Sample Origin Effect on Chemical Reactivity of Tajogaite Volcanic Ashes for Ancient Mortar Repair

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

the file is attached

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Several problematic sentences i marked in the review.

Author Response

We thank Reviewer 1 for their time, constructive feedback, and helpful suggestions, which have all

owed us to improve the scientific quality and clarity of our manuscript. We have carefully revised the manuscript in response to each of the comments. Below, we address each of the reviewer’s points in detail.

Reviewer comments are shown in blue, and our responses are in bold. All corresponding changes in the manuscript are marked and highlighted in the revised version.

Reviewer 1:

Comment:
The study has potential for industrial application. Unfortunately, the report raises numerous questions. I suggest that the authors have possibilities to improve the quality of the presentation.

Response:
We appreciate the reviewer’s overall assessment. We have carefully revised the manuscript to improve its structure, clarity, and scientific rigor, addressing all specific issues raised in the following points.

Comment:

The study has potential for industrial application. Unfortunately, the report raises numerous questions. I suggest that the authors have possibilities to improve the quality of the presentation.

Major drawbacks are absence of major chemical analyses of ash. Although the data on trace elements are also of importance. It is likely that such data are available from geological organizations. Using SEM it is possible to get the data on the amorphous phases. It is not clear why it was not done. The volume ratio of amorphous phases was not obtained. According to figure 8, glassy/amorphous phases strongly dominate. These data are of primary importance for industrial application and for the clarification of study results.

 

The title of the paper is very strange. It is impossible that studied samples show any relation with geolocation. They were collected directly after and during the volcanic explosion. Thus, they are not altered. Moreover, the reported area of ash deposit is about 10 km2 (it is not so according to figure 1). They sampled from different heights, but there is nothing about hydrological and climate conditions. So, any potential to discuss geolocation of samples is absent.

Response:

Thank you for your comment. We agree that, since the samples were collected immediately during and after the volcanic eruption, there was limited time for environmental factors to cause significant alterations. Consequently, we acknowledge that the potential to discuss geolocation-based variability is low, as has been just confirmed in this study; previously it was only a potential result assumption. We have modified the title accordingly to better reflect the content and removed/discussed more cautiously any reference to spatial differentiation due to geolocation. We also corrected the inconsistency regarding the ash deposit area and clarified sampling conditions in the revised manuscript; The Figure 1 shows only an image detail of 1 position of the 15 collected samples.

“Tajogaite Volcanic Ashes as Pozzolanic Additions in the Conservation of Ancient Mortars: A Reactivity Study”

Comment:

Table 1. AAVV samples?

Response:

This is the abbreviation for volcanic ash, although this nomenclature has not been indicated in the text before. The caption of the table has been corrected to enhance its clarity.

Comment:

153 Augite is clinopyroxene and not related to feldspar family.

Response:

The text has been updated to correct this misconception by accurately classifying augite.

Comment:

According to minerals revealed in the samples, the composition of tephra is likely basalt or alkali basalt. The serious drawback of the study was that it did not determine the crystalline and amorphous ratio of rocks. For example, it is possible to evaluate making rock thin sections. It is possible to use epoxy as cementing agent and then prepare a thin section. Such technique is well known. Using an optical microscope, it is possible to get such data.

Response:

We appreciate the reviewer’s comment regarding the determination of the crystalline and amorphous phases in the tephra samples. However, in this study we employed the Fratini test and the lime saturation method (DSC), which are well-established procedures specifically designed to assess the pozzolanic reactivity and cementitious behavior of lime-based materials. Given that the samples were analyzed in powdered form, the chosen methodology—widely accepted in binder chemistry—does not require thin section analysis.

While petrographic analysis of thin sections impregnated with epoxy would undoubtedly provide interesting complementary information, especially regarding the microstructure, it would significantly expand the scope of the present study. Although the determination of crystalline and amorphous phases through thin section microscopy would offer valuable additional information, such analysis falls outside the scope of the present work. Future studies may include this approach to complement the results presented here with a more detailed mineralogical characterization.

Comment:

Figure 6 and other figures. The authors must use mineral abbreviation symbols. For example, Au is a symbol of gold.

Response:

The reviewer's feedback is understandable; the use of abbreviations that coincide with chemical elements can lead to misunderstandings. The Kretz abbreviations have been used to modify both figures and legends in instances where this error has occurred.

Comment:

223 SEM

Response:

It has been modified

Comment:

Figure 8. What are scales?

Response:

Figure 8 has been modified to improve the visibility of the scales.

Comment:

Figure 9. Minor meaning of such data. These are data on different mineral phases shown in the mixing manner. According to SEM these are mainly data on olivine, clinopyroxene, and albite. More interesting is the composition of amorphous phases.

Response:

We appreciate the reviewer’s comment regarding Figure 9. In our study, the identification of the main crystalline phases (such as olivine, clinopyroxene, and albite) serves only as general background information to describe the nature of the tephra. However, since our objective was to assess the pozzolanic and cementitious behavior of the material, the detailed characterization of crystalline versus amorphous phases was not required. The methods used—Fratini and lime saturation (DSC)—are specifically designed to evaluate the reactivity of amorphous components in powdered samples, without needing phase quantification. Nevertheless, we agree that further investigation into the composition and structure of the amorphous phase would be valuable in future studies aimed at a more comprehensive understanding of the material’s reactivity.

We agree with the reviewer that understanding the chemical composition of the amorphous phases is essential, as these are the most reactive components. In our study, the EDX analyses were not limited to crystalline phases such as olivine or augite, but were also conducted on the ash matrix, also targeting amorphous phases areas. This approach provided a more complete picture of the overall composition of the samples, including both crystalline and amorphous phases.

Comment:

232 I suggest that such ratios are of meaning if they reflect the composition of amorphous phases.

Response:

As previously stated, the specific identification and quantification of amorphous phases was not necessary for the scope of this study. The methods applied—Fratini and lime saturation (DSC)—are standardized tests that effectively evaluate the pozzolanic reactivity of powdered materials without requiring detailed phase differentiation.

Comment:

283-284 Such statement must be supported by the available data on the rate of alkali feldspar dissolution.

Response:

We thank the reviewer for the helpful comment. Upon reviewing the cited literature, we acknowledge that references [63–64] focus on the role of iron-bearing phases and do not address the dissolution behavior of alkali feldspars such as albite. The sentence has been revised accordingly to reflect the actual content of the sources and to remove unsupported claims.

Comment:

asFigure 10. Both ratios correspond to alkali feldspar. Are these data obtained on amorphous phases?

Response:

We thank the reviewer for the question. The Si/Al and Na/Al molar ratios presented in Figure 10 were obtained from EDX analyses performed on various areas of the samples, selected based on morphological differences observed under SEM. However, EDX alone does not allow us to determine whether the analyzed areas correspond to amorphous or crystalline phases. Therefore, we cannot confirm whether these specific data points are from amorphous phases, although some regions may contain a glassy matrix.

Comment:

306 and below. Is it in wt.%?

Response:

We thank the reviewer for the observation. The data presented in line 306 and below are expressed in volume percent (vol.%), not weight percent (wt.%). This has now been clarified in the revised manuscript and explicitly indicated in the caption of Table 2 to avoid any confusion.

Comment:

308 The reference is on incorrect number of the figure.

Response:

That has been solved.

Comment:

321-323 It seems that the amorphous phase ratio is possible to evaluate by the wt.% of fixed CaO. I suggest that this value is known for silica fume. So, the comparison is possible.

Response:

The comment of reviewer is accurate. As indicated by the references provided, the percentage of CaO that can be fixed by silica fume is known. This indicates that the pozzolanic activity of the volcanic ash under study is comparable to that of silica fume. The original sentence may not have been clear enough. For this reason, it has been rephrased to ensure that there are no misunderstandings.

Comment:

Table 2. Is it in wt.%?

Response:

We thank the reviewer for the observation. The data presented are expressed in volume percent (vol.%), not weight percent (wt.%). This has now been clarified in the revised manuscript and explicitly indicated in the caption of Table 2 to avoid any confusion.

Comment:

349 Fratinni test was formerly abbreviated Ft.

Response:

It has been amended.

Comment:

357-359 Is it possible to show data on figure 12?

Response:

We appreciate the reviewer’s suggestion and agree that including those data in Figure 12 will improve the clarity and completeness of the information. The figure has been updated accordingly in the revised version of the manuscript.

Comment:

Figure 12. What units are used for both axis?

Response:

It has been amended.

Comment:

406 From where are these data?

Response:

Thank you for your valuable comment. We have clarified the text to improve its readability by specifying that hydrotalcite was detected by X-ray diffraction (XRD). Furthermore, we indicate that its composition corresponds to [Mg₆Al₂CO₃(OH)₁₆·4(H₂O)], with carbonate anions in the interlayer, likely due to the excess of carbonates present in the aqueous medium.

Ref 1:  Cavani, F., Trifirò, F., & Vaccari, A. (1991). Hydrotalcite-type anionic clays: Preparation, properties and applications. Catalysis Today, 11(2), 173-301. https://doi.org/10.1016/0920-5861(91)80001-B

Ref 2:  Evans, D. G., & Slade, R. C. T. (2006). Structural aspects of layered double hydroxides. Structure and Bonding, 119, 1–87. https://doi.org/10.1007/430_006

Ref 3:  Rives, V. (Ed.). (2001). Layered Double Hydroxides: Present and Future. Nova Science Publishers.

Comment:

Figure 13. Ac is in the caption but not shown. XRD shows no indication that clinopyroxene, albite, and olivine participate in the reaction.

 

Response:

Please note that the reference to Ac has been removed from the caption, as it does not appear in the figure. We concur with the reviewer's assertion that the crystalline phases do not appear to be implicated in the reaction. You can find more information about this in paragraphs 386 to 394, where Figure 13 is mentioned for the first time.

Comment:

427-431 Not clearly written.

Response:

The text has been changed for better understanding

Comment:

461-464 At first high Ca/Si ratio and then low CaO content and higher SiO2.

Response:

We have rewritten the paragraph to ensure optimal clarity. The concept is that elevated Ca/Si ratios result in the formation of short polymeric chains. The samples analysed by EDX have low Ca/Si ratios, indicating the likelihood of obtaining long polymer chains. As that the signal due to Q1 units is weak, indicates that long chains are being formed.

Comment:

535-537 not clear.

Response:

The sentence has been changed to make it clearer what it means.

Comment:

There is used eclectic mode of ref list presentation.

Response:

References have been updated in accordance with the MDPI References Style Guide.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper examines volcanic ashes coming from the area surrounding the Tajogaite volcano in the Canary Islands. The samples were collected from different locations after the eruption in 2021. A comprehensive characterization has been performed using XRD, FT-IR, and SEM/EDS. A direct method has been used to prove the pozzolanic activity of the sampled volcanic ashes.

I would suggest some changes before the publication. The following are my comments and suggestions:

- The novelty of this study needs to be better highlighted. In fact, the method applied to assess pozzolanic activity is known, and the pozzolanic activity of Tajogaite volcanic ashes has already been demonstrated (https://doi.org/10.1016/j.conbuildmat.2024.135498).

- The aspects related to sustainability should be emphasized.

- The title does not feel effective. On the other hand, the paper focuses only on the characterization of the ashes and on their pozzolanic activity; the influence of geolocation is a secondary aspect instead and there has been no evidence of any correlation.

- Why was an effect of geolocation expected?

- Why were these ashes suggested only for use in ancient mortars?

- Use the terms 'SEM/EDS' or 'SEM/EDX' consistently throughout the text.

- Keywords: remove the numbers.

- Line 92: explain the meaning of “VVAA”.

- Table 2: what refers to coordinates X and Y? What is the method for pinpointing the coordinate plane?

- Line 100, provide a definition for the acronym DSC.

- Line 135: replace “phases” with “components”.

- Line 136: add the operating conditions. Which acquisition mode has been used? Have pellets been prepared?

- Line 148: do you mean "diffraction lines"?

- Lines 223-224: rephrase this sentence as its meaning is unclear.

Comments on the Quality of English Language

The language is not poor, but the English style requires revision.

Author Response

This paper examines volcanic ashes coming from the area surrounding the Tajogaite volcano in the Canary Islands. The samples were collected from different locations after the eruption in 2021. A comprehensive characterization has been performed using XRD, FT-IR, and SEM/EDS. A direct method has been used to prove the pozzolanic activity of the sampled volcanic ashes.

I would suggest some changes before the publication. The following are my comments and suggestions:

Comment:

- The novelty of this study needs to be better highlighted. In fact, the method applied to assess pozzolanic activity is known, and the pozzolanic activity of Tajogaite volcanic ashes has already been demonstrated (https://doi.org/10.1016/j.conbuildmat.2024.135498).

Response:

We appreciate the reviewer’s comment and the opportunity to clarify the novelty of our study. While it is true that the pozzolanic activity of Tajogaite volcanic ash has been previously addressed, we would like to point out several key differences between our work and the referenced study.

First, the method we used to assess pozzolanic activity is the Frattini test (EN 196-5:2011), which is a standardized chemical test, while the referenced paper evaluates pozzolanicity through mechanical strength development of cementitious mortars. This makes the two approaches complementary, but methodologically distinct. Additionally, our experimental design does not include cementitious mixes for the Frattini test, unlike the approach in the cited work.

Second, our study includes a long-term evaluation of pozzolanic activity through strength development up to 90 days, whereas the referenced work reports mechanical strengths only at 28 days. This allows us to provide additional insight into the pozzolanic reactivity over time.

We also acknowledge the relevance of the amorphous content quantification presented in the cited study, which complements our findings. For completeness and to strengthen our discussion, we have now cited this paper in the revised manuscript and highlighted the methodological differences and complementary nature of the two studies.

Finally, we believe both studies must have been developed in parallel, as the cited paper was published very recently and after we had completed the main stages of our own research.

 

 

Changes made in the manuscript:

• The aforementioned reference has been added to the Discussion section.
• A paragraph has been included to clarify the novelty of our work in relation to the cited study.

 

Comment:

- The aspects related to sustainability should be emphasized.

Response:

This text has been included in the introduction section:

The valorization of volcanic ash as a supplementary cementitious material aligns with key principles of sustainability in the construction sector. By utilizing a locally available and abundant natural by-product of the Tajogaite eruption, the environmental impact associated with the extraction and processing of conventional raw materials is significantly reduced. Moreover, the partial replacement of Portland cement with volcanic ash contributes to lowering CO₂ emissions, as cement manufacturing is one of the largest industrial sources of greenhouse gases. This approach supports circular economy strategies by transforming a potentially problematic volcanic residue into a valuable resource for eco-efficient building materials. The methodology adopted in this study, based on standardized tests and extended durability analysis, reinforces the feasibility of sustainable material integration in real-world applications. - The title does not feel effective. On the other hand, the paper focuses only on the characterization of the ashes and on their pozzolanic activity; the influence of geolocation is a secondary aspect instead and there has been no evidence of any correlation.

In this sense, the conclusions section has been updated.

Comment:

- Why was an effect of geolocation expected?

Response:

An effect of geolocation was expected due to possible variations in cooling rates and deposition conditions during the eruption, which could influence the ash's chemical composition and amorphous content.

Comment:

- Why were these ashes suggested only for use in ancient mortars?

 

Response:

These ashes are not limited to use in ancient mortars; their pozzolanic properties make them suitable for broader applications in modern cementitious systems. However, our focus on historical mortars stems from our research context, as volcanic ash was traditionally used in Roman construction. Studying its behavior in lime-based mortars allows us to assess its compatibility with heritage materials and support restoration practices using historically accurate and sustainable solutions.

Comment:

- Use the terms 'SEM/EDS' or 'SEM/EDX' consistently throughout the text.

Response:

Done

Comment:

- Keywords: remove the numbers.

Response:

Done

Comment:

- Line 92: explain the meaning of “VVAA”.

Response:

This is the abbreviation for volcanic ash, although this nomenclature has not been indicated in the text before. The caption of the table has been corrected to enhance its clarity.

Comment:

- Table 2: what refers to coordinates X and Y? What is the method for pinpointing the coordinate plane?

Response:

In Table 2, coordinates X and Y refer to the UTM (Universal Transverse Mercator) coordinates of the sampling locations, expressed in meters. These coordinates are based on the ETRS89 datum, Zone 28N, which is commonly used for geospatial referencing in the Canary Islands. This method allows precise geolocation of the ash collection points within the affected area of La Palma.

 

Comment:

- Line 100, provide a definition for the acronym DSC.

Response:

We apologize for the confusion caused by the use of the Spanish acronym. The term has been corrected throughout the manuscript. We now refer to it as the Accelerated Chemical Method with Saturated Lime Solution (ACM–SLS), and this has been clarified in the Methods section.

Comment:

- Line 135: replace “phases” with “components”.

Response:

As suggested by the reviewer, the term "phases" has been replaced by "components."

Comment:

- Line 136: add the operating conditions. Which acquisition mode has been used? Have pellets been prepared?

Response:

Infrared spectra were acquired using the ATR technique. It has already been specified in the document that this was the technique used.

Comment:

- Line 148: do you mean "diffraction lines"?

Response:

The change has been made to “diffraction lines"

Comment:

- Lines 223-224: rephrase this sentence as its meaning is unclear.

Response:

The sentence has been modified to enhance its clarity and coherence.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I paid attention to the fact that most of my minor questions and suggestions have been addressed by the authors. Major remarks directed, as I think, for the improvement of the sounding of the paper were ignored maybe by the objective reasons.

I noted that the figure 13 still needed correction (it uses wrong abbreviations of minerals). The beginning of the title of the paper looks extremely strange.

Comments on the Quality of English Language

English is not my native language