## 4 Hydrotalcites as a Part of the Building Materials
### Hydratcalcites in Reinforced Concrete as Corrosion Inhibitive Addition  
The main problem in reinforced concrete structures is the corrosion of the steel reinforcement, especially in coastal environments [84; 85; 86]. Usually, the reinforcement steel in solid concrete structure is passivated due to the high pH and therefore does not tend to corrode (see Figure 4) [87]. Nevertheless, in the presence of a sufficient level of chloride ions, the inner steel reinforcement can be de-passivated so the corrosion of the structure may start. This may occur especially after the exposition of the material to the seawater or in general, in the near-sea environment. The overall expenses of corrosion of reinforcement are considerable, as it involves repairing, restoration, and the safety monitoring activities, as well as aesthetics of concrete structures and components. Hence, it is vital to project concrete resistant to chloride corrosion. In these days, there are several approaches of preventing the corrosion of concrete reinforcement. The first one is the use of surface coatings, which act as a barrier resistant to the chemical agents which can affect the concrete. The most common materials used for the coating are epoxy resin, polyurethane, acrylic coatings, polymer emulsion coatings, and chlorinated rubbers. The most important aspects which are being studied are their ability to adhere to the concrete, chloride permeability, the crack-brinding ability, and resistance to moisture and temperature variations. Previous studies showed high applicability  
Figure 4: Scheme of the corrosion process. Figure 3: Scheme of application of hydratcalcites in building materials. of epoxy resin, which exhibits a good adhesion to concrete, as well as quite high effectiveness in reducing the water absorption causing the reduction of damage of the concrete reinforcement [88; 89; 90]. Another way of the anti-corrosion protection of concrete and concrete reinforcement is use of admixtures. The possible materials which can be used as an admixture are waste materials, such as coal fly ash or polymers, in the form of particles, short fibers, or organic liquids. Such materials not only work as an anti-corrosion agents but also improve the mechanical properties [91; 92]. Another possible sort of material which can be used as an admixture are the hydrolcalcites. Hydrolcalcites can be easily applied in some strategies of corrosion protection of concrete steel reinforcements [93; 94]. The results of experiments showed that some configurations of Al-Mg hydrolcalcites, with carbonate anions in the raw hydrolcalcites substituted by different anions (e.g., p-aminobenzoate, pAB), displayed better corrosion inhibiting result in extended time before the beginning of the corrosion. It also showed a higher chloride threshold in comparison to the same free ions in solution. The mechanism is based on ion exchange between free chloride ions and the intercalated inhibitive anions in hydrolcalcite matrix and simultaneous release inhibitive anions. These anions act as active inhibitors, causing the increase of a threshold concentration of chloride ions. The anionic exchange capacity of hydrolcalcites is quite high (2-4.5 miliequivalents/g), making the exchange of interlayer ions easily achieved [95; 96]. In addition, the influence of the addition of the anticorrosion material on mechanical properties is important. This statement was already previously mentioned in the literature [97; 98]. The addition of 5% of pAB hydrolcalcite in mortar specimens resulted in considerably improved resistance of the material to the chloride diffusion, accompanied with small decrease of compressive and flexural strength in units of MPa. Another promising way, besides the mentioned 5% mass substitution of cement by pAB hydrolcalcite in the bulk mortar, is the coating of reinforcing steel by part of cement paste coating 20% pAB/ NO\({}_{2}\)[99].