Foam glass
Foam glass or expanded glass is a porous glass foam material. It is used as a light weight, moisture- and fireproof building material with thermal and acoustic insulating properties.
It is made by heating a mixture of crushed or granulated glass and a blowing agent (chemical foaming agent), often carbon or carbonates such as limestone. Near the melting point of the glass, the blowing agent releases a gas, producing a foaming effect in the glass. After cooling the mixture hardens into a rigid material with gas-filled closed-cell pores comprising a large portion of its volume. Foam glass gravel is produced by letting the glass mass fracture during the cooling process. Often recycled glass, sometimes from disused CRTs, is used as a base material. Igneous rock such as obsidian and industrial waste slag may also be used.
While the term porous glass often indicates glass with pores in the nanometre- or micrometre-range the pore size of foam glass is usually within 0.5 to 5 mm, and the pores make up 80%~90% of the total volume.
Chemical foaming agents facilitate the release of the gaseous phase upon heat treatment. In general, these additives are either a) redox and neutralization agents, or b) decomposing agents. Redox and neutralization agents include nonoxide materials, e.g. carbides or nitrides. Decomposing agents include sulfates, e.g. CaSO4•nH2O, organic compounds, and carbonates, e.g. CaCO3. These materials release gas following decomposition and/or burning.
Expanded glass is widely used in the building industry and for other industrial insulation applications as well as a filler in composite materials.