Optimization and Hydration Mechanisms of Self-Activated Cementitious Binders Prepared from Ground Granulated Blast Furnace Slag, Carbide Slag, Desulfurized Gypsum and Silica Fume

The development of low-carbon binders from industrial solid wastes can reduce Portland cement use and support sustainable construction. However, the combined roles of alkalinity supply, sulfate reaction, and reactive silica supplementation in ground granulated blast furnace slag (GGBS)–calcium carbide slag (CCS)–desulfurized gypsum (DG)–silica fume (SF) binders remain unclear. A Box–Behnken design was used to evaluate the 3 d and 28 d mechanical properties, and response surface methodology (RSM) was subsequently applied to optimize CCS, DG, and SF contents based on the 28 d flexural and compressive strengths. Hydration and microstructure were characterized by isothermal calorimetry, XRD, TG–DTG, and SEM–EDS. Within the investigated ranges, DG exhibited the strongest quadratic effect on both 28 d strength responses, while CCS showed a pronounced effect on compressive strength and SF exhibited significant dosage-dependent effects. The optimized mixture contained 16.1% CCS, 9.7% DG, 9.5% SF, and 64.7% GGBS, achieving experimentally validated 28 d flexural and compressive strengths of 10.2 and 35.9 MPa, respectively. CCS promoted GGBS dissolution, DG facilitated ettringite formation, and SF enhanced C-(A)-S-H formation and pore filling. Their coordinated action produced a denser gel–crystal framework, providing guidance for the design of multi-solid-waste binders for low-carbon construction applications.