Abstract: A water-resistant prefabricated structural element comprising a substrate based on cured plaster, said substrate being able to be obtained by at least the following steps: (a) homogeneously mixing a dry material, comprising mostly at least one hydratable calcium sulfate, a water-repellent agent comprising at least one silicone compound, and water and preforming said substrate in the wet state; (b) drying the preformed substrate in order to obtain said substrate formed in the solid and dry state; wherein the homogeneous mix, prior to the preforming of the substrate in the wet state, also includes a mineral additive comprising at least one mineral component having a synergistic water-repellency affinity with said silicone compound.