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.