Visual Characterization And Quantitative Analysis Of Ion Migration Process In Cement-based Materials

Cement is widely used as a building material,and its reinforced concrete structure has become the most common form of construction.In addition to considering the safety and applicability of the reinforced concrete structure,its durability has increasingly become the focus of attention.In many coastal environments,the corrosion of steel bars in concrete structures caused by chloride ions in seawater has become the most prominent durability problem,even directly affecting the safety of the structure and causing serious economic losses.Therefore,effectively and accurately characterizing the distribution characteristics(depth and concentration)of chloride ions in cement-based materials is of great significance for establishing an effective chloride ion transmission model and predicting the service life of cement-based materials.To this end,many scholars have studied different test methods to characterize the distribution of chloride ions in cement-based materials.However,most methods have certain defects,such as insufficient test accuracy,detrimental testing,time-consuming and labor-intensive,and not suitable for samples containing chloride ions.In order to explore an accurate,non-destructive and visual method for testing chloride ion migration,in this study,X-ray tomography technique(XCT)was used to non-destructively and accurately detect the migration behavior of chloride ions in cement-based materials.At the same time,micro-testing methods(mercury intrusion testing,SEM testing,etc.)are used to explain the results of XCT technology which can measure the resistance of cement-based materials to ion migration improvement by fly ash.The main research and results are as follows:(1)The method of characterizing the distribution of chloride ions in cement-based materials by XCT technology was established.The feasibility of using iodine ion instead of chlorine ion to characterize ion migration in XCT was verified.Then,based on the electromigration of iodide ions,the appropriate DC voltage(30V)and interval test duration(1 h)are determined to drive the migration of iodide ions insmall samples,so that the effective and obvious test results are obtained.The effective test parameters(X-ray source parameters,relative distance between light source and sample as well as detector,test duration,etc.)are determined by using the XCT technology to carry out the XCT test,and then the effective XCT image is measured.(2)In the process of using XCT technology to quantify ion migration,a better and systematic image processing method for CT data is proposed,including image reconstruction,X-ray hardening correction,CT value normalization,CT value concentration quantification,etc.With this method,the migration depth and concentration distribution of ions in cement-based materials driven by electric field can be measured without damage and accurately.Compared with the automatic potentiometric titration method,the accuracy of XCT in measuring ion migration was verified.(3)In this study,XCT technology is used to visualize and quantify the ion migration in cement-based materials with fly ash.XCT technology can visualize the migration results of ions in cement paste samples with different mixed proportions(0%,10%,20%,30%)of fly ash from a two-dimensional perspective.With the increase content of fly ash,the ability of cement-based materials to resist ion migration is improved,which can be presented through the quantitative concentration distribution curve and visual results comparison with XCT technology.