Hydration mechanisms of calcium sulfoaluminate (CSA) cements: the role of barium carbonate in sulfate-carbonate balance

This study investigates the partial and total replacement of CaSO 4 by BaCO 3 in calcium sulfoaluminate (CSA) cement systems through a systematic compositional series to examine the coupled effects of decreasing initial sulfate content, carbonate incorporation, and BaSO 4 formation on hydration. Six pastes with different BaCO 3 /CaSO 4 ratios were evaluated by isothermal calorimetry, X-ray diffraction, FTIR, thermogravimetry, SEM–EDS, and nitrogen adsorption. The progressive replacement of CaSO 4 by BaCO 3 altered hydration kinetics and phase assemblage as a function of replacement level. At 25% substitution, hydration was accelerated without reducing 28-day compressive strength compared with the CaSO 4 -containing reference. At higher replacement levels, barite and carbonate-AFm phases formed, ettringite stability decreased, and residual BaCO 3 increased. At 75% substitution, enhanced strätlingite formation was associated with improved compressive strength, whereas complete replacement led to slower hydration, higher amorphous content, and monocarbonate-containing phase assemblages. The results provide mechanistic insight into how sulfate availability, carbonate incorporation, sulfate immobilisation through BaSO 4 formation, and phase stability jointly control hydration pathways in CSA systems.