Experimental Study On Physical Mechanics, Deformation Characteristics And Microstructure Of MICP-cured Sand Based On Magnesium And Calcium Ion Concentration

With the rise of microbial induced carbonate precipitation（MICP）,it provides us with a new foundation reinforcement method in real life.Compared with the traditional foundation reinforcement method,it has the characteristics of energy saving,environmental protection,high efficiency and sustainable development.In the natural environment,microorganisms are often found in the soil and marine environment.Biomineralization is common in fossils,shells and limestones.These mineral materials are used in different engineering fields with their own excellent characteristics,and can also change some engineering properties of soil as by-products.MICP technology is widely used in cultural relic crack repair,heavy metal treatment and sand liquefaction resistance.It also has broad application prospects.The difference of metal ions in nutrient solution will lead to the difference of mineral composition,which will change the crystal form and combination mode of mineral materials,and then affect the difference of strength and deformation of solidified sand.In this paper,based on the nutrient solution（calcium chloride—urea）commonly used in the reinforcement treatment of MICP technology,magnesium chloride was added to keep the concentration of urea unchanged.Seven different concentrations of magnesium and calcium ions were set with the total concentration of metal ions of 0.5mol/l as the benchmark.The Mg²⁺/Ca²⁺ratios were 0.0/0.5,0.1/0.4,0.2/0.3,0.25/0.25,0.3/0.2,0.4/0.1 and 0.5/0.0.The effects of different Mg²⁺and Ca²⁺concentrations on the physical and mechanical properties,deformation characteristics and microstructure of MICP solidified sand were studied.The main research results of this paper are as follows:（1）Through physical tests and unconfined compressive strength tests,the effects of different concentrations of magnesium and calcium ions on the physical and mechanical properties of MICP solidified sand were studied.The test results show that the dry density and carbonate content of solidified sand decrease with the increase of magnesium ion concentration.Addition of magnesium ions can reduce the water absorption performance of solidified sand.When the concentration of Mg²⁺is low,Mg²⁺and Ca²⁺have a synergistic effect,which can improve the unconfined compressive strength of solidified sand.When Mg²⁺/Ca²⁺=0.1/0.4,the compressive strength is the highest.MICP solidified sand may have three failure modes:45°inclined section failure,local failure and global penetration failure.There is also a certain correlation between the physical and mechanical properties of solidified sand.For example,the dry density shows a linear growth trend with the increase of carbonate content,and the unconfined compressive strength and secant modulus show an approximate linear correlation.（2）The effects of different concentrations of magnesium and calcium ions and dynamic shear stress ratio on the dynamic characteristics of MICP solidified sand were studied by laboratory dynamic triaxial tests.The test results show that the dynamic pore pressure-vibration number curve of the test sample develops in an elongated shape,and the dynamic pore pressure may remain stable,low and then rise,and grooves in the later period.Most of the dynamic strain-vibration curves of the test samples developed in the shape of a bell,and the resistance to deformation was improved under the condition of a certain concentration of magnesium and calcium ions.The effective stress path of Mg-Ca solidified sand develops to the left,and the average effective stress cannot be completely zero in the late loading period.The deviatoric stress attenuation may occur in the deviatoric stress-dynamic strain curve of MICP cured sand.The dynamic strength curve also reflects that the anti-liquefaction ability of the MICP-cured sand is improved.When the magnesium and calcium ion concentration is 0.2/0.3,the curve is at the top,and the anti-liquefaction ability of the sample is the most favorable.（3）By uniaxial compression test,studied the different magnesium and calcium ion concentration and load size influence on sand where curing compression features,test results show that:where solidification sample under per level load deformation development with magnesium and calcium ion concentration present nonlinear variation,under the same load,low concentration of magnesium ions can effectively embody the reduce of the effect of specimen deformation;The compression curve shows that the pore ratio decreases linearly with the increase of the logarithmic load,and the two can satisfy the logarithmic relationship well.At the same time,the compression index of the sample shows a trend of decreasing first,then increasing and then decreasing,showing an obvious"W"shape,etc.（4）The effects of different concentrations of magnesium and calcium ions on the microstructure of MICP solidified sand were studied by microstructure tests.The results showed that the crystal development process was as follows:rhomboid shape→bitter melon shape（with a few lumps）→fibrous shape→netlike shape→cauliflower shape.EDS test shows that lower magnesium ion concentration can promote the increase of calcium ion content,while higher magnesium ion concentration reduces calcium ion content.The XRD results show that the crystal development has undergone a transition process from calcite and aragonite to calcite,aragonite and magnesium calcite to calcite and magnesium calcite to magnesite.