Effect of high-content steel slag on granulated blast furnace slag–desulfurized gypsum-based cementitious materials: Long-term volume stability and hydration mechanisms

• Excellent soundness performance is achieved in cementitious materials incorporating 70 % steel slag content. • Long-term volume monitoring reveals initial expansion peaking at 0.1168 % and subsequent contraction, maintaining a stable net expansion of 0.0936 % after 1200 days. • Concrete specimens develop significant compressive strength, reaching 39.8 MPa at 28 days and 49.5 MPa at 2 years. • The 70 % SS content establishes the necessary alkaline environment and calcium source, driving an AFt-formation-dominant hydration mechanism that effectively activates slag reactivity. This study optimized the mix proportion of a high-content steel slag (SS)-granulated blast furnace slag (GBFS)-desulfurization gypsum (DG) cementitious system through orthogonal experiments. The optimal mix ratio was determined to be A3B1C1: 70 % SS, 22.5 % GBFS, and 7.5 % DG, with an SS specific surface area of 600 m 2 /kg. The optimized system demonstrated excellent mechanical properties, achieving a 28d compressive strength of 40.2 MPa and exhibiting superior volume stability. Notably, its long-term strength continuously increased, reaching 49.5 MPa after 2 years for the 70 % SS content. Volume stability analysis of mortar specimens revealed an initial expansion phase peak of 0.1168 % attributed to rapid ettringite (AFt) crystallization, followed by a contraction phase linked to the densification of C − S − H gel, gel contraction, and self-desiccation shrinkage. Microstructural characterization showed that axial growth of AFt crystals at 7d and radial coarsening at 28d enhanced material compactness, while increased polymerization of C − S − H gel further reduced porosity. Mechanistic studies indicated that the 70 % SS content provided the alkaline environment and calcium ion source crucial for activating GBFS by promoting aluminosilicate dissociation. Although Ca(OH) 2 was rapidly consumed without significant accumulation, this process, alongside sulfate ion consumption, drove AFt formation as the primary hydration mechanism, effectively activating GBFS's latent reactivity and contributing to long-term volume stability. This offers a novel approach for developing high-SS, reduced-GBFS cementitious materials.