Carbon capture, utilization, and storage (CCUS) is one of the most prominent emerging technologies for mitigating global climate change. In this study, a comparative evaluation for CO2 fixation by carbonation of steelmaking slag was performed by life cycle assessment (LCA) using Umberto 5.5.4 software, with the Swiss Eco-invent 2.2 database. Six scenarios of carbonation for basic oxygen furnace slag (BOFS), steel converted slag (SCS), and blended hydraulic slag cement (BHC) in different types of reactors and/or method were established. The environmental impacts for each scenario are quantified using the valuation system of ReCiPe, where global warming potential (GWP), ecosystem quality potential (EQP), and human health potential (HHP) were evaluated. In addition, sensitivity analysis was carried out to evaluate the relevant uncertainties of heating efficiency on the GHG emissions in direct carbonation processes. According to the results of LCA and sensitivity analysis, the direct carbonation of steelmaking slag in a slurry reactor was found to be the most attractive method, since the GWP was the lowest among the selected scenarios. Furthermore, the best available technology (BAT) for CO2 capture by carbonation processes of alkaline wastes was proposed according to the key performance indicators (KPIs) with respect to engineering considerations and environmental impacts. It was concluded that the accelerated carbonation of steelmaking slag should be performed by combining the slurry reactor with a rotating packed bed (RPB) to maximize carbonation conversion and minimize environmental impacts and additional CO2 emissions.