Ambient-cured alkali-activated geopolymer mortars: influence of activator molarity and fly ash replacement on microstructure and mechanical performance

Abstract This study investigates the mechanical and microstructural behavior of ambient-cured geopolymer mortars synthesized with ground granulated blast furnace slag (GBFS) and Class F fly ash (FA) as binder materials. Sodium hydroxide (NaOH) solutions at molarities of 4 M, 6 M, and 8 M were used as alkali activators, while GBFS was partially replaced with FA at ratios ranging from 0% to 25% by weight. The fresh and hardened properties of the mortars were evaluated through flow table tests, flexural and compressive strength measurements, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and mercury intrusion porosimetry (MIP). Results indicated that higher activator molarity significantly enhanced both the flexural and compressive strengths, while increased FA content improved workability but adversely affected strength. Microstructural analyses revealed that greater FA replacement led to more microcracks and higher porosity, contributing to reduced mechanical performance. The formation of C-A-S-H and N-A-S-H gels was observed to be critical for strength development. The findings suggest that optimizing activator molarity and binder composition is essential for achieving durable, environmentally friendly geopolymer mortars under ambient curing conditions. Unlike most existing studies relying on elevated-temperature curing, this study offers a detailed analysis of ambient-cured systems using only industrial by-products, thus providing a novel framework for low-energy geopolymer mortar production suitable for real-world applications.