Abstract: Lightweight aggregate for use in the building industry is produced by pelletizing or granulating pulverized fuel ash or other siliceous material with a binder comprising lime in an amount less than 5% by weight based on the weight of the mixture of lime and siliceous material and curing the pellets or granules at a temperature within the range of from 35.degree. C. to 100.degree. C. in an atmosphere saturated with water vapor. The amount of lime added as binder is preferably within the range of from 1 to 4.5% by weight and curing temperature is preferably within the range of from 50.degree. to 85.degree. C. Preferably the pelletizing process is also carried out at an elevated temperature within the range of from 35.degree. C. to 100.degree. C.