The invention relates to a volume-modified casting compound based on a curable epoxide resin or a mixture of such resins, comprising an epoxide resin, a curing agent, optionally an accelerator and further additives such as fillers, flexibilizers and dyes. Said casting compound contains (a) a three-dimensionally cross-linked polysiloxane in a dispersed form and with a particle size in the range of from 0.02 νm to 50 νm which optionally carries reactive groups capable of chemically reacting with the epoxide resin and/or the curing agent, (b) a selected linear or branched siloxane compound that carries reactive groups capable of chemically reacting with the epoxide resin and/or the curing agent, and optionally (c) a low-molecular oligomeric siloxane compound. The invention further relates to the use of the inventive casting compound as a construction material or as an insulating material, and to the shaped parts and insulations produced therefrom.