Engineering and environmental behavior performance of magnesium potassium phosphate cement binder solidification/stabilization Zn-Cu contaminated soils

A large quantity of contaminated soil in abandoned industrial sites is polluted by heavy metal (HM) pollutants, among which zinc and copper pollution not only threaten human health but also damage the ecological environment. Currently, magnesium potassium phosphate cement (MKPC) is increasingly being used for the solidification/stabilization (S/S) of soils contaminated with a single HM pollutant. However, research on the application of MKPC for treating soils with composite HM pollutants remains limited. Therefore, this study investigates the application of MKPC in the S/S treatment of Zn
2+
, Cu
2+
, and composite contaminated soils, evaluating unconfined compressive strength (UCS), toxicity characteristic leaching procedure (TCLP), electrical conductivity (EC), and freeze-thaw cycle (F-T cycle) tests, as well as examining the mineral composition and microstructure based on the microscopic technique. When the concentrations of Zn
2+
and Cu
2+
were both 15000 mg kg
-1
, it was observed that the compressive strength of the solidified soil was approximately 4.0 times (0.99 MPa) that of the untreated soil (0.25 MPa), and EC decreased by 40.9% (from 6.92 to 2.82 mS cm
-1
) at the curing age of 28 days. The leaching concentrations of Zn
2+
and Cu
2+
were reduced to 6.56 mg L
-1
and 2.81 mg L
-1
, and were well below the USEPA standard of 15 mg L
-1
. However, the maximum leaching concentrations of Zn
2
⁺ and Cu
2
⁺ reached 43.23 mg L
-1
and 36.08 mg L
-1
after undergoing 9 freeze-thaw cycle tests, respectively, exceeding the regulatory limits. The formation of MKPC hydration products, MgKPO
4
·6H
2
O, effectively contributes to the S/S of Cu and Zn in the soil. Generally, the results of this study can offer useful guidance for the application of remediation techniques for composite contaminated soils.