As mentioned before, Microbially Induced Calcium Carbonate Precipitation is already a complex phenomenon that can be influenced by a variety of parameters. On the other hand, a large body of literature has been devoted to the study of S. Pasteurii as a sustainable construction material, investigating one or two parameters at a time. Hence, this section focuses only on the literature that presents parallel comparative methodologies and systematic characterizations and assessments. The preceding section (Sections 3) has covered the latest developments concerning the factors that impact the distinct characteristics of S. Pasteurii, as well as its behavior and viability in different environments. This section is dedicated to discussing the in-situ application of bio-cementation. It will cover relevant literature that explores the effects of different application methods and the challenges encountered in various application sectors. Among the application domains of bio-cementation, the soil stabilization and building construction sector has investigated this strategy the most.
4.1. Soil Stabilization
Traditional soil improvement strategies, such as adding natural and synthetic materials (e.g., recycled glass fibers, tires, fruit brunches, polypropylene, and polyester); injection of chemical grouting or deep mixing using cement and/or lime; and application of sand or stone columns, rely on synthetic substances that require significant energy for production and application and raise the pH of groundwater, causing serious environmental problems and contributing to ecosystem disturbance [
11]. With the widespread implementation of bio-cementation in soil stabilization applications, researchers have investigated the effects of different parameters, such as application method, nutrition media, and sand granularity, on the properties of
S. Pasteurii-treated soil at both laboratory and in-situ field scales. Regarding the application method, injection is the most common method for MICP-based soil stabilization. These findings underscore a significant aspect regarding the impact of grain size. While a higher level of porosity (100% coarse grain in Column A) does facilitate the migration of the bio-cementing agent throughout the treated column and may result in the highest
content, it does not necessarily yield the best mechanical properties. This is due to the fact that the bacterial solution tends to migrate to the bottom of the column under the influence of gravity, resulting in non-homogeneity throughout the column. Consequently, the top of the column exhibits the lowest
content, while the bottom exhibits the highest, contributing to the overall inferior mechanical properties of the column due to non-homogeneity.
As previously stated, soil stabilization is more advanced than other application fields of bio-cementation when it comes to scaling up the process for field application. Few studies have been conducted on the scaling up of bio-cementation to field scale [
1,
119,
120,
121,
122] ; therefore, it is important to highlight the most significant findings from these investigations. Omoregie et al. [
1] presented an economic strategy to scale up the production and cultivation of
S. Pasteurii under nonsterile condition using a custom-built stainless steel stirred tank reactor with a capacity of 3 m3. The scalability of the bacterial cells was investigated by increasing the volume of the seed cultures from 214 L to 2400 L and monitoring their growth for a duration of 90 hours. The results of the in-situ soil bio-cementation experiment demonstrated that bacterial cells, cultivated using this method, retained their ability to induce
precipitation even after being immobilized within sand specimens. The presence of
crystal formation within the treated sand particles was confirmed through the analysis of
content and microstructural data, which confirmed that the method presented in this study is as successful as the sterile condition.
Table 1 summarizes some of the findings from systematic investigations in the literature that used
S. Pasteurii for soil stabilization applications by injection. Table of literature that used other application methods to introduce
S. Pasteurii for soil stabilization are presented in Supplementary Materials Table S 1.
Although the injection method has proven to be quite effective in small specimens at the laboratory scale, significant problems have been noted at larger scales. Whiffin et al. [
10] investigated the impact of vertical injection of
S. Pasteurii along a 5-meter-long sand column. Their findings revealed that while calcium carbonate precipitation occurred throughout the entire treatment length, the concentration profile exhibited non-uniformity. In particular, higher quantities of calcium carbonate were observed at the injection points, with a subsequent decline in concentration along the length of the column. The impact of injection direction was assessed by Paassen et al. [
76] in a large-scale experiment involving the horizontal injection of
S. Pasteurii in a soil bed with a volume of 100 m
3. The study revealed a considerable variation in the peak strength of the unconfined compressive strength values, confirming again non-uniformity along the path of injection within the treated sand volume. In general, as the distance from the injection points increases, there is a decrease in the calcium carbonate content, which leads to an increase in porosity and a subsequent decline in the mechanical properties. This limitation of MICP application on a large scale arises from the occurrence of system clogging near the injection sites. This phenomenon is caused by the rapid precipitation of calcium carbonate by the bio-cementing agent, which leads to the closure of pores and hinders the migration of the bio-cementing agent throughout the entire treatment area, resulting in non-homogeneity. Hence, it is anticipated that the utilization of bio-cementing agents with low urease activity or the regulation of bio-cementation kinetics to achieve a slower rate would result in a reduced level of non-homogeneity in treatment, as it would facilitate the effective distribution of the bio-cementing agent along the entire length of the treatment. This disadvantage can be mitigated by employing a two-phase vertical injection procedure (alternating the injection of bacterial suspension and cementation or nutritional solution) and fine-tuning the injection parameters, such as decreasing the injection rate of bacterial suspension and increasing the flow rate of cementation solution [
10,
77].
Among the several research that evaluated the impact of particle size on the effectiveness of bio-cementation treatment aided by
S. Pasteurii, three stand out due to their systematic approaches [
46,
49,
118]. Lin et al. [
118] studied the effect of grain size on MICP-treated Ottawa sand by
S. Pasteurii, considering fine grains (D
10% = 0.26 mm and D
50% = 0.33 mm) and coarse grains (D
10% = 0.58 mm and D
50% = 0.71 mm). The findings of this investigation revealed that the coarse-grain soil had a lower
content in comparison to the fine-grain soil. However, it was observed that the coarse-grain soil demonstrated greater increases in S-wave velocity, peak shear strength, and cohesiveness when compared to the fine-grain soil. Moreover, the results obtained from triaxial testing indicated that the peak deviator stress of the coarse-grained soil containing 1.6%
and the fine-grained soil containing 1%
exhibited an average increase of 93% and 171%, respectively, as compared to their respective untreated specimens.
Hoang et al. [
46], on the other hand, investigated this effect on EICP-treated silica (SiO2)/quartz sand, considering fine grains (D
10% = 0.26 mm and D
50% = 0.36 mm) and coarse grains (D
10% = 0.61 mm and D
50% = 0.72 mm). The outcomes of this study highlighted that, similar to MICP-treated sand, the mechanical properties of EICP-treated sand, such as Young's modulus and UCS test, in coarse grain are higher than fine grain ( These findings underscore a significant aspect regarding the impact of grain size. While a higher level of porosity (100% coarse grain in Column A) does facilitate the migration of the bio-cementing agent throughout the treated column and may result in the highest
content, it does not necessarily yield the best mechanical properties. This is due to the fact that the bacterial solution tends to migrate to the bottom of the column under the influence of gravity, resulting in non-homogeneity throughout the column. Consequently, the top of the column exhibits the lowest
content, while the bottom exhibits the highest, contributing to the overall inferior mechanical properties of the column due to non-homogeneity.
As previously stated, soil stabilization is more advanced than other application fields of bio-cementation when it comes to scaling up the process for field application. Few studies have been conducted on the scaling up of bio-cementation to field scale [
1,
119,
120,
121,
122] ; therefore, it is important to highlight the most significant findings from these investigations. Omoregie et al. [
1] presented an economic strategy to scale up the production and cultivation of
S. Pasteurii under nonsterile condition using a custom-built stainless steel stirred tank reactor with a capacity of 3 m3. The scalability of the bacterial cells was investigated by increasing the volume of the seed cultures from 214 L to 2400 L and monitoring their growth for a duration of 90 hours. The results of the in-situ soil bio-cementation experiment demonstrated that bacterial cells, cultivated using this method, retained their ability to induce
precipitation even after being immobilized within sand specimens. The presence of
crystal formation within the treated sand particles was confirmed through the analysis of
content and microstructural data, which confirmed that the method presented in this study is as successful as the sterile condition.
Table 1). The trend of permeability reduction influenced by grain size indicated that permeability reduction in fine-grained bio-cemented sands steadily declined with the increase in
content, while for the same increase in
, the trend of reduction in permeability declination was comparatively less pronounced in coarse-grained sand [
46]. This phenomenon is often attributed to the smaller distances between particles, known as nucleation sites, in fine-coarse sand. The precipitation of calcium carbonate fills the small pores in fine grain rapidly, leading to the obstruction of pathways through which bacteria can move to continue the cementation in other pores. In coarse-grained sand, the interparticle spacing is greater. Through the precipitation of
, the contact points between the particles are initially joined, followed by the deposition of additional layers of calcium carbonate on top. This process facilitates the movement of bacteria or enzymes through the pores, enabling them to reach more profound or distant locations.
Mahawish et al. [
49] studied the effect of fine (0.075–9.5 mm) and coarse (2.36–16 mm) grain percentages in mixed Pakenham Blue Metal columns on the mechanical properties of treated sand. Five columns with different percentages of fine/coarse grains (A = 0/100, B = 25/75, C = 50/50, D = 75/25, E = 100/0 %) were subjected to MICP treatment by
S. Pasteurii. The average calcium carbonate precipitation for columns A and E was determined to be about 7% and 6.5%, respectively, and about 6% for other mixed columns (B, C, and D). However, column B exhibited the most uniform distribution of
along its length. The unconfined compressive strength test results for all three mixed columns (B, C, and D) were determined to be approximately 0.6 MPa, while for column A, despite having the highest
content, this value was lowered by half to 0.3 MPa, and for column E, it was dropped to nearly 0.5 MPa. These findings underscore a significant aspect regarding the impact of grain size. While a higher level of porosity (100% coarse grain in Column A) does facilitate the migration of the bio-cementing agent throughout the treated column and may result in the highest
content, it does not necessarily yield the best mechanical properties. This is due to the fact that the bacterial solution tends to migrate to the bottom of the column under the influence of gravity, resulting in non-homogeneity throughout the column. Consequently, the top of the column exhibits the lowest
content, while the bottom exhibits the highest, contributing to the overall inferior mechanical properties of the column due to non-homogeneity.
As previously stated, soil stabilization is more advanced than other application fields of bio-cementation when it comes to scaling up the process for field application. Few studies have been conducted on the scaling up of bio-cementation to field scale [
1,
119,
120,
121,
122] ; therefore, it is important to highlight the most significant findings from these investigations. Omoregie et al. [
1] presented an economic strategy to scale up the production and cultivation of
S. Pasteurii under nonsterile condition using a custom-built stainless steel stirred tank reactor with a capacity of 3 m
3. The scalability of the bacterial cells was investigated by increasing the volume of the seed cultures from 214 L to 2400 L and monitoring their growth for a duration of 90 hours. The results of the in-situ soil bio-cementation experiment demonstrated that bacterial cells, cultivated using this method, retained their ability to induce
precipitation even after being immobilized within sand specimens. The presence of
crystal formation within the treated sand particles was confirmed through the analysis of
content and microstructural data, which confirmed that the method presented in this study is as successful as the sterile condition.
4.2. Building Construction
Bio-cementation has been utilized in the field of building construction, primarily for producing new building materials rather than repairing damaged structures. The most common application method used in building construction is to employ bio-cementing solution as an admixture component of concrete.
Table 2 presents an overview of key findings derived from systematic investigations documented in the literature, which have employed
S. Pasteurii to develop self-healing concrete through mixing techniques. As can be observed, there is a notable disparity in the quantity of relevant publications utilizing
S. Pasteurii in comparison to soil stabilization (
Table 1 and Table S 1). Moreover, it is worth mentioning that in the context of building construction (
Table 2), the primary emphasis is placed on mechanical and permeability characterization of the treated material rather than the assessment of calcium carbonate content, compared to soil stabilization(
Table 1).
The experimental approach commonly used in the majority of the literature involves the following steps:
Introducing bacteria as an admixture component in concrete.
Curing the cast concrete for a period of 28 days.
Inducing cracks in the concrete by applying either uniaxial or flexural force.
Treating the damaged concrete with a bacterial nutrient media or treatment solution.
Monitoring the healing progress of the crack over time.
Conducting mechanical and permeability tests to characterize the healed concrete.
S. Pasteurii has shown remarkable effectiveness in repairing concrete cracks. A study by Sohail et al. [
128] utilizing this bacterium for the purpose of repairing cracks in concrete has demonstrated that not only does this bacterium have the capability to effectively fill cracks up to a width of 4 mm, but it can also successfully adhere two separate pieces of a concrete specimen that have become detached during a flexural test.
Various nutritional media, such as Lactose mother liquor [
129,
130], sodium carbonate [
130], calcium nitrate-urea, and calcium chloride-urea [
130,
131], have been employed in the cultivation of
S. Pasteurii within concrete mixtures. However, due to the variability in environmental conditions, concrete components, and treatment methods, it is unfortunate that a definitive conclusion cannot be reached on this matter. Kim et al. [
132] investigated the influence of concrete type on the process of
S. Pasteurii bio-cementation by measuring the relative mass gained as a result of treatment on normal and lightweight concrete, concluding that the relative weight gains observed in both normal and lightweight concrete were comparable. This suggests that the precipitation of calcium carbonate by
S. Pasteurii is not dependent upon the specific type of concrete used, as long as the environmental conditions necessary for bacterial growth (such as concrete pH and temperature) remain consistent.
Although the straightforward method of adding bacterial and nutrient solutions into concrete mixtures is commonly employed, researchers have explored alternative approaches to introduce bacteria into concrete mixtures. Chen et al. [
133] employed lightweight aggregates as carriers for
S. Pasteurii in concrete. In this study, the aggregates underwent several initial immersions in nutrient media (specifically, calcium lactate/yeast extract), followed by multiple drying cycles. Subsequently, the aggregates were immersed in a bacterial solution. After the aggregates were completely dried, they were combined with other components to produce concrete. The comparison of the healing process of induced cracks was conducted on three groups: the controlled group (A) without treated aggregate; the air-cured treated group (B) with treated aggregate but no additional supplement; and the immersion-cured treated group (C) with treated aggregate and immersion in a urea/calcium carbonate solution. The results showed that the precipitation of
by
S. Pasteurii can be reactivated in the presence of appropriate nutrition; in group C, the cracks were completely cured after 91 days, while in group B, only partial curing was observed, and no curing was observed in group A.
When using bio-cementing agents as a mixture component in concrete, it's important to keep in mind that an excessive bacterial concentration in the concrete solution may not always lead to the best permeability or mechanical properties. For instance, an investigation on the effect of
S. Pasteurii concentration (10
3, 10
5, and 10
7 cells/ml) in a concrete mixture on the mechanical properties of the concrete demonstrated that the compressive strength of concrete with 10
5 cells/ml bacterial concentration was 11% higher than that of 10
3 cells/ml and 13.5% higher than that of 10
7 cells/ml [
134,
135]. The observed decrease in compressive strength of concrete at higher cell concentrations of bacteria (10
7 cells/ml) can be attributed to the disruption of matrix integrity caused by excessive bacterial activity.
The study conducted by Zaerkabeh et al. [
136] is among the limited number of research efforts that have examined the impact of crack size on the effectiveness of self-healing concrete utilizing
S. Pasteurii. Cracks with widths of 0.5 and 1 mm and depths of 5, 10, and 15 mm were investigated in this study. The study's findings indicate that the flexural strength and energy absorption capability of the specimens dropped as the depth and width of the cracks increased. Notably, the specimen with a crack depth and width of 0.5 exhibited the highest healing efficiency after 7 and 28 days. While the authors acknowledge that impeding the flow of nutrients and oxygen by clogging the pores could potentially lead to a drop in the healing rate among samples tested after 28 days, it is important to note that this study did not take into account the volume of the crack, which could possibly contribute to the observed reduction in healing of cracks with larger dimensions. It is comprehensible that in a suitable environment, the quantity of bio-cementation, like any other chemical reaction, is a function of time. Since the rate of the chemical reaction involving the precipitation of
by
S. Pasteurii for all the studied specimens is constant, the quantity of precipitated crystals will increase over time. Hence, in the case of larger cracks, a more extended period of monitoring the healing process may have yielded more precise conclusions.
Table 2.
Selective literature employing S. Pasteurii for new construction material using mixing method.
Table 2.
Selective literature employing S. Pasteurii for new construction material using mixing method.