## 3 Results and Discussion
### Assessment of Structural Changes in Concrete after Cyclic Sulfate Attack  
Samples after cyclic tests with sulfate attack were examined using an optical microscope to compare the structure of concretes made using the three technologies under consideration--vibrating, centrifuging, and vibocentrifuging. Photos of the structure of concrete samples are shown in Figures 10-12.  
Figure 10 shows that the structure of vibrated concrete after 30 cycles (Figure 10a) and especially after 60 sulfate attack cycles (Figure 10b) has defects in the form of structure breaks, large pores, air channels, and cracks. Such defects are already much less pronounced, and they are not so noticeable in samples of centrifuged concrete (Figure 11). As for vibocentrifuged concrete, its samples have minor structural defects in the initial stage of development (Figure 12). In concrete, under the action of aggressive waters containing sulfates, destruction manifests itself in the form of swelling and curvature of structural elements. In this case, not only does the removal of components from the volume of cement  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Cycle Number**} & \multicolumn{3}{c}{**Manufacturing Technology**} \\ \cline{2-4}  & **Vibration** & **Centrifugation** & **Vibocentrifugation** \\ \cline{2-4}  & & **Compressive Strength (MPa)** & \\ \hline
0 & 32.6 & 35.9 & 38.1 \\ \hline
5 & 32.5 & 35.8 & 38.1 \\ \hline
10 & 32.3 & 35.6 & 37.9 \\ \hline
15 & 32.0 & 35.4 & 37.7 \\ \hline
20 & 31.7 & 35.1 & 37.5 \\ \hline
25 & 31.3 & 34.8 & 37.2 \\ \hline
30 & 30.8 & 34.4 & 36.8 \\ \hline
35 & 30.3 & 34.0 & 36.4 \\ \hline
40 & 29.6 & 33.4 & 35.9 \\ \hline
45 & 28.9 & 32.8 & 35.4 \\ \hline
50 & 28.2 & 32.3 & 34.9 \\ \hline
55 & 27.5 & 31.8 & 34.4 \\ \hline
60 & 26.9 & 31.3 & 34.0 \\ \hline \hline \end{tabular}
\end{table}
Table 10: Change in prism compressive strength of concrete samples depending on the number of sulfate attack cycles.  
stone occur, but, on the contrary, as a result of chemical reactions between it and substances coming from the external environment, new compounds are formed, the volume of which exceeds the volume of the solid phase of the components of the hardened cement paste. An example of such corrosion is the formation of a \"cement bacillus\"--calcium hydrosulfoaluminate. Calcium hydrosulfoaluminates occupy a volume approximately 2.5 times larger than that of the original calcium aluminate. As a result, internal stresses appear, causing the appearance of cracks. The result of this type of corrosion is sometimes the formation of bubbles on the concrete surface--the phenomenon of local delamination. It consists in the fact that flat round fragments begin to bounce off the concrete [56].  
Figure 11: Centrifuged concrete samples after cyclic sulfate attack at 8\(\times\) magnification: (**a**) after 30 cycles; (**b**) after 60 cycles.  
Figure 10: Samples of vibrated concrete after cyclic sulfate attack at 8\(\times\) magnification: (**a**) after 30 cycles; (**b**) after 60 cycles.  
In general, from the analysis of the structure of variotropic (centrifuged and vibrocentrifuged) concrete, it follows that the outer and middle layers of this type of concrete are the densest structure with fewer pores compared to the inner layer. The inner layer can be the main source of reduced concrete resistance. Structural pores in this layer are mainly represented by filtration channels, the number and size of which increase as they approach the inner cavity of the product. The hardened cement paste of this layer is characterized by a significant volume of capillary pores: approximately 25% of the total number of macropores in centrifuged concrete. Thus, by providing protection from the inner layer, it is possible to achieve high rates of sulfate attack [38; 57; 58; 59].