Open time and creep resistance of hydraulically settable inorganic construction materials such as mortars and tile adhesives are both improved at the same time by the addition of a polymer mixture in the form of an aqueous dispersion or water-redispersible powder, containing one or more protective colloid-stabilized polymers having units derived from 20% to 30% by weight of vinyl chloride, 50% to 75% by weight of vinyl acetate, 5% to 20% by weight of ethylene, and one or more protective colloid-stabilized, vinyl halide-free polymers of one or more ethylenically unsaturated monomers.