Aluminum-Mediated Chloride Binding during Tricalcium Silicate Hydration: Toward Highly Durable and Sustainable Seawater-Mixed Cement Blends

Durability and sustainability challenges have shaped the evolution of concrete over centuries, particularly in marine environments where on-site production using seawater and prolonged exposure in offshore and coastal regions are prevalent. Understanding chloride binding mechanisms during cement hydration and uncovering the molecular interactions between seawater ions and cement hydrates are thus crucial for advancing seawater-mixed concrete applications in marine infrastructures. Herein, we conducted ingeniously designed experiments and simulations to elucidate the complex hydration reactions of tricalcium silicate (C₃S) in varied chloride solutions. The findings reveal distinct chloride binding responses of C₃S pastes regulated by the presence of solution cations (Ca and Mg ions) and the addition of alumina nanoparticles (ANPs). In addition, these factors exhibit a synergistic effect on enhancing the formation of calcium-aluminosilicate-hydrates (C–(A–)S–H) and modifying their surface chemistry, thereby significantly improving chloride binding capacity. Ab initio calculations provide further insights into the critical roles of Ca/Mg ions and Al substitution in modulating the molecular structure and charge distribution of C–(A–)S–H. Ca and Mg ions act as electronic linkages, bridging surface O and adsorbed Cl ions. This effect is further amplified by Al substitution, which introduces cumulative negative charges around the bridge site and facilitates the formation of stable Al–O–M linkages that enhance chloride capture. Our outcomes advance the design of sustainable and durable seawater-mixed cement blends for resilient marine concrete infrastructures.