Effects of a two-step heating process on the properties of lightweight aggregate prepared with sewage sludge and saline clay

This paper investigates the effect of a two-stage heating process on the properties of lightweight aggregate (LWA) prepared with sewage sludge and saline clay. Initially, pyrolysis of sewage sludge and pelletized pellets was studied and it was found that the preheating temperature should be controlled between 400 and 600°C in order to decompose the contaminant organic compounds and ensure that sufficient gases evolve during the sintering process for bloating of the internal structure of LWA. A total of 11 different preheating (400-600°C) and sintering (1000-1150°C) temperatures with varying holding times were studied and compared. The preheating test results demonstrated that the crushing strength of LWA is mainly influenced by temperature rather than holding time. As for the sintering process, an 1150°C sintering temperature was a critical point to produce sufficient glassy phases and vitrified surface of LWA. Increasing the sintering temperature could enhance the crushing strength and density and resulted in a less permeable structure for water absorption. A shorter time (i.e. 5 min) of sintering is likely to provide better crushing strength; however, longer holding times could significantly reduce water absorption. The overall results suggested that the preheating temperature should be set at 400°C for 30 min followed by sintering at 1150°C for 15 min in order to produce lightweight aggregate with low bulk density (0.66 g/cm³), water absorption (1.06%) and comparable high strength (11.1 MPa).