Greater use of fly ash in concrete

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2011

An eco-friendly process being evaluated uses more fly ash in place of cement.

KEY CONCEPTS
• Fly ash has been used as a replacement for cement in concrete to a limited degree over the past 70 years.
• Testing has shown that up to 75% of the cement used in concrete can be replaced with fly ash to form a new material known as high-volume fly ash concrete (HVFA).
• HVFA exhibits comparable strength to conventional concrete but has some performance concerns that are in the process of being addressed.

Fly ash is a byproduct generated in coal-burning power plants during the combustion process. It is a combination of calcium, aluminate and silicon that is so light it would escape from the smokestacks if it were not captured by electrostatic precipitators.

Jeffery Volz, assistant professor in the department of civil, architectural and environmental engineering at Missouri University of Science and Technology in Rolla, Mo., says, “Fly ash contains very small spherical particles that have diameters ranging from 100 to 200 microns. They represent the minerals in coal left over after combustion.”

In the United States, there are more than 450 coal-burning electric power plants. They produce approximately 130 million tons of fly ash each year. Most of the fly ash captured is either stored in coal power plants or placed in landfills.

Volz says, “Approximately 43% of the fly ash is used as a component in either wallboard or concrete. In the latter application, up to 25% of the cement used to make concrete has been replaced by fly ash. The use of fly ash at this level has been going on for about 70 years. One of the first structures built with fly ash was the Hoover Dam, which is on the border between the U.S. states of Arizona and Nevada. The Hoover Dam was completed in 1936. ”

Fly ash is used because it reduces the temperature rise that occurs during the hydration of Portland cement. It has also been found to improve the durability of concrete used in infrastructure applications such as bridges and roadways.

Concrete is produced by mixing Portland cement, water and gravel or sand. One problem with using cement is that a large amount of carbon dioxide is generated during its production from heating limestone and other materials at elevated temperatures.

Volz says, “With our crumbling infrastructure, demand for concrete will certainly increase rapidly. We need to find a way to use more fly ash and less cement in order to develop a more environmentally favorable process.”

The use of more fly ash will reduce the amount that has to be placed in landfills and also reduce carbon dioxide emissions. Such a concrete has now been produced and is in the process of being evaluated.

HIGH-VOLUME FLY ASH CONCRETE
Volz is heading up a study to determine how much additional fly ash can be used in place of cement in concrete. He says, “We have been able to replace as much as 75% of the cement used in concrete with fly ash. We designate the new material as high-volume fly ash (HVFA) concrete.”

Volz and his co-workers are running tests to determine if HVFA concrete has comparable performance properties to conventional concrete. Volz says, “The No. 1 requirement for concrete is strength.” 

Compression strength testing is used to measure this property by steadily applying increasing loads on concrete until cracks appear. A beam of concrete evaluated in this fashion is shown in Figure 3.


Figure 3. In compression strength testing, high-volume fly ash concrete exhibits comparable strength to conventional concrete. (Courtesy of B.A. Rupert/Missouri University of Science and Technology)

Volz says, “The additional fly ash enables the resulting concrete to be denser. The strength of the concrete over the long-term is just as good. We have been able to develop HVFA concrete-containing up to 75% cement replacement that can reach compressive strengths of 4,000 to 5,000 pounds per square inch, which is comparable to conventional concrete.”

Volz indicates that the use of additional fly ash reduces the increase in temperature that occurs when cement is hydrated with water. He says, “During hydration, one pound of cement produces one pound of excess calcium hydroxide (lime) which tends to leach out of the concrete. Fly ash reduces this process and also lowers the ratio of water to cement that needs to be used in manufacturing concrete. In effect, fly ash acts in a similar fashion to a ball bearing by enabling concrete to flow better.”

One of the concerns facing Volz is that HVFA concrete takes longer to set. He says, “We are working to incorporate activators into the concrete formulation to accelerate the setting process.”

Another issue is that the HVFA concrete has inferior scaling resistance compared to conventional concrete. Volz says, “HFVA concrete does not perform as well in freeze-thaw cycles occurring in the presence of deicing salts. We believe that chloride anions are causing problems on the surface of the concrete, which is leading to pieces coming off of the material.” 

A further challenge for Volz is that the fly ash produced is dependent upon the type of coal used and may be unique to each coal-burning power plant. He says, “Fly ash is produced from either Class F—which is known as bituminous—or Class C, which is known as sub-bituminous or lignite coal.”

Future work will involve development of a test that can be used to determine how much fly ash produced from a specific power plant can be incorporated into HVFA concrete. Much of the work has been focused on power plants in the state of Missouri but Volz hopes to take his work nationwide.

Additional information is available in an article and video available on the Web here or by contacting Volz at volzj@mst.edu. 

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.