Cement is a term used to describe many different types of substance that are used as binders or adhesives.  The most important cement produced in terms of quantity is Portland cement, which is used in construction.  Other types of cement include masonry and oil well that are manufactured for specialist applications and calcium aluminate cement, which is used in refractories.  

The majority of these cements are inorganic and hydraulic which means they set, harden and do not disintegrate in water.  The primary feature of these cements is their ability to form relatively stable insoluble bonded aggregations of considerable strength and dimensional stability on hydration with water.  Organic cements include latexes and water-soluble polymerisable monomers but kaolin is not used in this type of cement.  

Hydraulic cements are manufactured by processing and proportioning different ratios of raw materials, followed by burning to produce hard nodules known as clinker.  The clinker is then ground to the desired fineness required for an adequate rate of hardening by reaction with water.  Normally two types of raw material are used, one rich in calcium and the other rich in silica.  Calcium materials used include limestone chalk, marl and shells.  The most common silica materials are either clay or shale.  Kaolin can be used because of its aluminium silicate component, but it is not essential and can be substituted by other minerals.  

1.1 Kaolin in cement

Pozzolanic materials have long been used in the cement industry.  The majority of these materials are co-products from other industries, such as coal fly ash, blast furnace slag or fumed silica.  Metakaolin, produced by calcining kaolin at 650-800°C, is a highly reactive pozzolan suitable for use as a cementing material in concrete.  Metakaolin particles are nearly ten times smaller than cement particles, which allows the production of a denser, more impervious concrete.  The use of metakaolin in concrete increases durability and also improves mechanical properties.