Cenosphere is a lightweight, hollow sphere made largely of silica and alumina and filled with air or inert gas, typically produced as a coal combustion byproduct at thermal power plants. The color of cenospheres varies from gray to almost white and their density is about 0.4–0.8 g/cm3 (0.014–0.029 lb/cu in), which gives them a great buoyancy.
Cenospheres are hard and rigid, light, waterproof, innoxious, and insulative. This makes them highly useful in a variety of products, notably fillers.
Cenosphere characteristics: Fine particles, hollow, light weight, high strength, wear resistance, high temperature resistance, thermal insulation.