## 3 Results
Figure 5: The 3D models of microporosity obtained within the IZ between the overlay made of cement mortar and the existing concrete obtained by the micro-CT method at different moments of overlay hardening for Samples (**a**) T1; (**b**) T2; (**c**) T3; and (**d**) T4. Figure 6: The microporosity \(\phi\) as a function along a sample height obtained within the IZ between the overlay made of cement mortar and the existing concrete substrate obtained using the micro-CT method at different moments of overlay hardening for Samples (**a**) T1; (**b**) T2; (**c**) T3; and (**d**) T4. It can be seen from Figure 6 that the microporosity at different moments of hardening of the overlay increases with time. It can be noted that only the microporosity, thanks to micro-CT, has been calculated. Based on the variation of the microporosity along the sample height, a different thickness of the IZ could be adopted for different surface treatment methods (Figure 6). Even though the differences are apparent, when comparing for further analysis, the IZ zone with a width of 2 mm was adopted. It is also visible that the microporosity \(\phi\) within the IZ, except for T3 with the ground concrete substrate (Figure 6c), is much higher than that within the overlay for most of the specimens. The highest changes between the microporosity of the overlay and microporosity within the IZ are visible for Sample T2 for the as-cast concrete substrate (Figure 6b) and the shotblasted concrete substrate (Figure 6d). It can be seen in Figure 7 that the number of pores \(n\) at different moments of hardening of the overlay increases with time. It is much more visible for Samples T3 and T4, where mechanical surface treatment has been applied. For Samples T1 and T2 (without special surface treatment), the changes are insignificant. It is visible that the number of pores at 28 days of hardening of the overlay for samples within the IZ is similar to the number of pores in the overlay. However, at 3 h of hardening of the overlay for Samples T3 and T4 (Figure 7c-d), the number of pores is higher at the IZ than within the overlay. Additional analyses, however, did not show any changes in the shape of the pores during time. In Figure 8, the change in microporosity \(\phi\) as a function along a sample height obtained within the IZ between the overlay and the existing concrete substrate is presented. This change was obtained using the micro-CT method at different moments of hardening of the overlay. Figure 7: The number of pores \(n\) as a function along a sample height obtained within the IZ between the overlay made of cement mortar and the existing concrete substrate obtained by the micro-CT method at different moments of overlay hardening for Samples (**a**) T1; (**b**) T2; (**c**) T3; and (**d**) T4. As can be seen in Figure 8, after the first scan at 3 h of hardening of the overlay, the change in microporosity between the overlay and the substrate in the IZ differ and depend on the surface treatment of the existing concrete substrate. There are no significant changes in the microporosity of the existing concrete substrate. The greatest changes, related to the first scan performed after 3 h, can be seen at 28 days of hardening of the overlay. For Sample T1, the microporosity increases by around 0.7%, while for the other samples the increase is higher than 1% after the first scan at 3 h of hardening of the concrete overlay. Taking the above into consideration, Figure 9 presents the average microporosity \(\phi\) as a function along a sample height obtained within the IZ between the overlay and the existing concrete substrate using the micro-CT method at different moments of hardening of the overlay. It can be seen in Figure 9 that a major jump in the microstructural evolution occurs at between 3 and 24 h, which can be accompanied with the rapid development of hydration. In addition, the average microporosity \(\phi\) is higher within the IZ than that within the overlay for all samples.