Abstract: High-calcium fly ash is an industrial by-product generated during the combustion of coal for energy production, which is characterized by higher concentrations of calcium, carbon and hydraulic minerals. Previous studies have proved that high-calcium fly ash can enhance the strength of heavy metal contaminated soils and lower the mobility of heavy metal ions. In this paper, laboratory experiments and theoretical analysis are used to investigate the properties of strength, leaching, drying and wetting cycle of Pb and Zn contaminated soils solidified/stabilized by high calcium fly ash. The results show that the unconfined compressive strength of soil decreases due to the presence of heavy metal ions. Adding high-calcium fly ash to the contaminated soils can significantly improve the strength and reduce the amount of heavy metal ions that filter out. The strength of Zn contaminated soils solidified with fly ash is superior to the Pb contaminated soils. Both Pb2+ and Zn2+ can be effectively immobilized at low concentration; while Zn2+ is more effectively immobilized at high concentration. With the times of wetting and drying cycle increases, the unconfined compressive strength increases first, and then decreases. The heavy metal concentration of leachate increases as the times of wetting and drying cycle increases when the content of heavy metal in the soil is low. Times of wetting and drying cycle have little effect on the leaching properties of heavy metal when it is at a high level in the soil.

Key words: high calcium fly ash, heavy metal contaminated soil, solidification/stabilization, cyclic drying and wetting