Insights into Cl− binding and phase evolution during the early hydration of cement pastes prepared with NaCl solutions: a study using high-field NMR and in situ XRD

Abstract

Using saline materials such as seawater in concrete promotes sustainability but raises durability concerns, particularly due to corrosion of steel reinforcement. However, the mechanisms regarding the early-stage binding of saline ions during cement hydration remain poorly understood. This study investigates sodium (Na⁺) and chloride (Cl⁻) ion binding in early hydration of tricalcium aluminate (C₃A) and tricalcium silicate (C₃S), and different types of cements (OPC and Slag cement) with or without sodium sulfate (0.5 M Na₂SO₄), in the presence of 1.0 M NaCl solution. High-field nuclear magnetic resonance (NMR), calorimetry, and in situ X-ray diffraction (XRD) were employed to analyze ion binding, hydration kinetics, and phase development. Results indicate slower Cl⁻ binding in C₃S pastes due to their gradual hydration, whereas rapid AFm (alumina-ferric oxide monosubstituted phases) formation in C₃A pastes promotes quick binding of saline ions. Slag cement, despite lower heat release, exhibits greater Cl⁻ binding than OPC, and sodium sulfate addition retards ions binding in both cements. Early-stage Cl⁻ binding predominantly occurs through physical adsorption, and NMR analyses reveal increased chloride mobility correlates with greater chloride leaching risks.