Study On Numerical Simulation Of Ion Transport Affecting Concrete Durability Based On Lattice Boltzmann Method

Calcium corrosion and chloride ion corrosion are two important factors affecting the insufficient durability of concrete.The calcium dissolution process generally refers to the dissolution and decalcification reaction of the hydration products inside the material,and the calcium ions produced by the dissolution diffuse outward under the action of pore water.As the weakest link of concrete materials,ITZ is easy to become the generation and expansion area of cracks,and the calcium dissolution phenomenon in this area is also more serious;Chloride ion migration in concrete is an extremely complex and slow process,and its migration rate and depth are closely related to the environment of concrete and the material factors of concrete itself.Due to the influence of the internal microstructure of the concrete and the service environment,the above two processes are accompanied by the coupling of multiple fields.Therefore,this paper focuses on the seepage corrosion of ITZ cracks and chloride ion erosion of concrete,and studies the multi field coupling problem of calcium ion and chloride ion in the process of internal transport in concrete.The main research contents are as follows:（1）A micro-scale numerical model of coupled fluid flow and chemical dissolution in interfacial transition zone（ITZ）of concrete fracture was established.The microstructure of ITZ was modeled by the quartet structure generation set.The evolution of fluid velocity and solute concentration were simulated by lattice Boltzmann method with double distribution functions.The accuracy in dealing with the convection-diffusion and reaction-diffusion problems were verified by two classical examples.Finally,the fluid flow and dissolution coupling mechanism of ITZ fracture were discussed considering the effects of seepage velocity,Ca（OH）₂ content and Ca（OH）₂ arrangement.The results showed that the wall corrosion happened quickly under faster initial flow velocity,resulting in acceleration of overall porosity increase.As to the Ca（OH）₂ content,the higher Ca（OH）₂ content in ITZ could add the contact area between the ITZ wall and fluid,which increased the dissolution access and lead to larger Ca²⁺concentration in the fracture.In addition,when the dissolved Ca²⁺cannot migrate in time,the dissolution of Ca（OH）₂ will be restrained.In conclusion,the coupled fluid flow and dissolution process of ITZ fracture was controlled by both the comprehensive action of Ca（OH）₂ content and Ca²⁺concentration.For different arrangements of Ca（OH）₂,the results showed that microstructure of ITZ affected the relative permeability,which was the largest in horizontal growth,followed by uniform growth,and the smallest in vertical growth under steady dissolution.（2）For the concrete structure exposed to chloride environment,the corrosion effect of chloride ion under the external temperature is very complex,which affects the corrosion process of reinforcement and its durability.Therefore,based on the lattice Boltzmann method the double distribution functions are applied to describe the temperature evolution of concrete and the concentration evolution of chloride ion respectively,and a numerical model of chloride ion erosion concrete is established considering the time varying effect of temperature.On this basis,the effects of water-cement ratio,saturation and temperature difference between day and night on chloride ion erosion mechanism and service life of concrete are discussed.The results show that the diffusion coefficient of chloride ion increases with the increase of water-cement ratio,which intensifies the erosion of chloride ion.The higher the saturation of concrete is,the faster the diffusion rate of chloride ion is,which reduces the service life of concrete,and the trend tends to be stable when the saturation is more than 75%.In addition,the greater the temperature difference between day and night,the more significant the effect on the diffusion coefficient,but the effect of temperature difference between day and night on the service life of concrete is not obvious.（3）At present,most of the researches on the numerical simulation of calcium ion and chloride ion transport in concrete are based on the underlying code In parameter setting,it is more complicated to modify the underlying code,and it is often necessary to use third-party software to draw the curve of related data or display the calculation cloud diagram.Therefore,on the basis of the above research,this paper uses the GUI interface function in MATLAB software,combined with the unique programming advantages of the lattice Boltzmann method,and develops two visualizations of ITZ fracture seepage-dissolution and chloride ion erosion concrete.Calculation Software,The software has higher calculation accuracy and calculation rate,parameter setting is easier to get started,and real-time interaction with users during the calculation process.The post-processing module of the calculation can also draw related curves and export data.