Carbonation based leaching assessment of recycled concrete aggregates

Abstract

The leaching characteristics of seven different recycled concrete aggregates (RCA) samples derived from building demolition waste, concrete pavement, stockpiled, and freshly crushed concrete were evaluated focusing on the effects of carbonation, liquid-to-solid ratio (L/S), and particle sizes. Batch water leach test (WLT), toxicity characteristic leaching procedure (TCLP), and synthetic precipitation leaching procedure (SPLP) were performed to assess the pH, electrical conductivity (EC), alkalinity, and the leached concentrations of Ba, Ca, Cr, Mg, and SO4 in RCA effluent. The leaching efficiency of the test methods at different RCA carbonation levels was also evaluated. Results indicated that the effluent pH, EC, and alkalinity decreased, while the leached fractions of elements increased with an increase in L/S ratio. An increase in calcium carbonate content tended to increase the leaching of Mg, Cr, and SO4. For highly carbonated RCA samples, effluent pH, EC, alkalinity, and Ca concentrations were higher for particle sizes of 1.19 mm–0.149 mm, while fresher RCA samples resulted in higher values from particles finer than 0.149 mm. Carbonated RCA samples leached higher Ca, Mg, and Ba in TCLP, whereas the maximum concentrations of Cr and SO4 were found in WLT effluent. For less carbonated RCA samples, Ca concentrations in WLT and TCLP effluents were comparable, SPLP leached higher amounts of Mg, Ba concentrations were maximum in WLT, and TCLP concentrations of Cr and SO4 were the most critical ones. TCLP alkalinity increased, whereas WLT and SPLP alkalinity decreased with an increase in calcium carbonate content of the RCA.