Abstract：Major mineralogical composition in steel slag including slow cooling slag, gas quenching slag and hot stew slag were analyzed respectively by X-ray diffraction. The microstructure of mineralogical phase in these three kinds of steel slag was observed through metallographic optical microscope, and their grain sizes with the same morphological characteristic phase were analyzed by parallel intercept method. The results showed that the main phases in these slags were all dicalcium silicate, RO phase and calcium ferrite, and a little tricalcium silicate, f-CaO and f-MgO because of their similar chemical composition. In these three steel slags, tricalcium silicate was black tabular crystal, dicalcium silicate was black round granular and dendritic crystal, calcium ferrite was gray amorphous state and continuously filled in other phases. The iron and magnesium phase mostly appeared white amorphous state and was continuously spread in blank phase and gray phase. The cooling intensity in high temperature segment had great influence on the grain size of typical minerals. The tendency was as follows: higher cooling intensity, smaller grain size. Therefore, the grain size was the largest in hot stew slag, the smallest in gas quenching slag, and moderate in slow cooling slag.
