Method For Modifying Mixed Construction Muck And Surplus Sludge Based On Microbial Induced Carbonate Precipitation(MICP)

At present,the resource utilization rate of construction muck and surplus sludge is low,and a large amount of construction muck and surplus sludge are disposed of at will.The mixed modification treatment of the construction muck and surplus sludge can improve the soil mechanical stability and increase the air permeability of the surplus sludge,if the heavy metal pollution can be reduced,it is expected to be used as reclaimed greening soil.In this paper,Microbially Induced Carbonate Precipitation(MICP)technology is used to reduce heavy metal pollution and improve the environmental geotechnical characteristics of construction muck and surplus sludge,so as to realize the resource utilization of construction muck and surplus sludge.In this paper,the environmental geotechnical characteristics of construction muck,surplus sludge and their mixtures were systematically tested,and the modification direction of the mixture was proposed;carried out a simulated column test using MICP technology to modify five different proportions of construction muck and surplus sludge mixture.The shear wave velocity,urease activity,and CaCO3 content of the mixture were monitored during the MICP modification process;the mechanism of improving the strength of the mixture,the change of hydraulic characteristics,and the stabilization of heavy metals were revealed during the modification of the MICP.The main contents and conclusions are as follows:(1)The construction muck sample belongs to sandy silt,with a large range of main particle sizes,low water content,and high strength and permeability coefficient;the surplus sludge sample belongs to high liquid limit clay,which has a small range of main particle sizes high water volume,low strength and low permeability coefficient.With the decrease of the mixing ratio of construction muck,the permeability coefficient and internal friction angle of the mixture decrease,and the water holding capacity and cohesive force increase.When the mixing ratio of construction muck is 90%,the permeability coefficient of the mixture can meet the requirements of the "Greening Planting Soil Standard".When the mixing ratio of construction muck is 70%,the organic matter and electrical conductivity EC of the mixture meet the requirements of the above standards.The heavy metal content of different proportions of the mixture does not meet the requirements of the above standards,and measures need to be taken to reduce its potential heavy metal pollution.(2)The microbial urease activity was high and the MICP reaction was strong 12 to 20 hours after the completion of the microbial bacterial fluid and cement fluid infusion.Within 10 hours after the completion of the infusion of microbial bacterial fluid and cement,the shear wave velocity of the five different proportions of the mixture continued to increase and remained stable after reaching a certain shear wave velocity;The highest shear wave velocity and calcium carbonate content of the mixture gradually increase with the higher incorporation ratio of construction muck.After the perfusion of microbial bacterial liquid and cement liquid was completed,the calcium carbonate content in the simulated column showed a trend of rapid increase first and then slow increase.(3)The unconfined compressive strength of the modified mixture is 66.4?129.7 kPa;compared with the mixture before modification at the same ratio,the permeability coefficient of the modified mixture and the leaching toxicity of heavy metals are significantly reduced,and the water retention Performance,pH,organic matter content,and electrical conductivity EC all increased.When the mixing ratio of construction muck in the mixture reaches 90%,except for As and Ni,the leaching toxicity of the heavy metal of the modified mixture meets the"groundwater quality standards" Class ? water and "surface water environmental quality standards" V Water-like requirements.When the mixing ratio of construction muck in the mixture is less than 80%,the organic matter content can meet the requirements of the "Greening Planting Soil Standard".The conductivity EC of the modified mixture cannot meet the requirements of the above standards.Based on the dynamic test results of the hybrid modified simulation column,it is recommended that the optimal mixing ratio of construction muck and surplus sludge is 80:20,and the indicators of the mixture can basically meet the requirements of greening and planting soil.(4)With the increase of calcium carbonate content,the shear wave velocity of the mixture shows a clear linear increase trend.Therefore,the variation law of the calcium carbonate content in the mixture can be obtained indirectly by monitoring the shear wave velocity.With the increase of calcium carbonate content,the saturation permeability coefficient gradually decreases;this is because the calcium carbonate crystals generated by the MICP process fill the pores between the particles of the mixture,reducing the porosity of the mixture,thereby reducing the percolation channel.With the increase of calcium carbonate content,the unconfined compressive strength of the mixture gradually increases;this is because the MICP process on the one hand reduces the porosity of the mixture by calcium carbonate filling on the one hand,making it more dense,and on the other hand by carbonic acid Calcium cementation increases the contact area between particles and enhances the adhesion between particles.In addition,the MICP process fixes heavy metals through carbonation and physical sealing,thereby reducing the leaching concentration of the heavy metals in the mixture.