The demand for quartz sand, which is used in the construction industry, but also in other branches of industry, e.g., in foundries, where it is used for the production of casting molds, is constantly increasing. On the other hand, the rehabilitation of depleted open sand pits requires a large amount of material. To avoid the use of virgin material for remediation, the use of a geotechnical composite material produced by mixing of non-hazardous waste foundry slag and foundry dust with clay residues from sand extraction and processing was investigated. Studies have shown that an environmentally acceptable foundry waste based geotechnical composite with suitable geotechnical properties can be produced and used as an alternative to natural clay or sand in the remediation of open sand pits. In this way, waste from the foundry industry is minimized and natural resources are preserved.