Impact of Fe doping on the reactivity of γ-dicalcium silicate: Insights from DFT calculations

This study investigates the impact of Fe doping on the reactivity of gamma-dicalcium silicate (gamma-C2S) using density functional theory (DFT) calculations. We examined isolated point defects and defect complexes in various charge states to assess substitution tendencies. The results reveal that substituting Si with Fe is energetically more favorable than substituting Ca, due to smaller distortions and minimal impact on Fe-O bonds compared to Ca-O bonds. We systematically analyzed the impact of Fe impurities on gamma-C2S reactivity through atomic charges, chemical bonds, and ab initio molecular dynamics (AIMD) simulations. On the one hand, Fe strengthens nearby Ca-O bonds, reducing the effective charge of oxygen atoms and inhibiting reactivity. On the other hand, hydroxylation of Fe-O groups, followed by dehydroxylation, decreases the coordination number of certain surface Ca ions, enhancing reactivity. The overall reactivity of Fe-doped gamma-C2S balances inhibition and enhancement, depending on the reacting molecules. In pure H2O systems, the inhibitory effect prevails, while in the presence of H2CO3, the enhancement effect dominates. Experimental observations corroborate these findings. This study provides insights into impurity doping mechanisms in calcium silicate minerals.