The expansion phenomenon, which is a major issue when using steelmaking slag as a roadbed material and civil engineering material, is caused by the volume expansion during hydration of free CaO (undissolved CaO and precipitated CaO) and free MgO (undissolved MgO and precipitated MgO) contained in steelmaking slag. From the weight change of MgO reagent hydrated in distilled water, it was clarified that hydration of undissolved MgO proceeded slowly. In addition, experiments of hydrating MgO-FeO solid solution using an autoclave revealed that with increasing FeO concentration in the solid solution, the hydration degree became lower. The FeO content solved in solid MgO at 1550-1650 °C was experimentally determined and expressed by the following equation:From these results, the conditions for the formation of free MgO phase that is not hydrated (or easily hydrated) were discussed.(mass%FeO) in MgO = −0.477(mass% CaO)∕ mass% SiO 2 + 0.756(mass%T.Fe) + 0.695(mass% MgO) + 437000∕T − 224.3The contributing editor for this article was Sharif Jahanshahi.