Abstract: The invention relates to a hydroexpansible composite material, the method of preparing same and a composition for implementing this method, as well as to the uses thereof. The composite material includes a solid flat support having, on at least one of its faces, a coating with a high proportion of a hydroexpansible polymer powder fixed to the support by means of a hydrosoluble binder, the coating being essentially formed of:95 to 55% by weight of hydroexpansible polymer powder;5 to 23% by weight of hydrosoluble binder; and0 to 22% by weight of surfactant.This hydroexpansible material may advantageously be used in medicine, surgery agriculture and especially as sealing agent in cables, particularly electric cables.FIG. 2 shows how, in a cable, this material (11, 13) forms a sealing ring (13) about the point of infiltration of the water, thus preventing its longitudinal propagation.