Benefits of Metakaolin As a Cement Substitute
Question:
What is metakaolin, and why should I consider using it in concrete countertops?
Answer:
Metakaolin is a cement substitute often used in concrete countertop mixes to boost the physical properties of the concrete. It is a manufactured pozzolanic mineral admixture made by calcining (thermally activating) purified kaolin clay. By itself, kaolin clay has very little reactivity. But once the clay is chemically altered by heat, it reacts very aggressively with calcium hydroxide, a normal cement hydration byproduct, to form additional cementitious compounds.
Metakaolin has similar effects on concrete as silica fume (a pozzolan made from recycled waste), permitting compressive strengths of 10,000 psi or higher. While this strength increase is impressive, it is not the sole reason for using metakaolin. Concrete countertop fabricators often look for ways to boost production efficiency and increase the quality of their products. Metakaolin does both by producing higher early strengths, which allows for earlier form stripping, earlier processing and quicker turnaround. In addition, metakaolin decreases porosity, increases tensile and flexural strengths, reduces drying shrinkage, and aids in finishability when the countertop is troweled.
Metakaolin is manufactured as a nearly white or slightly creamy off-white powder, so it often is used as a white substitute for silica fume. Metakaolin may or may not have an effect on concrete color, depending primarily on the color of the cement, how the metakaolin is dosed, and the color and dosage of any pigments. If the concrete is made with white cement, there is virtually no perceptible change, even at high metakaolin loadings. However, if the concrete is made with gray cement, greater substitutions of metakaolin for cement will result in a lightening of the base concrete color.
Metakaolin can also influence the intensity and appearance of acid staining. Depending on the age of the concrete when it is acid stained, the amount of metakaolin used, and the strength of the acid stain, the resulting color may be weaker than if no metakaolin was used. Higher acid concentrations and longer exposure times might be necessary for deep acid stain colors.
Metakaolin is often used as a cement replacement in dosages of 5% to 20%, but the optimal dosage is about 10% to 15%. At dosages lower than 10%, many of the benefits are not realized. At loadings higher than 15%, there is a diminishing return on benefits versus the cost increase of using the extra metakaolin.
What is metakaolin, and why should I consider using it in concrete countertops?
Answer:
Metakaolin is a cement substitute often used in concrete countertop mixes to boost the physical properties of the concrete. It is a manufactured pozzolanic mineral admixture made by calcining (thermally activating) purified kaolin clay. By itself, kaolin clay has very little reactivity. But once the clay is chemically altered by heat, it reacts very aggressively with calcium hydroxide, a normal cement hydration byproduct, to form additional cementitious compounds.
Metakaolin has similar effects on concrete as silica fume (a pozzolan made from recycled waste), permitting compressive strengths of 10,000 psi or higher. While this strength increase is impressive, it is not the sole reason for using metakaolin. Concrete countertop fabricators often look for ways to boost production efficiency and increase the quality of their products. Metakaolin does both by producing higher early strengths, which allows for earlier form stripping, earlier processing and quicker turnaround. In addition, metakaolin decreases porosity, increases tensile and flexural strengths, reduces drying shrinkage, and aids in finishability when the countertop is troweled.
Metakaolin is manufactured as a nearly white or slightly creamy off-white powder, so it often is used as a white substitute for silica fume. Metakaolin may or may not have an effect on concrete color, depending primarily on the color of the cement, how the metakaolin is dosed, and the color and dosage of any pigments. If the concrete is made with white cement, there is virtually no perceptible change, even at high metakaolin loadings. However, if the concrete is made with gray cement, greater substitutions of metakaolin for cement will result in a lightening of the base concrete color.
Metakaolin can also influence the intensity and appearance of acid staining. Depending on the age of the concrete when it is acid stained, the amount of metakaolin used, and the strength of the acid stain, the resulting color may be weaker than if no metakaolin was used. Higher acid concentrations and longer exposure times might be necessary for deep acid stain colors.
Metakaolin is often used as a cement replacement in dosages of 5% to 20%, but the optimal dosage is about 10% to 15%. At dosages lower than 10%, many of the benefits are not realized. At loadings higher than 15%, there is a diminishing return on benefits versus the cost increase of using the extra metakaolin.
posted by Jeff Girard
at
3:59 PM
3 Comments:
Hi,we are able to obtain super pozalins which are derived and graded from the chimney stacks of our coal burning power stations. The recomended percentages are between 10-15% of the cement content. In your experience would it have the same effect on counter top mixes as Metakaolin. Heinz
By
Heinz-Afristone, at 1:19 PM
Hello Jeff,
Do you have a preference in Metakaolin brands? I’ve been using MetaMax but each manufacturer seems to claim their Metakaolin is superior. Do you think there much difference between manufacturers?
Thank you.
Benjamin Cunin
www.portlandconcretecountertops.com
By
Benjamin Cunin, at 11:52 PM
Heinz,
It sounds like you're talking about fly ash. Fly ash is a widely used pozzolan, both in "ordinary" concrete and in concrete countertops. Metakaolin is another type of pozzolan. Fly ash, at those doses, may not produce the same results at an early age as metakaolin, and because most fly ash is gray, may affect the color of the concrete.
Benjamin,
I find that all of the metakaolin brands I've used produce very satisfactory results. There might be slight different color, gradation and reactivity differences between brands, but all reputable brands perform as one would expect.
There is an article I wrote on metakaolin on the Concrete Connections website: https://www.concretecountertops.net/library.item.51/how-to-use-metakaolin-in-concrete-countertops.html
By
Jeff Girard, at 10:15 AM
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