Abstract: In producing an additive for a building material, lightweight granules (e.g., polystyrene beads) are first coated with a waterproof adhesive by mixing in a suitable mixer. When the granules are satisfactorily coated, they are further coated with a wet cementitious material, for example aggregate and wet cement, or with wet plaster, which is added to the mixture. Only sufficient moisture is added to allow the cement or plaster to initiate the chemical reaction which results in the curing of the coating material on the granules. The wet material acts to separate the granules while forming a coating thereon. The granules are then removed from the mixture and allowed to dry and set. The dry granules may then be added to a conventional concrete or plaster mix.