The present invention relates to a process for preparing, starting from a slag material, a filler for construction materials which contain bitumen or a hydraulic binding agent. The slag used to prepare the filler contains γ-dicalcium silicate. The process comprises the step of removing from the slag material a finer fraction formed by particles of a size between 0 and at least 0.75 mm so as to reduce the γ-dicalcium silicate content of the slag material; and the step of finely milling at least a portion of the remaining coarser fraction of the slag to obtain the filler. The invention also relates to the obtained filler, to the use thereof for preparing construction materials and to concrete or mortar compositions and asphalt compositions containing the filler. By removing the finer fraction from the slag material, a filler is obtained which has a considerably lower water content so that it becomes suitable for use in asphalt or concrete or mortar compositions, in particular in self-compacting concrete compositions.