Pre-Carbonated Steel Slag Aggregates as Reactive Carbon Sinks in Mortar: A Circular Approach to CO₂ Sequestration and Structural Enhancement

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript systematically investigates the carbon sequestration potential of pre-carbonized steel slag aggregate in mortar and its effect on mechanical properties and durability. The experimental content is relatively complete, but the main problems of the current manuscript are focused on some of the conclusions are expressed in a strong way suggesting additional clarification and language convergence. It is recommended that the manuscript be accepted with minor revision.

1. Line 19-23 The statement “permanent sequestration of CO₂ and significant enhancement of material properties” in the Abstract is too definitive, and it is recommended that a qualifying statement be added to make it clear that the conclusion is based on the laboratory scale and specific carbonization conditions.
2. Lines 27-30 The CO₂ uptake rates (14-19 wt%) given in the paper should be clarified in terms of the basis for their calculation (whether they are based on aggregate mass or not) in order to avoid ambiguity for the reader.
3. Lines 72-76 describe the mechanism by which carbonation improves material properties in a general way, and it is recommended that the main mechanisms (e.g., pore filling, ITZ densification, etc.) be briefly pointed out to enhance the logical continuity.
4. Lines 94-98 are not clear enough about the innovative nature of “pre-carbonized aggregate” as opposed to carbonization and conservation or carbonization and curing, and it is recommended that the differences between this paper's methodology and existing studies be clearly distinguished.
5. Lines 168-172 do not monitor the relative humidity in the reactor during the carbonization process, and it is recommended that the potential impact of this limitation on the uniformity of carbonization be explained in the methodology or discussion.
6. Line 186-194CO₂ uptake calculations assumed CaCO₃ as the main carbonation product, and it is recommended that it be made clear whether the effect of other possible phases, such as Mg carbonate, was ignored.
7. Line 276-279 The comparison of carbonation efficiency of different steel slags is clearer, and it is recommended to emphasize that the ranking is obtained under the carbonation conditions in this paper to avoid excessive extrapolation.
8. Lines 305-314 attribute the strength enhancement mechanism to both hydration and carbonation reactions, and it is suggested that it be clarified whether the current data are sufficient to distinguish between the relative contributions of the two, or that the interpretation remains qualitative.
9. Lines 429-438 are a little too strong on “significantly improved freeze-thaw durability”, and it is suggested to revise it to “better freeze-thaw stability under the present test conditions”.
10. Lines 629-635 Conclusion is a bit lengthy and it is recommended to condense it and highlight the core finding, i.e., the effect of different slag chemical compositions on the synergistic effect of carbon sequestration potential and engineering performance.

 

Author Response

Please see attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

Your manuscript titled "Pre-Carbonated Steel Slag Aggregates as Reactive Carbon Sinks in Mortar: A Circular Approach to CO₂ Sequestration and Structural Enhancement." The study investigates the use of carbonated steel slag aggregates as partial sand replacements in mortar for CO₂ sequestration and performance improvement. Four slag types were subjected to moisture-assisted carbonation, achieving CO₂ uptake levels of 14–19% by mass. The results indicate that carbonated basic oxygen furnace and blast furnace slags enhance mortar strength, durability, and density, while the observed benefits depend strongly on slag chemistry, highlighting the importance of tailored carbonation treatments.

Based on the presented work, several points require further attention, as outlined below.

 

Comment 1
The description of the carbonation procedure lacks sufficient detail and supporting references. The manuscript states that carbonation was performed at a pressure of 2–3 bar absolute for 6 hours at 25 ± 2 °C, but it is unclear how these parameters were selected. Please provide a clearer justification for the chosen pressure, duration, and temperature. In addition, the method used to ensure uniform 8% moisture content is not adequately described. The CO₂ pressure control remains vague, as a range is reported rather than a fixed value, and no information is provided on gas flow rate or how CO₂ saturation was maintained during carbonation.

Comment 2:
Figure 3 requires improvement in clarity, as some parts of the figure are difficult to discern. Please enhance the resolution and readability of all elements.

Comment 3:
The results section lacks XRD analysis to confirm the mineralogical changes responsible for the observed strength variations. If XRD data are available, they should be included. Otherwise, please explain how this limitation is addressed and how the conclusions are supported without this evidence.

Comment 4:
The SEM analysis is primarily qualitative. Although the images suggest microstructural densification, no quantitative image analysis is provided. Including metrics such as porosity or pore size distribution would strengthen the claims related to pore refinement and provide statistical support for the discussion.

 

Comment 5:
The conclusion needs to be revised, as it does not adequately reflect the results presented in the manuscript. It should be more clearly grounded in the experimental findings. In addition, the discussion of future research directions is limited and should be expanded.

Comment 6:
Please revise the references section, as there is duplicate numbering that needs to be corrected.

Author Response

Please see attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents a well-conceived and timely study on the pre-carbonation of steel slag aggregates for CO₂ sequestration and performance enhancement in mortar. The research is novel, methodologically sound, and addresses a significant gap in the literature concerning the upstream treatment of aggregates. The findings are clearly presented and supported by comprehensive experimental data. Several points should be addressed to further improve clarity, rigor, and impact.

1- The manuscript would benefit from explicitly stated research questions or hypotheses in the Introduction,

2- The discussion of results is comprehensive but can be made more compelling by better synthesizing findings across sections. For instance, the mechanistic framework (Section 3.7) is excellent but could be introduced earlier and more explicitly linked to the individual results (e.g., the poor performance of Skim Slag) to create a more cohesive narrative throughout the discussion.

3- Minor inconsistencies exist, such as figure callouts appearing before the figures themselves (e.g., reference to Figure 3(c) on page 2, while the figure is described on page 6). A careful review to ensure all figures and tables are cited in sequential order would enhance professionalism.

4- Some mechanistic claims (e.g., hydration–carbonation competition, pore blocking effects) would benefit from additional citations to reinforce interpretation.

5- Engage a native English speaker or professional editing service to polish the language without altering the technical content.

6- The manuscript appropriately highlights scalability, but the final discussion could further elaborate on: Practical challenges in industrial-scale aggregate carbonation and Comparative CO₂ sequestration efficiency relative to other carbon utilization strategies

Comments on the Quality of English Language

• Reduce sentence length and repetition

• Improve consistency in tense and terminology (e.g., “pre-carbonation” vs “carbonation”)

• Correct minor grammatical and typographical issues

Author Response

Please see attachment

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Review of the article: “Pre-Carbonated Steel Slag Aggregates as Reactive Carbon Sinks in Mortar: A Circular Approach to CO₂ Sequestration and Structural Enhancement”

This article addresses a highly relevant topic: the use of pre-carbonated steel slags in cementitious mortars to improve their mechanical and microstructural properties while simultaneously sequestering CO₂. The authors investigate several types of slag – BFS, BOF, Skim, and Rockport – and demonstrate that properly conducted carbonation can significantly enhance mortar strength, chemical resistance, and microstructural densification. The topic is timely and has strong publication potential, particularly in journals focusing on construction materials, carbon capture and storage (CCS), and steel slag recycling.

Strengths of the work:

- Comprehensive experimental approach: The authors assess mechanical properties (compressive strength, UPV), chemical properties (TGA, ASTM C88), and microstructure (SEM), providing a thorough understanding of the impact of pre-carbonation on mortar performance.

- Comparison of different slag types: Studying multiple slags allows for practical conclusions regarding which slags respond best to carbonation.

- Carbonation procedure: The method is sound and clearly described, including moisture content, CO₂ pressure, duration, temperature, and TGA-based quantification. The experiment is reproducible and scientifically robust.

- Practical relevance: The study demonstrates that the process could be implemented in industrial mortar production without modifying standard practices, increasing the practical value of the findings.

- Literature grounding: The references cover relevant topics such as steel slag chemistry, carbonation strategies, and sustainable cementitious materials. However, there are minor but important issues: duplicate references ([13] and [18] – Smołka-Danielowska et al., 2019), some entries lack publication years (listed as n.d.), and citation formatting is inconsistent. These should be corrected for clarity and accuracy.

Comments and suggestions:

1. Sample terminology: The naming of samples is inconsistent (e.g., “NaOH Carbonated Slag” vs. “Rockport Slag”), which may confuse readers. A simple, uniform naming system like [Slag Type] – [Activation if any] – [Carbonated/Non-carbonated] would improve clarity.
2. Skim Slag and NaOH-activated slag: Skim Slag shows poor carbonation performance, and the current explanation (“high sulfate content”) is speculative. Additional chemical characterization or literature references would strengthen the argument. For NaOH-activated slag, more detail on concentration, its effect on carbonation and mechanical properties, and potential limitations is needed.
3. Reproducibility and statistics: The manuscript does not report the number of repetitions or standard deviations. Including these would increase result reliability.
4. Discussion of results: Some sections repeat similar mechanisms (e.g., densification improving both UPV and compressive strength). Condensing these sections and focusing on differences between slag types would improve readability.
5. TGA interpretation: Clarify how percentage mass loss translates into actual CO₂ uptake in mortar to facilitate practical application.
6. SEM image interpretation: Some SEM interpretations are questionable. For example, in Figure 12, a feature indicated as a microcrack does not clearly appear as one. On several images, specific phases are identified without point-wise elemental analysis (EDS), which reduces the reliability of these claims.
7. Conclusions: Clear, but could be strengthened by highlighting limitations and future research directions: mechanistic understanding of Skim Slag, long-term durability, and optimization of carbonation parameters for different slags.
8. Editing, figures, and units: Ensure consistent figure captions (e.g., “FIGURE 3.” vs. “Figure 2”), use MPa instead of psi for strength results (UPV units are fine as reported), and standardize citation formatting.
9. Keywords: “Mortar” would be more accurate than “concrete,” as the study focuses on cementitious mortars, not concrete.

Summary:

This is a valuable and interesting article that convincingly demonstrates pre-carbonation of steel slags as a practical, eco-friendly method to improve mortar performance and sequester CO₂. Experimental procedures are solid, and results are thoughtfully interpreted. Minor improvements in sample naming, statistical reporting, SEM interpretation, explanation for Skim and NaOH-activated slags, citation consistency, duplicate references ([13] and [18]), and the use of MPa instead of psi would strengthen the manuscript. After addressing these points, the article could be suitable for publication and would contribute to the ongoing research on sustainable construction materials, CO₂ sequestration, and steel slag recycling.

Author Response

Please see attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for addressing the review comments and revising the manuscript accordingly.

Best regards,

Reviewer 3 Report

Comments and Suggestions for Authors

No more comments.