**3.3 Water quality**

The water utilized for the preparation of cement plays a significant role in corrosion protection. The water content salts, minerals, impurities, and chemicals, such as sulfides and chlorides, lead to steel corrosion in concrete.

The localized chloride ions break down the passive film on the steel reinforcement of concrete. Alkaline conditions provided by the passivity can be destroyed by the chloride ions, even if a high level of alkalinity remains in the concrete. Chloride ions de-passivate the metal and promote active metal dissolution. Chloride reacts with the calcium aluminate and calcium aluminoferrite in the concrete to form insoluble calcium chloroaluminate and calcium chloroferrites.

*Corrosion Protection and Modern Infrastructure DOI: http://dx.doi.org/10.5772/intechopen.111547*

**Figure 19.** *Chloride attack.*

$$\text{Cl}^{(-)} + 2\text{CaO}.\text{Al}\_2\text{O}\_3 \rightarrow \text{Ca}\_2\text{Al}(\text{OH})\_6(\text{Cl}, \text{OH}).2\text{H}\_2\text{O} \tag{9}$$

$$\text{Cl}^{(-)} + 2\text{Ca}\_2(\text{Al}, \text{Fe})\_2\text{O}\_5 \rightarrow \text{3CaO}. \text{Fe}\_2\text{O}\_3. \text{CaCl}\_2. \text{10H}\_2\text{O} \tag{10}$$

Calcium chloroaluminate and calcium chloroferrites have a non-active form of chloride. After this conversion of chloride, some active soluble chloride always remains in equilibrium in the aqueous phase of the concrete.

**Figure 19** shows the electrochemical process of chloride attack. Moreover, the presence of calcium chloride in water reduces the electrical resistance of the concrete and promotes the electrochemical process of corrosion. Further, calcium chloride is used to shrink cracks in concrete. This additive as an accelerator causes steel corrosion in concrete.

$$\text{Fe}^{(2+)} + 2\text{Cl}^{(-)} \rightarrow \text{FeCl}\_2 \tag{11}$$

$$\text{FeCl}\_2 + 2\text{H}\_2\text{O} \rightarrow \text{Fe(OH)}\_2 + 2\text{HCl} \tag{12}$$

The soluble sulfates present in the water reacts with the tricalcium aluminate of cement, causing the expansion of concrete and the corrosion of steel reinforcement. The sulfate attack is already discussed in earlier sections. The common reduction of sulfate, resulting in the formation of gypsum (CaSO4.2H2O) and calcite (CaCO3) is as follows.

$$\text{CaCO}\_3 + 2\text{H}^{(+)} + \text{SO}\_4^{(2-)} + \text{H}\_2\text{O} \rightarrow \text{CaSO}\_4.2\text{H}\_2\text{O} + \text{CO}\_2\tag{13}$$

#### **3.4 Carbonation**

As discussed in earlier sections, cement hydration hardens the concrete with the liberation of calcium hydroxide. This calcium hydroxide set up a protective layer around the steel reinforcement. However, this free hydroxide in the concrete reacts with carbon dioxide present in the environment to form calcium carbonate. The overall carbonation of concrete can be summarized as follows.

**Figure 20.** *Concrete carbonation.*

$$\text{Ca(OH)}\_{2} + \text{CO}\_{2} \rightarrow \text{CaCO}\_{3} + \text{H}\_{2}\text{O} \tag{14}$$

Further, this calcium carbonate accelerates the electrochemical reaction of corrosion. Moreover, the absorbed carbon dioxide into the moisture present in the concrete form a mildly acidic solution. This reduces the alkalinity of concrete and breaks the protective layer on reinforced steel. The reaction is also known as carbonation. Hence, carbonation results in the corrosion of steel reinforcement specifically for highpermeable concrete (**Figure 20**).

#### **3.5 Electrolysis**

The generation of direct current due to not grounded high voltage or current leakages can cause corrosion in steel reinforcement. This generated direct current directly accelerates the electrochemical reaction of corrosion.

$$\text{2H}\_2\text{O} + \text{O}\_2 + 4\text{e}^{(-)} \text{ (Direct Current)} \rightarrow \text{4OH}^{(-)} \tag{15}$$

Further, the presence of highly conducive electrolytes like saline water also accelerates corrosion in steel reinforcement.

$$\text{Fe}^{(2-)} + \text{OH}^{(-)} \text{ (Electrolyte)} \rightarrow \text{Fe(OH)}\_{2} \tag{16}$$

#### **3.6 Alkali aggregate**

The silicon components of aggregates react with alkalis like sodium oxide (Na2O) and potassium oxide (K2O) present in the cement and forms soluble and viscous alkali-silica gel around and within the aggregate. The alkali-silica gel further absorbs water from the surrounding concrete and expands, causing internal stresses and leading to cracking in concrete.

SiO2 þ Na2O ! Na2SiO3*:*nH2O Sodium silicate gel ð Þ (17)

$$\text{SiO}\_2 + \text{K}\_2\text{O} \rightarrow \text{K}\_2\text{SiO}\_3.n\text{H}\_2\text{O} \text{ (Potassium silica gel)}\tag{18}$$

Hence, increasing the porosity of the concrete and increasing the probability of forming corrosion of steel reinforcements.
