This study presents a systematic investigation into the role of corundum aggregates (CA) in enhancing the thermal resilience of metakaolin and silica fume-based geopolymer (GP) mortars. The primary objective was to improve the high-temperature performance of GP mortars by replacing standard sand (SA) with thermally stable CA by focusing on mechanical properties, microstructural integrity, and thermal compatibility under exposure to temperatures up to 1000 °C. Seven mix designs incorporating metakaolin, silica fume, potassium silicate, SA, and varying proportions and gradations of CA were prepared and evaluated. The experimental program included compressive strength, mass and volume changes, capillary water absorption (CWA), pore structure, and nanomechanical assessments. Advanced characterization techniques such as X-ray Diffraction (XRD), Thermogravimetric (TGA) and Derivative Thermogravimetric (DTG) analyses, Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), porosimetry, and nanoindentation, were employed to investigate phase transformations and microstructural evolution. A one-way ANOVA at a 5% significance level and Tukey’s Honest Significant Difference (HSD) post-hoc test were conducted to statistically assess the influence of CA content and gradation on strength and mass loss. The results revealed that CA incorporation markedly enhanced thermal stability and strength retention. The SA0-CA50-M mix achieved 83 MPa at 1000 °C, representing 143% strength retention and a 302% improvement compared with SA50-CA0. A finer CA gradation (SA0-CA50-S) further improved performance, yielding 139 MPa with 264% strength retention. Higher CA content reduced shrinkage by up to 40% and decreased porosity from 30.43% to 16.18% through sintering. Nanoindentation confirmed a ~ 20 GPa increase in elastic modulus and a 3.5 GPa rise in hardness near CA particles. Microstructural analyses (SEM, XRD, FTIR) revealed the development of thermally stable crystalline phases, confirming superior structural integrity. ANOVA results validated that improvements in strength and mass loss were statistically significant.
Al-Naghi et al. (Mon,) studied this question.