Study on mechanical and permeability characteristics of nickel-copper-contaminated soil solidified by CFG

The study introduces the potential use of a novel curing agent formed by mixing cement, fly ash, and desulfurization gypsum (CFG) for the stabilization/solidification of nickel-copper-contaminated soil. According to the tests of the unconfined compressive strength (UCS), permeability, and scanning electron microscope (SEM), the samples' characteristics, which include UCS, deformation, and permeability, are investigated under different curing time, ion concentration, and curing agent content. The correlation between the UCS and permeability of samples is estimated. The results demonstrates that the gradual increase of the UCS and impermeability of the samples can be recognized with the increase of curing age; however, the failure strain decreased gradually but the trend is reversed with the increase of heavy metal concentration. Besides, greater amounts of CFG results in better mechanical and impermeability properties. The correlation between UCS and permeability coefficient reveals that the UCS of contaminated soil is significantly negatively correlated with permeability coefficient, and the prediction formulas of permeability coefficient based on age and CFG content are given respectively. Moreover, the microscopic analysis demonstrates that the hydration products calcium silicate hydrate(C-S-H) and ettringite (AFt) of CFG curing agent are the main reasons for the enhancement of the UCS and impermeability of contaminated soil. However, the increase of nickel and copper concentration leads to the decrease of hydration products and the compactness of solidified soil, which is the reason for the weakening of the compressive strength and impermeability of contaminated soil.