Abstract: The unstable substances such as f-CaO,f-MgO and RO phases in steel slag are the main substances affecting its stability,which limits the large-scale resource utilization of steel slag.The direct research method indicated that a large number of f-CaO with spherical large size and concentrated distribution,exist in the medium or high basicity steel slag,and f-CaO content will be decreased and particle size will be reduced by reducing steel slag basicity and utilizing the treatment methods of gas quenching,hot stuffy and so on.The chemical synthesis research method incicated that Fe and Mn ions doping in CaO by chemical synthesis will increase the compact density of f-CaO and reduce its grain size,and Mn2+ has a stronger reduction effect on the f-CaO grain size.Further,status in the hydration characteristics of f-CaO in steel slag,the volume expansion rate of steel slag,and the corresponding mathematical model of f-CaO hydration expansion and the volume expansion evolution model of steel slag aggregate,and then the relationship between the autoclave pulverization rate of steel slag and the expansion force of single steel slag particle and so on are introduced.Then,the microstructure and hydration characteristics of f-MgO and RO phases in steel slag are reviewed,and the results indicated that the increase of steel slag basicity can promote the formation of magnesia,and will be beneficial to the MnO incorporating into MgO-FeO solid solution;besides,the hydration activity of RO phases is directly related to basicity and KM=nMgO/n(FeO+MnO),RO phases in steel slag are unstable,therefore,when KM≥1,RO phases may cause stability damage.In the end,the future research direction is pointed out.Such as,under the different basicity,treatment process,aging time and hydration degree of steel slag,the hydration rate of unstable substances in steel slag,ions transport process and the force conduction mechanism at the microstructure level;and then effect of hydration product ions of active minerals in steel slag on the hydration process of unstable substances and its relationship with expansion characteristics or long-term stability of steel slag;besides,the criteria standard and evaluation methods of steel slag stability and its relationship with long-term stability.

Key words: f-CaO, f-MgO, RO phase, microstructure, hydration and expansion properties