Abstract: The present invention relates to a method for improving the water separation and compression strength of cement mortar and concrete, and for extending the time taken for the cement mortar and concrete to stiffen, by adding to the mortar and concrete a waste sludge obtained by neutralizing with alkali mother liquor and optionally also contaminated gas-washing liquors obtained when producing aluminum fluoride from aluminum hydroxide and hexafluorosilicic acid. The waste sludge is added to the cement mortar and concrete in quantitites corresponding to less than 10% of a prescribed cement weight, preferably 1-5%.