Structural stability of belite sulfoaluminate clinkering polymorphs

Belite is a major phase of cement clinker that has attracted great attention due to its applicability in the production of environmental friendly cement. In this work, we investigate and compare properties of γ- β- polymorphic forms of belite. γ-belite is the ground state phase. Whereas, β-belite is found to be metastable phase of belite and it is the most present phase of belite in ordinary cement. γ-belite is a non-reactive phase with water of belite; while β-belite clearly reacts with water. First-principles methods based on Density Functional Theory within the pseudopotential formalism have been used here to study the structural, mechanical and electronic properties of both belite phases. The influence of the crystallographic structure of belite on the reactive sites distribution under nucleophilic and electrophilic attacks has been highlighted. In addition, a noticeable difference of the reactive sites localisation of the two phases has been pointed out. Our results are in agreement with the observed high-water reactivity of β-belite compared to γ-belite. However, our calculations show very slight effect of the belite polymorphism on mechanical properties. In particular, the elastic behaviour of γ- and β-belite shows important similarities regarding the elastic parameters obtained for the two belite phases. Nevertheless, our results predict the γ-belite to be slightly more rigid and resistant to mechanical loading compared to β-belite