Intensification of mineral carbonation was studied to determine its effectiveness on carbonation of Canadian kimberlite tailings and wollastonite ore and to clarify the carbonation reaction pathways of stainless-steel slag (Argon Oxygen Decarburization (AOD) and Continuous Casting (CC) slags). The incubator carbonation and slurry carbonation experiments were considered to be “mild”, while pressurized carbonation and simultaneous ball milling carbonation were intensified by increasing the reaction temperature and pressure and applying milling to further activate the solid materials during carbonation, respectively. In situ X-Ray diffraction (XRD) was conducted during mineral carbonation of the AOD and CC slags, as well as their constituent silicate minerals in either water-saturated (wet) subcritical CO2(g), or wet supercritical CO2(SC). The results indicate that kimberlite tailings and wollastonite ore sequester increasing amount of CO2 depending on the intensification of the carbonation methodologies and in situ XRD revealed the transient and unstable phases of stainless-steel slags that evolve during carbonation.