Study on the Influence of Carbonation on the Microstructure of Cement-based Materials Based on BSE Technique

Background: The influence of carbonation on the interfacial transition zone (ITZ) microstructure of cement-based materials was significant. However, the width of ITZ is about tens of microns, and studying its micro-characteristics (such as porosity, hydration products, content of unhydrated cement, etc.) by macro test was difficult. Methods: Backscattered electron (BSE) imaging technology and gray scale analysis method were used to analyze the cement-based materials with water-binder (W/B) ratios of 0.53 and 0.35, respectively. Results: BSE and gray scale analysis showed that in the ITZ, the porosity of 0.53P (Portland cement paste), 0.35P (Portland cement paste), 0.53F (fly ash), and 0.35F (fly ash) decreased by 24.1%, 28.9%, 49.5%, and 64.2% respectively, whereas the content of hydration products increases after carbonation, and the matrix also shows the same rule. At the same time, the smaller W/B ratio, the greater the porosity reduction, and the filling effect of carbonation on the specimens with supplementary cementitious material (SCM) was more significant than that of pure cement specimens. Conclusion: The porosity of the ITZ decreased after carbonation, however it remained higher than that of the matrix. Consequently, the ITZ remained a vulnerable zone with a greater diffusion rate of CO2 compared to the matrix even after carbonation.