A composition comprising component A and component B wherein component A comprises: - 50-80 parts gypsum, - 20-99 parts of an alkaline polymer resin, and - 0.1-2.0 parts of a hydration inhibitor, based on the total weight of component A and wherein component B comprises: - 20-99 parts of an acidic polymer resin, - 30-50 parts of a filler, of the kind such as herein before described and - 0.1-6.0 parts of an activator, based on the total weight of component B.