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Calcined kaolin - why use it?  
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Calcined kaolin - why use it? 

OK, the bug has bitten – the Soccer World Cup bug that is. I bought a Vuvuzela. If you don’t know what a Vuvuzela is, I predict you will know (and hate) the thing by the end of June.

Picture a long plastic trumpet that makes a terrible blaring noise. Then picture a full stadium of soccer fans all blowing it at the same time. If there ever was a time for earplugs, this will be it!

Things are getting crazy here in South Africa. Not only is World Cup Soccer fever mounting, the ports are at a standstill. This is due to a transport workers strike and it is getting a lot of us into a tizz. Factories may come to a stop without essential imports, not to mention all the World Cup stuff stuck in containers in the port. Stress levels are rising by the day….

So to keep your mind off it, let’s discuss calcined kaolin. What is it and why should we use it in our paints, paper, plastics, rubber and ceramics?

Naturally-occurring kaolin is known as “hydrous kaolin” because it contains water within its crystal lattice. This water cannot be removed by simply drying the kaolin.

Calcining a mineral is to heat it to a temperature where a phase transition occurs, usually the loss of a volatile component. In the case of kaolin it loses its waters of crystallisation.

If you only heat kaolin to 800ºC, you form an intermediate product called Metakaolin. Metakaolin is an interesting material in its own right. When you add it to concrete, it acts as a pozzolan. This means it enhances the strength of the concrete, a useful property in civil engineering and oil-well cementing.

To form calcined kaolin, however, you need to continue heating it all the way to 1100ºC. Various mineral phase changes occur, but what we need to know is that calcining causes the kaolin structure to collapse and become denser. This improves its opacity and its whiteness. The particles become sharp and hard, but also more porous because of the voids formed where the water molecules were (hydroxyl groups, to be correct). Flash calcining is a method where the kaolin is heated very quickly. This increases the number of voids compared to conventional methods of calcining.

These changes give calcined kaolin some useful properties in various applications:


Better opacity and whiteness make calcined kaolin a great extender for titanium dioxide. The hard calcined kaolin particles help to improve the structural strength and scrub-resistance of a coating. They also add corrosion-resistance and fire-resistant properties to the paint.  

The disadvantage is the cost. Calcination and the subsequent milling of the hard calcined particles both require a lot of energy. This makes calcined kaolin around three times more expensive than hydrous kaolin. However it is still less than half the price of titanium dioxide. So it remains a good option as an extender, particularly in paints above CPVC (Critical PiGMent Volume Concentration).


Good whiteness is useful in ceramics too. However the most useful property to the ceramist is the fact that calcined kaolin is already calcined. Hence it will have no plasticity and there will be no expansion or contraction of this material. It imparts mechanical strength plus a fine white surface texture.

It is easy to tell calcined kaolin from hydrous kaolin by looking at their data sheets. Just look at the LOI (Loss on Ignition) value. For water-washed kaolin it is around 13%, but for calcined kaolin it will be zero. The LOI test removes the waters of crystallisation and reports that mass loss as a percentage.

Rubber and plastics

Using calcined kaolin as a filler in rubber and plastics helps impart tensile strength. Hence it is a reinforcing filler instead of being non-reinforcing like hydrous kaolin. The fire-resistant property of calcined kaolin plus the fact that it is an electrical insulator is especially useful in the plastic covering of electric cables. It also imparts uv-resistance so it is often added to greenhouse film and garden furniture.


An unexpected use of calcined kaolin is as a pesticide. When sprayed onto fruit, the fine sharp calcined kaolin particles deter pests by getting into their joints and irritating them. At the same time it reflects the sun and acts as a sunscreen.

Calcined kaolin has many other uses such as in paper coating and in petrochemical catalysts. All in all, a very useful material.

Well I hope you enjoy the World Cup Soccer. You may need some coping strategies, though: Buy your own Vuvuzela so you can drown the others out. Take 5 weeks off and stay at home with the kids. Get inspired to visit South Africa sometime. Keep your head down and hope the craziness passes you by. Or get into the spirit and have a big party. Go Bafana Bafana!

Warmest regards,



May 2010


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