Experimental Study of MICP Solidified Sand Under the Coupling of Multiple Test Factors

To optimize the test factors in the process of solidifying sand through the microbial induced carbonate precipitation(MICP)method, 25sand column tests, which used round siliceous sands as the test material, were conducted on the basis of the orthogonal design of the central grouting method with 6 test factors that were coupled.The Ca2+ concentrations before and after the reaction in the MICP process were detected, and the CaCO3 crystal contents in different parts of the solidified sand columns were measured.Moreover, the microstructure of the crystals was observed through the scanning electron microscopy(SEM)method.The analysis of the influence of the test factors on Ca2+ utilization showed that the major factors affecting Ca2+ utilization were the concentration of the bacterial suspension, concentration of the cementing solution, and reaction time.In addition, high concentration of the bacterial suspension, low concentration of the cementing solution and long reaction time result in high Ca2+ utilization.Bacterial cells sink during the retention time of the bacterial suspension, which affects the formation position of CaCO3 crystals.CaCO3 produced in the MICP process migrates with the flow in sands, affecting the distribution of crystals in the sand columns.A high flow rate of the solutions can result in a high average CaCO3 content and a uniform CaCO3 distribution.The SEM images illustrate that the CaCO3 crystals in the solidified sand were mainly calcite and the concentration of the cementing solution can affect the crystal size.When the concentration of the cementing solution was higher than 750 mmol/L, the crystals showed stacking-type structures.Moreover, vaterite were detected in the test cases with high bacterial concentration and short retention time of the cementing solution.The strength of the MICP solidified sand increases with the increase in the CaCO3 content;however, it is also influenced by other factors, such as the distribution and size of the crystals.The optimization results of the six test factors are as follows:the OD600 value of the bacterial suspension is 0.5-1.0, the retention time of the bacterial suspension is 3 h, the concentration of the cementing solution is lower than 500 mmol/L, the flow rate of the solutions is related to the transport distance, and the retention time of the cementing solution ensures that Ca2+ is completely consumed and utilized. © 2020, Editorial Board of Journal of Tianjin University(Science and Technology). All right reserved.