NMR investigations on Cl⁻ and Na⁺ ion binding during the early hydration process of C₃S, C₃A and cement paste: A combined modelling and experimental study

Using materials containing sodium chloride(e.g., seawater) to make concrete is sometimes unavoidable. However, the associated ion binding mechanisms and structural evolution during the early hydration process are not fully understood. This study investigated Cl⁻ and Na ⁺ binding behaviour and microstructure of C₃S, C₃A and cement paste within the first 48h using a high-field NMR setup. Results show that during hydration, KOH can reduce Cl⁻ binding but enhance Na ⁺ ion binding in C₃S paste, while increasing gypsum dosage in C₃A paste retards both Cl⁻ and Na⁺ binding. Retarders reduce Cl⁻ and Na⁺ binding, while accelerators slightly promote Cl⁻ binding but reduce Na⁺ binding in cement paste. It is also found that boundary nucleation and growth(BNG) modelling is the most suitable for fitting the ion binding. Finally, the microstructure assessed by the T₁ relaxation rate is related to ion binding in various ranges, indicating that hydrates during hydration have various binding efficiencies.