Silicate, Synthetic

any of the simple or complex salts of silicic or aluminosilicic acids. Synthetic silicates are constituents of, for example, cements, refractory materials, slags, common and silica brick, porcelain and faience, glass, glazes, enamels, adsorbents, and catalysts. The synthetic silicates formed in the process of heating a starting mixture, whose composition includes, for example, SiO₂, A1₂O₃, CaO, MgO, and K₂0, in many ways determine the properties of the final commercial product. Synthetic silicates can also be formed as a result of undesirable processes, for example, the corrosion of refractory materials by smelted metallurgical slag. Unlike natural silicates, synthetic silicates can be prepared as single-phase products free of impurities. Nearly all analogues of natural silicates have been synthesized, as well as a large number of silicates that do not occur in nature.

Synthetic silicates of alkali metals are relatively fusible and are freely soluble in water. Silicates of Na and K in the form of water, or soluble, glass are used in the manufacture of glue, paints, and various types of putty, as well as in soapmaking. The synthetic silicates of bivalent and especially trivalent metals exhibit high refractoriness and chemical stability. The calcium silicates 3CaO • SiO₂ and 2CaO•SiO₂ are the basic components of portland cement. Calcium hydrosilicates, formed upon hydration of cement as well as during the hydrothermal treatment of silica brick and silicate concrete products in autoclaves, determine the strength of hardening materials. Calcium meta-silicate, CaO•SiO₂, serves as a filler in paper manufacture. Magnesium orthosilicate, 2MgO•SiO₂, is a basic component of for-sterite refractory materials and is also present in magnesite refractory materials and slags. Magnesium metasilicate, MgO•SiO₂, forms the crystalline phase in electrically insulating steatite ceramics. The aluminum silicate mullite, 3Al₂O₃•2SiO₂, ensures strength at high temperatures and resistance to corrosion in grog and mullite refractory materials, which are widely used in metallurgy and glassmaking. This same silicate crystallizes to form extremely slender needles upon firing of porcelain products and serves as a special reinforcement for porcelain.

Aluminosilicates of lithium and magnesium, with low thermal expansion coefficients, are basic crystalline phases of ceramic materials that are stable to thermal shock. The ability of silicate melts to solidify in glass form without crystallization makes it possible to obtain glass for various purposes. However, conditions for glass crystallization are sometimes specially created during the production process; crystalline glass materials referred to as devitrified materials are obtained as a result.