Effect of chloride attack on strength and leaching properties of solidified/stabilized heavy metal contaminated soils

The deterioration of cement-based materials due to chloride attack is a well-known phenomenon attracting extensive attention worldwide. However, the effects of chloride attack on the strength and long-term stability of stabilized/solidified heavy metal contaminated soils have not been studied in detail. In the present work, systematic tests were developed to investigate the effects of chloride attack on the strength and leaching characteristics of heavy metal contaminated soils treated with cement and fly ash. The test results suggested that increasing the NaCl concentration from 0% to 15% had a detrimental influence on the unconfined compressive strength (UCS) of solidified/stabilized contaminated soil. The UCS of the specimens decreased during the initial 7 days of soaking and then increased at longer soaking periods. In leaching tests, the concentration of Pb2+ in the leachate decreased with the increasing NaCl concentration, while that of Zn2+ and Cr3+ increased. The leached Pb2+ concentration reached its maximum level after 7 days of soaking and then decreased significantly at longer soaking times. In contrast, the leached Zn2+ and Cr3+ concentrations decreased significantly during short-term immersion. The leachability of the three contaminants followed the order: Zn2+ > Cr3+ > Pb2+. The mineralogical analysis performed by X-ray diffraction revealed that high concentrations of heavy metal contaminants significantly inhibit the development of hydration under such an aggressive chloride environment. The microstructure analysis performed by scanning electron microscopy indicated that Friedel's salt was formed during chloride attack, resulting in larger porosity and a looser structure of the solidified soils. More compact and uniform structures were observed for the specimens soaked for 28 and 90 days than that soaked for 7 days.