Metakaolin:
The Pozzolan for Decorative Concrete
by Michael Chusid
You may never have heard of metakaolin, but chances are it's coming your way. This admixture - now being used in everything from fashionable floors to custom countertops - makes concrete stronger, brighter and more workable while decreasing the amount of portland cement needed in the mix.
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Metakaolin is a type of pozzolan, materials that enhance the strength, density and durability of concrete. While decorative concrete contractors are at the vanguard of many concrete innovations, some have been slow to discover these materials. This is changing, however, since high reactivity metakaolin, a relatively new type Of pozzolan, offers features unavailable from other highly reactive pozzolans; - improved concrete appearance and workability.
Over half of the portland cement concrete used in the United States now contains pozzolans, according to the National Ready Mix Concrete Association. Fly ash and silica fume, the most common pozzolans, are problematic in decorative concrete: They are typically dark in color and, because they are industrial by products, are not uniform in color. Silica fume is a highly reactive pozzolan capable of producing very strong and durable concrete, but results in "sticky" mixtures that are difficult to place and finish and require special curing techniques.
Andrew Goss crafts jewelry, such as this bracelet, with metakaolin-enriched concrete.
High-reactivity metakaolin (HRM), on the other hand, is a white powder that brightens concrete. It is manufactured especially for use in concrete and to assure consistent appearance. It is highly reactive and meets or exceeds the performance of silica fume. But unlike silica fume, it creates a creamy cement paste that is easier to pump, trowels "like butter," and can be cured with ordinary curing techniques.
Metakaolin is produced from kaolinite, a mineral found in kaolin clay (also known as china clay). The material is purified to remove compounds that could discolor concrete, and then calcined under controlled heat to create an amorphous aluminosilicate that is reactive in concrete. Generally used to replace 5 percent to 20 percent of the portland cement in a mixture, metakaolin also reduces emission of greenhouse gases associated with portland cement manufacturing. A growing number of concrete producers have metakaolin on hand, and it is carried by distributors selling plastering materials to swimming pool contractors.
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Metakaolin can be used in white portland cement concrete without compromising its brilliance Mixed into ordinary gray pordand cement, it gives the concrete a lighter value that can create new aesthetic opportunities. It can also be used in integrally colored concrete to lighten and intensify color, and its whiteness and uniformity make it easier to match colors.
Concrete appearance
In addition to brighter and more consistent color, metakaolin improves concrete appearance in several important ways:
• The improved workability of metakaolin-enriched concrete enables contractors to achieve smoother, denser finishes without over troweling. This results in less trowel bum, reduced mottling, and fewer surface blemishes.
• Metakaolin makes concrete denser so it is less likely to absorb stains that could disfigure a surface.
• As a highly reactive pozzolan, metakaolm consumes lime that might otherwise cause efflorescence. And it resists moisture penetration that can dissolve and transport lime and other soluble minerals to the surface of concrete.
• Alkali silica reaction (ASR) is a chemical reaction between the alkaline lime in concrete and the silica in some types of aggregate. If not mitigated, ASR can crack concrete. Metakaolm is one of the most effective admixtures for preventing ASR and allows crushed recycled glm to be used in decorative concrete.
• Improved durability: Nothing ruins the appearance of decorative concrete more dim cracking, corrosion, spallmg, or other forms of concrete deterioration, By boosting concrete strength and density, metakaolin provides a safety factor to help keep your work looking its best Metakaolin-enhanced concrete.
Metakaoline-enhanced concrete
One concrete contractor who has embraced metakaolin is Alan Bouknight, president of Azzarone Contracting Corp. in Mineola, N.Y. He says, "I made samples with metakaotin and put them out in the yard to watch how they fared. After more than three years of exposure to the elements, they show none of the pitting and deterioration we get with typical concrete in our harsh urban environment."
Since then, Bouknight has used metakaolin in exposed concrete floors for a fashionable Armani Casa showroom, to create a sculptural concrete stairway for the Museum of Modem Art, and for pool decks for a custom beachfront residence.
"Metakaolin gives the concrete more 'butte? and enables me to coax out the inherent beauty of concrete in subtle ways," he says. "It makes concrete more consistent so we can create an ultra smooth surface without flow lines or objectionable blemishes." Used with white portland cement, metakaolin "can produce beautiful opalescent finishes. Plus, the bright white metakaolin helps intensify the color we are after."
Tom Ralston, president of Tom Ralston Concrete in Santa Cruz, Calif , is experimenting with metakaolin for concrete countertops, Using dosages of up to 10 percent or 15 percent of the cement weight. "Metakaolin will prove to be a. very important additive as we continue to improve our countertops," he says. "It fills in voids and supports the sand particles to ease finishing. Instead-of taking three or four swipes with a trowel, you might-need only one or two to get a smooth hard finish."
A Primer on Pozzolans
Portland cement consists primarily of calcium silicates. When mixed with water, a chemical reaction known as hydration creates calcium silicate hydrate (CSH). Needle like crystals of CSH interlock to hold concrete together, almost like a glue. Hydration also produces calcium hydroxide, also known as lime. In fact, approximately 15 percent to 25 percent of hydrated portland cement is lime. Lime does not contribute to the strength of concrete and can even be detrimental to a project. For example:
• Lime migrates out of concrete to form efflorescence on the concrete surface.
• When lime leaches out of concrete, it creates voids that increase the porosity of concrete, allowing moisture penetration and corrosion.
• Lime is highly alkaline and contributes to alkali silica reaction (ASR) that can crack concrete.
Pozzolans are supplementary cementitious admixtures that react with lime to form more CSH. The additional CSH increases the cementitious bonds in the concrete to make it stronger. This also makes concrete denser and less porous so concrete is more resistant to penetration by water and chemicals. And because pozzolans consume lime, there is less efflorescence and ASR. Highly reactive pozzolans like metakaolin outperform other pozzolans and can produce high-performance mixtures with higher strength and lower porosity than could otherwise be achieved.
In this simulated microscopic view of cement paste, hydration begins when Portland cement particles are surrounded by water.
The Portland cement begins to dissolve in the water, forming a gel.
Crystals begin to form as a result of chemical reactions.
Inside the concrete, the highly alkaline lime can react with certain types of aggregate to form alkali silica reaction (ASR). ASIR expands in the presence of water, creating forces that can crack concrete.
Calcium silicate hydrate (CSH) shown here in white, creates interlocking crystals that give concrete its strength. However, as much as 25 percent of the products of hydration become calcium hydroxide (lime), shown in green, and does not contribute to the strength of concrete. Voids are formed by excess water in a mixture.
The voids create pores that allow water to penetrate concrete. Since lime is soluble, it dissolves to create even larger pores and allow more liquid and gases to penetrate into the concrete where they can accelerate corrosion. The lime can also be deposited on the concrete surface where it forms efflorescence and disfigures concrete.
As an alternative to ordinary concrete mixtures, high reactivity metakaolin can be used to replace 5 percent to 20 percent of the Portland cement in a mixture. The small size of the metakaolin particles fills in the gaps between cement particles to create a denser and creamier cement paste.
As a pozzolan, metakaolin reacts with lime formed during cement hydration to form additional CSH. Because there is less lime, the potential for efflorescence and ASR are reduced.
The additional CSH formed by metakaolin results in a denser concrete that is stronger and has greater durability due to lower porosity. Plus, the white color of metakaolin brightens the concrete.
Images © Engeelhard Corporation
A unique animation explaining how pozzolans improve concrete is available at www.engelhard.com as part of an online continuing education program.
Metakaolin is an affordable alternative to using white portland cement to lighten the color of concrete, Ralston says. 'The colors you can produce with metakaolin are in a range you don't ordinarily see; we use it if customers want concrete that is just a little more pastel." He suggests that the higher early strength due to metakaolin could reduce the waiting period before concrete is hard enough to polish, and that the denser concrete may make countertops more stain resistant.
Metakaolin is also finding its way into the fine arts. For example, Andrew Goss, the author of Concrete Handbook for Artists, uses metakaolin in concrete sculpture and jewelry. He says, "Metakaolin can double the compressive strength of the concrete, lowers permeability and increases density but also has advantages over silica fume. Metakaolin makes the mix creamier, less sticky, and is plain white in color."
Guidelines for use
Concrete with metakaolin can be placed, finished and cured in pretty much the same manner as concrete without HRM. It complies with ASTM C 618—Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete, Class N and is accepted under ACI 318—Building Code Requirements for Structural Concrete and Commentary.
According to Anthony Reed, market development manager for Engelhiard Corp.'s MetaMax high-reactivity metakaolin, "At an HRM replacement level of 8 percent to 10 percent, concrete is generally more cohesive and less likely to bleed. As a result, the effort required for pumping and finishing processes is reduced. Compressive strength and density of hardened concrete are also increased. Optimum pozzolanic benefits of metakaolin can be obtained by replacing 10 percent to 20 percent of the portland cement in the mixture with HRM. These levels produce concrete with lower porosity and improved durability and resistance to impact and abrasion."
Reed also points out that metakaolin reduces concrete slump and generally requires the addition of a high-range water reducer (HRWR). "Metakaolin should generally be added to batch after portland cement and before HRWR is added," he says. "Air entraining admixtures may require somewhat increased dosage when used with metakaolin."
When using integrally colored concrete, metaknolin brightens the shade of colored concrete by creating a lighter cementitious base color, according to Nick Paris, marketing manager for Davis Colors.
Pool contractors use metakaolin to "make plaster more resistant to erosion and corrosive chemicals. The white additive brightens plaster and improves workability to reduce trowel bum and mottling.
While metakaolin should have no impact on surface, applied coatings, special consideration must be given when using penetrating stains, Paris explains. "Because metakaolin increases the density of concrete, it may reduce the ability of stain to penetrate the surface. In addition, pozzolans consume some of the lime in concrete and can reduce the ability of chemically active stains to react with lime and develop their color."
David Kucera, president of David Kucera Inc., Gardiner, N.Y., a fabricator of precast concrete ornamentation and architectural cladding, points out a further tip on finishing concrete with metakaolin. "Metakaolin makes my concrete more resistant to acid rain, but it also makes it more difficult to use an acid wash to expose the aggregate. I now have to work twice as hard to remove the cement paste." He admits that this is an ironic testimony to the effectiveness of metakaolin in improving the density of concrete. On the other hand, he says, metakaolin simplifies sandblasting. "Me harder concrete makes it easier to keep an edge on a piece, and comers don't get rounded off as easily.
Tom Ralston polishes a countertop created with metakaolin. The additive prevents alkali silica reaction (ASR), making it possible to use recycled glass as aggregate.
Contractors who have used metakaolin advise that before doing a project with the additive, decorative concrete craftsmen should prepare trial batches using proposed materials in order to refine their mix and technique. "Once they get used to working with it," Bouknight says, "the guys on my crew love metakaolin, especially how it finishes. It makes them prouder of the work they do."
Michael Chusid, FCSI, is an architect and serves on the ACI Committee 124 - Concrete Aesthetics. He is a technical and marketing consultant to building product manufacturers and can be reached at www.chusid.com.