This review discusses the carbonation of steel slag, focusing on its properties, mechanisms, and applications. Steel slag, a by-product of the steel industry, contains high levels of free calcium oxide (f-CaO) and free magnesium oxide (f-MgO), which can cause soundness issues when used as a binding material or aggregate. Carbonation treatment transforms these oxides into calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃), reducing the risk of expansion and improving the utilization of steel slag. Additionally, carbonation captures and stores CO₂, helping to reduce carbon emissions. Steel slag is primarily composed of CaO, SiO₂, FeₓOᵧ, Al₂O₃, and MgO, with minor oxides such as MnO, P₂O₅, Na₂O, and SO₃. Its physical and chemical properties, including specific gravity, water absorption, and soundness, are important for its application. The carbonation process involves the reaction of CO₂ with the mineral phases of steel slag, leading to the formation of CaCO₃ and MgCO₃. This process can be carried out through direct or indirect methods, with direct carbonation being more common. The carbonation of steel slag is influenced by internal factors such as mineral composition, particle size, and chemical composition. External factors including carbonation period, temperature, and CO₂ partial pressure also play a role. Increasing the carbonation period and temperature generally enhances the carbonation depth and rate, while excessive CO₂ partial pressure can lead to rapid precipitation, blocking pores and reducing further CO₂ contact. Carbonation improves the soundness of steel slag by transforming f-CaO and f-MgO into stable carbonate compounds, reducing the risk of expansion. It also enhances the mechanical properties of steel slag-based materials, making them suitable for use in bricks, supplementary cementitious materials, and aggregates. Potential applications include wastewater treatment, thermal insulation, and artificial stone production. The review highlights that carbonation is an effective method for promoting the utilization of steel slag, reducing environmental hazards, and capturing CO₂. However, challenges remain in optimizing the carbonation process for practical applications, including the need for further research on the relationship between carbonation parameters and steel slag composition, as well as the balance between soundness and hydration activity.This review discusses the carbonation of steel slag, focusing on its properties, mechanisms, and applications. Steel slag, a by-product of the steel industry, contains high levels of free calcium oxide (f-CaO) and free magnesium oxide (f-MgO), which can cause soundness issues when used as a binding material or aggregate. Carbonation treatment transforms these oxides into calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃), reducing the risk of expansion and improving the utilization of steel slag. Additionally, carbonation captures and stores CO₂, helping to reduce carbon emissions. Steel slag is primarily composed of CaO, SiO₂, FeₓOᵧ, Al₂O₃, and MgO, with minor oxides such as MnO, P₂O₅, Na₂O, and SO₃. Its physical and chemical properties, including specific gravity, water absorption, and soundness, are important for its application. The carbonation process involves the reaction of CO₂ with the mineral phases of steel slag, leading to the formation of CaCO₃ and MgCO₃. This process can be carried out through direct or indirect methods, with direct carbonation being more common. The carbonation of steel slag is influenced by internal factors such as mineral composition, particle size, and chemical composition. External factors including carbonation period, temperature, and CO₂ partial pressure also play a role. Increasing the carbonation period and temperature generally enhances the carbonation depth and rate, while excessive CO₂ partial pressure can lead to rapid precipitation, blocking pores and reducing further CO₂ contact. Carbonation improves the soundness of steel slag by transforming f-CaO and f-MgO into stable carbonate compounds, reducing the risk of expansion. It also enhances the mechanical properties of steel slag-based materials, making them suitable for use in bricks, supplementary cementitious materials, and aggregates. Potential applications include wastewater treatment, thermal insulation, and artificial stone production. The review highlights that carbonation is an effective method for promoting the utilization of steel slag, reducing environmental hazards, and capturing CO₂. However, challenges remain in optimizing the carbonation process for practical applications, including the need for further research on the relationship between carbonation parameters and steel slag composition, as well as the balance between soundness and hydration activity.