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It is fired (calcined) under carefully controlled conditions to create an amorphous aluminosilicate that is reactive in concrete. Like other pozzolans (fly ash and silica fume are two common pozzolans), metakaolin reacts with the calcium hydroxide (lime) byproducts produced during cement hydration.
Calcium hydroxide accounts for up to 25% of the hydrated Portland cement, and calcium hydroxide does not contribute to the concrete’s strength or durability. Metakaolin combines with the calcium hydroxide to produce additional cementing compounds, the material responsible for holding concrete together. Less calcium hydroxide and more cementing compounds means stronger concrete.
It is very fine and highly reactive, gives fresh concrete a creamy, non sticky texture that makes finishing easier.
Our experience has shown that optimal performance is achieved by replacing 10% to 15% of the cement with metakaolin. While it is possible to use less, the benefits are not fully realized until at least 10% metakaolin is used. The advantage of replacing some of the cement with metakaolin, rather than simply adding metakaolin to the mix, is that any existing color formulas or mix designs won’t change, or will only very slightly change. This is because the dosage of pigments and superplasticizers are based on the cement content in the concrete. Of course, it is ok to simply add metakaolin to an existing mix, but it’s important to realize that the total equivalent cement content will increase. Be aware that this will affect not only the pigment and admixture dosages but also the water to cement ratio, a critical factor in mix design.
How well metakaolin behaves depends on your mix design and, more critically, on how much mix water is used. Keep in mind that any admixture must be used wisely. All the other factors (good mix design, proper reinforcing, etc) must also be properly designed and accounted for in order to take advantage of the benefits metakaolin can give you.
For example, making a very high compressive strength concrete is pointless if the reinforcing is inadequate.
Metakaolin is compatible with most concrete admixtures, such as superplasticizers, retarders, accelerators, etc. If questions arise as to compatibility with any admixtures you use in your mix, consult with the admixture manufacturer for guidance.