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Metakaolin in Concrete

What is kaolin?

Kaolin or kaolinite clay is a layered dioctahedral phyllosilicate clay that occurs naturally when minerals like feldspar decompose. In a pure form, kaolin is white, though it is typically found with some naturally formed oxides in kaolin clay. These oxides influence the color, giving it a pink, red, orange, or even yellow hue. Kaolin is sometimes called china clay because of its extensive use in ceramics, fine china and porcelain.


What is metakaolin?

Metakaolin is the product of calcining the raw kaolin between 530 and 570 C and milled to a -325 mesh material. The process produces a highly pozzolanic powder.


Why is metakaolin used in concrete?

Metakaolin is used mainly for its pozzolanic reactivity, producing additional calcium silicate hydrates and calcium aluminosilicate hydrates. These minerals provide cementitious properties that enhance the durability and strength of concrete.


What is an effective replacement dosage?

It is always recommended to run a trial, or multiple trials, to ensure the mix has the desired properties that you’re looking for. That being said, metakaolin can replace up to 20% of cement.


Is metakaolin pozzolanic?

Yes, metakaolin is pozzolanic, meaning that when the fine siliceous and aluminous material reacts in the presence of calcium oxide (slaked lime) and moisture to form calcium silicate hydrates and calcium aluminosilicate hydrates, these are the cementing chemicals that give it the strength.


How is metakaolin produced?

The journey to producing metakaolin starts with mining kaolinite clay, a naturally occurring mineral found in hot moist climates. When feldspar and other aluminous silicates decay, kaolin is the result. It’s one of the most naturally occurring minerals found in most parts of the world. Once the kaolin is mined, it is heated to remove the hydroxide groups (water) from the clay creating metakaolin.


What is High Reactivity Metakaolin?

High reactivity metakaolin is the same as metakaolin except that it contains the highest percentages of silica and alumina (90% or more). It is the premium version of metakaolin. The high concentrations of silica and alumina make the cement more workable, has a creamier finish, and is brighter in color, making it suitable for architectural applications and high-performing concrete finishes.

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