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 yeʹelimite and gypsum revealed that ettringite formation (α ~ 70% at 50 h) is limited by yeʹelimite 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. Yeʹelimite, in the non-active-BCSA system, dissolves at higher pace (α ~ 25% at 1 h) than in the active-BCSA one (α ~ 10% at 1 h), 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.