Mechanical Properties and Microstructural Evolution of CO2-Mineralized Cured Steel Slag-Fly Ash Composite Cementitious Materials
This study investigates the optimization of CO2 mineralization curing on the performance of a ternary cementitious system comprising steel slag, fly ash, and Portland cement. Specimens were fabricated with varying residual water-to-cement ratios (r/w), steel slag and fly ash contents, curing pressures, and durations, then subjected to standard curing and CO2 mineralization curing. Compressive strength and carbon sequestration rate were measured, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to analyze mineral composition and microstructural evolution. Results indicate that compressive strength first increases then rapidly decreases with increasing residual water-to-cement ratio, with an optimal r/w below 0.15. Both compressive strength and carbon sequestration rate increase with higher steel slag content; the 50% steel slag + 10% fly ash formulation exhibited the highest values. Microstructural analyses revealed that CO2 mineralization primarily consumes hydration products such as Ca(OH)2, C-S(Al)-H, and AFt, generating abundant calcium carbonate that densifies the pore structure, thereby enhancing mechanical properties. Lower residual water-to-cement ratios, higher steel slag content, or extended curing durations increase the content and crystallinity of calcium carbonate. SEM observations confirmed the presence of densely packed, well-crystallized rhombohedral calcite in specimens with lower water-to-cement ratios and higher steel slag content. These findings provide a mechanistic basis for the engineering application of CO2 mineralization curing in ternary solid-waste cementitious materials.