Study On Engineering Characteristics And Thermal Conductivity Of Solidified Sand By Microbially Induced Calcium Carbonate Precipitation

Ground source heat pump technology(GSHP)is a green cooling and heating technology.One of its key technical parameters is the thermal conductivity of soil.In order to improve the thermal conductivity of the soil,the fine sand was treated by MICP injection(Microbially induced calcium carbonate precipitation).A series of theoretical and experimental studies have been carried out for this study and the following conclusions are drawn:(1)The factors affecting sporosarcina pasteurii were studied.In the precipitation reaction of calcium carbonate induced by microorganisms,a certain concentration of cement solution promotes the amount of calcium carbonate formed.However,when the concentration exceeds1.25 mol/L,the highly concentrated ion solution causes a decrease in the amount of calcium carbonate formed.When the pH of the MICP reaction system is between 6 and 9,the effect of the MICP reaction is not much different.When the temperature of the reaction system is between 10 and 40 ~oC,the amount of calcium carbonate formed increases with increasing temperature.The salinity of the MICP reaction system inhibits the activity of sporosarcina pasteurii.For an average marine salinity of 35‰,the amount of calcium carbonate formed is9.2%lower than that of a salt of 0.(2)The improvement effect of fine sand on MICP technology were studied.A series of tests were carried out on MICP treated sand column for 1-3 times.It was found that the more the injection cycles,the higher the ultrasonic wave velocity through the sand column,the higher the unconfined compressive strength,and the higher the calcium carbonate content.The unconfined compressive strength of the MICP treated sand column for 3 times reached3.65 MPa,and the calcium carbonate content reached 10.41%.The low field NMR experiment found that MICP has a lowering effect on the porosity of the sand column.The more the number of injection,the smaller the porosity and the smaller the main pore radius.(3)Transient method was used to test the thermal conductivity of MICP treated soil and the relationship between thermal conductivity and fine sand properties was analyzed.The thermal conductivity of MICP treated sand is improved compared to untreated sand.Compared with untreated sand,the thermal conductivity of dry MICP treated sand for 1-3times is increased by 63.1%,93.24%and 119.52%,respectively.The effect of saturation degree on the thermal conductivity of MICP treated sand was studied.The thermal conductivity of saturated untreated sand is 382.57%higher than that of dry sand.The thermal conductivity of saturated sand treated by MICP for 1-3 times is228.49%,212.39%and 203%higher than that of dry sand,respectively.The relationship between thermal conductivity of sand and dry density,porosity,calcium carbonate content and other properties are summarized.It is found by scanning electron microscopy(SEM)image analysis that calcium carbonate acts as a"thermal bridge"between sand particles,which increases the contact surface between soil particles and improve the heat transfer efficiency between sand particles,and then resulting in the improvement of thermal conductivity of sand.This study not only explores the great potential of MICP in engineering applications,but also provides a method reference for the practical application of GSHP.