Abstract
Coal combustion byproducts such as high-carbon fly ash (HCFA) are commonly used in highway construction as a stabilizing agent or soil amendment. Annually, 131 million tons of fly ash is produced in the United States. Unfortunately, only 40% of this fly ash is recycled, most of which are Class C and Class F types. These types of fly ash are generally used in concrete and cement production. However, HCFA cannot be used in such processes because of its high carbon content. Therefore, large amounts of HCFA are being put into landfills and causing significant economic loss and loss of valuable land space. Reuse of HCFA in geotechnical structures, particularly in road embankment construction, keeps these materials out of landfills. However, it is important to determine the environmental suitability of HCFA as a soil amendment because it poses a great potential for metal leaching. To evaluate environmental suitability, a series of column leach tests were conducted to determine the leaching of metals (aluminum, arsenic, and chromium) from soils amended with fly ash. In addition, numerical computer modeling programs were used to predict the metal concentrations in the soil vadose zone and ground-water. Results from laboratory tests indicated that metal concentrations increased with an increase in fly ash content and did not exceed the maximum concentration limits for drinking water, with few exceptions. Numerical modeling techniques predicted that leached metal concentrations decreased in the soil vadose zone with time and that concentrations of leached metals would be below environmental regulatory limits.
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