This experimental study aims to investigate the impact of elevated temperatures up to 900 ℃ on compressive strength of Metakaolin-based geopolymer mortars compared to Ordinary Portland cement (OPC) based mortar. It employs two distinct experimental approaches: destructive testing to assess compressive strength before and after exposure to heat, and nondestructive testing to evaluate mass loss resulting from heat exposure. The study considers specimens of both OPC and Metakaolin based geopolymer mortars aged for 7 and 28 days, subjected to elevated temperatures, for two-hours in unsealed conditions, ranging from 100 ℃ to 900 ℃ with 100 ℃ increment. Results for 7 days of age are discussed herein, other tests are still ongoing. For the geopolymer mixture, the binder comprises metakaolin, activated by an alkaline solution of sodium hydroxide (SH, NaOH) and sodium silicate (SS, Na2SiO3) with a ratio of SS/SH equals 2.5. Notably, the experiments demonstrate a continuous decrease in compressive strength for 7-day old OPC mortar when subjected to temperatures exceeding 100℃ compared to ambient conditions. A decrease of more than 50 % is observed at temperature of 450 ℃ for OPC mortar of 7 days age. The OPC specimens were completely burst at temperature of 600 ℃. However, the metakaolin-based geopolymer exhibits an impressive 45% increase in strength when subjected to up to 400℃ compared to ambient conditions, highlighting its superior ability to retain strength at higher temperatures. Additionally, the mass loss for 7-day old OPC mortar exceeds that of the Metakaolin-based geopolymer mortar by approximately 14.33%, while the latter demonstrates minimal mass loss despite exposure to elevated temperature.