Calcined clays

Metakaolin is produced by heating sources of kaolin (clay, paper sludge etc.)  to between 650°C and 750°C to produce a material that can be added to cement in place of clinker.

Calcined clay – or metakaolin – is produced by heating a source of kaolinite to between 650°C and 750°C. Kaolin is both naturally occurring, as in china clay deposits and some tropical soils, as well as in industrial by-products, such as some paper sludge waste and oil sands tailings. It is a pozzolanic material with smaller particle size than Portland cement.

Blends of Portland cement with metakaolin are fairly widely tested and accepted in standards around the world, including in Europe and North America.

Metakaolin in much more reactive that other pozzolans (e.g. fly ash but similar to silica fume), helping to accelerate the reactions that harden concrete. This improves the early strengths of concrete with metakaolin, making it suitable for precast applications.

As with other pozzolans, metakaolin increases the long-term strength of concrete and its durability, reducing the effects of alkali-silica reactions (which can be severely damage concrete) and increasing resistance to chemical attack. It also lightens the colour of concrete, making it possible to add an integral coloured tint.

The need to activate metakaolin through heating the raw material to relatively high temperatures make the production of metakaolin a more costly exercise that other supplementary cementitious materials (SCMs). More problematically, the use of metakaolin can increase water demand during concrete production by over 20% unless suitable water-reducing admixtures are used.

Metakaolin can also increase the risk of carbon (black) steel reinforcement corrosion by decreasing resistance to carbonation.

There is more limited field experience with metakaolin cements that other SCMs and, due its limitations, its use (as with silica fume) seems limited to high-strength lightweight concretes and in the precast/poured mould sector.