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By Bill Imhoff
Using ash in concrete is nothing new. The Romans used volcanic ash in their spectacular construction projects long before the introduction of Portland cement, having discovered its value as a hardening agent when mixed with lime. The ancient Romans used volcanic ash as an admix to erect buildings such as the Pantheon and Coliseum, roads, and aqueducts. Remember, these structures are more than 2,000 years old.
Fly ash concrete was first used in the U.S. in the 1920s for dam construction, when engineers found that it reduced the total cement requirement. Today, as more projects seek LEED certification, ash is resurfacing as a popular environmentally-friendly alternative to Portland cement. No longer are we using volcanic ash, of course, but coal fly ash is gaining ground as the green alternative of choice for LEED projects. Many of the projects and the millions of square yards of LEED flooring we’ve installed in recent years used ash in the cement mixture. While fly ash holds benefit as a recycled component, it also presents flooring contractors with unique challenges.
To understand the green benefit and challenges that fly ash brings to the flooring process, you must understand fly ash. It’s a byproduct of coal power plants, which account for more than half of the electricity we consume in the United States, according to Headwaters Resources in South Jordan, Utah, which supplies materials derived from coal combustion products, including fly ash, nationwide.
“Fly ash is composed of the non-combustible mineral portion of coal. When coal is consumed in a power plant, it’s first ground to the fineness of powder. Blown into the power plant’s boiler, the carbon is consumed — leaving molten particles rich in silica, alumina and calcium. These particles solidify as microscopic, glassy spheres that are collected from the power plant’s exhaust before they can “fly” away — hence the product’s name: fly ash,” according to a Headwaters report.
Fly ash is considered the modern-day “Pozzolan,” a word that comes from the Italian city, Pozzuoli, which most consider the birthplace of ash concrete technology. Pozzolan is a siliceous material that by itself possesses no cementitious value. However, with the presence of moisture, it reacts with calcium hydroxide to form a compound with cementitious properties.
Concrete made with fly ash is strong, durable and resistant to chemical attack, as evidenced by the many early Roman buildings still standing.
Traditionally-produced Portland cement, while extremely versatile and strong, is not environmentally friendly. According to the Green Resource Center, the manufacture of Portland cement accounts for 6% to 7% of the total carbon dioxide (CO2) humans produce. That equates to the greenhouse-gas equivalent of 330 million cars, each driving 12,500 miles per year.
Coal fly ash is a coal-fired power plant waste byproduct, which otherwise would end up in a landfill. Its use in cement reduces the energy demands of manufacturing other concrete ingredients, reducing energy consumption, costs and greenhouse emissions, and slowing the depletion of natural resources. In fact, according to Headwaters research, more than 12 million tons of coal fly ash are used in American concrete products each year.
Because of coal fly ash’s green qualities, its use in concrete can generate points toward LEED certification by the U.S. Green Building Council:
In the past several years, we’ve gained extensive experience in commercial LEED projects, many of which used fly ash in the concrete. We found that, while fly ash benefits concrete durability and strength, makes it more workable, and is the green thing to do, its use as an additive in concrete can generate some fairly challenging bonding issues with flooring. Here’s why:
Fly ash is one of the residues generated in coal combustion. Depending on the source and type of coal being burned, fly ash makeup varies considerably, but all fly ash contains substantial amounts of silicon dioxide and calcium oxide.
If you’ve ever tried to attach anything to a silicon-based product, you know firsthand that it won’t stick. Similarly, calcium oxide, also known as quicklime or burnt lime, is a caustic, alkaline crystalline byproduct — and like other high-alkalinity products, plays havoc with flooring adhesives. “The fly in the ointment, or in this matter, the fly ash, is that nothing will stick to this type of concrete regardless of what you do,” wrote Lew Migliore, president and owner of The Commercial Flooring Report in March 2008. He advised flooring contractors to get involved early in the specification process to prevent specifying or trying to install products that simply won’t stick.
The American Coal Ash Association and the flooring industry have been working to address this problem since 2008. In response to issues raised in that 2008 article, ACAA Executive Director David Goss said, “There have been many successful projects in which coal fly ash (CFA) has replaced more than the typical 20% to 30% replacement levels. However, as the replacement levels are increased, a change in the typical concrete placement and curing process should be observed. In other words, more can be better, provided the unique characteristics of fly ash are accounted for during the engineering phase of the program.