The water-proofing of a gypsum based product formed from a plaster based compound consists of adding at least one compound incorporating a polysiloxane and at least one compound incorporating a pregelified starch. Independent claims are also included for: (1) the plaster based water-proofed compound containing a polysiloxane and a pregelified starch; (2) the preparation of the plaster based water- proofed compound; (3) a molded object, tile or plate made from the compound; (4) a product to be conjointly utilized in the water-proofing operation.
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