The thermal evolution of metakaolin geopolymers: Part 2 – Phase stability and structural development

The structural evolution of a systematic series of geopolymers derived from metakaolin is explored. The onset temperature of crystallization, phase composition and densification of the specimens in the current work are determined by quantitative XRD, FTIR and dilatometric analyses. Alkali cation is observed to have no significant effect on the onset temperature of crystallization and quantity of crystalline phase developed in geopolymers, though the phase composition of NaK- and K-geopolymers is more complex than Na-geopolymers, as is expected from phase diagrams. The amount of crystalline phase decreases in all systems with increasing Si/Al ratio, which is proportionally related to the extent of densification observed in Part 1 of the work. Despite similar phase composition of all geopolymers, the onset temperature of densification is observed to be higher in all K-geopolymers compared to the NaK- and Na-specimens of analogous Si/Al ratio. It is proposed that the increase in densification temperature is due to the increase in gel viscosity of K-geopolymer compared to Na- and NaK-geopolymers. The effect of heating rate on the densification mechanisms of geopolymers is analyzed, showing that the amount of crystalline phase and gel viscosity of geopolymers are critical in determining the extent of shrinkage during densification.