High-temperature performance of alkali-activated binders of fly ash and calcium aluminate

This paper studied the effect of elevated temperatures on the compressive strength of alkali-activated fly ash (so-called geopolymer) with the addition of calcium aluminate cement (CAC). In the elevated temperature tests (600 and 800 °C), the unstressed test method was adopted. Taguchi analysis was used to study the effect of three parameters (considered in mix design) on the hot strength. These parameters include the replacement ratio of fly ash by CAC, molarity of sodium hydroxide solution, and alkaline solution to fly ash ratio. The first parameter was found to be the most influential factor. XRD, TG/DSC and SEM techniques were then carried out to investigate the effects of the CAC content on the high-temperature performance of the geopolymer. The phase transformation taking place during the heat exposure was found to affect the compressive strength in the hot state directly. The retained strength of the optimal geopolymer mortar at 600 °C was around 2.5 times higher than that of ordinary Portland cement mortars with a similar original strength level.