Properties and microstructure of lightweight aggregate produced from sintered sewage sludge ash

The properties of lightweight aggregate (LWA) manufactured from the ash produced by fluidised bed incineration of sewage sludge have been investigated. The ash was mixed with a clay binder, formed into approximately spherical pellets and rapidly sintered in a rotary tube furnace at temperatures between 1020 and 1080 °C. Selected physical properties of sintered ash pellets relevant to use as lightweight aggregate have been determined, including density (apparent specific gravity), water absorption and compressive (crushing) strength. These have been compared to the properties of a commercially available lightweight aggregate (Lytag). Sewage sludge ash (SSA) pellets sintered over a range of temperatures were found to have lower densities than Lytag and low water absorption when fired between approximately 1050 and 1080 °C. Individual pellet strengths were comparable to Lytag. Major crystalline phases present in both as-received and sintered sewage sludge ash were quartz (SiO2), the calcium magnesium phosphate mineral whitlockite (Ca7Mg2P6O24) and hematite (Fe2O3). Manufacturing lightweight aggregate from sewage sludge ash may become increasingly viable as landfill disposal costs continue to increase and the costs of alternative natural aggregates obtained from increasingly distant locations also increase. Sewage sludge incinerators tend to be situated in urban areas, close to where construction activity is occurring. The results indicate the potential for manufacturing high quality lightweight aggregate from the sterile, inert ash produced by sewage sludge incineration, using relatively simple processing and low temperature sintering.