Steelmaking slags are usually used as roadbed and civil engineering materials. However, the
expansion phenomenon, which is caused by the volume expansion during hydration of free CaO and
free MgO contained in steelmaking slag, is indispensable. Since the hydration of free MgO in
steelmaking slag is much slower than that of free CaO, it is qualitatively considered that free MgO
contributes to the hydration expansion of steelmaking slag over a long period of time. Similar to free
CaO, free MgO consists of undissolved MgO and crystallised MgO in steelmaking slag. The latter
precipitates during slag cooling. When divalent metal oxides (FeO, MnO, CaO) are dissolved in the
crystallised MgO to form a MgO-based solid solution, it is expected to inhibit the hydration reaction
similar to precipitated CaO. From the previous hydration test of Magnesioferrite, which was calcined
after mixing MgO and Fe2O3 reagents, it was found that the higher the Fe2O3 concentration, the
slower the hydration reaction progressed. In this study, by clarifying the influence of calcination
conditions of MgO-FeO and MgO-FeO-MnO solid solutions, which were prepared by the calcination
of mixture of MgO, FeO and MnO reagents, on their hydration reactivity, the effect of composition
and thermal treatment on the formation of free MgO that is not hydrated (or easily hydrated) were
discussed. To investigate the influence of MgO grain size, coarse MgO particles, which were
obtained by crushing MgO crucible, were also used for heat treatment. With increasing the amount
of FeO and MnO in the MgO-FeO-MnO solid solution, the hydration reaction became more
suppressed. The expansion suppression effect was higher with solid-solved FeO than with solidsolved
MnO. When the calcination temperature of the MgO-FeO solid solution was increased, the
Fe3+/Fe2+ concentration ratio increased, resulting in a higher hydration rate.