**3. Autogenous self-healing of cement and concrete**

Autogenous self-healing in cement was spotted early in the twentieth century by Lauer and Slate [34], and the concept was gradually established by different researchers [35, 36]. The crystallisation of calcium carbonate within the crack is the primary process in autogenous self-healing of matured concrete [35]. Reactions involved in the deposition of calcium carbonate are presented in Eqs. (1)–(3). In those reactions, CO2 dissolved in water from the air, and the calcium ion Ca2+ is derived from concrete.

$$\rm H\_2O + CO\_2 \leftrightarrow H\_2CO\_3 \leftrightarrow H^+ + HCO\_3^- \leftrightarrow 2H^+ + CO\_3^{2-} \tag{1}$$

$$\text{Ca}^{2+} + \text{CO}\_3^{2-} \leftrightarrow \text{CaCO}\_3 \text{ (pH}\_{\text{WATER}} > 8) \tag{2}$$

$$\text{Ca}^{2+} + \text{HCO}\_3^- \leftrightarrow \text{CaCO}\_3 + \text{H}^+ \text{ (7.5} \times \text{pH}\_{\text{water}} < 8\text{)}\tag{3}$$

Reasons for autogenous self-healing proposed by different researchers [36] are: (i) Further reaction of the unhydrated cement, (ii) expansion of the concrete in the crack flanks, (iii) crystallisation of calcium carbonate, (iv) closing of the cracks by fine particles existing in the water and (v) closing of the cracks by spilling off loose concrete particles resulting from the cracking. This five action model is schematically presented in **Figure 4**.

The understanding and improvement of autogenous self-healing have developed in four major directions (**Figure 2**). These are: (i) manipulation of existing

#### **Figure 4.**

*A model of five steps taking place within three processes, physical, chemical and mechanical (Reproduced from [1]).*

### *Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

Self-healing performance in concrete is assessed using visual observation, mechanical strength recovery, permeability, durability improvement and microstructural evaluation (**Figure 3**). There are three fundamental factors in evaluating the self-healing: visual crack sealing and the identification of healing compounds causing it, the improvement of the durability performance and the recovery of mechanical strength properties [3, 15–21]. The mechanical strength recovery is limited in most of the concrete self-healing process. Hence, the most reliable selfhealing performance is based on the physical crack closure, durability improvement, that is, permeability reduction parameters, and microstructural evaluations.

Autogenous self-healing in cement was spotted early in the twentieth century by

Reasons for autogenous self-healing proposed by different researchers [36] are: (i) Further reaction of the unhydrated cement, (ii) expansion of the concrete in the crack flanks, (iii) crystallisation of calcium carbonate, (iv) closing of the cracks by fine particles existing in the water and (v) closing of the cracks by spilling off loose concrete particles resulting from the cracking. This five action model is schemati-

The understanding and improvement of autogenous self-healing have developed

in four major directions (**Figure 2**). These are: (i) manipulation of existing

*A model of five steps taking place within three processes, physical, chemical and mechanical (Reproduced*

� \$ 2H<sup>þ</sup> þ CO3

<sup>2</sup>� \$ CaCO3 pHWATER <sup>&</sup>gt; <sup>8</sup> (2)

� \$ CaCO3 <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>7</sup>*:*5<pHwater <sup>&</sup>lt;<sup>8</sup> (3)

<sup>2</sup>� (1)

Lauer and Slate [34], and the concept was gradually established by different researchers [35, 36]. The crystallisation of calcium carbonate within the crack is the primary process in autogenous self-healing of matured concrete [35]. Reactions involved in the deposition of calcium carbonate are presented in Eqs. (1)–(3). In those reactions, CO2 dissolved in water from the air, and the calcium ion Ca2+ is

**3. Autogenous self-healing of cement and concrete**

H2O þ CO2 \$ H2CO3 \$ H<sup>þ</sup> þ HCO3

Ca<sup>2</sup><sup>þ</sup> <sup>þ</sup> CO3

Ca2<sup>þ</sup> <sup>þ</sup> HCO3

derived from concrete.

*Advanced Functional Materials*

cally presented in **Figure 4**.

**Figure 4.**

*from [1]).*

**194**

conditions, such as age, compressive stress and curing condition (e.g. wet-dry cycle); (ii) fibres to restrict cracks (e.g. ECC); (iii) shrinkable polymers to initiate internal stress after cracking to shrink the cracks and (iv) cement-compatible mineral additives.
