Cenospheres

These hollow spheres are a naturally occurring byproduct of the burning process at coal-fired power plants. The spheres have a hard shell made largely of silica and alumina; they therefore have a low weight combined with a high compressive strength.

What are cenospheres used for:

Cenospheres are used as a structural lightweight filler in concrete and plastics. The spheres are mixed with resin to make lightweight syntactic foam used as a core material for sandwich panels, tooling blocks, and buoyancy foam. They are used to make heat-resistant refractory tiles and ceramic coatings with a low thermal conductivity. Metal-coated cenospheres are added to EMI shielding paints.

More information:

The spheres are sized between 5 and 500 microns with a loose bulk density of 0.35 to 0.48 g/cm³. They can withstand high temperatures between 1200 and 1400 °C and the thermal conductivity ranges from 0.10 to 0.18 W/m·K. A variety of standard grades with specific size distribution, density, color, and chemistry are available.