The solidification of chromium-contaminated soil using polyurethane (PU) was systematically investigated. The unconfined compression test was conducted to investigate the effects of the curing time, PU dosage and the content of chromium ions on the unconfined compressive strength (UCS) of chromium-contaminated soil. The effect of the PU dosage on the pore structure was investigated using nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), and the mechanism of strength change was revealed by combining the strength law with the pore structure development law. In addition, the ability of the PU to solidify the chromium-contaminated soil was studied by the toxicity characteristic leaching procedure (TCLP). According to the above test results, the UCS and the ability of the PU to solidify the chromium ions increased with the increase in curing time. The NMR tests showed that with the increase in PU dosage, the porosity decreased and the soil became more compact, hence increasing the strength. When the chromium ion content was 2000 mg/kg and the PU dosage was 8%, the strength of the sample was 0.37 MPa after curing for 24 h, which met the requirement of 0.35 MPa set by the U.S. Environmental Protection Agency. Consequently, PU is a solidification agent with high-early strength.

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With the application of fiber-reinforcement technology, the mechanical properties of silty clay are improved with fiber reinforcement. However, the variation of permeability coefficient and other parameters of flax-fiber reinforced silty clay have not been sufficiently studied. In this study, the permeability of flax-fiber reinforced silty clay soaked with zinc-contaminated solution under different osmotic pressure was tested by a flexible-wall permeameter, and the effects of zinc-ion concentration and confining pressure on the permeability of flax-fiber reinforced silty clay were studied. Genius XRF was employed to detect the types and quantity of metal elements in the specimens, thereafter, the reasons for the change of permeability were explained from chemical and microscopic perspective. The results showed that the permeability coefficient of flax-fiber reinforced silty clay decreased significantly with the increase of zinc-ion concentration in a low concentration (about 1-10 mg L −1). While in a high concentration (about 100 mg L −1), the permeability coefficient of flax-fiber reinforced silty clay changed little with the increase of zinc-ion concentration. While the flax-fiber reinforced silty clay was not soaked with zinc-ion solution, the permeability coefficient of the specimen increased with the increase of confining pressure. However, when the flax-fiber reinforced silty clay was soaked with zinc-contaminated solution, the permeability coefficient first decreased and then tended to be constant with the increase of confining pressure. With the increase of confining pressure, the porosity of flax-fiber reinforced silty clay decreased, and with the increase of zinc-ion concentration, the porosity of flax-fiber reinforced silty clay first increased and then decreased. In recent years, with the rapid development of industry, the pollution degree of industrial development area has been increasing 1-3. There are some engineering projects built on the ground of demolished landfills, and residual contaminants in the subgrade always change the physical and chemical properties of the ground soil 4-6. Depending on the survey bulletin of soil pollution in China published on April 17, 2018, the survey covers all of China's inland farmland, which included woodlands, grasslands, unused land and construction land, about 6.3 × 10 12 m 2. The result showed that the environmental in industrial wasteland was seriously polluted, and the soil pollution was common around the world. Among those pollutants, one class of the most serious pollutants is heavy metals. However, the eight kinds of heavy metals, including Cr, Ni, Cu, Zn, As, Cd, Hg and Pb, which accounted for the main part of inorganic pollution, and the average concentrations of them were 63.

Currently, less than 15% of waste iron tailings are utilized. Iron tailings can be used as fine aggregate in concrete, but this kind of concrete has no coarse aggregate, resulting in low strength. Additionally, iron tailings contain some heavy metals, which will cause environmental pollution if improperly treated. In this study, the mechanical properties, sulfate resistance, and pore structure distribution of basalt fiber-biochar-concrete (PFB) were studied. Where basalt is to enhance the mechanical properties of samples, and biochar is to adsorb heavy metals in iron tailings, to prepare environmentally friendly materials. Unconfined compressive strength (UCS) test, flexural strength (FS), sulfate immersion test, leaching behavior, and mercury intrusion porosimetry (MIP) test were used to study the performance of the samples, and X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), and scanning electron microscope (SEM) was used to characterize the samples, explaining the change mechanism of the macroscopic test. The results show that the compressive strength of PFB increased by 2.5% but the flexural strength increased by 12%. The basalt and biochar improve the pore size distribution of samples, that is, the pore size greater than 10 nm is reduced while the pore size between 2 and 6 nm is increased. Biochar can effectively adsorb heavy metals of Cu, Zn, Pb, and Cd, and their leaching concentration is reduced by 50–70%. Basalt fiber improves the mixing performance of concrete, while biochar with a small particle size fills the micro pores in concrete; this paper provides a new idea of sustainability for the preparation of environmentally friendly materials and the utilization of waste iron tailings.

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