Geopolymer foams: An overview of recent advancements
Introduction
Geopolymers are an exciting class of aluminosilicate binders [1], [2] synthesised at low temperatures, typically below 100 °C [3], but often ambient temperature has been employed to decrease the binder’s production cost [4], [5], [6]. The term “geopolymer” was coined by Davidovits in 1978 [7] to describe mineral polymers synthesised by geosynthesis, i.e., the chemical reaction of aluminosilicate precursors with alkali polysilicates, yielding Al-O-Si bonds [8]. The alkaline route is the most common approach towards the chemical activation of Si- and Al-rich precursors, but acidic conditions can also be used (e.g. phosphoric acid) [9], [10], [11]. Geopolymers have also been referred to in literature as inorganic polymers and alkali-activated materials [12], [13], [14], [15]. The distinction between the different terminologies is beyond the scope of this review, but can be found in literature [3], [16], [17], [18], [19], [20], [21]. In fact, these materials can all be synthesised using the same chemistry [18], and their use in the applications considered in this review is suitable regardless of the calcium content in the precursors. Therefore, here the term “geopolymer” will be used generically.
Geopolymers were first envisioned for fire resistance applications [22]. However, since the early 1980′s these materials have been mainly considered as an alternative to ordinary Portland cement, primarily due to their lower embodied CO2 [23], [24], but also to performance advantages (e.g. early compressive strength; greater chemical and heat resistance) [13], [14], [25]. The use of various industrial waste streams as raw materials, such as fly ash [26], [27], metallurgical slags [28], [29], glass wastes [30], [31], is another advantage, particularly considering the new circular economy paradigm. Nonetheless, and despite tremendous efforts, the full potential of geopolymers has yet to be fully exploited [32], [33], which is attributed to legislation, social and technical barriers [34]. A key driver to foster the use of geopolymers is by extending their application range to areas where Portland cement does not meet the requirements. In recent years, the use of porous geopolymers, intentionally designed to contain pores, for high-added value applications, such as thermal [35], [36] and acoustic insulation [37], [38], fire resistance [39], [40], adsorbents [41], [42] and pH regulators [43], [44] has been considered. However, in other industrially relevant applications such as their use as catalysts or catalyst supports [45], [46], [47], [48], [49] this strategy (e.g. deliberately introducing porosity in the specimens) has yet to be explored. The interest in geopolymer foams (lightweight solid materials) has sharply increased in recent years, as demonstrated by the number of publications on this topic (see Fig. 1). Review papers focusing exclusively on porous geopolymers are uncommon, the exceptions being [32], [50], [51]. Nevertheless, these reviews were mainly focused on the processing and properties, while the applications were only briefly addressed. Here, the most promising and recent advances in the use of highly porous geopolymer foams are discussed, and the main challenges for future research identified, enabling their wider use.
Section snippets
Methods for identifying relevant literature
A systematic review of literature published over the past 20 years (between 1998 and 2018) focusing on geopolymers was carried out using Scopus in December 2018. Scopus database has around 70 million items and covers ~22,000 peer-reviewed journals. Papers were filtered using the keyword “geopolymer”. A total of 4295 papers, including articles, conference papers and reviews, and excluding book chapters and reprints, were identified. Then, the title and abstract of these papers was screened, by
Geopolymer foams synthesis
In this section, the most relevant synthesis protocols to produce geopolymer foams will be briefly described. It is not our intention to provide a detailed description, as this has already been reported in literature [50].
The most common synthesis route to produce geopolymer foams is by the incorporation of a foaming agent (e.g. hydrogen peroxide, fine metallic powders) [55], [56], [57], [58] into the geopolymeric slurry, usually known as the chemical foaming technique. This strategy takes
Thermal and acoustic insulators
Geopolymer foams have important technical advantages (e.g. high thermal stability, green synthesis; non-flammability) over conventional low thermal conductivity materials and, therefore, a significant number of articles focusing on this topic is available. One of the first investigations was performed by Prud’homme et al. [36] in 2010. Silica fume was used as foaming agent to produce geopolymer foams with density of 534 kg/m3 and thermal conductivity between 220 and 240 mW/m K. In the same
Future prospects and challenges
Over recent decades, geopolymers have been considered as a potentially more eco-friendly substitute for Portland cement, yet recently the possibility of tailoring their porosity, adsorption and leaching has boosted the interest in using them in environmental remediation applications (e.g. wastewater treatment), renewable energy production (e.g. biogas) and as energy-saving building materials (e.g. thermal insulation).
Geopolymers have demonstrated interesting potential as low thermal
Conclusions
The use of geopolymer foams in environmental remediation applications, renewable energy production, and as multifunctional and energy-saving building materials might be a driver to allow the widespread of this technology. Geopolymer foams have shown promising thermal insulation performance. However, to set them apart from other conventional building materials, multifunctional foams combining thermal and acoustic insulation, coupled with thermal stability and moisture buffering capacity must be
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
R.M. Novais wishes to thank FCT for supporting his work (Ref. CEECIND/00335/2017). R.C. Pullar thanks the FCT for supporting this work through grant IF/00681/2015. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, FCT Ref. UID/CTM/50011/2019, financed by national funds through the FCT/MCTES. The authors are grateful to the JECS Trust for funding Contract 2017146. We thank the publishers (Elsevier and Springer Nature) for grating us copyright
Dr. Rui Novais has currently a Research Fellow position at the Department of Materials and Ceramic Engineering and CICECO-Aveiro Institute of Materials at University of Aveiro. He obtained his PhD in Polymer Science and Composites, in 2012 from the University of Minho. His current research interest is in the synthesis of geopolymers for environmental remediation applications, wastes valorization, synthesis of ecoceramics for energy production, and on activated carbons. He has published 54
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Dr. Rui Novais has currently a Research Fellow position at the Department of Materials and Ceramic Engineering and CICECO-Aveiro Institute of Materials at University of Aveiro. He obtained his PhD in Polymer Science and Composites, in 2012 from the University of Minho. His current research interest is in the synthesis of geopolymers for environmental remediation applications, wastes valorization, synthesis of ecoceramics for energy production, and on activated carbons. He has published 54 papers, 46 of which are in JCR SCI journals, 39 in the top quartile (Q1), and 34 are in the top 10% of their areas. He is also the first author of a book chapter on lightweight inorganic polymers. He has more than 670 citations, and an h index of 15 (Scopus).
Dr. Robert Pullar is a Principal Investigator at the Department of Materials and Ceramic Engineering and CICECO - Aveiro Institute of Materials at the University of Aveiro. He works in the areas of Sustainable Materials Science and Green Chemistry, sol-gel and nanosynthesis, and magnetic, dielectric, piezoelectric, photocatalytic, photo/electrochromic, biomimetic/biomorphic and biomaterials made in bulk, powder and nanoparticle forms. He has published 157 papers, 89 of which are in JCR SCI journals in the top quartile (Q1), 57 are in the top 10%, and 42 are in journals in the top 3 of their areas. He has a large number of citations (>4400), an h index of 34 (Scopus), and a g index of 62. In 2012 he published a major invited review on hexaferrites in Progress in Materials Science, which was the 3rd most cited PMS article over its first 5 years, has over 30,000 views and has been cited more than 960 times.
Professor João Labrincha is an Associate Professor at the Department of Materials and Ceramic Engineering and CICECO-Aveiro Institute of Materials at University of Aveiro. Following his PhD in Materials Science and Engineering (University of Aveiro, 1993) he implemented a research line on Wastes Recycling and Sustainable Use of Resources, teaching disciplines on related topics. He is an expert in ceramics processing. His current research interest is in the development of multifunctional eco-materials, waste-based products (geopolymers, cements and mortars, pigments) for a circular economy, and on novel photocatalytic materials active under visible-light. He has 22 patent applications (two of those as International PTC), more than 300 papers (Science Citation Index), >6470 citations, and an h index of 42 (Scopus). He is Associate Editor of Clay Minerals and of Journal of Sustainable Metallurgy. He is author/co-author of 8 book chapters and co-editor of 7 books.