## 4 Conclusions and Discussion  
In the approach presented in this article, microporosity tests were performed to study the microstructural evolution of the interphase between hardening overlay and existing concrete substrate. Based on the presented research, the following conclusions can be drawn:  
* The results presented in this work confirm the potential of high resolution X-ray micro-CT to observe the microstructural evolution within the interphase between hardening mortar and existing concrete substrate. A simple methodology was illustrated. If carefully applied, the procedure is a useful tool for evaluating the microstructural features within the IZ. * The average values of microporosity within the IZ differ in relation to the microporosity in the hardening overlay. The highest difference is for surfaces without special surface treatment (T1 and T2), and the lowest for ground surfaces (T3). There are no significant changes in the microporosity of the concrete in the existing substrate. * The average number of pores increases within the IZ at different moments of hardening of the overlay. It is much more observable for Samples T3 and T4, where mechanical surface treatment has been applied. 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. * Due to the size of the sample and the duration of the test, the obtained resolution allows microporosity to be measured. Thus, the obtained values of microporosity will be lower compared to the overall macroscopic porosity values. * The number of micropores exhibits a more progressive increase than the microporosity. It could be due to the fact that more new micropores appear later. Their volume is small when compared to the overall volume of micropores. Later, the changes in morphology of the smaller pores could still occur at a lower scale of observation, while the changes in morphology of the greater pores at a higher scale are not meaningful. * If microporosity is increasing, it is likely that changes could also occur in nanoporosity. This is evidenced by the latest test using mercury intrusion porosimetry (MIP), which is presented in [63]. However, we are not able to verify it using micro-CT because of the available resolution and required size of the sample. * Based on the variation of the microporosity along the sample height, a different thickness of the IZ can be adopted for different surface treatment methods. The thickness of the IZ cannot be constant and depends on the mix design and existing concrete substrate surface condition and may be influenced by the overlay properties (thickness, aggregate size, etc.). * This approach does not cover the pull-off adhesion between mortar and existing concrete substrate. It does, however, give the strength of the IZ between the hardening overlay and the existing concrete substrate, which is quantified by microporosity measurements. Moreover, critical aspects related to sample preparation and experimental conditions have to be discussed. The main technical limitation is represented by the limited size (less than 20 mm in this particular case) of the samples that can be investigated. To obtain a satisfactory resolution, a scanning time of approximately one to two hours is required. Thus, no changes in the concrete can be recorded in the early stages of concrete hardening. This size also limits the maximum diameter of aggregate, which should be 5-10 times less than the sample size. Therefore, scanning concrete is more challenging in contrast to cement paste, where there is no aggregate and the particles are much smaller. These aspects significantly affect the results and should be taken into consideration in future investigations. This work was supported by the National Centre of Science, Poland (grant No. 2014/15/D/ST8/00550 \"Evaluation of the Interlayer Bond of Variably Thick Concrete Layers Based on Nondestructive Tests Using Artificial Intelligence\").