Steel slag was reconstructed by using lime as the single controlling material at a high temperature,and XRD,petrographic analysis and SEM techniques were adopted to determine the mineral composition of the modified steel slag as well as the transformation of the RO phase in the modification process.Moreover,the stability and the activity index of the modified steel slag were tested.The results demonstrate that (1) the RO phase primarily reacts with CaO,in which FeO phase is decomposed into Fe₃O₄ ,CF (calcium ferrite),C₂F (dicalcium ferrite), Ca₂ (AlFe)O₅ and C₄ AF (tetracalcium alumino-ferrite) successively with the increase of CaO content,and a part of MgO phase exists in the form of MgO crystal,and the others behave as the solid solution in the silicate and liquid phases; (2) when the CaO/SiO₂ mole ratio is not large enough,C₃ S (tricalcium silicate) in the original steel slag is resolved into C₂ S (dicalcium silicate) in the modification process; (3) no C₃ S generates even when the CaO/SiO₂ mole ratio reaches 3 because CaO primarily reacts with high-content iron; (4) only when the iron reacts completely,may the redundant CaO react with C₂ S to produce C₃ S; and (5) both the stability and the cementitious activity of the steel slag significantly improve after the modification by lime.