A Coupled Load–Damage–Diffusion Model for Sulfate Transport in Concrete Under Static and Fatigue Loading with Drying–Wetting Cycles

External sulfate attack constitutes a major durability challenge for structural concrete, yet the influence of mechanical loads on sulfate transport remains incompletely understood. This study develops a coupled sulfate–water transport model that embeds the effects of axial static loads and fatigue loads into the effective sulfate diffusion coefficient through physically motivated correction factors, without explicitly resolving chemical reactions or physical crystallization. Numerical simulations were performed for coupled scenarios of static loading, fatigue loading, and drying–wetting cycles. Three principal findings emerge. First, drying–wetting cycles produce a stable convection enrichment peak at 1 to 3 mm from the exposed surface, rather than a monotonic decay profile. Second, axial static loads exhibit pronounced tension–compression asymmetry: tensile stress enhances sulfate transport, while compressive stress exerts a comparatively weak inhibition effect. Third, fatigue loading introduces time-dependent accumulation and nonlinear acceleration driven by the dynamic evolution of microcrack connectivity. The tensile zone consistently exhibits markedly higher sulfate enrichment and deeper penetration than the compressive zone, identifying it as the dominant region for coupled deterioration. These findings provide a quantitative foundation for durability assessment of concrete structures under complex loading environments.