Abstract: Method for impregnating mineral building materials, with aqueous solutions of silanols and/or siloxanols is disclosed. The solution is obtained by continuous hydrolysis of alkoxysilanes and/or alkoxysiloxanes, of the general formula ##EQU1## wherein R.sup.1 =alkyl with 1 to 8 carbon atoms, at least 90% of the R.sup.1 groups in the average molecule being alkyl groups with 1 to 4 carbon atoms,R.sup.2 =methyl or ethyl,a=0 to 1.5,b=1.0 to 4.0,a+b=2.0 to 4.0in an amount corresponding to the expected consumption. The solution obtained is applied on the mineral building material within 3 to 30 minutes after the reactants are mixed. Suitable hydrolysis apparatus is also disclosed.