The effect of high temperature will trigger the evolution of a series of phase compositions and microstructures within metakaolin-fly ash-based geopolymers, resulting in changes in the deterioration of mechanical properties. To study the high-temperature resistance of metakaolin-fly ash-based geopolymers, the ratio of metakaolin and fly ash was adjusted. This article studied compressive strengths, visual appearance, mass loss ratio, permeability, products, and microstructure. The results showed that as the temperature increases, the compressive strength first increases and then decreases. The geopolymers have the highest compressive strength after exposure to temperatures of 200 °C. The change in compressive strength is determined by both the increase caused by the polymerization reaction and the decrease caused by microcracks. Moreover, metakaolin-fly ash-based geopolymers containing 75% metakaolin (GFMK-75) have the best high-temperature resistance. The residual compressive strengths of GFMK-75 after exposure to 200, 600, 800, and 1000 °C for 2 h are 183.1%, 95.2%, 75.8%, and 65.2%, respectively. Other analyses all confirmed the above conclusions. In summary, the residual compressive strength of metakaolin-fly ash-based geopolymers is highest after exposure to 200 °C, and GFMK-75 shows the best high-temperature resistance compared to others. The findings offer a theoretical foundation for the high-temperature resistance of metakaolin-fly ash-based geopolymers.
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