The invention relates to copolymers which comprise, in copolymerized form, a1) 30 to 90% by weight of at least one monoethylenically unsaturated C3-C8-carboxylic acid or of an anhydride or water-soluble salt thereof, a2) 3 to 60% by weight of at least one monomer comprising sulfo groups, a3) 3 to 60% by weight of at least one nonionic monomer of the formula (I) H2C═C(R1)COO—[R2—O]o—R3  (I)  in which R1 is hydrogen or methyl, R2 are identical or different, linear or branched C2-C6-alkylene radicals which may be arranged in blocks or randomly, and R3 is hydrogen or a straight-chain or branched C1-C4-alkyl radical and o is a natural number from 3 to 50, a4) 0 to 30% by weight of one or more further ethylenically unsaturated monomers which are polymerizable with a1), a2) and a3), where the sum of a1), a2), a3) and a4) adds up to 100% by weight.