<p>The main reason for the underutilization of steelmaking slags is their instability, which is influenced by their complex phase structure. Recycling and sustainable steelmaking slag face challenges due to the presence of magnesium oxide (MgO), a crucial composition in steelmaking slag (MgO–CaO–Fe_<i>x</i>O–SiO₂–Al₂O₃–P₂O₅–MnO–TiO₂). This study investigates the phase evolution of steelmaking slag during the cooling process with 8–12 mass% MgO contents, using high-temperature experiments and FactSage thermodynamic equilibrium calculations. XRD and SEM–EDS analyses identified the main phases in steelmaking slag, including Ca₂SiO₄ (C₂S), RO (MgO–FeO solid solution), Ca₂Fe₂O₅ (C₂F), and Ca₃SiO₅ (C₃S). The study reveals that RO increases as the MgO content increases. C₃S precipitates from steelmaking slag with 12 mass% MgO content at a reduced temperature of 1400&#xa0;°C. It is important to note that C₃S remains in the steelmaking slag even after slow cooling. This may be due to substituting Ca²⁺ in C₃S by Fe²⁺ and Mg²⁺ ions. Intriguingly, the precipitation of <i>f</i>-CaO in steelmaking slag with 10 mass% of MgO content occurred at room temperature (annual average relative humidity 80%, temperature 18℃) after one year, while there was no manifestation of <i>f</i>-CaO precipitation in the steelmaking slag with 12 mass% of MgO content even after one year. This discrepancy suggests a potential stabilizing effect of an appropriate amount of MgO in steelmaking slag. However, it is crucial to emphasize that this inference lacks supporting data, necessitating further research to validate this hypothesis. This work is significant for revealing the complexities of steelmaking slag behavior, offering crucial insights to optimize recycling processes.</p> Graphical Abstract <p></p>