Strength and microstructure characteristics of cement-soda residue solidified/stabilized zinc contaminated soil subjected to freezing-thawing cycles

The remediation efficiency of the solidification/stabilization (s/s) method on soil contaminated with heavy metals may deteriorate when the treated soil is exposed to freezing-thawing (F-T) cycles. In this study, a series of laboratory tests were conducted to investigate the strength and microstructural characteristics of Zn-contaminated soil treated by cement-soda residue under cyclical F-T conditions. Test results showed that the stress-strain curves were able to be characterized by a strain softening model and the failure mode could be classified as brittle failure before subjecting to F-T cycles. After exposure to the F-T cycles, the stress-strain curves gradually began to resemble the hardening model, and the compressive strength decreased significantly. Unconfined compressive strength (UCS) of the specimens obtained the maximum value at Zn2+ concentration of 2000 mg/kg, regardless of their exposure to the F-T cycles; sample Zn0.2 was the most vulnerable. After being subjected to F-T cycles, micro-cracks formed in the specimens. These cracks would be enlarged by the continuous exposure to F-T cycles, which resulted in the reduction of strength. F-T cycles could significantly damage the crystalline structures of cement hydrates; the damage could also contribute to the deterioration of soil structure. Microstructural analysis results showed that the size of the samples' micro-pores decreased due to the migration of pore water during freezing, while the size of larger pores increased due to ice lens formation and expansion.