THE ROLE OF SiO_2 Al_2O_3 ON THE PROPERTIES OF GEOPOLYMERS WITH AND WITHOUT CALCIUM

摘要

The rapid setting properties of high calcium fly ash based geopolymers (HCFG) have restricted potential application of these binders compared to those derived from bituminous coal or metakaolin. In calcium-free systems, initial SiO_2 and Al_2O_3 contents are known to play a key role in controlling setting and final properties of these binders. This study investigates the effect of SiO_2 and Al_2O_3 contents on setting and hardening of HCFG and the outcomes are compared with calcium free metakaolin-based systems (MG).A series of HCFG mixes were formulated with varying amounts of SiO_2 and Al_2O_3 to achieve SiO_2/Al_2O_3 in the range 2.7 -4.8. It has been shown that the effect of varying SiO_2 and AI2O3 in HCFG on setting and hardening is markedly different from that observed for MG systems. Increases in either SiO_2 or Al_2O_3 content appear to shorten the setting time of calcium-based systems unlike MG where increasing Al_2O_3 accelerates setting. When calcium is present, the setting process was associated primarily with CSH or CASH formation and there appears to be a prevailing SiO_2/Al_2O_3 ratio that prolongs setting. Furthermore, in HCFG, SiO_2/Al_2O_3 ratios in the range of 3.20-3.70 resulted in products with highest strengths and longest setting times. The results suggest that the initial predominance of Ca~(2+) and its reactions effectively help maintaining a SiO_2/Al_2O_3 ratio at which amorphous geopolymer phase is stable.