Experimental Study On Multiple Solidification Methods Of Sludge Based On MICP

Most of the plains in Fujian Province are distributed along the coast and contain deep silt layer.Therefore,its deep foundation pit engineering,offshore wind power generation foundation will involve the solidification of silt soil.The way to solidify the sludge is mostly to add cement to make cement soil.However,the mass production of cement will deepen the concentration of carbon dioxide in the atmosphere.Microbial induced calcium carbonate precipitation（MICP）refers to the use of physiological activities of microorganisms to produce calcium carbonate precipitation under a certain environment to solidify sand particles,which is an environmentally friendly foundation reinforcement method.In this paper,the MICP function of Bacillus pasteuricus and the combination of additives such as gypsum and cement are considered to strengthen the sludge,aiming at using a large amount of waste phosphogypsum,replacing or reducing the amount of cement,and achieving the effect of green environmental protection,energy conservation and emission reduction.The main contents of this article are as follows:（1）The changes of physical and mechanical properties and microscopic morphological characteristics of MICP reinforced sludge（MICP sludge）at different ages,bacterial solution concentration(OD₆₀₀)and cement concentration were investigated.The results show that the strength of the strengthened sample increases with the increase of the bacterial solution concentration.When the OD₆₀₀is 4 and the cement solution concentration is 2M,the reinforcement effect is the best.After 7 days,the unconfined compressive strength（UCS）of the sample can reach 104.96 k Pa.However,the reactants and products of MICP will reduce the liquid-plastic limit of the sample.The hexahedral calcite produced by MICP binds the surrounding flaky silt particles into aggregates,but the quantity of cemented silt particles is small,resulting in smaller aggregates,so the cementation efficiency is low.（2）MICP and phosphogypsum were used to reinforce the sludge,and the OD₆₀₀of the bacterial liquid was 4.The physical and mechanical properties and microscopic morphological characteristics of the sludge at different ages,gypsum content and cement concentration were investigated.The test shows that phosphogypsum can provide a certain amount of calcium ions and help solidify the sludge.The strength of the strengthened sample increases with the increase of various influence factors.When the gypsum content is 18%,the cement concentration is 1.5M,and the age is 7 days,the maximum UCS can reach 178.64 k Pa,while the liquid-plastic limit only increases with the increase of gypsum content.The microscopic test results of the materials show that the strip-like calcium sulfate crystals interlace to form a skeleton,while a large number of rod-like aragonites produced by MICP connect the sludge particles between the flaky silt particles to form a"connection-open"structural unit,which is constantly superimposed and grows into aggregates,and the agglomerates adhere to the calcium sulfate skeleton to solidify the sludge,but there are still a lot of pores in it.（3）The mechanical mixing method is used to reinforce the sludge with MICP and cement.The concentration of bacteria solution used is OD₆₀₀=4.The physical and mechanical properties and microscopic morphology of MICP-cement-soil and cement-soil formed by MICP-cement-soil are compared to explore the reinforcement mechanism of MICP-cement-soil.The test results show that when the cement content is 18%,the urea concentration is 1.5M and the calcium ion concentration is 0.5M,the effect is the best,and the increase is the largest in 0-7days,and then the increase is slow,and the UCS is up to 21.51MPa after 90 days.The micro-material test shows that the cement hydration products cement the sludge into aggregates and form a skeleton.The calcite produced by the MICP action covers the surface of the aggregates and fills the pores,and the staggered calcites bite each other,which increases the mechanical strength of MICP-cement-soil.In addition,the products and reactants of MICP accelerate the hydration reaction of cement and play the same role as the early strength agent of carbonate system and alkaline earth metal system in MICP-cement soil.The research work in this paper reveals the reinforcement mechanisms of three types of silt reinforcement methods based on MICP,providing a reference for the future exploration and application of the three types of silt reinforcement methods,and providing a certain idea for MICP reinforcement of fine grained soil.