Effect Of Carbonation On Behavior Of Cement Stabilized/solidified Clays Contaminated With Heavy Metal

Stabilization/solidification technology has been extensively used for remediation of heavy metal contaminated soils.Physical and chemical erosions from complex external environment can affect the long-term effectiveness of solidified contaminated soils.Carbonation is one of the main factors that can change the properties of solidified contaminated soils under unsaturated condition.It is necessary to study the behavior of solidified contaminated soils during carbonation process in order to ensure safety of further utilization of heavy metal contaminated sites.The study can provide theoretical basis for carbonation control and utilization of solidified heavy metal contaminated soils,and is very useful to facilitate the design of stabilization/solidification technology and to improve the remediation level of heavy metal contaminated sites in China.Studies on the influence of carbonation on the behavior of solidified contaminated soils are very limited.Based on this background,with the financial support of the National Natural Science Foundation of China（Grant No.51578148）and the Research Innovation Program for College Graduates of Jiangsu Province（No.KYLX16＿0241）,the carbonation effect of cement-solidified clays contaminated with heavy metal is systematically studied,through the combination of indoor experiment and theoretical analysis,and the combination of macro test and micro experiment.The research contents include the test and evaluation of carbonation depth of cement-solidified soils,the influence of carbonation on microstructure,strength and leaching behavior of solidified contaminated soils,and the evaluation method of leaching behavior of solidified contaminated soils considering carbonation effect.The main works and conclusions can be given as follows:（1）A series of indoor accelerated carbonation tests are carried out to analyze the variations of carbonation depth,carbonation coefficient,diffusion coefficient of CO₂ and electrical resistivity of cement-solidified soils.Based on the air permeability model of unsaturated soils and Papadakis carbonation model,a power function correlation between diffusion coefficient of CO₂ and air-filled porosity of solidified soils is obtained,and a prediction model of carbonation depth for solidified soils is established.An evaluation method of carbonation depth of cement-solidified soils is proposed based on the electrical resistivity measurement,considering the linear relationship between electrical resistivity and carbonation depth of solidified soils.（2）A series of X-ray diffraction（XRD）,scanning electron microscopy（SEM）,thermogravimetric analysis（TGA）and mercury intrusion porosimetry（MIP）tests and pore solution chemical tests are carried out to analyze the influence of carbonation on cement reaction products,mineral forms of lead and zinc,micromineral morphology,cumulative pore volume and pore distribution density.The mass distributions of Ca CO₃,CSH and CH,the carbonation degree distribution of cement-solidified soils,and the variations of conductivity,p H value and ORP potential of pore solution during carbonation process are also analyzed.A linear relationship between carbonation degree and p H value of pore solution is obtained.The microstructure mechanism of carbonation for solidified soils is revealed.（3）The effects of carbonation,lead content,cement content and dry density on physical properties,unconfined compressive strength,deformation modulus,failure strain and stress-strain characteristic of cement-solidified soils are investigated by unconfined compressive tests.A power function relationship between unconfined compressive strength and porosity of solidified soils is obtained.The internal control mechanism of the variation of strength characteristic for cement-solidified clays contaminated with heavy metal under carbonation is discussed.（4）A series of acid/base titration tests,p H-dependent leaching tests and sequential extraction procedures are carried out to investigate the effects of carbonation on acid/base buffering capacity,alkalinity of solidified contaminated soils and solubility of lead and zinc.The relationship between solubility of lead and zinc in solidified soils under carbonation and p H value is revealed.The effects of carbonation and acid rain on chemical speciation of lead and zinc in solidified soils are analyzed and verified with the variation of solubility of lead and zinc.Based on the variation of microstructure parameters,the mechanism of the variation of solubility of lead and zinc in solidified soils under carbonation is discussed.（5）A series of semi-dynamic leaching tests are performed on carbonated and non-carbonated specimens using simulated acid rain leachates with different p H values.The variations of p H value,ORP potential,mass of solidified soils and lead concentration in the leachate are analyzed.The relationship between lead concentration and leachate p H is discussed.The influence and internal mechanism of carbonation on lead release behavior from solidified soils under different p H conditions are investigated.The diffusion coefficient of lead is calculated.The research results can be used to evaluate the long-term release potential of lead from cement-solidified contaminatd clays.（6）Based on an integrated framework for evaluating leaching in waste management and utilization of secondary materials,considering the influence of carbonation on leaching behavior of heavy metal,a three-tiered framework considering carbonation effect for evaluating leaching in solidified contaminated soils is proposed.Based on the evaluation framework and the prediction model of carbonation depth of solidified soils,the calculation methods of long-term release of pollutants considering carbonation effect are given for chemical equilibrium-controlled and mass transfer-controlled scenarios.