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Calcined Kaolin (calcined clay)

For listing of Imerys Calcined Kaolin Products click here...


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.



Calcination occurs in two steps.

At 700°C the dehydroxylation of the kaolin is complete forming a poorly crystalline metakaolin. Products produced at this temperature include PoleStar™ 450  which can improve the dielectric properties of PVC cable insulation, and MetaStar™  which acts as a pozzolanic concrete additive.

At 980°C an amorphous defect spinel is formed which undergoes a transformation, with mullite recrystallising in an amorphous glass at temperatures above 1100°C. Products produced within this temperature range provide excellent properties in rubber compounds, especially when coated with silane, improving mechanical properties and chemical resistance. Silane treatment further enhances rigidity, toughness and dimensional stability in polyamide mouldings.


Markets for Calcined Kaolin

When formulated into film compounds Imerys calcined kaolin improves the thermal properties of agricultural films, giving the potential to reduce heating costs, reduce the planting to cropping time and increase the length of the growing season. In film that requires antiblock additive, calcined kaolin offers an ideal and cost effective balance of antiblock, haze and clarity performance.

In plastics the use of calcined kaolin is in PVC cable to improve electrical performance and in it's coated form as Polarite 102A, as a functional filler in engineering thermoplastics.


PoleStar™ 200P is a very effective opacifying extender in paint, partially replacing TiO2, reducing the overall cost of paint formulations but maintaining paint film quality.


IMERYS offers a range of high performance calcined kaolins from its production plants in the UK and USA.


Quick View

Mineralogy: Metakaolin or amorphous, aluminium silicate (defect spinel)
Particle Shape: Irregular, with surface voids
Specific Gravity: 2.6
Refractive Index: 1.56
MOH Hardness: 4-5
Moisture: (max %) 0.5
pH: 6-8




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©Imerys Minerals Ltd. 2012 | Registered office: Par Moor Centre - Par Moor Road - Par - Cornwall - PL24 2SQ UK | Company No: 269255