The invention relates to a process for the body-hydrophobization of substrates by means of organosilicon compounds (O) which are solid at 20°C and can be prepared by reacting one molar equivalent of silane (S) selected from among hydrocarbyltrihalosilane, hydrocarbyltrihydrocarbyloxysilane and mixtures thereof and partial hydrolysates thereof with polyhydroxy compounds (P) in such a ratio that from 0.3 to 1.3 molar equivalence of hydroxy radicals are present per molar equivalent of halo or hydrocarbyloxy radical. The invention further provides building material mixtures containing organosilicon compounds (O) and the substrates to be hydrophobicized.