Use of Sulfur Waste in the Production of Metakaolin-Based Geopolymers

Round 1

Reviewer 1 Report

I recommend the work for publication. 

Perhaps, to clarify the distribution of elements on the surface it is worth to make EDX analysis.

Supplement the list of references with more relevant articles

Author Response

1. I recommend the work for publication.

• Thank you!

2. Perhaps, to clarify the distribution of elements on the surface it is worth to make EDX analysis.

• Currently, the EDX analysis is not available, so the ICP analysis was conducted on geopolymers as reported in section 3.2.

3. Supplement the list of references with more relevant articles.

• The list of references was modified and updated with more relevant articles [Please see references: 9, 10,11, 13, 22, and 28].

Reviewer 2 Report

The paper presents a preliminary study on the incorporation of sulfur waste into geopolymer matrices. Geopolymers were synthesized using sulfur in varying proportions in the alkaline activator and mixed with metakaolin. The effects on compressive strength, microstructure, crystalline phases, and other properties were investigated. The study addresses an interesting aspect of using sulfur waste in geopolymer production. The findings regarding its impact on compressive strength and microstructure are noteworthy. However, there are some aspects that need clarification and improvement before publication.

1. Clarity and Organization:

The abstract provides a concise overview of the study but could be improved by clarifying the objectives and significance of the research right at the beginning.

In the conclusion, reiterate the main findings and their implications. The current conclusion is a bit repetitive and could be streamlined.

2. Introduction and Objectives:

The introduction should provide a clearer context for the study. Why is incorporating sulfur waste into geopolymers important? What gaps in knowledge does this study aim to address?

State the specific objectives of the study to guide readers through the research's purpose.

3. Methodology:

Provide more detailed information about the synthesis process. Include the specific steps involved in dissolving sulfur in the alkaline activator and the mixing process with metakaolin. This will help other researchers replicate the study.

Mention the testing methods used for analyzing mechanical properties and microstructure. Provide references for these methods.

4. Results and Discussion:

Clearly present the results, especially those related to the effects of sulfur on compressive strength and microstructure. Consider using tables and figures for visual clarity.

When discussing results, explain the mechanisms behind the observed effects. How does sulfur interact with aluminum ions? What causes the increase in compressive strength up to 5% wt sulfur and the subsequent decrease?

Expand on the significance of the new crystalline phase (Al2·H10·O17·S3) identified in XRD analysis. How does this phase relate to the observed changes in properties?

Elaborate on how the denser and more compact microstructure after adding sulfur translates to the observed mechanical improvements.

Provide a more detailed explanation of the interference bands observed in FTIR spectra. How do these bands relate to the interaction between sulfur, sulfate, silica, and aluminum?

Clarify the implications of the TG analysis results. How does the absence of unreacted sulfur in the microstructure impact the durability and long-term performance of the geopolymer?

5. Conclusion:

Clearly summarize the key findings and their implications.

Consider discussing potential applications of this research in real-world environmental and engineering solutions.

 

Overall Recommendation:

The paper presents valuable insights into the incorporation of sulfur waste into geopolymer matrices. To enhance its clarity and impact, I recommend revising the introduction, methodology, and discussion sections for better organization and detail. Providing additional explanations for observed effects and mechanisms will enhance the contribution of this study to the field. Addressing these points will greatly improve the overall quality of the paper.

Author Response

matrices. Geopolymers were synthesized using sulfur in varying proportions in the alkaline activator and mixed with metakaolin. The effects on compressive strength, microstructure, crystalline phases, and other properties were investigated. The study addresses an interesting aspect of using sulfur waste in geopolymer production. The findings regarding its impact on compressive strength and microstructure are noteworthy. However, there are some aspects that need clarification and improvement before publication.

• Thanks for your valuable comments.

1. Clarity and Organization: The abstract provides a concise overview of the study but could be improved by clarifying the objectives and significance of the research right at the beginning. In the conclusion, reiterate the main findings and their implications. The current conclusion is a bit repetitive and could be streamlined.

• The novelty of this work is explained in the abstract [Lines 14-16].
• The conclusion section was rewritten.

2. Introduction and Objectives: The introduction should provide a clearer context for the study. Why is incorporating sulfur waste into geopolymers important? What gaps in knowledge does this study aim to address? State the specific objectives of the study to guide readers through the research's purpose.

• The specific objectives and the significance of this study are added [lines 90-95].

3. Methodology: Provide more detailed information about the synthesis process. Include the specific steps involved in dissolving sulfur in the alkaline activator and the mixing process with metakaolin. This will help other researchers replicate the study. Mention the testing methods used for analyzing mechanical properties and microstructure. Provide references for these methods.

• The Methodology was reviewed and organized [Fig. 1: experimental procedure step-by step, Table 1.: composition, and section 2.2: preparation]. 
• Testing methods are mentioned.

4. Results and Discussion:

Clearly present the results, especially those related to the effects of sulfur on compressive strength and microstructure. Consider using tables and figures for visual clarity.

When discussing results, explain the mechanisms behind the observed effects. How does sulfur interact with aluminum ions? What causes the increase in compressive strength up to 5% wt sulfur and the subsequent decrease?

Expand on the significance of the new crystalline phase (Al2·H10·O17·S3) identified in XRD analysis. How does this phase relate to the observed changes in properties?

Elaborate on how the denser and more compact microstructure after adding sulfur translates to the observed mechanical improvements.

Provide a more detailed explanation of the interference bands observed in FTIR spectra. How do these bands relate to the interaction between sulfur, sulfate, silica, and aluminum?

Clarify the implications of the TG analysis results. How does the absence of unreacted sulfur in the microstructure impact the durability and long-term performance of the geopolymer?

• The discussion was reviewed and improved as much as possible. ICP analysis was carried out and discussed  in section 3.2. the mechanism and the effect of sulfur on the microstructure and the correlation with mechanical properties was reported in [ Fig. 3]. 

5. Conclusion:

Clearly summarize the key findings and their implications.

Consider discussing potential applications of this research in real-world environmental and engineering solutions.

• The conclusions were reviewed and modified according to reviewer's comments.

Overall Recommendation: The paper presents valuable insights into the incorporation of sulfur waste into geopolymer matrices. To enhance its clarity and impact, I recommend revising the introduction, methodology, and discussion sections for better organization and detail. Providing additional explanations for observed effects and mechanisms will enhance the contribution of this study to the field. Addressing these points will greatly improve the overall quality of the paper.

• I appreciate your reviews and comments

Reviewer 3 Report

The authors in the research paper “Use of sulfur waste in the production of metakaolin-based geopolymer” present a straightforward and interesting experimental evaluation of encapsulation of Sulphur waste into geopolymer matrix. The reviewer commends the authors for the novelty of the work carried out. The paper is of high importance to readers working in the domain of geopolymers and can be accepted for publication given grammar and language of the manuscript are thoroughly corrected. There are however some suggestions and recommendations the reviewer likes the authors to consider. Below are the comments.

1.      The authors are advised to run a thorough English and grammar check on the manuscript as some sentences are hard to follow. Lines 38-40 and 50-53 need to be rephrased for better understanding.

2.      The authors chose to dissolve sulfur in alkaline activator instead of using it as filler citing mechanical failure due sulfur being exposed to water as a reason. This is crux of the study, and the authors are advised to elaborate this in detail in the introduction section citing appropriate references.

3.      Why did authors choose to move ahead with a 6M NaOH solution (Line 101) when they were clear about dissolving sulfur in it. This would reduce the alkalinity of solution further. Also, from Table 2, given the quantities of NaOH and water used, it doesn’t seem like the molarity of the solution is 6. The authors are advised to kindly recheck it and address the issue adequately.

4.      Lines 180-181 need rephrasing.

5.      Kindly mention the specific gravity and particle size of metakaolin used in the study.

6.      How did the authors check if the reaction was geopolymerization and not mere alkali activation given that the mix was cured at 40C. This needs to be included in the manuscript.

English of the manuscript should be improved thoroughly. Few lines are hard to comprehend

Author Response

The authors in the research paper “Use of sulfur waste in the production of metakaolin-based geopolymer” present a straightforward and interesting experimental evaluation of encapsulation of Sulphur waste into geopolymer matrix. The reviewer commends the authors for the novelty of the work carried out. The paper is of high importance to readers working in the domain of geopolymers and can be accepted for publication given grammar and language of the manuscript are thoroughly corrected. There are however some suggestions and recommendations the reviewer likes the authors to consider. Below are the comments.

• We appreciate the encouraging comments. Thank you!

 

1. The authors are advised to run a thorough English and grammar check on the manuscript as some sentences are hard to follow. Lines 38-40 and 50-53 need to be rephrased for better understanding.

• Lines 38-40 and 50-53 have been rephrased, and the English has been carefully reviewed by a MDPI English Editining

2. The authors chose to dissolve sulfur in alkaline activator instead of using it as filler citing mechanical failure due sulfur being exposed to water as a reason. This is crux of the study, and the authors are advised to elaborate this in detail in the introduction section citing appropriate references.

• The novel experimental procedure was added to the abstract (lines 14-16), also a related section was added to the introduction [lines 90-95].

3. Why did authors choose to move ahead with a 6M NaOH solution (Line 101) when they were clear about dissolving sulfur in it. This would reduce the alkalinity of solution further. Also, from Table 2, given the quantities of NaOH and water used, it doesn’t seem like the molarity of the solution is 6. The authors are advised to kindly recheck it and address the issue adequately.

• Thanks for the valuable comment. Indeed we found an error in the molar ratios and it was corrected [line 118].
• The alkalinity of the geopolymer solution was measured experimentally, and it was slightly reduced by dissolving sulfur.

4. Lines 180-181 need rephrasing.

• Done

5. Kindly mention the specific gravity and particle size of metakaolin used in the study.

• A reference to the kaolinite (metakaolin) was added.

6. How did the authors check if the reaction was geopolymerization and not mere alkali activation given that the mix was cured at 40C. This needs to be included in the manuscript.

This experimental procedure is based on our previous studies, so the related references were added.

Reviewer 4 Report

This preliminary study introduces the incorporation and chemical stabilization of sulfur waste into a geopolymers matrix and explores the concept of material production for further environmental and engineering solutions. The paper has research significance and clear logic, but some problems need to be explained and supplemented.

1. Some typos and formatting errors need to be corrected. Such as, in Line 157, Table 1 should be Table 2; there is a problem with the cross-page in Table 2; the line numbers in Table 3 are unclear; and there is no content in Line 207~212.

2. The Figure quality needs to be improved, especially Figure 1, Figure 2, and Figure 8.

3. The author has a detailed description of most of the experimental procedures. However, Why the machine-head speed select 1mm/min?

4. Why the sample dimensions were height=40 mm, width=20 mm, and length=20 mm?

5. In Figure 2, there is only one set of data for compressive strength. The data on compressive strength at different sulfur contents need to be added.

6. The author stated that ‘adding sulfur from 0 wt.% to 5 wt.%, compared with metakaolin, was found to lead to an increase in the compressive strength of the geopolymer from 22.5 MPa to 29.9MPa. When sulfur is between 5 wt.% and 15 wt.%, a decrease in compressive strength was observed to 15.7 MPa, which can be explained by defects and voids in the geopolymer microstructure through the solubility of excess sulfur and thus the formation of defects and voids in the geopolymer microstructure.’ I thought it was an interesting work. However, is it possible to add a ‘discussion’ to clear the competing mechanisms of these two effects?

7. To improve the significance of sulfur waste in the geopolymers matrix, it is suggested to add a part about the impact of compressive strength of other waster materials into the geopolymers matrix in the introduction.

8. The following literature maybe useful for improving your quality:

① Shear mechanical responses of sandstone exposed to high temperature under constant normal stiffness boundary conditions. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2021, 7:35.

② Numerical investigation on the fatigue failure characteristics of water-bearing sandstone under cyclic loading. Journal of Mountain Science, 2021, 18(12): 3348-3365.

This preliminary study introduces the incorporation and chemical stabilization of sulfur waste into a geopolymers matrix and explores the concept of material production for further environmental and engineering solutions. The paper has research significance and clear logic, but some problems need to be explained and supplemented.

1. Some typos and formatting errors need to be corrected. Such as, in Line 157, Table 1 should be Table 2; there is a problem with the cross-page in Table 2; the line numbers in Table 3 are unclear; and there is no content in Line 207~212.

2. The Figure quality needs to be improved, especially Figure 1, Figure 2, and Figure 8.

3. The author has a detailed description of most of the experimental procedures. However, Why the machine-head speed select 1mm/min?

4. Why the sample dimensions were height=40 mm, width=20 mm, and length=20 mm?

5. In Figure 2, there is only one set of data for compressive strength. The data on compressive strength at different sulfur contents need to be added.

6. The author stated that ‘adding sulfur from 0 wt.% to 5 wt.%, compared with metakaolin, was found to lead to an increase in the compressive strength of the geopolymer from 22.5 MPa to 29.9MPa. When sulfur is between 5 wt.% and 15 wt.%, a decrease in compressive strength was observed to 15.7 MPa, which can be explained by defects and voids in the geopolymer microstructure through the solubility of excess sulfur and thus the formation of defects and voids in the geopolymer microstructure.’ I thought it was an interesting work. However, is it possible to add a ‘discussion’ to clear the competing mechanisms of these two effects?

7. To improve the significance of sulfur waste in the geopolymers matrix, it is suggested to add a part about the impact of compressive strength of other waster materials into the geopolymers matrix in the introduction.

8. The following literature maybe useful for improving your quality:

① Shear mechanical responses of sandstone exposed to high temperature under constant normal stiffness boundary conditions. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2021, 7:35.

② Numerical investigation on the fatigue failure characteristics of water-bearing sandstone under cyclic loading. Journal of Mountain Science, 2021, 18(12): 3348-3365.

Author Response

1. This preliminary study introduces the incorporation and chemical stabilization of sulfur waste into a geopolymers matrix and explores the concept of material production for further environmental and engineering solutions. The paper has research significance and clear logic, but some problems need to be explained and supplemented. Some typos and formatting errors need to be corrected. Such as, in Line 157, Table 1 should be Table 2; there is a problem with the cross-page in Table 2; the line numbers in Table 3 are unclear; and there is no content in Line 207~212.

• English has been carefully reviewed by a MDPI English Editing, the tables were reviewed and corrected accordingly.

2. The Figure quality needs to be improved, especially Figure 1, Figure 2, and Figure 8.

• Figures 1, 2, and 8 were modified and corrected accordingly. 

3. The author has a detailed description of most of the experimental procedures. However, Why the machine-head speed select 1mm/min?

• This speed is based on the ‘impact time’ before breaking the specimens, which should be between 30s to 60s.

1. Why the sample dimensions were height=40 mm, width=20 mm, and length=20 mm?

• These dimensions are based on ASTM D695, ISO 604, where the height of cubic samples should be twice as width or length.

2. In Figure 2, there is only one set of data for compressive strength. The data on compressive strength at different sulfur contents need to be added.

• The 5 sets were added to Fig. 2.

3. The author stated that ‘adding sulfur from 0 wt.% to 5 wt.%, compared with metakaolin, was found to lead to an increase in the compressive strength of the geopolymer from 22.5 MPa to 29.9MPa. When sulfur is between 5 wt.% and 15 wt.%, a decrease in compressive strength was observed to 15.7 MPa, which can be explained by defects and voids in the geopolymer microstructure through the solubility of excess sulfur and thus the formation of defects and voids in the geopolymer microstructure.’ I thought it was an interesting work. However, is it possible to add a ‘discussion’ to clear the competing mechanisms of these two effects?

• Section 3.2 was added to explain the effect of sulfur contents on the microstructural and mechanical properties.

4. To improve the significance of sulfur waste in the geopolymers matrix, it is suggested to add a part about the impact of compressive strength of other waster materials into the geopolymers matrix in the introduction.
5. This preliminary study aims to explore a method for incorporation of sulfur in the geopolymer matrix. Unlike other waste, sulfur can not be used as a filler because of its high solubility in water. We hope in our next paper to explore the optimal sulfur content and to compare it with other wastes.
6. The following literature maybe useful for improving your quality:

① Shear mechanical responses of sandstone exposed to high temperature under constant normal stiffness boundary conditions. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2021, 7:35.

② Numerical investigation on the fatigue failure characteristics of water-bearing sandstone under cyclic loading. Journal of Mountain Science, 2021, 18(12): 3348-3365.

• The references were added.

Round 2

Reviewer 2 Report

The authors have addressed my comments.