A method for treating a raw steel plant slag to transform it into a hydraulic binder at least equivalent to a Portland cement clinker, comprises the following steps: oxidizing treatment with input of oxygen, air or a mixture thereof at a pressure ranging between 1 to 15 bars, at a temperature ranging between 1650 and 1400 C., of raw steel slag containing, relative to the raw slag total weight, at least 45 wt. % of CaO and less than 30 wt. % of Fe₂O₃; and adding to the slag a lime source optionally with silica and/or alumina, so that the slag, after tranformation and at room temperature, has a propotion of Fe₂O₃ of less than 13 wt. % and a mineralogical composition comprising at least 40 wt. % in C₃S mineralogical phase and more than 10 wt. %, of calcium ferrite, relative to the final slag total weight.