Cation substitution induced reactivity variation on the tricalcium silicate polymorphs determined from first-principles calculations

Alite is one of the most important phases in cement clinker. It is a solid solution of tricalcium silicate (Ca₃SiO₅, C₃S for short, C = CaO, S = SiO₂)which has seven polymorphs. M1-C₃S and M3-C₃S are the most common polymorphs in cement clinker. Much experimental research had identified that cations doping affected the polymorph of alite and the hydration properties of cement clinker. This article systemically investigated the influence of cation doping on crystal structures and electronic structures of M1-C₃S and M3-C₃S employed by first-principles calculations based on density functional theory. Non-bonding electrons dispersed on the p orbitals of Oi (the isolated oxygen atoms)and Oc (covalent bonded oxygens in nesosilicate tetrahedrons)in C₃S were determined and quantified by the integration of partial density of states at specified energy levels. Non-bonding electrons are the most probable electrons that will be transferred when the oxygen atoms suffer from electrophilic attack as minerals contact with water. The results showed that there were more non-bonding electrons in both of Oi and Oc atoms in M1-C₃S than that in M3-C₃S, which lead to M1-C₃S more reactive. The substituted cations with specific valence electrons configurations and weak electronegativity are determined, which will typically increase the amount of non-bonding electrons in the systems generally. These would help to understand how cation doping affected the hydration of alite, and furthermore, improve the quality of cement clinker.