In situ stabilizing soil with fly ash has become a practical and economical solution for construction on soft ground. However, leaching of trace elements from stabilized soil can be a concern. Understanding the pH-dependent leaching behavior and mechanisms controlling release of elements from soil-fly ash mixture is important for assessing the environmental impacts associated with using fly ash in soil stabilization. In this study, pH-dependent leaching tests were conducted to investigate the leaching behavior of soilfly ash mixtures used in roadway construction. The soils included organic clay, silt, clay, and sand. The fly ashes included Class C and off-specification high-carbon fly ashes. Four leaching patterns as a function of pH were observed: (i) leaching of Ca, Cd, Mg, and Sr follows a cationic pattern; (ii) leaching of Al, Fe, Cr, Cu, and Zn follows an amphoteric pattern; (iii) leaching of As and Se follows an oxyanionic pattern for some mixtures and anomalous leaching patterns for other mixtures; and (iv) leaching of Ba presents amphoteric-like pattern but less pH-dependent. Modeling results from MINTEQA2 indicated that release of the elements, except As and Se are solubility-controlled. For a given element, the solubility-controlling solids were found to be very consistent. Oxide and hydroxide minerals control leaching of Al, Fe, Cr, and Zn, whereas carbonate minerals control leaching of Mg and Cd. Leaching of Cu is controlled by oxide and/or carbonate minerals. Both carbonate and sulfate minerals are controlling solids for Ca, Ba, and Sr depending on pH of the leachate. The difference and inconsistency between the release behavior for As and Se and the other elements are probably due to different controlling mechanisms, such as sorption, or solid-solution formation.
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