Insight the essential role of manganese impurity to the initial hydration of Mn-doped C4AF at the atomic level

The growing focus on reducing CO₂ emissions has promoted the use of alternative raw materials in cement production. Most heavy metal ions from these materials are preferentially incorporated into tetracalcium aluminoferrite (C₄AF), making it important to understand the effect of this incorporation on C₄AF hydration. In this work, we examined the hydration properties of manganese (Mn)-doped C₄AF by combining well-defined density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. The results indicate that Mn doping stabilizes the Mn–O octahedral structure, reducing charge transfer between water molecules and the surface. This change lowers adsorption energy and interfacial bond strength, weakening the potential for dissociative water adsorption. Mn doping reduces the initial hydration reaction rate of C₄AF, with the following hierarchy of reaction rates observed: pure C₄AF > Mn-doped C₄AF surface > Mn-doped C₄AF interphase. The hydrogen bonding density at the interface becomes localized upon Mn addition, reducing charge transfer between the water and the surface. Mn near the interface further delays the reaction by anchoring water molecules and hydroxyl groups. These findings provide valuable insights into the early hydration of Mn-doped C₄AF.