The aim of this research is to study the effect of different percentages of potassium feldspar (KF) (0%, 5%, 10% and 20% by weight) on the proficiency of fly ash (FA)-based geopolymer paste when exposed to elevated temperatures of 300 °C, 600 °C and 900 °C. The percentages (by weight of the source material) of both Na2O and SiO2 were fixed at 8% each. The behaviour was evaluated on the behalf of residual strength, weight loss, volumetric shrinkage and physical changes at various temperatures. To observe the changes in microstructure and mineralogy during the thermal exposure, scanning electron microscopy (SEM) images of the samples were studied. After observing the results, good durability was revealed at elevated temperatures when KF was incorporated in FA-based geopolymer paste. At 900 °C, the residual compressive strengths of adding of KF @ 0%, 5%, 10% and 20% by weight were 22%, 25%, 29% and 42% of those at room temperature, respectively. At 900 °C, the geopolymer matrix was observed to be most densified, which imparts superior durability to the specimens.