Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag Cement

Excess-sulfate phosphogypsum slag cement (EPSC), offering the potential for large-scale phosphogypsum (PG) utilization, has drawn significant attention. However, its susceptibility to salt erosion in marine/saline environments remains unquantified, hindering engineering applications. This study, therefore, systematically investigates the effect of various salts (NaCl, MgCl2, KCl, and Na2SO4) at different concentrations (0.5–1.5%) on the hydration mechanism and performance of EPSC using rheometry, strength tests, and microstructural characterization (XRD/SEM-EDS). The findings reveal that EPSC exhibits low initial yield stress and plastic viscosity, both of which increase over time. The addition of Na+, Cl−, and SO42− ions promotes hydration and flocculent structure formation in the EPSC paste, thereby enhancing the yield stress and plastic viscosity. In contrast, Mg2+ and K+ ions inhibit the hydration reaction, although Mg2+ temporarily increases the plastic viscosity by forming Mg(OH)2 during the initial stage of the reaction. Both Na2SO4 and NaCl improve mechanical properties when their concentrations are within the 0.5–1.0% range; however, excessive amounts (>1%) negatively impact these properties. Significantly, adding 0.5% NaCl significantly improves the mechanical properties of EPSC, achieving a 28-day compressive strength of 51.06 MPa—a 9.5% increase compared to the control group. XRD and SEM-EDX analyses reveal that NaCl enhances pore structure via Friedel’s salt formation, while Na2SO4 promotes the early nucleation of ettringite. However, excessive ettringite formation in the later stages of the hydration reaction due to Na2SO4 may negatively affect compressive strength due to the inherent abundance of SO42− in the EPSC system. Therefore, attention should be paid to the effect of excessive SO42− on the system. These results establish salt-type/dosage thresholds for EPSC design, enabling its rational use in coastal infrastructure where salt resistance is critical.