Bio-Mitigation of Sulfate Attack and Enhancement of Crack Self-Healing in Sustainable Concrete Using Bacillus megaterium and sphaericus Bacteria

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the authors investigated the effectiveness of Bacillus megaterium (BM) and Bacillus sphaericus (BS) in enhancing the durability of sustainable concrete against sulfate attack through microbially induced calcium carbonate precipitation (MICP).  The paper was interesting and well organized. However, there are some comments to be addressed. It can be published after some revision.

(1) In the section of Introduction, before the last paragraph, the caption of “1.1 Research significance” is suggested to be removed.

(2) Why did the authors select the different concentrations of 2%, 5%, and 10% for sulfate solutions?

(3) In this study, two kinds of bacteria were used in the concretes. Why did the authors use different dosages for the two bacteria? The illustration should be presented.

(4) The units used in this paper should be checked and revised. For example, KN should be changed as kN.

(5) The detailed procedure on pre-cracking preparation should be added.

(6) In the section 3, the authors listed the testing results for every mixture in the full text. However, the same results were shown in the figures. The testing results should not be shown repeatedly.

(7) The quality of Figures 32 and 33 should be improved, in which the labeled text can not be seen clearly.

(8) Based on the testing results, the authors conclude that Bacterial inclusion mitigated sulfate damage through microbially induced calcium carbonate precipitation (MICP), sealing cracks, and bolstering durability. The related mechanism analysis should be added in the full text.

Author Response

Response to Reviewer 1 comments

 

General: The paper was interesting and well organized. However, there are some comments to be addressed. It can be published after some revision.

Response: The authors greatly appreciate the reviewer's critical comments and helpful input on our paper. We have answered each comment in full below, along with the changes we made to the document as a result.

Point 1: In the section of Introduction, before the last paragraph, the caption of “1.1 Research significance” is suggested to be removed.

Response 1: Thank you for your suggestion. We have removed this caption from the revised manuscript's introduction section.

Point 2: Why did the authors select the different concentrations of 2%, 5%, and 10% for sulfate solutions?

Response 2: We are very grateful to Reviewer 1 for their helpful and insightful comments. The 2%, 5%, and 10% Mg₂SO₄ solutions were chosen on purpose to show a range of sulfate exposure levels, from low to very aggressive, which are frequent in real life. The 2% sulfate content, for example, mimics moderate exposure, like soils or groundwater that have a little bit of sulfate contamination. The 5% sulfate concentration shows aggressive conditions, such as places close to industrial sewage, while the 10% sulfate concentration shows excessive exposure, like soils with a lot of sulfate or wastewater streams that are quite concentrated. The study uses 2%, 5%, and 10% sulfate to cover moderate, aggressive, and extreme situations. This lets us fully evaluate bacterial self-healing concrete and get a better idea of how strong it is in real field conditions.

.This range aligns well with common studies in the literature:

https://doi.org/10.3390/ma17194678

https://doi.org/10.3390/ma17143388

https://doi.org/10.3221/IGF-ESIS.71.14

 

Point 3: In this study, two kinds of bacteria were used in the concretes. Why did the authors use different dosages for the two bacteria? The illustration should be presented.

Response 3: The authors appreciate your comment. In this study, two bacterial species from the Bacillus family (Bacillus megaterium and Bacillus sphaericus) were chosen because of their well-documented efficiency in bacterial self-healing of concrete, and they are locally available. They have a high activity and can precipitate calcium carbonate under alkaline conditions. Both Bacillus megaterium (BM) and Bacillus sphaericus (BS) were used at the same contents (1.0% and 2.5%) of cement weight. Depending on the previous studies that showed that these rates achieve the best performance of healing 0 % was chosen as the control mix [7,57]. This aimed to evaluate the effect of content and bacterial type performance under similar conditions, ensuring a fair comparison. Each type of bacteria was assessed for its impact on mechanical properties, sulfate resistance, and self-healing efficiency via MICP.

This range aligns well with common studies in the literature:

       https://doi.org/10.1080/21650373.2022.2101156.

https://doi.org/10.1016/j.cscm.2024.e03188

 

Point 4: The units used in this paper should be checked and revised. For example, KN should be changed as kN.

Response 4: Thank you for your suggestion. The units have been modified in the revised manuscript.

Point 5: The detailed procedure on pre-cracking preparation should be added.

Response 5: The authors value your constructive feedback. In the portion of the document called "Creation of Cracks," we go into great length about the steps involved in preparing for pre-cracking. This includes the cracking technique, how to observe cracks, and the conditions for curing, commencing with the demolding of specimens. Figure 6 also shows this information.

 

Point 6: In the section 3, the authors listed the testing results for every mixture in the full text. However, the same results were shown in the figures. The testing results should not be shown repeatedly.

Response 6: We value the reviewer's feedback. Each result figure shows how different parameters affect and change things, taking into account the percentage of growth or decrease, as indicated in the figures in Section 3. We want to stress, nonetheless, that we did not provide a table with all the test findings for each mixture in the text of the results section. The results are only shown in graphs to show trends and make comparisons. The paragraphs in Section 3 only talk about how to analyze and understand the graphical results, not how to duplicate them in numbers. There is no overlap of raw data between the text and the figures overall.

Point 7: The quality of Figures 32 and 33 should be improved, in which the labeled text can not be seen clearly.

Response 7: We appreciate the reviewer’s observation. The quality of figures has been improved.

Point 8: Based on the testing results, the authors conclude that Bacterial inclusion mitigated sulfate damage through microbially induced calcium carbonate precipitation (MICP), sealing cracks, and bolstering durability. The related mechanism analysis should be added in the full text.

Response 8: We thank the reviewer for this observation. We have added a detailed explanation of the mechanism by which bacterial inclusion mitigates sulfate-induced damage through microbially induced calcium carbonate precipitation (MICP) into the discussion section:

The mechanism of MICP is responsible for the crack healing and durability improvement observed in bacterial concrete. Bacteria from the Bacillus family, like Bacillus megaterium and Bacillus sphaericus, hydrolyze urea in concrete's alkaline environment to produce ammonium and carbonate ions. The carbonate ions react with calcium ions, which are present in the pore solution or released during hydration of cement; calcium carbonate (CaCO₃) is precipitated. CaCO₃ crystals effectively seal holes and microcracks, preventing the development of expansive ettringite and restricting external sulfates. Moreover, the biomineralisation process improves sulfate resistance and matrix density with time.

This range aligns well with common studies in the literature:

https://doi.org/10.3221/IGF-ESIS.71.14

10.33945/SAMI/JCR.2019.4.5

https://doi.org/10.1016/j.conbuildmat.2019.03.079

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This study explored the potential of using two bacteria (BM and BS) to enhance the resistance of concrete to sulfate attack and self-healing cracks, and the topic has application value. However, there are significant deficiencies in the clarity of the description, the depth of the experimental design, and the rigor of the data presentation. Major modifications are needed to improve the scientific quality and readability.

1. The readability of the introduction part needs to be improved, and the logical context is not clear enough to effectively explain the core innovation points and clear research significance of this study (especially for the biological mitigation of high sulfate environment). It is recommended that the author completely reorganize this part and remove the secondary title “1.Research Significance”.
2. The interior of cement-based materials is generally strongly alkaline (pH>12). The manuscript should include the growth and reproduction of two bacteria at different pH ( especially high pH range ) and their effects on mineralization efficiency.
3. The core premise of this study is that microorganisms can survive and function effectively in high sulfate erosion environment. However, the authors failed to provide convincing evidence that the selected bacteria (BM and BS) could survive, grow and reproduce and maintain mineralization activity at high sulfate concentrations. This is the key to determine the scientific value and application feasibility of this study.
4. This paper points out that “The reason for the increase in compressive strength is the precipitation of calcium carbonate, which fills the pores in concrete and causes the size of the pores to decrease”. Please deeply analyze and clarify why BM can produce more calcium carbonate than BS under the same/similar conditions? Is this difference due to the metabolic characteristics of the strain itself, the adaptability to the environment or other factors?
5. Please clarify why the compressive strength of concrete samples decreases with the increase of sulfate solution concentration at the same age?
6. In this paper, it is mentioned that there is a large amount of dolomite in the samples, please explain the source of dolomite and the reasons for its formation or existence.
7. Why is there only 6 sets of data for crack healing efficiency at 120 days?
8. The effective numbers of the data need to be carefully checked and kept consistent. Please correct according to the measurement accuracy and instrument resolution.
9. There are many writing errors in the manuscript that affect readability and professionalism, including but not limited to: subscript error, inconsistent fonts, letter case errorsand other pen errors and grammatical errors. Please be sure to carry out extremely detailed full-text proofreading and language polishing before the revision is submitted.

Author Response

Response to Reviewer 2 comments

 

General: This study explored the potential of using two bacteria (BM and BS) to enhance the resistance of concrete to sulfate attack and self-healing cracks, and the topic has application value. However, there are significant deficiencies in the clarity of the description, the depth of the experimental design, and the rigor of the data presentation. Major modifications are needed to improve the scientific quality and readability.

Response: We sincerely thank Reviewer 2 for the thoughtful and constructive comments provided. We are grateful for your valuable feedback, which has contributed significantly to improving the quality and clarity of our work. We have answered each comment in full below, along with the changes we made to the document as a result.

Point 1: The readability of the introduction part needs to be improved, and the logical context is not clear enough to effectively explain the core innovation points and clear research significance of this study (especially for the biological mitigation of high sulfate environment). It is recommended that the author completely reorganize this part and remove the secondary title “1.Research Significance”.

Response 1: Thank you for your suggestion. We have removed this caption in the revised manuscript and modified and enhanced the introduction section.

Point 2: The interior of cement-based materials is generally strongly alkaline (pH>12). The manuscript should include the growth and reproduction of two bacteria at different pH ( especially high pH range ) and their effects on mineralization efficiency.

Response 2: We are thankful to Reviewer 2 for the valuable and constructive feedback. Research indicates that B. megaterium grows exceptionally well within a pH range of 7.0 to 11.5 and retains urease activity on the verge of cementitious pH, despite slight declines in enzyme performance as pH elevates beyond 11.5; on a similar note, B. sphaericus could withstand high pH, as its spores may germinate and metabolize at levels up to 12.5. Normally, concrete has a pH of between 12 and 13, but during mixing, the exothermic cement hydration causes the pH to rise to 13 and 90°C, which may have an effect on bacterial survival and nutritional availability. This specific aspect was not experimentally addressed in the current study; we recognize its significance and plan to investigate the effect of alkaline pH levels on bacterial viability, urease activity, and calcium carbonate precipitation efficiency for both Bacillus megaterium and Bacillus sphaericus in future work.

This range aligns well with common studies in the literature:

https://doi.org/10.1007/s11756-024-01751-0

https://doi.org/10.1021/acsami.9b21465

10.1016/j.jmrt.2024.01.261

 

 

Point 3: The core premise of this study is that microorganisms can survive and function effectively in high sulfate erosion environment. However, the authors failed to provide convincing evidence that the selected bacteria (BM and BS) could survive, grow and reproduce and maintain mineralization activity at high sulfate concentrations. This is the key to determine the scientific value and application feasibility of this study.

Response 3: We appreciate your inquiry. The survival and growth of bacteria can be inferred through:

1. 1. The study's test results indicate a higher mechanical property increase in concrete samples containing bacteria Bm and BS than those without bacteria at different age stages and under different environmental conditions.
2. The results of the SEM, EDS, and XRD tests, conducted 120 days after the pouring date, showed calcite formation in samples containing BM and BS bacteria exposed to sulfates, indicating continued bacterial activity.
3. The results show that the average healing percentage of M10, which contained 2.5% bacteria BM and was exposed to 5% sulfate, was 90.03%.

This demonstrates bacteria's ability to survive over time, which has a positive impact on the strength of concrete.

 

Point 4: This paper points out that “The reason for the increase in compressive strength is the precipitation of calcium carbonate, which fills the pores in concrete and causes the size of the pores to decrease”. Please deeply analyze and clarify why BM can produce more calcium carbonate than BS under the same/similar conditions? Is this difference due to the metabolic characteristics of the strain itself, the adaptability to the environment or other factors?

Response 4: We are thankful to Reviewer 2 for the valuable and constructive feedback. The observed difference is attributed to several interrelated factors:

1- Higher activity (~690 U/mL vs. lower values in related strains)

2- Greater alkaline adaptability, owing to its protective spore structure, high resistance to alkaline environments, and broader pH tolerance

3- After moisture exposure, Bacillus megaterium spores germinate faster than B. sphaericus, enabling earlier calcium carbonate precipitation.

4- Enhanced nucleation efficiency, facilitated by larger cell surface area and exosporium features that promote CaCO₃ crystallization.

https://doi.org/10.1038/s41598-025-07323-9

https://doi.org/10.4014/jmb.1212.11087

https://doi.org/10.3390/buildings15060943

 https://doi.org/10.1155/2022/6188680

 

Point 5: Please clarify why the compressive strength of concrete samples decreases with the increase of sulfate solution concentration at the same age?

Response 5: We appreciate your inquiry. The reduction in compressive strength with increasing sulfate concentration at the same curing age is a well-documented phenomenon, primarily caused by sulfate-induced chemical degradation of the cementitious matrix. We have clarified this in the revised manuscript with the following explanation:

Sulfate ions (SO₄²⁻) penetrate the concrete and react with hydration products. Ettringite and gypsum are also formed, which are expansive by-products. These products cause internal stress, microcracking, and matrix cohesion loss. Higher sulfate concentrations accelerate these reactions and damage. These actions result in greater deterioration of the cement matrix and reduction in compressive strength over time.

This range aligns well with common studies in the literature:

https://doi.org/10.3390/buildings14072187

https://doi.org/10.3390/ma13143179

https://doi.org/10.3221/IGF-ESIS.71.14

 

Point 6: In this paper, it is mentioned that there is a large amount of dolomite in the samples, please explain the source of dolomite and the reasons for its formation or existence.

Response 6: We are thankful to Reviewer 2 for the valuable and constructive feedback. The large amount of dolomite in the XRD results is generated mainly from the natural mineral composition of the aggregates used in the production of concrete mixes. The coarse and fine aggregates used were sourced from carbonate-rich formations, which naturally contain dolomite as a dominant mineral. Also, due to raw material composition, small amounts of dolomite may also be contained in the cement.

The strong XRD peaks are attributed to natural dolomite's high crystallinity and purity, which generates intense reflections even at moderate content. Moreover, the relatively low occurrence of amorphous phases in the specimens improves the visibility of crystalline minerals such as dolomite in XRD patterns. These observations confirm that dolomite was present as a primary mineral in raw materials, not a product of secondary formation during hydration or microbial activity.

 

Point 7: Why is there only 6 sets of data for crack healing efficiency at 120 days?

Response 7: The authors appreciate your inquiry. They submitted only six data sets on crack healing efficiency at 120 days because they purposefully selected representative mixes with the most important variables in our study.

The six mixes included:

Control mix (M0) for baseline comparison, optimal bacterial mixes (i.e., those that performed the best), and mixes subjected to different sulfate exposure levels to observe healing under aggressive conditions. By showing these six mixed, we were able to provide a balanced representation of critical variables (bacterial type, bacterial content, and environmental exposure) while avoiding duplication. We really wanted to prioritize and show these representative cases to highlight long-term healing trends while maintaining scientific integrity and realism.

 

Point 8: The effective numbers of the data need to be carefully checked and kept consistent. Please correct according to the measurement accuracy and instrument resolution.

Response 8: We appreciate your inquiry. The data numbers have been checked, kept, and corrected consistently according to measurement accuracy and instrument resolution.

Point 9: There are many writing errors in the manuscript that affect readability and professionalism, including but not limited to: subscript error, inconsistent fonts, letter case errorsand other pen errors and grammatical errors. Please be sure to carry out extremely detailed full-text proofreading and language polishing before the revision is submitted.

Response 9: Thank you for your valuable comment. We have conducted comprehensive and detailed proofreading of the entire text. We believe these revisions have significantly improved the quality and clarity of the manuscript.

 

 

 

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Ibrahim et al. presents a well-structured experimental investigation into bacterial self-healing concrete, addressing the critical issues of sulfate attack and crack healing using Bacillus megaterium (BM) and Bacillus sphaericus (BS). The manuscript is well written but requires some revisions as follows:

 

1. The manuscript lacks details on bacterial spore viabilityduring concrete mixing/curing. High pH (12–13) and temperature (90°C during hydration) could impair bacterial activity. Is it possible to add a section on bacterial survivability under simulated concrete conditions?

 

2.The full name of SEM etc. should be given in abstract.

 

3. Line 46-47, Line 211, Line 220: Minor mistakes

 

4. The Introduction is too long. Please emphasize the important point and delete the unnecessary content.

 

5. How did the authors ensure the phase in SEM image without EDX?

 

6. Line 740-741: Based on what criteria did the authors choose samples for XRD test.

 

7.Mix Design (Table 6): "M0–M23" labels are confusing (e.g., M0 vs. M4 both listed as 0% bacteria). Clarify how silica fume/fly ash were incorporated (replacement vs. addition).

 

8. Why does BM outperform BS? Discuss biochemical differences(e.g., urease activity, CaCO₃ yield)? EDS data (Fig. 34) show elevated Ca/Si ratios under sulfate exposure. Elaborate on how MICP competes with ettringite formation. I suggest that the authors should expand the "Results and Discussion" to link microstructural findings (SEM/XRD) to performance mechanisms.

 

9. Figures & Tables:

Label axes clearly (e.g., Fig. 8: "Bacteria content" should specify "% by cement weight").

Include unit for crack widths (Table 7: "μm").

Unify terminology (e.g., "Bacillus sphaericus" vs. "Bacillus subtilis" in SEM section, p. 31).

 

This manuscript presents valuable contributions to self-healing concrete technology. With revisions addressing bacterial viability, statistical rigor, and mechanistic clarity, it will be suitable for publication in Infrastructures. I recommend major revision and reevaluation after amendments.

 

Author Response

Response to Reviewer 3 comments

 

General: Ibrahim et al. presents a well-structured experimental investigation into bacterial self-healing concrete, addressing the critical issues of sulfate attack and crack healing using Bacillus megaterium (BM) and Bacillus sphaericus (BS). The manuscript is well written but requires some revisions as follows

Response: We sincerely thank Reviewer 3 for the thoughtful and constructive comments provided. We are grateful for your valuable feedback, which has contributed significantly to improving the quality and clarity of our work. We have answered each comment in full below, along with the changes we made to the document as a result.

Point 1: The manuscript lacks details on bacterial spore viability during concrete mixing/curing. High pH (12–13) and temperature (90°C during hydration) could impair bacterial activity. Is it possible to add a section on bacterial survivability under simulated concrete conditions?

Response 1:

We are thankful to Reviewer 3 for the valuable and constructive feedback. Research indicates that B. megaterium grows exceptionally well within a pH range of 7.0 to 11.5 and retains urease activity on the verge of cementitious pH, despite slight declines in enzyme performance as pH elevates beyond 11.5; on a similar note, B. sphaericus could withstand high pH, as its spores may germinate and metabolize at levels up to 12.5. Normally, concrete has a pH of between 12 and 13, but during mixing, the exothermic cement hydration causes the pH to rise to 13 and 90°C, which may have an effect on bacterial survival and nutritional availability. This specific aspect was not experimentally addressed in the current study; we recognize its significance and plan to investigate the effect of alkaline pH levels on bacterial viability, urease activity, and calcium carbonate precipitation efficiency for both Bacillus megaterium and Bacillus sphaericus in future work.

This range aligns well with common studies in the literature:

https://doi.org/10.1007/s11756-024-01751-0

https://doi.org/10.1021/acsami.9b21465

10.1016/j.jmrt.2024.01.261

 

Point 2:  The full name of SEM etc. should be given in abstract.

Response 2: Thank you for your helpful suggestion. We have updated the abstract to include the full names of the abbreviations.

 

Point 3:  Line 46-47, Line 211, Line 220: Minor mistakes.

Response 3: Thank you for your helpful suggestion. We have conducted a comprehensive and detailed proofreading of the entire text.

 

 

Point 4:  The Introduction is too long. Please emphasize the important point and delete the unnecessary content.

Response 4: Thank you for your helpful observation. We have carefully revised the Introduction to highlight the key objectives and significance of the study, while removing repetitive or non-essential content.

 

Point 5:  How did the authors ensure the phase in SEM image without EDX?

Response 5: Thank you for the thoughtful question. EDS was conducted in the revised manuscript. EDS analysis provides valuable insights into self-healing concrete mixes' chemical composition and microstructural characteristics, particularly when subjected to sulfate, which simulates extreme environmental conditions. Figure 34 displays the EDS test findings.

 

Point 6:  Line 740-741: Based on what criteria did the authors choose samples for XRD test.

Response 6: We appreciate you asking about the rationale for selecting samples for XRD testing. The samples we selected for X-ray Diffraction (XRD) analysis were selected based on their representative mechanical performance and microstructural features we observed previously. Samples were also chosen for XRD based on the following:

Control mix (M0), which allowed for a baseline comparison.

Samples with distinct bacterial content and curing conditions (e.g., M2, M3, M8, M10, M11, M16, M17, M20, and M21) to reflect the potential influence of different treatments on mineralogical composition. Samples that show considerable differences in compressive and flexural strength, especially those that indicate meaningful capacity for self-healing or resistance to sulfate exposure. Ultimately, the intent was to analyze a possible set of behaviors from natural to maximum enhancements of samples to identify performance trends with mineral phase advances (e.g., calcite, changes in portlandite or ettringite content). Overall, this characterization approach aided in gaining meaningful and comparable XRD results.

 

Point 7:  Mix Design (Table 6): "M0–M23" labels are confusing (e.g., M0 vs. M4 both listed as 0% bacteria). Clarify how silica fume/fly ash were incorporated (replacement vs. addition).

Response 7: We sincerely appreciate the reviewer's helpful comment. Concrete samples were prepared containing silica fume/fly ash, with a different type and content of bacteria, and cured in freshwater and varying concentrations of magnesium sulfate (0%, 2%, 5%, and 10%). Accordingly, the mixes shown in Table 6 were identified. For example, Mix M0 contains no bacteria, contains silica fume, and is cured in freshwater. Mix M4 contains no bacteria, contains silica fume, and is cured in 2% magnesium sulfate solution. Mix M8 contains bacteria and silica fumes and is cured in a 5% magnesium sulfate solution. Mix M12 contains no bacteria, contains silica fume, and is cured in 10% magnesium sulfate solution. We clarify that silica fume and fly ash were included in this study as additions to the mixture, not as replacements for cement. We intended to include these materials to increase the pozzolanic reaction, improve microstructural densification, and promote bacterial activity by refining the pore structure.

Point 8:  Why does BM outperform BS? Discuss biochemical differences(e.g., urease activity, CaCO₃ yield)? EDS data (Fig. 34) show elevated Ca/Si ratios under sulfate exposure. Elaborate on how MICP competes with ettringite formation. I suggest that the authors should expand the "Results and Discussion" to link microstructural findings (SEM/XRD) to performance mechanisms.

Response 8: The authors sincerely appreciate the reviewer's insightful feedback. The noted variation is ascribed to multiple interconnected elements:

1- Higher activity (~690 U/mL vs. lower values in related strains)

2- Greater alkaline adaptability, owing to its protective spore structure, high resistance to alkaline environments, and broader pH tolerance

3- After moisture exposure, Bacillus megaterium spores germinate faster than B. sphaericus, enabling earlier calcium carbonate precipitation.

4- Enhanced nucleation efficiency, facilitated by larger cell surface area and exosporium features that promote CaCO₃ crystallization.

This range aligns well with common studies in the literature:

https://doi.org/10.1038/s41598-025-07323-9

https://doi.org/10.4014/jmb.1212.11087

https://doi.org/10.3390/buildings15060943

 https://doi.org/10.1155/2022/6188680

 

As far as that is concerned, the calcium-to-silicon (Ca/Si) ratio is a convenient indicator of the quality and kind of resulting hydration products formed, especially the all-important calcium silicate hydrate (C-S-H) gel responsible for strength development and durability. Also, Ca/Si ratio values that are too high are said to indicate the existence of excess calcium phases, which would be unfavorable to mechanical strength and durability. In situations where the concrete is exposed to contact with solutions of sulfates, such as in mixes indicated by sulfate additions, the foreign sulfates would react with calcium hydroxide and other hydrating phases to produce expansive forms such as ettringite and gypsum. The reaction can add more Ca to the matrix or cause low-density, poor structures, affecting mechanical performance and durability.

This range aligns well with common studies in the literature:

https://doi.org/10.1016/j.dib.2019.104394.

https://doi.org/10.1088/1757-899X/1116/1/012168.

 

We have added a detailed explanation of the mechanism by which bacterial inclusion mitigates sulfate-induced damage through microbially induced calcium carbonate precipitation (MICP) into the discussion section:

The mechanism of MICP is responsible for the crack healing and durability improvement observed in bacterial concrete. Bacteria from the Bacillus family, like Bacillus megaterium and Bacillus sphaericus, hydrolyze urea in concrete's alkaline environment to produce ammonium and carbonate ions. The carbonate ions react with calcium ions, which are present in the pore solution or released during hydration of cement; calcium carbonate (CaCO₃) is precipitated. CaCO₃ crystals effectively seal holes and microcracks, preventing the development of expansive ettringite and restricting external sulfates. Moreover, the biomineralisation process improves sulfate resistance and matrix density with time.

This range aligns well with common studies in the literature:

https://doi.org/10.3221/IGF-ESIS.71.14

10.33945/SAMI/JCR.2019.4.5

https://doi.org/10.1016/j.conbuildmat.2019.03.079

 

From the analysis of the results from the tests on compressive and flexural strength, it can be concluded that there is increased strength within concrete samples containing bacteria. The sample that demonstrated the highest strength contained 2.5% Bacillus megaterium (M2). The Scanning Electron Microscopy (SEM) showed that bacteria were evident in the mixes that were exposed to sulfate, as well as bacterially precipitated calcite and ettringite, which filled pores and cracks, even under sulfate attack. Energy Dispersive X-ray Spectroscopy (EDS) gave examples of the chemical composition and microstructure of the self-healing concrete in sulfate environments. The Ca/Si ratio indicates both mechanical strength and durability, and the lowest Ca/Si ratio, 1.94, was recorded for the mix containing 2.5% Bacillus megaterium (M2) and indicates high pozzolanic and microorganism activity. The mineral identification and crystalline phase of the self-healing concrete were determined through X-ray Diffraction (XRD) analysis, and we could see a strong effect of microbial-induced calcium carbonate precipitation (MICP) at 2.5% Bacillus megaterium (M2). This indicates complete agreement in the results of the compression, flexural, and microstructural tests.

As shown in figures (8 -19) and (32 - 35).

 

Point 9:  Figures & Tables:

Label axes clearly (e.g., Fig. 8: "Bacteria content" should specify "% by cement weight").

Include unit for crack widths (Table 7: "μm").

Unify terminology (e.g., "Bacillus sphaericus" vs. "Bacillus subtilis" in SEM section, p. 31).

This manuscript presents valuable contributions to self-healing concrete technology. With revisions addressing bacterial viability, statistical rigor, and mechanistic clarity, it will be suitable for publication in Infrastructures. I recommend major revision and reevaluation after amendments.

 

Response 9: Thank you for your thoughtful and constructive feedback. We have carefully reviewed and updated figures and table labels to ensure clarity and consistency. The unit for crack widths in Table 7 has been added. We have also standardized bacterial nomenclature. All suggestions from the review have been addressed and incorporated into the revised manuscript.

 

 

 

 

 

 

 

 

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The paper can be accepted for publication since all the comments have been addressed.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been sufficiently improved to warrant publication in Infrastructures

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have revised the manuscript accordingly.