Long-term chloride transport resistance of one-part alkali-activated slag–fly ash concrete under ambient drying and carbonation exposure

The long-term durability of one-part alkali-activated slag–fly ash (AASF) concrete under controlled ambient exposure remains poorly understood, despite its promise as a low-carbon alternative to Portland cement concrete. This knowledge gap is especially important because prolonged drying and carbonation may substantially modify pore structure, pore-solution chemistry, and chloride transport, yet these effects are rarely captured by early-age performance indicators. This study provides a two-year investigation of ambient exposure effects on one-part AASF concrete, using mixtures with comparable 28-day compressive strengths to isolate the effects of slag/fly ash ratio, solid activator chemistry, and shrinkage-reducing admixture (SRA). Chloride transport was assessed using three complementary transport tests, supported by carbonation depth measurements, microstructural characterisation, and pore-solution analysis. Reducing the slag content from 75% to 50%, together with partial replacement of sodium metasilicate by sodium carbonate, increased the volume of permeable voids and produced an approximately threefold increase in the apparent chloride diffusion coefficient after two years. In contrast, the mixes containing 3–5% SRA exhibited lower chloride transport, reduced carbonation depth, and a refined pore structure relative to the reference mix. Chloride transport indices showed strong linear correlations with carbonation depth and pore-structure variations, while preferential chloride ingress through the carbonated outer region indicated that carbonation locally weakened chloride transport. FTIR, TGA, and pore-solution analyses suggested that this deterioration was associated with decalcification of reaction products, alkali redistribution and leaching, and progressive microstructural degradation. These findings show that similar 28-day properties do not guarantee similar chloride transport after two years in the mixes tested, and that coupled drying–carbonation effects strongly govern durability evolution. The findings also identify SRA as an effective admixture-based strategy for improving long-term durability under ambient exposure, with direct implications for the design of more durable low-carbon concrete systems.