Alkali-activated cement systems (Geopolymers) are essential alternatives to OPC in the sustainable concrete production industry. In general, the alkali-activated mortar and concrete have good fire resistance due to their ceramic-like properties. Due to the limited resources investigating POFA-GGBS based geopolymer exposed to elevated temperatures, this study will help researchers for the first time understand the geopolymeric binder characteristics after exposure to elevated temperature on strength, thermo-physical stability, and microstructure properties. The main objective is to investigate the role of calcium coupled with aluminum and their effect on geopolymeric gel stability. The alkali-activated mortar was synthesized with liquid sodium silicate and sodium hydroxide. Cubic specimens were heated to a range of temperatures between 100 degrees C and 800 degrees C to evaluate strength loss and gel thermal microstructure damage. Scanning electron Microscopy test confirmed the finding of the ability of POFA with GGBS to be used as a novel environmentally-friendly material to produce sustainable mortar and concrete with advanced performance under high temperatures. Thermal-physical stability of POFAGGBS mortar due to the spherical pore system allows the vaporized water to be released outside the samples. FTIR spectra results revealed two mechanisms explaining the effect of GGBS by lowering the wavelength through shifting Si-O peaks with the aluminium substitution producing C-A-S-H and changing the FTIR spectra by generating new peaks. TGA/DTG and DSC tests confirmed the formation of a robust gel with a higher degree of crystallinity and very high thermal stability due to the Ca and crosslink effect of Al in the geopolymeric system. The excellent thermal performance of POFA-GGBS mortar under elevated temperature would increase the applications for this novel environmentally-friendly material.