Metakaolin is a recently developed supplementary cementing materials. Adding metakaolin will improve concrete properties and enhance concrete durability. Metakaolin produce by the calcining of kaolinite clay at a temperature of 650C to 800 C to drive off the chemically bound water in the interstices of kaolin and destroy the crystalline structure, this process will transfer the kaolinite to MK phase. Unlike other pozzolans, the metakaolin carefully refined to produce a lighter color, remove impurities, and control particle size. The metakaolin has a particle size of less than 2 µm. The particle size of metakaolin is significantly lesser than the cement particle size. However, the silica fume pozzolans are finer than metakaolin.
Table no:1 showing the typical chemical composition of metakaolin. We can notice that the silicon dioxide and aluminum oxide forms almost 93.29% of the total chemical composition of metakaolin. The metakaolin can replace 5-20% of cement in concrete. Metakaolin can improve concrete properties by packing effects and by reacting with calcium hydroxide to form C-S-H. It was found that metakaolin improve concrete properties while enhancing concrete workability. Concrete with metakaolin pozzolans will require less water-reducing admixture compared to concrete modified by silica fume.
Furthermore, adding metakaolin to the concrete mix will reduce the rate of diffusion of sodium and chloride. Adding 8% of metakaolin to concrete will reduce the diffusion coefficient by nearly 50%. The ingress of chemicals such as chloride will reduce concrete durability. Chloride will cause reinforcement rust, and this will result in concrete cracking and spalling. In general, adding metakaolin to the concrete mix will enhance concrete strength, reduce permeability, reduce free drying shrinkage, and improve concrete toughness. The white color of metakaolin will not darken concrete as silica fume does. This makes the metakaolin suitable for color matching. The price of metakaolin is lesser than the price of silica fume, which makes it an excellent competitor to silica fume.