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This study investigates the adsorption mechanism of CO3^(2−) on the (111) surface of tricalcium silicate (C3S) using density functional theory simulations. Two distinct adsorption configurations were identified: a tilted alignment with localised bonding to Ca ions and concentrated charge transfer, and a parallel orientation with delocalised interactions involving multiple Ca ions. Charge density analysis revealed charge transfer from the surface to the carbonate molecule, with electron accumulation around oxygen atoms of CO3^(2−). Partial density of states analysis showed significant changes near the Fermi level after adsorption, indicating the formation of new bonding states. Molecular dynamics simulations demonstrated that the tilted configuration stabilises the surface by reducing Ca ion mobility, while the parallel configuration leads to increased ion mobility and higher surface reactivity. These findings emphasise the importance of site-specific interactions and electronic structure changes in understanding CO2 mineralisation mechanisms in cementitious materials.