## 10 Bacteria-Activated Self-Healing Cementitious Materials
### Self-Healing Approach  
The secretion of the healing agents can be triggered after occurrence of a crack or damage within an optimal autonomously healing system. The restoration of the micro-cracks in a concrete can potentially be handled by the autonomous healing techniques. Recently, the cracks located on the concrete's surface have been efficiently manage with the autonomous healing approaches. After the incorporation of bacteria, calcium carbonate precipitates were molded into the earlier cracks' layer inside the concrete [12; 27]. Resistance to alkali environments is a required characteristic of the incorporated bacteria in the concrete structures [120; 121]. The binding of the gravel and sand in concrete is facilitated by the microbiological induction of calcium carbonate precipitation that also aids the plugging-in of the formed micro-cracks [122]. The durability of concrete can be enhanced by incorporating various microorganisms-mediated calcite precipitations reactions. Within a highly alkaline environment, Bacillus Sphaericus can precipitate calcium carbonate in concrete via the conversion of urea into carbonate and ammonium [13]. Thus, such concrete is capable of self-healing the cracks that are below 0.2 mm in size. The entrance of deleterious materials is not halted when the sizes of the cracks surpass 0.2 mm because the self-healing actions are not sufficient. The hibernation stage of the bacteria within the concrete is disrupted upon the appearance of cracks in autonomously healing concrete. The cracks are filled via the autonomous healing as the calcium carbonate precipitates into them due to the metabolic activities of bacteria. The hibernation stage recommended once the calcium carbonate sealed the formed cracks entirely. Therefore, any new cracks can be filled due to the activation of the bacteria. Microbiologically-induced calcium carbonate precipitation (MCIP) is a mechanism that describes the nature of bacteria as a long-term healing agent. The results in Table 8 indicate that the metabolic pathways of multiple bacteria determine the formation of calcium carbonate. Autotrophic processes are said to generate less precipitated calcium carbonate than heterotrophic processes. The requirement of organic carbon sources for the growth is characteristic of heterotrophs that are incapable of using carbon to make their own organic compounds. Conversely, simple substances are used to generate the complex organic compounds, such as chemosynthesis and photosynthesis, for autotroph organisms. The bacterial cells are protected by a mineral layer when the calcium carbonate is generated by the microbial [123; 124]. Urealysis can generate CaCO\({}_{3}\) precipitate; multiple microorganisms can be used to achieve the autonomous self-healing. A comprehensive literature review revealed the specific implementations of various types of bacteria in concrete as self-healing agents. Graphite nanoplatelets together with lightweight aggregates in concrete can improve the strength after Bacillus Subtilis are added [21]. Improvement in the concrete's durability was observed when rice husk ash concrete was incorporated with Bacillus Aerius bacteria [116]. The value of CS was increased by 25% when Bacillus Megaterium bacteria was incorporated into the concrete [117]. Additionally, the durability of the concrete was enhanced due to the inclusion of Bacillus Sphaericus into the concrete that produced CaCO\({}_{3}\) precipitates [46]. Following the autonomous healing, both durability and mechanical strength of the Sporoscarcina Pasteurii bacteria-incorporated FA-based concrete was improved [118]. Similarly, the same bacteria revealed comparable effects when added to silica fume concrete [118]. Another option for the surface treatment monitoring of the concrete was discovered via the introduction of Bacillus Sphaericus into it [125]. Recent studies hinted that the encapsulation and direct application are the two principal approaches for applying the healing agent into the concrete.