Influence of Seawater-Derived Salts on Ye’elimite Hydration Kinetics and Microstructural Evolution: Toward Developing Sustainable Calcium Sulfoaluminate Cement-Based Materials

Calcium sulfoaluminate (CSA) cement offers a strategic low-carbon alternative for seawater sea-sand concrete, reducing both CO2 emissions and freshwater demand. However, the ambiguous mechanisms governing seawater salts’ impact on its hydration kinetics and microstructural development hinder its practical engineering. This study systematically investigated the influence of three key seawater salts─NaCl (NC), Na2SO4 (NS), and MgCl2 (MC)─on the hydration kinetics and microstructural evolution of ye’elimite (the primary CSA cement clinker) compared to deionized (DI) water. Results revealed that seawater salts altered the hydration kinetics of C4A3 via a dual effect characterized by early-stage acceleration, followed by later-stage retardation. The NS system demonstrated the most pronounced dual effect, while the MC system had the least impact. Microstructural analysis revealed that these salts significantly modify phase evolution and crystal morphology. Specifically, the AH3 content ranked as MC > NS > DI > NC. AFm was present across all systems, with the highest content in DI and the lowest in NC, while AFt content shifted from an early-stage ranking of NC > MC > NS to a late-stage ranking of NC > NS > MC. Friedel’s salt formed only in Cl–-containing systems, with the highest concentrations consistently observed in the NC system. Furthermore, both NC and NS systems fostered larger AFt and AFm crystals compared to the DI system, while the MC system generated smaller crystals. While all salt systems increased the macropore volume, the NC and MC systems reduced micropores. Additionally, AH3 exhibited higher crystallinity in the NS/NC systems and the lowest in the MC system; the AH3 phase exhibited a comparatively stronger affinity for cations, with the adsorption following the order of Mg2+ > Ca2+ > Na+, while the uptake of anions by AH3 remained weak. These findings elucidate the fundamental mechanisms for the development of next-generation, low-carbon CSA-based composites.