Date of Award
Doctor of Philosophy
Civil, Construction, and Environmental Engineering
Industrial byproducts, fly ash and steel slag are widely used in pavement subgrade soil stabilization. Similarly, recycled concrete aggregates (RCA) are one of the most commonly used waste materials in pavement base constructions. These materials are known to leach heavy metals and other elements of environmental concerns. On the other hand, cement is frequently used as an activator during fly ash and slag stabilization, which considerably increase the material pH, and hence, the leaching potentials of toxic constituents. The pH of RCA greatly varies with age, source and storage conditions due to carbonation. Additionally, the influence of matric potential on the leaching behaviors of RCA remained unknown. This study investigated the leaching characteristics of RCA, and cement activated fly ash and steel slag stabilized soils through laboratory batch water leach test, toxicity characteristic leaching procedure, synthetic precipitation leaching procedure and pH-dependent leach tests. The effluent concentrations of calcium (Ca), magnesium (Mg), aluminum (Al), barium (Ba), chromium (Cr), copper (Cu), iron (Fe), zinc (Zn), manganese (Mn), sulfur (S), dissolved inorganic/organic carbon (DIC/DOC), sulfate (SO4), pH, electrical conductivity (EC) and alkalinity were quantified. Moreover, geochemical modeling was performed to evaluate the leaching mechanisms of these elements. Results indicated that the effluent concentrations of Ca, Ba, Al, SO4 increased; Mg, Cu and DIC concentrations decreased; Fe, Zn and DOC fluctuated; and Cr and Mn concentrations remained unaffected by cement addition. Cement activation altered the pH dependent release of Ca, Cr, Ba, Zn, S, SO4, DIC and DOC, noticeably. Multiple batch tests were required for the comprehensive leaching assessment. Geochemical modeling indicated that, except for Cr, the releases of elements from cement activated fly ash and slag stabilized soils were solubility controlled. In case of RCA, leaching characteristics were considerably influenced by carbonation, particle sizes and liquid to solid ratios (L/S). Distinct pH dependent releases of elements were observed relying on degree of carbonation. The selection of appropriate leach tests depended on RCA carbonation and constituent of potential concern. Matric potentials at different saturation conditions were found to have an impact on the release of elements from RCA. The highest solution pH and leached concentrations of Ca and Ba were originated in the matric potential range of 2 kPa to 5 kPa.
Mahedi, Masrur, "Geoenvironmental aspects of recycled materials in pavement applications: Leaching characterizations" (2019). Graduate Theses and Dissertations. 17257.