Effect of calcium sulfate source and dosage on the rheology and early-age hydration of supersulfated cement

Abstract Supersulfated cement (SSC) has emerged as a sustainable and promising binder. However, studies addressing its behavior in the fresh state and at an early age remain limited, particularly regarding the influence of calcium sulfate source and dosage on fresh-state behavior. This work investigated the effect of calcium sulfate source and dosage in SSC composed of slag (75–85%), Portland clinker (5%), and anhydrite and gypsum as calcium sulfate sources (10–20%). Hydration kinetics were analyzed using isothermal calorimetry and thermogravimetry during the first 40h of hydration. Complementary analyses, including FTIR and quantitative XRD with Rietveld refinement, were also performed, while rheological behavior was assessed using rotational rheometry. Early-age analyses indicate that ettringite is the dominant hydration product in SSC during the first 20h of hydration, whereas from approximately 30h onwards hydration proceeds through overlapping AFt evolution and C–S–H precipitation. No gypsum precipitation was detected during early hydration. Anhydrite-based formulations showed higher cumulative heat release after 72h, greater formation of ettringite and C–S–H, particularly at 15% anhydrite, and significantly lower dynamic yield stress (up to 70%) and equivalent viscosity (up to 52%) than gypsum-based systems. In contrast, gypsum promoted faster ion saturation, restricting hydration. Calcium sulfate source and dosage strongly influenced both fresh-state rheology and early-age hydration in SSC. Anhydrite promoted a more favorable balance between workability and ongoing hydration than gypsum, highlighting sulfate selection as an important parameter for SSC mixture design.