Study On Chloride Ion Transport And Carbonation Mechanism Of Concrete Based On Molecular Dynamics

CO₂ in the atmosphere of coastal cities and Cl^-in the ocean in China make concrete structures face extremely harsh corrosive environment,in which the erosion of water molecules and chloride ions is the main factor to reduce the durability of concrete.Macroscopically,the research on the coupling erosion mechanism of chloride ion and carbonation in concrete structures has become mature,but there is relatively little research on the micro-erosion mechanism.Therefore,based on molecular dynamics theory,this thesis mainly studies the diffusion and adsorption of water molecules and chloride ions in cement-based structures and their effects on structural mechanical properties.The kinetic transport model is mainly composed of the upper C-S-H nanogel and the bottom erosion solution.The effects of pore size,solution concentration and fatigue load on the diffusion and adsorption characteristics of water molecules and chloride ions,as well as the mechanical properties of C-S-H structure were explored by constructing gel channels with different sizes,different corrosion solution concentrations and applying fatigue load to the model.In addition,a mixed erosion solution model with different concentrations of Na Cl and Na₂CO₃ was constructed to explore the transport,adsorption and mechanical properties of chloride ion and carbonate ion under the coupling effect of carbonization and fatigue load.The main research contents are as follows:（1）The C-S-H dynamic models with different pore sizes are established to explore the influence of pore size on the transport and adsorption characteristics and structural mechanical properties of water molecules and chloride ions.It is found that with the increase of C-S-H pore size,the diffusion coefficients of water molecules and chloride ions are significantly improved.Although the adsorption capacity of pore surface to water molecules decreased,the retention time of sodium ions on the surface of C-S-H pore and the formation rate of Ca-Cl ion clusters were increased.It leads to the decrease of bulk modulus,shear modulus and Young’s modulus in all directions of C-S-H structure.With the adsorption process,the C-S-H structure will be gradually destroyed,and the durability of concrete will be reduced.（2）The C-S-H model of solutions with different Na Cl concentrations was established,and the effects of corrosive solution concentration on the transport and adsorption of water molecules and ions and the mechanical properties of C-S-H structure were explored.It is found that with the increase of concentration,a large number of ions remain on the surface of C-S-H pores,occupying the adsorption space of water molecules,which weakens the adsorption of water molecules on the surface of C-S-H.Chloride ions are more likely to form ion clusters with calcium ions,which will lead to the decrease of bulk modulus,shear modulus and Young’s modulus of C-S-H structure,accelerate the destruction of C-S-H structure and trigger the formation of new cracks.（3）The influence of diffusion and adsorption characteristics of water molecules and chloride ions on the mechanical properties of C-S-H structure under fatigue load is studied.It is found that due to the continuous extrusion deformation of the C-S-H structure and the continuous relaxation of the pore channels,water molecules and chloride ions in the solution are continuously squeezed into the pore channels,which leads to a significant increase in the diffusion coefficient of water molecules and chloride ions.At the same time,a large number of calcium ions in the C-S-H structure will escape into the pore channels under the load,which makes the bulk modulus,shear modulus and Young’s modulus of the C-S-H structure decrease.（4）The adsorption and diffusion characteristics of chloride ions and carbonate ions and the influence of C-S-H structural mechanical properties under carbonation and load were studied.It is found that the increase of carbonate ion concentration will hinder the formation of Ca-Cl bond and further reduce the diffusion coefficient and adsorption capacity of chloride ions in pores.Similarly,with the increase of chloride ion concentration,it will also affect the formation of Ca-CO₃ ion clusters and reduce the diffusion coefficient of carbonate ions in pores.With the increase of carbonate ion concentration,the volume modulus,shear modulus and Young’s modulus of C-S-H structure gradually increase,indicating that carbonation can improve the performance of concrete structures.