Porous geopolymer composites: A review
Introduction
Geopolymers, originally termed and largely developed by Joseph Davidovits in the 1970s, are produced by the polycondensation of aluminosilicate materials in the presence of an alkali or acid activating solution [1]. Geopolymers have been considered to possess a typical polymeric structure, consisting of many repeated subunits with high molecular weight, although it could also be described as akin to that of an alumino-silicate glass, based on a continuous random network possessing only short-range order [1]. Geopolymers (inorganic polymers) have emerged as one of the most promising inorganic non-metallic materials over the past few years, due to their remarkable advantages such as low cost and low CO2 emissions for their production,[2], [3] facile synthesis protocol,[4] good formability and local availability of the raw materials,[3], [5], [6] superior thermal and chemical resistance,[7] lightweight porous structure,[6], [8] rapid hardening at low temperature with an excellent resultant strength,[9] etc. [1], [10] They can also be used for coatings and adhesives,[11], [12], [13] new binders for composites,[14], [15] new cement for concrete [16], [17] and waste encapsulation,[18] new precursors for zeolite[19] and ceramics,[20], [21] new template for carbon,[22] and others [23], [24]. They can be thus used in various applications including: green construction and building materials,[17], [25] repair and strengthening materials for infrastructures and heritage structures,[26] heat-resistant structural components,[27] toxic and radioactive waste containment materials,[28], [29], [30] low-cost ceramics tiles and refractories,[31] artworks and artifacts,[1], [32] etc. The research in geopolymer technology, which encompasses mineralogy, colloid chemistry, modern inorganic chemistry, physical chemistry and most engineering technology fields, has sharply increased in last 30 years, as evidenced by the increasing number of research groups working all over the world on this topic (see Fig. 1).
In last decades, porous geopolymers and porous geopolymer composites (PGCs) have attracted increasing attention by widely expending the application domains where dense geopolymer components cannot meet the demands. There has been a series of reviews providing information on the fabrication, property, and application of porous geopolymers,[6], [8], [10], [24], [33], [34], [35], [36] and geopolymer composites [16], [19], [37], [38]. However, to the best of our knowledge, no review on porous geopolymer composites has been published.
Fillers and reinforcing and/or functional components have been added to improve or optimize the properties of porous geopolymers or to endow them with new functionalities. In particular, hybrid materials or composites can combine the different characteristics or performance of two or more constituents. The interest in porous geopolymer composites has sharply increased in recent years. A content analysis approach was used to identify the literature (only journals based on the science citation index were considered) in this review.
In a broad sense, powder-type materials containing mostly amorphous silica (SiO2) and/or alumina (Al2O3) can be directly used for geopolymer synthesis [39]. Hence, lots of aluminosilicate materials including natural pozzolanic materials and industrial waste by-products have been used for geopolymer synthesis around the world [1], [39]. Except for silica and alumina, other oxides such as TiO2, Fe2O3, MgO, CaO, etc. and residues are more or less present. Normally, there are regarded as a part of the geopolymer matrix. In this review, the impurities deriving from raw materials were considered as a part of the matrix as well. It should be noted that some aluminosilicate materials (zeolite, additional or fiber-type or sphere-type oxides or/and aluminosilicates, aluminosilicates with low reactivity) are categorized as the second phase with respect to the geopolymer matrix.
Due to the large number of publications in the field, this review focuses solely on porous geopolymer composites in the form of granules, monoliths, membranes, etc., discussing their fabrication, main properties, and applications.
Section snippets
Fabrication of porous geopolymer composites
Generally, porous geopolymers can be obtained by four main methods, which are: 1) direct foaming, 2) the replica route, 3) the sacrificial filler route and 4) additive manufacturing, as indicated in a previous review [6]. Due to the excellent rheological properties of an aluminosilicate polymer paste, which are analogous to those of cement and organic resins, fillers forming a second-phase or acting as reinforcement can be added directly to the slurry; i, e., the synthesis methods used to
Properties and applications of porous geopolymer composites
In this section, the current progress on the mechanical, thermal, adsorption, and other properties of porous geopolymer composites is reported. Factors affecting the properties of the PGGs obtained by different processing routes and/or different additives were addressed. Applications such as water purification, membrane support, thermal insulation materials, lightweight parts, etc. were summarized and discussed as well.
Future perspectives and challenges
Many fabrication routes (direct foaming, embedding lightweight (porous) fillers, additive manufacturing, immersion and impregnation method, reactive emulsion templating-based method, particle compaction method, post-grafting method, etc.) and their corresponding pore forming mechanisms have been successfully employed for the manufacturing of porous geopolymer composite components. In summary, every approach has its pros and cons, which in turn offers important research opportunities and
Conclusions
To summarize, this article systematically reviews research papers concerning porous geopolymer composites published until the end of 2020. It presents an overview of the main manufacturing strategies (direct foaming, embedding lightweight (porous) fillers, additive manufacturing, immersion and impregnation method, reactive emulsion templating-based method etc.), properties (mechanical, thermal, adsorption etc.) and corresponding applications (thermal insulation, adsorption, filtration, etc.).
A
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
This work was supported by the National Natural Science Foundation of China [52002090], the Heilongjiang Postdoctoral Science Foundation Funded Project (LBH-Z19051), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Heilongjiang Province (2019QD0002) and the Fundamental Research Funds for the Central Universities [3072020CF1001, 3072019CFJ1003]. The authors are grateful to Prof. Joseph Davidovits (Geopolymer Institute, 02100 Saint-Quentin, France) for kindly
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