Abstract
This study investigates the durability and microstructural evolution of alkali-activated slag-fly ash (AASF) concretes exposed to a combined sulfate and chloride environment. Concrete specimens with ground granulated blast-furnace slag (GGBS) to fly ash (FA) ratios of 80:20 (SF82), 50:50 (SF55), and 20:80 (SF28) were activated using a sodium silicate and sodium hydroxide solution. The specimens were immersed in a combined solution containing 5 wt.% Na2SO4 and 3.5 wt.% NaCl for up to 365 days. Degradation was monitored through compressive strength changes, mass loss, and free chloride penetration profiles. Microstructural changes were characterized using X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), and mercury intrusion porometry (MIP). The results indicate that high-slag mixes (SF82) initially exhibit superior resistance to chloride penetration due to a highly refined pore structure; however, they suffer significant decalcification and microcracking under long-term sulfate exposure. High-fly ash mixes (SF28) show lower mechanical properties but superior volumetric stability owing to the chemical resilience of the N-A-S-H gel network. The 50:50 blend (SF55) achieved the optimum balance, maintaining high strength retention, low chloride diffusion, and minimal microstructural damage. These findings provide critical insights for designing resilient, low-carbon alkali-activated structures in marine and saline-soil environments.
