Phase transformation and microstructure evolution of a kaolin-based precursor

Geopolymer composites were synthesized with different Si:Al molar ratios and cured for different periods to investigate the correlation between phase transformation, microstructure evolution, and flexural strength. Phase transformation and microstructure evolution are correlated; sodalite content (72.8 wt%), i.e., kaolinite to sodalite transformation, is maximized at a Si:Al ratio (expressed as a decimal) of 1.94 and curing time of 24 h. The resulting products are characterized by a dense microstructure, and comparatively high relative density (54%) and flexural strength (20 MPa) owing to enhanced crosslinking and condensation during geopolymerization. However, increasing the Si:Al molar ratio to 2.22 reduces flexural strength to 16 MPa owing to a reduction in sodalite content (60 wt%) attributed to the impeditive effect of excessive silica on geopolymerization and the transformation of sodalite to cancrinite.