Assessing mechanical and microstructural performance of ultra-high performance geopolymer mortar with micro activators under thermal curing

This research develops and evaluates ultra-high performance geopolymer mortar (UHPGM) incorporating micro-activators and municipal solid waste incineration fly ash (MFA) as a sustainable supplementary cementitious material under thermal curing (60°C for 48 h). The study aims to enhance mechanical performance, durability, and eco-efficiency by optimizing binder composition and curing conditions. Five mix designs containing varying proportions of silica fume (SF), metakaolin (MK), and MFA were tested for workability, density, compressive strength, drying shrinkage, capillary water absorption, and elastic modulus. Microstructural and mineralogical analyses were conducted using SEM/EDS, XRD, and TGA/DTG to elucidate hydration and gel formation mechanisms. Results revealed that MFA improved workability by 36.1% owing to its low water absorption and fine particle morphology. All UHPGM mixes achieved compressive strengths above 100 MPa, with the SF-rich mix reaching 134.5 MPa. Ternary blends containing MFA and MK demonstrated a favorable balance of strength, density, and dimensional stability. SF significantly reduced drying shrinkage (by ~24%) and capillary absorption, forming dense C-(A)-S-H and (C,N)-A-S-H gels that enhanced microstructural compactness. Thermal curing promoted calcite formation and secondary hydration, reducing porosity and strengthening interfacial bonding. The findings highlight MFA's potential as a low-cost, eco-friendly alternative in producing sustainable UHPGC, capable of delivering high mechanical performance and durability while supporting waste valorization and carbon reduction goals in modern construction materials.