Effect of solid-solved FeO and MnO on hydration of free MgO in steelmaking slag

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.