Mechanical Properties and Microstructural Evolution of CO2-Mineralized Cured Steel Slag-Fly Ash Composite Cementitious Materials

• • Optimal residual water-to-cement ratio (r/w) ≤ 0.15, curing pressure ~0.3 MPa, and curing time ~5 h yield a balanced combination of compressive strength and carbon sequestration; higher pressures increase CO2 uptake but provide diminishing or negative strength returns. • • Under constant total solid waste content, steel slag content dominates the synergistic improvement of strength and carbon sequestration; formulations with ≥40% steel slag perform well, with 50% steel slag + 10% fly ash identified as the 'synergistic optimal zone'. • • CO2 mineralization promotes the formation of highly crystalline calcite that fills pores, reduces interfacial transition zone (ITZ) continuity and crack connectivity, leading to crack deflection/blunting and enhanced fracture energy, thereby driving strength gains. • • Lower residual water-to-cement ratios, higher steel slag content, and longer curing durations increase both the content and crystallinity of CaCO3, as confirmed by XRD and TG-DTG; the mass loss increase in the 500–950 °C region corresponds to highly crystalline CaCO3, supporting the crystallinity–mechanical response correlation.