Development of Geopolymer Foams for Multifunctional Applications

Round 1

Reviewer 1 Report

The manuscript is describing the functionality of foam geopolymer for multi-functional purposes. The overall manuscript was interesting, and sufficient results are presented. However, some changes should be made before being accepted for publication. The corrections should be made as follow:

1. For the introduction part, please separate your paragraph so that the reader does not have difficulty to read.
2. The introduction part is too long and hard to understand. Please highlight the novelty of this manuscript.
3. What software did you use to quantify the XRD analysis?
4. In the materials and method section, please present the XRD (Rietveld refinement) and XRF result of MK and slag in a table form.
5. The authors should know how to differentiate between the materials and method section. As I have asked in questions no 3 and 4, the authors have mentioned in the method section, which means after the material section. So the authors should rearrange these parts so that the readers will not confuse.
6. Based on lines no 127-128, on what basis did you replace the MK with slag to reduce the shrinkage?
7. Please avoid any grammatical errors. The manuscript is hard to read and understand. The manuscript should be checked by a suitable and qualified person.

Author Response

Comments from Reviewer 1

The manuscript is describing the functionality of foam geopolymer for multi-functional purposes. The overall manuscript was interesting, and sufficient results are presented. However, some changes should be made before being accepted for publication. The corrections should be made as follow:

Response: Thank you for your constructive comments and suggestions. I would like to answer your questions and comments point by point.

• Comment 1: For the introduction part, please separate your paragraph so that the reader does not have difficulty to read.

Response: Thank you for pointing this out. We agree with this comment. In the Introduction part, we have divided the previous long paragraph into several smaller ones. We hope that this will make easier to read. If there is any hard-to-read section left, please let us know which one it is and we will try to change it.

• Comment 2: The introduction part is too long and hard to understand. Please highlight the novelty of this manuscript.

Response: We have revised the Introduction part and modified the wording for the sake of clarity. We have tried to better highlight the novelty of the present research. We hope that the Introduction part will be acceptable in its present state.

• Comment 3: What software did you use to quantify the XRD analysis?

Response: As described in 2.3. Methods section, the X’Pert Highscore Plus software and ICDD PDF-2 reference database were used to evaluate X-ray diffractograms and to carry out Rietveld Analysis. If the reviewer feels the need, we will of course insert this information in the right place of section 2.1. Materials.

• Comment 4: In the materials and method section, please present the XRD (Rietveld refinement) and XRF result of MK and slag in a table form.

Response: The mineral and chemical compositions of MK and slag were given in a table form as requested.

• Comment 5: The authors should know how to differentiate between the materials and method section. As I have asked in questions no 3 and 4, the authors have mentioned in the method section, which means after the material section. So the authors should rearrange these parts so that the readers will not confuse.

Response: Thank you for pointing this out. We partially agree with this comment. Although Crystals does not regulate the structure of the section "Materials and Methods", we see in the light of the articles previously published in this journal that most authors present each subsection in the following order:

1. Description of raw materials,
2. Mixture preparation steps,
3. Presentation of measurement methods.

For this reason, we have followed the above mentioned sequence. However, we admit you are right; the methods indicated in the section of "Materials" may confuse the reader. For the sake of clarity, we have modified the relevant parts of Section 2.1. Hopefully, this is no longer a confusing part for the reader. However, if the reviewer feels that further modification is needed in relation to the section "Materials and methods", please let us know and we will try to improve it.

• Comment 6: Based on lines no 127-128, on what basis did you replace the MK with slag to reduce the shrinkage?

Response: Based on results by Samson et.al, the replacement of MK with slag can be advantageous in reducing the shrinkage tendency. In a later part of the manuscript (Section 3.3.3) we referred to the work of the mentioned authors, however, this was omitted here. Thank you for pointing this out. We have added the appropriate information to the sentence in question.

• Comment 7: Please avoid any grammatical errors. The manuscript is hard to read and understand. The manuscript should be checked by a suitable and qualified person.

Response: We are sorry that we have made many grammatical errors. We have subjected our manuscript to an extensive English revision. During the professional proofreading, we improved the spellings and simplified the sentences. We hope this will make our paper easier to read and understand. We have marked all revisions to the manuscript using the “Track Changes” function in MS Word, such that any changes can be easily viewed by the editors and reviewers.

Reviewer 2 Report

I have to emphasize that the paper reflects me a feeling of reading a laboratory report rather than a scientific paper. There are experimental works that are already stated in the references given by the authors; moreover, the experimental outcomes in the paper indicate the opposite findings of references without any scientific interpretation. Authors find out some of the treatment on GP foam has even negative effect in the point of mechanical strength and again without any scientific interpretation. If so why indicate those findings in a scientific paper.   Under the sight of my above comments, I suggest major revision.  

1. The paper needs professional proofreading, there are long sentences hard to read and understand. There are also a few miswrites.
2. Abstracts may be written in different ways to make them easy to read and understand. Please consider rewriting page 1 line 8 and lines 11-12
3. Page 3 line 115, please rearrange the spectrometry values in a table.
4. Page 6 line 257; The effects of different pre-cursors on compressive strength and porosity need to be explained scientifically. According to Fig1, there is a strong CS and porosity variation between different stabilizers, which was not stated unambiguously.
5. Page 6 lines 283-284-285; what is the authors' interpretation about the opposite outcome here stated as different findings between current work and given reference.
6. Page 12 line 9; According to generally accepted judgment, additive slag must increase the mechanical properties of GP foam. However, the authors have indicated the opposite outcome against general judgment. The negative effect of slag addition into GP foam without scientific explanation has no impact on the overall paper and needs to be removed completely from it.
7. Page 15 Figure 10b, Tyme of analysis, time or type?
8. The authors need to explain the uniqueness and value-added of the scientific paper in the conclusion part.
9. I would also suggest the authors to familiarize themselves with the possibility of foaming aluminum from recyclate: Ercoli, R.; Laskowska, D.; Nguyen, V.V.; Le, V.S.; Louda, P.; Łoś, P.; Ciemnicka, J.; Prałat, K.; Renzulli, A.; Paris, E.; Basilici, M.; Rapiejko, C.; Buczkowska, K.E. Mechanical and Thermal Properties of Geopolymer Foams (GFs) Doped with By-Products of the Secondary Aluminum Industry. Polymers 2022, 14, 703. doi: 10.3390/polym14040703

Comments for author File: Comments.pdf

Author Response

Comments from Reviewer 2

I have to emphasize that the paper reflects me a feeling of reading a laboratory report rather than a scientific paper. There are experimental works that are already stated in the references given by the authors; moreover, the experimental outcomes in the paper indicate the opposite findings of references without any scientific interpretation. Authors find out some of the treatment on GP foam has even negative effect in the point of mechanical strength and again without any scientific interpretation. If so why indicate those findings in a scientific paper. Under the sight of my above comments, I suggest major revision.

Response: We thank the reviewer for their constructive comments and suggestions. We have revised our paper accordingly and feel that the reviewer’s comments helped clarify and improve our paper. I would like to answer your questions and comments point by point.

• Comment 1: The paper needs professional proofreading, there are long sentences hard to read and understand. There are also a few miswrites.

Response: Our manuscript has been comprehensively proofread by a suitable and qualified person. During this process, we have corrected grammatical errors, misspellings, and shortened sentences that were too long. All revisions to the manuscript have been marked with the “Track Changes” function in MS Word for easy viewing by editors and reviewers. Hopefully, with the changes made, our manuscript will be easier to follow and understand for the reader.

• Comment 2: Abstracts may be written in different ways to make them easy to read and understand. Please consider rewriting page 1 line 8 and lines 11-12.

Response: We have rewritten the sentences in question. For the sake of clarity, we have redrafted and simplified the Abstract. Hopefully we were able to improve readability. However, if the reviewer feels that further modification is needed in relation to this section, please let us know and we will try to change it.

• Comment 3: Page 3 line 115, please rearrange the spectrometry values in a table.

Response: The chemical composition of MK was given in a table form as requested. In addition, at the request of the other reviewer, the mineral composition was also tabulated. (Data of slag are also included in Tables 1 and 2.)

• Comment 4: Page 6 line 257; The effects of different pre-cursors on compressive strength and porosity need to be explained scientifically. According to Fig1, there is a strong CS and porosity variation between different stabilizers, which was not stated unambiguously.

Response: We agree with the reviewer that we did not really detail the effect of each stabilizer on strength and porosity. The aim of a comparative study with different foam precursors was to find the composition with which the best strength-porosity relationship can be achieved. In this sense, we did not consider it important to explain why stabilizers act differently on the physical properties of foams. Although with different absolute values, the course of the curves of foams prepared with 3 different precursors – relevant physical properties as a function of hydrogen peroxide concentration – is very similar. Differences in absolute values may be due to different compositions of soap precursors. The surfactant and foaming properties of soap precursors are greatly influenced by the properties of the fatty acids used. Although both olive oil and sunflower oil are sources of vegetable fatty acids, while olive oil is rich in monounsaturated chains, sunflower oil is rich in polyunsaturated chains. Sodium oleate is the sodium salt of oleic acid, a monounsaturated fatty acid. The nature of the fatty acids used greatly influences the surfactant and foaming properties of the soap precursors, which ultimately determine the physical properties of the foamed product. Taking into account the reviewer's comment, we have expanded the explanation of Figure 1.

• Comment 5: Page 6 lines 283-284-285; what is the authors' interpretation about the opposite outcome here stated as different findings between current work and given reference.

Response: The question is justified; we really did not provide an explanation for this. The reason for the different findings between current work and given reference may be due several factors:

1. The composition of the activating solutions was not the same in the two studies. While Bai et al. used a potassium-based activator solution for their experiments; we prepared a sodium-based activator solution for our research. In addition to the properties of the fatty acids used for saponification, the type of alkali used also greatly influences the foaming properties of soap precursors. Sodium-based foams are harder (more viscous) while potassium-based foams are less viscous [29].
2. The oils used came from other sources and therefore did not have the same composition. While in the given reference both olive oil and sunflower oil came from Italy; we used an olive oil from Spain and a sunflower oil from Hungary for our experiments. It should be noted that in the cited article, the authors did not provide information on the composition of the oils.

• The speed and time of mechanical mixing in the production of foams were not the same:

- Bai et al.: alkaline activating solution and MK were stirred at 800 rpm for 30 min, then oil and H₂O₂ were added to this mixture, the former was homogenized at 1200 rpm for 10 min and the latter at 600 rpm for 5 min.

- present study: alkaline activating solution and MK were stirred at 900 rpm for 5 minutes, then after oil addition the slurry was homogenized at 1200 rpm for another 5 minutes, and finally the H₂O₂ was added to this mixture and stirred at 600 rpm for 1 minute.

Overall, it can be said that the differences between the results reported in the given reference and in the present study can be influenced by the composition of the activating solution, the quality of the oils and the differences in the mixing parameters. A deeper interpretation of the phenomenon would require further experiments. Taking into account the reviewer's question, we have supplemented the relevant part of the manuscript.

• Comment 6: Page 12 line 9; According to generally accepted judgment, additive slag must increase the mechanical properties of GP foam. However, the authors have indicated the opposite outcome against general judgment. The negative effect of slag addition into GP foam without scientific explanation has no impact on the overall paper and needs to be removed completely from it.

Response: The comment is justified; a more detailed explanation is needed to understand this phenomenon. Partial replacement of MK with ground granulated blast furnace slag (GGBFS) is generally considered to have a beneficial effect on the mechanical properties of geopolymers. This general judgment is true for "dense" (non-foamed) geopolymers, especially when they are mixed with some aggregate (e.g. sand). In our experience, aggregate-free geopolymers tend to crack. The cited article [38] was interesting to us because it could have provided a solution for creating an aggregate-free yet non-cracking structure. Although it should be noted, the given reference only discusses the shrinkage/cracking properties of bulk (non-foamed) geopolymers. However, the shrinkage tendency of foams was not reported by the authors. It should also be noted that none of the relevant literature deals with the shrinkage/cracking behavior of blended MK-GGBFS based geopolymer foams.

In our study, we found that the shrinkage tendency of blended MK-GGBFS based geopolymer foams increases with increasing slag content (no aggregates were used to produce foams). The development of the properties in this way is, in our opinion, due to the fact that the geopolymerization processes take place faster in the case of GGBFS. Slag has high calcium content and thus has a higher pozzolanic reactivity than MK. However, the faster reaction results in the formation of a microstructure with a high pore content and thin cell walls. This weak microstructure cannot provide sufficient strength to handle the stresses resulting from volume change caused by shrinkage, which leads to cracking of the structure.

Shrinkage at high "rates" is also facilitated by the fact that the water content - required for incorporation but quasi-excess for geopolymerization - can escape faster from the connected pore structure formed during foaming. For this reason, the process of the above mentioned volume change leads to more intense cracking of the foams.

For the sake of better comprehensibility, we have supplemented the manuscript with the explanation given here.

• Comment 7: Page 15 Figure 10b, Tyme of analysis, time or type?

Response: I'm sorry I misspelled the x-axis caption in Figure 10b, thank you for drawing my attention to this error. The correct version is the Time of analysis; I have corrected this in the manuscript.

• Comment 8: The authors need to explain the uniqueness and value-added of the scientific paper in the conclusion part.

Response: Thank you for pointing this out. At the request of the reviewer, we have revised the "Conclusion" section of the manuscript. We have tried to better highlight the novelty and value-added of the research work.

• Comment 9: I would also suggest the authors to familiarize themselves with the possibility of foaming aluminum from recyclate: Ercoli, R.; Laskowska, D.; Nguyen, V.V.; Le, V.S.; Louda, P.; Łoś, P.; Ciemnicka, J.; Prałat, K.; Renzulli, A.; Paris, E.; Basilici, M.; Rapiejko, C.; Buczkowska, K.E. Mechanical and Thermal Properties of Geopolymer Foams (GFs) Doped with By-Products of the Secondary Aluminum Industry. Polymers 2022, 14, 703. doi: 3390/polym14040703

Response: Thank you for this suggestion. The proposed article contains valuable, useful information; it would have been interesting to explore this foaming process. However, in the case of our study, it seems slightly out of scope because in the literature review, we have shown that foams produced using only aluminum (or H₂O₂) are thermodynamically unstable systems with typically closed pores and inhomogeneous pore size distributions.

In the case of our study, the aim was to develop foam with controlled properties – high strength and high open porosity – and a homogeneous pore size distribution. The production of foams with such properties requires the use of a surfactant, which ensures adequate stability of the wet foam. In the proposed article, the authors used chopped carbon fibers to reinforce the foam. The foaming process was carried out with by-products of the secondary aluminum industry. Although high-strength foams can be produced by fiber reinforcement, the inhomogeneity of the pore size has not been improved (this fact is well observed in Figure 7).

We have also tried to improve the mechanical properties of the foams with fiber reinforcement (kaolin wool fibers were added to the wet foam). However, we found that fiber addition had no effect on the physical properties; the strength, porosity and thermal conductivity results of the fiber-free and fiber-reinforced specimens were very similar. For this reason, fiber reinforcement is not recommended for the production of the high porosity foams we have developed. However, if the reviewer feels that the proposed article may provide valuable information for understanding the present study, we will, of course, include this article in the right place of "Introduction" section.

Round 2

Reviewer 2 Report

I would like to thank the authors for including my suggestions in the publication. I wish you much success in your further scientific work.
I accept the paper as it stands.