Evaluation of chloride effects on sulfate attack in cement paste: Insights into phase composition and micromechanical degradation across corrosion depths

Concrete in marine environments is highly susceptible to multisalt-induced physicochemical degradation, particularly from sulfate and chloride. However, the role of chloride during sulfate exposure in cement remains limited understanding. This study systematically investigates chloride effects on sulfate attack in cement pastes using a multi-technique approach. Samples were exposed to 5 wt% Na₂SO₄ solutions containing 0, 3, and 6 wt% NaCl for one year. Depth-resolved ionic transport, phase reactions, C-S-H composition, and micromechanical degradation were analyzed, with particular focus on the coupled chemical-micromechanical response. Results showed that chloride preferentially stabilized monosulfate, promoting 4-7% Friedel's salt formation while suppressing crystalline ettringite and gypsum. Chloride-sulfate competition intensified at 3% chloride concentration, shifting the system toward chloride-dominant phases at 6 wt% NaCl. C-S-H transformed from gel clusters to fibrous, higher Si/Ca structures, while Al/Ca remained largely unchanged. Chloride exacerbated sulfate-induced corrosion by reducing the fraction of high-density C-S-H and lowering indentation modulus and hardness by up to 43% and 42%, respectively, within the 1-10 mm surface layer compared with sulfate-bearing exposure. Increased 3-6% chloride content further enlarged the coefficient of variation in nanoindentation up to 31.4%, reflecting higher phase heterogeneity. Unlike the sulfate-bearing attack, which was primarily controlled by crystallization pressure and localized C-S-H decalcification, chloride-sulfate coupling drove deeper C-S-H decalcification, enhanced ion transport, and progressive nanostructural weakening. The relationship among chloride dosage, phase evolution, and micromechanical weakening provides a reference for evaluating the durability of concrete in marine environments.