| Cement-based materials are commonly utilized in the construction of bridges,ports,and other infrastructure that must withstand the impacts of seawater and mild erosion while in service.One of the most important factors in the assessment of the durability of cement-based materials is the chloride ion diffusion coefficient.The "bottom to up" strategy is used in the study to investigate the relationship between the structural properties of cement-based materials and the chloride ion diffusion coefficient.The following are some of the contents of the study:In the case of cement paste,the mercury intrusion porosimetry(MIP)was used to discuss the fractal characteristics of pore structure in the systems of fly ash-cement,silica fume-cement,and fly ash-silica fume-cement matrix,and a method for predicting chloride ion diffusion coefficient in the system was developed by Computed Tomography(CT)and fractal theory.In the case of cement mortar,CT was used in conjunction with a correlation function,a distribution function approach,and a standard calculation method to investigate the aggregate distribution law.The pore size distribution features of mortar were investigated using three different pore size characterisation methods:Nuclear magnetic resonance(NMR),nitrogen adsorption technique(NAD)and MIP.The chloride ion diffusion coefficient calculation formula was established using the Mori-Tanaka homogenization process.The following are the primary conclusions:The MIP experiment results show that the cement paste pore structure corresponds to the solid phase volume fractal,meets the scope of gel pore and capillary pore χ∝d3-DS,that as the water cement ratio decreases,gel pore and capillary pore fractal dimension increase,that lower porosity,fly ash,and silica fume on the influence law of fractal dimension are similar,and that as dosage increases,gel pore fractal dimension increases.The effective porosity feff and mercury injection porosity fmip meet the linear relationship fmip=1.5 8 feff-0.12 in the digital image model of cement paste based on CT,which proves that the effective porosity obtained by CT gray averaging has a certain accuracy.The Drcm of RCM is compatible with the Deff of chloride ion diffusion coefficient predicted by the model.It rises when the water cement ratio rises and falls as the fly ash and silica fume content rises and falls.The link between effective chloride ion diffusion coefficient Deff and porosity fmip was developed as Deff=2.75-16.34 fmip+86.7fmip2 based on the correlation between fmip and gray distribution.Based on the CT image of mortar,the results obtained by correlation function and distribution function are close to the representative element obtained by mean fraction,and the size of the representative unit is independent of the aggregate type and volume fraction.The aggregate type can affect the microstructure imaging of digital image.Compared with the mechanical sand,the volume fraction of natural sand under the representative unit is closer to the design value,indicating that the design value of natural sand volume fraction can be used as the microstructure parameter.The pore structure of mortar is similar to that of a pure cement paste system,according to NMR,NAD,and MIP.The critical pore size of mortar with various sand-cement ratios is between 50 and 60 nanometers,the maximum pore size of gel is around 4 nanometers,and the maximum pore size of capillary is about 46 nanometers,all of which meet the solid volume fractal.In a certain range,the sand volume fraction φagg and the total porosity fmip meet a linear relationship fmip=0.1859-0.0789φagg.As the sand is brought into the interface,the effective porosity fbase of cement matrix increases with the increase of the sand volume fraction φagg,so as the chloride diffusion coefficient Dcp,base.The effective chloride diffusion coefficient Dcon obtained by Mori-Tanaka homogenization calculation coincides with the Drcm of RCM test,and decreases with the sand volume fraction φagg. |
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