High energy X-ray scattering reveals the short-range order of proto-C-S-H: Implications for the nucleation of cement hydrates

The heterogeneous nature of the cement clinker, its complex coupled dissolution/re-precipitation process and the complex defect chemistry of the hydrates formed during cement hydration, make that the nucleation and growth mechanisms of cement hydrates are not fully understood. Recent studies have suggested the existence of a disordered precursor prior to the crystallization of C-S-H. Here, a combination of X-ray scattering and electron microscopy experiments are used to study the structure and crystallization kinetics of this amorphous intermediate. Our results suggest that proto-C-S-H is predominantly composed of 6-fold oxygen-to-calcium coordinated CaO_x polyhedra, in contrast to the 7-fold coordinated polyhedra in C-S-H with a tobermorite-like structure. The addition of gluconate, a widely used additive, resulted in an increased kinetic persistence of proto-C-S-H. Overall, our study provides new insights into the formation and structure of proto-C-S-H, and suggests that it plays an important role in the crystallization mechanism of C-S-H.