Fly ash is used to improve the durability and quality of precast concrete products.
By Debbie Sniderman
Chances are you use fly ash at your plant or know a plant that does. But how much do you really know about where it comes from or why it works so well in your mix? Fly ash is a byproduct of burning coal during combustion at power plants and has been used as a cementitious material in the United States on a large scale since 1948.
Fly ash is not technically cement but a pozzolan with particle sizes like cement. When mixed with water, it chemically reacts with calcium hydroxide to form compounds possessing cementitious properties. And, when added as part of the concrete cocktail, it provides a number of well-known benefits to precast concrete producers, including being more economical and sustainable, slowing down the rate of corrosion and making precast more durable.
How fly ash works in concrete
Fly ash increases concrete’s durability by the pozzolanic reaction. The hydration reaction of water and cement produces calcium silicate hydrate and calcium hydroxide. When fly ash is introduced, it reacts with the CH, producing more CSH.
This is advantageous because CH doesn’t benefit concrete and is often easily dissolvable, allowing chloride ions to enter. Anything that reduces the amount of CH in concrete helps improve durability and strength.
Also, concrete with more CSH has smaller sized pores. With reduced pore size distribution, concrete has lower permeability from outside agents such as chlorides. Chlorides typically enter through large pores and can travel several inches to react with rebar or other materials inside the concrete. With finer pore sizes, it’s harder for chlorides to enter.
More CSH also substantially slows the ingress of sulfates into concrete by another mechanism different from permeability. If concrete walls or columns are used in soil that containing larger than usual amounts of sulfates, adding fly ash can reduce expansive reactions and cracking. In addition, when fly ash forms more CSH, it prevents the well-known alkali silica reaction from happening by trapping the alkalis in the concrete.
What’s important about fly ash
According to Tom Adams, executive director of American Coal Association, fly ash helps makes a more dense and less permeable concrete.
“This is important in the eye of the audience that precasters appeal to, as their customers are in it for the long term,” Adams said. “For applications such as parking decks and other heavy precast work, long-term durability and maintenance are important concerns, and one of the most outstanding contributions of fly ash to quality.”
The fly ash coming out of power plants today isn’t significantly different from earlier decades, but the way it is used in the marketplace has evolved.
“Cement chemistry has changed over the last several decades,” he said. “The grinding and chemistries have evolved to accommodate a number of concerns, including U.S. Environmental Protection Agency requirements. As cement has evolved, fly ash hasn’t.”
Class C fly ash, from subbituminous coal, has lower heat values, is more reactive and has much less sulfur content than Class F fly ash from bituminous coal. As more plants have shifted to subbituminous coal, it has made an impact on fly ash use.
“The biggest change over the last 10-20 years is the more widespread use of subbituminous coal from the western part of the USA than the bituminous coal mined from parts of the country east of the Mississippi,” he said.
While there are many types of fly ash available, some are not suitable for use in concrete. ASTM C618 is the standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete. It defines the specifications and class limits. Both Class C and Class F fly ash have to comply.
Adams said precasters need to know which type to use when mixing concrete for a specific application. Most Class C fly ash will set much quicker than Class F, and it doesn’t take a lot of adjustment in chemistry to manage.
“Marketers have to inform customers where the fly ash is from, because it behaves differently from different parts of the country. Some are a blend of western and eastern coal,” he said. “Precasters need to be aware of the different types because they react differently.”
Why fly ash is important to use
The big bang for using fly ash is in improving durability. Decades of evidence shows that fly ash improves durability and compressive and flexural strengths, and it is the best way to deal with the ASR problem. Adams said that departments of transportation in the U.S. spend more than $5 billion per year on fly ash, as it’s one of the many tools that allow structures to be built to last 75-100 years.
“Fly ash generally costs less than Portland cement depending on the market and delivery logistics,” Adams said. “Even when using additional admixtures to get it to set in an acceptable way, fly ash still offers a lower cost concrete.”
In addition to cost savings, increased fly ash in a concrete mix provides sustainability benefits. Fly ash is a big help for those that are producing for clients who care about recycling or reusing materials that would have otherwise be landfilled. Every ton of fly ash used instead of Portland cement keeps it out of a landfill and cement in concrete, thus reducing carbon from carbon dioxide emissions.
How fly ash advances a precaster’s work in the plant
Fly ash is commonly specified in precast jobs, reducing surface imperfections and improving workability. Reducing the water-cementitious material ratio allows a precaster to increase the fly ash content and increase the early-age strength. Chemical admixtures are also used as an affordable way to offset the effect of fly ash, helping the concrete quickly achieve strength.
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company.
ASTM C618, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete,” astm.org/Standards/C618.htm
American Coal Ash Association, acaa-usa.org
Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland-Cement Concrete, astm.org/Standards/C311.htm