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A 'cenosphere' is a lightweight, inert, hollow sphere filled with inert air or gas. The color of ''cenospheres'' varies from gray to almost white and their density is about 0.4–0.8 g/cm³, which gives them a great buoyancy. They are mainly produced as a byproduct of burning coal in thermal power plants.
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 then other types of foam materials. Such composite materials are called syntactic foam. Aluminum based syntactic foams are finding applications in the automotive sector.

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Production

The process of burning coal in thermal power plants produces fly ash containing ceramic particles made largely of alumina and silica. These particles form a part of the fly ash produced in the burning process. They are produced at the high temperature of 1,500 to 1,750 degrees Celsius through complicated chemical and physical transformation. Their chemical composition and structure varies considerably depending on the composition of coal that generated them.
The ceramic particles in fly ash have three types of structures. The first type of particles are solid and are called precipitator. The second type of particles are hollow and are called cenospheres. The third type of particles are called plerospheres, which are hollow particles of large diameter filled with smaller size precipitator and cenospheres. Due to the hollow structure cenosphres have low density.