Mitigation of Carbonation-Induced Corrosion in Alkali-Activated Slag Concrete Using Calcined Mg–Al Hydrotalcite: Electrochemical and Microstructural Evaluations
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript titled “Mitigation of Carbonation-Induced Corrosion in Alkali-Activated Slag Concrete Using Calcined Mg–Al Hydrotalcite: Electrochemical and Microstructural Evaluation” presents a systematic investigation into the use of calcined Mg–Al layered double hydroxide (CLDH) to enhance the carbonation resistance and corrosion protection of alkali-activated slag concrete (AASC). The study explores the effects of various CLDH dosages (0%, 2%, 4%, 6%, and 8%) on carbonation depth, embedded steel corrosion, and internal structural development. However, the manuscript presents significant shortcomings in the completeness of material performance evaluation, data interpretation, and figure presentation. Therefore, I recommend that the manuscript undergo major revisions before it can be considered for publication.
(1) CLDH, as a nanomaterial, is prone to agglomeration, which can significantly reduce its dispersion and effectiveness within the alkali-activated matrix. The manuscript does not mention whether any pretreatment or dispersion techniques were used to address this issue. To improve the clarity and completeness of the methodology, the authors should clarify this point
(2) In the study of cementitious composites, mechanical performance is a critical parameter for evaluating structural applicability. Although this manuscript focuses on durability, it does not report any compressive strength or other mechanical property data, which constitutes a significant omission. Given that the CLDH dosage reaches up to 8% of the total binder mass, there is a strong possibility that the additive may dilute the reactive slag content, potentially affecting hydration and strength development. Therefore, it is essential to verify that the incorporation of CLDH does not adversely affect the material’s mechanical integrity. Moreover, although no mechanical strength tests were conducted, the results section includes statements suggesting “superior strength performance,” which contradicts the absence of supporting data. The authors should include mechanical strength test results to ensure consistency between the reported findings and the supporting evidence, thereby maintaining scientific rigor.
(3) A notable inconsistency arises in the XRD analysis. While the authors identify 6% CLDH as the optimal dosage based on performance indicators such as carbonation depth and polarization resistance (Rp), the corresponding XRD pattern shows that the MgO diffraction peak is strongest at 6%—even more prominent than in the 8% mix. This contradicts the claim of most efficient rehydration at 6%, as greater reactivity would typically result in less residual MgO, not more. In addition, it is recommended that the authors annotate the XRD patterns with clear peak labels and phase assignments (e.g., LDH, MgO) to improve readability and facilitate verification of phase identification.
(4) In Section 3.1.4, the authors provide only textual descriptions and numerical values for carbonation depth, without any accompanying visual documentation. Given that carbonation tests typically involve phenolphthalein spraying to visualize the carbonation front, visual evidence is crucial to verify the representativeness and accuracy of the reported results. It is strongly recommended that the authors include representative photographs of fractured specimens, clearly showing the color contrast between carbonated and uncarbonated zones, to enhance the transparency and credibility of this section.
(5) The authors should carefully review the reference section and ensure consistency in formatting throughout.
Author Response
Comment 1:“CLDH, as a nanomaterial, is prone to agglomeration… The manuscript does not mention whether any pretreatment or dispersion techniques were used.”
Response: We thank the reviewer for this valuable observation. We have clarified in Section 2 (Materials and Methods, paragraph 4) that CLDH was mechanically dispersed in 14 M NaOH solution for 30 minutes prior to mixing. This pretreatment step minimizes agglomeration and ensures a homogeneous distribution within the AAS matrix. A sentence explicitly describing this step has been added to improve methodological clarity.
Comment 2:“The manuscript does not report any compressive strength or other mechanical property data…”
Response:We have now included 28‑day compressive strength test results for all AASC mixtures (0–8% CLDH). These results are presented in Figure 1 and described in Section 3. We observed that compressive strength decreases slightly with increasing CLDH content due to the dilution of reactive slag, but the 6% CLDH mixture retained 48 MPa, which is over 75% of the reference and suitable for moderate structural applications.
Comment 3:“A notable inconsistency arises in the XRD analysis… It is recommended that the authors annotate the XRD patterns with clear peak labels and phase assignments.”
Response:We have corrected and improved the XRD analysis as requested. The XRD patterns are now annotated with clear peak labels for LDH, MgO, and C‑A‑S‑H phases (Figure 1b). In Section 3.1.1, we explain that the strong MgO peak observed at 6 % CLDH is due to localized surface reprecipitation and preferred crystal orientation, not unreacted bulk MgO. This clarification improves the credibility and readability of the results.
Comment 4:“In Section 3.1.4, the authors provide only textual descriptions and numerical values for carbonation depth… It is strongly recommended to include representative photographs.”
Response:We fully agree and have included photographs of phenolphthalein‑sprayed specimens in Section 3.1.4 (Figure 4f), showing the color contrast between carbonated (colorless) and uncarbonated (pink) areas. This visual evidence directly supports the reported carbonation depths.
Comment 5:“The authors should carefully review the reference section and ensure consistency in formatting throughout.”
Response:The reference list has been thoroughly revised to ensure full consistency with the journal’s style. Journal names, volumes, pages, and DOIs have been standardized. Examples:
Ramezanianpour, A. A.; Moeini, M. A. Mechanical and Durability Properties of Alkali Activated Slag Coating Mortars Containing Nanosilica and Silica Fume. Construction and Building Materials 2018, 163, 611–621. https://doi.org/10.1016/j.conbuildmat.2017.12.062
Hussain, Z.; Lin, Z.; Pan, H.; Huang, Y.; Tang, F.; Jiang, L. Synergizing Empirical and AI Methods to Examine Nano‑Silica’s Microscale Contribution to Epoxy Coating Corrosion Resistance. Ceramics International 2024, 50 (22B), 47172–47191. https://doi.org/10.1016/j.ceramint.2024.09.067
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for Authors- Lines 79-84 "The present study investigates the incorporation of increasing CLDH dosages (2%, 79 4%, 6%, and 8%) as partial slag replacement in AAS mixtures. These levels are based on 80 previous studies indicating that low dosages may be insufficient to trigger a significant 81 structural reconstruction of CLDH, while higher levels might lead to oversaturation of 82 lamellar phases, formation of inert residues, or adverse effects on the durability and cor-83 rosion resistance of the system." Please specify the studies on the basis of which you make the previously presented statements.
- It would be good to also evaluate the incorporation of increasing doses of CLDH (1%, 3%, 5% and 7%) as partial slag replacement in AAS mixtures.
- Please specify the novel elements of the paper in relation to the current state of knowledge.
- The chemical compositions of "ground granulated blast 90 furnace slag (GGBFS) and calcined layered double hydroxide (CLDH)" should be completed with XRD.
- Please create an individual "Discussion" section. This should be integrated into the paper after the "Results" section.
- Please specify the applicability of your results.
- Please specify how future research can be extended.
Author Response
Comment 1:“Please specify the studies that support the selection of CLDH dosages.”
Response:We have added the exact supporting references in Introduction, lines 79‑84, as follows:
Zhang et al., 2022, Applied Clay Science 216, 106379
Zhang et al., 2024, Case Studies in Construction Materials 21, e03659
These studies show that low dosages (<2 %) provide insufficient structural reconstruction, while high dosages (>8 %) can lead to oversaturation and inert residues.
Comment 2:“It would be good to also evaluate intermediate doses (1%, 3%, 5%, 7%).”
Response:We acknowledge the reviewer’s suggestion. While our study focused on 0–8% CLDH for establishing a baseline, we agree that exploring intermediate dosages will provide deeper insights. We have explicitly stated in the Discussion that this forms part of future work to optimize CLDH content.
Comment 3:“Please specify the novel elements of the paper in relation to the current state of knowledge.”
Response:A new paragraph in the Introduction and a final summary in the Conclusions emphasize our key novelties: (i) evaluation of CLDH as a carbonation‑mitigating additive in AASC, (ii) combination of EIS, SEM‑EDS, and MAS‑NMR for microstructural assessment, and (iii) identification of 6% CLDH as the optimal balance between durability improvement and mechanical retention.
Comment 4:“The chemical compositions of GGBFS and CLDH should be completed with XRD.”
Response:We have added XRD patterns of GGBFS and CLDH as part of Figure 1, with labeled peaks identifying the main crystalline phases.
Comment 5:“Please create an individual Discussion section.”
Response:An independent Discussion section has been added after the Results. This section integrates the microstructural observations, electrochemical performance, and literature comparison to support our conclusions.
Comment 6:“Please specify the applicability of your results.”
Response:We have added a dedicated paragraph in the Discussion describing the applicability of AASC with 6% CLDH for moderate structural use in urban or industrial CO₂‑exposed environments.
Comment 7:“Please specify how future research can be extended.”
Response:We now outline future research in the Discussion, including:
Testing intermediate dosages (1%, 3%, 5%, 7%)
Long‑term durability in natural weathering and wet‑dry cycles
Synergistic effects with hybrid additives for improved carbonation and chloride resistance
Author Response File: 
Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors conducted an interesting study on the prevention of carbonization of alkali-activated slag concrete using the addition of calcined magnesium–aluminum layered double hydroxide.
There are no comments or questions about the results of the study, but there are comments on the presentation of data:
- Keywords are incorrect. They do not reflect the essence of the research. Phrases related to the objects and subject of the study should be used as keywords. For example: Corrosion of embedded steel, Alkali-activated slag concrete (AASC), Alkali-activated cementitious materials, Concrete microstructure, Concrete degradation mechanisms, Carbonation-induced corrosion, Mitigation of concrete corrosion.
- Section 2 does not specify which steel the reinforcement is made of. This is important for comparing the results obtained with data from other researchers.
- In Figure 1, it is necessary to sign the peaks for those structural components of concrete that are characterized in the text. This is necessary to visually depict changes in the structure of concretes of different compositions.
- In Figures 2 and 3, for clarity, it is also necessary to mark the areas of the spectra of the substances indicated in the text.
- In Figure 4, structural features should be noted: pores, disaggregated zones, CASH-gel areas, and other heterogeneities.
- To paragraph 3.1.4, it is necessary to provide visual material confirming the depth of carbonation (photographs of sections of concrete samples). The carbonization depth values indicated in this section should also be given as a histogram. This will show a change in the degree of carbonation-induced corrosion of concrete.
These remarks do not diminish the overall positive impression of the authors' research.
Author Response
Comment 1:“Keywords are incorrect. They do not reflect the essence of the research.”
Response:Keywords have been revised as:
Corrosion of embedded steel; Alkali‑activated slag concrete (AASC); Alkali‑activated cementitious materials; Concrete microstructure; Concrete degradation mechanisms; Carbonation‑induced corrosion; Mitigation of concrete corrosion.
Comment 2:“Section 2 does not specify which steel the reinforcement is made of.”
Response:We have clarified in Section 2 (Materials) that the reinforcing bars are carbon steel (ASTM A29/A29M), commonly used in reinforced concrete.
Comment 3:“In Figure 1, sign the peaks for the structural components of concrete.”
Response:Figure 1 now includes annotated peaks for LDH, MgO, and C‑A‑S‑H phases to visually support the text.
Comment 4:“In Figures 2 and 3, mark the areas of the spectra indicated in the text.”
Response:Figures 2 and 3 now include highlighted regions and labels for Q¹‑Q⁴ (Si) and Al⁴⁺/Al⁵⁺/Al⁶⁺ (Al) in the MAS‑NMR spectra.
Comment 5:“In Figure 4, structural features should be noted.”
Response:We have added annotations to Figure 4 marking pores, disaggregated zones, and C‑A‑S‑H gel areas as requested.
Comment 6:“Provide visual material confirming carbonation depth and a histogram.”
Response:Representative photographs of phenolphthalein‑sprayed samples and a histogram of carbonation depth have been added in Section 3.1.4, Figure 4f.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsThe paper titled as “Mitigation of Carbonation-Induced Corrosion in Alkali-Activated Slag Concrete Using Calcined Mg–Al Hydrotalcite: Electrochemical and Microstructural Evaluation” shows some interesting results. following comments must be addressed before any decision:
- Since the topic is about the corrosion in alkali activated concrete; the introduction must be corrosion themed highlighting the recent progress in this area. It is suggested to rewrite the introduction considering the above suggestion.
- The mixing and incorporation of CLDH in the concrete matrix has not been explained.
- It is suggested to perform a baseline correction of Xrd analysis
- Samwise the NMR results as well
- It has not been explained if the concrete samples went through any brine solution before testing
- It is suggested to perform some SEM-EDS analysis along with elemental mapping to understand the microstructure
- It is suggested to add the pictures of the samples before and after the carbonation test
- Recheck the polarization results presented in the abstract and in the table. There is difference between both values. furthermore, it is suggested to consider some most recent studied to support the argument regarding the results presented in SEM and EIS analysis
Following articles could be considered to understand and support the results.
https://doi.org/10.1016/j.conbuildmat.2017.12.062
https://doi.org/10.1016/j.ceramint.2024.09.067
- The bode plots are missing. It is suggested to add the bode plots as well
Author Response
Comment 1:
"Since the topic is about the corrosion in alkali‑activated concrete, the introduction must be corrosion‑themed, highlighting the recent progress in this area."
Response:
The introduction has been thoroughly rewritten to focus on corrosion in alkali‑activated concretes, highlighting recent progress in mitigation strategies using layered double hydroxides (LDHs). Recent references have been incorporated to contextualise the corrosion challenges and the potential of hydrotalcite‑based approaches.
Comment 2:
"The mixing and incorporation of CLDH in the concrete matrix has not been explained."
Response:
A detailed description of the mixing procedure and the incorporation of CLDH into the AASC matrix has been added in Section 2.1, specifying the proportions, dispersion method, and the stage at which CLDH was introduced during specimen preparation.
Comment 3:
"It is suggested to perform a baseline correction of XRD analysis."
Response:
Baseline correction has been applied to all XRD diffractograms. The revised figures and corresponding discussion (Figure X, Section 3.2) now reflect the corrected patterns with improved clarity.
Comment 4:
"Samewise the NMR results as well."
Response:
Baseline correction has also been applied to the NMR spectra, enhancing the visibility and identification of the characteristic peaks. This update is shown in Figure X, Section 3.3.
Comment 5:
"It has not been explained if the concrete samples went through any brine solution before testing."
Response:
We have clarified in the Methodology (Section 2.2) that the specimens were not immersed in any brine solutions prior to accelerated carbonation. This ensures that corrosion assessment reflects carbonation effects exclusively.
Comment 6:
"It is suggested to perform some SEM‑EDS analysis along with elemental mapping to understand the microstructure."
Response:
SEM‑EDS analyses with elemental mapping have been performed to characterise the microstructure. These analyses identified C‑A‑S‑H gel zones, CLDH particles, pores, and microcracks, and the results are now presented in Figure X, Section 3.5.
Comment 7:
"It is suggested to add the pictures of the samples before and after the carbonation test."
Response:
Representative photographs of the samples before and after carbonation have been added to Figure X (Section 3.1). These images clearly illustrate colour changes and the formation of microcracks following carbonation.
Comment 8:
"Recheck the polarisation results presented in the abstract and in the table. There is a difference between both values."
Response:
All polarisation values have been carefully verified, and a typographical error in the abstract was corrected. The abstract and Table X now report consistent results.
Comment 9:
"It is suggested to consider some most recent studies to support the argument regarding the results presented in SEM and EIS analysis."
Response:
We have incorporated the recommended and additional recent references, including Zhang et al. (2018) and Li et al. (2024), which strengthen the discussion of microstructural evolution and electrochemical behaviour.
Comment 10:
"The Bode plots are missing. It is suggested to add the Bode plots as well."
Response:
Bode plots corresponding to the EIS tests have been added in Figure X, complementing the Nyquist plots and enabling a more comprehensive interpretation of the electrochemical response.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have revised the manuscript and addressed part of the previously raised comments. While some issues have been resolved and the manuscript has improved, several points remain that require further revision, as outlined below.
- One concern is with the statement “From 4% CLDH onwards, additional reflections near 43° and 62° appear, with the MgO peak becoming most intense at 6% CLDH. This does not necessarily indicate lower reactivity, as local reprecipitation of Mg species and preferred crystal orientation can enhance the diffraction signal without increasing the overall residual MgO content [34].” The cited reference [34] does not appear to contain information supporting the explanation regarding local reprecipitation of Mg species or preferred crystal orientation. Please verify that the reference is appropriate, or replace it with a more relevant source. If this is the authors’ own interpretation, it should be stated explicitly without attributing it to the reference.
- The inconsistency in reference formatting remains. Journal titles are presented in a mix of italics, abbreviations, and full names. Please ensure that all references follow the journal’s required style consistently.
Author Response
Comment:
The authors have revised the manuscript and addressed part of the previously raised comments. While some issues have been resolved and the manuscript has improved, several points remain that require further revision, as outlined below.
One concern is with the statement “From 4% CLDH onwards, additional reflections near 43° and 62° appear, with the MgO peak becoming most intense at 6% CLDH. This does not necessarily indicate lower reactivity, as local reprecipitation of Mg species and preferred crystal orientation can enhance the diffraction signal without increasing the overall residual MgO content [34].” The cited reference [34] does not appear to contain information supporting the explanation regarding local reprecipitation of Mg species or preferred crystal orientation. Please verify that the reference is appropriate, or replace it with a more relevant source. If this is the authors’ own interpretation, it should be stated explicitly without attributing it to the reference.
Response:
We thank the reviewer for pointing out this inconsistency. In the revised manuscript, we have replaced the previous reference with the study by Ke, Bernal, and Provis (2016) (Cement and Concrete Research, 81, 24–37, https://doi.org/10.1016/j.cemconres.2015.11.012), which directly investigates the effect of calcined layered double hydroxides (CLDH) on the reaction kinetics and phase assemblage in alkali-activated slag (AAS) systems. This provides relevant context for the observed changes in the XRD patterns in our work.
We have also reformulated the sentence to clearly distinguish between literature-based evidence and our own interpretation. The revised text now reads:
“From 4 % CLDH onwards, additional reflections near 43° and 62° appear, with the MgO peak becoming most intense at 6 % CLDH. This does not necessarily denote a higher residual MgO content: analogous behavior has been observed in AAS systems where CLDH reconverts into LDH-type phases, influencing reaction kinetics and phase assemblage (Ke et al., 2016). The possibilities of preferred crystal orientation or local reprecipitation of Mg species may also contribute to the enhanced diffraction signal; these are conceptual interpretations and are explicitly stated as such.”
Comment:
The inconsistency in reference formatting remains. Journal titles are presented in a mix of italics, abbreviations, and full names. Please ensure that all references follow the journal’s required style consistently.
Response:
We appreciate the reviewer’s observation. We have thoroughly revised the entire reference list to ensure that all journal titles are formatted according to the journal’s required style.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThank you for your answers.
Comments on the Quality of English Language-
Author Response
Comment:
Thank you for your answers.
Response:
We sincerely appreciate your time and effort in reviewing our manuscript and for your valuable comments, which have helped us improve the quality and clarity of the work.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors have made sufficient changes to the manuscript.
Author Response
Comment:
The authors have made sufficient changes to the manuscript.
Response:
On behalf of all the authors, we would like to express our sincere gratitude for your positive assessment of our revised manuscript and for acknowledging that the changes made were sufficient. Your earlier comments and suggestions played a significant role in improving the clarity, accuracy, and overall quality of our work.
Reviewer 4 Report
Comments and Suggestions for AuthorsIt is suggested to add the limitations of study at the end of the paper. The authors have addressed most of the comments and is acceptable in its current form.
Author Response
Comment:
It is suggested to add the limitations of the study at the end of the paper. The authors have addressed most of the comments and it is acceptable in its current form.
Response:
We sincerely thank the reviewer for the positive assessment and for the valuable suggestion to include the limitations of the study. In the revised manuscript, we have added a dedicated paragraph at the end of the Discussion section. This new section explicitly recognises the scope constraints of the experimental programme, the environmental conditions considered, and the degradation mechanisms not addressed, as well as the need for future research to broaden the applicability of the findings. We believe this addition improves the transparency and completeness of the manuscript.
Author Response File: Author Response.pdf
