Synchronous hot-pressed ceramic-like geopolymer mortars: Multiscale synergy and enhanced high-temperature performance

Geopolymer is a low-carbon binder with better thermal stability than ordinary Portland cement, yet conventionally cast mortars show slow early strength gain and severe degradation above 800℃. This study extends synchronous hot pressing (SHP) from paste to geopolymer mortar to enhance mechanical properties and high-temperature resistance. Using metakaolin, fly ash, and silica fume as precursors with a composite alkali activator, the binder-to-sand ratio was optimized. At the optimal ratio of 1:1.5, SHP-treated mortar achieved 28-day compressive and flexural strengths of 70.31 MPa and 18.01 MPa, 73% and 68% higher than controls. At 800 °C, SHPGPM retained a compressive strength of approximately 35.3 MPa, remaining above 50% of the corresponding 25 °C reference strength in the elevated-temperature test series. Based on a 50% residual compressive-strength criterion and piecewise linear interpolation, the estimated 50%-retention temperature increased from approximately 690 °C for the Control to approximately 840 °C for SHPGPM. Moreover, SHP mortar retained structural integrity up to 1000℃, while controls cracked severely at 600℃. Mechanistically, the integrated SHP route was associated with a denser microstructure, altered local aluminosilicate bonding environments, and a modified temperature-dependent structural evolution, which are consistent with the reduced crack development and improved high-temperature performance observed relative to the conventionally cast route. This work provides a basis for designing ceramic-like geopolymers for high-temperature structural use.