XCT Analysis Of One-dimensional Carbonation,chloride Ion And Sulfate Corrosion Of Cement-based Materials

Industry XCT can non-destructively detect microstructure of cement based material,however,experiments proved that raw results obtained from the XCT include errors induced by the equipment and external factors,which cause significant influences and should not be ignored.After testing and analysing XCT results for samples experiencing one-dimensional Cl^-corrosion,sulfate corrosion and carbonation,the author has established a rational data processing method to minimise the above mentioned errors.Through the revised data of the analysis,the regularity of microstructural change in hardened cement paste experiencing the three durability conditions is revealed,and the mechanism of the microstructural change is also discussed.The main research findings of this dissertation can be summarized as follows:?1?In the analysis of durability-related XCT data,samples far away from the exposed surface those are unaffected by external environment and merely has negligible continuous hydration,can be used as reference for adjustment in result analyses.Based on the hypothesis that volume of each component in the reference position does not change during the one-dimensional exposures,adjusting the gray scale range of each component at different testing ages can minimising the errors caused by the instrument and test conditions.?2?In discussing the Cl^- ingress in hardened cement pastes,the XCT results processed by this method cannot determine the depth of Cl^-ingress.However,the obtained results can reflect the degree of Ca dissolution within the samples.Meanwhile,it can be approved that the microstructure of the samples was getting loosen gradually during the Cl^-immersion from exposure surface to inner of the samples,and a further hydration of cement particles and its depth were also detected.The phenomenon of the gradual loosening of the microstructure caused by immersion in 0.5mol/L NaCl solution had a limit state,once reached this limit,the microstructure was almost stablised.Once immersion of samples in the same NaCl solution,change of microstructure in the specimen with W/C of 0.35 was lower than that in the 0.5specimen.In two samples with different W/C,it has an approximate linear relationship between the depth of microstructural change and the square root of immersion duration.In addition,the XCT results of samples immersed in 0.5mol/L NaCl+saturated Ca?OH?₂ solution showed that the dissolution of Ca was obviously alleviated,and the gradual loosening of materials was weaker than that in 0.5mol/L NaCl solution.When the sample is immersed in seawater,it may be due to the low concentration of Cl^-and the presence of multiple irrelevant ions in seawater,the XCT results showed that it had no significant change in the microstructure of the sample.?3?The results of carbonation experiments showed that the microstructure of hardened cement paste was initially compacted and then getting loosen caused by the exposure to 5%of CO₂.The XCT results analysis method proposed in this dissertation can accurately allocate the depth of CO₂ ingress.In addition,the TG and XCT experimental results showed that the ingress of CO₂ in the samples with a W/C of 0.35 and 0.5 was basically the same,which is different from widely accepted carbonation theory.However,it needs to be noted that depth related results obtained from the XCT analyses and the phenolphthalein method have different theoretical meanings and the XCT results confronts with several existing theories.?4?In the research of sulfate attack,the depth of surface crack development and sulfate ingress depth in samples can be justified by the proposed analysis method.In which,the crack depth of the material surface obtained by the analysis method is very close to the cracking depth of the visualised XCT images;also,the depth of sulfate ingress obtained by XCT analysis is in good agreement with the distribution of sulfate ions obtained by Ion Chromatography.Finally,to assist the use of this XCT data processing method for other scholars,the author of this dissertation has developed a VBA program based on Microsoft Excel,which may quickly processing the XCT results for one-dimensional ingress of substances in hardened cement paste.After adjusting the grayscale threshold for different components,with the data exported from Avizo,the change in contents and ingress depth for external substances at different ages can be quickly justified.