking phase, determining the action of the self-healing mechanism in the recovery of initial mechanical properties, namely the strength and the stiffness. The precracking mechanical properties of self-healing HPC compared with a reference HPC mix can be easily evaluated by means of flexural and compressive strength standard tests executed at a specific age.

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0.08

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0.04 0.03 0.02 0.01 0.00 0.001

0.01

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1 10 Pore diameter (μm)

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Figure 3.4 Pore size distribution of UHPC after 28 days of curing. REF: without self-healing system; MIX 5: with low content of self-healing system; and MIX 10: with high content of self-healing system (García Calvo et al., 2017).

Nevertheless, the partial or total recovery of mechanical properties of HPC due to selfhealing capacity, mainly strength or stiffness, can demand more specific tests. The strength in precracking load phase can be conditioned by the self-healing system introduced in concrete. In studies carried out on self-healing fiber reinforced UHPC a modification of precracking mechanical properties (both in compressive and in flexural strengths) measured on prismatic samples at 7 and 28 days of curing (Fig. 3.5) due to the inclusion of a silica encapsulated and silica nanoparticles based self-healing system was clearly detected (García Calvo et al., 2017). In this research the mechanical properties of a reference mix with silica fume were compared with two self-healing concretes of similar compositions. In the latter examples the SF was partially substituted by two proportions of the self-healing system, that is 5% and 10% of epoxy resin filled silica microcapsules (CAP) and the corresponding amount of amine functionalized nanosilica (NS), as mentioned in Section 3.3.1. In the self-healing fiber reinforced UHPC specimens the values obtained in the flexural and compressive strength were lower than in the reference sample. In fact, the higher the amount of self-healing additions, the higher the decrease in the mechanical strength. The effect was even more pronounced in the case of the flexural strength. It is interesting to note in Fig. 3.5 the small strength gain of the self-healing UHPC from 7 days to 28 days. Such effect was previously reported in cement pastes with similar CAP and NS contents (Pérez et al., 2015a). In fact, initially the self-healing additions induced a faster strength gain as a consequence of the higher pozzolanic reactivity of CAP and NS with respect to SF derived from their larger surface area (Pérez et al., 2015b). However, such effect did not last long because NS was rapidly consumed and the pozzolanic activity of the surface area of CAP dropped when its pores were filled by cement hydrates, preventing further reaction. It has

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40

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Figure 3.5 Average values of flexural (left) and compressive strength (right) obtained on fiber reinforced UHPC. REF: without self-healing system; MIX 5: with low content of self-healing system; and MIX 10: with high content of self-healing system (García Calvo et al., 2017).

to be underlined that CAP contained epoxy compound as well as silica, so the total silica content (SF plus silica from CAP and NS) of the resulting self-healing concretes was slightly lower than that of the silica fume of the REF concrete. Thus, the lower total silica content of the self-healing concretes compared to the silica content of the reference mix also influenced the final strength values obtained. Moreover, the silica microcapsules could act as weak points thus decreasing the strength values. This negative effect in the mechanical strength has been reported in other self-healing encapsulated systems such as in melamine/polymer based systems (Wang et al., 2014) although the microcapsule contents were lower than those used by García Calvo et al. (2017). Therefore, this suggests that the use of silica microcapsules limits the negative effect in the mechanical properties, either because they are more compatible with the cement matrix than the polymeric ones or because they have a higher mechanical strength. Anyway, the silica functionalized nanoparticles used in the study mentioned above did not seem to limit the mechanical performance. Numerous references gather different tests used to induce the cracks and evaluate the mechanical recovery capacity of the self-healing systems in HPFRCCs (Tang et al., 2015; Zakari Muhammada et al., 2016; Ferrara et al., 2018). According to them, it is advisable to use the same test method to induce the cracking and evaluate the mechanical recovery capacity attributable to the self-healing system. One of the most commonly used tests for evaluating the mechanical recovery due to a selfhealing system is the three-point bending test on prismatic notched specimens with control of the crack mouth opening displacement (CMOD) (Hilloulin et al., 2014; Granger et al., 2007; García Calvo et al., 2017). This test allows for controlling not only the place where the crack appears but also the width of the crack. Apart from the crack width, the effect of the self-healing capacity in mechanical properties recovery strongly depends on self-healing time and first cracking age, although this last factor has more influence on autogenous self-healing (Granger et al., 2007).

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Ferrara et al. (2018) reported that the three-point bending test on notched prismatic specimens with control of CMOD generated important variations in the residual crack width after unloading. The mentioned variations were attributed to the precision and control capacity of the measuring equipment that can compromise the effective evaluation of the self-curing capacity. Nevertheless, other studies have reported similar residual crack widths between the pairs of specimens corresponding to the same concrete after the unloading by using an extensometer with micron precision (Granger et al., 2007; Wang et al., 2014; García Calvo et al., 2017). Fig. 3.6 shows the results from the CMOD controlled three-point bending test where the flexural load capacity of the fiber reinforced UHPC mentioned previously (García Calvo et al., 2017) was determined after two curing ages up to a CMOD of 1000 mm. In Fig. 3.6 an influence of the self-healing additions and their content in the mechanical performance before and after the concrete matrix failure is clearly observed. In general, the flexural peak load associated to concrete occurs at an average CMOD of about 20e25 mm. At this point, the crack appears and the mechanical behavior becomes governed by the fibercementitious matrix adhesion performance. In the mentioned samples, the concrete peak load increased with increasing curing time and it was lower in the self-healing concretes. After this peak, the load dropped due to the initiation of cracks in the microconcrete matrix. The drop after the peak load was greater in the reference mixture, particularly after 28 days of curing, than in self-healing concretes. Therefore, selfhealing additions reduced the stiffness of the matrix probably because silica microcapsules acted as weak points that prevented too much energy concentration previous to crack initiation. Thus, the properties of the cementitious matrix were modified with the inclusion of the self-healing additions and they possibly changed the load transmission and the fiber-matrix adherence. These changes of the properties are confirmed when observing the strainestress curves after the peak load since the initial load of the curve was larger in the reference concrete mix than in self-healing concretes, especially at 28 days. This aspect is linked to higher recovery capacity and higher initial strength gain in the reference mixture due to a better fiber-cementitious matrix adhesion compared to self-healing UHPC performance. In fact, a load/toughness capacity decrease was detected when increasing the dose of healing agent added, probably due to the adhesion loss between the matrix and the fiber. The small load reduction detected in the postfailure curve of the reference mix, particularly at 28 days of curing, is also remarkable. This load decrease is related to fiber sliding, very common in fiber reinforced concretes. This behavior was also detected in the self-healing concretes but in a lesser degree due to their lower stiffness. In fact, a stiffer cementitious matrix is expected to present lower capacity for adjusting the deformations promoted by fiber sliding thus limiting the load transmission/distribution to other elements when the located fiber sliding failure takes place. In cementitious matrices with lower stiffness (as the self-healing concrete mixtures mentioned), their higher deformation capacity promoted good distribution of the stress suffered by a specific fiber thus transmitting part of this stress to the surrounding fibers during the fiber sliding. Several studies on autogenous self-healing mechanisms have proven them to be effective in mechanical recovery of strength or stiffness for crack widths not exceeding 30 microns either by the formation of new CeSeH gel (Hilloulin et al.,

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REF

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Figure 3.6 Flexural load capacity of fiber reinforced UHPC up to a CMOD ¼ 1000 mm at 7 (up) and 28 days of curing (down). REF: without self-healing system; MIX 5: with low content of selfhealing system; and MIX 10: with high content of self-healing system (García Calvo et al., 2017).

2014, 2016; Granger et al., 2007) or by the precipitation of salts in induced cracks in concretes (Ferrara et al., 2017; Gupta et al., 2018). Moreover, Van Tittelboom et al. (2011) reported a recovery of more than 50% of the original strength and stiffness after self-healing using tubular capsules filled with healing agent in conventional mortars. However, not much research has been done in HPC focusing on autonomous selfhealing or on crack widths bigger than 30 mm and the little that has been done did not focus on the use of nanoparticles (Li et al., 1998; Hilloulin et al., 2014; Huang et al., 2011).

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The capacity of mechanical recovery and efficiency of self-healing systems of the same UHPC developed by García Calvo et al. (2017) have been studied when 150 and 300 mm crack widths were induced on notched prismatic specimens. The effectiveness of this silica encapsulated plus functionalized silica nanoparticles based self-healing system has been evaluated after 28 and 56 days of healing. It must be taken into account that once cracked, the specimens were stored in sealed plastic bags for the healing period so as to prevent carbonation and/or hydration promoted by further external water penetration thus limiting the autogenous self-healing. No significant influence of healing time on improving the mechanical properties of the UHPC studied was observed. Fig. 3.7 shows the results obtained for 300 mm crack width. According to the results mentioned in this section, one of the most limiting factors when evaluating the mechanical recovery of HPC seems to be the width of the crack opening. More research focusing on the healing of wider cracks are needed as well as the improvement of the self-healing systems based on nanoparticles in order to

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Figure 3.7 Average values of flexural strength obtained after two healing periods of 28 and 56 days on cracked notched prismatic fiber reinforced UHPC specimens. REF: without selfhealing system; MIX 5: with low content of self-healing system; and MIX 10: with high content of self-healing system.

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enhance the recovery of the mechanical properties after the crack healing, namely the strength and the stiffness.

3.3.3

Modifications in the self-healing capacity

As previously mentioned, HPC show significant autogenous self-healing performance due to their special composition. However, very few cases have been reported in the literature focused on the use of nanoparticles in order to improve the self-healing ability of high performance concretes. In fact, in some cases the self-healing effect was a consequence of another functionality that was looked for. For instance, Falikman et al. (2012) reported smog eating and self-healing properties of UHPC incorporating TiO2 nanoparticles to obtain a photocatalysis effect. In 2010, Qian et al. (2010) published possibly the first study focused on the use of nanoparticles for improving the selfhealing properties of high performance concretes. They investigated the behavior of ECC focusing on the influence of curing condition and precracking time by utilizing nanoclay. They evaluated the feasibility of nanoclay particles to act as internal water reservoirs to promote self-healing behavior of ECC, thus eliminating the dependence on the external water supply. The work was based on the one published by Valcke et al. (2009) who tried to use nanoclay as internal water supply for precracked mortar samples to heal themselves. According to their study, in a homogenized clay/water mix, the nanoclay took up to 95%e98% of its own mass as interlayer water so the extra water was free water that can be potentially utilized for cement hydration. Qian et al. (2010) used Portland cement CEM I 42.5N in the mixture with blast furnace slag, limestone powder, and nanoclay. The nanoclay was a purified Na-montmorillonite with a small particle size (z500 nm). The introduction of this nanoclay significantly increased the water content needed to achieve a similar workability than the one obtained in ECC with similar composition but without nanoclay. The reloading deflection capacity, that is, the recovery levels of the ECC after cracking and subsequent healing, was improved by the use of the nanoclay. However, and despite the fact that the self-healing effect improved in comparison to samples without the nanoclay, it continued being dependent on the curing conditions. In particular it was less effective when curing took place in air compared to water, CO2, and wet/dry conditions. Furthermore, healing capacity was reduced over time as a consequence of the gradual exhaustion of cementitious material for continuous hydration. The healing activity of a self-healing system that mixed the use of silica microcapsules with epoxy and amino-functionalized silica nanoparticles has been tested in UHPC (García Calvo et al., 2017). Cracks of two different sizes (150 and 300 mm) were created and left to heal for 7 or 28 days. The self-healing capacity was evaluated by measuring the water absorption rate of the cracked specimens and an important self-healing capacity was deduced since the water absorption rates of cracked reference concretes were higher than those of cracked self-healing concretes. In this sense, the UHPC with 5% of silica microcapsules and the corresponding amount of the functionalized nanoparticles showed lower absorption rate and total water increase than the concretes with 10% of silica microcapsules. Thus, an increase in the selfhealing system inclusion does not always improve the self-healing performance of

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the developed materials. In all the cases, both the final CMOD defined and the healing period influenced the self-healing efficacy. Regarding the healing time, after 28 days lower water absorption rates were observed than after 7 days. In fact, after 28 days of healing the initial slope of the water absorption curves of the selfhealing concretes was similar to that obtained in the uncracked samples. This fact and the partial reduction of the capillary suction coefficient with healing time detected in samples without self-healing system implied the existence of an autogenous self-healing mechanism in the UHPC developed. Since the samples were sealed in plastic bags in order to avoid any water ingress and/or carbonation during the healing period and the water to binder ratio was really low (0.24), the significant autogenous self-healing capacity of UHPC was again highlighted. Anyway, as it has been already confirmed even in large scale tests, autogenous crack healing mechanism is not capable of healing cracks as large as the ones considered in the mentioned study (Van Tittelboom et al., 2016) so the superior performance of the autonomous selfhealing system under such conditions is evident. Regarding the influence of the final CMOD in the above study, the self-healing observed in the cracks of 300 mm was less effective than that observed in cracks of 150 mm, but the healing period was more important for the formers so possibly longer healing periods would allow to improve the healing of the 300 mm cracks. The corresponding SEM analysis combined with X-ray CT analysis clearly showed that the resulting healing did not totally heal the cracks promoted but partially filled/sealed them with the epoxy compound. This behavior is presented in Fig. 3.8 which shows a BSEM image of a UHPC with 5% of silica microcapsules and the corresponding amount of silica nanoparticles, cracked up to a CMOD of 300 mm and healed for 28 days. The compositional mapping considering Si, Ca, and C signals shows the presence of the epoxy organic compound outside the

Figure 3.8 BSEM image (500x) of a crack of a self-healing UHPC specimen cracked up to a CMOD of 300 mm and healed for 28 days (García Calvo et al., 2017).

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capsules and blocking the crack in certain points. The reason why the epoxy did not fill the cracks completely is simply that there was not enough epoxy. In fact, since the crack size was comparable to the diameter of the capsules the concentration of the capsules should be much higher. The not complete crack healing phenomenon has been also described by Van Tittelboom et al. (2016) in high performance concrete (z60 MPa) with self-healing systems but without nanoparticles in them (one system based upon the encapsulation of polyurethane and the other system relied upon the addition of superabsorbent polymers). At last, the use of silica nanoparticles and nanofibers for healing the cracks of existing structures has become a common technique for protecting the structures from decay (Venkat Rao et al., 2015). For example, fiber wrapping is a technology used to increase the strength and durability of existing structures. This technology is based on the use of fiber sheet containing silica nanoparticles and hardeners. When the fiber sheet is wrapped with the concrete surface, the nanoparticles enter into the concrete cracks and they close the cracks on the surface of the concrete. According to the studies mentioned in this section, the inclusion of nanoparticles in high performance concretes to improve their self-healing behavior has not been extensively evaluated yet. However, the results published are promising so in the future nanotechnology will play an important role in this regard.

3.4

Durability of self-healing HPC under aggressive environments

Final application of self-healing HPC is especially important in the case of highly demanding infrastructures in which crack repair is especially expensive or difficult. This is the case of underground applications, in marine environments or in arctic areas, characterized by especially severe and damaging environmental conditions. Considering this, it is of primary importance to study the durability of self-healing HPC under aggressive environments to define the viability of its efficient application to increase the service life of demanding infrastructures. Within this context, one of the main causes of concrete deterioration is freezeethaw action, especially in cold climates with high humidity and low temperatures, that affects the tightness of the concrete matrix and consequently permeability to aggressive agents (Penttala, 2006). The effect of chloride ingress must also be considered as cracks often act as preferential paths giving rise to higher chloride attack upon crack appearance (Kim et al., 2014; Berrocal et al., 2015). In fact, this is known to be one of the major durability issues in concrete infrastructures, especially in marine environments. Several works have been published on the durability of HPFRCC and especially in the ECC category. The self-healing mechanism in these types of materials is mainly autogenous and is based on carbonation and hydration reactions at crack surfaces upon contact with the environment. Consequently, the self-healing efficiency is greatly influenced by the specific environmental humidity, temperature, and carbonate ions content. Works published by Yang et al. (2009, 2011b) investigated the self-healing

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capacity of ECCs subjected to different wet and dry conditioning regimes. They found a significant recovery of the mechanical performance upon healing in these environments, and healing to a lower extent in the case of higher temperatures in the drying stages. In addition, stiffness recovery by crack healing was lower in specimens precracked in the early stages of curing than in mature specimens since in the latter case they had more ability to maintain tighter crack width even at a higher predamage level. Regarding self-healing UHPC involving nanoparticles, the published references (Pérez et al., 2015c; Guerrero et al., 2015; Pérez et al., 2017) have studied the durability of the previously described material based on epoxy encapsulated in silica microcapsules and amine-functionalized silica nanoparticles. The behavior of the UHPC was addressed under freeze and thaw cycles, and saline environment with sodium chloride attack. Cylindrical specimens of 100 mm in diameter and 25 mm thick of a UHPC cured for 28 days were cracked to specified CMOD of 150 and 300 mm using the Brazilian splitting tension test. Different sets of self-healing concrete specimens were studied in order to analyze the sealing efficiency of the proposed system and the durability of the UHPC once healed: •





• •

Specimens were subjected immediately after cracking to a freezeethaw (FT) durability test consisting of 150 cycles of 3 h in dry ambient at 20 C and 3 h under water at 20 C. This set was used to analyze sealing efficiency of the proposed self-healing system in cold climates, with high humidity and low temperatures. Similar specimens subjected to a salt spray (SS) test were used to predict material selfhealing in marine environments with high chloride attack. A climatic chamber complying with the UNE-EN-ISO9227-2007 (UNE, 2007) standard was used with a NaCl solution of 5% concentration at 23 C during a period of 90 days. Specimens stored in controlled laboratory conditions (LC) in sealed plastic bags at 20 C after cracking during the 28-days healing period were used as a reference. These conditions prevent carbonation and moisture exchange between the concrete and the environment, so that any self-healing process was due to the action of the epoxyeamine self-healing system. Specimens were stored in controlled LC during the 28-days healing period and subsequently subjected to the previously described FT cycles in order to evaluate the durability of the HPC once the cracks were healed by the self-healing system. Similar specimens were stored in controlled LC during the 28-days healing period and subsequently subjected to the previously described SS test.

Tightness of the different sets of self-healing specimens was evaluated by capillary water absorption tests and the results were compared with those of uncracked selfhealing concrete. The specimens subjected to the FT cycles immediately after cracking did not show scaling or degradation and, in the case of nominal crack width of 150 mm, the results of capillary suction test were similar to those of specimens healed at laboratory conditions (Guerrero et al., 2015). In fact, the values of capillary absorption coefficient (K) obtained after 24 h were 0.009 and 0.010 kg/m2min0.5 for the samples stored in laboratory conditions (LC) and subjected to FT cycles, respectively. In the case of the specimens cracked up to 300 mm, the absorption rate in the first part of the experiment was higher for samples subjected to FT cycles. This could be due to the pressure

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exerted by ice inside the partially healed crack during freezing part of the cycle that may produce internal microcracks and also probably compress the healing adhesive into pores and hollow microcapsules. Consequently, during the thawing part of the cycle a higher water uptake was possible than in samples stored in the laboratory with no effect of ice. Nevertheless, the final mass increase observed for the sample subjected to the FT test was lower than the corresponding to the analogue sample stored in LC. This behavior was related to the enhancement of autogenous healing of the cracks by water entering during the thawing periods, giving rise to a denser structure. No significant external damage was noticed in the concrete specimens subjected to the SS test immediately after cracking (Pérez et al., 2015c), although an increase of the measured weight with the test time was observed related to the corrosion of the fibers and to penetration of the pulverized saline solution. The results from the subsequent capillary absorption test in cracked self-healing HPC samples after the salt spray test and in those maintained in LC were very similar, thus discarding a significant influence of the chloride attack in the self-healing efficacy. Fig. 3.9 shows the curves obtained from the capillary water absorption test performed on self-healing (SH) concrete specimens stored in controlled laboratory conditions (LC) for healing during 28 days and specimens stored in these conditions and subjected to freezeethaw (FT) cycles (Pérez et al., 2017). After 24 h (time0.5 equal to 38 min0.5) the mass increase of self-healing concrete was similar with or without the FT cycles at both crack widths and only slightly higher than the value corresponding to the uncracked concrete. At longer test times the only effect observed was that mass increase was lower in the specimens cracked to 300 mm when subjected to FT cycles after healing in LC conditions. This behavior confirmed a densification of the concrete matrix due to secondary hydration in the thawing cycles. 10

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8

6

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0 0

20

40

60

80

100

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Figure 3.9 Mean mass increase during the water absorption test of SH concrete after a 28-day healing period in controlled laboratory conditions (SH-LC) and subsequently subjected to FT cycles (SH-FT). The results of uncracked samples are included for comparison (Pérez et al., 2017).

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The results of water absorption corresponding to SH concrete subjected to the SS test after the 28-day healing period are shown in Fig. 3.10 and compared to those of the set at LC. The behavior of both sets of self-healing concrete specimens cracked to a CMOD of 150 mm was similar during the initial stages of the water absorption test, indicating that chloride environment did not affect the effective sealing of these thinner cracks. Nevertheless, the final mass increase was clearly lower in the case of SH-SS specimens compared to the SH-LC set, which confirmed the densification induced by autogenous healing during the durability test. Regarding the samples cracked to a CMOD of 300 mm, the initial mass increase of the SH-SS specimens was higher and faster than that of the SH-LC set. However, as the capillary water absorption test proceeds, SH-LC specimens showed higher mass increase values. These results may be due to a noncomplete sealing of the 300 mm wide cracks during the storage time in LC conditions. The saline solution may penetrate these cracks partially filled with the epoxyeamine adhesive; this phenomenon could produce a slight degradation of the sealing that results in the fastest initial mass increase observed in Fig. 3.10 for SH-SS specimens. At the same time, in samples with well-healed thinner cracks, a densification effect induced by the saline environment on the concrete matrix was observed. This densification effect could be responsible for the lower final mass increase in the SH-SS set. Crack surfaces obtained by splitting the specimens with CMOD of 300 mm into two halves were analyzed by backscattered electron microscopy. The presence of sodium containing crystals, which was observed in cracked specimens of reference concrete, was not observed within the cracks of self-healing concrete (Pérez et al., 2017). In conclusion, according to the obtained results, it is confirmed that the efficiency of the autonomous self-healing mechanism tested is not affected by the two aggressive environments tested. Moreover, it provides a UHPC with clearly enhanced resistance to damage by freezeethaw cycles and salt spray test (to sodium chloride penetration). 10

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8 6 4 150 μm 300 μm

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0 0

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60 80 t0.5 (min0.5)

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Figure 3.10 Mean mass increase during the water absorption test of SH concrete after a 28-day healing period in controlled laboratory conditions (SH-LC) and subsequently subjected to SS test (SH-SS). The results of uncracked samples are included for comparison (Pérez et al., 2017).

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This improved durability implies an increase of the service life of reinforced concrete infrastructures built with the proposed self-healing concrete in humid cold climates and marine environments.

3.5

Future trends

The development of self-healing HPC, or even UHPC, is mainly based on the necessity to guarantee durable concrete structures under extreme environmental and/or operating conditions. The concept is based on combination of two innovative technologies, HPC and autonomous self-healing concretes, in order to develop long lasting concrete structures. As mentioned before, concrete developed in the future will be expected to last under severe operating and environmental conditions with high durability requirements, such as high mechanical fatigue and extreme temperatures. In fact, several new infrastructure projects will require long service life spans that often will exceed those formulated in standards, for example, concretes to be used in offshore structures or underground structures (where large temperature gradients and high pressure are expected), or concrete structures to be installed along coast lines (high chloride contents), in subarctic or arctic areas (low temperatures and ice-abrasion), desert areas (high temperatures and drastic temperature changes between night and day), etc. In most of these mentioned situations, service life longer than 100 years will be required but these periods significantly exceed the current conventional service life design. Since service life can be extended more economically with a durable initial design than by future rehabilitation, self-healing HPC will strongly reduce the maintenance costs in these special situations although they are expected to increase the initial cost of the infrastructure. According to the studies already published, the development of self-healing high performance concretes is a fact. After cracking, the healing of crack widths above 300 mm is possible and this crack healing effectively ensures the tightness of the concrete structure thus limiting or even avoiding the penetration of aggressive agents. However, the recovery of the initial mechanical properties has not been obtained yet. Since in conventional concretes only partial recovery of the mechanical properties after healing the cracks has been obtained, this aspect will be more difficult in HPC, due to the higher initial mechanical properties. Moreover, the healing of wider cracks must be also attempted and this will be essential when very aggressive environments or aggressive operating conditions of the concrete structures are considered. In order to achieve both milestones, self-healing systems very compatible with the concrete matrix must be used. In this sense, the use of the nanotechnology will be key as the preliminary studies mentioned in this chapter have already demonstrated. In fact, the use of self-healing systems based on nanoparticles is expected to influence less negatively the resulting initial mechanical properties than those based on microparticles (such as microcapsules). The scaling up of the developed self-healing technology in HPC, from laboratory to real conditions, has not been made yet. Clearly, this scaling up will depend on the specific environmental and operational conditions expected in every structure. In many cases, the combination of different self-healing systems will be needed and overall the improvement of the joint action of the autogenous and autonomous approaches.

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This last issue is a clear advantage when considering HPC since the autogenous healing is inherent to this type of concrete. But not only is the scaling up of the evaluation of self-healing HPC in real conditions needed but also the scaling up of the production of the self-healing system in order to ensure its commercialization. Finally, and considering self-healing cement based materials in general, not just the high performance ones, the unification of applicable, reliable, and more accurate evaluation criteria to better characterize the self-healing strategies developed is really needed. Nevertheless, in view of the significant advances already reported, the selfhealing HPC will undoubtedly be implemented in many concrete structures in the near future.

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The impact of graphene oxide on cementitious composites

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Alyaa Mohammed, Jay G. Sanjayan, Ali Nazari, Nihad T.K. Al-Saadi Swinburne University of Technology, Hawthorn, Melbourne, Victoria, Australia

4.1

Introduction

Service life and design of concrete structures should normally take into account both strength and durability to control the efficiency of the final product. All sorts of applied loads including compressive, shear, tensile, etc. can show the capacity of concrete structure under specific requirements. Different factors including but not limited to mix design, structural design, and curing affect the strength of the final concrete product (Mehta and Monteiro, 1993; Neville, 2011). A literature survey shows that in the past two decades, there have been extensive efforts to formulate the strength properties of concrete. Keeping on researching for suitable formulations with high attention shows the importance of finding reliable concrete in every practice. Attempts to increase the strength of concrete has led to the advent of ultra-high performance concrete with compressive strength values of 150 MPa or higher (Allena and Newtson, 2011; EL-Attar et al., 2015). Although improvements in attaining high compressive strength are remarkable, other strength features of unreinforced concrete including tensile and flexural capacities show less success. This is because of the heterogeneous nature of concrete where a weak point can initiate the crack and abrupt fracture of unreinforced concrete. Additives such as nanomaterials can assist raising tensile and flexural strength of concrete significantly. Durability, the other primary property of cementitious materials, is the ability of concrete to resist chemical attack, alteration of weather condition, and any other service life challenges. To have a durable concrete structure, planned functions must be constantly maintained for the predicted service life of the structure (Mehta and Monteiro, 1993). Although Ordinary Portland cement (OPC) is a significant contributor of greenhouse gas emission to the environment, concrete technology commonly uses it as the binder of most concrete structures. OPC concrete has always been considered as a sustainable material that can resist various environmental conditions. However, during harsh environmental situations, it is very hard to have sustainable concrete without modifying the mixture by relevant and advanced additives. However, OPC concrete itself cannot be considered as a durable material in many ecological circumstances. Enormous studies have been conducted on improving the strength and durability of OPC concrete; however, slightly effective progresses have been reported yet (Gjørv, 2011; Tang et al., 2015; Hensher, 2016). The aim of this chapter is to investigate the new approach of enhancing the durability of concrete structures by incorporating Nanotechnology in Eco-efficient Construction. https://doi.org/10.1016/B978-0-08-102641-0.00004-9 Copyright © 2019 Elsevier Ltd. All rights reserved.

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graphene oxide (GO) reinforcements. A literature survey shows that a wide variety of nanoscale particles and nanomaterials such as nanosilica, nanoalumina, carbon nanotubes, and GO have been used in concrete structures (Nasution et al., 2015). Nanoparticles are able to improve the concrete pore structure and hence, enhance the resistance of concrete structure to chemical attacks through reducing permeability of corrosive ions (Guo et al., 2017; Ghosal and Chakraborty, 2017). They have also the ability of making mechanical or chemical bonds with the cementitious paste and increase concrete strength by appropriate mechanisms. The difference between GO and other nanomaterials is in the form of the structure where GO has a sheet-like structure with a microscale size and nanoscale thickness and other materials are in terms of few molecules and spatial nanoscale size (Kim et al., 2010a, 2011). Therefore, effects of incorporating such an interesting material into concrete structures could represent a prolific research area. Various microscale particles such as fly ash and carbon fibers have been used in concrete structures to enhance their properties (Pelisser et al., 2010; Yusof et al., 2013). There is also significant attention to the use of nanoscale materials such as carbon nanotubes and nanosilica to attain a durable concrete (Du et al., 2014; Siddique and Mehta, 2014). Using graphene-based materials including GO in concrete has been seriously examined in the past decade. This chapter reviews the effects of incorporation of GO on properties of concrete specimens. First, some brief information about nanotechnology is given; it is obligatory because nanotechnology demonstrates the ways of production and characterization of nanomaterials. After that, properties of graphene and GO and the method of synthesis of GO will be addressed. A cutting-edge usage of GO in cementitious materials will then be discussed. Finally, a discussion about the influence of GO in developing an outstanding new construction material is presented. The term “nanotechnology” in the literature is a representative of multiple applications because various fields of study such as construction, materials science, biotechnology, medicine, and so on use it (Mohammed et al., 2018c). However, it is widely accepted that nanotechnology recognizes very small particles where the size of consideration is generally less than 100 nm (Ramsden, 2016). Any material with this dimension has different characteristics, features, and functions which differentiate it from its original bulk material. Therefore, the incorporation of these very tiny materials in an appropriate matrix can result in promising outcomes that are unattainable through adding mesoscale or even microscale additives. Gogotsi (Nanomaterials handbook, 2006) has presented a thorough definition of nanotechnology: If size and shape could be managed in nanodimension, then the resultant technology for designing, manufacturing, and characterizing of materials applicable in any equipment, system, or structure is called nanotechnology (Mohammed et al., 2018c). Countless additional inclusive representations of the nanotechnology concept could be found in the literature, for instance, it is the generation of materials and equipment and prevailing of the target substance in very small scales such as atoms and molecules, and in a more general case, supramolecular structure (Ramsden, 2016). In a more nonexclusive sense, it is the manner of extremely tiny elements of substances to produce innovative large-scale components (Singh, 2014).

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Besides these thorough definitions, it is worthy to mention that nanotechnology incorporates the practice of using nanoparticles as appropriate additives/reinforcements in suitable host substances.

4.2

Graphene materials

Graphene is a 2D material and its atoms are structured in a hexagonal lattice. It can be considered that all types of graphite-based materials are composed of graphene where the size and arrangement of graphene sheets alter. Many forms of graphene are used nowadays by changing the size or shape of graphene sheet in an appropriate manner. Some examples include carbon nanotubes which are rolled-up graphene sheets with metallic and semiconducting properties, zero-dimensional fullerenes which are obtained by wrapping up graphene layers, and 3D graphite structures which are a form of stacked graphene sheets to form a semimetallic structure (Geim and Novoselov, 2007; Stoller et al., 2008). There are a wide variety of 2D materials other than graphene such as borophene, germanene, silicene, etc., however, the primary focus of research community is on graphene and its components. This is as a result of the unique properties of graphene including its superior strength, and suitable electrical and heating properties which make it appropriate for various fundamental applications (Lee et al., 2008). Graphene is used in different forms including but not limited to GO, reduced GO, and nanoparticle of graphene (Allen et al., 2010). Majority of the aforementioned graphene types exhibit superior properties, which enable them to improve mechanical, chemical, and electrical properties of a structure (Balandin, 2011; Kim et al., 2010b). Among them, GO, which was first employed to create graphene particles, could be taken as one of the most common derivatives of graphene. Nonetheless, GO is now considered as a prominent material because of its unique behavior in a wide variety of applications (Mohammed et al., 2018c; Stankovich et al., 2007; Dreyer et al., 2010). It is not only due to appropriate mechanical, electrical, and physical properties of GO, but as a result of the functionalizing ability of this interesting material.

4.3

Graphene oxide

GO is one of the proficient derivatives among a wide range of graphene-based substances. To produce GO, graphite is oxidized into graphite oxide and then, GO is derived by exfoliation of the resultant oxide (Geim and Novoselov, 2007; Gilje et al., 2007). In fact, synthesis of GO is in compliance with graphite oxide because the latter is a stacked structure of GO single layers. Although there are extensive research on GO in the literature, attempts to synthesize graphite oxide date back to 1850s. An Oxford Chemist, Brodie Dating, oxidized graphite by a combination of KClO3 and HNO3 to produce graphite oxide (Brodie, 1859). Staudenmaier then used an H2SO4 oxidizing component to modify Dating’s method (Staudenmaier, 1898). Although both methods were effective, they were left because of their timeconsuming nature and hazardousness of the chemical agents. Hummers and Offerman

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were the next researchers who, in 1957, introduced a faster procedure (Geim and Novoselov, 2007). They changed the oxidizing components and used sulfuric acid (H2SO4) and sodium carbonate (NaNO3) as their chemicals (Hummers and Offeman, 1958). Hummers’ method is now one of the prominent methods which is extensively used to fabricate GO; the method is constantly improving and the most recent ones are supplemented by preoxidation phases to the original Hummers’ method (Mohammed et al., 2018c; Chen et al., 2013; Shahriary and Athawale, 2014; Marcano et al., 2010).

4.3.1

GO synthesis

Because the method introduced by Hummers and Offerman is one of the most acceptable methods in research, a brief description of it is given in this section: 23 g of sulfuric acid (H2SO4) with 98% purity is kept below 0 C for 30e45 min using an ice bath. A mixture of 1 g of graphite (powder form) and 0.5 g of NaNO2 is then progressively added to H2SO4. Mixing of these three chemical agents takes place for 45 min under continuous stirring. During this 45 min, the mixture container is placed in the ice bath to acquire the desirable mixing. At the next stage, 3 g of another chemical agent namely potassium permanganate (KMnO4) is steadily supplemented to the mixture. Occurrence of a strong oxidation in the mixture due to the addition of KMnO4 caused a color change to greenish black (from black). Temperature increases due to this reaction but it must be kept under 20 C, by adding some more ice to the ice bath for instance. Stirring for 15 min is still necessary to have a homogeneous mixture. Then, the temperature of the mixture must be reached to the ambient temperature so it is detached from the ice bath and is put in an oil bath. The stirring again continues and a color change to brown occurs. To finalize the reaction, 140 mL of ultrapure water together with 10 mm of hydrogen peroxide (H2O2) is introduced into the mixture. The constant stirring of the mixture will be maintained for another 15 min until its color changes to dark brown. The mixture is then left until the next day. During this time, GO is settled and is separated from the mixture by centrifugation. To remove impurities from the produced GO, the resultant material is washed by 5% HCl solution several times (Hummers and Offeman, 1958). Bothe Hummers and modified Hummers methods are presented in Fig. 4.1 (Marcano et al., 2010).

4.3.2

Structure of GO

Carbon atoms are arranged in a hexagonal packed structure to form GO. Further, GO has lots of functional groups. Epoxide and hydroxyl functional groups are mostly within GO’s basal plane while GO edges mostly incorporate carboxyl and carbonyl functional groups (Yang et al., 2009; Li et al., 2013; Zubir et al., 2014). Functional groups have the ability to react/interact with a wide variety of inorganic or organic molecules. Therefore, GO functional groups can react with many materials and form strong chemical bonds such as ionic or covalent bonds (Ong et al., 2015). A monolayer of GO which is deposited on a silica substrate is shown in Fig. 4.2 (G omez-Navarro et al., 2007). It is very hard to observe GO sheets in mixture through optical or electron microscopy because it has a pale

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NOx Hummers

3 KMnO4 H2SO4, 0.5 NaNO3

Improved

6 KMnO4 9:1 H2SO4/H3PO4

Sifted/filtered Oxidized materials HGO IGO HGO+

Hummers modified

6 KMnO4 H2SO4, 0.5 NaNO3 NOx

Hydrophobic carbon material recovered

Figure 4.1 Production of GO by Hummers and modified Hummers methods (Marcano et al., 2010).

Figure 4.2 A monolayer of GO deposition on a silica substrate (G omez-Navarro et al., 2007).

nature; it is solely observable when the deposition of GO occurs on an appropriate substrate. There are some proposals on the structure of GO and some of them are depicted in Fig. 4.3. Although literature reports a wide range of GO application in various fields of study, the molecular structure of GO has not been finalized yet. Many reasons have caused this deficiency of knowledge and among them are dissimilar synthesis methods, degree of oxidation, and nonstoichiometric and amorphous nature of GO (Szab o et al., 2006; Talyzin et al., 2014). Moreover, the current techniques to identify 2D materials have not been well-developed and the resolution of equipment might not be sufficient to characterize GO sheets (Casabianca et al., 2010; Becerril et al., 2008).

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Hofmann

Scholzboehm

Nakajima-matsuo

Ruess

Lerf-klinowski

Décány

Figure 4.3 GO molecular structure proposed in the literature (Szab o et al., 2006).

4.3.3

Hygroscopic nature of GO

As mentioned before, GO contains various functional groups in both basal plane and edges and therefore, GO has a hygroscopic nature and absorbs water. Water molecules are primarily trapped in GO’s interlayer vacancies. GO monolayers tend to agglomerate and stack; the interaction between water molecules and GO oxygenated groups (epoxide, hydroxyl, carboxyl, and carbonyl) creates hydrogen bonds. This reaction results in significant changes in the physical, mechanical, and structural properties of GO (G omez-Navarro et al., 2007; Si and Samulski, 2008a). The concentration of available GO solutions ranges between 1 and 4 mg/mL (Si and Samulski, 2008a,b). It is also interesting that besides water, GO is capable of dispersing in many solvents including dimethylformamide, tetrahydrofuran, N-methyl-2-pyrrolidone, etc. (Chuah et al., 2018; Paredes et al., 2008).

4.4 4.4.1

Effects of GO incorporation into cementitious composites Effects on mechanical properties

The usage of GO-incorporated concrete dates back to less than a decade ago. Lv et al. (2013a,b, 2014) seem to be the first researchers who initiated using GO in cementitious

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materials. They studied mechanical properties as well as cement hydration of GOincorporated cement matrix. The results show that the addition of GO in very small quantities (0.01e0.03 wt%) can increase strength values significantly; this was found in the form of 143% increase in flexural strength and 129% increase in compressive strength of control mix (Al-Saadi et al., 2017d). One of the aspects that Lv et al. (2013a,b, 2014) emphasize is the formation of flower-like crystals because of the nucleation of cement hydrate and CeSeH gel on GO nanosheets. Gong et al. (2014), Babak et al. (2014), Chuah et al. (2014), and Sedaghat et al. (2014) were next researchers who studied effects of GO incorporation on the compressive strength of concrete and achieved more than 40% increase in strength values in their studies. All these three groups mention that the main reason to increase the strength is the formation of cement hydrates and strengthening gels. Gong et al. (2014), Babak et al. (2014), Chuah et al. (2014) mention that the increase in strength is also related to the refinement of pore structure. Sedaghat et al. (2014) also emphasize that the adhesion between GO nanosheets and cement paste causes this increase in strength. The reason presented by Sedaghat et al. (2014) is claimed by Duan et al. (2018) as well. Horszczaruk et al. (2015) studied the effect of adding high dosages of CO (3%) to cement paste. Surprisingly, they could not achieve any remarkable strength raise. The only success was the increase in Young’s modulus which was at least double the value of their control sample. Some other studies were conducted in 2015 (Li et al., 2015; Pan et al., 2015; Sharma and Kothiyal, 2015; Wang et al., 2015) and all demonstrated that GO is more effective on increasing flexural strength rather than compressive strength. There was a boom in studying GO/cement composites from 2016 onward and lots of research (Duan et al., 2018; Sharma et al., 2016; Abrishami and Zahabi, 2016; Meng and Khayat, 2016; Lu et al., 2016; Wang et al., 2016a,b; Cao et al., 2016; Zhao et al., 2016, 2017; Qin et al., 2017; Lu and Ouyang, 2017; Yang et al., 2017; Li et al., 2017a,b,c; Mokhtar et al., 2017; Kang et al., 2017; Gholampour et al., 2017; Han et al., 2017; Tragazikis et al., 2018; Kim et al., 2018; Long et al., 2018) were carried on to study strength capabilities of these interesting materials. Almost all of these studies show a remarkable increase in all strength values. Most of them mention that adding GO particles as the nucleation sites for cement hydrates is the main reason for increasing strength values.

4.4.2

The influence of GO on durability

The structure of concrete could be considered as a heterogeneous composite with a huge percentage of pores. Concrete is normally faced with aggressive environmental conditions and hence, the durability of this material primarily depends on its transport properties. Sorptivity, permeability, and diffusion are nominated as crucial mechanisms that can describe the movement of aggressive elements in pore structures of cement paste (Basheer et al., 2001; Yang et al., 2004). These transport properties are affected by the microstructure of cementitious matrix expressed by both volume and connectivity of the pore network (Marchand et al., 2001; Shekarchi et al., 2010). Thus, it can be concluded that the most effective way to improve concrete

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durability can be through careful design and selection of materials of the concrete mix which result in high quality concrete less prone to deterioration by chemical attacks (Shi et al., 2012). In practical terms, fluids do not transport in concrete in a single phenomenon nor in a single type of ion. Thus, fluids movement through concrete occurs not only within the porous system but also by sorption and diffusion (Lane et al., 2010; Zhutovsky and Kovler, 2012). The transport mechanisms that are associated with the movement of different chemical species in concrete can be described as follows: There is a direct influence on concrete durability involving the mobility of fluids within the concrete. Thus, it is better to have a sound understanding of this phenomenon (Hilsdorf and Kropp, 2004; Mehta, 1991). There are three major fluids including water (either in the pure case or with aggressive ions), oxygen and carbon dioxide, which have the ability to enter the concrete and affect its durability. Durability can be connected to the ease with which both liquids and gasses can enter concrete, in another term “permeability” (Abobaker, 2015). It is measurable by calculating the rate of penetration of soluble ions as well as oxygen and water into the interior structure of concrete under a pressure gradient to reach a certain level. In other words, permeability is the transport of a fluid under hydrostatic pressure, it can be described by Darcy’s law (Mehta, 1991). The permeability of cementitious materials depends mostly on their porosity, tortuosity, and continuity of pores aligned with size, shape and pore distribution. Durability of cementitious materials incorporated with GO nanosheets has been studied by some researchers and their results indicate that some improvement happens. Water sorptivity of cementitious mixtures has been shown to reduce by adding GO nanosheets especially when a dosage of 0.03% is used (Mohammed et al., 2015). It has been also shown that chloride ingress in the pastes decreases with adding any amount of GO. Our previous studies (Mohammed et al., 2016, 2017d) also considered freezeethaw properties of cementitious mixtures containing different concentrations of GO. Those studies showed the remarkable effects of GO on increasing freezeethaw cycles of concrete especially when 0.06% of GO was used. The mechanism described in those studies was the creation of air voids due to GO reactions; surprisingly, the additional air voids showed no negative impact on the mechanical performance of the studied mixtures. This is where that in normal concrete without GO, the addition of air entraining agents and subsequent increase in air voids significantly reduce the strength performance. Tong et al. (2016) have also studied the effect of adding GO in different mortars and have concluded that weight loss during freezeethaw cycles and chemical attack occasions rescues by 80% and 30% respectively. Du et al. (2016) and Du and Dai Pang (2015) studied the effect of GO in concrete samples on chloride ion penetration. They achieved 80% reduction in chloride penetration as a result of pore refinement and increased tortuosity of cement matrix. However, higher dosages of GO beyond 1.5% caused agglomeration of these particles and hence reduction in the effectiveness of GO. Resistance to carbonation is another durability index of concrete structures. Carbon dioxide diffuses into the concrete structure and reacts with the cement mixture. Due to the occurrence of various reactions, the nature of cement matrix changes and reduction in strength values happens. This results in rapid degradation of the concrete mixture.

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Lower carbonation depth shows higher resistance to aggressive environmental conditions. There are some studies from authors of this chapter in this area and their results show the huge improvement in the reduction of carbonation depth (Mohammed et al., 2017a, 2018a). The very limited carbonation has been related to the refinement of pore structure and interlocking of GO nanosheets to carbonate and calcium ions. GO has the ability of increasing resistance of concrete to high temperature exposures (Mohammed et al., 2017b). The refinement of pore structure which was in the form of a raise in gel pores and reduction in capillary pores was considered as the main reason for this performance.

4.5

Some structural applications of GO/cement composites in repairing of reinforced concrete

Carbon fiber reinforced polymer (CFRP) is one of the composite materials which is used in both repairing and strengthening of reinforced concrete structures. The usage of epoxy-based adhesives (and organic ones in general) incorporates disadvantages such as flammability of the resin or the issue of poisonous fumes (T€aljsten and Blanksv€ard, 2007). Moreover, there are other issues such as susceptibility to exposure to very low temperatures such as 70 C (Gamage et al., 2006), sunlight, and UV radiations (Ombres, 2011). When the surface has some humidity or the environmental temperature is lower than 10 C, the application of these composites is problematic (D’Ambrisi and Focacci, 2011). Therefore, it seems necessary to use an alternative adhesive such as polymer cement-based to overcome these issues. There are some evidences that show the success of using these alternative materials in the method of strengthening by CFRP (Al-Saadi et al., 2016). Nonetheless, polymer cement-based adhesives might have performance affected by the situation in which the polymer is used such as its hydrothermal situation. Another type of adhesive is a nonpolymer cementitious one, which has remarkable properties such as suitable bonding and appropriate resistance to ecological situations. Moreover, disadvantages such as flammability or release of poisonous fumes are not associated with these adhesives (Hashemi and Al-Mahaidi, 2012). This section presents some applications in which GO has been used to produce nonpolymer cementitious adhesives. Innovative high-strength self-compacting nonpolymer cement-based adhesive (IHSSC-CA) is a mixture incorporating GO nanosheets and was used in a previous study (Mohammed et al., 2016a). It is a very high strength mixture showing outstanding tensile and compressive strength values of 18 and 116 MPa after 28 days respectively (Mohammed et al., 2018c). Other properties of IHSSC-CA are given in Table 4.1. IHSSC-CA has been used in our previous studies and the results obtained for all applications including stiffness, ductility, bond strength, etc. are promising. Strengthening of fiber-reinforced concrete with the near-surface mounted CFRP (NSM-CFRP) technique at different loading conditions has been studied. We showed that it is more convenient to apply IHSSC-CA for NSM-CFRP than other common adhesives such as epoxy-based and cement-based polymer ones. This is because of the unique properties

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Table 4.1 Properties of the innovative cementitious adhesive (IHSSC-CA) (Mohammed et al., 2016a)

Flow

Initial setting time (min)

Final setting time (min)

28-day tensile strength (MPa)

28-day compressive strength (MPa)

Bond strength (MPa)

Permeability 3 10L16 m2

7.5%

120

420

13.8

101

1.2

0.023

of IHSSC-CA including better flowability and workability (Al-Saadi et al., 2018). Further, IHSSC-CA showed better pull-out and bond strength results in NSM-CFRP applications than epoxy-based and cement-based polymer adhesives (Al-Saadi et al., 2018). Fig. 4.4 shows the application of IHSSC-CA with CFRP strip and Table 4.2 shows the pull-out test results. Another feature of using IHSSC-CA is its ability to reduce stress concentration. Physical analysis (Fig. 4.5) of CFRP connected to reinforced concrete through the application of IHSSC-CA showed a lower stress concentration than samples with epoxy-based and cement-based polymer adhesives (Mohammed et al., 2017c). After the pull-out test, a remarkable change in the pore network and structure of IHSSCCA was observed. The behavior is in contrast with epoxy-based and cement-based polymer adhesives which show slight change in the pore structure under loading. This implies the suitable behavior of the applied IHSSC-CA adhesive under the applied load (Fig. 4.6) (Mohammed et al., 2017c). Further, 3D laser profilometry analysis results shown in Fig. 4.7 reveal that a very rough surface is acquired for IHSSCCA adhesive after pull-out tests rather than epoxy-based and cement-based polymer adhesives (Mohammed et al., 2018b).

Figure 4.4 Application of IHSSC-CA with CFRP strips (Al-Saadi et al., 2018).

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Table 4.2 Pull-out test results (Al-Saadi et al., 2018) Ultimate pull-out force (kN)

Ultimate bond strength (MPa)

Ultimate axial stress (MPa)

CFRP strip utilization

Innovative cementitious adhesive (IHSSC-CA)

34.5

4.80

1233

0.34

Rupture of CFRP strip

Polymer cement adhesive

22.3

3.09

794

0.21

Pull-out of CFRP strip

Epoxy adhesive

41.1

5.70

1467

0.39

Debonding of CFRP strip

Specimens ID

(a)

Failure mode

(b)

Almost even thickness of a thin layer of IHSSC-CA

Uneven thickness of PCA layer with a number of voids disturbed randomly

(c)

Gravel particles from concrete substrate

Unsmooth surface of epoxy adhesive layer with uneven thickness

Figure 4.5 Images of the bond area after pull-out testing (Mohammed et al., 2017c). 16

14

IHSSC-CA-after the testing

12

IHSSC-CA-before the testing

14 Va/cm3(STP) g-1

Va/cm3(STP) g-1

16

10 8 6 4

PCA-after the testing PCA-before the testing

12 10 8 6 4 2

2

0

0 0

0.2

0.4

0.6 p/p0

0.8

1

1.2

0

0.2

0.4

0.6 p/p0

0.8

1

1.2

Figure 4.6 Nitrogen adsorption isotherms for IHSSC-CA and PCA (Mohammed et al., 2017c).

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(a)

0.20

(b)

(c)

Figure 4.7 3D and 2D images of the topographic surface of CFRP strip: (a) with IHSSC-CA, (b) with PCA, and (c) with epoxy (Mohammed et al., 2018b).

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High temperature testing of IHSSC-CA showed that this adhesive is able to maintain around 60% of its strength even at elevated temperatures (Mohammed et al., 2016b). Fig. 4.8 shows the compressive and tensile strengths at different temperatures for normal-strength concrete (NSC) and IHSSC-CA. The adhesive used in this study had a thickness of 5 mm. Further experiments showed that this thickness is insufficient to protect samples from the heat. An increased thickness of 20e25 mm was then applied as the protective heat-resistance cover. Test results showed better bond strength when the thickness increased (Fig. 4.9). It was revealed that higher temperatures require higher thickness of the IHSSC-CA adhesive where for 600 and 800 C heat exposure, the application of at least 20 and 25 mm (respectively) of adhesive is essential (Mohammed et al., 2016b).

Compressive strength (Mpa)

120 IHSSC-CA

100

NSC 80 60 40 20 0 21 °C

400 °C 600 °C Temperature

800 °C

Tensile strength (Mpa)

16 14

IHSSC-CA

12

NSC

10 8 6 4 2 0 21 °C

400 °C 600 °C Temperature

800 °C

Figure 4.8 Compressive and tensile strengths of tested specimens (Mohammed et al., 2016b).

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5

Without protective cover

4.5

With protective cover fitting curve Without protective cover fitting curve

Bond strength (MPa)

4 3.5

T = 4.597–0.002T–4.88E–06T2 R2 = 0.96

3 2.5 2 1.5

T = 4.675–0.006T–2.893E–11T2 R2 = 0.97

1 0.5 0

0

100

200

300 400 500 600 Temperature (°C)

700

800

900

Figure 4.9 Average bond strength versus temperature relations (Mohammed et al., 2016b).

The results of fatigue tests on concrete samples joined to NSM-CFRP through the application of IHSSC-CA were better than samples with epoxy-based and cementbased polymer adhesives, especially at longer lives (Fig. 4.10). The analysis of pore structure of specimens subject to fatigue loading shows a better composite behavior for IHSSC-CA than other polymer-based adhesives (Al-Saadi et al., 2017a). Physical examination after fatigue and post-fatigue tests (Fig. 4.11) shows uniform stress distribution and therefore, suitable composite behavior is observed between NSM-CFRP and IHSSC-CA (Mohammed et al., 2018c). This is against epoxy-based and cement-based polymer adhesives where a nonuniform stress distribution was observed, as shown in Fig. 4.12 (Al-Saadi et al., 2017a).

Load range (La) (kN)

20

15 FR20IC-1 FR20C-1 FR10C-1 FS20C-1 FS10C-1

10

5

0 10

100

1000 10000 100000 Fatigue life (N) (Cycles)

1000000

10000000

Figure 4.10 Load range-fatigue life relationships for IHSSC-CA (FR20IC) and polymer cement-based (FR20 C) adhesives (Al-Saadi et al., 2017a).

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16 R20IC R20C

14 Va/cm3(STP) g-1

12 10 8 6 4 2 0 0

0.2

0.4

0.6

0.8

1

1.2

P/PO

Figure 4.11 Nitrogen adsorption isotherms for IHSSC-CA (R20IC) and polymer cement-based (R20 C) adhesives (Al-Saadi et al., 2017a).

Figure 4.12 Images of CFRP strip and a bond area of tested specimens: (a) IHSSC-CA and (b) polymer cement-based adhesive (Al-Saadi et al., 2017a).

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Load (kN)

120 100 MC MSC MRC MSE MRE

80 60 40 20 0

0

20

40

60 Deflection (mm)

80

100

120

Figure 4.13 Load versus midspan deflection relations (Al-Saadi et al., 2017b).

In a previous study, deflection capacity (ductility) and flexural strength of reinforced concrete beams strengthened by NSM-CFRP using IHSSC-CA adhesive showed improvement compared to a control specimen (Al-Saadi et al., 2017b). Despite IHSSC-CA, strengthening by using epoxy-based adhesives showed decreased ductility and flexural strength as shown in Fig. 4.13 (Al-Saadi et al., 2017b). Fig. 4.14 shows the deformation behavior of reinforced concrete beams strengthened by both epoxy-based and IHSSC-CA adhesives. This figure shows the suitability of using IHSSC-CA adhesive in creating large deformations compared to epoxy-based adhesive which caused abrupt rupture (Al-Saadi et al., 2017b). Further, as Fig. 4.13 shows, the application of epoxy-based adhesive to strengthen reinforced concrete beams causes the appearance of no residual strength after the ultimate strength. The post-cracking behavior of specimens strengthened by IHSSC-CA is different and a large amount of residual strength (close to 87%) sustains after reaching its ultimate strength (Al-Saadi et al., 2017b). Results of our previous studies show the effectiveness of using IHSSC-CA as adhesive for repairing and strengthening of reinforced concrete beams. Values such as deflection and strain are higher in structures strengthened by epoxy-based adhesives. Further, the proposed IHSSC-CA adhesive has the capability for in-situ applications (Al-Saadi et al., 2017c,d). Analysis of the fatigue behavior of reinforced concrete beams strengthened by the IHSSC-CA adhesive shows the effectiveness of this binder as better stress transfer and bond strength are attainable compared to epoxy-based adhesives, as shown in Figs. 4.15 and 4.16. This behavior shows once again that the utilized IHSSC-CA adhesive has the ability of maintaining composite action even under fatigue loads (Al-Saadi et al., 2017c,d). Besides mechanical performance, IHSSC-CA is more convenient for on-site applications compared to epoxy-based adhesives because its workability, flowability, and self-compacting properties are better and create a uniform and smooth bonding layer (Al-Saadi et al., 2017c).

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(a) Crushing of concrete

Rupture of CFRP strips Rupture of CFRP strips

Rupture of CFRP strips

Transverse cracks

(b)

Beginning of concrete cover seperation at the cut-off point of the CFRP strips

Intersection between flexure and shear cracks

Main horizontal crack along the level of tension steel reinforcement

Figure 4.14 Failure modes of tested beams: (a) with IHSSC-CA and (b) with epoxy adhesive (Al-Saadi et al., 2017b).

Fig. 4.17(a) shows that reinforced concrete beams with IHSSC-CA adhesive upon the rupture of CFRP strip. This happens once the concrete cover in the compression zone crushes and reinforced steel bars reach to their yield strength. In other words, there is no degradation between the adhesive and CFRP strip under fatigue loads (Al-Saadi et al., 2017d). The results for CFRP strips joined to reinforced concrete

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18 16

Deflection (mm)

14 12 FRE FSE FC FRC FSC

10 8 6 4 2 0 0

0.5

1

1.5

2

2.5

3

Number of cycles (million)

Figure 4.15 Midspan deflection versus fatigue life relations for tested beams (Al-Saadi et al., 2017c). 0.8 0.7

Crack width (mm)

0.6 0.5 0.4 0.3 FC FRE FSE FRE FSC

0.2 0.1 0

0

0.5

1 1.5 2 Number of cycles (million)

2.5

3

Figure 4.16 Crack width versus fatigue life relations for tested beams (Al-Saadi et al., 2017c).

through epoxy-based adhesive in Fig. 4.17(b) show an abrupt failure after the crushing of concrete cover and the yielding of steel bars. In other words, because of bond degradation between CFRP and epoxy-based adhesive, this type of failure occurs (Al-Saadi et al., 2017d). Ductility of beams reinforced by IHSSC-CA also increased compared to epoxy-based adhesives (Al-Saadi et al., 2017d). Less deformation of

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Figure 4.17 Failure modes of tested post-fatigue beams: (a) with IHSSC-CA and (b) with epoxy adhesive (Al-Saadi et al., 2017d).

the surface of CFRP strips was proved by studying the results through 3D laser profilometry, as shown in Fig. 4.18. The less deformation means higher capacity of repaired and strengthened reinforced concrete to resist fatigue loading (Al-Saadi et al., 2017d).

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(a) µm

21.8 15.0 10.0 5.0 0.0 –5.0 –10.0 –15.0 –20.0 –25.0 –30.0

1.27 mm –35.0

0.95

–40.0 –46.0

(b) µm

38.5

30.0 25.0 20.0 15.0 10.0 5.0 0.0 –5.0 –10.0 –15.0 –20.0

1.27 mm 0.95

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Figure 4.18 2D and 3D images of the topographic surface of CFRP strip (a) with IHSSC-CA and (b) with epoxy adhesive (Al-Saadi et al., 2017d).

4.6

Summary of the chapter

This chapter reviewed effects of incorporating GO on the performance of cementbased materials. Further, an innovative application of GO-incorporated mixture applicable in repairing and strengthening of reinforced concrete sections was presented. Review of the current literature showed that GO is suitable for the modification of cement paste microstructure. The reduction in the amount of capillary pores and the raise of gel pores are the direct benefits of using GO. As a result of this microstructural refinement, cement hydration improves and hence, mechanical properties of concrete are enhanced remarkably. GO addition was also effective in increasing durability of concrete by reducing water transport in concrete. It was also shown that concrete samples reinforced by GO has better freezeethaw behavior. This is attributed to the surfactant effect of GO. One interesting feature of adding GO was about sustaining mechanical strength of air-entrained concrete. Adding air to

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concrete to increase its freezeethaw ability reduced the mechanical strength of normal concrete, adding GO prevents this decrease in strength. Carbonation of concrete is also improved by adding GO. Large surfaces of GO trap carbon dioxide diffusing into concrete. GO-incorporated cement mixtures as adhesive for joining CFRP strips to reinforced concrete revealed unique results compared to common adhesives. Better adhesion, higher compressive and tensile strength, better ductility, lower deflection, and better fatigue performance of beams were the advantages of using this binder.

Acknowledgment A review paper on effects of graphene oxide on cementitious materials has been already published by the authors as “Mohammed, A., Al-Saadi, N.T.K., Sanjayan, J., 2018. Inclusion of graphene oxide in cementitious composites: state-of-the-art review. Australian Journal of Civil Engineering, 1e15.” This is to confirm that the current chapter again reviews this effect. However, the majority of information in this chapter are new.

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Application of nanomaterials in alkali-activated materials

5

Q.L. Yu Department of the Built Environment, Eindhoven University of Technology, Eindhoven, The Netherlands

5.1

Introduction

During the transition of one material from macro- to nano-range size, significant changes will occur in electron conductivity, optical absorption, chemical reacting activity, and mechanical properties, as well as in surface energy values and surface morphology of the composites (Falikman, 2011). Nanotechnology is the ability to control and restructure the matter at the atomic and molecular levels in the range of approximately 1e100 nm, and exploit the distinct properties and phenomena at that scale as compared to those associated with single atoms or molecules or bulk behavior (Roco et al., 1999). Nanotechnology has gained increasing attention among the industrial sector and researchers in the last decades (Lazaro et al., 2016), especially because the development of appropriate methods of properties and reactions control in nanoscales can lead to creation of new materials, technologies, and devices. Nanotechnology has been already widely investigated in traditional Portland cement based concrete system (Falikman, 2011; Lazaro et al., 2016). The hydrated phases present in cement paste have particles in the nanorange of 1e100 nm in size. Reducing the dimensions of concrete ingredients and formation of specific continuous filamentary structures in the three-dimensional structure result in a radical improvement in its performance. Besides, because of the dimensions controlled in the transitional zone between atom and molecule, nanomaterials can contribute to concrete with extra functionalities, including for example enhanced durability and mechanical properties, adjustable conductivity, or air purifying property (Lazaro et al., 2016). The production of Portland cement is continuously increasing and the global demand would have increased almost 200% by 2050 from 2010 (Naik, 2008). However, the production of Portland cement consumes a large amount of primary resources, and meanwhile its production involves high energy-consumption and greenhouse gasemission (Duxson et al., 2007). Approximately, 7% of the global CO2 emissions are associated with the cement production and the manufacture of each ton of cement produces up to 900 kg of CO2 (Sumesh et al., 2017). In the last years, extensive efforts have been spent on searching for alternatives to replace Portland cement (Provis et al., 2015), and alkali-activated materials (AAM) have drawn great attention because of their superior properties and environmental benefits (Pacheco-Torgal et al., 2008). Weil et al. (2009) demonstrated that the global warming potential of alkali-activated

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concrete is 70% lower compared to Portland cement concrete. This can be further reflected by the steadily increasing publications, the returned number is 4691 in 2017 while it was only 1798 in 2007 by searching the keyword “alkali-activated materials” in Sciencedirect.com owned by Elsevier (Keywords, 2018). The concept of AAM uses alkali solutions or salts to dissolve and activate wastes or industrial by-products to obtain sustainable building materials. Many factors can affect the performance of AAM, e.g., alkali natures (Jimenez et al., 1999), alkali concentrations (Heikal et al., 2014), reactivity and physiochemical properties of raw materials (Lee and Lee, 2013), curing methods, etc. Various subjects concerning alkali-activated materials have been deeply addressed, with experimental or modeling approaches. Some examples are for instance chemistry (Garcia-Lodeiro et al., 2015), mix design (Gao et al., 2016), use of different solid precursors such as fly ash (FA) (Gao et al., 2017a) and slag (Yuan et al., 2017c), rheology (Leonelli and Romagnoli, 2015), mechanical properties (Yuan et al., 2017b), water absorption (Zhang et al., 2017a), pore structure (Yuan et al., 2017c), shrinkage (Gao et al., 2016; Yuan et al., 2017c), durability aspects such as frost resistance (Cyr and Pouhet, 2015), carbonation resistance (Bernal, 2015), corrosion behavior (Criado, 2015), chemical attack resistance (Bascarevc, 2015), fire resistance (Panias et al., 2015), sustainability concerning recycling (Chindaprasirt and Cao, 2015) and life cycle analysis (Ouellet-Plamondon and Habert, 2015), functionalities such as air purification (Zhang et al., 2015), additive manufacturing applying 3D technology (Panda et al., 2017), etc. Among these investigations, nanotechnology has also been applied, for instance, nanosilica, nanotitania, carbon nanotube, etc. are applied in alkaliactivated materials in order to acquire different functions or properties. But up till now no published literature is available on thoroughly summarizing the application of nanotechnology in alkali-activated systems. This chapter aims to provide a comprehensive review on the application of nanotechnology in alkali-activated materials. In Section 5.2, nanotechnology in alkaliactivated materials is analyzed. The performance enhancement by nanomaterials in alkali-activated materials is evaluated and the potential health and environmental issues induced by nanomaterial applications are discussed. Subsequently in Sections 5.3e5.6, applications of different nanomaterials in alkali-activated materials are reviewed and compared. In Section 5.7, brief conclusions and future research trends are provided.

5.2

Nanotechnology in alkali-activated materials

Feynman in 1959 pioneered the idea nanotechnology by describing the possibility of material synthesis via direct manipulation of atoms (Feynman, 1992), while Taniguchi invented the term “nanotechnology” in 1974 (Saba et al., 2016). Certain properties of materials would be affected when their sizes reduce to the nanorange, for example, size can strongly impact melting point, fluorescence, chemical reactivity, electrical conductivity, etc., due to quantum mechanical effects or other physical effects (e.g., high specific surface area) (Lazaro et al., 2016). The amount of atoms at the surface compared

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to the number of atoms in the inner part becomes significant for nanomaterials, increasing the amount of material exposed to the surrounding medium, increasing the reactivity of nanomaterials (Roco et al., 1999), and one typical example is the increased solubility of silica with the decrease of its particle size. Although humans have already worked with nanoparticles (e.g., natural waxes, clays, and paints) since the antiquity, the big difference is that now we have a better understanding of the physical and chemical phenomena of nanotechnologies (Lazaro et al., 2016). Consequently, materials with new properties can be formulated and produced, allowing researchers to take advantage of the special properties that occur in the nanoscale. Nanomaterials are new emerging materials in the field of civil engineering and nanotechnology has resulted in a variety of products and applications. Among the various benefited industrial directions, the construction sector has observed significant progress in the past few decades. Thanks to the extra-brought properties, the application of nanomaterials in construction materials, for instance cement, concrete, coating, paint, glass, etc., is becoming more and more popular. Most of the recent researches on applying nanomaterials in concrete aim to alter the cement material structure and it has been recognized that addition of nanomaterials to concrete matrix can significantly affect its properties. For instance, adding nanosilica to concrete both improves the packing of the ingredients of concrete mixture (Falikman, 2011), leading to improved strength and refined microstructure (Wu et al., 2017), and accelerates the cement hydration, resulting in faster formation and transformation of CeSeH gel that later will also enhance the durability aspects (Quercia et al., 2014). The application of nanotechnology in alkali-activated materials has also gradually drawn attention, although it is still limited compared to traditional cementitious materials. Researches have been carried out to investigate the influence of nanoparticles on key properties of alkali-activated materials, including for instance workability (Gao et al., 2018; Deb and Sarker, 2016; Rodríguez et al., 2013), setting (Gao et al., 2015b; Chindaprasirt et al., 2012), density and porosity (Gao et al., 2016; Rodríguez et al., 2013; Lo et al., 2017; Yang et al., 2015), microstructure (Rodríguez et al., 2013; Lo et al., 2017; Yang et al., 2015), mechanical properties (Lo et al., 2017; Yang et al., 2015; Lyu et al., 2014), durability (Patel et al., 2015), shrinkage (Gao et al., 2016; Yang et al., 2015), fire resistance (Gomez-Zamorano et al., 2016), etc. Various nanomaterials have been applied, including for instance nanosilica (Rodríguez et al., 2013; Gao et al., 2015b, 2017b), nano-Al2O3 (Guo et al., 2014; Riahi and Nazari, 2012), nanoclay (Assaedi et al., 2016b; Khater et al., 2013), nano-TiO2 (Zhang et al., 2015, 2017c), and nanocarbon tubes (Rovnaník et al., 2016; Saafi et al., 2013; Abbasi et al., 2016). It is observed that the addition of nanosilica leads to reduced workability (Deb et al., 2016; Gao et al., 2015a). The nanosilica has a clear contribution to the mechanical property of fly ash based geopolymer (Deb et al., 2016), but much less significant contribution to slag based alkali-activated binder (Gao et al., 2015a). Chindaprasirt et al. (2012) reported that nano-Al2O3, with the size of 13 nm, contributes to a shorter setting time of high calcium fly ash based geopolymer paste, and the more the addition of nano-Al2O3, the faster the setting. Phoo-ngernkham et al. (2014) reported that nano-Al2O3 contributes to improved bonding of geopolymer and the concrete substrate. Yang et al. (Yang et al., 2015) observed that nano-TiO2 enhances the

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strength and reduces the shrinkage with a dosage of 0.5% by mass. Assaedi et al. (Assaedi et al., 2016b) reported that adding 1%e3% nanoclay reduces the porosity and water absorption up to 7.21% and 17.35% respectively. Saafi et al. (2015) developed fly ash based geopolymers applying reduced graphene oxide and observed that graphene oxide refines the microstructure and improves mechanical properties of the geopolymer. Nevertheless, negative effects on workability were also often observed when applying nanomaterials to alkali-activated materials because of their high specific surface area that leads to a higher water demand (Deb et al., 2016; Gao et al., 2015a). Therefore, the selection of appropriate superplasticizer is crucial in achieving the desired workability. However, available literature shows that the current superplasticizers show no or rather low improvement concerning workability that could be associated with their physical and chemical incompatibility or rapid chemical oxidation in a high alkali system (Rashad, 2014). Superplasticizers mainly enhance the workability of alkali-activated systems for a short period of time and work better to fly ash based system while an increasing slag content and/or silicate activator content will strongly reduce the workability (Keulen et al., 2018). In addition, nanomaterials can sometimes cause potential harmful effects to human health, animal health, and environment. With evolution in time, the construction designs are becoming more complicated and the construction workers are therefore increasingly exposed to complex environment (Panda et al., 2017). Construction materials where nanomaterials are applied can release nanoparticles (NPs) into the atmosphere under different conditions (Azarmi et al., 2014; Gottschalk and Nowack, 2011), which in turn might cause severe health and environmental problems (PachecoBlandino et al., 2012). The potential routes of human exposure of nanomaterials are mainly inhalation and absorption. Tjoe Nij et al. (2003) reported that exposure to crystalline silica (e.g., quartz) in the construction industry regularly exceeds occupational exposure limits and the risk of early signs of quartz dust related pneumoconiosis is increasing among construction workers, however hardly no research has been performed on this issue. The toxicity of nanomaterials depends on a number of factors, including particle size, surface area, crystallinity, surface chemistry, and particle aggregation/agglomeration tendency (Pavlidou and Papaspyrides, 2008). Nanofibers can potentially lead to pulmonary illnesses (Byrne and Baugh, 2008). TiO2 nanoparticles can penetrate into the skin of hairless mice, causing different pathological lesions in several organs (Wu et al., 2009). The interaction of nanomaterials with human is still a significant matter of research although a number of preventive measures and safety measures have been proposed and applied (Pavlidou and Papaspyrides, 2008). Therefore, more efforts are still needed to further understand nanomaterials in order to better make use of them for advancement of construction materials.

5.3

Effects of nanosilica on alkali-activated materials

Nanosilica (NS) has been widely applied in Portland cement concrete for the improvement of microstructure (Quercia et al., 2014; Neville, 1995), mechanical properties (Assaedi et al., 2016b), and durability (Quercia et al., 2014), due to its nucleation effect

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to accelerate the cement hydration process (Shaikh et al., 2014) and pozzolanic activity (Qing et al., 2007). Another reason to replace cement with nanosilica in concrete is to reduce the CO2 footprint of concrete (Lazaro et al., 2016). Recently, more and more attention has also been paid to apply nanosilica in alkali-activated systems due to the variously enhanced properties caused by nanosilica. Phoo-ngernkham et al. (2014) investigated the performance of fly ash-based geopolymers containing nanosilica. Sodium hydroxide (NaOH) modified sodium silicate (Na2SiO3) was used as the alkali activator with the Na2SiO3/NaOH ratio of 2. A liquid/ binder ratio of 0.6 was used and NS was added with the dosage of 1%e3% (S1eS3 in Fig. 5.1). Results show that increasing the nanosilica content results in the decrease of setting time, with the initial setting time reducing from 30 min till 12 min and final setting time from 58 to 26 min at the NS dosage of 3%. The XRD patterns (see Fig. 5.1) of the geopolymer pastes containing nano-SiO2 are comparable to that of fly ash. Increased quartz is shown, attributed to the added nanosilica. The presence of the CeSeH phase is confirmed by the presence of peaks at 29.5 and 32.05 o 2thea, which contributes to the mechanical property of the geopolymer matrix. Microstructural analysis revealed that a nanosilica content of up to 2% increases the reaction product and densifies the matrix, but higher nanosilica contents show negative effects. The addition of NS with the dosage of 1% shows the best contribution in terms of compressive strength while further increasing the NS to 3% shows negligible effect. Lo et al. (2017) investigated the effect of nanosilica on the alkali-activated characteristics of spent catalyst metakaolin-based geopolymer. Water glass modified by sodium hydroxide was used as the alkali activator and nanosilica was used up to 2% by mass. It is found that up to 0.5% addition, nanosilica contributes to a refined microstructure, resulting in improved compressive strength, while further increase in the

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nanosilica dosage results in negative effects. The enhanced effect is attributed to the enhanced compactness by nanosilica, confirmed by the MIP test results. Gao et al. (2013) applied nanosilica up to 3% by mass in metakaolin-based geopolymers, using water glass modified by sodium hydroxide as the alkali activator. Increased reaction products, amorphous hydrated aluminosilicate, due to nanosilica addition were observed and an optimum nanosilica content of 1% in terms of strength and porosity was found. Adak et al. (2014) investigated the effect of nanosilica on strength and durability of a fly ash-based geopolymer, the used nanosilica content was up to 10% by mass. The results showed that a nanosilica content of 6% exhibits appreciable mechanical properties under ambient temperature, as well as less water absorption and reduced charge passing in rapid chloride ion penetration test. The enhancement is attributed to the transformation of amorphous reaction products to crystalline phases. Deb et al. (2016) studied the effect of nanosilica on sorptivity and acid resistance of ambient-cured geopolymer mortars. Low calcium fly ash was used as the main aluminosilicate source and ground granulated blast furnace slag (GGBS) and/or Portland cement were used to accelerate the setting time. Sodium hydroxide modified sodium silicate solution was used as the alkali activator. Sorptivity tests were performed on samples without nanosilica and with 2% nanosilica following standard ASTM C1585. Results show that the sorptivity coefficient of the samples without nanosilica ranges from 3.575  103 mm/s0.5 to 3.980  103 mm/s0.5, while that of samples with 2% nanosilica ranges from 1.247  103 mm/s0.5 to 2.157  103 mm/s0.5. Such a significant improvement indicates the improvement of porosity, which can be explained as: firstly the nanosilica improves the packing of the mix and secondly the nanosilica is involved in the reaction that leads to an increased amount of aluminosilicate gel. The geopolymer samples were immersed in 3% sulfuric acid solution for 90 days and the changes in mass were determined on a weekly basis. The results show that the reference geopolymer lost 5.41% after 90 days but the mass loss of samples containing 2% nanosilica is only 1.9%. The compressive strength loss was also determined on the samples after acid exposure. The compressive strength of the reference reduced from 29.0 to 19.1 MPa while that of samples containing 2% nanosilica was only very slightly reduced from 60 to 58 MPa, which is attributed to the improved acid resistance of hydrated gel by the lower porosity. Gao et al. (2015a) investigated the effect of nanosilica on alkali-activated slag/fly ash blends. GGBS and Class F fly ash were used as the solid precursors. Sodium hydroxide modified sodium silicate was used as the alkali activator, with an equivalent sodium oxide (Na2O) content of 5% by mass of the binder and an activator modulus of 1.4. Two slag/fly ash mass ratios of 30/70 and 70/30 were used and nanosilica was added up to 3% by mass. Results show that a lower slag/fly ash ratio leads to better workability and increasing the nanosilica content results in a dramatic reduction of workability. Heat release monitored by an isothermal calorimeter shows retardation effect by the used nanosilica, which can be explained by: (1) the dissolved nanosilica during the early age leads to an increment of the initial silica content in the solution, which delays the reaction of slag and fly ash and (2) the incorporation of nanosilica

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leads to a decrease of effective slag content in the system, which leads to the decrease of the reaction intensity. Gel character analyses conducted by FTIR and TG/DSC show that the main reaction product is a CeAeSeH type gel with chain structure, and the addition of nanosilica slightly increases the chemically bound water content. A nanosilica addition of up to around 2% benefits the compressive strength at typical curing ages of 3, 7, and 28 days, while a further higher nanosilica content shows negative effects. As suggested by Provis and Van Deventer (2014) among other researchers (Gao et al., 2015b; Yuan et al., 2015, 2017a), many alkali activators including for instance MOH, M2SiO3, M2CO3, and M2SO4 work well with various precursors but in overall M2SiO3 provides the best activation and the most commonly used cation is Na due to its relatively low cost and availability (Wang et al., 1994; Bernal et al., 2013). However, the production of sodium silicate involves the calcination of sodium carbonate and quartz sand at temperatures between 1400 and 1500 C, generating a large amount of CO2 of 403e540 kg/ton and an energy consumption between 420 and 1250 MJ/ton (Gao et al., 2017b). Therefore, this process substantially increases the embodied energy of silicate-activated binders, reducing sustainability (Rodríguez et al., 2013). Efforts have been spent on investigating silicate-based alkali activators with alternative silica sources (Gao et al., 2017b, 2018; Rodríguez et al., 2013; Puertas et al., 2015; Zhang et al., 2017b, 2017c; Sturm et al., 2015). Rodríguez et al. (2013) developed a silicate-based alkali activator by mixing analytical sodium hydroxide or potassium hydroxide with a commercial nanosilica suspension, reaching a SiO2/M2O ratio of 1.16 and Na2O content of 12%/K2O content of 18.2%. A commercial sodium silicate solution based activator was designed as the reference. Fly ash containing the main phases of SiO2 (39%), Al2O3 (28.01%), Fe2O3 (15.43%), and CaO (10.27%) was used as the solid precursor. Results show that the water demand of the nanosilica-based binder is lower than the reference binders and the K-based reference shows lower water demand than the Na-based binder. The lower water demand of nanosilica-based binder can be explained by the structural differences of the used silica sources while the lower water demand of Kseries is attributed to the lower silica content. It is shown that at 28 days of curing, the compressive strength of nanosilica-based binders is lower than those of references, attributing to the lower dissolved silica content of the nanosilica/MOH solutions compared with the commercial silicate solutions. The pore structures of the studied systems were evaluated using the MIP technique (see Fig. 5.2). It is seen that the K-based binder shows in overall lower porosity, which is in line with its lower water demand, while the nanosilica based binder shows lower porosity compared with the commercial silica-based binder. This can be explained by the slower silica dissolution in the system, which is consistent with the slower strength development of this system. Gao et al. (2017b) considered further the sustainable silicate-based alkali activator by exploring an eco-nanosilica as the silicate source. In this research, an econanosilica, produced by the dissolution of olivine, exhibits advantages with regard to carbon emission, energy consumption, and total costs (Lazaro et al., 2012). This type of amorphous nanosilica is produced under temperatures lower than 95 C and

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