A technosol is an artificially created planting/structural medium from manmade materials, such as construction debris and compost. This article from Ecological Engineering journal says a mix of 20% “excavated deep horizons” (in layman’s terms, I think this is just dirt from construction sites), 70% crushed concrete, and 10% compost might work. If we truly want green cities, and we don’t want to reduce natural habitats to wastelands by harvesting materials from them to green our cities, this could be a good approach.