ReviewA review of elevated-temperature properties of alternative binders: Supplementary cementitious materials and alkali-activated materials
Graphical abstract
Introduction
As the main binder ingredient in concrete production, cement contributes to about 7% of global anthropogenic CO2 [1], [2], great efforts have been made to find alternative materials that can replace cement as new binder systems in concrete. For the past few decades, supplementary cementitious materials (SCMs) derived from industry by-products or natural pozzolan materials have been used to partially substitute cement, including fly ash (FA), ground granulated blast furnace slag (GGBS), silica fume (SF), metakaolin (MK), limestone, fine glass powder, etc. [3], [4], [5], [6]. Likewise, agricultural wastes like rice husk ash (RHA), palm oil fuel ash (POFA), rice straw, cotton stalk ashes, sugarcane bagasse ash have also been used to partially replace cement [7], [8], [9]. In SCMs-cement binder system, apart from a filling effect, SCMs also have a pozzolanic effect due to the reaction between the silica in the SCMs and the Ca(OH)2 derived from cement hydration, producing secondary calcium silicate hydrate (C-S-H) gels.
In general, the incorporation of SCMs will enhance the fire resistance of cement because the consumption of Ca(OH)2 by SCMs-induced pozzolanic reaction effectively avoids the formation of a loose microstructure caused by the decomposition of crystals at high temperatures [10]. In addition, C-A-S-H gel (generated by the aluminum-rich SCMs induced pozzolanic reaction) has a higher stability at elevated temperatures than C-S-H gel [11]. Moreover, at a relatively low high temperatures, unreacted SCMs particles also promote the formation of C-S-H minerals to gain a higher strength. Poon et al. [12] reported that unhydrated pulverized fly ash (PFA) or GGBS reacted with CaO at about 200 °C to form tobermorite (a hydrated silicate) which was 2–3 times more stable than C-S-H gel. Khan et al. [13] and So et al. [14] also found that the formation of tobermorite led to a decrease in porosity and contributed to the maximum strength of concrete at 200 °C. However, the addition of SF and MK with finer particles resulted in a more compact microstructure of cement and could lead to a greater vapor pressure, which compromised the improved fire resistance induced by the pozzolanic reaction [15], [16].
SCMs by itself cannot hydrate. Thus, cement can only be partially replaced by SCMs to ensure a satisfactory performance. Hence, to produce cementless binder, alkaline solution can be used to activate silicon/aluminum-rich SCMs [5], [17], [18], [19]. This kind of alkali-activated materials can be generally divided into two categories: 1) geopolymer with a three-dimensional silicoaluminate structure, and 2) high calcium alkali-activated binder (AAB) containing mainly C-A-S-H gel. The commonly used geopolymers are formed by the activation of FA (Class F), SF, and MK, while high calcium AAB is formed by the activation of calcium-rich slags (such as FA (Class C), GGBS and steel slag) [20], [21], [22], [23], [24]. Fundamentally different from cement, alkali-activated materials have better strength and durability. Fig. 1. illustrates the use of SCMs in cement and the production of alkali-activated materials.
Excellent elevated-temperature resistance has been demonstrated by alkali-activated materials due to the stable network structure of abundant oxides and the presence of mineral phases that are hard to decompose at elevated temperatures [17], [25], [26]. Geopolymer is supported by N-A-S-H polymeric chains, which has a higher stability than C-S-H in conventional cement-based materials at elevated temperatures [27]. Meanwhile, geopolymer will form a ceramic-like product through viscous sintering to densify the system [26]. Junaid et al. [26] observed that sintering occurred at 550 to 850 °C in the FA-based geopolymer, possibly producing minerals such as albite (Na2O·Al2O3·6SiO2), nepheline ((Na, K)AlSiO4), sillimanite (Al2[SiO4]O), and labradorite (NaAlSi3O8). As for AAS, C-A-S-H is the main gel, which is similar to cement and has a lower thermal stability than N-A-S-H. However, Rashad et al. [25] observed a high thermal stability in AAS exposed to 80–1000 °C due to the formation of akermanite (Ca2MgSi2O7) and mullite (3Al2O3·2SiO2), which had a low thermal expansion coefficient and low creep rate. It is noteworthy that the elevated-temperature performance of the alkali-activated material incorporated mortar or concrete is quite different from that of the binder itself due to the more prominent effect of thermal incompatibility between binder and aggregate [28]. The main physio-chemical changes of SCMs and alkali-activated materials are summarized in Fig. 2. As a SCMs-cement binder, the dissociation of AFt/AFm and shrinkage of C-(A)-S-H occur below 300 °C, in addition to evaporation of free water. At 400–500 °C, calcium hydroxide crystals decompose into lime and water, resulting in the degradation of the internal microstructure of the binder. Calcium carbonate is more stable with a dissociation temperature above 600 °C. At 950 °C, crystals and C-(A)-S-H completely decompose and form amorphous products. At above 1000 °C, SCMs and cement particles melt to form a gel-like substance. As alkali-activated materials, the change of high calcium AAB with C-A-S-H as the main product at high temperatures is similar to that of SCMs except for negligible crystal decomposition. However, N-A-S-H and K-A-S-H in geopolymer decompose and crystallize at temperatures of above 800 and 1200 °C, respectively. In practical engineering, the compatibility of aggregate and binder should also be considered as a key factor affecting strength, apart from the degradation of binder itself at elevated temperatures [10]. Quartz is the most commonly used aggregate in mortar or concrete, which has a phase transition at 573 °C from α-quartz to β-quartz with a large volume expansion. Incompatibility of thermal expansion between the aggregates and the binders leads to weakness in the interface transition zone (ITZ), which is more sensitive in N-A-S-H and K-A-S-H [28], [29].
Unlike cement, alternative binders undergo complex physical and chemical changes under elevated temperatures. To broaden the application of alternative binders, a comprehensive review of recent scientific progress on the elevated-temperature performance of SCMs-cement binder system and alkali-activated materials system is of great importance and significance. In this review, the macro properties, mineral alteration, and microstructure of SCMs and alkali-activated materials exposed to elevated temperatures are explored and summarized to provide guidance for future research in this field. This review concludes by identifying the limitations of the existing research and providing instructive perspectives on the future directions.
Section snippets
Heating methods
To investigate the temperature-dependent properties of binders, heating tests are usually conducted in muffle furnaces. Different heating methods have distinct influences on the properties of binders. Currently, two heating methods are widely adopted. One is to simulate a real fire and heated to nearly 800 °C in less than 30 min. Worldwide, there are various standards for simulating real fires in logarithmic increments [39], [40]. Among them, the standard fire curves given by ASTM E119 and
Elevated-temperature properties of SCMs-cement binder system
SCMs are natural or artificial siliceous and aluminous fine powder materials, including industrial waste products, natural pozzolans and activated minerals. Generally, SCMs can be used to partially replace cement as a cementitious material, effectively reducing carbon emissions and improving the durability of the host materials [45]. Industrial by-products are the most widely used SCMs, such as FA from coal-fired power plants, GGBS from iron production, and SF from the ferrosilicon industry.
Glass powder as SCMs
Glass contains a high content of SiO2. When finely ground, it can be used as an alternative binder to cement [5], [57], [87], [88]. Li et al. [6] examined the mechanical properties of cement paste with 30% glass powder upon exposure to elevated temperatures. The results showed that at 300 °C, the residual compressive strength ratios of cement pastes containing glass powder were about 1.44 times higher than their original strength at ambient temperature, which were also much higher than that of
Elevated temperature properties of alkali-activated slag
Alkali-activated slag prepared with high Ca-containing but low hydraulic SCMs, such as GGBS, have other attractive properties for applications in the construction materials. The alkaline activator facilitates the dissolution of vitreous phases in slag. Then, the dissolved ions further dissociate, precipitate, and condensate to form various polymers, including C-S-H and calcium C-A-S-H gels, to underpin the major macro performance [110]. The main gel system of such materials is quite different
Elevated temperature properties of alkali-activated blended system
Recent researches have focused extensively on the alkali-activated binder systems with a mixture of calcium (GGBS) and aluminosilicate (FA and MK) sources. In contrast to the sole activation of FA or MK, the blended system is room-temperature fabricable; whereas, the preparation and curing of geopolymer generally requires high temperatures [27]. Furthermore, the blended system possesses similar properties to AAS and geopolymer, and has higher chemical resistance and elevated-temperature
Conclusions
The review provides an update on recent experimental results on the behavior of alternative binders, including SCMs, geopolymer, alkali-activated slag, and alkali activator Ca-Si-based blended system, at elevated temperatures. Special emphasis is placed on the temperature-dependent macro-properties, microstructure, and mineral composition of alternative binders exposure to elevated temperatures. Based on the literature review, the following conclusions are drawn:
SCMs exert an impact on the
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
The authors would like to acknowledge the financial support of the China Postdoctoral Science Foundation (2018M632219), and the National Natural Science Foundation of China (Grants No.52078202).
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