A novel ultra-high residual strength-enhanced geopolymer incorporated corundum aggregates after elevated temperature exposure

In order to improve the high temperature resistance of building materials and develop a material that does not need to be repaired after fire, this paper develops a novel high temperature resistant geopolymer with metakaolin-fly ash blended precursor and corundum aggregates, characterized by thermal enhanced ultra-high residual strength. The surface morphology, mass loss, volume shrinkage, compressive strength, mineral composition, nanomechanical properties, microstructure, and pore distribution of geopolymer mortar before and after exposure to elevated temperatures ranging from 20 to 1000 °C are tested and analyzed. The effects of different aggregate types, volume fractions and particle size gradations on performance evolution and thermal incompatibility in geopolymer mortar are clarified and discussed. The results show that corundum aggregates utilization can significantly improve the mechanical properties and high temperature resistance of geopolymer mortar. The finer corundum aggregates with a smaller fineness modulus of 1.55 tends to better alleviate aggregates deterioration and mitigate thermal incompatibility between aggregate and geopolymer paste, thus improves the microstructure and pore distribution, residual compressive strength of the geopolymer mortar after high temperatures. With the increase of exposure temperatures, the designed geopolymer materials innovatively experiences a continuously enhanced strength attributed to the excellent precursor sintering and aggregate thermal compatibility, instead of severe strength degradation in most normal cementitious materials. By using corundum aggregate and optimized particle size distribution, the ultra-high residual compressive strength and strength retention rate could be achieved after exposure to 1000 °C for 1 h up to 148.5 MPa and 272 %, respectively.