Strategies to control the high temperature shrinkage of fly ash based geopolymers

•Fly ash based geopolymer composites were investigated for thermal resistance.•Si:Al ratio <2 geopolymers showed good increases in strength after firing at 1000 °C.•The addition of alumina and wollastonite fillers reduced thermal shrinkage.•The composites exhibited potential for fire and thermal resistant applications.

Strategies to control thermally induced shrinkage in fly ash based geopolymers are investigated. The use of low (<2) Si:Al ratios in the geopolymer provides superior thermal properties and the addition of fillers delivered additional improvements resulting in impressive thermal resistance. Two thermally stable inorganic fillers, α-alumina and wollastonite, were used because of their high melting points and relatively low thermal expansion. Shrinkage events at elevated temperatures revealed that samples containing filler exhibited 30–35% less volume shrinkage than the control at 1000 °C. Initial compressive strength was moderate (∼30 MPa), but increased markedly when samples were fired in excess of 600 °C (>100 MPa). Wollastonite addition also improved the flexural strength of the samples which was attributed to its acicular shape imparting fibre-like qualities. Microstructural investigations showed that unfired samples predominantly consisted of spherical particles coated with reaction product. Firing to 1000 °C converted the morphology to a more compact phase with increased pore size.