## 3 Research Results and Discussion
### Carbonization  
According to the review article written by [PERSON] and [PERSON]. [30], carbonation, or the process of formation of calcium carbonate from calcium hydroxide, is a key factor reducing the durability of reinforced concrete in the course of operation. It also boosts the strength of concrete products [126] and modifies the alkaline reaction of the concrete stone environment to neutral [30; 140; 141], which may boost corrosion of reinforcement in concrete. It is generally assumed that depassivation of steel commences at a pH below 11.8, while a decrease in pH to 9 is observed during carbonization [30; 59]. For most civil buildings, the process of carbonation is mainly characterized by the external sorption of carbon dioxide. This process is quite slow (up to 1 mm per year), since the concentration of carbon dioxide in the air is not high [30]. In turn, gamma radiation accelerates the process of carbonation in concrete structures and triggers its relatively ubiquitous development in concrete. It also reduces porosity but boosts cracking [30; 54; 60; 69; 70; 77; 142]. In general, carbonation products include calcite and vaterite/aragonite [30; 143]. Cement matrix pores are filled with vaterite, and this is the main reason why the strength properties of the cement matrix improve [142]. However, it is found that gamma irradiation (at a dose of 1.5 MGy) causes microstructural changes in the hardened cement mass, characterized by a change in the carbonation phase from vaterite to calcite [60], which, in turn, is explained by the fact that different molar volumes change porosity and can accelerate carbonation [60]. The equilibrium between calcium hydroxide and calcite depends on irradiation [143], while the proportion of calcite increases due to irradiation [128]. As calcite is formed in the pore space, the structure of pores changes, and the total effective porosity decreases; it contributes to a decrease in mass transfer through the cement stone or a decrease in access of aggressive substances to the reinforcement [59]. The difference between the unit volume of calcite and calcium hydroxide can lead to shrinkage of the cement stone and, accordingly, cracking [59]. Moreover, carbonation is accelerated by limestone [60], used as a plasticizer in the process of concreting, as well as an aggregate for protective reinforced concrete structures of shields. An increase in the relative humidity of the structure boosts carbonation. The highest carbonation rate was observed in the range of 55% to 70% [144; 145; 146]. At the same time,the rate of corrosion decreases if the structure is in a humid state due to a decrease in the oxygen diffusion process in concrete [147; 148]. However, this occurs at a relative humidity of 70% [149]. The diffusion coefficient of oxygen in air (open pores) is approximately 104 times greater than in the liquid phase (pores filled with water) [147]. The rate of reinforcement corrosion of carbonated mortar or concrete can also be reduced by decreasing the water-cement ratio: experimental results show that a decrease in the water-cement ratio from 0.8 to 0.55 results in a 2.5-fold decrease in the surface current density at a relative humidity of 100% [150]. However, as a rule, protective reinforced concrete structures of buildings and facilities where radiation and nuclear materials are handled have much lower water-cement ratios due to the need to meet strict strength requirements. The following trend is revealed in [151], whose authors study water-cement ratios close to those used in nuclear energy facilities (0.25, 0.3, and 0.35): as the water-cement ratio decreases, so does the overall probability of corrosion. However, prevailing factors include the extent of pore saturation and the effective area of contact between steel and water-filled pores [144]. Hence, processes of radiation-triggered heating, carbonization, and radiolysis of chemically bound and unbound pore water are of predominant importance in the mechanistic modeling of processes reinforcing steel corrosion in concrete.