Distribution of oxides in fly ash controls strength evolution of geopolymers

•Geopolymer synthesized from six different types of Australian fly ash.•Uniform distribution of oxides in fly ash is the key role of synthesis of geopolymers.•Geopolymers with same SiO2 and Al2O3 distribution pattern have higher strengths.

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 and sodium hydroxide solution (NaOH). Concentration of NaOH for all mixtures was 8 M. Specimens were cured at 60 °C 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.