This invention relates to an apparatus for sealing a passage through a concrete floor and about a conductor, such as an insert device for cables adapted to be positioned in structural openings, such as, for example in holes drilled into or formed in floors and other planar surfaces of structures such as buildings, to facilitate connection of power lines, cables and wires from a source into a cable receiving junction box at the underside of the floor and through the insert device and thence to a service head at the upper face of the floor, for connection of various accessory devices thereto, a fire retardant hollow member, comprised of water-containing alkali metal silicate, being provided to seal the device in the floor opening and to seal the cables therein, automatically foaming to effectuate the sealing action described, on occurrence of fire or excessive temperatures, to effectively so seal the assembly. The cables on such occurrence, are thus sealed in the insert device in air-tight, spaced relation to each other; spread of fire and excess heat therethrough will be automatically prevented, and the fire protection rating of the floor wherein the insert device is so mounted and used will be substantially equivalent to the original rating of the floor.