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Kaolin

What is kaolin?

Kaolin rock

Kaolin, or ‘china clay’ as it is commonly called, is a hydrated aluminum silicate crystalline mineral formed over many millions of years by the hydrothermal decomposition of granite rocks. Hydrous kaolin is characterized by its fine particle size, plate-like or lamellar particle shape and chemical inertness. Metakaolin is manufactured by the calcination of kaolin to form an amorphous pozzolanic (define) white mineral additive for use in cement based products.

Calcined Kaolin is an anhydrous aluminium silicate produced by heating ultrafine natural kaolin to high temperatures in a kiln. The calcination process increases whiteness and hardness, improves electrical properties, and alters the size and shape of the kaolin particles.

Geology

Kaolin is formed when the anhydrous aluminum silicates found in feldspathic rocks, like granite, are altered by weathering or hydrothermal processes. The process which converted the hard granite into the soft matrix found in kaolin pits is known as ‘kaolinization’. The quartz and mica of the granite remain relatively unchanged whilst the feldspar is transformed into kaolinite. Smectite may also form in small quantities in some deposits. The refining and processing of the fine fraction of the kaolinized granite yields predominantly kaolinite with minor amounts of mica, feldspar, traces of quartz and, depending on the origin, organic substances and/or heavy minerals.

Physical properties

SEM photograph kaolin
  • High brightness
  • Non abrasive
  • Naturally fine particles
  • Refractory, remains white after calcination
  • Weak conductivity (heat and electricity)
  • Hydrophilic and easily dispersed in water
  • Personal care
  • Construction
  • Personal care

    Kaolin is prized in face creams, facemasks and make up where it provides superior sebum and impurities absorption and acts as a texturizing, matting and mattifying agent. It is an ideal natural opacifying agent in gels and shampoos.

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    Shampoo bottle
  • Construction

    In the construction industry, kaolin, metakaolin and calcined kaolin are used to improve the mechanical properties, porosity and appearance of various types of concrete and cement.

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For other markets please visit Imerys Kaolin or Imerys Ceramics.