Alkali-Activated Metakaolins: Mineral Chemistry and Quantitative Mineral Composition

Round 1

Reviewer 1 Report

Manuscript number: minerals-1920367

Title: Alkali-Activated Metakaolins: Effect of Phase Composition on Stability in Water and Compressive Strength

Reviewer's comments:

The present study focused on two objectives. The first objective was to evaluate phase composition from the bulk chemical analysis using the CQMA procedure and the stability of metakaolin alkali-activated for 28 days. The second task was to search for possible relationships between the quantitative amount of crystalline and non-crystalline phases in alkali-activated metakaolins and compressive strength. As for the manuscript, the structure is reasonable and the method is appropriate. 

This is an interesting and meaningful topic, but there are many problems with grammar and the language needs to be further improved. 

(1)As for Title, "……Stability in Water and Compressive Strength". The expression is ambiguous: "……Stability in Water and Stability in Compressive Strength" or "……Stability in both of Water and Compressive Strength". Please correct.

(2)In the Abstract, the full name is given as "CQMA". The explanation is given for "CS 27 values".

(3)The introduction does not provide an overview of why the necessary stability in water was studied. Suggested addition and clarification to align with the two objectives of the abstract.

(4)Line 86, "dried at 65 for 20 h. "should be" dried at 65℃ for 20 h."

(5)Line 87 and Line 88, "2, 3 and 28 days". Please check hours or days.

(6)Line 146, "stirring of 1 g of sample and 100 mL of distilled water", Please specify whether sample is a powder or a block?

(7)Line 179, Line193, Line 211, for Figure 1, Figure 2, and Figure 3, the provided pictures are not clear, which affects the reviewers' review.

(8)Line 230, Figure 4, What are "MIR spectra" and "NIR spectra"?

Comments for author File: Comments.pdf

Author Response

Reviewer #1 (R1): The present study focused on two objectives. The first objective was to evaluate phase composition from the bulk chemical analysis using the CQMA procedure and the stability of metakaolin alkali-activated for 28 days. The second task was to search for possible relationships between the quantitative amount of crystalline and non-crystalline phases in alkali-activated metakaolins and compressive strength. As for the manuscript, the structure is reasonable and the method is appropriate. 

Response:

Thank you for your positive comments given in 7 points. The corrections in the manuscript (Action) are marked as R1-1, R1-2,……R1-7.

• As for Title, "……Stability in Water and Compressive Strength". The expression is ambiguous: "……Stability in Water and Stability in Compressive Strength" or "……Stability in both of Water and Compressive Strength". Please correct.

Response: We considered aspects of the title suggested by the reviewers. The title has therefore been changed and we believe that it will be more in line with the content of the article (see below for details).

Action:

The title was changed to:

“Alkali-Activated Metakaolins: Mineral Chemistry and Quantitative Mineral Composition” R1-1

• In the Abstract, the full name is given as "CQMA". The explanation is given for "CS values".

Response: Thank you for pointing out this deficiency.

Action:

Chemical Quantitative Phase Analysis (CQMA) was corrected, R1-2.

Compressive Strength (CS) was corrected, R1-2.

• The introduction does not provide an overview of why the necessary stability in water was studied. Suggested addition and clarification to align with the two objectives of the abstract.

Response: Yes, in abstract and in the objectives clarification was added.

Action:

It was inserted into the abstract: The first objective was to evaluate mineral chemistry and quantitative mineral phase composition from the bulk chemical analysis using the Chemical Quantitative Mineral Analysis (CQMA) procedure and the comparison of the chemistry of metakaolins after alkali-activation for 28 days according to the elements Al, Si, Na and K using the leaching test in distilled water. R1-3.

In Introduction, the following text has been added to the objective. R1-3.

Quantitative calculation of phases of metakaolins and phases formed after their alkaline reaction with NaOH after 28 days is carried out after testing the stability of the structures during the leaching of elements Al, Si, Na and K in distilled water.

• Line 86, "dried at 65 for 20 h. "should be" dried at 65℃ for 20 h."
• Line 87 and Line 88, "2, 3 and 28 days". Please check hours or days.

Response: The 2.1. Samples preparation was corrected and rewritten.

Action:

The text was added and marked as R1-4,5:

Alkali-activated metakaolin samples AM1, AM2 and AM3 were prepared by mixing of each metakaolin M1, M2 and M3 (10 g) with 8M NaOH (8 mL). The fresh mixtures were formed into cubic mold 2×2×2 cm and left to activate for a period of 2, 3 and 28 days under laboratory conditions. Samples for analysis were taken out of the molds after each time of 2, 3 and 28 days and marked for 2 days as AM1-2, AM2-2, AM3-2, for 3 days as AM1-3, AM2-3, AM3-3 and for 28 days as AM1-28, AM2-28, AM3-28.

• Line 146, "stirring of 1 g of sample and 100 mL of distilled water", Please specify whether sample is a powder or a block?

Action:

Sample was specified: "stirring of 1 g of a powder sample and 100 mL of distilled water" marked R1-6.

• Line 179, Line193, Line 211, for Figure 1, Figure 2, and Figure 3, the provided pictures are not clear, which affects the reviewers' review.

Figures have been modified:

Figure 1

Figure 2

Figure 3

(8) Line 230, Figure 4, What are "MIR spectra" and "NIR spectra"?

Response: It was explained in Methods, L123:

…“the mid-IR (MIR, 4000–400 cm⁻¹) or near-IR (NIR, 8000–4000 cm⁻¹) range.  

Reviewer 2 Report

Overall comment:

In this paper, there are a lot of vague and wrong statements, which makes the whole manuscript hard to follow. Besides, the entire text needs a rigorous language revision since the whole English language is poor. Overall, the novelty and analysis are weak, and I recommend rejecting this article in this form.

·                       Line 15-17, this sentence is vague, and it is hard to follow.

·                       Line 17- 18; this sentence is not grammatically correct.

·                       Line 25-27,^ . The compressive strengths from 6.42±0.33 to 9.97±0.50 MPa related positively to H2O+ and H2O bound in HS and TN, 26 while negatively to the amount of alkali-activated metakaolinite (MKA) and Z-A.^ the whole sentence does not make sense.

·                       The whole introduction does not have coherency and fluency. Besides poor english language, authors start with kaolin and metakaolin then they jump into crystal analysis, then they jump back to introducing metakaolin, and …. (Sentence in line talks about the same idea as line 62-66)

·                       Line 80-82, *Kaolin samples 1, 2 and 3 were sieved to the size fraction ˂ 40 µm and then were heat-treated at 800 °C in the air for 3 h to the metakaolin (M) samples which were denoted as M1, M2 and M3, respectively. * this sentence is vague and wrong( heat-treated at 800 °C in the air????????)

·                       Line 83, no one uses the word *doughs* for the geopolymer or cement. All use paste when you have only binder without aggregate.

·                       Line 409-414, the whole paragraph does not make sense.

….

Technical Comments

•                          Technically, the authors claim that they want to relate the phase analysis to the mechanical properties of metakaolin-based geopolymer. Since many factors can affect the materials' properties, like the sample's porosity, the powder's reactivity, particle size, specific surface area, the morphology of particles, and so forth, the authors did not consider them. And even based on their data in Tables 1 to 4, all samples have almost the same chemical composition and crystal amounts, but the compressive strength of AM3-28 is 40 percent lower than AM1-28 and AM2-28. As a result, the whole discussion is not correct.

•                          It is ubiquitous to use the XRD test for crystallography, and this method has been used for many years and has acceptable accuracy. Here authors used XRF and CQMA procedures for measuring crystal amount. Still, they must validate their method with at least an XRD test (they only presented the XRD spectrum without any internal or external standards and did not do any quantitative analysis on it).

•                          In figure 9, the authors have only three data, and they did a regression analysis on three that need more data to have a significant result.

Author Response

In this paper, there are a lot of vague and wrong statements, which makes the whole manuscript hard to follow. Besides, the entire text needs a rigorous language revision since the whole English language is poor. Overall, the novelty and analysis are weak, and I recommend rejecting this article in this form.

• Line 15-17, this sentence is vague, and it is hard to follow. It was deleted.
• Line 17- 18; this sentence is not grammatically correct.

The reaction products resulting from alkali-activation of metakaolin are impacted by the composition of the initial kaolin, amount of alkali-activated kaolinite and water. The present study focused to analyze these parameters on the metakaolins

• Line 25-27,^ . The compressive strengths from 6.42±0.33 to 9.97±0.50 MPa related positively to H2O+ and H2O bound in HS and TN, 26 while negatively to the amount of alkali-activated metakaolinite (MKA) and Z-A.^ the whole sentence does not make sense.

The compressive strengths (CS) from 6.42±0.33 to 9.97±0.50 MPa has been related positively to H₂O+ and H₂O bound in HS and TN.

• The whole introduction does not have coherency and fluency. Besides poor english language, authors start with kaolin and metakaolin then they jump into crystal analysis, then they jump back to introducing metakaolin, and …. (Sentence in line talks about the same idea as line 62-66).

Introduction was rewritten:

Metakaolin from high-quality kaolin is formed by calcination at 650–850 °C [6–9]. The reaction products of metakaolin alkali-activation process confirmed the presence of remaining kaolinite not fully reacted with the alkali-activator in metakaolin [10–11]. The alkaline activation of metakaolin with NaOH solution involves reactions that produce an amorphous hydrated sodium aluminosilicates of the general formula Na₂O∙3SiO₂∙Al₂O₃∙3H₂O with a predominant three-dimensional structure [12].

X-ray diffraction (XRD) and quantitative phase analysis have limited capabilities for studying amorphous phases. Traditionally, powder XRD deals with the characterization of polycrystalline materials that contain amorphous or weakly crystalline phases. The contributions of the intensities of the amorphous phases to the diffraction pattern are not always obvious, especially at low concentrations. Available method for quantitative phase analysis of these materials is quantitative XRD coupled with Rietveld analysis. However, their accuracy is highly dependent on the 'degree of crystallinity' of the solids present. X-ray amorphous phase cannot be directly measured and the crystalline phases in the presence of amorphous/unidentified crystalline phases may be significantly overestimated [13].

 Currently, the quantitative phase analysis of amorphous components in the mixture was performed using the direct-derivation method including a background-subtracted halo pattern together with profile models for crystalline components [14].

The fraction of reacted metakaolin has been determined quantitatively for four geopolymer samples using two XRD methods and point counting by SEM [15]. Another method integrates XRD data of identified minerals with relevant chemical analysis using the Chemical Quantitative Mineral Analysis (CQMA) procedure [16,17]. CQMA procedure performs recalculation of the bulk chemical analyses (XRF) to the quantitative number of phases occurring in all crystallization stages and refines the mineral composition.

The present study focused on the experimental dehydroxylation of kaolins and the formation of metakaolins at 800 °C. The alkali-activation of metakaolins to geopolymer precursors was characterized for 2, 3 and 28 days. The first objective was to evaluate mineral chemistry and quantitative mineral phase composition from the bulk chemical analysis using the CQMA procedure and the comparison of the chemistry of metakaolins after alkali-activation for 28 days according to the elements Al, Si, Na and K using the leaching test in distilled water. The second task was solving the relation between the quantitative mineral composition of metakaolins alkali-activated for 28 days and compressive strengths.

• Line 80-82, *Kaolin samples 1, 2 and 3 were sieved to the size fraction ˂ 40 µm and then were heat-treated at 800 °C in the air for 3 h to the metakaolin (M) samples which were denoted as M1, M2 and M3, respectively. * this sentence is vague and wrong( heat-treated at 800 °C in the air????????)
• Line 83, no one uses the word *doughs* for the geopolymer or cement. All use paste when you have only binder without aggregate.
• Line 409-414, the whole paragraph does not make sense.

Response:

The text was rewritten Kaolin samples 1, 2 and 3 were sieved to the size fraction ˂ 40 µm and then were calcined at 800 °C for 3 h to the metakaolin (M) samples which were denoted as M1, M2 and M3, respectively. Alkali-activated metakaolin samples AM1, AM2 and AM3 were prepared by mixing of each metakaolin M1, M2 and M3 (10 g) with 8M NaOH (8 mL). The fresh mixtures were formed into cubic mold 2×2×2 cm and left to activate for a period of 2, 3 and 28 days under laboratory conditions. Samples for analysis were taken out of the molds after each time of 2, 3 and 28 days and marked for 2 days as AM1-2, AM2-2, AM3-2, for 3 days as AM1-3, AM2-3, AM3-3 and for 28 days as AM1-28, AM2-28, AM3-28, dried at 30 °C for 12 h, ground for 1 min on a vibrating mill, and then manually pulverized with an agate mortar and pestle.

Technical Comments

• Technically, the authors claim that they want to relate the phase analysis to the mechanical properties of metakaolin-based geopolymer. Since many factors can affect the materials' properties, like the sample's porosity, the powder's reactivity, particle size, specific surface area, the morphology of particles, and so forth, the authors did not consider them. And even based on their data in Tables 1 to 4, all samples have almost the same chemical composition and crystal amounts, but the compressive strength of AM3-28 is 40 percent lower than AM1-28 and AM2-28. As a result, the whole discussion is not correct.

Response: Yes, we understand that many other factors can affect the mechanical properties. From many of these parameters, this work is addressed to chemistry and phase composition, which has been solved sporadically in the literature so far.

In order not to mislead the intention of this work, the title has been changed to:

 “Alkali-Activated Metakaolins: Mineral Chemistry and Quantitative Mineral Composition”.

Although there are only 3 samples of metakaolins, a test of their mechanical resistance after 28 days of alkali activation was performed. Similarly, in the literature, only 4 samples of metakaolins were tested for MPa, when it was found that the fraction of reacted metakaolin has been varied from 2.6 to 38% for geopolymers with compressive strengths varying from 3.1 to 67 MPa [15].

In this work, the trend of different values was similar to published [45].

“And even based on their data in Tables 1 to 4, all samples have almost the same chemical composition and crystal amounts, but the compressive strength of AM3-28 is 40 percent lower than AM1-28 and AM2-28. As a result, the whole discussion is not correct.”

We really cannot agree with this statement which contradicts the given results and their discussion in this work.

 .

Author Response File: Author Response.pdf

Reviewer 3 Report

The article is about three different metakaolin used in alkali-activated materials and the effect of phase composition on properties. The results are interesting, but the authors should revise some points before publishing. The article is written appropriately, with minor revisions needed. The main comments about the paper are addressed in the attached document.  

Comments for author File: Comments.pdf

Author Response

Reviewer #3:

Mistakes on the L19, 23, 28, 46 and 49 were corrected.

Sample preparation on L86-90: The text was revised:

Alkali-activated metakaolin samples AM1, AM2 and AM3 were prepared by mixing of each metakaolin M1, M2 and M3 (10 g) with 8M NaOH (8 mL). The fresh mixtures were formed into cubic mold 2×2×2 cm and left to activate for a period of 2, 3 and 28 days under laboratory conditions. Samples for analysis were taken out of the molds after each time of 2, 3 and 28 days and marked for 2 days as AM1-2, AM2-2, AM3-2, for 3 days as AM1-3, AM2-3, AM3-3 and for 28 days as AM1-28, AM2-28, AM3-28.

L143-152: Yes, the topics were placed as 2.2.1 and 2.2.2

Figures 1, 2, 3 and 5 were corrected:

Figure 1

Figure 2

Figure 3

Figure 5

In Table 4: The alkali-activated materials are mainly formed by amorphous materials. In this case, it is expected that some parts of the material are not MK and are not zeolites or other crystalline phases from raw materials. Why this amorphous alkali-activated material is not considered here.

Please, accept the text in 3.5. CQMA Calculations, which I hope will explain the correct understanding of the given results:

The bulk chemical analyses of metakaolin samples alkali-activated for 28 days (AM-28 in Table 2), mineral phases identified in XRD patterns (Figure 3) and their crystallochemical formulas were used for the calculation of quantitative number of phases (Equation (2)). Chemistry of crystalline and metastable fragments (amorphous) of proto-zeolitic phases of zeolites Z-A (Na₁₂Al₁₂Si₁₂O₄₈) and HS (Na₆Al₆Si₆O₂₄·8H₂O) was calculated according to the reaction of metakaolinite with NaOH solution, according to Equation (6):

Regarding the quantity of accompanying minerals M, Q, and F (Figure 7b), the wt.% in MK (left bar) and in alkali-activated metakaolins AM-28 (right bar) are in good agreement which confirms their-non-reactivity during alkaline activation.

Figure 7. Graphical comparison of phases (wt. %) calculated by CQMA in: (a) MK in metakaolins (M) (left bar) and alkali-activated AM-28 products TN, HS, Z-A and MK_Un-28 (right bar); (b) muscovite (M), quartz (Q) and orthoclase (F) in metakaolins (M) (left bar) and in alkali-activated AM-28 products (right bar).

Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

The authors have answered the questions posed and revised the manuscript. Recommended for acceptance.

Author Response

Reviewer #1 (R1): The present study focused on two objectives. The first objective was to evaluate phase composition from the bulk chemical analysis using the CQMA procedure and the stability of metakaolin alkali-activated for 28 days. The second task was to search for possible relationships between the quantitative amount of crystalline and non-crystalline phases in alkali-activated metakaolins and compressive strength. As for the manuscript, the structure is reasonable and the method is appropriate. 

Response:

Thank you for your positive comments given in 7 points. The corrections in the manuscript (Action) are marked as R1-1, R1-2,……R1-7.

• As for Title, "……Stability in Water and Compressive Strength". The expression is ambiguous: "……Stability in Water and Stability in Compressive Strength" or "……Stability in both of Water and Compressive Strength". Please correct.

Response: We considered aspects of the title suggested by the reviewers. The title has therefore been changed and we believe that it will be more in line with the content of the article (see below for details).

Action:

The title was changed to:

“Alkali-Activated Metakaolins: Mineral Chemistry and Quantitative Mineral Composition” R1-1

• In the Abstract, the full name is given as "CQMA". The explanation is given for "CS values".

Response: Thank you for pointing out this deficiency.

Action:

Chemical Quantitative Phase Analysis (CQMA) was corrected, R1-2.

Compressive Strength (CS) was corrected, R1-2.

• The introduction does not provide an overview of why the necessary stability in water was studied. Suggested addition and clarification to align with the two objectives of the abstract.

Response: Yes, in abstract and in the objectives clarification was added.

Action:

It was inserted into the abstract: The first objective was to evaluate mineral chemistry and quantitative mineral phase composition from the bulk chemical analysis using the Chemical Quantitative Mineral Analysis (CQMA) procedure and the comparison of the chemistry of metakaolins after alkali-activation for 28 days according to the elements Al, Si, Na and K using the leaching test in distilled water. R1-3.

In Introduction, the following text has been added to the objective. R1-3.

Quantitative calculation of phases of metakaolins and phases formed after their alkaline reaction with NaOH after 28 days is carried out after testing the stability of the structures during the leaching of elements Al, Si, Na and K in distilled water.

• Line 86, "dried at 65 for 20 h. "should be" dried at 65℃ for 20 h."
• Line 87 and Line 88, "2, 3 and 28 days". Please check hours or days.

Response: The 2.1. Samples preparation was corrected and rewritten.

Action:

The text was added and marked as R1-4,5:

Alkali-activated metakaolin samples AM1, AM2 and AM3 were prepared by mixing of each metakaolin M1, M2 and M3 (10 g) with 8M NaOH (8 mL). The fresh mixtures were formed into cubic mold 2×2×2 cm and left to activate for a period of 2, 3 and 28 days under laboratory conditions. Samples for analysis were taken out of the molds after each time of 2, 3 and 28 days and marked for 2 days as AM1-2, AM2-2, AM3-2, for 3 days as AM1-3, AM2-3, AM3-3 and for 28 days as AM1-28, AM2-28, AM3-28.

• Line 146, "stirring of 1 g of sample and 100 mL of distilled water", Please specify whether sample is a powder or a block?

Action:

Sample was specified: "stirring of 1 g of a powder sample and 100 mL of distilled water" marked R1-6.

• Line 179, Line193, Line 211, for Figure 1, Figure 2, and Figure 3, the provided pictures are not clear, which affects the reviewers' review.

Figures have been modified:

Reviewer 2 Report

Comments:

The introduction is still vague and needs to be revised significantly. The introduction should talk more about the topic under concern in the article. I recommend revising and adding these statements:

Include this statement in the first paragraph “Over the last years, the research focus has been on the feasibility of using this “green” binder instead or as a supplement to conventional cement applications” References: Aldred, James, and John Day. "Is geopolymer concrete a suitable alternative to traditional concrete." Proceedings of the 37th Conference on Our World in Concrete & Structures, Singapore. 2012. And Ranjbar, et al. "Rheological characterization of 3D printable geopolymers." Cement and Concrete Research 147 (2021): 106498. And "Effects of heat and pressure on hot-pressed geopolymer." Construction and Building Materials 231 (2020): 117106.

Include this statement in the second paragraph “In theory, the whole aluminosilicate precursor could potentially be dissolved and polymerized. However, this has rarely been reported even for pure and highly reactive precursors such as metakaolin (MK)” References: Kuenzel, C., et al. "Influence of metakaolin characteristics on the mechanical properties of geopolymers." Applied Clay Science 83 (2013): 308-314. and navid et al"Hardening evolution of geopolymers from setting to equilibrium: A review." Cement and Concrete Composites 114 (2020): 103729.

Page 2, line 52, how about Backscattered electron image analysis?  

Change “MIR” to “IR” in the whole manuscript. 

Author Response

[email protected] Reviewer #2:

Comments and Suggestions for Authors

Comments:

The introduction is still vague and needs to be revised significantly. The introduction should talk more about the topic under concern in the article. I recommend revising and adding these statements:

Include this statement in the first paragraph “Over the last years, the research focus has been on the feasibility of using this “green” binder instead or as a supplement to conventional cement applications” References: Aldred, James, and John Day. "Is geopolymer concrete a suitable alternative to traditional concrete." Proceedings of the 37th Conference on Our World in Concrete & Structures, Singapore. 2012. And Ranjbar, et al. "Rheological characterization of 3D printable geopolymers." Cement and Concrete Research 147 (2021): 106498. And "Effects of heat and pressure on hot-pressed geopolymer." Construction and Building Materials 231 (2020): 117106.

Include this statement in the second paragraph “In theory, the whole aluminosilicate precursor could potentially be dissolved and polymerized. However, this has rarely been reported even for pure and highly reactive precursors such as metakaolin (MK)” References: Kuenzel, C., et al. "Influence of metakaolin characteristics on the mechanical properties of geopolymers." Applied Clay Science 83 (2013): 308-314. and navid et al"Hardening evolution of geopolymers from setting to equilibrium: A review." Cement and Concrete Composites 114 (2020): 103729.

Dear Reviewer, thank you for your suggestions. We have enriched the Introduction for additional information.  Our article is focusing on specific area of phase analysis using several key analytical techniques along with explanation of chemical composition. To keep the study clear and straight for the reader, we didn’t want to go for further applicational options.

You are right in the sense of future applications, to mention all parameters of recently applicable geopolymers. We have decided to include two publications suggested in the first paragraph dealing with general knowledge of substitutability traditional concrete with geopolymers: 1. Aldred, James, and John Day. "Is geopolymer concrete a suitable alternative to traditional concrete." Proceedings of the 37th Conference on Our World in Concrete & Structures, Singapore. 2012 and 2. Ranjbar, et al.: Hardening evolution of geopolymers from setting to equilibrium: A review." Cement and Concrete Composites 114 (2020): 103729.

Page 2, line 52, how about Backscattered electron image analysis?  

In our work, the BSE images to quantify different microstructural estimation at macro-scale levels is not considered.

Change “MIR” to “IR” in the whole manuscript. 

The abbreviation Mid-IR is regularly explained (2.2. Characterization Techniques), and we wish to use this abbreviation in our manuscript.