Geopolymers are appreciated for their good resistance to fire temperatures. This study investigates the results of the effect of elevated temperature exposure at 800 °C on the chemical and structural changes of metakaolin hardened in phosphoric acid (MP) and sodium hydroxide (MA) reactions at 800 °C. The cured specimen strength of MA is 15.08 MPa, whereas MP is 46.2 MPa, which meets suitable mechanical properties and a compact microstructure character. The thermal stability of the sample was investigated by TGA/DTA, followed by ex-situ analysis using XRD, XPS, and SEM with EDAX. The results revealed that the matrix of MP comprised amorphous phases of silico-aluminium phosphate, albeit with some non-reacted crystalline phases at ambient temperature, while in MA there was a notable crystalline phase of the zeolitic type embedded in gel. In the XPS line of MAT, an upward shift of O1s BE from MA is due to the loss of Si–O–Na and oxygen in the sialate-siloxo structure. In MPT, the partial inclusion of [PO4] in [Si–O–T: T–Si/Al] results in desilication, finally resting in SiO2 phases. Thus, the thermal treatment and annealing manifested a loss of three-dimensional networks in both samples. Still, the most heat-resistant phases were found to be nepheline in MAT, aluminium phosphate, and the phospho cristobalite phase of SiO2 in MPT. Thus, the microstructural results are useful for predicting the strength retention mechanism of geopolymers.