A method of preparing a hydrophobic mineral and/or filler material, for
example a gypsum material, comprises (i) preparing a hydrolyzate
containing polysiloxanes by hydrolyzing a hydrolysable silane or
hydrolysable silane mixture in the presence of an acid hydrolysis
catalyst; and (ii) combining the polysiloxane hydrolyzate with the
mineral and/or filler, and optionally water and/or a catalyst for
condensation of the hydrolysate.