## 3 Sustainability and Self-Healing Process of Cementitious Materials  
Concrete remains one of the most widely used construction materials due to its excellent strength performance and ease of use. However, generation of cracks in different types of concretes is unavoidable due to its weakness in the tension. As a result, the lifespan of concrete structures is often compromised because of the development of cracks and ingress of harmful substances through these cracks. Crack repair is generally laborious, time consuming and costly [19]. Conversely, the creation of self-healing concrete is an engineered approach that leads to an autonomous healing of the cracks when applied. Bacteria with calcium nutrient source are incorporated in the concrete to achieve autonomous healing of the cracks, wherein the calcium carbonate precipitates upon the formation of a crack, thus, filling the crack volume. It is known that the traditional concrete has less strength and durability than bacterial concrete. In this view, calcite precipitation into the formed cracks within concrete may be a basis of the biotechnological approach to enhance the durability and strength of the concrete structures [20; 21]. Rebar corrosion, drying shrinkage, external loading, plastic shrinkage, thermal stress, or a combination of these factors can lead to the formation of cracks. For instance, shrinkage can cause micro-crack formations and the presence of an external mechanical load enables the cracks proliferation into a network at lower stress level. Chemical degradation of the concrete structures can occur when chemicals and moisture ingress into the concrete through such network of cracks. In addition, high levels of precipitation and moisture can aggravate such issues. Substantial costs, complicated accessibility, and the production of pollution are among the issues linked with manual interventions to repair the cracks in concrete. Currently, a range of cement-based and chemical repair materials are utilized to solve the cracks-related degradation of concrete structures. Environmental and health risks, as well as the material unsuitability, are associated with the use of chemical healing agents. Presently, about 7% of the anthropogenic CO\({}_{2}\) emission worldwide is due to cement production [22; 23; 24]. Significant structural or durability problems frequently arise as microcracks go undetected when they form at the initial or later stages of construction. It has been realized that recurrent maintenance of the concrete structures can prevent this loss, yet it comes with a significant financial toll. Thus, making manual intervention obsolete viaa sustainable cost-efficient method of repairing the formed cracks in the concrete structures became necessary. Over the last decade, the utilization of the autonomous healing techniques in the building structures has drawn much acclaim owing to their capacity in repairing the deterioration within materials of high caliber. Additional economic and environmental benefits can occur when self-healing techniques are implemented to relieve the need for human-assisted maintenance operations. Thus, researchers have focused on the precipitation mechanism of calcium carbonate stimulated by the microbes as a sustainable scheme to stop the crack formations. Bacillus Subtilis is one such bacteria and an autonomously healing microbe that directly acts upon a calcium compound such as calcium lactate to facilitate the cracks filling in concrete via the precipitation of calcium carbonate [22]. Such precipitation also occurs through a ureolytic bacteria such as Bacillus Sphaericus, causing the decomposition of urea [25; 26]. The setting of concrete occurs in an environmentally sustainable manner during the process of microbe-induced calcium carbonate precipitation in the cracked concrete structure [27]. Moreover, there is no health risk associated with the bacteria Bacillus Sphaericus [28]. The likelihood of reinforcement corrosion is further decreased as oxygen is consumed in the mentioned procedure.