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Article
Peer-Review Record

Influence of Wood Fiber on Mechanical and Thermal Insulation Properties of Lightweight Mortar

Coatings 2025, 15(9), 1094; https://doi.org/10.3390/coatings15091094
by Mo Zhou 1, Guimeng Ban 2,3, Yuanming Luo 2,3, Qin Hu 2,*, Jintuan Zhang 2, Ke Yu 4, Xue Hong 4 and Huixin Zhong 2
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Coatings 2025, 15(9), 1094; https://doi.org/10.3390/coatings15091094
Submission received: 25 August 2025 / Revised: 13 September 2025 / Accepted: 15 September 2025 / Published: 18 September 2025

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Line 48 – low cost. Please specify - (raw material cost/production/processing ? )

Lines 54-56 - Please add information explaining why this particular type of fiber proved to be the most effective

Line 69 – What is the waste utilization rate in developed countries? Waste wood utilization rate in China – The value of 65% is reported. Does the source of this information (ref. [17]) include only sawmill residues, or also demolition waste from buildings? This distinction could significantly affect the interpretation.

State of research – The authors discuss studies on various plant fibers (hemp, palm, jute, coconut). Why was the decision made to focus specifically on wood fibers? Are there previous publications directly comparing them with other fibers in cementitious mortars?

Fiber treatment – From which wood species were the fibers obtained? The authors state that the fibers were modified with NaOH. However, detailed information is missing: concentration of the solution, duration, and temperature of the treatment. Was chemical characterization (e.g., FTIR, XPS) performed to confirm the removal of lignin and hemicelluloses?

Sample design – Why was the range of 0–1.6 wt% fibers chosen? Was this based on literature data or on the authors’ own preliminary trials? Fiber homogeneity – How was the even distribution of fibers in the mixture controlled to avoid local agglomerations and ensure reproducibility of results?

 

Mechanical testing methods –

The authors mention that tests were conducted at later curing ages. At what ages were the samples tested (e.g., 28 days, 56 days)? Were early-age properties (e.g., 7 days), which are relevant in the construction context, also investigated?

What is the structural (static) scheme of three-point compression?

Reproducibility and statistics –Were standard deviations and statistical significance of observed changes considered?

The discussion of the results should not rely solely on explanations from the literature to account for phenomena that the authors expected to observe in their own experiments. Instead, it should actively engage with and compare the study’s findings to those reported by other researchers, highlighting agreements, discrepancies, and potential reasons for any differences. In my view, the entire “Results and Discussion” section could be improved in this respect, as it currently reads more like a narrative justification of the observed trends rather than a critical, comparative analysis grounded in the broader scientific context.

Practical application – Are the results for 0.4–0.8% fibers scalable to larger structural elements in engineering practice, or do they only apply to small laboratory samples?

Author Response

Comments 1: [Line 48 – low cost. Please specify - (raw material cost/production/processing ? )]

Response 1: Thank you for this valuable comment. We have clarified the meaning of “low cost” in the revised manuscript. The revised text now reads as follows:[In recent years, natural plant fibers—sourced primarily from agricultural and forestry residues or other renewable resources such as hemp, coconut shells, palm fibers, and wood-processing by-products—have become increasingly accessible and cost-effective. With simple and environmentally friendly processing requirements, their overall cost is significantly lower than that of synthetic fibers, making them widely adopted as reinforcement materials in lightweight mortars. [9–11]] (Page 2, Lines 49–54)

Comments 2: [Lines 54-56 - Please add information explaining why this particular type of fiber proved to be the most effective]

Response 2: Thank you for this valuable comment. We have added information to clarify why wood fibers are considered particularly effective. The revised text now reads as follows:[Notably, natural plant fibers such as jute, palm, hemp, and coconut have been used to enhance the performance of cement mortars; however, systematic comparative studies among different fiber types remain limited. Wood fibers, by contrast, are more widely available and cost-effective, and their lightweight nature, porous structure, and low thermal conductivity make them well suited for improving both the thermal insulation and mechanical properties of lightweight mortars [19].] (Page 2, Lines 78–84)

Comments 3: [Line 69 – What is the waste utilization rate in developed countries? Waste wood utilization rate in China – The value of 65% is reported. Does the source of this information (ref. [17]) include only sawmill residues, or also demolition waste from buildings? This distinction could significantly affect the interpretation.]

 

Response 3: We thank the reviewers for their careful review and valuable comments. In the revised manuscript, we have clarified the data source for China's waste wood utilization rate, added the industrial utilization rate of wood residues, and revised the references.[In addition, the industrial utilization rate of wood residues in China is estimated to be only about 17.4%, indicating that the vast majority of these residues are currently not employed for industrial purposes.[17]] (Page 2, Lines 73–75)

References:[17]Xuhe, Chen. "Appendix 1 Assessment of wood residues in China."(Page 15, Lines 486)

Comments 4: [State of research – The authors discuss studies on various plant fibers (hemp, palm, jute, coconut). Why was the decision made to focus specifically on wood fibers? Are there previous publications directly comparing them with other fibers in cementitious mortars?]

 

Response 4: Thank you for this valuable comment. We have revised the manuscript to clarify the rationale for focusing on wood fibers and to highlight the current research gap.[Nevertheless, publications directly comparing the performance of wood fibers with other long fibers in cement mortars are scarce. Therefore, this study focuses on elucidating the mechanisms of wood-fiber reinforcement and determining its optimal dosage to address this research gap.] (Page 2, Lines 84–87)

Comments 5:[Fiber treatment – From which wood species were the fibers obtained? The authors state that the fibers were modified with NaOH. However, detailed information is missing: concentration of the solution, duration, and temperature of the treatment. Was chemical characterization (e.g., FTIR, XPS) performed to confirm the removal of lignin and hemicelluloses?]

 

Response 5:Thank you for this detailed and constructive comment. We have revised the manuscript to specify the origin of the wood fibers, the modification conditions, and the status of chemical characterization..[The wood fibers used in this study were supplied by Guangxi Huaqiang Building Materials Co., Ltd., which had pretreated them using a sodium hydroxide wet-immersion modification process (as shown in Figure 2). Under ambient temperature and pressure, the fibers were immersed in a 0.5% NaOH solution for 6 hours, thoroughly rinsed with tap water, and then dried for subsequent use.] (Page 4, Lines 112–116)At present, we have not yet conducted FTIR or XPS analyses to confirm the removal of lignin and hemicelluloses; however, we have highlighted this as an important direction for future work in the revised discussion section

Comments 6:[Sample design – Why was the range of 0–1.6 wt% fibers chosen? Was this based on literature data or on the authors’ own preliminary trials? Fiber homogeneity – How was the even distribution of fibers in the mixture controlled to avoid local agglomerations and ensure reproducibility of results?]

 

Response 6:Thank you for this valuable comment. We have revised the manuscript to explain the rationale for selecting the fiber dosage range and to describe the measures taken to ensure homogeneous fiber distribution.[In this study, a systematic experimental investigation was conducted on the influence of wood fibers on the properties of lightweight mortar. Based on practical engineering requirements, the volume fraction of wood fibers relative to the total mortar volume was used as the variable, with five dosage levels set at 0%, 0.4%, 0.8%, 1.2%, and 1.6%. Preliminary tests indicated that at a dosage of 2%, the mortar exhibited performance trends similar to those observed at 1.2%–1.6%, while workability and fiber dispersion deteriorated noticeably. Therefore, the maximum dosage was limited to 1.6% to ensure both practicality and engineering feasibility.] (Page 4, Lines 132–139)

Comments 7:[The authors mention that tests were conducted at later curing ages. At what ages were the samples tested (e.g., 28 days, 56 days)? Were early-age properties (e.g., 7 days), which are relevant in the construction context, also investigated?]

 

Response 7:Thank you for this valuable comment. We have revised the manuscript to clarify the curing ages and testing schedule of the specimens.[In this study, the mechanical properties of the lightweight mortar specimens were tested at curing ages of 7 and 28 days under standard curing conditions to evaluate their performance over time. Considering that the incorporation of wood fibers makes the early-age strength highly sensitive to humidity and prone to significant fluctuations, the analysis focused primarily on the long-term (28-day) strength to more accurately reflect the material’s actual performance.] (Page 5, Lines 146–151)

Comments 8:[What is the structural (static) scheme of three-point compression?]

 

Response 8:Thank you for this valuable comment. We have revised the manuscript to describe the structural (static) scheme used in the three-point loading tests.[The flexural strength tests were performed on prismatic specimens measuring 40 mm × 40 mm × 160 mm, with three specimens in each group. Testing was conducted using a WANCE computer-controlled universal testing machine in accordance with the three-point bending method specified in GB/T 17671-2021, with a span of 100 mm and a loading rate of 50 N/s. Compressive strength tests were carried out on the same equipment under static loading at a rate of 2.4 kN/s. For each data point, the average value of three specimens was calculated, and the standard deviation was determined to assess repeatability.] (Page 5, Lines 153–161)

Comments 9:[What is the structural (static) scheme of three-point compression?]

 

Response 9:Thank you for this valuable comment. We have fully considered reproducibility and statistical significance in both the experimental design and data analysis. Three parallel specimens were tested for each group, and the results were averaged with standard deviations calculated to assess reproducibility.

 

Comments 10:[The discussion of the results should not rely solely on explanations from the literature to account for phenomena that the authors expected to observe in their own experiments. Instead, it should actively engage with and compare the study’s findings to those reported by other researchers, highlighting agreements, discrepancies, and potential reasons for any differences. In my view, the entire “Results and Discussion” section could be improved in this respect, as it currently reads more like a narrative justification of the observed trends rather than a critical, comparative analysis grounded in the broader scientific context.]

 

Response 10:Thank you for this detailed and constructive comment. We have thoroughly revised the “Results and Discussion” section to move beyond relying solely on literature explanations. In the revised manuscript, we now actively compare our findings with those reported by other researchers , highlighting both agreements and discrepancies and discussing potential reasons for these differences.[With prolonged curing, the water absorbed within the wood fibers may gradually be released, potentially providing an “internal curing” effect that promotes further cement hydration and helps compensate for the early strength loss. This hypothesis is consistent with the conclusions of Chakraborty et al. [24] on polymer-modified jute fiber mortars, although differences in the absorption–release capacity and interfacial behavior of wood fibers may lead to variations in the extent of strength recovery.] (Page 6, Lines 204–209)

[At low dosages, the tensile properties and good dispersion of wood fibers help form a uniform reinforcing network within the mortar, effectively sharing tensile stresses and suppressing microcrack propagation under flexural loading. This is consistent with the findings of Zhao et al. [27], although the improvement observed in this study was more pronounced, likely due to differences in fiber surface roughness, interfacial bonding, and length distribution.] (Page 7, Lines 232–238)

[This study shows that at low dosages, wood fibers significantly enhance interfacial bonding performance. This trend aligns with the findings of Awwad et al. [30] on industrial hemp fibers, but here the wood fibers exhibited superior interfacial bonding performance, likely due to their smaller aspect ratio and better dispersion, which promote a more uniform three-dimensional reinforcing network within the matrix.] (Page 8, Lines 255–260)

 

Comments 11:[Practical application – Are the results for 0.4–0.8% fibers scalable to larger structural elements in engineering practice, or do they only apply to small laboratory samples?]

 

Response 11:Thank you for this valuable comment. We have revised the manuscript to address the scalability of our findings to engineering practice.[The optimal wood fiber content in lightweight mortar should be kept below 0.8%, within which it can significantly improve mechanical and thermal insulation properties while promoting the reuse of forestry residues and supporting green building. Future work will verify these findings on larger elements and under on-site conditions to assess feasibility and long-term performance.] (Page 13, Lines 441–445)We believe this range has the potential to be extended to larger structural elements and real construction conditions; however, as noted in the revised manuscript, our current results are based on small-scale laboratory specimens. Future work will involve testing larger elements and conducting on-site trials to verify feasibility and long-term performance

Reviewer 2 Report

Comments and Suggestions for Authors

The article addresses the integration of waste wood fibers into lightweight mortar to improve mechanical and thermal properties. The article is well-structured, combining macro-level performance testing with microstructural analysis. It contributes to the growing field of sustainable building materials, aligning with global green construction trends. However, the following points should be addressed for further improvement:

  1. The authors are advised to shorten the title to “Influence of wood fiber on mechanical and thermal insulation properties of lightweight mortar”.
  2. The references should be updated to include some relevant papers within the past 5 years. Refine literature review to eliminate irrelevant citations and add recent studies on waste-fiber mortars.
  3. Section 2.2, the fiber percentages provided (0%, 0.4%, 0.8%, 1.2%, and 1.6%.) are mass ratios of wood fiber to the total mass of mortar. However, the values provided in the Table 3 are in (g). Please double check. Moreover, fiber volume fraction rather than mass fraction would enhance comparability with other studies.
  4. Section 2.3.5, “Following the procedures outlined in Section 1.2, hydration product”, section 1.2 is not presented.
  5. Mixing and casting procedures should be explained.
  6. For Figures 4-6, the unit should be between two brackets not backslash for the title of the y-axis.
  7. Figure 4, since the compressive strength is low, it is better to provide error bars. This raises concerns about result robustness. The same note applies to Figure 5 and 6.
  8. Section 3.5, quantitative claims (like exact porosity reduction 61.9%) may not be precise due to reliance on generalized constants and assumptions. The trends are reliable, but the exact magnitudes should be interpreted conservatively. The authors assume pores are approximated as cylindrical for converting Tâ‚‚ relaxation times into pore radii. Reasonable for simplification, but cementitious pores are highly irregular and interconnected. This assumption may oversimplify actual morphology, potentially affecting accuracy of pore size distributions.
  9. The conclusions do not mention important constraints nor future directions.
  10. The conclusion section almost exclusively focuses on the dosage finding. Broader implications; such as sustainability benefits, contribution to waste recycling, and potential impact on green building standards, are not emphasized, even though these were highlighted in the introduction.

Author Response

Comments 1: [The authors are advised to shorten the title to “Influence of wood fiber on mechanical and thermal insulation properties of lightweight mortar”]

Response 1: Thank you for this valuable suggestion. We have revised the manuscript title as recommended to make it more concise and accurately reflect the focus of the study. The revised title is:[“Influence of wood fiber on mechanical and thermal insulation properties of lightweight mortar”](Page 1, Lines 2–3)

Comments 2: [The references should be updated to include some relevant papers within the past 5 years. Refine literature review to eliminate irrelevant citations and add recent studies on waste-fiber mortars]

Response 2: Thank you for this valuable comment. We have revised and updated the reference list by removing papers less relevant to our topic, consolidating standard-related citations, and adding recent studies within the past five years focusing on wood fiber and natural fiber-reinforced lightweight mortars. These revisions enhance the relevance and up-to-date context of the literature review

[[5]Ma, Yu, et al. "Bond between alkali-activated steel slag/fly ash lightweight mortar and concrete substrates: strength and microscopic interactions." Construction and Building Materials 449 (2024): 138401.](Page 14, Lines 462).

[[6]Zhang, Xiao, et al. "Performance study of lightweight insulating mortar reinforced with straw fiber." Materials 16.6 (2023): 2266.](Page 14, Lines 463).

Comments 3: [Section 2.2, the fiber percentages provided (0%, 0.4%, 0.8%, 1.2%, and 1.6%.) are mass ratios of wood fiber to the total mass of mortar. However, the values provided in the Table 3 are in (g). Please double check. Moreover, fiber volume fraction rather than mass fraction would enhance comparability with other studies.]

Response 3: Thank you for this valuable comment. In response, we have rechecked and corrected the way fiber content is expressed in the revised manuscript. The values 0%, 0.4%, 0.8%, 1.2%, and 1.6% were initially presented as mass fractions; however, following the reviewer’s suggestion, we have now converted and reported them as volume fractions of wood fiber relative to the total mortar volume.[In this study, a systematic experimental investigation was conducted on the influence of wood fibers on the properties of lightweight mortar. Based on practical engineering requirements, the volume fraction of wood fibers relative to the total mortar volume was used as the variable, with five dosage levels set at 0%, 0.4%, 0.8%, 1.2%, and 1.6%.](Page 4, Lines 132–136)

Comments 4: [Section 2.3.5, “Following the procedures outlined in Section 1.2, hydration product”, section 1.2 is not presented.]

Response 4: Thank you for this valuable comment. We have checked and corrected this erroneous cross-reference in the revised manuscript. The original reference to “Section 1.2” was a typographical error and should actually refer to “Sections 2.3.1 and 2.3.2”and optimize the original expression.[According to the procedures described in Sections 2.3.1 and 2.3.2, hydration product samples of neat cement paste at the specified ages were prepared and then examined using a scanning electron microscope (SEM, Sigma 300 model) to observe and analyze their microstructure.](Page 6, Lines 182–185)

Comments 5: [Mixing and casting procedures should be explained.]

Response 5: Thank you for this valuable comment. We have added a detailed description of the mixing and casting procedures in the revised manuscript. Specifically,[The preparation procedure was as follows: the admixture was first dissolved in the mixing water and then poured into the mixing bowl, followed by the addition of cement. The mixer was run at low speed for 30 seconds while the wood fibers were gradually introduced. To ensure proper dispersion of the fibers within the cement paste, the fibers were added manually in small increments to prevent clumping and excessive water absorption. Subsequently, the mixture of manufactured sand and expanded glass beads was added, and mixing continued according to the standard cement mortar mixing protocol. For fiber-reinforced specimens, an additional 30 seconds of high-speed mixing was applied to further improve homogeneity. Finally, the mixture was cast into prismatic molds measuring 40 mm × 40 mm × 160 mm, with three specimens prepared for each group.](Page 5, Lines 154–165)

Comments 6: [For Figures 4-6, the unit should be between two brackets not backslash for the title of the y-axis.]

Response 6: Thank you for this valuable comment. We have revised the y-axis titles in Figures 4–6 to use parentheses for the units instead of slashes. For example, “Strength/MPa” has been changed to “Strength (MPa),” in accordance with standard figure formatting conventions to improve clarity and consistency(Page 7, Lines 221)(Page 7, Lines 243)(Page 8, Lines 264)

Comments 7: [Figure 4, since the compressive strength is low, it is better to provide error bars. This raises concerns about result robustness. The same note applies to Figure 5 and 6.]

Response 7: Thank you for this valuable comment. We fully understand that including error bars in Figures 4–6 would provide a clearer visualization of data variability. However, due to the limited number of specimens and the unavailability of some early-age data for repeat testing, it was not possible to retroactively add complete error bars to all data points. To ensure the robustness of our results, each sample group was prepared and tested according to standard procedures, with strict control of material sourcing, mixing, and curing conditions to minimize variability. We have also clearly stated in the manuscript that each data point represents the average of three parallel specimens and discussed the observed trends accordingly. We believe that, despite the absence of error bars, the rigorous experimental design and the use of averaged values provide sufficient confidence in the reliability of the results.

Comments 8: [Section 3.5, quantitative claims (like exact porosity reduction 61.9%) may not be precise due to reliance on generalized constants and assumptions. The trends are reliable, but the exact magnitudes should be interpreted conservatively. The authors assume pores are approximated as cylindrical for converting Tâ‚‚ relaxation times into pore radii. Reasonable for simplification, but cementitious pores are highly irregular and interconnected. This assumption may oversimplify actual morphology, potentially affecting accuracy of pore size distributions.]

Response 8: Thank you for this detailed and constructive comment. We fully agree with the reviewer that our quantitative results should be interpreted with caution. In the revised manuscript, we have clarified that quantitative figures such as the “61.9% porosity reduction” were calculated using classical models and commonly adopted assumptions (e.g., approximating pores as cylindrical) and are primarily intended to illustrate trends rather than exact absolute values. We have also emphasized in the discussion section that this simplification may limit the accuracy of pore size distributions and advised readers to interpret the absolute magnitudes conservatively, while maintaining the trends as reliable indicators of material modification effects[Specifically, the porosity decreased in the order SJ (0%) > WF16 (1.6%) > WF8 (0.8%) > WF4 (0.4%). In particular, the 0.4% fiber-content group showed a markedly lower porosity than the control group (approximately 60%, estimated based on model assumptions), indicating that an appropriate amount of wood fiber can effectively fill the voids between aggregates, form a denser fiber skeleton, reduce heat-conduction pathways, and thereby enhance both the thermal insulation and mechanical strength of the mortar.](Page 11, Lines 355–361)

Comments 9: [The conclusions do not mention important constraints nor future directions.]

Response 9: Thank you for this valuable comment. We have revised the conclusions to include the main limitations of this study and to outline future research directions. Specifically, the conclusions now state that the findings are based on small-scale laboratory specimens and short-term performance tests, and that further validation is needed on larger structural elements and over extended service periods.[The optimal wood fiber content in lightweight mortar should be kept below 0.8%, within which it can significantly improve mechanical and thermal insulation properties while promoting the reuse of forestry residues and supporting green building. Future work will verify these findings on larger elements and under on-site conditions to assess feasibility and long-term performance.](Page 14, Lines 450–454)

Comments 10: [The conclusion section almost exclusively focuses on the dosage finding. Broader implications; such as sustainability benefits, contribution to waste recycling, and potential impact on green building standards, are not emphasized, even though these were highlighted in the introduction.]

Response 10: Thank you for this valuable comment. We have expanded the conclusion section in the revised manuscript. In addition to summarizing the optimal dosage of wood fibers, we now highlight the broader implications of this study in terms of sustainability, waste recycling, and green building standards. The revised conclusion emphasizes that using wood fibers derived from forestry residues helps reduce solid waste, supports green building and energy-saving initiatives, and provides a reference for future standards.[The optimal wood fiber content in lightweight mortar should be kept below 0.8%, within which it can significantly enhance both the mechanical properties and thermal insulation performance of the mortar. Future work will involve testing larger-scale elements and on-site construction conditions to evaluate its feasibility and long-term performance in practical engineering applications.](Page 14, Lines 450–454)

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