Class F fly ash, formed from the burning of anthracite and bituminous coal, is one of two types of fly ash that were investigated as replacements for cement. The research revealed that concrete mixtures in which more than 50 percent of the cement has been replaced by the industrial waste product can be made to outperform conventional concrete mixtures while maintaining their inherent cost and carbon savings. University of Kentucky Center for Applied Energy Research
The early strength of high-volume fly ash concrete can be dramatically improved by the substitution of limestone for a portion of the ash.
September 17, 2013—In recent laboratory testing, concrete mixtures in which more than 50 percent of the cement has been replaced by the industrial waste product fly ash can be made to outperform conventional concrete mixtures while maintaining their inherent cost and carbon savings. The performance enhancements were made by replacing 25 percent of the fly ash, by volume, with a fine limestone powder and in some cases specifying Type III portland cement.
“A lot of the industry is using 15, 20, even 25 percent fly ash in their mixtures, but this research demonstrates that you can go beyond that and maybe replace 40 up to 60 percent of your cement with fly ash and limestone,” says Dale Bentz, a chemical engineer for the National Institute of Standards and Technology (NIST).
Bentz collaborated with Jussara Tanesi, Ph.D., a contracted project manager with the Federal Highway Administration (FHWA), and Ahmad Ardani, P.E., a concrete materials research engineer for the FHWA, to conduct laboratory performance tests of 14 different mixtures of high-volume fly ash (HVFA) concrete; the tests included one conventional mixture as a control.
The researchers began with nine basic mixtures: the control concrete and mixtures with either 40 or 60 percent fly ash by volume, the fly ash being classified as either Class C or Class F by ASTM International. Some of the mixtures substituted fine limestone powder for 25 percent of the fly ash by volume.
Fly ash is a by-product of combustion; most commercially available fly ash is produced in coal-burning power plants. Fly ash varies in chemical composition based on the type of coal that was burned. Class F fly ash forms from the burning of anthracite and bituminous coal. Class C fly ash results from lignite or subbituminous coal.
“These are two vastly different fly ashes in terms of their calcium oxide contents and also in terms of the way they behave in cementitious systems,” Bentz says. “The F ash is basically inert at early ages and the C ash produces extensive retardation. So it’s really a problematic ash to use.”
The team performed mixture proportioning and replacements on a volumetric basis, Bentz says. In the industry, mass-based replacement is more common, but creates complications because fly ash is typically much less dense than cement and thus changes the mixture proportions, water content, and yield.
On the basis of the first round of testing, the team conducted a second phase of testing on some of the formulas with either a lower water-to-powder ratio, or with the Type I/II cement replaced by Type III (high early strength) cement.
“Those mixtures, in general, exceeded the strengths of the control mixture and still had a significant reduction in their carbon footprint—and were also economical,” Bentz says.
The highest-performing mixtures in the study had 3,880 psi of compressive strength at day one, compared to 2,870 psi for the conventional control mixture. That advantage held over the course of the test period, with a strength of 8,840 psi for the C fly ash Type III mixture at day 28, compared to 6,750 psi for the control mixture. The fly ash mixture is estimated to cost $48.41 per cubic yard, compared to $53.61 per cubic yard for the control.
The limestone serves an important function in a concrete mixture, offering clear benefits and synergisms, Bentz notes. “The cement grains partially dissolve, go into solution, and then form reaction products that precipitate back on the cement grains,” Bentz explains. “Some nucleate and grow in the water-filled spaces. The limestone gives extra surfaces for that precipitation. It turns out that the hydration products like to precipitate on limestone just as much as they do on the cement particles.”
“Limestone powder provides a better backbone, because it’s the growth of this network of hydration products that’s going to give you the setting and strength in your material,” Bentz adds. “The limestone accelerates the hydration by providing more surfaces for that precipitation to occur. And it also partially reacts in the system with some of the aluminate phases that are present in the cement or the fly ash. And they tend to form reaction products that are more voluminous and stiffer, so that decreases porosity and increases strength.”
The results of the research, “Ternary Blends for Controlling Cost and Carbon Content,” were published by the American Concrete Institute in the August 2013 issue of the journal Concrete International. NIST will soon release Technical Note 1812, “Best Practices Guide for High-Volume Fly Ash Concretes: Assuring Properties and Performance.”