The Role of Cement–Water Interaction on Chloride Ingress in Sustainable Cement-Based Systems

The durability of concrete structures exposed to chlorides is critically governed by chloride ingress, which induces reinforcement corrosion. While ordinary Portland cement (OPC) has been extensively studied, alternative binders such as calcium aluminate cement (CAC) and calcium sulfoaluminate (CSA) cement offer distinct microstructures that may enhance chloride resistance. Understanding how cement type and the water-to-cementitious material ratio (w/cm) influence the apparent chloride diffusion coefficient (Da) is paramount for designing durable infrastructure. This study systematically quantifies the influence of cement type (OPC, CAC, CSA) and w/cm ratio (0.45, 0.55, 0.65) on Da using a full-factorial experimental design and bulk diffusion testing. The results reveal a significant interaction between cement type and w/cm (p < 0.001). OPC exhibits predictable porosity-driven degradation, with Da increasing exponentially from 8.42 × 10−12 to 1.46 × 10−10 m2/s as w/cm increases from 0.45 to 0.65. CAC shows a non-linear response, with optimal performance at w/cm = 0.55 (Da = 2.90 × 10−11 m2/s) due to microstructural refinement from hydrate conversion but suffers severe degradation at w/cm = 0.65. CSA cement is extremely water-sensitive, with Da increasing by an order of magnitude (from ~4 × 10−11 to 3.37 × 10−10 m2/s) at w/cm = 0.65, marking a critical failure threshold. Model fit quality (R2) serves as a leading indicator of microstructural instability, which is corroborated by XRD phase analysis. This study concludes that CSA presents a high risk for field applications with poor w/cm control, while OPC offers more predictable performance, advocating for cement-specific, performance-based specifications beyond traditional OPC-centric limits.