Mechanistic insights into the anion-driven suppression of the initial cement reaction of tricalcium aluminate

The hydration process of ordinary Portland cement (OPC) involves intricate interactions among ions in the pore solution, dissolving clinker phases, and precipitating hydrates. Tricalcium aluminate ((Formula presented.)) shows the highest reactivity of the cement clinker phases during the initial hydration stages of OPC. The kinetic control of this reaction is of fundamental importance. In commercial OPC, cement producers add calcium sulfates to suppress the initial reactivity of (Formula presented.). Dissolution suppression strongly depends on the chemical nature of the added anion. This study explores the early hydration behavior of (Formula presented.) in the presence of three sodium salts (NaCl, (Formula presented.), and (Formula presented.)) using in situ isothermal calorimetry and in situ X-ray diffraction. Among the sodium salts studied, phosphate exhibits the strongest inhibition of early (Formula presented.) hydration, followed by sulfate and chloride. The experimental data are combined with two theoretical methods: thermodynamic modeling and molecular dynamics simulations. The molecular dynamics simulation implies the very fast formation of a hydroxyapatite-like surface layer on the (Formula presented.) surface. It also yields the interfacial tensions of the salt-containing systems, which corresponds to the experimentally observed order. Time-dependent thermodynamic modeling of the hydrating systems provides insights into the equilibrium phase composition. This work can provide pathways for further studies bridging experimental in situ methods and theoretical approaches such as all-atom molecular simulations and thermodynamic modeling.