Experimental Study Of MICP Combined With Nanomaterials To Modify Solidified Island Calcareous Sand In South China Sea Environment

In the past decade,MICP reinforcement technology has gradually developed into a new branch of geotechnical engineering.Compared with traditional cement grouting,MICP reinforcement technology has the advantages of low grouting pressure,short curing time,natural raw material source,and environmental friendliness.Therefore,as a new geotechnical material and reinforcement method,MICP solidified sand has brought new opportunities for green environmental protection construction,which has attracted more and more scholars’attention and in-depth research.In this paper,the South China Sea island reef calcareous sand is taken as the research object,combined with microbial mineralization technology and nano-material modification technology to reinforce the calcareous sand.In this study,the influence of ambient temperature on the unconfined compressive strength of calcareous sand samples before and after reinforcement was studied by setting different temperature gradients;Nano-TiO₂of different quality was added into soil,liquid culture medium and nutrient solution respectively to explore the effect of Nano-TiO₂on the action process of MICP;Finally,the natural seawater after sterilization was used to replace the deionized water required for preparing liquid culture medium and nutrient solution for MICP reinforcement,and the application of MICP engineering in seawater environment was studied.The main conclusions are as follows:（1）The reinforcement effect is best at 30℃.With the increase of temperature,the formation rate of Ca CO₃,the conversion rate of Ca²⁺and the unconfined compressive strength first increase and then decrease.The bacterial adhesion rate continues to increase,and the water absorption rate continues to decrease.The increase of temperature will affect the physiological state of bacterial cells.When the MICP reinforcement is carried out at30℃,the formation of Ca CO₃minerals is more dense,The gap between adjacent sand particles is less,and the bond is tight,which can better play a bridging role,and the strength of the cured sample is also higher.（2）Adding an appropriate amount of Nano-TiO₂into the soil or nutrient solution can improve the reinforcement effect of the sample to a certain extent,while the excessive amount of Nano-TiO₂will play a negative role.With the increase of the amount of Nano-TiO₂,the unconfined compressive strength of the calcareous sand sample shows a trend of increasing first and then decreasing.The reinforcement effect is best when 1g Nano-TiO₂is added into the nutrient solution.Adding Nano-TiO₂into the liquid medium will inhibit the growth and reproduction of Sporosarcina pasteurii,resulting in a lower concentration of urease in the environment,and the higher the amount,the weaker the ability of urease to hydrolyze urea in the environment.In this way,the sample strengthened by MICP is not shaped,so it is not feasible to add Nano-TiO₂into the liquid medium for the cultivation of Sporosarcina pasteurii.Nano-TiO₂mainly plays the role of destroying the cell membrane structure and killing bacterial cells.After the cell membrane is destroyed,the urease in the cell overflows,the urease concentration in the environment increases,and the mineralization is enhanced.At the same time,the incorporation of Nano-TiO₂may cause the crystal form of Ca CO₃to change,thus improving the reinforcement effect of MICP.When the content of Nano-TiO₂in the environment is too high,it will cause the inactivation of biological enzymes and inhibit the mineralization of microorganisms.（3）Both direct domestication and gradient domestication can enable Sporosarcina pasteurii to adapt well to seawater environments.Compared to the untamed Sporosarcina pasteurii,after domestication,the production of Ca CO₃,Ca²⁺conversion rate,and unconfined compressive strength of the MICP reinforced samples were effectively improved,but their reinforcement effect still cannot compare to that under pure water conditions.Adding Nano-TiO₂to seawater nutrient solution cannot improve the reinforcement effect of the sample.The reason may be that natural seawater contains a large amount of other ions compared to freshwater environment,and the composition and content of substances in the environment change,which has adverse effects on the state and related properties of Sporosarcina pasteurii.Adding Nano-TiO₂to seawater nutrient solution will have an inhibitory effect on the solidification effect of MICP.Using natural seawater to carry out island and reef engineering construction according to local conditions can not only effectively alleviate the shortage of freshwater resources,but also reduce transportation and construction costs,which is of great research significance.Further exploration is needed for the application of MICP engineering in seawater environments.