Mechanism Analysis on Carbonation of Hardened Cement Paste with XCT

X-ray computed tomography (XCT) was used to monitor microstructure changes of hardened cement paste, with different water/ cement ratios during a one-dimensional carbonation process. After removing equipment-induced measurement errors by a volume-constant method, the pores, dense hydration products, loose hydration products, and unhydrated cement in the sample were classified. Consequently, variation of each component with depth and carbonation duration were obtained, based on which mechanism of carbonation on hardened cement paste was conducted. Results showed that microstructure changes in hardened cement paste under CO2 effects is a coexisting simultaneous densification and cracking process. Within the entire carbonation process, the changes exhibited a general alternate pattern of densification-cracking-redensification, leading to connection of micropores into macro ones in hardened cement paste (i.e., the so-called carbonation-induced cracking); entire volume of the pores bigger than 4.4 mu m did not present obvious changes during carbonation; a CO2 influence depth can be obtained by this method, which has an approximate linear relationship with the square root of carbonation duration and has limited influence with the water/cement ratio used in this study.