Characterization of CDW types by NIR spectroscopy: Towards an automatic selection of recycled aggregates

The petrographic and mechanical heterogeneity of construction and demolition waste (CDW) strongly affects the physic-mechanical performance of new concrete and mortar mixes (RAC: recycled aggregate concrete and RAM: recycled aggregate mortar) made with these recycled aggregates (RA). It is thus extremely important to quickly discriminate and sort them in groups of materials with homogeneous petrography, similarly to primary natural raw materials. Eighteen CDW samples, divided in six groups (natural stones, concrete, bricks, perforated bricks, roof tiles and tiles), from the Abruzzo region (central Italy) are analysed by both micro-X-ray fluorescence energy-dispersive spectroscopy (XRF-EDS) and hyperspectral imaging (HSI), in particular in the short wavelength infrared range (SWIR). Every SWIR spectrum is characterized by three regions of absorption at the following wavelength ranges: 1330–1680 (I), 1830–2140 (II) and 2140–2400 nm (III), displaying the lowest, highest and intermediate intensities, respectively. The first and second wavelength regions are linked to O–H and H–O–H motions, whereas the highest wavelength range is linked to atoms bonded to the O–H environment and/or to the carbonate group. According to previous analysis and to the micro-XRF-EDS surface chemistry, the amount of CaCO3 is rich to very rich for concrete and natural stones, intermediate to absent for masonries and absent for tiles. The intensity or area (%) of the absorption component vibrating at about 2345 nm is directly proportional to the amount of CaO (CaCO3 and CO2), and inversely to SiO2 and Al2O3. Therefore, SWIR can rapidly detect and separate CDW as a function of their carbonate to aluminosilicate contents, i.e. concrete and natural stones made with limestone vs masonries and tiles. The attained outcomes are of paramount importance either for routinely screening or in post-disaster circumstances.