Producing greener concrete with metakaolin

New greener building standards
Widespread adoption of new building standards aimed in part at reducing greenhouse gas emissions is presenting the concrete industry with a difficult question: how do you make concrete with constructible properties and a smaller carbon footprint?

Cement manufacturing has received the infamous distinction of being among the top five sources of industrial global greenhouse gas emissions in a CO₂ obsessed world. Today, approximately 5% of global manmade CO₂ emissions are produced by cement manufacturers. For every tonne of cement manufactured in North America, one tonne of CO₂ is emitted, drawing the ire of environmentalists and green-minded politicians. New regulations and laws, like California’s AB-32 Global Warming Solutions Act, are mandating reductions in greenhouse gas emissions. This, in turn, is pushing cement producers and concrete contractors to seek solutions such as less CO₂ intensive supplementary cementing materials (SCM) in order to reduce the carbon footprint of their operations.

Whitemud Resources Inc. offers the concrete industry an economical and environmentally friendly path to achieve that goal. Its branded SCM, WhitemudMK, is a highly reactive metakaolin pozzolan created from kaolin clay. It can replace up to 20% of the cement in concrete.

Metakaolin
“As a highly reactive pozzolan, metakaolin works well with slag or flyash, allowing larger cement replacements to be achieved and resulting in environmentally-friendly concrete that performs,” says Professor Doug Hooton, NSERC/CAC Industrial Research Chair in Concrete Durability and Sustainability at the University of Toronto. “In addition, it results in increased durability to sulfate attack, chloride ingress, and alkali-silica reaction, thus increasing sustainability through increased service life.”

Metakaolin has long been widely accepted as an SCM across North America and around the world. It is recognised for its ability to shorten concrete setting and stripping times, while increasing compressive and flexural strength and improving durability. These are properties needed to meet the demanding loads of today’s “super” structures at building sites from Dubai to China.

According to Kelly Babichuk, Whitemud’s President and Chief Operating Officer, contractors have known about metakaolin’s performance advantages for years. Until recently, however, the economics just did not work. Whitemud Resources, however, has cracked the economic nut, enabling metakaolin to be profitably supplied to the concrete industry.

Reduced emissions
Generating enough heat to fuse the various elements into cement clinker produces major greenhouse gas emissions. Even more CO₂ is produced from the reaction caused by burning the limestone. The kaolin clay to metakaolin reaction, however, does not produce any CO₂. In addition, the Whitemud kiln temperature of 800 °C is just over half the 1500 °C required to produce cement clinker, reducing energy requirements and greenhouse gas emissions.

Secure resource
The main issue with other SCMs, such as flyash, granulated blastfurnace slag (GBS) and silica fume is that they are all waste materials of other industrial processes. Babichuk believes that although utilising waste products from other industries is important, there are drawbacks to relying on a manufacturer for whom SCM is a secondary business. “If you’re using SCMs that are the waste products of industrial processes used to produce steel or electricity, your supply of SCM is dependent upon your supplier’s success in marketing their primary products,” he asserts. “If the silicon-metal producer you’re using closes its plant, for example, there goes your supply of silica fume. That has been a major problem in some markets. Whitemud offers a stable supply of metakaolin worldwide. Supplying contractors and concrete producers with metakaolin is our only business.”

Metakaolin in the mix
In addition to its numerous environmental benefits, Rick Ketcheson, Whitemud’s Director of Technical Services and Product Development, sees metakaolin as having many operational advantages over other SCMs. “Metakaolin contains the highest percentage of alumino-silicates among the SCMs, allowing it to maximise the amount of durable binders in concrete,” Ketcheson explains. “It reacts with lime more aggressively than flyash, and is much cheaper than silica fume. Flyash may take up to two months to react with the lime in concrete. Using metakaolin, the reaction may occur within two weeks.”

Increased structural lifetime
Whitemud’s ability to increase concrete’s resistance to alkali-silica reactivity (ASR) is another way that it helps to prolong the longevity of structures. ASR is causing infrastructure throughout North America to crumble. Metakaolin increases concrete’s resistance to ASR by increasing its density and reducing its pore size and permeability. Substituting metakaolin for up to 20% of cement also improves concrete’s sulfate resistance.

Babichuk admits that metakaolin is not usually foremost in the minds of contractors and engineering firms as an ASR solution, yet it is proven to be the most ASR-resistant pozzolan because of its relatively pure alumina-silicate composition. It could improve the safety of concrete structures, such as bridges and office buildings, by reducing the instances of catastrophic failures where concrete has broken away and fallen into the street. He suggests the need for pozzolan concrete additives is going to increase as the better aggregate quarries in Canada and the US are depleted.

Whitemud Resources Inc.

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