Clay And Its Classifications
Purest clays consist mainly of kaolinite (2SiO2.Al2O3.2H2O) with small quantities of minerals such as quartz, mica, felspar, calcite, magnesite, etc. By their origin, clays are subdivided as residual and transported clays. Residual clays, known as Kaolin or China clay, are formed from the decay of underlying rocks and are used for making pottery.
The transported or sedimentary clays result from the action of weathering agencies. These are more disperse, contain impurities, and free from large particles of mother rocks.
On the basis of resistance to high temperatures (more than 1580°C), clays are classified as
refractory, high melting and low melting clays. The refractory clays are highly disperse and very
plastic.
These have high content of alumina and low content of impurities, such as Fe2O3, tending to lower the refractoriness. High melting clays have high refractoriness (1350–1580°C)
and contain small amount of impurities such as quartz, felspar, mica, calcium carbonate and
magnesium carbonate.
These are used for manufacturing facing bricks, floor tiles, sewer pipes,etc.
Low melting clays have refractoriness less than 1350°C and have varying compositions.
These are used to manufacture bricks, blocks, tiles, etc.
Admixtures are added to clay to improve its properties, if desired. Highly plastic clays which require mixing water up to 28 per cent, give high drying and burning shrinkage, call for addition
of lean admixtures or non-plastic substances such as quartz sand, chamottee, ash, etc. Items of lower bulk density and high porosity are obtained by addition of admixture that burn out.
The examples of burning out admixtures are sawdust, coal fines, pulverized coal. etc. Acid resistance items and facing tiles are manufactured from clay by addition of water-glass or alkalis.
Burning temperature of clay items can be reduced by blending clay with fluxes such as felspar, iron bearing ores, etc. Plasticity of moulding mass may be increased by adding surfactants such as sulphite-sodium vinasse (0.1–0.3%).
Thanking you*************************