Experimental And Application Studies On Soft Soil Carbonated And Stabilized By Reactive Magnesia

With the needs of global environmental protection,energy conservation and emissions reduction,the research of low-carbon,environmental-protection and high-efficiency soft soil foundation treatment technology is the development trend of modern engineering construction.The use of reactive magnesia?MgO?and carbon dioxide?CO2?as a new curing agent instead of traditional Portland cement to reinforce the soft soil is an innovative idea and technology.The research has groundbreaking scientific significance and application prospects in energy conservation,emission reduction and sustainable development of civil engineering.Supported by the National Natural Science Fund Project?51279032?,the "twelfth five-year"National Science and Technology Support Programme?2012BAJ01B02-01?,and Graduate Student Scientific Research Innovation Program of Jiangsu Province?KYLX₀147?,this dissertation studies the application of reactive MgO carbonation-stabilization technology in the typical soft soil,revealing the carbonization reinforcement mechanism and engineering characteristics of the reactive MgO-treated soils;through indoor tests,microscopic tests and model tests,this dissertation studies systematically the influence of reactive MgO content,carbonation time,initial water content,MgO activity index,natural soil type and CO2 pressure on the physical,chemical,electrical resistivity,mechanical strength and microscopic characteristics of carbonated reactive MgO-treated soils,and estabilishes the basic models of carbonated reactive MgO-stabilized soils,as well as puts forward the construction technology of carbonated reactive MgO-stabilized soils.Listed below are the main research results:?1?The study summarizes systematically the present research situation of soft soil foundation reinforcement,MgO cement and carbonation/solidification,clarifies the application prospect of reactive MgO in soil carbonization reinforcement and CO2 emission reduction,analyzes the research achievement of carbonization effect,durability and microscopic mechanism of reactive MgO-treated soils,and points out the problems that need to be further studied.?2?The paper studies systematically the influence laws of MgO content,initial water content?or water-MgO ratio,wo/c?,carbonation time,MgO activity index,natural soil type and CO2 pressure on the physical properties of carbonated reactive MgO-treated soils,and reveals the physical characteristics of carbonated soils.The results show that:the carbonation process of reactive MgO-treated soils is an intense exothermic process,and the increase of temperature exceeds 25 centigrade degrees;when the MgO content and MgO activity index are low,the content of fine grained soil is high,or the aeration time is long enough,transverse cracks are likely to form on the side of the soil samples;the weight,volume and density of reactive MgO-treated soils have increased to different extents after carbonation while the water content,specific gravity,porosity and saturation have reduced obviously after carbonation;the study establishes the power function relationship between the physical indicators of carbonated soils and wo/c under different conditions.?3?Through the tests of the electrical resistivity,pore fluid conductivity and pH value on carbonated reactive MgO-treated soils,this paper explores the influence laws of MgO content,initial water content?or wo/c?,carbonation time,MgO activity index and natural-soil liquid limit on the electrical resistivity,pore fluid conductivity and pH value of the carbonated soils;this study analyzes the correlation between the electrical resistivity and some physical indicators such as water content,porosity and saturation,and builds the electrical resistivity prediction equation based on wo/c and carbonation time;on the basis of Archie's resistivity generalized model,the study establishes electrical resistivity-porosity-saturation model and analyzes the relationship between electrical resistivity and pore fluid conductivity and volumetric water content,demonstrating the feasibility of generalized Archie's resistivity model in the application of carbonated reactive MgO-treated soils.Research results show that:the carbonated soil's electrical resistivity increases significantly compared to the uncarbonated soil,and the relationship between electrical resistivity and all physical indicators could be fitted by power function;the pH value?10.41-11.03?of carbonated reactive MgO-treated soils reduces after carbonation and is far lower than that of the lime or cement stabilized soils.?4?Through the uniaxial unconfined compression tests and permeability tests of carbonated reactive MgO-treated soils,the paper studies the influence laws of MgO content,carbonation time,initial water content,MgO activity index,natural soil type and CO2 pressure on the unconfined compressive strength,stress-strain,failure strain and deformation modulus of carbonated reactive MgO-treated soils;this paper establishes a strength prediction model under the relative conditions,and illustrates the growth mechanism of unconfined compressive strength of the carbonated soils.Research results show that:the strength of reactive MgO-stabilized silt after 6.0 hour's carbonation is higher than that of the 28 day-cement stabilized silt with the same dosage;the unconfined compressive strength increases the increase of MgO content,carbonation time or MgO activity index,but decreases with the increase of initial water content or wo/c.The carbonated soils,according to the order from high to low from their corresponding strength,are silt,silty clay,clay,muddy silty clay and muddy clay,and the strength of carbonated silt is much higher than that of carbonated muddy clay.Increasing CO2 pressure?200 or 300 kPa?can accelerate the carbonation rate of reactive MgO-treated soils.The reactive MgO-stabilized soils present a trend of developing from elastic-plastic material to brittle material in the carbonation process;the failure strain range?0.5%-2.6%?of the carbonated soils decreases after carbonation and the deformation modulus generally increases with increasing the unconfined compressive strength.The unconfined compressive strength of the carbonated soils reduces with increasing water content and pH value while it increases with increasing dry density;there exists a linear relationship with positive correlation between unconfined compressive strength and electrical resistivity,indicating that it is feasible to evaluate the strength of carbonated MgO-stabilized soils by the non-destructive electrical resistivity method.?5?Through the permeability tests of carbonated reactive MgO-treated soils,the paper studies the changing laws of permeability coefficient of carbonated reactive MgO-treated soils under different conditions;the permeability coefficient of carbonated/solidified soils decreases with increasing the curing agent dosage,and the permeability coefficient of carbonated soils is slightly higher than that of cement soil,and the permeability coefficient of carbonated silt is greater than that of carbonated silty clay.The permeability coefficient of carbonated silt reaches the minimum at the carbonation time of 6.0 h,while that of carbonated silty clay grows slowly;with the increase of initial water content,the permeability coefficient of carbonated silt slightly reduces while that of carbonated silty clay increases;the permeability coefficient of carbonated soils has the first decrease after increasing trend with the increase of CO2 pressure and increases with increasing wo/c,water content or void ratio,and the relationship between the permeability coefficient and void ratio can be fitted by the power function.?6?Through X-ray diffraction analysis?XRD?,scanning electron microscope?SEM?,energy spectrum analysis?EDS?and mercury injection porosimetry?MIP?tests,this paper studies the evolution law of mineral composition,microstructure and pore characteristics of carbonated reactive MgO-treated soils,and analyzes the relationship between the unconfined compressive strength and the XRD peak intensity of crystalline nesquehonite,revealing the microscopic mechanism of carbonated soils.Based on HNO3 acidification and thermal analysis tests,the study explores the carbonation degree or CO2 uptake of reactive MgO-treated soils under different conditions,and reveals the chemical reaction mechanism of the stabilized soil.Research results show that:the main mineral compositions of carboanted soils are elongated prismatic or rod-like nesquehonite,needle-or rose-flaky reticulate hydromagnesite/dypingite among which nesquehonite accounts for the largest;with the increase of carbonation time,brucite first converts into hydromagnesite/dypingite and then changes into nesquehonite crystals.With increasing carbonation time,MgO activity index and CO2 pressure,the pore volume of the carbonated soils decreases while the CO2 uptake increases;with increasing wo/c and natural soil's liquid limit,the pore volume decreases while the CO2 uptake increases;and moreover,there exists the optimal initial moisture content?25%?and MgO content?20%?,which make the pore volume soils minimum and the CO2 uptake the highest.The XRD peak of nesquehonite crystals and the unconfined compressive strength show a positive correlation,confirming the importance of nesquehonite crystals to the strength improvement of the carbonated soils;the reinforcement mechanism of carbonated reactive MgO-treated soils mainly includes the hydration reaction,ion-exchange adsorption,carbonation reaction and filling effect;the paper puts forward the structure models of carbonation reinforcement of silt and silty clay in the end.?7?Through indoor model tests,this study explores the reinforcement effect of reactive MgO carbonation technology on weak foundation soils,and puts forward the overall construction technology for reinforcing foundation soils by using reactive MgO and carbonation.Research results show that:the temperature from the carbonation reaction is higher than that from the hydration reaction,and the temperature,strength and carbonation degree of the upper foundation soils increase significantly compared with the lower foundation soils while the water content and pH value of the upper foundation soils are less than those of the lower foundation soils;the smaller the distance from the air supply is,the higher the carbonation temperature,subgrade resilience modulus,penetration resistance,strength and carbonation degree are;the temperature rising rate,resilience modulus and penetration resistance of the carbonated silt are higher than those of the carbonated silty clay;the pipe-cover overall carbonation technology can realize the carbonation and reinforcement of the on-site weak soils,but it should be noted that the CO2 should be injected at the bottom and the distance between the pipe and the CO2 supply should be less than 40 cm in practical application.