The silicate industry

The silicate industry makes materials such as glass, cement, ceramics and concrete from natural silicates and silica. Ordinary glass is made by melting a mixture of sand (SiO₂), soda (Na₂CO₃) and limestone (CaCO₃): Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂↑, CaCO₃ + SiO₂ → CaSiO₃ + CO₂↑; the product is a mixture of sodium and calcium silicates with SiO₂, roughly Na₂O·CaO·6SiO₂. On cooling, glass thickens slowly, so it can be shaped in many ways. Replacing soda with potash (K₂CO₃) gives a hard-to-melt chemical glass, and adding PbO gives crystal glass; Co(II) oxide colours glass blue, Cr(III) oxide green and Cu(I) oxide red. For cement, limestone and clay are ground and fired in a rotary kiln at about 1450 °C; the resulting clinker is ground to cement, which sets when mixed with water, even under water. Sand and gravel mixed into cement give concrete, and steel reinforcement placed inside gives reinforced concrete. Ceramics (pottery, porcelain, faience, brick) are shaped from clay, kaolin and sand and fired; the surface is glazed to make it waterproof. Pottery has long flourished in Uzbek cities such as Samarkand, Bukhara, Khiva and Rishton, and kaolin is mined at Angren.