The invention relates to a building material composition which is suitable in particular for use as a bitumen-free sealant for sealing structures and contains a polymer dispersion or a corresponding redispersion powder, in particular based preferably on emulsion polymers which are redispersible in an aqueous medium, together with polystyrene particles, hollow ceramic and synthetic microspheres and, if appropriate, at least one binder and, if appropriate, further additives. The sealant according to the invention can in principle be formulated as a 1C or 2C application system. In addition to excellent sealing properties, as can conventionally be achieved only with bitumen-containing sealants, the bitumen-free building material composition according to the invention shows in particular excellent shrinkage behavior after application.