Assessing the suitability of three Australian fly ashes as an aluminosilicate source for geopolymers in high temperature applications

Fly ash characteristics cannot be assumed to be constant between power stations as they are highly dependent on the coal source and burning conditions. It is critical to understand the characteristics of fly ash in order to produce geopolymers suitable for high temperature applications. We report on the characterisation of fly ash from three Australian power stations in terms of elemental composition, phase composition, particle size, density and morphology. Geopolymers were synthesised from each of the fly ashes using sodium silicate and sodium aluminate solutions to achieve a range of Si:Al compositional ratios. Mechanical properties of geopolymer binders are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. It was found that the twenty eight day strength of geopolymers is largely dependent on the sub 20 μm size fraction of the fly ash. Strength loss after high temperature exposure was found to be dependent on the concentration of iron in the fly ash precursor and the Si:Al ratio of the geopolymer mixture.