A Geochemical and Agronomic Evaluation of Technosols Made from Construction and Demolition Fines Mixed with Green Waste Compost

Construction and demolition fines (C&D-fines) and green waste compost (GWC) are two commonly generated urban waste materials that represent repositories of geochemical value. Here technosols were produced from volumetric mixtures of these materials ranging from 0–100% C&D-fines, with the remaining proportion comprised of GWC. Agronomic assessment was carried out by way of pot and rhizobox plant growth experiments with ryegrass, barley and pea to determine germination, plant mass and rooting behaviours. Geochemical and mineralogical evaluation was achieved by soil pore water solution measurements combined with X-ray powder diffraction analyses respectively, to characterise the technosols and their distinct deviations from a reference agricultural geogenic soil (soil). The results demonstrated that germination, growth and root mass/surface area of vegetation were up to 80-fold greater after 30-days in the technosol composed of equal volumes of the two materials (50% C&D-fines: 50% GWC) compared to the soil. High concentrations of Ca and Mg in pore waters (550–800 mg·L−1) were dominant features of the technosols, in contrast to the soil (<50 mg·L−1), resulting from gypsum and calcite enrichment of the C&D-fines. In contrast, the GWC represented a source of soluble K (450–1000·mg·L−1). Highly elevated Ca concentrations in extended leaching tests of the C&D-fines reflected ongoing gypsum dissolution, whereas soluble Mg and K were rapidly depleted from the GWC. In summary, short-term performance of the technosols as plant growth substrates was strong despite their geochemical and mineralogical distinction from soil. Gleaning additional geochemical value from combining urban wastes in this way is potentially suited to myriad scenarios where geogenic soils are contaminated, sealed or otherwise absent. Further assessment will now be needed to determine the geochemical longevity of the technosols before wider scale applications can be recommended.