Leaching Behavior of Lime–Fly Ash Mixtures

Abstract

As part of a larger investigation that included numerical and field-based components, the use of lime to reduce the leachability of a coal combustion fly ash was evaluated in the laboratory. The focus of this paper is on the experimental assessment of lime–fly ash leachability through sequential leach (SL), freeze–thaw (FT), and wet–dry (WD) leaching as well as multileachant sequential extraction (SE) tests. The objectives were to study the leachability of trace elements, including arsenic (As), cadmium (Cd), chromium (Cr), and selenium (Se). These results suggest that lime addition reduces the leachability for Cd, Se, and to some extent As. They also suggest that Cr is rendered more leachable with increasing lime content, for the conditions and low levels tested. It appears that there is a threshold lime content (>1.0%) that must be exceeded prior to reducing the leachability of As and Se. In particular, this threshold likely corresponds to the level at which appreciable cementitious reactions have developed. For example, in the case of As after the first cycle of leaching, the concentration was below the reporting limit (10 µg/L) for 0% lime. However, at 0.5% lime amendment, the leached concentration increased to nearly 50 µg/L. Subsequent lime additions reduced this concentration. No such threshold was observed for Cd leachability as was expected as a direct consequence of hydroxide precipitation, which is well established under the measured pH conditions. As such, Cd mobility is insensitive to the extent to which cementitious reactions are initiated. Overall, the results suggest that while lime stabilization may be effective for reducing leachability, sufficient amounts must be added; otherwise, the leachability of some constituents can actually be exacerbated.