# Constructed Technosols: A Strategy toward a Circular Economy
## 4 Waste Materials in Technosols Design
### Waste Materials Employed as Structural Components  
To produce fertile Technosols, natural toposil is often used, but an alternative with a lower environmental impact could be suitable mining and building wastes from the city or the countryside. Among the recycled mineral materials, the most used are the wastes deriving from construction or demolition building activities (C&DW) (Table 1). Produced in large quantities, they represent about 30-40% of all wastes produced by countries belonging to the Organization for Economic Co-operation and Development (OECD) [85]. The European Union alone, in 2016, produced 374 million tons of C&D waste, excluding excavated soil [86]. Other structural materials have been used as soil matrix for constructed soils: terrestrial or glacially deposited sediments [63], aquatic sand and sediments from lakes, rivers or dams [87; 88], bricks [42; 47], concrete and demolition rubble [47], sand and recycled ferrihydrites [89], coal waste [90; 91], mining wastes [65; 69], ashes [92], recycled bentonite [93], crushed rock [94], usually with the addition of natural or native toposils [90; 93; 94]. The excavated material from deep soil horizons could be also exploited for this purpose, due to their great availability in every country. At the European level, it has been estimated that more than 500 million tons are produced every year [47], a part of which would definitely achieve the quality to be used in Technosol formulations. Excavated subsoils had already been successfully used in some projects [42; 44; 47; 62], even if not so extensively [95]. The reuse of wastes, incorporating them in constructed technosols, may also be useful to reduce the impact of residues such as mine tailings [65]. Mining wastes represent the 26% of the total wastes produced yearly in Europe [66], a part of which is contaminated while most of the residues is an inert material able to be used as structural material for the reclamation of the closed mines as proposed by [PERSON] et al. [29] or of degrades areas. Also the study by [PERSON] et al. [69] about the use of mine residues in addition to marble waste and biochar showed positive results for the reduction of metal mobility from the starting material to constructed soils. [PERSON] et al. found that coal mining wastes with high sulfur levels could be used in technosol formulations for the reclamation of mining areas [91]. Similarly, [PERSON] et al. [96] proposed the rehabilitation of mining sites through the combined use of constructed technosols and phytoremediation activities. The solution allowed the germination and growth of plant species whereas the content of contaminants observed in the plants was below hazardous levels. Regarding their agronomic properties, these wastes alone cannot be assimilated to a natural topsoil or a substrate for plant growth, even if they present a potential fertility [47]. Nevertheless, these recycled materials contribute to the agronomic properties of the constructed soil even at low use ratios [47; 91]. All the reported materials concur to soil structure and texture, water drainage, and nutrient release. Some specific wastes could also provide a consistent contribution of useful elements or substances. For example, steel mill slag, from industrial process of ore-smelting for cast iron, is widely used in Japan and Brazil agriculture not only as structural material but also as amendment. In fact it is a source of Ca, Mg, P, Fe, Mn, and Si and it is a corrective for soil acidity [97]. Also, Fe (hydro)oxides from mining and from goethite production have some use in improving the nutrient content as they can be used to coat sand particles. The result is a substrate whose nutrient retention capacity has been increased, as well as its capability to absorb and retain pollutants [89]. One of the aims of the study by [PERSON] et al. was to predict the main agronomic properties of a constructed soils using mathematical models starting from the properties of the constituting wastes [47].