Distribution of oxides in fly ash controls strength evolution of geopolymers

Compressive strength of six types of geopolymer pastes made from different fly ash sources were studied. Alkali activation was performed by a mixture of sodium silicate sodium hydroxide solution (NaOH). Concentration of NaOH for all mixtures was 8 M. Specimens were cured at 60 oC and 100% RH for 24 h. The effect of different parameters on compressive strength was studied via contour analysis. Microstructural analysis of both fly ashes and geopolymers reveals that strength evolution mainly depends on the distribution of SiO2 and Al2O3 in the starter fly ash rather than their ratios. Localized short order diffusion for geopolymerization necessitates both uniform distribution of SiO2 and Al2O3 in the whole fly ash powder, and presence of these oxides in approximately same pattern.