This study investigates the characteristics of lightweight aggregates sintered from sewage sludge ash by modifying the proportion of the main components (SiO2-Al2O3-flux). The ash of incinerated sludge from a municipal sewage treatment plant (STP) was used as the tested material and sintering temperature ranged from 1050 to 1100 degrees C within a time span of 10-30 min. The sludge ash appeared to have a high proportion of SiO2 (44.89%) Al2O3 (11.62%) and Fe2O3 (6.81%) resembling the dilatable shale. When the sintering temperature was raised to above 1060 degrees C, the blowing phenomenon appeared. The aggregates become lighter in weight by prolonging the sintering time and raising the temperature. Cullet powder (amorphous SiO2), Al2O3, and fly ash were added to sludge ash to analyse the characteristic changes of the aggregates. The results showed that amorphous SiO2 lowered the melting point and increased foaming; Al2O3 raised the compression resistance; fly ash lowered the sintering temperature required. However, the composition of fly ash can vary dramatically, resulting in a less predictable characteristic of aggregates. (c) 2005 Elsevier B.V. All rights reserved.