System for spraying light insulation concrete, which comprises: - a container equipped with a tank and at least one endless screw feed rotatably mounted in the tank, the tank being provided with at least one primary outlet orifice in line with the endless screw and a secondary outlet orifice in line with the endless screw feed; - a compressor; - a hose made from rubber or any other pressure-resistant elastomer; - a spray lance equipped with ○ a gun, ○ a coupling to which the hose is connected in a fluidtight manner; ○ a pipe connecting the coupling to the gun; ○ a Venturi nozzle to which the compressor is connected by means of a hose, this Venturi nozzle being in the form of a constriction produced in the pipe alongside the gun and comprising a narrowed section followed by a splayed section through which the Venturi nozzle emerges in the gun, which has a diameter very much greater than that of the pipe; ○ a primary inlet manifold and a secondary inlet manifold both symmetrical with respect to the gun and emerging therein in line with the Venturi; - a primary feed pipe connecting the primary outlet orifice of the vessel to the primary inlet manifold of the lance; - a secondary inlet pipe connecting the secondary outlet orifice of the vessel to the secondary inlet manifold of the lance; - and wherein, downstream of the junction between the inlet manifolds ( 43A, 43B) and the gun, the latter comprises a section with a smaller diameter that opens into a section with a large diameter, the said section with a smaller diameter comprising a straight portion, and an angled portion that extends the straight portion and forms therewith an angle, said large-diameter section partially overlapping the smaller-diameter section, and having an angled external portion connected to said smaller-diameter section upstream of its mouth, so as to create, around the angled portion, a decompression chamber intended to cause turbulences in the flow of concrete in order to slow the latter down. - a container equipped with a tank and at least one endless screw feed rotatably mounted in the tank, the tank being provided with at least one primary outlet orifice in line with the endless screw and a secondary outlet orifice in line with the endless screw feed; - a compressor; - a hose made from rubber or any other pressure-resistant elastomer; - a spray lance equipped with ○ a gun, ○ a coupling to which the hose is connected in a fluidtight manner; ○ a pipe connecting the coupling to the gun; ○ a Venturi nozzle to which the compressor is connected by means of a hose, this Venturi nozzle being in the form of a constriction produced in the pipe alongside the gun and comprising a narrowed section followed by a splayed section through which the Venturi nozzle emerges in the gun, which has a diameter very much greater than that of the pipe; ○ a primary inlet manifold and a secondary inlet manifold both symmetrical with respect to the gun and emerging therein in line with the Venturi; ○ a gun, ○ a coupling to which the hose is connected in a fluidtight manner; ○ a pipe connecting the coupling to the gun; ○ a Venturi nozzle to which the compressor is connected by means of a hose, this Venturi nozzle being in the form of a constriction produced in the pipe alongside the gun and comprising a narrowed section followed by a splayed section through which the Venturi nozzle emerges in the gun, which has a diameter very much greater than that of the pipe; ○ a primary inlet manifold and a secondary inlet manifold both symmetrical with respect to the gun and emerging therein in line with the Venturi; - a primary feed pipe connecting the primary outlet orifice of the vessel to the primary inlet manifold of the lance; - a secondary inlet pipe connecting the secondary outlet orifice of the vessel to the secondary inlet manifold of the lance; - and wherein, downstream of the junction between the inlet manifolds ( 43A, 43B) and the gun, the latter comprises a section with a smaller diameter that opens into a section with a large diameter, the said section with a smaller diameter comprising a straight portion, and an angled portion that extends the straight portion and forms therewith an angle,