PURPOSE: To provide an element for a composite structure which shows a superb mechanical rigidity, light weight, a high heat resistance and a remarkable reactivity to fire by forming at least, one sheet fixed to both faces of a cellular structural core, of a mica reinforcing material.

CONSTITUTION: The structural element 1 is formed of a core 3 made of a sponge-like material such as a honeycomb material, for example, an aluminum or a polyurethane foamed material. The core 3 has two faces, each of which is covered with a sheet 5, 7 of mica paper. This mica paper is impregnated with an impregnation binder such as silicone, epoxy resin or polyamide. The sheet 5, 7 is fixed to the core 3 with a binder and an adhesive. Thus it is possible to obtain a highly rigid and very light finished product panel by constituting the panel as described. Besides, the whole panel is fire-resistant as the mica is nonflammable.