New Coal Ash Use: Strong, Lightweight Alternative to Concrete Without Cement

by John Toon, Georgia Institute of Technology

Each year, coal-burning power plants, steel factories and similar facilities in the United States produce more than 125 million tons of waste, much of it fly and bottom ash left from combustion. Mulalo Doyoyo has plans for that material.

An assistant professor in Georgia Tech’s School of Civil and Environmental Engineering, Doyoyo has developed a structural material based on leftovers from coal burning. Cenocell is high-strength and lightweight—without cement, an essential ingredient of conventional concrete. With broad potential applications and advantages such as good insulating properties and fire resistance, the green material could replace concrete, wood and other materials in construction, transportation and aerospace applications.

Mulalo Doyoyo, assistant professor in the Georgia Tech School of Civil and Environmental Engineering, poses with samples of the new material produced from coal ash. (Georgia Tech Photo: Gary Meek)
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“Dealing with the ash left over from burning coal is a problem all over the world,” Doyoyo said. “By using it for real applications, our process can make the ash a useful commodity instead of a waste product. It could also create new industry and new jobs in parts of the world that need them badly.”

Fly ash is composed of small particles removed from combustion gases by pollution-control systems. Most must be disposed of as waste, though types of fly ash can replace a portion of the cement used in conventional concrete.

Cenocell, produced from either fly or bottom ash in a reaction with organic chemicals, requires none of the cement or aggregate—sand and rock—used in concrete. And unlike concrete, it emerges from curing ovens in final form and does not require a lengthy period to reach full strength.

“This is a new material very different from concrete,” Doyoyo said.

Because it uses what is now considered a waste material to replace cement, which generates carbon dioxide—a greenhouse gas—the new material is considered an asset to the environment. The material can have a wide range of properties that make it competitive with concrete, especially the new classes of autoclaved lightweight concrete.

For instance, specific densities range from 0.3 to 1.6, and the material can be manufactured to withstand pressures of up to 7,000 pounds per cubic inch. The properties can be controlled by choosing the proper ash particles size, chemical composition and curing time, which can range from three to 24 hours.

Georgia Tech researchers prepare to test samples of Cenocell, a new material produced from coal ash. Strong and lightweight, Cenocell could replace concrete in certain applications. (Georgia Tech Photo: Baary Meek)
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“We have a wide range in terms of texture, properties, performance and applications,” Doyoyo said. “The possibilities for this material are very broad.”

Among potential applications for the material are:

  • Building and construction industry—infrastructure materials that provide sound, crash and fire barriers; permeable pavements; drainage fillers; ultralight truss stiffeners, foam, wood and concrete replacements in residential and commercial buildings; and acoustical tiles. Cenocell is lighter than most lightweight concrete, and lightweight versions can be machined and cut with standard band saws.
  • Transportation industry—cores for shock and crash absorbers; fillers for trailer floors or b-pillars in vehicle frames.
  • Aerospace industry—ultralight heat shielding.
  • Protective installations—fireproof blast walls or structural fillers for hazardous fluids.

Though for competitive reasons he won’t disclose the precise chemical composition of Cenocell, Doyoyo said the processing involves mixing ash with organic chemicals. The chemical reaction produces foaming and results in a gray slurry that resembles bread dough. The material is then placed in forms and cured in ovens at approximately 100 degrees Celsius until the desired strength is attained.

Cenocell is a new material produced from coal ash. Material properties can be varied by controlling the chemical composition and curing time. (Georgia Tech Photo: Baary Meek)
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“We form a final compound through a combination of chemical and mechanical processes,” Doyoyo said. “Once it comes out of our process, it is ready to go and does not continue to change over time.”

Unlike concrete, which remains a mixture of materials held together by chemical bonds, Cenocell is a homogenous material. The cell sizes and final strength depend on the curing time and size of the ash particles used. Estimates suggest the material could be manufactured for an average cost of $50 per cubic yard.

Doyoyo and his research team, which includes Paul Biju-Duvall, Julien Claus, Dereck Major, Rolan Duvvury and Josh Gresham, have made only small samples for testing.

Scanning electron microscope image of the Cenocell material(Image Courtesy of Mulalo Doyoyo)
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They are working with a Georgia-based maker of autoclaved concrete to produce larger samples for additional testing. Large-scale manufacturing could be done with the same equipment now used to make autoclaved concrete, Doyoyo said.

He presented information about the material May 4-7 during the World of Coal Ash meeting.

“We are focusing a lot on the construction industry,” Doyoyo said. “When this material is used to build a structure, it will save a lot of energy for heating and air conditioning because of its good insulating properties.”

A native of South Africa who was educated at the University of Cape Town, Brown University and Massachusetts Institute of Technology, Doyoyo sees value beyond the reuse of a waste material. He thinks Cenocell could provide low-cost housing in developing countries and economic development impact from a new industry.

“This material could help develop communities by allowing people living near coal-burning facilities to create a new industry and new jobs,” he said. “This could be an engine of development for people who have been struggling.

“It really is a material with a social conscience.”


John Toon is the manager of the Research News & Publications Office of Georgia Tech Institute of Technology. Reach him at

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