An investigation of major factor of influence of strength gain or loss of geopolymer when exposed to 800°C

When fly ash based geopolymer mortars were exposed to an elevated temperature of 800°C, it was found that the strength after the exposure sometimes decreased, but at other times increased compared to the original strength. The aim of this investigation is to find the reason for this contrasting behaviour. Following exposure to high temperature, residual strengths of specimens prepared with two different fly ashes, with initial strengths ranging from 5 MPa to 60 MPa, were investigated. The parameter that was found to have a major influence on the contrasting behaviour was the ductility of the mortars. The results indicate that the higher the ductility the lower the strength loss. This correlation is attributed to the fact that mortars with high ductility could provide higher capacity to accommodate thermal incompatibility than mortars with low ductility. Beyond the particular threshold of ductility, some mortars even increased strength after the exposure, possibly due to sintering.