The largest share of input raw materials in the foundry is silica sand, which constitutes more than 90% of the molding mixture required for the production of molds necessary for castings production. Almost all silica sand that enters the foundry ends its material cycle in a landfill, either after a single use (molding mixture with an organic binder) or after a certain period of time. Used foundry sand represents more than 80–90% of the waste produced by the foundry. Disposal of this waste requires relatively high costs. Although used silica sand with bentonite binder does not represent a significant burden on the environment, its quantity is very large, which is why there is an effort to use this material in other industrial sectors. The article compares 3 types of sands: high-quality new foundry sand, sand intended for concrete production, and used foundry sand, which is a mixture of silica sand, binder (bentonite), and carbonaceous substances that has lost its properties important for the production of molds. Their chemical composition, granulometry and morphology were compared. Cement, concrete sand and the used foundry sand were used in the production of concrete, in which the concrete sand was gradually replaced in the concrete mixture by the used foundry sand. Subsequently, the properties of the concrete prepared in this way (compressive strength, flexural strength and Schmidt strength) were monitored. The results obtained show that the replacement of concrete sand with the used foundry sand reduces the compressive strength of concrete, but the replacement of sand with the used foundry sand up to 15% maintains the required strength of concrete. The replacement of up to 30% of concrete sand did not have a negative impact on the flexural strength of concrete.