h i g h l i g h t s 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. a b s t r a c t 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. Combina-tion 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. Ó 2014 Published by Elsevier Ltd.