Valorization of washing aggregate sludge and sewage sludge for lightweight aggregates production

Washing aggregate sludge (WAS) from a gravel pit and sewage sludge (SS) from a wastewater treatment plant (WWTP) were analysed in terms of their physical, mineral and chemical characteristics. Both waste materials were mixed, milled and made into granules, pre-heated for 2 and 5 min and sintered in a rotary kiln at temperatures between 1175 °C and 1275 °C for different dwell times ranging from 1 to 30 min. The effects the raw material characteristics, pre-heating and heating temperatures and dwell times had on loss on ignition (LOI), mineralogy, bloating index (BI), loose bulk density (ρb), apparent and dry particle density (ρa, ρd), water absorption after 24 h (WA24h) as well as the compressive strength (S) of the aggregates were all studied. The products obtained were classified as lightweight aggregates (LWAs) in accordance with the Standard EN-13055-1 (loose bulk density ⩽ 1.20 g cm−3 or particle density ⩽ 2.00 g cm−3). Their water absorption values were between 23.54% and 38.36% and compressive strength values between 1.23 MPa and 3.03 MPa. These properties were affected by the heating temperature, pre-firing and firing dwell times. The compressive strength values obtained were compared to those typically found for commercially available expanded clay aggregates (EXCAs) manufactured in Spain. According to these, it was possible to establish two groups of LWAs. The LWAs obtained in this research could potentially be used for the following commercial applications or for similar uses: horticulture, insulation, gardening, lightweight insulating concretes and/or prefabricated lightweight structures.