A carbon-sequestering cementitious binder: Synergistic mechanisms of carbonated steel slag and carbonated carbide slag on hydration and strength development

The complementary use of carbonated carbide slag (CCS) and carbonated steel slag (CSS) enables substantial cement substitution, but their composition-driven hydration synergy remains unclear. In this study, CCS and CSS were incorporated individually and jointly into Portland cement (PC) at a fixed total replacement of 30 wt%, with systematically varied proportions. Isothermal calorimetry, Krstulović-Dabić kinetic modelling and pore-solution analysis showed that increasing CSS progressively redistributed hydration from strong early nucleation toward sustained later-stage reaction, balancing CaCO 3 -assisted nucleation with Si/Al-mediated dissolution-precipitation. At the optimum CCS10CSS20 composition, the 28-d TGA-derived apparent degree of hydration reached 92.63%, while monocarboaluminate increased to 17.40 wt%. The gel-associated Q 1 + Q 2 Al + Q 2 proportion reached 65.15%, compared with 52.47% in CCS30, indicating enhanced C-(A-)S-H formation and polymerization, accompanied by pore refinement. This evolution reflects the complementary roles of CCS-derived carbonate nucleation and the sustained chemical contribution of CSS-derived reactive Si and Al. These coupled changes increased the 1-d and 28-d compressive strengths by 26% and 82%, respectively, relative to CCS30, while maintaining a 36.5% CO 2 reduction relative to PC. The results establish a composition link among hydration kinetics, hydrate assemblage, structural evolution and strength, providing a mechanistic basis for optimizing composite cements.