Characteristics of alkali-activated slag mortar under combined freeze-thaw cycles and sulfate exposure: Microstructural and mechanical insights

The growing demand for durable and sustainable construction materials in cold and sulfate-rich environments underscores the limitations of traditional Portland cement (PC), particularly under combined freeze-thaw cycles and sulfate attacks. While alkali-activated slag mortar (AASM) presents an eco-friendly alternative, its long-term performance under these coupled stressors remains inadequately explored. This study investigates the mechanical and microstructural behavior of AASM activated by calcium oxide and sodium carbonate (1:1 molar ratio) and compares it with PC mortar. A total of 300 freeze-thaw cycles in a 5 % sodium sulfate solution were conducted, followed by assessments of surface damage, dynamic elastic modulus, compressive strength, microhardness, pore structure, and hydration products. The findings reveal that AASM, particularly the low water-to-binder AAS2 mixture, demonstrated markedly superior resistance to freeze-thaw damage, with minimal surface deterioration, higher retention of dynamic modulus and mechanical properties, and denser microstructure compared to PC mortar. XRD and SEM analyses showed that the AAS system formed a dense C-(A)-S-H gel network without crystalline Ca(OH)₂, while EDS confirmed the low Ca/Si ratio in the product (ca. 0.58), correlating with improved dimensional stability. Additionally, the absence of ettringite and CH in hydration products and reduced porosity further enhanced durability. This study provides valuable insights into the synergistic effect of Na₂CO₃-CaO activation and mix design optimization on freeze-thaw under sulfate attack durability of AAS mortars. These findings broaden the understanding of AAS behavior in erosive environments and provide a promising low-carbon alternative for durable construction in cold and chemically aggressive regions.