This invention relates to a process of a lightweight, heat insulating, high compressive strength, water insoluble material by heat treatment at temperatures between 650-950.degree. C., possessing a low bulk density (from 300 to 800 kg/m.sup.3) and improved heat insulating capacity (.lamda.=0.07 to 0.25 W/mdeg.) when it has been used to form an article having a compressive strength of from 0.49 to 5.59 Mpa. The starting chemical composition comprising more than 90% by weight of either perlite, pumice or pumice related group of materials or mixture of them and an additive in quantities less than 10% by weight containing Na.sub.2O and B.sub.2O.sub.3.