Abstract: The present invention relates to copolymers consisting of (a) 1-99% by weight of structural units of the formula (1)  where R1 is hydrogen or methyl, R4 is C1-C22-alkylene, R5 is C1-C22-alkyl or hydrogen, and X is ammonium, lithium, sodium, potassium, an amine or a mixture of these substances, and (b) 99-1% by weight of structural units of the formula (2)  where R1 is hydrogen or methyl, R2 and R3, independently of one another, are hydrogen, or C2-C22-alkyl, wherein component b) is not hydrolysed and the copolymers have been prepared by a solution precipitation polymerization in a nonaqueous solvent or a water-miscible, organic solvent having a low water content which is a precipitant for the copolymer. The invention furthermore relates to a process for cementing subterranean wells using a cement slurry comprising the following components: a) water of various salinity cement a novel copolymer in a concentration of 0.01-5% bwoc, preferably from 0.05 to 0.