Study Of Chloride Migration And Chloride Binding Isotherms In Cement-based Materials With Ultra-low Water To Binder Ratio

The influences of supplementary cementitious materials and curing regimes on the microstructure,compressive strength and chloride ion permeability of cement-based materials with ultra-low water to binder ratio were investigated,and then the correlations between microstructure,compressive strength and chloride ion permeability of cement-based materials with ultra-low water to binder ratio were also investigated in this paper.The test results show that: Under standard curing conditions,with the prolongation of curing age,the microstructure of cement-based materials with ultra-low water to binder ratio is gradually improved,the porosity is gradually reduced,and the compressive strength and chloride penetration resistance of cement-based materials with ultra-low water to binder ratio are effectively improved.Compared with standard curing conditions,the hot water curing has a more significant effect on the improvement of the microstructure of the cement-based materials,which significantly reduces the porosity of the cement-based materials,thereby improving the compressive strength and chloride penetration resistance of the cement-based materials with ultra-low water to binder ratio.Within a reasonable range of content,supplementary cementitious materials can effectively improve the microstructure and pore structure of the matrix,make the matrix more uniform and dense,reduce the porosity effectively,and thus significantly improve the compressive strength and chloride penetration resistance of the cement-based materials with ultra-low water to binder ratio,in which the reinforcing effect of silica fume,silica fume and fly ash or slag are most significant.There is a good linear relationship between compressive strength and porosity of cement-based materials with ultra-low water to binder ratio;chloride diffusion coefficient has good nonlinear relationship with porosity and capillary porosity,respectively,among which chloride ion diffusion coefficients correlation with capillary porosity is the best and the corresponding empirical formulas are determined.The equilibrium chloride ion concentrations of cement-based materials with ultra-low water to binder ratio exposed to different concentrations of sodium chloride solutions were determined by the isothermal adsorption method,and the isothermal adsorption curve of chloride binding of cement-based materials with ultra-low water to binder ratio were obtained by data regression analysis.For cement-based materialswith ultra-low water to binder ratio,the chloride binding capacity was calculated,and the influence of different supplementary cementitious materials on the chloride binding capacity of cement-based materials was studied.The experimental results show that for cement-based materials with ultra-low water to binder ratio,the chloride binding isotherms were best described by the Freundlich nonlinear isotherm as a whole.Moreover,chemical binding also can be described by Freundlich isotherm while physical adsorption can be described by Langmuir nonlinear isotherm.The Linear binding isotherm does not reflect the chloride ion binding isotherm of cement-based materials under actual conditions.The total chloride binding capacity and chemical binding capacity of cement-based materials with ultra-low water-to-binder ratio in different cementitious systems gradually increase with the increase of free chloride ion concentration,but physical adsorption capacity decrease,and both change nonlinearly.In the process of chloride ion binding of cement-based materials with ultra-low water-to-binder ratio,chemical bonding is dominant and physical adsorption is second,and chemical binding capacity is much higher than physical adsorption capacity.The effects on chloride binding capacity are different with different supplementary cementitious materials.Silica fume has negative effect on the chloride binding capacity of cement-based materials with ultra-low water to binder ratio and decreases the chloride binding capacity.When silica fume is compounded with fly ash or slag,it demonstrated negative synergistic effects on the chloride binding capacity,thereby decreasing the chloride binding capacity of the matrix.Fly ash and slag have positive effect on the chloride binding capacity of cement-based materials with ultra-low water-to-binder ratio,which effectively improves the chloride binding capacity of the matrix,especially when fly ash and slag are added,it demonstrated positive synergistic effects on the chloride binding capacity of the matrix whose effect is most significant.