Microstructure of cement paste at different carbon dioxide concentrations

Numerous studies have been performed on the carbonation model based on the boundary (carbonation front line) between the partially carbonated and non-carbonated zones. However, a significant difference exists between the traditional phenolphthalein spray test results and the prediction model results on carbonation depth. The difference is mainly caused by the partially carbonated zone. In this study, the changes in the microstructure of cement paste during the carbonation process at different carbon dioxide (CO₂) concentrations (0·03, 3 and 20%) were determined through the phenolphthalein spray test, thermogravimetric analysis and mercury intrusion porosimetry (MIP). The results showed that carbonation depth and the square root of carbonation time have a linear relationship. Compared with that in the natural carbonation condition, the effect of 20% carbon dioxide concentration on the size of the partially carbonated zone was significant. However, no effect was seen for 3% carbon dioxide concentration. MIP showed that, in the same carbonation zone, low carbon dioxide concentration led to low total porosity; there was a clear refinement in the pore size and there was a high degree of carbonation.