Molecular Simulation of Chloride Ion Binding Mechanisms to Na-Doped Tobermorite 14 Å as a Model System for Sodium-Containing Cements

Alternative cements, such as alkali-activated materials (AAMs), produced from the alkaline activation of industrial byproducts, represent a potential lower-CO₂ substitute for ordinary Portland cement (OPC). However, the interaction of the main binder phases, such as calcium-silicate-hydrate-type gels, with corrosive ions from the surrounding environment remains largely unexamined. This is particularly the case for gels that contain alkalis, such as sodium, which exist in OPC blended with recycled sodium-rich glass and high-Ca AAMs. In this study, we use force field molecular dynamics simulations and 5% Na-doped tobermorite 14 Å as a model structure to understand the adsorption mechanisms of chloride ions with the interface of alkali-doped calcium-silicate-hydrate (C-S-H) gel. We adopt an adiabatic shell model to represent the polarizability of oxygen ions and a flexible model for water molecules. From the simulations, we show that the existence of Na⁺ ions in the structure enhances the adsorption of solvated Cl^- ions and thus hinders their subsequent ingress into the concrete in two ways. First, new and stable adsorption sites are introduced. This is attributed to the charge-balancing mechanism taking place when substituting Ca²⁺ ions with Na⁺ ions in the solid, where H⁺ ions are included in the substitution event (if at moderate pH levels), leading to the formation of silanol groups (Si-OH). Second, introducing the Na⁺ ions, which are known to be structure-making cations, leads to slower dynamics for interfacial water and Cl^- ions because of the tendency of Na⁺ ions at the tobermorite/water interface to be slightly displaced into the solution. These results indicate that the existence of alkalis in the C-S-H gel will lead to enhanced chloride binding, delaying the onset of detrimental chloride-induced steel corrosion.