Bacteria-Based Self-Healing Concrete

Bacteria-Based Self-Healing Concrete: History

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Subjects: Engineering, Civil

Contributor:

Sardar Kashif Ur Rehman

Faisal Mahmood

Mohammed Jameel

Nadia Riaz

Muhammad Faisal Javed

Abdelatif Salmi

Youssef Ahmed Awad

Cracking is one of the main ways that concrete ages, allowing pollutants to seep within and potentially lowering the physical and mechanical strength and endurance of concrete structures. One of the healing procedures that merits research is the use of bacterially generated calcium carbonate precipitation in concrete mixtures to mend concrete cracks.

MICP
self-healing concrete
bacterial concrete
cracking
Sealingability

1. Introduction

Concrete has become one of the world’s utmost widely utilized substances due to its unique properties, which include its good compressive strength, availability, and adaptability, its economical nature, its suitability with reinforced steel bar, its being flame retardant, its superior caloric weight (Highly resistant to weight loss against rise in temperature [1], and the ability to cast in specific forms and dimensions [2,3,4]. Concrete, however, is prone to cracking and corrosion, and these fissures and corrosion, which are caused by mechanical and environmental forces, considerably degrade the structure’s performance, serviceability, and longevity [5,6,7]. Despite the fact that passive crack treatments (such as chemical and polymer sealants) are available on the market, they are typically time-consuming and non-sustainable and don’t repair the inside of crack [8,9]. Currently, microbiologically influenced corrosion (MIC), a technique that describes the involvement of microorganisms in the corrosion process, has received the greatest attention in the scientific literature. If concrete cracks are not promptly and properly treated, reinforced concrete structures are susceptible to collapsing. Concrete should not crack because, if it does, water and oxygen [10] will enter the concrete, accessing the steel bars, corroding it in the early stages of construction. Corrosion is a process that is significantly influenced by temperature, which enhances the pace of reaction [11]. The most prevalent cause of RC constructions failing early is rusting of the reinforcements [12]. Chlorides, which are created by chemical manufacturers and are present in seawater, antifreeze, and other goods, are primarily responsible for rusting [13]. Steel reinforcement in concrete commonly corrodes due to concrete carbonation [14]. The most frequent stages of RC construction deterioration are as follows: [15]: It will eventually collapse due to loss of passivity, cracking, corrosion, flaking of the protective coating, decreased adhesion between the reinforcements and the concrete, and other factors.

2. Factor Affecting the MICP

2.1. Nucleation Site

Bacteria’s importance as crystal nucleation sites [44] has long been recognized [45]. Because of their large surface-to-volume ratio, bacteria are said to behave as crystal nucleating agents, according to geological data [46,47]. The first stage in crystal formation is the molar ratio contact of metallic ions with chemically reactive compounds created by microorganisms, which are found largely in the murein [22]. These locations nucleate the accumulation of additional mineral as chemical expedite after complexation. Only tiny-grain precipitates can be produced in this location due to the narrow intervals between wall polymers, whereas the outside surface sites have no such restriction. As a result, large-grain precipitates can form if enough metal ions are available [48]. In order to promote mineral precipitation, the hydrophilic power between the bacterium and the mineral must be lower than the interaction tension between the mixture and the mineral [49].

With his staining technique, Christian Gram established the essential systematic distinction between gram-positive and gram-negative bacterial kinds. Unaware of it, he had also distinguished between the various eu-bacterial wall types and their chemical and structural composition [50,51].

2.2. Bacterial Type

Based on the many metabolic pathways used by microorganisms, the MICP process can be divided into four categories: ureaolysis, denitrification, sulphate reduction, and CO₂ hydrolysis mechanism [72]. Due to its effectiveness in the deposition reaction of calcium carbonate by urease bacteria, ureaolysis has been studied in considerable detail in MICP. To increase CaCO₃ deposition efficiency, different urease bacteria, like Bacillus sphaericus, have their enzyme activity and production studied [73,74], including Bacillus subtilis [75,76], Bacillus megaterium [77,78], Bacillus aerius [79], Sporosarcina pasteurii [80,81], Bacillus cereus [82], and Bacillus cohnii [83,84]. Because of its efficient enzymatic activity and biosynthesis, Sporosarcina pasteurii has emerged as one of the most popular urease bacteria research respondents among them. For calcium carbonate precipitation, several genera of bacteria discussed above can be chosen based on abiotic conditions. Urease-positive bacteria, for example, are needed to produce urease, which stimulates urea decomposition in the ureaolysis biochemical process [85]. The urease enzyme, found in bacteria like Bacillus, initiates the conversion of urea to ammonia to CO2 [86]. Urease-positive bacteria can exist in a variety of places, including the human body and soil. Besides determining whether the bacteria is suitable for the metabolic route, its feasibility and efficacy in various scenarios must also be examined. The viability of microorganisms plays a crucial role in the process efficacy when MICP is used. Discovered under anoxic conditions, the activity of Sporosarcina pasteurii (NCIMB 8841) was suppressed [87]. Bacillus species, however, exhibit increased activity in a variety of environments [88]. The results show that B. sphaericus is the bacterium most frequently used in MICP operations that use a urea-fortified medium. B. mega-terium is one of the other species in this genus [89] and B. lentus [90], have also been identified as MICP-causing microorganisms.

2.3. Bacterial Concentrations and Ureolytic Activity

The efficiency of the MICP process and the crystals produced are influenced by bacterial concentration and ureolytic activity. The hydrolysis of urea is a very slow process (3 × 10⁻¹⁰ s⁻¹), despite the fact that the urease enzyme can greatly speed up the process (3 × 10⁻⁴ s⁻¹) [91]. As a result, the precursor with enhanced ureolytic activity should be chosen for increased calcium carbonate generation. Aside from that, the amount of bacteria present can influence calcium carbonate formation which seal the crack result in the strength recovery provided. The number of bacterial cells participating in the fermentation process affects how quickly urea decomposes; a greater numeral of bacterial cells in the zymolysis activity leads to increased urease enzyme synthesis, which adds to MICP’s efficiency [92]. Because urease-positive bacteria are predominantly aerobic, oxygen-limiting circumstances hinder bacterial growth and calcium carbonate production [43]. Denitrifying bacteria, however, can start MICP anaerobically by utilizing nitrate as an electrophile to oxidize organic materials to produce calcium carbonate [23]. Denitrifying strains have a reduced ability to cause calcium carbonate precipitation than ureolytic bacteria, which may influence the functionality of MICP activities [43].

2.4. PH

Despite the fact that microbes affect microbial decomposition by changing almost any deposition factor [49], The ability of bacteria to create an alkaline environment through a variety of biological activities has been credited as the primary function in the formation of calcium carbonate [29]. Calcium carbonate precipitation and dissolution are mediated by microbial activity in response to environmental circumstances [101]. Generally, precipitation is influenced by the saturation index (SI) [102]; Using the formula in Equation (8), which is based on the interactions of temperature, calcium hardness, total alkalinity, and pH, which is connected to PH medium, one may calculate the saturation index, which is a number.

2.5. Nutrients

Since nutrients provide the energy needed for bacterial production and biochemical activity, their availability has a significant impact on calcium carbonate bio mineralization [43,108]. Bacteria have a wide range of sources and amounts of critical components that are required for their functioning [109].

Calcium carbonate bio mineralization is dependent on the presence of free Ca²⁺ in the nearby habitat, in addition to the carbon source [114]. The disintegration of 1 mole of urea develops the creation of 1 mole of calcite, as demonstrated in Equations (1)–(3) and(7) [115]. It is obvious that a higher urea content enhances mineral precipitation. In addition, it has been discovered that inclusion a large amount of calcium salt reduces enzyme activity and, as a result, causes calcium carbonate precipitation [116,117,118,119,120,121]. Employing Bacillus genera in the existence of urea, [2] studied the influence of varying Ca²⁺ causes on the deposition of calcium carbonate. According to the authors, CaCl₂ is the best calcium supply for biogenesis of calcium carbonate. The presence of soluble Ca²⁺ may cause a calcite production and urease functioning, which could be explained by adding ions species to the suspension media [116]. It’s also important to remember that, in order to obtain the maximum amount of calcium carbonate, reagent concentrations must be kept within acceptable ranges to prevent microbial growth inhibition [102].

2.6. Temperature

The temperature of the incubation chamber is one of the most important (the incubation chamber temperature is the laboratory static and shaking incubator temperature for the bacteria growth that is kept 37 °C for effective growth of bacteria; Higher temperature may affect the growth of bacteria) operating parameters that might influence bacterial growth and, as a result, the bio-mineralization process [2]. The catalysis of urea hydrolysis, like other enzymatic reactions, is a temperature-dependent process [128]. The ideal temperature, however, lies between 20 and 37 °C, depending on the surrounding conditions and the concentrations of chemical reagents in the fermentative media [129] [130,131]. The rate of urea disintegration is increased by 5 and 10 times, respectively, when the reaction temperature is raised from 10 to 20 °C and between 10-15 °C, according to previous research [132]. In a solution medium with 0.02 gram/Liter of catalyst, we discovered that increasing the temperature from 20 to 50 °C enhances calcium carbonate production [133]. However, a relatively high temperature had a negative impact on bacterial metabolism and enzyme function [91]. Despite a consistent urease activity at comfortable temperature (35 °C), we discovered that at a high temperature (55 °C), the enzyme activity significantly declines [134].

3. Tests for Assessing the Bio-Mineralized Calcium Carbonate Based SELF-Healing Concrete

Several methods for evaluating the crack self-healing effect were put forth in order to evaluate the potential of microbial self-healing concrete (MSHC) to close cracks. For instance, X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy (XRD) [137,138] were employed to examine the shape of crystals that precipitated at the break. The ability of the bio-mineralized self-healing precipitate was further evaluated using water permeability, porosity, and chloride ion permeation resistance [139,140] and nondestructive detection methods including ultrasonic and acoustic emission were used to assess the effectiveness of bio-self-healing. concrete [141,142,143,144]. TEM analysis was used to assess the sealing ability or deposition of MICP by [124] on archeological gypsum plasters employed M. xanthus bacteria. However, characterization techniques like SEM, XRD, and water permeability are only suitable for testing in the lab and not real engineering since they cannot concurrently satisfy the demands of operability and nondestructive testing. Acoustic emission and ultrasonic technology are expected to be heavily used in practical engineering due to the advantages of simple detecting equipment, practical operation, and economical detection. The majority of MSHC research is based on previously published experiments.

However, several scientists have chosen to use measurements of the deposited minerals to confirm the encapsulation of bacteria in self-healing concrete [91,145,146,147,148]. In the realm of encapsulating materials in self-healing concrete, techniques of testing based on the restoration of mechanical characteristics by the transmission of ultrasonic waves are still unexamined. Quantification of precipitated crystals in cementitious and biochar materials has been investigated using TGA for concrete or mortar samples.

Microstructure tests are carried out using tools including scanning electron microscopes (SEMs), field-emission scanning electron microscopes (FESEMs), and X-ray diffraction to identify and describe embedded materials after self-healing (XRD) [173] used bio-cement with a variable urea-CaCl2 and bacterial cell density concentration. Based on its 40% greater compressive strength than conventional concrete, improved material finish, aesthetic qualities, and environmental effect, the authors came to the conclusion that bio-cement has potential as a sustainable design material.

ATRIR [149,152] revealed the chemical composition of the deposit, which consisted of a calcite and aragonite combination in addition to two CaCO₃ polymorphs. For more confirmation of the characterization results, the XRD is frequently employed, along with SEM-EDX by various researchers provided in Table 2. This method was utilized to search for verified healing agents in the precipitates based on macrostructural and SEM inspection. The most prevalent microstructural test used by investigators to trace deposition products in crack specimens was SEM analysis [175]. In addition, for qualitative and quantitative elemental analysis, several researchers have combined EDS with SEM [163,164,176,177]. X-ray tomography, Raman spectroscopy, and Nuclear Magnet Resonance (NMR) can be utilized to monitor and study crack healing qualitatively and quantitatively besides the existing microstructure level research [178,179]. The nanotechnology level study of self-restoration effectiveness by bacteria encased materials has yet to be completed. These tests are performed to ensure that the results of the microstructural testing are as reliable as possible. Nanoscale tests should be performed to determine the bonding strength within the fractures at the interface between the deposited minerals and the cement substrate [180].

4. Sealing Ability and Recovery of Mechanical and Durability Properties

The crack width healed, the precipitate’s bonding with the mixture and structure, and the precipitate’s strength would all influence the restoration of original concrete properties. For efficient self-healing, properties that are as close to those of the original concrete as possible are desirable. The capability of bio-based self-healing concrete to heal is dependent on a variety of parameters, such as the curing condition, concentrations of doable spores and nutrients, the age of the concrete, and the amount of time it takes for the concrete to heal. Healing can occur in two different formats: calcium carbonate precipitation to close the fractures, and carbon dioxide produced by bacteria metabolism reacting with unreacted portlandite at the crack region to make more deposits. The use of a bio-based restoring chemical was found to cure a wide variety of fracture widths. Although it is dependent on a variety of conditions, healing efficiency is optimal when crack width is kept between 100 and 200 µm.

4.1. Recovery of Mechanical Properties

Wang [190] used glass tubes to encapsulate bacteria cells in PU and silica gel. The crack width used to measure mechanical strength recovery was around 0.35 mm. In the case of silica gel, there was more calcium carbonate precipitation, but only around 5% strength recovery. PU-containing specimens recovered between 50 and 80 percent of their potency. Conversely, the bacterial role in regaining mechanical strength was questioned because the strength recovery for live and dead bacteria cells was not significantly different. The precipitate quantity was larger in silica gel than in PU, although PU had a higher strength recovery. It’s reasonable to assume that PU, as an excellent sealing agent, played a major role in the mechanical strength recovery.

4.2. Recovery of Durability Properties

In concrete, effective self-healing means that the durability and mechanical strength are totally or almost restored to the original specimen. Water permeability and water absorption tests are frequently used to assess durability. Healing cracks also entails sealing any voids or linked pores through which foreign chemicals from the air or water could enter. As a result, water permeability and absorption are reduced. Pore blockage by calcium carbonate, which has a relatively low solubility, causes absolute permeability reduction through bacterial action [193,194,195].

Wang [151] used hydrogel as an encapsulation for bacteria spores and bio-reagents and reported a significant reduction in permeability of about 68 percent. Even for 0.3–0.4 mm cracks, the maximum crack size of 0.5 mm was repaired, albeit there was a wide range of healing ratios (40–90%). However, when compared to when only hydrogel was used, there is an improvement. Due to the proportionate dispersion of spores and bio reagents when encased along in hydrogel, better healing may be expected. Because of this encapsulation approach, bacteria would have fast reach to nutrients and precursor compounds in the case of cracking. Furthermore, in addition to bacterial precipitation, some autogenous healing helped by internal hydrogel curing may improve permeability reduction.

5. Field Application of Bio-Mineralized Self-Healing Concrete

To test the capacity of Sporosarcina pasteurii cells to self-heal, broken rock was repaired with the cells [198]. In 17 h of handling, a sizable amount of calcite precipitated (around 750 gramme), and a water permeability test of a single fracture over a large area revealed a significant reduction. These results suggested that MICP can be utilised to reduce the porosity of cracked rock in practise, suggesting that a MICP-based strategy would be a suitable choice for reducing unwelcome groundwater flow through fractured channels. In another investigation, investigators used bacteria to treat the faces of limestone at various temperatures in attempt to discover the optimal microorganism for practical use [199]. The limestone was reinforced to depths of 30 mm after a surface modification with bacteria twice in 12 h. The treatment cost was reduced to within the reach of common consolidates by optimizing urease dosages and carbonate precursor solutions. Subsurface drilled well fluid leaking in typical oil and gas exploitation or carbon capture technologies can be stopped by using the MICP a novel way by sealing gaps and porosity and lowering the system’s liquid penetration. So because activation solutions used in MICP-based sealants are water-soluble and have lower viscosities than those used in cement-based sealants, they are simpler to move into the consolidation deposit and are therefore more appropriate for usage with cracked structures. This study used conventional fluid conveyance methods to treat sandstone strata fractures 340.8 m below ground level with a S. pasteurii culture and a urea-calcium mixture (packer and bailer). Leading to decreased in insertion rate of flow from 1.9 to 0.47 L/min and a reduction in well pressure gradient from more than 30 percent to 7 percent. Moreover, following MICP therapy, the crack extension stress while re-fracturing improved in comparison to before MICP administration. The applications for which bio-concrete might be especially advantageous are shown in Figure 7. These applications’ main goals were to cut maintenance costs and prevent water infiltration. Additional uses included those for hard-to-reach locations, the nuclear sector, water-retaining structures, and airports [200].

Figure 7. Proposed application areas of self healing bioconcrete [201].

This entry is adapted from the peer-reviewed paper 10.3390/cryst12091222

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