Supplementary cementitious materials (SCM) have been around the concrete industry for decades. The leading SCMs in the ready mix industry are fly ash and blast slag. As the push for green building continues, you’ll see the use of these mixes increase. As a contractor, you’ll want to know how these mixes affect your job and how to take advantage of their benefits.
Fly ash, a fine powder that’s a by-product of burning coal, has been used in concrete in the United States since the 1940s. It was first used in projects where a reduced heat of hydration was desired, such as dam building. In the 1970s, when the Environmental Protection Agency (EPA) imposed stricter emissions controls on electricity manufactures, they started collecting more fly ash and increasingly began marketing its use. “The quality control and use of fly ash in concrete has grown more than 10 percent each year since the 1970s,” says John Ward, vice president of marketing and government affairs with Headwaters, Inc., the nation’s leading supplier of fly ash.
“The use of fly ash is increasing in a couple of ways,” Ward continues. “More and more people who didn’t use it before are starting to use it, and people who did use it before are learning ways to use it more.”
About 15 million tons of fly ash was used in concrete as a cement replacement in 2006.
According to the Slag Cement Association, blast slag, a by-product of manufacturing iron in a blast furnace, has roots in concrete applications going back to the 1890s, but its availability increased in the 1980s with the opening of a granulation facility in Baltimore and the establishment of ASTM specification soon after. Blast slag’s popularity remained in the mid-Atlantic states for much of the 1980s and early 1990s, but its popularity has increased in recent years as more facilities around the United States and Canada have opened, making the product more widely available.
In 2007, more than 3.5 million metric tons of slag was sold for concrete use.
Both fly ash and blast slag are used as supplemental cementitious materials and can replace a certain amount of Portland cement in a concrete mix. While the products are chemically different (fly ash is a pozzolan and blast slag a hydraulic cement), they lend some similar properties to a concrete mix.
“They can give you higher later-age strengths, reduce the permeability of concrete and improve the durability of concrete from the aspects of alkali-aggregate reaction, sulfate resistance and reduced heat of hydration,” says Colin Lobo, senior vice president of engineering with the National Ready Mixed Concrete Association (NRMCA). “There also seems to be some benefits in preventing corrosion of imposing steel because of the reduced permeability.”
Concrete mixes using blast slag and fly ash can also offer contractors increased workability and flow and better finishing. The addition of SCMs delay early strengths, but in most cases those issues can be overcome with planning.
Concrete has many properties that contribute to sustainable design - it includes local materials, paired with insulating products concrete can achieve high energy savings and it’s long-lasting. The addition of supplemental cementitious materials makes it even more attractive to those involved in sustainable building projects.
The use of fly ash and blast slag contributes to attaining points in the U.S. Green Building Council’s (USGBC) LEED program (LEED stands for Leadership in Energy and Environmental Design). As a recycled post-industrial waste, blast slag and fly ash can contribute to gaining points under LEED’s Materials and Resources category, while certain SCM mixes can achieve points under the Innovation in Design category. Points are also available under the Sustainable Sites category, namely for concrete’s light coloring which contributes to minimizing the heat island effect. The Slag Cement Association adds that blast slag is a very light material and further increases concrete’s ability to reflect light and reduce the heat island effect over other concrete mixes.