Degradation-front evolution and micromechanical heterogeneity in cement pastes incorporating expansive agents under sodium sulfate attack

Expansive agents (EAs) are widely used in concrete but have the potential to reshape the physicochemical pathways under external sulfate attack (ESA) due to the internal introduction of calcium-aluminum-sulfur. ESA induces a depth-dependent degradation front that drives progressive microstructural and chemical degradation; however, the evolution of this reaction front in EA-modified systems remains poorly understood. This study analyzed depth-dependent, micro-level alterations, coupling nanomechanical, chemical, and porosity evaluations to elucidate the reaction front in plain and EA-modified cement pastes under ESA. Specimens containing 0% and 8% EA were immersed in 5% sodium sulfate solution for 200 days, and penetration was analyzed at 2 mm, 10 mm, and 20 mm from the surface. Results showed that, despite a ~1 wt% increase in sulfate in the EA-cement, the degradation front remained confined to 13–20 mm. EA incorporation increased the average Ca/Si ratio by 31% (2 mm) and 20% (10 mm) and elevated the Al/Si ratio by 15–45%. Portlandite was depleted in the degradation zone but progressively accumulated toward the interior. In the degradation zones, EA increased homogenized modulus and hardness by 3.4–12.5%, enhanced high-density C-S-H up to fourfold, and more than doubled portlandite/C-S-H, as hydration-released calcium migrated outward and redeposited portlandite. EA addition reduced microporous phase fractions by 19% (2 mm) and 42% (10 mm) and decreased gel porosity by up to 18%, while increasing local alkalinity and calcium activity, which enhanced portlandite buffering and slowed C-S-H decalcification. These findings provide mechanistic insight into the degradation-front evolution of EA-modified cement under ESA.