Abstract: Upper and lower plates, as well as other parts, for a slide gate valve are formed by casting into a mold, preferably a metal mold, a castable refractory composition containing from 1 to 5% of a long chain glassy polyphosphate, preferably one having an average chain length of about 20 phosphorous atoms, from 0.1 to 0.5% boric acid, the balance of the castable being sized periclase refractory grain.

Abstract: A magnesium aluminate spinel-bonded refractory can be formed which does not exhibit undue expansion (due to the reaction of magnesia and alumina to form magnesium aluminate spinel) between the raw, compacted state and the fired, ceramically bonded state when fired to a temperature of 1400.degree. C if a very finely divided alumina with dense particles is used. More specifically, the alumina used has an average particle size of less than 5, preferably less than 2, microns, and a specific surface of less than 30 m.sup.2 /g, preferably about 5 m.sup.2 /g (i.e., the small crystallites or particles are dense and do not have a high surface area, as do finely divided active aluminas).