The Effects of Xanthan Gum and Guar Gum on the Mechanical Properties of Sand–Bentonite Mixtures
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
The research is very interesting and provides an innovative idea for the application of bio based materials in civil engineering. The overall structure of the manuscript is complete. However, some areas need to be improved.
Figure 3 needs to be modified to increase readability.
The introduction needs to be reorganized and the research innovation needs to be clarified.
The conclusion part needs to highlight the key points and be more logical and organized.
Author Response
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Comments 1: [The research is very interesting and provides an innovative idea for the application of bio-based materials in civil engineering. The overall structure of the manuscript is complete. However, some areas need to be improved. Response 1: [Thank you for your positive and constructive feedback. We appreciate your recognition of the innovation and structure of the study. We have addressed all specific comments below to enhance the quality and clarity of the manuscript.] Comments 2: Figure 3 needs to be modified to increase readability. Response 2: We agree with this comment. Therefore, we have revised Figure 5 by increasing the font size, adjusting axis labels, and repositioning overlapping elements. This has improved its clarity and visual presentation. [Page 9, Figure 5] Comments 3: The introduction needs to be reorganized and the research innovation needs to be clarified. Response 3: We thank the reviewer for this suggestion. Accordingly, we have added two new paragraphs to the Introduction section that (1) highlight the existing research gap in long-term, concentration-based biopolymer applications to sand-bentonite mixtures, and (2) explicitly state the innovation of our dual experimental and FTIR-based approach. These paragraphs improve the structure and clearly position the novelty of this study. [Page 2 Lines 66-71, Page 3, Lines 91–98] Comments 4: The conclusion part needs to highlight the key points and be more logical and organized. Response 4: Thank you for this valuable and constructive comment. In response, we have revised the Conclusion section to improve clarity, structure, and emphasis on the study’s key contributions. Without changing the original findings or meaning, we strategically added two short paragraphs to: Emphasize the comparative performance of xanthan gum versus guar gum in long-term applications, and Reinforce the significance of combining mechanical evaluation with FTIR analysis to offer a more comprehensive understanding of the stabilization mechanisms. These additions help transition the conclusion from a summary of results to a clearer interpretation of the research's practical relevance. The new content has been highlighted in yellow in the revised manuscript. [Page 20, Lines 562–565, Lines 568-570]
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Reviewer 2 Report
Comments and Suggestions for Authors
Line 15 (abstract): Are you sure that the 75% sand in your mixtures is represented by SiO2 with minimal impurities of iron, Al and what so else?. Also, are thy totally quartz or there are some amorphous silica phases?.
Line 57 (Introduction): Some references are given in full, i.e. are not numbered according to style of MDPI journals.
Page 6: Please remove the green background from Fig. 2.
Line 229-237: Are you sure that the resultant FTIR spectra have well preserved CO and OH vibrations although you heated the sample to 105 °C before measurements?.
Page 7: Please unify the number of digits (two only) in the MDD values in Table 2 and else.
In Figure 3, please resolve the overlap of values and notation of the Y-axis. Same exists in Fig. 5 (page 10) and Fig. 6 (page 11).
Page 9, Fig. 4: Insert a horizontal line at the base of columns in the histogram graphs, and also remove the light green background. Also, for Fig. 7 in page 12.
Page 11, Fig. 6: Keys of legend should not be outside frame.
Please combine two paragraphs in page 14.
Line 411-414: The O-H stretching vibrations in the FTIR spectra are so weak and doubtful. Probably heating of samples before measurement is the main cause. Please try to solve this problem.
Page 16: Different font types and size exist, so please unify.
Page 18: Discussion is not bad but it needs more elaboration and enhancement.
The reference list in page 19-20 needs careful re-editing considering punctuation and some other issues. For example, journal names are given in full for some, e.g. [1] and [5] but it is abbreviated in others, e.g. [2].
Comments for author File: 
Comments.pdf
Author Response
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Comment 1: Line 15 (abstract): Are you sure that the 75% sand in your mixtures is represented by SiO2 with minimal impurities of iron, Al and what so else?. Also, are they totally quartz or there are some amorphous silica phases? Response 1: Thank you for pointing this out. We agree that mineralogical characterization is important to accurately describe the material used. Therefore, we have conducted X-ray diffraction (XRD) analyses on both the sand and bentonite used in this study. The XRD results confirm that: The sand is primarily composed of quartz (Q) with additional peaks corresponding to carbonates (C), attapulgite (A), feldspar (Fr), and kaolinite (K) (see Figure 1, Page 4). This indicates a silicate-rich sand with minor clay and carbonate content. The bentonite sample is dominated by montmorillonite (M) and contains illite (I), feldspar (F), quartz (Q), and minor carbonates (C) (see Figure 2, Page 4).This information has been added to the Materials section, under the subheadings “Sand” and “Na-bentonite clay,” to provide a clear mineralogical profile of the components used in the mixtures. [Updated on Page 4, Section 2.1.1 and 2.1.2] Comment 2: Line 57 (Introduction): Some references are given in full, i.e. are not numbered according to style of MDPI journals. Response 2: Thank you for your observation. We have corrected all in-text references to follow the numerical citation format in accordance with MDPI guidelines. [Page 2, Line 57 and throughout the manuscript] Comment 3: Page 6: Please remove the green background from Fig. 2. Response 3: Thank you. We have removed the green background from Figure 2 to enhance clarity and maintain visual consistency. [Fig. 4 Page 7] Comment 4: Line 229-237: Are you sure that the resultant FTIR spectra have well preserved CO and OH vibrations although you heated the sample to 105 °C before measurements? Response 4: Thank you for pointing this out. We agree with this concern. Therefore, we re-analyzed three representative samples (0.5%, 1%, and 2% xanthan gum) using FTIR after air-drying at room temperature, instead of oven-drying at 105 °C. The new spectra (Figure 16 Page 18) show sharper and more intense O–H and C=O bands, confirming that these functional groups are better preserved without thermal exposure. This update has been added to the FTIR analysis section [Page 17, Lines 428–438], and the comparison is discussed to highlight the influence of drying method on functional group visibility. Comment 5: Page 7: Please unify the number of digits (two only) in the MDD values in Table 2 and else. Response 5: Thank you. All values in Table 2 and elsewhere have been standardized to show only two decimal digits. [Page 8, Table 2] Comment 6: In Figure 3, please resolve the overlap of values and notation of the Y-axis. Same exists in Fig. 5 (page 10) and Fig. 6 (page 11). Response 6: We appreciate this comment. The Y-axis labels and overlapping notations in Figures 5, 7, 8, and 10 have been corrected for clarity. [Pages 9, 11, 12, and 13] Comment 7: Page 9, Fig. 4: Insert a horizontal line at the base of columns in the histogram graphs, and also remove the light green background. Also, for Fig. 7 in page 12. Response 7: Thank you. A horizontal line has been added at the base of the histogram bars and the green background has been removed in Figures 9 and 11. [Pages 13 and 14] Comment 8: Page 11, Fig. 6: Keys of legend should not be outside frame. Response 8: We have repositioned the legend in Figure 8 to remain within the frame. [Page 13] Comment 9: Please combine two paragraphs in page 14. Response 9: Thank you. The two paragraphs on page 16 have been merged into a single cohesive paragraph for improved readability. [Page 15 Lines 372-381] Comment 10: Line 411-414: The O-H stretching vibrations in the FTIR spectra are so weak and doubtful. Probably heating of samples before measurement is the main cause. Please try to solve this problem. Response 10: Thank you for this important observation. We agree that the weak O–H stretching signals in the original FTIR spectra warranted further investigation. The initial samples were oven-dried at 105 °C prior to FTIR testing in accordance with common practice in soil and materials analysis. This was done to ensure moisture consistency and eliminate interference from free or absorbed water, which can obscure relevant peaks in the 3000–3600 cm⁻¹ region. Such drying procedures are aligned with ASTM and ISO guidelines for FTIR solid sample preparation. However, as you correctly noted, oven drying may also lead to the suppression of heat-sensitive functional groups—particularly hydrogen bonds and structurally bound water—present in biopolymers like xanthan gum and guar gum. To address this, we performed a second FTIR analysis using air-dried samples at room temperature, avoiding thermal alteration of labile groups. The revised spectra (now shown in Figure 16) revealed sharper and more distinct O–H and C=O bands, especially at higher biopolymer content. This confirmed that mild drying better preserves the hydrogen-bonding features that play a role in biopolymer–soil interaction. We have updated the FTIR Discussion section accordingly to present a more accurate interpretation of these molecular features. [Page 17, Lines 428–438]. Comment 11: Page 16: Different font types and size exist, so please unify. Response 11: We appreciate this observation. All font types and sizes in the manuscript have been standardized depend on MDPI format. [Throughout the manuscript] Comment 12: Page 18: Discussion is not bad but it needs more elaboration and enhancement. Response 12: Thank you. We have rewritten and expanded the Discussion section to provide deeper insights, clearer interpretation, and stronger linkage between mechanical and chemical findings. [Page 19,20 Lines 506-543] Comment 13: The reference list in page 19-20 needs careful re-editing considering punctuation and some other issues. For example, journal names are given in full for some, e.g. [1] and [5] but it is abbreviated in others, e.g. [2]. Response 13: Thank you for bringing this to our attention. Initially, we used Mendeley to generate and format the references automatically. While this helped in maintaining citation integrity, it unfortunately introduced inconsistencies in journal name formatting—some titles were given in full while others were abbreviated. To resolve this, we manually reviewed and edited the entire reference list to ensure consistent application of the MDPI citation style. This included standardizing journal name abbreviations, correcting punctuation, and verifying citation details. The revised reference section now adheres fully to MDPI formatting guidelines. [Revised on Pages 21–22]
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Reviewer 3 Report
Comments and Suggestions for Authors
Each device has its own measurement accuracy. The results have its own precisely defined measurement error.
1. The table presents the results of mechanical measurements of samples even to the fourth decimal place, which is incorrect (e.g. E=4477.612). The degree of accuracy of the device should be taken into account.
2. Mechanical properties do not change regularly, as the Authors note, e.g. stress decreases with the amount of rubber and time, but for small times it increases.
3. The worst results concern FTIR studies. Functional groups such as hydroxyl and carboxyl are the result of the sample oxidation process, but do not change with the cross-linking time. The results are not uniform and it is difficult to say that they are the result of cross-linking because the characteristic peaks for these groups do not change with the cross-linking time, they are very weak (their position or height do not change).
4. Since FTIR confirms chemical interactions between biopolymers and additional components, it is difficult to observe.
5. It would be good to propose a chemical mechanism for cross-linking with biopolymers.
Author Response
Comments 1: The table presents the results of mechanical measurements of samples even to the fourth decimal place, which is incorrect (e.g. E=4477.612). The degree of accuracy of the device should be taken into account.
Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have revised all numerical values in the tables, including E₅₀ and UCS, to display no more than two decimal places in accordance with the accuracy of the measurement instruments used. This change improves clarity and consistency of data presentation. [Revised on Pages 8–19,20, Tables 2–3 and corresponding text in the Results section] All UCS and modulus values are now rounded to one decimal place to reflect device precision.
Comments 2: Mechanical properties do not change regularly, as the Authors note, e.g. stress decreases with the amount of rubber and time, but for small times it increases.
Response 2: Thank you for this important observation. We would like to clarify that the mechanical properties reported in the manuscript—particularly unconfined compressive strength (UCS)—are based on real, unaltered experimental data. These values reflect the actual performance of the sand-bentonite mixtures treated with different biopolymer concentrations and curing durations. In response to your comment, we have revised the language in the Results and Discussion sections to more accurately describe the behavior as observed, rather than implying a uniform trend. Specifically, we clarified that while the overall strength generally increases with curing time, some non-monotonic variations, particularly at earlier stages, are a true reflection of the material behavior, possibly due to initial hydration effects, gel formation kinetics, or temporary structural rearrangement.
Comments 3: The worst results concern FTIR studies. Functional groups such as hydroxyl and carboxyl are the result of the sample oxidation process, but do not change with the cross-linking time. The results are not uniform, and it is difficult to say that they are the result of cross-linking because the characteristic peaks for these groups do not change with the cross-linking time, they are very weak (their position or height do not change).
Response 3: We appreciate this valuable observation. We acknowledge that the initial FTIR spectra obtained from oven-dried samples exhibited relatively weak and inconsistent O–H and C=O signals, which limited our ability to confidently track changes over time. This may be attributed to the thermal sensitivity of biopolymer functional groups, which can degrade or partially dissociate during high-temperature drying, as supported by previous studies. In response, we conducted a second FTIR analysis using samples that were air-dried at room temperature in order to preserve labile functional groups and avoid thermal alteration. The updated spectra (presented in Figure 16) show noticeably improved definition in the hydroxyl and carbonyl absorption bands, particularly at higher biopolymer concentrations. Although the characteristic peak positions remained largely unchanged over time, the increased clarity of these bands reinforces the interpretation that the dominant interactions in the system are physical in nature, primarily hydrogen bonding and electrostatic attractions, rather than covalent cross-linking. These revised findings and interpretations have been incorporated into the updated FTIR analysis and discussion section. [Updated on Page 17, Lines 428–438]
Comments 4: Since FTIR confirms chemical interactions between biopolymers and additional components, it is difficult to observe.
Response 4: Thank you for pointing this out. We agree that while FTIR can provide important insight into functional group interactions, its capacity to detect subtle or overlapping signals, especially in complex composite systems like biopolymer-treated sand, can be limited. In our case, the spectra confirmed the presence of key functional groups (e.g., hydroxyl and carboxyl), but the data did not provide direct evidence of covalent bonding. We have clarified this limitation in the revised FTIR discussion, emphasizing that the observed changes are best interpreted as indicative of physical interactions, including hydrogen bonding and electrostatic attraction between biopolymer molecules and clay minerals. These forms of bonding, although less permanent than covalent interactions, contribute significantly to the development of a cohesive and stable soil-biopolymer matrix. [Clarified on Page 18, FTIR Discussion] Comments 5: It would be good to propose a chemical mechanism for cross-linking with biopolymers.
Response 5: We appreciate this suggestion. To address it, we have added a section in the FTIR discussion outlining the most plausible mechanisms by which biopolymers interact with soil constituents. While the data do not indicate covalent cross-linking, the hydroxyl (–OH) and carboxylate (–COO⁻) groups in xanthan gum and guar gum are known to form hydrogen bonds with mineral surfaces and can engage in cation-bridging interactions with divalent and trivalent metal ions commonly found in clay minerals (e.g., Ca²⁺, Mg²⁺). These mechanisms are well-documented in the literature and offer a reasonable explanation for the enhanced structural integrity and cohesion observed in treated specimens, particularly at higher polymer concentrations. Supporting references have been added to substantiate this interpretation.
[Added on Page 20, Lines 528-534] The proposed interaction mechanism involves the formation of hydrogen bonds between the biopolymer’s functional groups and hydroxylated mineral surfaces, as well as ionic bridges mediated by exchangeable cations in the clay. These interactions contribute to network formation and improved mechanical behavior.
Round 2
Reviewer 2 Report
Comments and Suggestions for Authors
In the caption of Figure 1, please replace “of” instead of “for”. Also, correct Fr to F as in the figure's key. Carbonate minerals are represented by calcite only as displayed by the very characteristic peaks, so better to specify.
It is better if you can insert a scale bar for the materials used in the study.
No need for the frame for the graph in Figure 4 and others.
Can you insert a scale for Figure 13 too?.
Comments for author File: Comments.pdf
Author Response
Comment 1:
In the caption of Figure 1, please replace “for” with “of”. Also, correct Fr to F as in the figure's key. Carbonate minerals are represented by calcite only as displayed by the very characteristic peaks, so better to specify.
Response 1:
Thank you for your observation. We have corrected the caption of Figure 1 by replacing “for” with “of” and have standardized the abbreviation “Fr” to “F” to maintain consistency with the figure legend. Additionally, we have specified that the carbonate minerals are represented by calcite, in line with the prominent peaks identified in the XRD profile.
[Updated on Page 4, Figure 1,2]
Comment 2:
It is better if you can insert a scale bar for the materials used in the study.
Response 2:
We appreciate your suggestion. A scale bar has been added to the relevant material images in the manuscript to provide a clearer reference for the dimensions of the materials analyzed.
[Updated on Page 5, Figure3]
Comment 3:
No need for the frame for the graph in Figure 4 and others.
Response 3:
Thank you for the helpful comment. We have removed the outer frame borders from Figure 4 and all other related graphs to improve visual clarity and consistency with academic standards.
[Updated on Page 7, 10, 12, 13, and 15, Figures 4, 5, 7, 8, and 10]
Comment 4:
Can you insert a scale for Figure 13 too?
Response 4:
Thank you for pointing this out. A scale bar representing 38 mm (the diameter of the cylindrical samples) has now been added to Figure 13 to provide a visual reference for size. The caption has also been updated accordingly.
[Updated on Page 17, Figure 13 and its caption]
Reviewer 3 Report
Comments and Suggestions for Authors
I do not have any comments. THe paper now is good.
Author Response
Comment:
I do not have any comments. The paper now is good.
Response:
We sincerely thank the reviewer for their positive feedback and appreciation of the revised manuscript. We are grateful for your time and thoughtful review throughout the process.
