The invention provides a method for preparing a gypsum composition containing a cross-linked polymer comprising the following steps: preparing a slurry of the gypsum composition by mixing the various constituents of the composition, or precursors thereof, with water, in a mixer; hydraulic setting; and drying and cross-linking in the presence of heat. The invention applies to the manufacture of gypsum boards. The invention further provides a composition of gypsum containing a cross-linked polymer, and further comprising a water resisting additive in an amount such that the water uptake is below 10%.