A mixture Ia which comprises a composition IIa consisting of a) 1 to 95% by weight of a solid III, preferably a basic solid III, having a primary particle size of 5 nm to 20 microns, and b) 5 to 99% by weight of a polymeric mass IV obtainable by polymerizing b1) 5 to 100% by weight, based on the mass IV of a condensation product V of .alpha.) at least one compound VI which is capable to react with a carboxylic acid or a sulfonic acid or a derivative thereof or a mixture of two or more thereof, and .beta.) at least one mole per mole of the compound VI of a carboxylic acid or a sulfonic acid VII which exhibits at least one radically polymerizable functional group, or a derivative thereof or a mixture of two or more thereof and b2) 0 to 95% by weight, based on the mass IV, of a further compound VIII having an average molecular weight (number average) of at least 5000 and having polyether segments in the main or side chain, wherein the proportion by weight of the composition IIa in the mixture Ia is 1 to 100% by weight.