Coal Ash-based Material Could Replace Concrete
Mulalo Doyoyo, an assistant professor in Georgia Tech's School of Civil and Environmental Engineering, has developed a new structural material based on fly ash and bottom ash from coal burning. Known as Cenocell™, the material is strong and lightweight without the use of cement.
With advantages such as good insulating properties and fire resistance, the material could replace concrete, wood, and other materials in construction, transportation, and even aerospace.
"Dealing with the ash left over from burning coal is a problem all over the world," said Doyoyo. "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 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 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.
Because it uses what is now considered a waste material to replace cement -- which generates carbon dioxide -- the new material is considered an asset. It 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 the curing time, which can range from three to 24 hours.
"We have a wide range in terms of texture, properties, performance and applications," said Doyoyo. "The possibilities for this material are very broad."
Among the potential applications for the material are:
• Infrastructure materials that provide sound, crash and fire barriers; permeable pavements; drainage fillers; ultra-light 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.
• Cores for shock and crash absorbers; fillers for trailer floors or b-pillars in vehicle frames.
• Ultra-light heat shielding.
• Fireproof blast walls or structural fillers for hazardous fluids.
Though for competitive reasons he won’t 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 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.
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.
Doyoyo will present information about the material at the World of Coal Ash meeting May 4-7, 2009.
"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 re-use of a waste material. He believes 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."