Cenosphere

A cenosphere is a lightweight, inert, hollow sphere made largely of silica and alumina and filled with air or inert gas, typically produced as a byproduct of coal combustion 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. Cf. glass microspheres.

Cenospheres are hard and rigid, light, waterproof, innoxious, and insulative. This makes them highly useful in a variety of products, notably fillers. Cenospheres are now used as fillers in cement to produce low-density concrete. Recently, some manufacturers have begun filling metals and polymers with cenospheres to make lightweight composite materials with higher strength than other types of foam materials. Such composite materials are called syntactic foam. Aluminum-based syntactic foams are finding applications in the automotive sector.

Silver-coated cenospheres are used in conductive coatings, tiles and fabrics. Another use is in conductive paints for antistatic coatings and electromagnetic shielding.

The word cenosphere is derived from two Greek words kenos (hollow, void) and sphaera (sphere), literally meaning "hollow spheres".

The process of burning coal in thermal power plants produces fly ash containing ceramic particles made largely of alumina and silica. They are produced at temperatures of 1,500 to 1,750 °C (2,730 to 3,180 °F) through complicated chemical and physical transformation. Their chemical composition and structure varies considerably depending on the composition of coal that generated them.

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