Microbial self-healing of cracks in cement-based materials and its influencing factors

Abstract

Cement-based materials are brittle and crack easily under natural conditions. Cracks can reduce service life because the transport of harmful substances can cause corrosion damage to the structures. This review discusses the feasibility of using microbial self-healing agents for crack healing. Tubular and spherical carriers can be used to load microbial self-healing agents and protect microbes, which prolongs the self-healing time. The area self-healing ratio, permeability, mechanical strength, precipitation depth method, numerical modeling, and ultrasonic method can be employed to identify the self-healing effect of cracks. Moreover, the self-healing mechanism is systematically analyzed. The results showed that microbial self-healing agents can repair cracks in cement-based materials in underground projects and dam gates. The difficulties and future development of self-healing cracks were analyzed. A microbial self-healing agent was embedded in the cement-based material, which automatically repaired the developing cracks. With the development of intelligent building materials, self-healing cracks have become the focus of attention.

Graphical abstract

Keywords

cement-based materials / cracks / microbial self-healing agent / mechanism / intelligent building materials

1 Introduction

Cement-based materials have controlled strength, good durability, and good impermeability. They can be widely applied in the fields of road construction, ocean engineering, underground engineering, and bridge engineering because of the abundance of their raw materials, simple production processes, and low prices [1–7]. Hence, cement-based materials inevitably suffer from external corrosion during service, which results in the development of cracks. Furthermore, their inherent brittle defects can aggravate crack development [1,8–11].

The structure of cement-based materials is typically damaged by cracking under the influence of external forces. Cracks provide channels for the transmission of chemical substances, such as carbon dioxide, chloride ions, and sulfate ions [12–14]. These chemical substances can cause a loss of carrying capacity [15–19], a decrease in alkalinity [20], a decrease in corrosion resistance [21–25], and volume expansion [26–32] in reinforced concrete. They may act simultaneously in cracks, further accelerating the damage process [33,34]. Therefore, the durability decreases. This not only causes huge economic losses but also seriously threatens personal safety.

A continuous network structure can be formed owing to an increase in the number of microcracks in cement-based materials, which causes an increase in permeability. Repairs should be conducted to improve the impermeability and resist the corrosion of harmful media in cement-based materials containing cracks [35–37]. Currently, the methods for crack repair in cement-based materials mainly include active repair and self-repair [38,39]. Active repair involves the injection of inorganic nonmetallic cementitious materials, organic cementitious materials, and inorganic biological cementitious materials into cracks to seal them. Self-healing indicates that materials can automatically trigger a repair mechanism to heal cracks when they appear [40]. Self-healing agents and active repair reagents are of the same type. First, a self-healing agent is embedded in the material. Self-healing proceeds spontaneously when cracks are formed. With the development of intelligent building materials, self-healing cracks have become the focus of attention.

2 Types of self-healing agents and self-healing mechanism

The self-healing agent for the corresponding crack should be selected based on the service environment of the material. When cracks develop, the agent is released to fill and seal the cracks to improve mechanical strength, impermeability, corrosion resistance, and other properties. Currently, the types of agents mainly include inorganic nonmetallic cementitious materials, organic cementitious materials, and biological cementitious materials [40–43], as shown in Tab.1. These types are the same as those of the active repair agents (Tab.2). When cracks occur, the built-in agent can quickly flow to the cracks to repair them, requiring the agent to exhibit green environmental protection, good viscosity, and fluidity. Therefore, solution-based repair agents are primarily used in morphological studies [41]. As most cracks are exposed to natural conditions, air-curing or solvent-volatile repair agents can be used [41]. The viscosity is usually required to be in the range of 100–500 mPa·s [41].

In recent years, microbial self-healing agents have exhibited good environmental friendliness and self-healing potential in repairing cracks and have attracted extensive attention worldwide [67–73]. The concept of biological self-healing agents originates mainly from microbially induced carbonate precipitation (MICP). Natural MICP conditions were created in the cracks, and calcium carbonate was produced to seal the cracks automatically. Microorganisms in biological self-repairing agents are mainly urease-producing bacteria (UPB) and carbonic anhydrase bacteria (CAB) [74,75]. UPB and CAB are commonly used, and mineralize and heal cracks in cement-based materials, as shown in Fig.1 [42,43,69,71]. The microorganisms commonly used as self-healing agents for UPB, CAB, and nitrifying bacteria are Sporosarcina pasteurii, Bacillus mucilaginous, and Diaphorobacter nitroreducens, respectively [42,69,71]. These microbial self-healing agents can also be supported by carriers, and the conditions required for carriers are more stringent than those for inorganic nonmetallic and organic self-healing agents. For example, a highly alkaline carrier (pH > 12) inactivates microorganisms. Therefore, the alkalinity of the carrier should be maintained below a pH of 11 [76]. The main feature of biological self-healing agents is the production of carbonate ions under the specific enzymatic action of microorganisms, combined with calcium ions at the cracks to form biological calcium carbonate, thus cementing and filling the cracks (Eq. (4)). The substrates of Sporosarcina pasteurii, Bacillus mucilaginous, and Diaphorobacter nitroreducens were urea, carbon dioxide, and a mixture of calcium formate and calcium nitrate [42], respectively, and their corresponding biochemical reactions are shown in Eqs. (1)–(4):

(1)Sporosarcinapasteurii:\mathrm{C}\mathrm{O}{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{N}{\mathrm{H}}_4^{+}+\mathrm{C}{\mathrm{O}}_3^{2-}

(2)Bacillusmucilaginosus:\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+{\mathrm{H}}^{+}

(3)\begin{array}{rl}& Diaphorobacternitroreducens:\\ & 5\mathrm{H}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+2\mathrm{N}{\mathrm{O}}_3^{-}\to {\mathrm{N}}_2+3\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+2\mathrm{C}{\mathrm{O}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}\end{array}

(4)\mathrm{C}\mathrm{e}\mathrm{l}\mathrm{l}-\mathrm{C}{\mathrm{a}}^{2+}+\mathrm{C}{\mathrm{O}}_3^{2-}=\mathrm{C}\mathrm{e}\mathrm{l}\mathrm{l}-\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3\left(\mathrm{s}\right)

Inorganic self-healing reagents are readily available and inexpensive, and can be mixed directly with cement-based materials. However, some reagents pollute the environment, and the repair speed is slow. The preparation of organic self-healing reagents is complex. Their repair speed is fast, and they have good effectiveness; however, they are relatively expensive. They are not easy to mix with cement-based materials, they usually require a carrier for loading, and they age easily under natural conditions. The price of microbial self-healing agents lies between that of inorganic and organic self-healing agents, and microorganisms are easy to prepare. Bacterial powders can be directly mixed with cement-based materials. Microbial self-healing agents have a positive effect on the self-healing of early cracks and result in a faster self-healing speed, making them an environmentally friendly and ideal self-healing material.

3 Carriers of self-healing agents

Microbial self-healing agents exist mainly in the form of a solution, and a carrier is required to load and mix them into materials. When cracks occur, these liquid self-healing agents flow out and react with the air and calcium hydroxide at the crack. Hydration products that can fill and bind cracks are produced. Hence, the carrier choice is scientific. Self-healing agents should be automatically released from the carriers when cement-based materials develop cracks. Generally, the incorporation of a carrier does not affect the performance of cement-based materials, and the carrier chemically reacts with the cement hydration products and self-healing agents.

Self-healing agent carriers mainly have tubular and spherical structures [40,41,77]. The main materials of tubular carriers are glass and ceramics. A self-healing agent was injected into a hollow tubular carrier and mixed with cement-based materials by stirring [40,41]. The length of the tubular carrier can reach 100 mm [77]. However, the hollow tubular carrier containing the self-healing agent broke easily during stirring. Moreover, glass or ceramic tubular carriers are not convenient for molding specimens. A lightweight aggregate, expanded clay, and other materials containing porous structures can be used for the encapsulation or immobilization of bacteria. Therefore, a porous spherical carrier is often used to load the self-healing agent [40,41]. Spherical carriers with a diameter of ≤ 1 mm are often used for the self-repair of cracks [42]. Spherical carriers have little effect on the workability of cement-based materials, and are convenient for stirring and shaping. The manufacture of spherical carriers is easier than that of tubular carriers, as they can be easily prepared under laboratory conditions. Spherical carriers contain a wide range of raw materials, such as waste concrete, ceramsite, expanded vermiculite, low-alkali cement, porous plastics, and ceramics. Compared with tubular carriers, spherical carriers can heal cracks in multiple directions and avoid the wastage of self-healing agents.

4 Influencing factors of microbial self-healing agents

The main factors that affect the ability of the microbial self-healing agent to heal cracks in cement-based materials are the state of the self-healing agent, cement type, carrier type, microorganism type, content and state, substrate types and content, calcium ion types and content, temperature, and moisture content. These factors significantly affect the self-healing of cracks in cement-based materials. Therefore, these influencing factors should be investigated to determine the optimal working conditions.

4.1 Effect of types of microbial self-healing agents

There are two main types of microbial self-healing agents: liquids and solids. A liquid-type agent is loaded in a carrier in the form of a solution, and it is automatically released when the occurrence of a crack is sensed, which triggers a mineralization reaction to form calcium carbonate, which can heal cracks [78]. Solid-type microbial self-healing agents mainly consist of microorganisms and substrates in cement-based materials, or carriers in the form of powders [79,80]. When cracks are generated, a mineralization reaction is triggered around them in the presence of a water/nutrient solution, which fills the cracks [79]. Microorganisms are usually prepared as liquids and can easily be used to prepare liquid-type self-healing agents, which have the best healing effect on early cracks [81]. After 28 d, the enzymatic effect of microorganisms decreased gradually, resulting in the deterioration of the mineralization healing effect [81]. However, microorganisms in solid-type agents exist in the form of spores, which protect them for a long period (months to years) and allow them to exist in the carrier or cement-based material in a dormant state until cracks occur [82]. At this time, the spore microorganisms are resurrected in the presence of a water/nutrient solution, and a mineralization reaction occurs around them to form calcium carbonate, which fills cracks [79,82]. Cement-based materials, which are similar to living bodies, have long service lives. During the entire life process, cracks may occur at different times, which requires microorganisms to maintain the characteristics of life for a long time using a microbial self-healing agent. Therefore, solid-type agents have more practical application prospects than liquid-type agents.

4.2 Effect of cement types

This type of cement has an impact on the microbial self-healing agent because of its alkalinity. The pH of the pore solution at the cracks is usually approximately 9–10, which is conducive to the deposition of calcium carbonate [83]. When microorganisms in the self-healing agent have no carrier protection, high-alkali cement (pH = 12–13) will inactivate the microorganisms, rendering the microbial self-healing agent unable to heal cracks [84,85]. For example, compared with high-alkali ordinary Portland cement (OPC), low-alkali sulfo-aluminate cement, MgO cement, and alkali-activated cement are conducive to the survival of microorganisms and can be used as carriers [79,82,86]. Hence, the type of cement affects the ability of the microbial self-healing agent to heal cracks.

4.3 Effect of temperature

In microbial self-healing agents, the enzymatic activity of the microorganisms is significantly affected by temperature. Therefore, they must be loaded into the carrier to avoid the temperature (93 °C) generated by OPC hydration to kill the UPB or CAB [87]. Multiple studies have confirmed that the ideal temperature for biocement involving UPB and CAB is in the range of 20–35 °C [88–90]. The size and morphology of calcium carbonate precipitated by UPB differ at 5, 25, and 50 °C [89]. Spherical calcite of 15–20 µm is produced at 25 °C in the MICP process, and it can bond well with sands and heal the cracks [89]. The activity of carbonic anhydrase produced by CAB was the highest at 30 °C, and the effect of the calcification was the best [90]. The production of CaCO₃ induced by CAB can well heal cracks in cement-based materials [91,92]. In summary, the best operating temperatures of UPB and CAB are 25 and 30 °C, respectively.

4.4 Effect of moisture

Microorganisms cannot be revived without the presence of moisture in the carriers. Hence, moisture curing is the best method for self-healing concrete [93]. This is because microorganisms require moisture for growth and reproduction during the self-healing of cracks [94]. When a cement-based material cracks, the enzyme in the microorganism is activated to hydrolyze the substrate, and biocement is then produced to heal cracks under moisture [95]. Moisture is essential for microbial self-healing. Different relative humidity levels exhibit different repair effects.

4.5 Effect of carrier types

The carrier types are the same as the cement types and affect the ability of the microbial self-healing agent. Carriers can effectively protect microorganisms and improve the self-healing effect of cracks in the highly alkaline environment of cement-based materials [95,96]. The most commonly used carriers are core–shell or porous structures that can carry a certain amount of microbial self-healing agents [95]. The pH of the carrier is compatible with microorganisms and does not trigger biochemical reactions with microbial self-healing agents [97]. Hence, the compatibility of carrier materials with microbial self-healing agents and cement-based materials should be considered. Carriers with porous structures are most commonly used for loading UPB and CAB.

4.6 Effect of types, content, and state of microorganisms

The types, contents, and states of microorganisms have a significant impact on the effectiveness of microbial self-healing agents for the self-healing of cracks [81,82]. Regarding the state of the self-healing agent, the solid state of spore microorganisms is generally better than the liquid state. Currently, the most commonly used microorganisms are UPB and CAB (Tab.3) [98–100]. The bacterial solution or powder can be directly mixed with cement at a certain water–cement ratio [81,91,92]. These two microorganisms have different unit volume contents, hydrolyzed substrate types, and calcium ion types [81,82,100]. Hence, the crystal structure and properties of calcium carbonate may differ, causing differences in the self-healing effect. The best unit volume contents of UPB and CAB and the corresponding best substrate and calcium ion types were obtained through experimental screening. The cost of microbial self-healing reagents prepared using CAB is lower than that of those prepared using UPB because of the different types of calcium ions and hydrolysis substrates used.

4.7 Effect of types and content of substrates

The type and content of substrates have an important impact on biomineralization products [101]. The substrate has a vital influence on the biomineralization precipitation of calcium carbonate [90,101]. No enzymatic effects or precipitation were observed without the substrate. The concentration of the substrate also affects mineralization [90,101]. For example, different concentrations of urea can affect the mineralization and cementation abilities of UPB [102,103]. The mass ratio of bacteria to urea and urea content also affect the mineralization and cementation ability of UPB. The concentration of urea affects the content of the biomineralization product, crystal type, and cementing ability. Typically, the concentration of urea is 800–1000 mmol/L [102]. The HCO₃^– concentration reached 45 mmol/L when the culture time of CAB was 44 h for CO₂ capture [90]. Hence, the optimal urea concentration in UPB and the culture time for CAB were 1000 mmol/L and 44 h, respectively.

4.8 Effect of types and content of calcium ions

The price, type, and content of calcium sources can also affect the mineralization and use of microorganisms in microbial self-healing agents, as shown in Tab.4 [104–106]. Calcium was purchased from the Aicaigou website. According to literature, the supply efficiency of calcium ions affects the precipitation amount and efficiency of calcium carbonate [83,107]. In cracks without an external calcium source, microorganisms can only induce calcium ion deposition in cement hydration products, which affects the mineralization and healing efficiency [83]. Therefore, a certain amount of Ca must be added.

In MICP-cemented sandy soil, Ca(CH₃COO)₂ as a calcium source has a better cementing effect than Ca(NO₃)₂ or CaCl₂ [106]. For steel-bar-reinforced cement-based materials, the chloride ion in calcium chloride causes the electrochemical corrosion of the steel bars and can be used as an early strength agent. The costs of other calcium sources are typically higher than that of calcium chloride (Tab.4). The calcium source for CAB that induces calcium carbonate precipitation is usually calcium nitrate [108]. The cost of organic Ca sources is higher than that of inorganic Ca sources, which are usually weak electrolytes, and the utilization efficiency of Ca ions is low [83]. Therefore, an ideal calcium source is selected for incorporation into a cement-based material or carrier according to the construction cost and environment of the construction project [109]. The optimal calcium ion content was determined from the effect of the concentration of calcium ions in the external calcium source on the self-healing of cracks at different ages [83]. The calcium ion concentration of 60 mmol/L is beneficial for the growth of CAB, and the CA activity is highest at this concentration [90]. The calcium ion concentration of 0.2 mol/L prepared using CaCl₂ can be used to produce calcium carbonate with excellent performance by UPB, which can better repair cracks [110]. In other words, the aforementioned influencing factors are determined through experiments according to the cost, durability, usage environment, and other conditions of the cement-based materials.

5 Evaluation of self-healing effects

The microbial self-healing effects on cracks in cement-based materials must be evaluated after self-healing. The commonly used quantitative evaluation methods include water penetration rate, mechanical strength, depth healing efficiency (DHE), area self-healing ratio (ASR), gas permeability, and Cl^– migration coefficient [43,58,111,112].

5.1 Water penetration

Before self-healing, water permeability and crack area should be greater than those after self-healing. After self-healing, the self-healing agent binds to and fills the cracks, reducing their widths. Compared with before self-healing, it takes more time for the same unit volume of water to penetrate the repaired cracks. Usually, the water penetration rates (mL/s) of a crack before and after repair are measured using the constant water head method. Prior to self-healing, the initial water penetration rate (V₀, mL/s) of the cement-based material with cracks was determined. At different self-healing times, the water penetration rate (V_t, mL/s) of the crack was measured, and the resistance to the water-permeating self-healing ratio (WST, %) was calculated according to Eq. (5) [43]:

(5)WST=\left(V_0-V_{\mathrm{t}}\right)/V_0\times 100\mathrm{\%}.

5.2 Mechanical strength

The evaluation methods of mechanical strength after microbial self-healing mainly include the evaluation of the compressive strength [58] and three-point bending strength [111]. The effect of self-healing is characterized by the ratio of the increase or decrease in the mechanical strength or the rate of change of the elastic modulus before and after self-healing. The self-healing depth of the cracks can be measured according to the initial crack depth (ICD) and crack depth on day n (CDN) [46]. The DHE was calculated using Eq. (6) [46]:

(6)DHE=\left(ICD-CDN/ICD\right)\times 100\mathrm{\%}.

5.3 Binary image

The direct acquisition of crack images is the most intuitive method for obtaining surface cracks. First, digital photos perpendicular to the crack surface were obtained before and after self-healing, and crack images were analyzed using the ImageJ software to obtain crack binarized images before and after repair. The number of pixels contained in the crack images was calculated using ImageJ before and after self-healing, that is, the healed area at different time periods. The ASR was calculated according to the number of pixels after self-healing (A_t) and the number of crack pixels before self-healing (A₀) at different time periods, as shown in Eq. (7) [43]:

(7)ASR=\left(A_0-A_{\mathrm{t}}\right)/A_0\times 100\mathrm{\%}.

5.4 Numerical modeling

The crack-healing ratio can be predicted using numerical modeling for different days and sizes. Laboratory data were compared with the predicted results to determine the optimal experimental parameters. The repair effect was evaluated based on the crack-healing ratio. For example, first-order ordinary and second-order partial differential equations have been used to simulate UPB-based self-healing concrete [113]. In this process, the finite difference and element methods can solve the biochemical–diffusive model well. The correlation coefficient, R, is 0.99. The laboratory data were similar to the results predicted in the same context [113]. Based on the concept of effective calcium ions, Qian et al. [62] predicted the deposition depth of calcium carbonate under cracks of different widths, and the predicted results were verified through experimental results, which were accurate for CAB-based self-healing concrete. CO₂ and Ca²⁺ reached the best matching reaction concentration within 1.0 mm of the crack surface, and CaCO₃ quickly precipitated to fill the cracks [100]. Zhuang and Zhou [114] used a machine-learning method to predict the self-repairing ability of bacteria-based self-healing concrete cracks and analyzed the importance of the parameters. The results showed that the initial crack width had a more significant impact on the crack-closure percentage than the healing time and number of bacteria. Zhang and Qian [115] established crack geometric characteristics using probability density function models and evaluated the probability distribution features of the spherical particles of microbial self-healing agents. This study can be used to calculate the theoretical dosage of self-healing particles and predict the effect of crack self-repair. Zemskov et al. [116] developed a mathematical model for the self-healing process of bacterial concrete based on a moving boundary problem. The simulation results indicated that the crack was fully healed after three days.

In addition, electron microscopy, X-ray tomography, nanoindentation, and ultrasound can qualitatively characterize the cementation and filling effects of cracks before and after self-healing.

6 Potential applications

The characteristics of microorganisms are as follows. 1) They can survive in carrier- or cement-based materials for a long time. The spores are loaded into carriers and can live up to approximately one year. The bacterial powder was directly incorporated and could survive for up to three months. 2) They should not negatively impact the environment or the performance of cement-based materials. For example, the selected microorganisms should be harmless to the human body and should not pollute water resources. In the absence of nutrients, these microorganisms become inactive and undergo ablation. 3) Active repair of cracks occurred. When cracks occur in cement-based materials, external moisture and oxygen enter the carrier, and the microorganisms in the carrier are revived and spread to the crack surface, thereby inducing calcium carbonate precipitation to repair the cracks. If there are no cracks in the short-term, moisture and oxygen cannot enter the carriers and the microorganisms remain dormant. To keep the biological activity, pH, temperature, and nutrients are very important. The characterization of the microbial self-healing agent is environmentally friendly, inexpensive, and effective. Bacillus mucilaginous and Sporosarcina pasteurii are often used to heal cracks. Microbial self-healing agents heal cracks using manual spraying and pre-embedding methods. They are typically used for both new and old cement-based materials that exhibit cracks with a wide range of self-healing widths. The range of width of self-healing cracks of artificial pre-embedded microbial self-healing agents is usually 0.1–1 mm. When the width of the crack reaches 0.8 mm, the crack cannot be repaired completely using microbial self-healing agents [93]. Compared with pure water repair, microbial self-healing agents can heal crack widths up to 0.46 mm [93,117]. Manual spray repair involves spraying a self-healing agent onto cracks [118], and microorganisms can work autonomously, which induces mineral precipitation to bind and seal the cracks. For example, Fig.2 shows that old cracks can be repaired using a UPB-based self-healing agent involving Sporosarcina pasteurii. When cracks are larger than 1 mm, fine sand can be filled first, and the large cracks can quickly heal after the MICP-based self-healing agent is poured into them, which increases their appearance and impermeability (Fig.2(a) and 2(b)). Cracks smaller than 1 mm in old cement-based materials can be healed by grouting without the addition of sand, as shown in Fig.2(b). Fig.3 shows that loose sand particles in old cracks were well connected by the MICP-based self-healing agent. Their permeability can be effectively reduced [118,119].

In artificial pre-embedded self-healing, Ramachandran et al. [120] first applied the MICP technology to heal cracks. This method can increase the stiffness and strength of cement mortar significantly, and the healing effect of surface cracks is better than that of deeper ones [121]. Subsequently, several researchers have used different types of microorganisms to induce mineral precipitation and heal cracks. They evaluated the effects before and after self-healing and attempted to apply them to actual projects [91,92]. Microcapsules as carriers to load microorganisms can better protect the microorganisms and induce a large amount of calcium carbonate at the cracks, which effectively improves impermeability [121,122]. Qian et al. [91] used a microbial spore powder-based self-healing agent to repair concrete cracks in underground engineering and observed that the self-healing agent did not negatively affect the mix ratio and performance of concrete. The agent can also effectively heal early cracks in the concrete of subway stations and the lock channel wall [91,92]. In practical engineering applications, wet-linen-containing nutrient solutions can better promote the self-healing of cracks than water [83]. Therefore, microbial self-healing agents can be commercially used.

7 Problems and development prospects

Microbial self-healing agents that heal material cracks involve multiple disciplines, such as civil engineering, chemistry, materials science, and microbiology [41]. It is often difficult for scholars to understand these subjects fully, causing technology to encounter bottlenecks. Therefore, cooperation and exchange between multiple disciplines is required to apply this technology to practical engineering on a large scale as soon as possible. Microbial self-healing agents have a certain shelf life, and research has often applied them to heal early cracks for a few days or years [62,123,124]. However, after decades of service with cement-based materials, how can a microbial self-healing agent still have the ability to heal cracks? The carriers affect the performance of cement-based materials. How do microorganisms in a microbial self-healing agent survive in highly alkaline environments for long periods? In actual engineering, how does one maintain high self-healing efficiency? How can the performances of cement-based materials be restored after self-healing? How does a microbial self-healing agent completely bind and fill cracks instead of self-healing the surface? These issues require scientists to continue their efforts to solve the problems of microbial self-healing agents and enable their large-scale application.

Cracks healed by microbial self-healing agents still have several key technical problems, but the self-healing of cracks has become a developmental trend [50,125–135]. For example, the self-healing of cracks can effectively alleviate potential hazards in the concrete of dams and underwater tunnels. Several studies have indicated that the mechanical properties of microbial self-healing concrete can be improved. For example, Williams et al. [136] studied the strength recovery of microbial concrete cracks after self-healing, and the results showed that the flexural strength recovery rate of the microbiome was 8%–30%, whereas that of the benchmark group was only 3%–5% [96]. Simultaneously, it reduces the high cost of manual monitoring and external repair and extends service life [96,137–139]. As intelligent self-healing materials, microbial self-healing agents are an important part of the cement-based material structure–intelligence integration. Different types of biocement-based self-healing agents [140–143] have considerable potential for the timely healing of cracks in major cement-based structures, which can improve their mechanical properties, safety, and durability.

8 Conclusions

The types of carriers, evaluation methods, and repair mechanisms used in microbial self-healing agents were reviewed, improving theoretical and technical guidance for future large-scale applications. The main mechanism of a microbial self-healing agent that repairs cracks is the enzymatic action of microorganisms on the substrate, which produces carbonate ions that react with calcium ions at the cracks to form calcium carbonate, thus filling the cracks. The self-healing effects were evaluated using the ASR and resistance to the water-permeating self-healing ratio. Tubular and spherical carriers can load and protect microorganisms for long periods. The spherical carrier material can be prepared according to the applied environment, and is controllable and adjustable to satisfy the corresponding requirements. The microbial self-healing agent can heal cracks with a width of 0.1–1.0 mm. The performance of the microbial self-healing agents was better for microcracks in the early stage than for large cracks in the late stage. Concrete cracks in underground projects and dam gates can be healed using microbial self-healing agents. The issues involving microbial self-healing agents in actual engineering can be better solved through multidisciplinary crossover. They are intelligent materials that are of great significance for improving the durability and safety of cement-based structures.

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