Effect of Zn²⁺-Cu²⁺ combined heavy metal on mechanical properties and microstructure of clayey soil

The strength deterioration mechanism of soil polluted by heavy metals plays a crucial role in the research of mine site pollution. In this study, an unconfined compressive strength (UCS) test, a pH test, a scanning electron microscopy (SEM) test, a low filed nuclear magnetic resonance (NMR) test, and an X-ray diffraction (XRD) test were conducted on Zn²⁺, Cu²⁺ and the combination of Zn²⁺ and Cu²⁺ polluted soil to investigate the strength deterioration mechanism. The results show that both the UCS and pH value of soil decrease with increasing heavy metal concentration. The UCS of Zn²⁺-Cu²⁺ combined polluted soil is between Zn²⁺ and Cu²⁺ polluted soil at the same total concentration. However, the deterioration rate of combined heavy metal polluted soil is less than the sum of deterioration rate of the two single polluted soils at the same total concentration. In addition, heavy metal cations in polluted soil cause flocculent gels of cosmids to shrink, the micropores to become smaller and the macropores to become larger. The porosity increases slightly with the increase of heavy metal concentration due to decreased pH value. The results from SEM, low field NMR, and pH could explain the dynamic evolution process of soil structure with different heavy metals and concentrations, which provides an experimental basis for mine-site polluted heavy metal treatment technology and the prediction of clayey soil strength deterioration.