Williams Sinclair are the market leader in this grade of Perlite

Sinclair Perlite Standard 2.0mm – 5.0mm. ... The air trapped in the perlite is insulated from temperature variations and therefore helps stabilise the compost temperature. Sinclair Perlite has a neutral pH of between 6.5 – 7.5, is chemically and biologically stable and does not lock up or promote nutrient release.