Abstract: Low density cementitious compositions are disclosed which are essentially the hydrated products from mixtures of inorganic hydratable binders such as gypsum and Portland cement with a novel lightweight aggregate material comprised of partially combusted bark particles obtained from bark-burning boiler furnaces of pulp processing concerns. Preferred compositions additionally incorporate conventional additives such as surfactants, fibrous materials, foam stabilizers and conventional lightweight aggregate such as expanded vermiculite to give products useful as lightweight thermal insulation concretes and fireproofings.