Technical and environmental properties of recycled aggregates produced from concrete sludge and excavation materials

This study presents a Re-Con Zero dry washing technology used to produce recycled aggregates from concrete sludge (RCZ) and combined as a feedstock in wet recycling excavation materials (EM) for unbound applications. A two-component admixture i.e, high water absorbing polymer and aluminum sulphate was used in producing RCZ. RCZ0, RCZ50 and RCZ100 were produced with EM within the range 0-100%. Analysis by the Los Angeles (LA) and micro-Deval (MD) test demonstrated RCZ to be of high-quality, hence a clear trend of improved performance as the content increased was observed in the LA and MD values. X-ray diffraction analysis performed on original and pulverized residues (<1.6 mm) showed that feldspar, quartz, phyllosilicates (mica and chlorite) and minor amounts of pyroxene (diopside) were present together with paste minerals such as portlandite and residual clinker, and calcite. Acid solubility results demonstrated the relation to increased paste content as particle size reduced. This observation was more prevalent in <1.6 mm RCZ fraction which further indicated the significant amount of that remained paste after wet treatment. Low levels of the content of chemical species of potential concern were in general found and were mostly complying to Norwegian regulations. Total Cr exceeded the criteria of 100 mg/kg, but the leachable Cr(VI) was not detected which showed that remaining Cr after recycling process was present mostly on the trivalent form. This indicated low leaching of Cr upon carbonation and a decrease in pH.