Influence of Seawater and Salt Ions on the Properties of Calcium Sulfoaluminate Cement

Calcium sulfoaluminate (CSA) cement is recognized for its rapid hardening, early strength development, low carbon, and eco-friendly characteristics. The impact of seawater ions on the properties of CSA is studied utilizing varying concentrations of sea salt containing four different ions (K+, Mg2+, SO42−, and Cl−). A comprehensive assessment of CSA-based materials includes mechanical performance, hydration products, and microstructure analysis through bending and compression strength tests, pH tests, X-ray diffraction (XRD), scanning electron microscopy (SEM), digital image correlation (DIC), and acoustic emission (AE) analysis. The study reveals that the addition of seawater enhances the strength of CSA cement-based materials, transitioning their performances from structural brittleness to ductility. It highlights the different impacts of salt ions on CSA mortar: chloride (Cl−) minimally affects strength, but in combination with magnesium (Mg2+), it leads to the encapsulation of ettringite by M-S-H gel, thereby weakening the overall strength of CSA cement. Furthermore, mixing CSA mortar with potassium sulfate (K2SO4) solution enhances early-stage strength (cured<7 days) but diminishes it in the later stages (cured>7 days). A synergistic effect is observed when SO42− and Mg2+ ions coexist, resulting in an overall strength reduction of CSA mortar. Notably, when K+, Mg2+, SO42−, and Cl− are combined in solution, the pH significantly decreases compared to natural seawater, leading to a notable reduction in compressive strength of CSA mortar.