The present invention relates to the use of epoxy derivatives which have been prepared by reacting a di-, tri- or tetraglycidyl compound (A) with an optionally unsaturated reactive component (B) consisting of a C.sub.8-C.sub.28-fatty acid, a C.sub.8-C.sub.28-alcohol or a secondary C.sub.8-C.sub.28-amine as additives to cementitious building materials. The epoxy derivatives proposed according to the invention are outstandingly suitable as compositions for preventing or suppressing efflorescence on surfaces of set cementitious building materials and/or for hydrophobing the corresponding cementitious systems. In addition, the cementitious products take up substantially less water owing to the additives proposed according to the invention, with the result that frost damage and rapid rusting of the reinforcing steel can be substantially reduced.