Mitigation and adaptation are viable strategies for resolving climate change issues which may pose significant challenges to both ecosystems and human populations around the world. Aqueous carbonation is a promising process for mitigating CO2, due to the permanent storage of gaseous CO2 into carbonate precipitations (CaCO3 and/or MgCO3). In this study, aqueous carbonation of blended hydraulic slag cement (BHC) for CO2 sequestration was investigated and evaluated under various operating conditions, i.e., different reaction temperatures and CO2 concentrations, in a slurry reactor. The suspension BHC slurry was strongly alkaline (pH ~11.4) before carbonation, whereas the pH of the slurry dropped rapidly to nearly a weakly acidic solution (i.e., pH ~6.3) after introducing CO2 gas into the reactor. The results show that the maximum CO2 capture capacity was 181 g CO2 per kg BHC at a reaction time of 120 min, a CO2 concentration of 10%, and a gas flow rate of 2.5 L/min at 65°C. The reaction temperature slightly influenced the carbonation conversion of BHC, with an increasing temperature resulting in relatively higher conversion. In addition, the SEM and XRD results suggest that the BHC should be carbonated with CO2 to form CaCO3 in a slurry reactor. It was thus concluded that the CO2 could be successfully captured by the carbonation of BHC in this manner. Furthermore, the experimental data were utilized to determine the rate-limiting mechanism based on the shrinking-core model (SCM), which was validated by the observations of SEM images. The SCM results indicate that the overall carbonation reaction of BHC in a slurry reactor was controlled by the ash-layer diffusion mechanism.