Material Provides New Use for Coal Ash
July 1, 2009
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 ash and bottom ash left over 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 these leftovers from coal burning. Known as Cenocell, the material offers attributes that include high strength and light weight -- without the use of 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.
Fly ash is composed of small particles removed from combustion gases by pollution control systems. Most of it must now be disposed of as a waste product, though certain types of fly ash can be used to replace a portion of the cement used in conventional concrete. Cenocell, produced from either fly ash or bottom ash in a reaction with organic chemicals, requires none of the cement or aggregate (sand and rock0 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.
Though for competitive reasons, he would not disclose the precise chemical composition of Cenocell, Doyoyo says the processing involves mixing the 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 212°F (100°C) until the desired strength is attained.
“We form a final compound through a combination of chemical and mechanical processes,” Doyoyo explains. “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 both 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 also includes Paul Biju-Duvall, Julien Claus, Dereck Major, Rolan Duvvury and Josh Gresham -- have so far made only small samples for testing. 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, he says.