Effects of w/b Ratio on Sodium Sulfate Crystallization Damage and Degradation Mechanisms in Semi-Immersed Alkali-Activated Slag Mortar

This study investigates the long-term durability and crystallization-induced degradation mechanisms of alkali-activated slag (AAS) mortars with varying water-to-binder ratios (w/b, 0.4, 0.45, 0.5) under semi-immersion in 5 wt.% sodium sulfate solution. Through 360 d of exposure, the evolution of physical-mechanical properties (mass change, open porosity, compressive/flexural strength) and ion migration patterns (SO₄^2-, Na⁺, Ca²⁺) were analyzed to unravel the interplay between pore structure, ion transport, and crystallization-induced deterioration. Results demonstrated that higher w/b ratios exacerbated surface crystallization and spalling due to accelerated ion transport and pore coarsening. Early-stage strength gains (up to 25.15% at 120-180 d) stemmed from pore refinement via sulfate deposition and continued slag hydration. However, prolonged exposure triggered microstructural degradation, with open porosity increasing by 58.9% and strength declining by 30.6% at 360 d for a w/b of 0.5 compared to a w/b of 0.4. This was driven by crystallization pressure and the decalcification of hydration products. Ion migration analysis revealed SO₄^2- enrichment in evaporation area and outward Na⁺ diffusion, establishing supersaturation gradients that aligned with crystallization damage progression. These findings provide critical insights for optimizing AAS mortar formulations to mitigate sulfate crystallization risks in semi-immersed environments.