Mechanical and microstructural evolution of solidified/stabilized heavy metal-contaminated soil under a hydro-chemical-mechanical coupling environment

Abstract

The long-term effectiveness of Solidification/Stabilization (S/S) under realistic disposal scenarios is a common controversy of its application. This study sought to investigate the evolution of the mechanical properties and microstructure of Pb-Zn-Cd composite contaminated soil solidified/stabilized by cement and fly ash (CSCS) under a hydro-chemical-mechanical coupling environment (HCM). An HCM simulation system was developed to reproduce the simultaneous action of hydraulic, chemical, and stress environments on CSCS. The mechanical and microstructural evolution of CSCS under HCM was assessed using vertical settlement monitoring, unconfined compressive strength (UCS) test, computed tomography (CT), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The increasing environmental acidity induces an increase in vertical displacement and a decrease in UCS. The highest final settlement was 0.96 mm at pH 3 and a pressure of 250 kPa, which was 39.13 % greater than the value at pH 5 and 7. The UCS decreased by 18.54 % after 168 h of HCM treatment at pH 7, while the value reached 45.83 % when the pH was lowered to 3. Seepage contributes to initial hydration as evidenced by an increase in the early E50 of the CSCS. Hydrodynamic scouring and chemical erosion increase the pore size and deteriorate the soil structure, whereas compaction helps to redistribute soil particles, thus improving the structural integrity and uniformity of the CSCS.