## 3 Results and Discussion  
### Changes in the Structure of the Pore Space of Concrete during Leading Experiments  
The porosity of the concrete was studied by a set of methods--uCT, SEM, and low-temperature nitrogen adsorption, which made it possible to consider a larger range of pore sizes. To observe changes in the porosity of the concrete, the same sample was used, taken before and after the experiment (Figure 2a). When processing the uCT data, a region with relatively low X-ray density was identified, which corresponded to the pores in the sample body (Figure 2b). According to the results of the analysis, it was found that the porosity (macropores) in the concrete sample before and after leaching increased from 29.23% of the volume to 34.80% (Figure 2c--areas are highlighted in yellow in the model, corresponding to the pores in the body of the sample by X-ray density). The largest increase in pores was reliably recorded in the edge part of the sample, while no changes were observed in the central part. This is most likely due to the short duration and static conditions of the experiment; the central zones were not affected by the leaching processes. At the same time, the average sizes of the pores and particles differed markedly from sample to sample (three duplicates were involved in the experiment), which can be explained by the high heterogeneity of the initial cement material (cementing conditions were strictly kept the same for all samples). Thus, it was decided to study the changes in the porosities of the cements by the CT and SEM methods in the same samples before and after leaching, considering the central zone was unchanged and the marginal zone was changed. Along with the change in the total porosity observed by the CT method, there was also a change in the pore morphology, traced by the SEM method. Figure 3b presents data on the change in the probability density (Ni/(N * 1)) of the form factor distribution (K\({}_{\text{f}}\)), obtained using SEM followed by analysis of the obtained images in one of the modules of the STIMAN program for complex quantitative morphological analysis of the microstructure of solids [65]. K\({}_{\text{f}}\) is defined as the ratio of the area to the perimeter of the pore in the SEM image. The K\({}_{\text{f}}\) of a circle is equal to 1. According to the presented histograms, it can be seen that in the edge part of the concrete sample, exposed to synthetic water, the shape of the pores is more spherical than in the central part, where leaching is practically not observed. At the same time, the maximum equivalent pore diameter (D) (equivalent pore diameter is the diameter of a circle equal in area to the analyzed pore) in the edge zone increased significantly compared to the central part, from 162 \(\upmu\)m to 259 \(\upmu\)m. In this case, the change in the contribution of pores of different categories to the total porosity of the sample is more complex: the contribution to the total porosity of pores with D > 10 \(\upmu\)m increased during the experiment, and the contribution of pores D = 1-10 \(\upmu\)m decreased (Figure 3a). Figure 2: The change in the porosity of concrete samples. (**a**)—photo of a concrete sample before and after leaching; (**b**)—model of concrete cylinder obtained from \(\upmu\)CT; (**c**)—relative porosity, identified using computer microtomography (% of the volume). The surface characteristics and micro-mesoporosity of the concrete samples were determined using low-temperature nitrogen adsorption on a Quadrasorb SI/Kr unit (Table 1). The samples were crushed prior to testing. That is, as a result of using the method, it became possible to observe changes in the microporosity (interparticle interactions) to which other methods are not sensitive. It was found that during the experiments, there was an increase in the surface area of the concrete by about 1.4 times, and the volume of the pore space by 1.2 times, while the average pore size decreased by 2.3 times.