Compressive strength and microstructural analysis of fly ash/palm oil fuel ash based geopolymer mortar under elevated temperatures

•POFA/FA based geopolymer mortar gain strength by subjecting to elevated temperature up to 500°C.•Increasing of POFA content expedite the start of micro-pores formation due to high temperatures.•Increasing in POFA content delays the temperature of maximum strength. This paper presents the effects of exposing palm oil fuel ash (POFA)/fly ash (FA) based geopolymer mortar to elevated temperatures at early stage in terms of microstructural and compressive strength. Combination of sodium silicate and sodium hydroxide is used as activator. The development of compressive strength of POFA/FA based geopolymers mortar was investigated using XRF, XRD, TGA/DTG, and FESEM. It was observed that replacement of the 0–100% of POFA in FA based geopolymer mortar expedite the start of micro-pore formation due to exposure to high temperatures and shift the strength peak from 300°C to 500°C.