Abstract: Aiming at evaluating the engineering feasibility of two self-hardening materials, slag-cement-bentonite (SCB) and magnesium oxide activated slag-bentonite (MASB), as vertical barriers for Cr(Ⅵ)-contaminated sites, the evolutions of their engineering properties and microstructures under the effect of K₂CrO₄ solution were investigated. Firstly, the changes in mass, appearance, thickness of the discoloration layer, unconfined compressive strength and permeability coefficient of the specimens after the immersion test were determined. Subsequently, based on the results of the macroscopic tests, some specimens were subjected to microscopic tests such as X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR) and elemental morphology sequential extraction. The results indicated that the masses of SCB and MASB specimens as well as the thickness of the discoloration layer increased with the increase of immersion time and the concentration of contaminated solution. This is mainly due to the generation of substances such as CaCrO₄ and MgCrO₄, as well as the physical adsorption of CrO₄^2- within the specimen. The unconfined compressive strengths of SCB and MASB specimens decreased with an increasing concentration, while the permeability coefficient increased with it. When the concentration of the contaminated solution reached 0.5 mol/L, the strengths of SCB and MASB specimens decreased by 42% and 60% respectively, while the permeability coefficients increased by more than 10 and 100 times respectively, which was largely due to the decrease in the generation of hydration products and the reduction of specimen densification under the effect of K₂CrO₄ solution. Comprehensive analysis revealed that although the two self-hardening materials, SCB and MASB, had good engineering performance under tap water immersion, their chemical compatibility with K₂CrO₄ solution was poor, and they were easy to deteriorate due to the effect of Cr(Ⅵ) contaminated solution. Therefore, when the concentration of Cr(Ⅵ) in the contaminated site reaches a large level, both materials can not directly meet the requirements for use, and modification studies need to be carried out.

Key words: self-hardening vertical barriers, slag-cement-bentonite, magnesium activated slag-bentonite, Cr(Ⅵ)-contaminated solution, macroscopic test, microstructure