In-situ early-age hydration study of sulfobelite cements by synchrotron powder diffraction

Eco-friendly belite calcium sulfoaluminate (BCSA) cement hydration behavior is not yet well understood. Here, we report an in-situ synchrotron X-ray powder diffraction study for the first hours of hydration of BCSA cements. Rietveld quantitative phase analysis has been used to establish the degree of reaction (α). The hydration of a mixture of ye'elimite and gypsum revealed that ettringite formation (α ~70% at 50h) is limited by ye'elimite dissolution. Two laboratory-prepared BCSA cements were also studied: non-active-BCSA and active-BCSA cements, with β- and α′H-belite as main phases, respectively. Ye'elimite, in the non-active-BCSA system, dissolves at higher pace (α ~25% at 1h) than in the active-BCSA one (α ~10% at 1h), with differences in the crystallization of ettringite (α ~30% and α ~5%, respectively). This behavior has strongly affected subsequent belite and ferrite reactivities, yielding stratlingite and other layered phases in non-active-BCSA. The dissolution and crystallization processes are reported and discussed in detail.