Experimental Study On The Effect And Mechanism Of Urease Inhibitors Applied In Microbially Induced Calcium Carbonate Precipitation For The Treatment Of Sandy Soil

Microbial Induced Calcium Carbonate Precipitation（MICP）has been developed gradually since 2004.As a new foundation treatment technology,MICP has become a research hotspot in geotechnical engineering.It is of great significance to carry out the research on MICP to meet the carbon peaking and carbon neutrality goals.MICP is limited by the life activities of microorganisms,therefore,there are many restrictions on the environmental conditions and grouting methods.Besides,the problem that the reaction speed is too fast to be effectively controlled is also a limit to the concentration and flow rate of the grout,and to the the cementation.Therefore,seeking a more efficient and convenient method to improve and optimize the MICP process is still an important topic in this field.In this dissertation,urease inhibitors,which are commonly used in agricultural and forestry production and clinical medicine,are introduced into MICP in geotechnical engineering for the first time to control and optimize the MICP reaction process.Firstly,a urease inhibitor,n-butyl thiophospho triamine（NBPT）,which is suitable for MICP reaction conditions was selected through theoretical study and microbial solution tests.Then,NBPT was applied to the MICP for the treatment of sandy soil.The mechanical properties of the treated sand columns,such as compressive strength,shear strength,permeability and uniformity,were tested to comprehensively evaluate the effect of NBPT,and the mechanism of NBPT in MICP reaction was analyzed.In addition,experimental sutdys on the common influencing factors of MICP were also carried out and the optimal values of influencing factors were proposed.The research results provided important basic datas and theoretical basis for the in-depth studys and practical applications of this method.The main contents and achievements of this dissertation are as follows:（1）The reaction mechanism of MICP was described in detail.The reaction includes two main steps,namely urease-catalyzed urea hydrolysis and calcium carbonate crystal precipitation.Based on the reaction mechanism,the way to solve problems brought by the fast reaction was explored.The idea of inhibiting urea hydrolysis rate and releasing urease activity was put forward.Urease inhibitors were introduced into the field of geotechnical engineering.The improvement effect of MICP on soil engineering properties was optimized from the reaction principle.（2）The action mechanism of urease inhibitor was studied in depth.The results of MICP solution tests suggested that NBPT was a suitable urease inhibitor for MICP reaction.It was also suggested that the concentration of NBPT should be within the range of 0.01%to 1%.（3）The effect of different concentrations of NBPT on MICP was studied by solution tests.The results showed that the involvement of NBPT in the reaction could reduce the hydrolysis rate of urea and prolong the time of calcium carbonate precipitation in the initial period of the reaction.It also showed that NBPT would influence the cementation level of MICP reaction products,and the effect had a relationship with the concentration of NBPT.The cementation level of the reaction products changed with the increase of NBPT concentration,and the peak value appeared when NBPT concentration was 0.1%.（4）Tests of MICP treating sandy soil with NBPT were carried out and comparative tests were designed to explore the influence of different concentration of NBPT on the effect of MICP-treated sand columns.The unconfined compressive strength,shearing strength,permeability,uniformity and the total amount of products of the treated sand columns were measured.The results showed that the effect was significantly affected by the change of NBPT concentration.The appropriate dosage of NBPT is 0.1%.（5）Scanning electron microscopy（SEM）,energy spectrum（EDS）and X-ray diffraction（XRD）were used to analyze the micromorphology of the products of MICP reaction in which NBPT participated.The results showed that NBPT could regulate the crystal morphology of Ca CO₃ and the effect of the regulation was related to the concentration of NBPT.In addition,the results also showed that NBPT and its hydrolysate would participate in the precipitation of organic precipitates,which promoted the formation of calcium carbonate crystal and organic matter copolymer,thus improving the cementation level of the products.The product has a certain impurity adsorption capacity.（6）The influencing factors such as NBPT concentration,cementing solution concentration,bacteria-cementing solution ratio,temperature and p H value were studied as the research variables.The orthogonal tests were carried out to explore the effect of these influencing factors on the MICP reaction with NBPT.The results showed that the optimum dosage of NBPT is 0.1%～0.2%,the reference concentration of cementing solution is 1mol/L,the ratio of bacteria to cementing solution should better be 1:1,the reaction temperature should be 30°C,and the initial p H value should be 7～8.These results provide data references for the subsequent researches and applications.