# Smart Materials for Green(er) Cities, a Short Review
## 2 Smart Materials for Green(er) Cities  
### Self-Healing Concrete  
Introducing autonomous repairaability and healability in cementitious materials represents an innovative strategy to extend the lifetime and reduce the CO\({}_{2}\) footprint of buildings [6; 7; 8; 9]. To date, concrete is the most widely applied material in the construction industry owing to its high mechanical properties paired with low production costs [10]. Concrete is formed by mixing cement, water, sand and coarse fillers. Subsequently, the cementitious phase is dried and cured, yielding calcium-silicate-hydrate (C-S-H) domains, which mainly contribute to the stiffness of the inorganic composite. While concrete benefits from a high compressive strength, its tensile properties (tensile strength and elongation) are limited [11]. Thus, concrete used for the construction of bridges, roads or buildings is prone to cracking during the lifetime of these structures [12]. Equipping concrete with a self-healing function is one way to ensure the structural integrity of buildings without costly and time-consuming maintenance and repair, as the material is able to close the damage zone without external intervention [13]. Here, it should be noted that cementitious materials--without a particular modification--can autonomously heal cracks, albeit at a limited crack size (<50 \(\upmu\)m) [14]. This so-called autogenous healing can be based on various mechanisms: (i) a delayed hydration of residual cement reagents [15], (ii) the carbonation of dissolved calcium hydroxide [16], (iii) a swelling of the C-S-H phases [17] or (iv) the physical closure of cracks by small particles present in water [18]. However, in all cases, water is required to come in contact with the damage zone and to react with the concrete matrix or to transport the required reagents. Another route to deposit materials and to physically seal small cracks in concrete is the precipitation of calcium carbonate with the help of selected bacteria [19]. Various types of bacteria have been reported that are able to survive the highly alkaline environment of cementitious materials and form calcium carbonate via different metabolisms (e.g., aerobic respiration of dissolved inorganic carbon, urea hydrolysis or nitrate reduction) [20]. While some bacterial spores can be directly mixed with cementitious materials, the majority of the reported concepts use carriers such as clay or activated carbon particles to extend the lifetime of the bacteria and protect them during the processing of concrete [21; 22; 23]. However, the limited lifetime of bacteria is only one issue when it comes to the industrial implementation of bacterial-based self-headable concrete. The bacteria also need optimum conditions to produce calcium carbonate via certain metabolic pathways. In particular, the availability of oxygen is vital for bacteria relying on aerobic respiration [24] and urea hydrolysis [25] to perform self-healing. In contrast, oxygen, which diffuses though microcapillaries in concrete, might become a problem for anaerobic bacteria [26]. In addition, some types of bacteria, such as _Bacillus pseudofirmus_, are reported to negatively affect the compressive and tensile strength of concrete [27]. In their review, [PERSON] et al. summarized that the potential of this type of self-healing cementitious material is in specific applications. These are aboveground, having slow crack-opening displacement rates. Indeed, crack closure can take up to a few months in the case of larger fracture sizes [6]. The ability to withstand larger deformation and to reduce crack propagation in both autogenous and bacteria-based self-healing concretes can be significantly improved by adding selected fibers based on carbon [28], cellulose [29], thermoplastics [30] or glass [31]. In the presence of the fibers, crack propagation can be prevented by fiber bridging, which is a well-established way in fiber-reinforced composites to improve their fracture toughness [32].