Abstract: The microstructure and content of f-CaO in slow cooling slag,gas quenching slag and hot closed treatment slag have been investigated through SEM and chemical analysis.The dispersed f-CaO in steel slag was mainly formed during the process of primary crystallization of C₂S and C₃S,formation of RO phase and decomposition of C₃S.Generally speaking,the quantity of dispersed f-CaO with smaller grain size was large,and its distribution was widely.The typical microstructure of f-CaO in slow cooling slag was gray spherical,thin film or strip shape.Dispersed f-CaO in gas quenching slag was gray thin film and strip shape,while which in hot closed treatment slag was strip shape.Cooling intension has significant influence on f-CaO content in steel slag.The f-CaO content in slow cooling slag undergone lower cooling intension was the highest and was 6.96%.The f-CaO content in hot closed treatment slag with higher cooling intension and hydrolysis was only 0.55%.While the f-CaO content in gas quenching slag was 2.28%.The f-CaO content in gas quenching steel slag decreased with the decline of particle size.Consequently,the comprehensive utilization of gas quenching steel slag could be improved through declining particle size of the solid slag.