Ammonia-free polymer dispersions containing at least one copolymer which contains, as polymerized monomers, a) from 40 to 80% by weight of at least one ester of acrylic acid with C2-C12-alkanols, b) from 10 to 40% by weight of at least one vinylaromatic monomer and/or at least one ester of methacrylic acid with C1-C4-alkanols, c) from 2 to 15% by weight of at least one monoester of a C2-C10-alkanediol with acrylic acid or methacrylic acid, d) not more than 1% by weight of ethylenically unsaturated monomers which have at least one acid group, or salts thereof, e) less than 5% by weight of &agr;,&bgr;-ethylenically unsaturated nitrites and f) less than 1% by weight of amides of &agr;,&bgr;-ethylenically unsaturated mono- and dicarboxylic acids, and the polymer powders or polymer granules obtainable from the polymer dispersion, are used as additives for building materials based on hydraulic binders. The building materials based on hydraulic binders and binder formulations which contain these polymers and a process for improving the mechanical strength, elasticity and processibility of building materials based on hydraulic binders are also described.