Solidification/Stabilization of Pb2+ and Cd2+ Contaminated Soil Using Fly Ash and GGBS Based Geopolymer

Geopolymer is an environment-friendly cementitious material, which can effectively immobilize heavy metals. However, the existing study on solidification/stabilization (S/S) of heavy metal contaminated soil using geopolymer is very limited. In order to investigate the effects of geopolymer produced by activating fly ash (FA) and ground granulated blast-furnace slag (GGBS) (FA + GGBS) with composite activator of Na2SiO3 and NaOH on environmental and engineering properties of Pb2+ and Cd2+ contaminated soil, a series of tests including soil pH, leachate pH and electrical conductivity (EC), toxicity leaching, UCS and pH-dependent tests were conducted at different curing times. Additionally, scanning electron microscopy/energy dispersive spectroscope (SEM/EDS), X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) tests were performed to analyze the micro mechanism of FA + GGBS based geopolymer solidified contaminated soil. The results indicated that the UCS and pH of soil increased steadily with FA + GGBS content and curing time increasing. Moreover, as FA + GGBS content and curing time increased, the leached Pb2+, Cd2+ concentration decreased significantly, and the concentration was closely related to the pH of leachate. Compared with Cd2+, the solidification ratio of FA + GGBS based geopolymer for Pb2+ was lower. Besides, the leached Pb2+ and Cd2+ concentrations decreased first and then increased with increasing pH of extraction fluid. Microscopic analysis showed that the gel products of FA + GGBS based geopolymer were mainly C-(N)-A-S–H gels. However, the geopolymer structure became looser due to the existence of Pb2+ or Cd2+. The formation of Cd(OH)2 was the primary solidification mechanism of Cd2+, while Pb2+ was mainly immobilized in geopolymer structure by ion exchange. The MIP results showed that the volume of inter-aggregate pores between 0.01 and 1 μm reduced with increasing FA + GGBS content of solidified soil.