Effect of Sodium Sulfate on Fracture Properties and Microstructure of High-Volume Slag-Cement Mortar

This study investigates the effect of added sodium sulfate on the performance of high-volume slag-cement mortar (HVSCM). Herein, Na₂SO₄ (0, 1, 2, and 4 wt.% Na₂O) was used to modify HVSCM. The compressive strength, fracture properties, microstructure, and environmental impact of the synthesized samples were analyzed. The results showed that the 1 day compressive strength of HVSCM can be improved by 345.5% with the addition of 4% Na₂O (as Na₂SO₄), compared to samples without Na₂SO₄. However, the 28 day compressive strength of Na₂SO₄-activated HVSCM was 14.3-26.4% lower than that of the non-activated HVSCM, though still comparable to OPC. Regarding fracture properties, the initial fracture toughness of non-activated HVSCM was 45.6% higher than that of Ordinary Portland cement (OPC) mortar. Furthermore, Na₂SO₄ activation further increased initial fracture toughness, with the sample containing 4% Na₂O showing a 101.1% improvement over OPC. In contrast, fracture energy was not significantly influenced by Na₂SO₄ addition. Microstructurally, the enhanced fracture properties of non-activated HVSCM were attributed to a higher degree of C-(A)-S-H polymerization and a denser binder phase. Sodium sulfate introduced sodium ions to strengthen electrostatic attraction and cohesion between C-(A)-S-H globules, offsetting reduced polymerization. Environmental assessment confirms that both activated and non-activated HVSCM substantially reduce embodied energy and CO₂ relative to OPC, while the additional embodied energy associated with Na₂SO₄ activation remains limited (<12%). Overall, this work provides a comprehensive understanding of the fracture behavior of Na₂SO₄-activated HVSCM, elucidating its capacity to enhance early-age strength and fracture toughness while highlighting its limited effect on long-term strength and fracture energy. These findings support the tailored use of Na₂SO₄ activation for sustainable construction applications.