In a method for producing puzzolanic or hydraulic grinding additives for the cement industry from basic oxidic slags, in particular steel slags, using a metal bath for the reduction of metal oxides contained in the slags, the basicities of the liquid slags are adjusted to values lying 0.1 to 0.5 below the basicity value (CaO/SiO.sub.2) of the target slag prior to the reduction by adding acidic corrective substances such as, e.g., quartz sand and/or blast furnace slag and/or SiO.sub.2 -containing corrective substances.