Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its light weight after processing.
When it reaches temperatures of 850–900 °C, perlite softens (since it is a glass). Water trapped in the structure of the material vapourises and escapes and this causes the expansion of the material to 7–16 times its original volume. The expanded material is a brilliant white, due to the reflectivity of the trapped bubbles.
Unexpanded (“raw”) perlite bulk density: around 1100 kg/m³ (1.1 g/cm³).
Typical expanded perlite bulk density: 30–150 kg/m³
Due to its low density and relatively low price, many commercial applications for perlite have developed.
In horticultural applications, perlite is used throughout the world as a component of soilless growing mixes where it provides aeration and optimum moisture retention for superior plant growth. For rooting cuttings, 100% perlite is used. Studies have shown that outstanding yields are achieved with perlite hydroponic systems. Other benefits of horticultural perlite are its neutral pH and the fact that it is sterile and weed-free. In addition, its light weight makes it ideal for use in container growing. Other horticultural applications for perlite are as a carrier for fertilizer, herbicides and pesticides and for pelletizing seed. Horticultural perlite is as useful to the home gardener as it is to the commercial grower. It is used with equal success in greenhouse growing, landscaping applications and in the home in house plants.
Perlite is also used in foundries, cryogenic insulation, as a lightweight aggregate in mortar (firestop) and in ceramics as a clay additive.
Perlite is an excellent filter aid. It is used extensively in Australia and New Zealand as an alternative to diatomaceous earth. The popularity of Perlite usage in this application is growing considerably worldwide.
Typical analysis of perlite
- 70-75% silicon dioxide: SiO2
- 12-15% aluminium oxide: Al2O3
- 3-4% sodium oxide: Na2O
- 3-5% potassium oxide: K2O
- 0.5-2% iron oxide: Fe2O3
- 0.2-0.7% magnesium oxide: MgO
- 0.5-1.5% calcium oxide: CaO
- 3-5% loss on ignition (chemical / combined water)
