Abstract: A lightweight aggregate comprising an alkali metal borosilicoaluminate, is in the form of discrete, porous and expanded particles of approximately spherical shape, has a specific weight between about 60 and about 600 kg/m.sup.3 and a compression strength of from about 140 kg/cm.sup.2 up to about 250 kg/cm.sup.2, is highly insoluble in water, steam, acids or bases, has a melting temperature above 1500.degree. C. and a low alkalinity, and chemically comprises a homogeneous admixture of acid, basic and amphoteric oxides.A highly preferred embodiment of the aggregate has an M.sub.2 O/B.sub.2 O.sub.3 ratio between about 3:1 to about 6:1, where M is an alkali metal.

Abstract: A porous lightweight particulate aggregate for use in concretes, mortars, building blocks and other building elements, comprises expanded and vitrified low alkalinity particles of a material composed of an alkali metal silicate having an M.sub.2 O/SiO.sub.2 ratio of from about 1:2 to 1:4 wherein M is an alkali metal; silica in an amount of from about 0.1 to about 50% by weight of said alkali metal silicate; and an alkaline earth metal silicate in an amount of from 0.1 to 200% by weight of said alkali metal silicate, said particles having a density of not more than about 60 g/dm.sup.3 and a compression strength of up to about 275 kg/cm.sup.2 and having an expansion ratio of up to 15 times their original collapsed volume. Suitable fillers and additives may be added to said aggregate to provide specific and useful properties for a variety of uses thereof.