Water vapor-assisted carbonation of steel slag: calcium source contributions and kinetic behavior

The efficiency of direct carbonation of steel slag, a promising technique for CO₂ mineralization, is often limited by incomplete calcium utilization. In addition, the reaction pathways of different calcium-bearing phases and evolution of kinetic behavior under intermediate-temperature conditions are insufficiently understood. To address these limitations, water vapor was introduced into the reaction atmosphere to promote interfacial reaction and effectively utilize reactive calcium phases. In this study, water vapor-assisted carbonation of steel slag was systematically investigated over 773-1073 K, and the highest net carbonation performance was achieved at 873 K. Water vapor significantly enhanced carbonation by increasing the carbon sequestration efficiency from 4.75% under dry conditions to a maximum value of 9.82%. The residual f-CaO decreased to below 0.2% under the selected conditions of 873 K and 25 vol% H₂O. Semi-quantitative estimation of the apparent calcium-source contributions further showed that the apparent contribution of calcium silicate-related phases increased from 0.01% under dry conditions to 4.31% in the presence of water vapor, indicating that water vapor promoted the participation of these phases in carbonation. The effects of mass transfer and pore accessibility were further clarified by examining the particle size and compaction pressure, and the results indicated that the apparent calcium silicate-related contribution was considerably more sensitive to these factors than free calcium carbonation. Kinetic analysis based on the shrinking core model suggested a possible stage-wise shift from an initial surface-reaction control regime to an increasingly diffusion-influenced regime at later stages. These findings clarify the calcium-source-dependent reaction pathways and kinetic behavior during steel slag carbonation and provide a mechanistic basis for optimizing intermediate-temperature mineral carbonation processes.