Fire-Clay Or Refractory Clay
Fire-clay is a term, loosely applied, to include those sedimentary or residual clays which vitrify at a very high temperature and which, when so burnt, possess great resistance to heat.
These are pure hydrated silicates of alumina and contain a large proportion of silica 55-75%, alumina 20-35%, iron oxide 2-5% with about 1 per cent of lime, magnesia and alkalis. The greater the percentage of alumina, the more refractory the clay will be. Fire clays are capable of resisting very high temperatures up to 1700 o C without melting or softening and resist spalling. The presence of a small percentage of lime and magnesia and alkalis help to melt the clay particles more firmly, whereas a large percentage of lime and magnesia tend to melt the clay at low temperatures. Iron oxide or other alkalis reduce refractory qualities of fire clay. The fire clay is used for manufacturing fire bricks used in furnance linings, hollow tiles, and crucibles.
Application Of Clay Products
Universal availability of raw materials, comparative simplicity of manufacture and excellent durability of ceramic materials have put them in the forefront among other constructional materials. The high strength and durability of clay products underlie their wide use in the various elements of buildings, such as walls, wall and floor facing materials, lining materials for chemical industry apparatus, chimney, light porous aggregates for roofing, and sewer pipes. The various applications of clay products in the building industry are as follows.
1. Wall materials. The examples are common clay brick, perforated clay brick, porous and perforated stiff-mud brick, hollow clay dry-press brick. Perforated plastic moulded ceramic stones and light weight building brick. Clay brick accounts for half of the total output of wall materials. Structural properties of hollow clay products and low heat losses through air-filled voids (particularly at subzero temperatures) provide great possibilities for reducing the thickness and the weight of exterior walls. Ceramic facing tiles remain the chief finishing material for sanitary and many other purposes and are still in great use for external facing of buildings.
Brick for special purposes. The example are curved clay brick, stones for sewage installations (underground sewer pipes) brick for road surface (clinker).
3. Hollow clay products for floors. The examples are stones for close-ribbed floors (prefabricated or monolithic), stones for reinforced ceramic beams, sub flooring stones (fillers between beams).
4. Facade decoration. The examples are glazed or non-glazed varieties subdivided in to facing brick and ceramic stones, floor ceramics, small-size ceramic tiles, ceramic plates for facades and window-sill drip stones.
5. Clay products for interior decoration. The examples are tiles for facing walls, built-in parts, large floor tiles and mosaic floor tiles.
6. Roof materials. The examples are common clay roof tiles for covering slopes of roofs, ridge tiles for covering ridges and ribs, valley tiles for covering valleys, end tiles ("halves" and "jambs") for closing row of tiles, special tiles.
7. Acid-resistant lining items. The examples are common acid-resistant brick, acid-resistant and heat-and-acid-resistant ceramic shaped tiles for special purposes, ceramic acid-resistant pipes and companion shapes.
8. Sanitary clay items. Sanitary ware items are manufactured mainly form white-burning refractory clay, kaolins, quartz and feldspar. There are three groups of sanitary ceramics: faience, semi-porcelain and porcelain, which differ in degree of caking and, as a consequence, in porosity. Items from faience have a porous shell, and items from porcelain, a solid shell, while those from semi-porcelain are of intermediate densities. The various degrees of caking of faience, porcelain and semi-porcelain, made of the same raw materials, are due to the latter's different proportions in the working mass.
Solid faience is used mainly to manufacture toilet bowls, wash basins, toilet tanks and bath tubs. Items are glazed, since unglazed faience is water permeable. Semi-porcelain items feature excellent hygienic and mechanical properties being intermediate between those of faience and porcelain. Porcelain outer shell is impervious to water and gases and possesses high mechanical strength and resistance to heat and chemical agent. Porcelain is used to manufacture insulators for power transmission lines, chemical laboratory vessels, etc.
9. Aggregate for concrete. Creamiste (manufactured from low-heat clay), a light weight porous material forms excellent aggregate for light weight concrete.
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