Metakaolin is obtained when kaolinite-containing soils, which are derived from the natural decomposition of Feldspars, are progressively calcined to undergo a series of transitional phases before reaching high temperatures, becoming stable and being cooled rapidly. The final product is becoming extensively used as a supplementary cementitious material which partially replace cement in concrete mixes, result- ing in more sustainable, economical and technically enhanced mixes. The high rate of pozzolanic reaction results in improved early strengths, while the pore-refining effect ensures that the high strengths are maintained in the long-term. The pore-refining effect also results in enhanced durability, highly densifying the concrete microstructure; the high rate of pozzolanic reaction also ensures a high consumption of Portlandite, which has beneficial effects on the mitigation of alkali-silica reactions, but adverse effects on carbonation resistance. Achieving the desired properties with respect to specific mechanical and durability properties lies in selecting an appropriate replacement level of cement by metakaolin, which for an overall positive effect seems to lie at approximately 15 %.
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