Restoration with mining wastes and construction and demolition wastes to improve circular economy and soil functions

Circular economy is based on the re-use of wastes. However, it also implies the application of solutions that can help to reduce the environmental and climate change impacts. On the one hand, the preparation of materials for construction from mining activities generates wastes, for instance in marble and stone industry. On the other hand, the use of these materials in construction also generates wastes, construction and demolition wastes (CDW), which account for more than a third of all waste generated in the EU. In the case of the SE of Spain, stone preparation (cutting, polishing, etc…) produces fine and coarse wastes, mainly from carbonate rocks and secondary from granites. The large building activity produces residues in a wide variety of materials (concrete, bricks, wood, glass, metals and plastics). 
Mining restoration could be an opportunity to reuse those materials, as many mining holes are abandoned and need to be refilled before the preparation of a soil cover and landscape recovery.
In this work, it has been studied the restoration of an ancient mining hole (up to 10 meters depth and 21000 m²), situated in the SE of Spain under a BSk climate -Köppen and Geiger classification-, with marble wastes and CDW, and finally the formation of a Technosol (IUSS Working Group WRB) which can support a vegetation cover. Several actions were done as follow: refilling of the mining hole first with marble wastes (calcareous materials mainly of different sizes), the second was the addition of CDW and the third was the mixing of CDW with topsoil (approximately last 50 cm depth of surface restoration) forming the surface and conditioning the topography to reduce soil erosion. This action was accompanied by the construction of a central drainage stone filled canal (50 cm depth). After two years, Pinus halepensis vegetation was introduced (261 pine trees). Two years later, soil samples were taken (0-5 cm to measure bulk density and 0-20 cm depth to measure pH, electrical conductivity and organic matter) in eight selected points distributed equidistant in the restored area. 
The results showed that the soil had a slightly basic pH, but not homogeneous along the surface (from 8.13 to 8.89). In the case of the electrical conductivity, the variation between samples was greater (0.49 to 2.41 dS/m) and probably controlled by the lithology of the area, in the north close to Triassic formations with soluble gypsum formations and in the middle to the south to calcareous rocks. The organic matter (between 1.13 to 1.59 %) and the bulk density (1.40 to 1.47 g/cm³) were more homogeneous in the restored area. The survival of the pine trees was successful and more than 80%, considering no irrigation applied excepting the first month.
The use of this materials could be of interest to reduce the impact, favoured the circular economy and restoration of ancient mining areas, recovering the landscape and the soil functions.