Enhancing durability of slag-based alkali-activated composites with dunite sand and carbon fiber: Sulfate, freeze–thaw, and thermal resistance optimization

The replacement of natural river sand with sustainable alternatives is essential to reduce environmental degradation associated with aggregate extraction. In this context, dunite sand (DS), a magnesium-rich ultramafic mineral, was investigated as an eco-efficient fine aggregate in thermally cured slag-based alkali-activated composites (AACs), in combination with carbon fiber (CF) reinforcement. The study aims to clarify how aggregate substitution and fiber addition interact to control matrix densification, crack-bridging efficiency, and durability under aggressive conditions. Sixteen mixtures incorporating varying DS (0–100%) and CF (0–1.00%) contents were prepared and cured at 80°C to promote rapid slag activation and microstructural development. Mechanical performance, high-temperature resistance, sulfate durability, and freeze–thaw behavior were systematically evaluated. The results demonstrate that moderate DS substitution (~25%) significantly improves matrix compactness and interfacial quality through a micro-filler effect, leading to enhanced strength and durability. CF effectively bridge micro cracks and improve flexural and thermal performance; however, excessive fiber contents impair workability and increase porosity due to fiber clustering. Durability results confirm that optimized DS–CF combinations reduce thermal degradation, limit sulfate-induced deterioration, and enhance resistance to cyclic freezing. Overall, 25% DS replacement combined with limited CF content (≤0.50%) provides the best balance between mechanical performance and durability. The findings highlight the importance of controlling aggregate–fiber synergy and curing strategy to develop durable, thermally stable, and sustainable alkali-activated composites for harsh service environments.