An apparatus for on-site manufacturing of lightweight concrete used for insulation in roof decks and roof systems of building structures. The apparatus is supported on a transportable trailer frame and includes a mixer, a concrete pump, and one or more tanks for holding a mixture of water and chemical products, such as foaming agents. The chemical agents/water solution is mixed with pressurized air according to a desired foam density and volume and is pumped through a foam cylinder ejector. The air/solution mixture exiting the ejector is directed into a carburetor which generates a foam product by mechanical impact of the air/solution mixture with beads contained in the carburetor. Concrete and the foam product are combined in the mixer, in accordance with a predetermined mixture ratio, and released into the concrete pump for forced discharge from the apparatus and application to a roof structure. A programmable logic controller controls the discharge rate and the amount of cement and foam product needed per batch according to the desired mixture ratio.