A plasticizer based on polycarboxylates is proposed, where the polycarboxylate component is a water-soluble, branched-chain and carboxyl-group-carrying copolymer which has at least one of the structural features selected from nitrogen-functionalized side chain and also, as linkage unit between main chain and side chain, an ester linkage, ether linkage and amide and/or imide linkage. The polycarboxylate component the molar ratio of acid groups to the side-group-carrying monomer is .gtoreq.2:1 and/or the side chain length n is <25, and the self-levelling screed has a sulphate content of .gtoreq.0.5% by weight, based on the total weight of the anhydrite fraction. The polycarboxylate component has a backbone provided with anionic functional groups and at least one non-ionic side chain has, preferably, an average molecular weight M.sub.n between 5,000 and 250,000 g/mol. A particularly good liquefying effect is achieved here using a plasticiser in powder form, which is present in an amount of from 0.1 to 5% by weight, based on the content of mineral constituents in the self-levelling screed.