A research team at the South Dakota School of Mines and Technology, led by Dr. M. R. Hansen, professor, civil and environmental engineering, has developed a sustainable type of concrete using 50 percent fly ash instead of cement. Cement, the critical ingredient in making concrete, is energy intensive to produce and contributes large amounts of carbon dioxide to the atmosphere. Fly ash is a waste product when coal is burned to generate electricity and is captured using electrostatic precipitators before it leaves the stack and is released into the environment. Concrete made with fly ash is sometimes called "green" concrete. If widely adopted, it could significantly help sustainability efforts by reducing energy consumption, reducing green house gasses and beneficially using a waste product.
Normally called High Volume Fly Ash (HVFA) concrete, this material is the culmination of a five-year research project at the School of Mines. Dr. Hansen, assisted by several civil engineering graduate students, tested many batches of concrete at various levels of cement replacement.
The factors important to concrete are chemical durability to combat alkali-silica reactivity and sulfate attack, strength gain and time of set. Fly ash has been used in concrete for many years but only in small amounts, replacing 10-20 percent of the cement by weight.
As described by Dr. Hansen, cement consists of tiny particles which are angular due to grinding, requiring extra water to make the concrete workable. Fly ash, on the other hand, consists of tiny particles which are round due to their formation in the hot gasses going up the stack. At the 50 percent replacement level, the ball-bearing effect of the fly ash spheres improves the concrete workability enough so that significant water can be removed from the mixture, reducing the water-cement ratio, increasing the early and long term strength, and outperforming the control concrete made without fly ash. Chemical durability must be verified by research and testing for each fly ash source.
Dr. Hansen has been invited as a keynote speaker to the International Conference on Advances in Concrete and Construction in February in Hyderabad, India to present a summary of this research. His refereed paper will be published in the proceedings. He will also present this report to the Concrete Technology Forum: Focus on Sustainable Development, in May 2008 at the Marriott Denver Tech Center. This research was partially supported by the National Science Foundation through the EPSCoR program and by Black Hills Corporation.