Lightweight building material board

A process for manufacturing a lightweight building material board comprised of a core and facing sheets, comprising the steps of:

• forming a mixture comprising the following ingredients, by weight of the mixture;
• about 40% to 65% expanded siliceous inorganic particles having a density in the approximate range of 1.5 pcf to 4 pcf and being of a size in the approximate range of 100 to 1000 microns;
• about 5% to 20% inorganic binder;
• about 10% to 60% water;
• and
• about 0% to 10% binder setting agent;
• depositing a layer of the mixture on a moving lower web of facing sheet material, the amount of water in the mixture being such that the consistency of the mixture is that of slightly damp powder;
• contacting the side of the layer opposite the lower web with an upper moving web of facing sheet material to form a sandwich of the webs and the mixture, both webs being sufficiently porous to permit the passage of moisture therethrough and both webs moving in the same direction;
• continuously applying pressure in the range of 30 psi to 500 psi to the moving sandwich by means of press rolls to compact the expanded inorganic particles, a substantial number of the particles being broken by the pressure and a substantial number of the particles remaining intact;
• and
• heating the compacted mixture in the range of 150.degree. F. to 400.degree. F. for a period of 0.25 to 6.0 hours to drive off water from the mixture through the facing sheet material and set the inorganic binder to form a core, whereby a board comprising the facing sheets and the core is formed;
• the degree of compaction and heating being such that the dry density of the core is in the range of 10 pcf to 30 pcf.