# Application of Foundry Wastes in Eco-Efficient Construction Materials: A Review
## 3 Overview of the Foundry Industry
### Foundry Industry Waste Management  
The increasing volume and complexity of waste from the modern economy are a significant threat to ecosystems and human health. Improper waste management, which includes inadequate collection systems and ineffective disposal, contributes to air, water, and soil pollution. The existence of open and unsanitary landfills intensifies the pollution of drinking water and facilitates the spread of infections and diseases, and the dispersion of waste negatively affects ecosystems. Moreover, hazardous substances from industrial waste pose risks to the health of urban populations and the environment [3]. The foundry industry can generate various types of waste. The origin of each type of waste is associated with distinct industrial practices, where different industries generate varying quantities of each residual material. A significant portion of this waste is composed of foundry sand, originating from the foundry molds and the slag resulting from iron ore smelting [35]. Although industries drive progress, their operations often lead to pollution, resource depletion, and significant waste production. Despite advances, the foundry industry, especially sand foundries, has remained behind in adopting more eco-friendly practices. Sand foundries not only release atmospheric pollutants such as dust and fumes, but also generate large quantities of sand as waste. The proper disposal of this sand is an issue that requires immediate attention and the development of sustainable solutions [36]. Since the early 1980s to the present day, industrial residual materials have been deposited in designated areas without the separation of different categories of waste. Recently, there has been a growing interest in the recovery of molding sands and scrap iron from landfills, due to their commercial value [37]. The foundry industry relies on high-quality silica sand for molding and casting, which results in valuable sand-based byproducts. This sand surpasses the quality of common or natural sand. Two predominant types of sand used in the foundry process, green sand (used in about 85% of foundries) and chemically bonded sand (a mixture of silica sand and chemical binders), influence the physical and chemical properties of foundry sand. These properties are also affected by various other factors, such as the type of casting process, the type of metal poured, the furnaces used, and the technology employed [38; 39]. In foundries, sand is recycled and reused multiple times, being utilized repeatedly until it no longer conforms to the necessary chemical characteristics, such as the presence of contaminants, and physical characteristics, such as grain size and shape. When the sand reaches the end of its reuse cycle, it is removed from the process and called waste foundry sand (WFS) [40; 41; 42]. Figure 4 illustrates the sand casting process. Figure 3: Investment casting process. The type of metal being cast is the primary factor affecting the chemical composition of WFS, which is predominantly composed of silica sand and coated with a thin layer of burnt carbon, residual binders (such as bentonite, sea coal, resins, or chemicals), and dust. The major oxides present in WFS include SiO\({}_{2}\) (over 80%), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\). Regarding heavy metals, chromium, nickel, and zinc are the most commonly detected elements in WFS [42]. In terms of physical properties, the particle shape of WFS varies from sub-angular to round, and its color is black [43]. The fineness modulus of WFS varies between 0.9 and 1.6, compared to 2.3 to 3.1 for standard sand [44; 45]. Additionally, the grain size distribution of WFS is very uniform. Regarding durability, WFS has demonstrated good performance [45]. The sand recovery process begins with the removal of external materials, followed by the disintegration of sand clumps and the removal of organic and inorganic binders through friction or thermal methods.