Abstract: A lightweight, thermally insulated block form for reinforced concrete construction, having voids extending between the top and bottom for the reception of concrete, is formed with a sheath of a non-combustible porous material, and a core of foamed plastic. The concrete receiving voids in the core are lined with circular cardboard tubes. In the finished construction the organically based plastic foam is enclosed in the inorganic non-combustible sheath and the surface of the sheath is sufficiently non-resilient so that when properly coated to seal and protect the surface it may act as a finished building surface. The cardboard tubes contain the concrete slurry so as to isolate the low tensile strength, low modulus of elasticity block against forces which might crack or distort the block.The block is formed by joining four rectangular slabs of the sheath material at their ends to form an open-ended rectangular tube.