Effect of SiO2 and Al2O3 on the setting and hardening of high calcium fly ash-based geopolymer systems

This study investigates the effect of silica and alumina contents on setting, phase development, and physical properties of high calcium fly ash (ASTM Class C) geopolymers. The characteristic rapid setting properties and, hence, limited workability range of high calcium fly ash geopolymers has restricted both development and potential application of these binder systems compared to conventional geopolymer binders derived from bituminous coal, i.e., (ASTM Class F) sources or from calcined kaolin feedstocks. For this study, control of setting and hardening properties were investigated by adjusting SiO2/Al2O3 ratio of the starting mix, via series of mixes formulated with varying SiO2 or Al2O3 contents to achieve SiO2/Al2O3 in the range 2.87–4.79. Foremost is the observation that the effect of varying silica and alumina in high calcium fly ash systems on setting and hardening properties is markedly different from that observed for traditional Class F geopolymer systems. Overall, increases in either silica or alumina content appear to shorten the setting time of high calcium-based systems unlike conventional geopolymer systems where increasing Al2O3 accelerates setting. The setting process was associated primarily with CSH or CASH formation. Furthermore, there appears to be a prevailing SiO2/Al2O3 ratio that prolongs setting, rather than Ca2+ ion content itself, while NASH primarily contributes to strength development. SiO2/Al2O3 ratios in the range of 3.20–3.70 resulted in products with highest strengths and longest setting times. These results suggest that initial predominance of Ca2+ ions and its reactions effectively help maintaining a SiO2/Al2O3 ratio at which amorphous geopolymer phase is stable to influence setting and initial strength development.