The study sets forth an experimental analysis aimed at evaluating the use of byproducts of industrial processes, in particular two types of sands coming from cast iron processing (called foundry or mold sands and sandblast sands), to partly replace the stone aggregates used to mix concretes going to build reinforced8 or prestressed concrete structures. The purpose is to wed advantages of an environmental nature, by reducing the amounts of material to be thrown away and raw materials used, with economic advantages, by diminishing the costs of waste disposal and of procuring natural aggregates. The results brought out that replacing natural fine aggregates with the byproduct coming from sandblasting operations (sandblast sand) provides the same starting workability as concrete mixed with natural aggregates only. This byproduct, added at 60 kg per cubic meter of concrete, provides superior workability over time relative to the reference concrete, with advantages in haul times and in pouring. Experimental tests also brought out however that replacing natural sands with waste materials coming from foundry forms (foundry sands) considerably increases the amount of water needed to achieve the same workability as the reference concrete. Furthermore, this byproduct involves a considerable loss of concrete workability during hauling, which advises against its use when mixing concretes going to build reinforced or prestressed concrete structures.