Geopolymer can be a viable alternative to Ordinary Portland Cement Binder because of their capacity to reduce CO₂ emission and utilization of industrial by-products. This paper aims to evaluate the effect of temperature on the mechanical and microstructure of the geopolymer from fly ash (FA) and ground granulated blast furnace slag (GGBS) with varying the alkali concentration (3.5 M, 5 M and 10 M) and chemical and mineral admixtures (Zirconia, Alumina, Zinc oxide and Silica fume). Using SEM analysis, morphology changes were identified before and after thermal exposure. The results revealed that the geopolymer mortars activated with the 10 M alkali exhibited higher compressive and split tensile strength in all combination of FA and GGBS studied. In addition to GGBS with FA, there is a strength improvement at ambient temperature (AT) whereas at elevated temperature it underwent strength reduction. The GP100 exhibited lower strength at AT (8 MPa) and the strength increased after 800 °C exposure due to the viscous sintering of the secondary sodium silicates which is correlated with the TGA studies. Among the admixture studied in FA geopolymer mortar, silica fume enhances the strength up to 20 ± 5 MPa. Thermal cycle studies revealed the rigidity of the fly ash geopolymer mortar with silica fume after the 20 cycles with strength retainment. The SEM analysis confirms the presence of more homogeneous gels at 800 °C without many cracks and pores. The pH of the stimulated pore solution of the GP100 produces a passive layer over reinforcement in geopolymer concrete. The split tensile strength achieved in the range of 4 MPa at AT and 1 MPa at 800 °C exposure.