Metakaolin is a pozzolan, probably the most effective pozzolanic material for use in concrete. It is a product that is manufactured for use rather than a by-product and is formed when china clay, the mineral kaolin, is heated to a temperature between 600 and 800ºC. Its quality is controlled during manufacture, resulting in a much less variable material than industrial pozzolans that are by-products. First used in the 1960s for the construction of a number of large dams in Brazil, metakaolin was successfully incorporated into the concrete with the original intention of suppressing any damage due to alkali-silica reaction.
When used to replace cement at levels of 5 to 10% by weight, the concrete produced is generally more cohesive and less likely to bleed. As a result pumping and finishing processes require less effort. The compressive strength of hardened concrete is also increased at this level of replacement.
Slightly higher replacement levels (up to 20%) produce a cement matrix that has low porosity and permeability. This results in improvements to resistance of the hardened concrete to attack by sulfates, chloride ions and other aggressive substances, such as mineral and organic acids. Freeze/ thaw resistance is improved and the risk of damage resulting from the effects of impact or abrasion is reduced for metakaolin concrete that has been finished and cured properly.