The Conditional Construction And Mechanism Based On Ettringite Stabilization/Solidified Heavy Metal Contaminated Soils

Solidification/stabilization is one of the main treatment methods for heavy metal contaminated sites.As a commonly used binder,cement hardly meets the specification requirements for the solidification/stabilization of high concentration heavy metal.This article is based on the National Natural Science Foundation of China?No.51578148?.This study develops ASC as a new ettrigite-based binder based on the research of the typical pollutants(Pb²⁺,Cd²⁺,Cr³⁺,and CrO₄^2-)in heavy metal contaminated sites.Through a series of physical and mechanical tests,leaching tests and microscopic tests,the ratio of binder ASC was determined based on the results of test.And combined with the analysis of the microstructure characteristics of the solidified solids,the solidification mechanism and dissolution mechanism of heavy metal pollutants solidified by binder ASC was revealed.The main contents and results are simply introduced as below.?1?Definitude the composition of the binder ASC.The influences of pH values,Ca²⁺,Al³⁺and SO₄^2-contents on the formation and stability of ettringite were studied when the ratios of binder groups were different.Under the conditions,the effect of the binder ASC on the physical,mechanical and leaching characteristics of the heavy metal contaminated soil were studied.Based on the solidification/stabilization effect,the optimization ratio of the curing agent ASC was proposed.The results show that a large amount of ettringite can be produced when the molar ratio of Ca/Al is greater than 2.7 and the molar ratio of S/Al is greater than 0.4.And the filling of ettringite and the hydration of hydrated calcium silicate are the main mechanisms frincreasing the strength of solidified soil.?2?The effect of cement and ASC binder on solidification/stabilization of heavy metal contaminated sites was compared and analyzed.The effects of different binder incorporation on moisture content,dry density,pore structure,pH value,stress-strain curve,unconfined compressive strength,deformation modulus,and leaching concentration of heavy metals were investigated.The results show that,with the increase of binder content,the strength and deformation modulus of ASC-cured soil is different from that of cement-cured soil.Due to the filling and expansion of ettringite,the critical blending amount of binder ASC is 10%,and when the blending amount of binder is greater than 10%,the heavy metal dissolution of ASC solidified soil is less than that of cement solidified soil.?3?The effect of binder ASC on Pb²⁺,Cd²⁺,Cr³⁺with different contents of heavy metals was studied.The effects of different heavy metal contents on the moisture content,dry density,porosity,pH value,stress-strain curve,unconfined compressive strength,deformation modulus,and leaching concentration of heavy metals were studied.The results show that the Cr³⁺-containing solidified soil has the lowest concentration of leaching,the highest strength,the best anti-deformation ability and stability,followed by the Cd²⁺solidified soil,but the solidification rate is all above 99.5%.?4?The mechanism of solidifying/stabilizing heavy metal contaminated soil by binder ASC was discussed.Through semi-dynamic leaching test,effective diffusion coefficient and leaching index were calculated,and the dissolution mechanism of heavy metals was analyzed.Using microscopic tests such as mercury intrusion test?MIP?,scanning electron microscopy?SEM?,and X-ray diffraction?XRD?,the solidification mechanism of the binder ASC were analyzed.The results showed that the curing mechanism of the binder ASC was dominated by ion exchange and negative electricity adsorption of ettringite,followed by the formation of alkaline precipitates.The main dissolution mechanism of heavy metals is dissolved.With the increase of heavy metal content,the effective diffusion coefficient decreases,the leaching index increases.The effective diffusion coefficient is in 8.74E-11¹.83E-09 cm²/s,and the leaching index is betwwen 8.65¹0.13.When the trivalent cationic content is high,ettringite can effectively fill the pore of the soil,improve the strength of solidified soil,reduce the leaching concentration of heavy metals.So the curing effect of ASC on Cr³⁺is better than that of Pb²⁺.