Abstract: A process and apparatus are disclosed for producing a cinder material useful as an aggregate in building materials such as building blocks or paving materials. The aggregate is produced from solid particles of waste, particularly bottom ash formed from the burning of coal in a power plant. Water is mixed as a vehicle with the bottom ash at the power plant to form a slurry, and the slurry is transported to a processing plant through a fluid conduit. At the processing plant the slurry is partly dewatered and excessive proportions of fine particles removed by passing the slurry through a baffled flume. Overside particles are removed from the slurry by passing it over a screen. The slurry is then fed into a screw classifier which separates out a solid particle component to provide a high quality cinder aggregate.