Factors Affecting Synthesized C-S-H CO₂ Uptake: Initial Alkalinity and Ca/Si

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. In order to make readers understand the content of the full text, explanations of W, L, H, LC, HC and WC should be added after Table 1. And remove the test method √.
2. Pay attention to the consistency of the abbreviation of the whole article.
3. Section 2.2 should add literature support for the method of C-S-H synthesis.
4. The basis of the start to end node of CaCO₃ decomposition temperature is completed. C-S-H with different Ca/Si ratio has different carbon fixation amount and different crystallinity, resulting in different decomposition temperature segments of CaCO₃.
5. In line 153, why can CaO and SiO₂ not completely react to produce C-S-H? The results of previous studies were compared.
6. In lines 228-229, does the low crystallinity of C-S-H indicate looseness at the microscopic level?
7. In line 283, the expression is incorrect and the language needs to be reorganized.

Author Response

Responses to reviewer

Dear reviewer,

Thank you very much for your huge contribution on the review of our paper. The comments and suggestions are very valuable and constructive comments for improving the quality of our work. Therefore, all your suggestions have been adopted and the related revisions have been made point by point in the corresponding positions. All the changes are marked with yellow background in the revised manuscript. All your queries have been answered properly point by point as blew:

1. In order to make readers understand the content of the full text, explanations of W, L, H, LC, HC and WC should be added after Table 1. And remove the test method √.

Response: Table 1 's test methods section has been removed and footnotes have been added to help readers better understand the full text.

2. Pay attention to the consistency of the abbreviation of the whole article.

Response: The full text has been checked and the silicon-oxygen tetrahedron has been changed to [SiO₄]^4-, which is the same as Figure 7 and Figure 12.

3. Section 2.2 should add literature support for the method of C-S-H synthesis.

Response: The purpose and advantages of the selected synthetic C-S-H method have been described in Section 2.2, and relevant references have been added.

4. The basis of the start to end node of CaCO₃decomposition temperature is completed. C-S-H with different Ca/Si ratio has different carbon fixation amount and different crystallinity, resulting in different decomposition temperature segments of CaCO₃.

Response: After consulting the literature, different Ca/Si ratio may lead to differences in the crystal form of CaCO₃ formed by C-S-H after CO₂ curing, which makes CaCO₃ have different decomposition temperatures. In view of such problems, the decomposition temperature range of different CaCO₃ crystal forms has been described in detail in Section 2.7. At the same time, the extension method used in thermogravimetric analysis has been explained to reflect the scientificity and rigor of the method for calculating CaCO₃ content.

5. In line 153, why can CaO and SiO₂not completely react to produce C-S-H? The results of previous studies were compared.

Response: This is mainly because when CaO and SiO₂ are used as raw materials for the synthesis of C-S-H, when the Ca/Si ratio is greater than 1.5, CaO in the synthesized C-S-H sample will not react completely due to excess, thus existing in the state of CH, and with the increase of the initial Ca/Si ratio, the content of CH in the synthesized C-S-H will gradually increase. This phenomenon mostly exists in the experiment of using calcium oxide and silicon oxide as raw materials to synthesize C-S-H by hydrothermal synthesis.

Chen, J, J.; Thomas, J, J. Solubility and structure of calcium silicate hydrate. Cem. Concr. Res. 2004, 34, 1499–1519. https://doi.org/10.1016/j.cemconres.2004.04.034

Wu J, Liao H, Ma Z, et al. Effect of different initial CaO/SiO2 molar ratios and curing times on the preparation and formation mechanism of calcium silicate hydrate[J]. Materials, 2023, 16(2): 717. https://doi.org/10.3390/ma16020717

6. In lines 228-229, does the low crystallinity of C-S-H indicate looseness at the microscopic level?

Response: By analyzing the crystallinity and average pore size, it is found that the average pore size of C-S-H samples with low crystallinity is also larger, indicating that the pore structure of C-S-H samples with low crystallinity is looser.

7. In line 283, the expression is incorrect and the language needs to be reorganized.

Response: The original legend of 283 lines has been modified.

 

Thanks again for your constructive comments on this article!

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The Author/s studied a good topic on the Factors of effecting synthesized C-S-H on CO2 uptake: initial alkalinity and Ca/Si; however, the manuscript should be corrected according to the below-mentioned comments prior to the decision:

 

1. The manuscript mentions significant research but would improve with a more comprehensive analysis of how its findings align with the existing literature.
2. More research on the function of other cementitious phases in CO2 sequestration outside of C-S-H would improve the contextual awareness of the outcomes.
3. More accessible description of crystallinity with respect to silicon-oxygen tetrahedral structures could be achieved with further schematic images.
4. Regression analysis or statistical validation could assist one more clearly estimate the linear association between CO2 absorption and pore size.
5. The work links variations in pore structure and silicate polymerization to trends in CO2 absorption.Still, the particular function of Ca2+ mobility and its interaction with CO32- ions may be clarified more.
6. Particularly for industrial uses, the effects of CO2 concentration and carbonation length on sequestration efficiency demand a deeper discussion.
7. The study notes that although it can not clearly explain mechanistically, under high pH (13.5) CO2 absorption does not favorably correlate with Ca/Si.More elaboration on the causes of this change is required.

Author Response

Responses to reviewer

Dear reviewer,

Thank you very much for your huge contribution on the review of our paper. The comments and suggestions are very valuable and constructive comments for improving the quality of our work. Therefore, all your suggestions have been adopted and the related revisions have been made point by point in the corresponding positions. All the changes are marked with yellow background in the revised manuscript. All your queries have been answered properly point by point as blew:

1. The manuscript mentions significant research but would improve with a more comprehensive analysis of how its findings align with the existing literature.

Response: Further analysis has been made on the similarities and differences between the results shown in the references in Sections 3.1, 3.2 and 4.2 and this paper.

2. More research on the function of other cementitious phases in CO2 sequestration outside of C-S-H would improve the contextual awareness of the outcomes.

Response: In cement-based materials, except for C-S-H gel phase, the carbon sequestration potential of other cementitious materials is lower than that of C-S-H.Therefore, C-S-H is mainly used as the research object in this paper. However, in cement hydration products, AFt has a certain carbon sequestration capacity and can also promote the strength of cement-based materials. Subsequently, the carbon sequestration capacity of AFt and its influencing factors will be studied.

3. More accessible description of crystallinity with respect to silicon-oxygen tetrahedral structures could be achieved with further schematic images.

Response: Figure 7 has been improved so that readers can have a clearer understanding of the schematic mechanism.

4. Regression analysis or statistical validation could assist one more clearly estimate the linear association between CO₂absorption and pore size.

Response: A linear fitting of the CO₂ uptake and the average pore diameter ( shown in Figure 10 ) has been performed, allowing the reader to more intuitively and clearly understand the correlation between the CO₂ uptake and the average pore diameter.

5. The work links variations in pore structure and silicate polymerization to trends in CO₂Still, the particular function of Ca²⁺ mobility and its interaction with CO₃^2- ions may be clarified more.

Response: The movement of calcium ions in C-S-H can be characterized by the length of silicon chain, and the interaction between calcium ions and carbonate ions needs to be further studied in the follow-up study.

6. Particularly for industrial uses, the effects of CO₂concentration and carbonation length on sequestration efficiency demand a deeper discussion.

Response: In order to further explore the carbon sequestration capacity of C-S-H in industrial applications, C-S-H has been mixed with cement-based materials as an admixture in the experiment to explore the promoting effect of C-S-H on the carbon sequestration capacity of cement-based materials, and different concentrations and carbonization time were set to explore the CO₂ absorption of the sample. The experimental results show that under different C-S-H content, the CO₂ absorption of cement samples with different C-S-H crystal seeds is mainly related to the initial porosity of the sample. In the next step, the effects of different C-S-H seeds on the hydration reaction and carbonization reaction of cement under CO₂ curing will be studied.

7. The study notes that although it can not clearly explain mechanistically, under high pH (13.5) CO₂absorption does not favorably correlate with Ca/Si.More elaboration on the causes of this change is required.

Response: The CO₂ absorption of C-S-H under different alkalinity conditions has been further analyzed and explained in Section 3.2 and Section 4.2.

 

Thanks again for your constructive comments on this article!

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

1- The paper lacks a clear justification for selecting pH values of 7.2, 12.0, and 13.5 and Ca/Si ratios of 1.0, 1.6, and 2.3. While previous studies have explored narrower ranges, an explanation for how these values were determined would improve the scientific rigor. A sensitivity analysis could strengthen the findings.

2- The curing conditions (CO2 concentration and exposure duration) are fixed at 90% CO2 for 6 hours, but the long-term carbonation behavior is not discussed. Have the authors considered prolonged exposure to CO2 to evaluate the progression of carbonation?

3- The XRD analysis in Figure 2 does not quantify the crystallinity index or provide Rietveld refinement results. A comparison of peak intensity ratios between C-S-H and CH would improve the clarity of mineral composition evolution.

4- The thermogravimetric data interpretation relies on fixed decomposition temperature ranges for CH and C-S-H carbonation. However, overlapping decomposition zones of C-S-H, CH, and CO₂-bound phases can lead to overestimation or underestimation of CO₂ uptake. How did the authors ensure accurate separation?

5- The mass balance calculation (Equation 10) for net CO₂ uptake assumes that CH carbonation is independent of C-S-H carbonation. However, CH dissolution influences pore solution chemistry and can affect C-S-H carbonation kinetics. A discussion of possible interactions would be valuable.

6- Additionally, the mean chain length calculations (Equation 6) should consider the relative error in deconvolution fitting. Was any validation conducted using reference materials?

7- The BET data in Figure 8 show an increase in specific surface area with pH but a decrease in CO₂ uptake at pH = 13.5. Is this due to pore blockage by secondary carbonation products? The authors should analyze whether CaCO₃ formation within C-S-H pores reduces available reactive sites.

8- The study attributes this to silicon depolymerization at high alkalinity, but could adsorption-desorption hysteresis loops provide more insight into whether pore connectivity limits CO₂ uptake?

9- The study focuses on CO₂ uptake within 6 hours, but does not address long-term kinetics. Since C-S-H carbonation is a diffusion-controlled process, does CO₂ uptake continue at a decreasing rate over time? Including kinetic modeling would provide deeper insights.

Author Response

Responses to reviewer

Dear reviewer,

Thank you very much for your huge contribution on the review of our paper. The comments and suggestions are very valuable and constructive comments for improving the quality of our work. Therefore, all your suggestions have been adopted and the related revisions have been made point by point in the corresponding positions. All the changes are marked with yellow background in the revised manuscript. All your queries have been answered properly point by point as blew:

1. The paper lacks a clear justification for selecting pH values of 7.2, 12.0, and 13.5 and Ca/Si ratios of 1.0, 1.6, and 2.3. While previous studies have explored narrower ranges, an explanation for how these values were determined would improve the scientific rigor. A sensitivity analysis could strengthen the findings.

Response: The background and reasons for determining pH and Ca / Si ratio have been described in the Introduction section. The reason for the selection of Ca/Si ratio and alkalinity : the alkalinity in cement is usually between 11 and 13, and the calcium-silicon ratio in cement is usually between 1.0 and 2.0. In order to simulate the influence of alkalinity and Ca/Si ratio on the micro-nano structure of C-S-H in the process of cement hydration more comprehensively and systematically, it is necessary to set a wider range of Ca/Si ratio and alkalinity range to study the influence of C-S-H micro-nano structure.

2. The curing conditions (CO₂concentration and exposure duration) are fixed at 90% CO₂ for 6 hours, but the long-term carbonation behavior is not discussed. Have the authors considered prolonged exposure to CO₂ to evaluate the progression of carbonation?

Response: After carbonization of C-S-H for 3d, 7d and 14d in subsequent experiments, it was found that under the condition of 90 % CO₂ concentration, the sample was completely carbonized after C-S-H curing time was more than 3d. The CO₂ uptake of C-S-H samples after complete carbonization only depends on the calcium-silicon ratio of C-S-H. The higher the calcium-silicon ratio of C-S-H samples, the higher the CO₂ uptake. After that, different CO₂ concentration gradients will be set up to explore the change rule of CO₂ uptake of C-S-H under different CO₂ concentration gradients.

3. The XRD analysis in Figure 2 does not quantify the crystallinity index or provide Rietveld refinement results. A comparison of peak intensity ratios between C-S-H and CH would improve the clarity of mineral composition evolution.

Response: The crystallinity of C-S-H has been recalculated and Figure 5 in Section 4.1 has been modified. At the same time, the causes of CH generation and the influence mechanism of different alkalinity on the relationship between CH and C-S-H are described in Sections 2.7 and 4.1.

4. The thermogravimetric data interpretation relies on fixed decomposition temperature ranges for CH and C-S-H carbonation. However, overlapping decomposition zones of C-S-H, CH, and CO₂-bound phases can lead to overestimation or underestimation of CO₂ uptake. How did the authors ensure accurate separation?

Response: After consulting the literature, different Ca/Si ratio may lead to differences in the crystal form of CaCO₃ formed by C-S-H after CO₂ curing, which makes CaCO₃ have different decomposition temperatures. In view of such problems, the decomposition temperature range of different CaCO₃ crystal forms and CH has been described in detail in Section 2.7. At the same time, the extension method used in thermogravimetric analysis has been explained to reflect the scientificity and rigor of the method for calculating CaCO₃ content.

5. The mass balance calculation (Equation 10) for net CO₂ uptake assumes that CH carbonation is independent of C-S-H carbonation. However, CH dissolution influences pore solution chemistry and can affect C-S-H carbonation kinetics. A discussion of possible interactions would be valuable.

Response: The research team has studied the hydration reaction and carbonation reaction of cement in the process of CO2 curing and the relationship between them. Subsequently, the experimental conclusions will be referred to, so as to further study the hydration kinetics and carbonization kinetics of C-S-H in the presence of pure phase C-S-H and CH, and the relationship between them.

6. Additionally, the mean chain length calculations (Equation 6) should consider the relative error in deconvolution fitting. Was any validation conducted using reference materials?

Response: The R square of the deconvolution fitting values in Fig.6 and Fig.11 is above 0.96, indicating that the deconvolution fitting effect is good, and the fitting value can accurately reflect the law of the actual data.

7. The BET data in Figure 8 show an increase in specific surface area with pH but a decrease in CO₂ uptake at pH = 13.5. Is this due to pore blockage by secondary carbonation products? The authors should analyze whether CaCO₃ formation within C-S-H pores reduces available reactive sites.

Response: Figure 8 only shows the positive correlation between specific surface area and pore volume, and does not show the relationship between specific surface area and pH. However, the specific surface area test results show that the specific surface area gradually decreases with the increase of pH. At pH = 13.5, the CO₂ uptake of 1.0H and 2.3H is much higher than that of other samples with lower pH. In theory, the calcium carbonate crystal produced by the carbonization reaction on the surface of C-S-H crystal will encapsulate the part of C-S-H that has not been carbonized, thus hindering the further carbonization of C-S-H. However, this situation is more likely to occur in large volume and fine pore structure samples. The C-S-H synthesized in this experiment is a fragmented small volume sample, and the C-S-H itself is a loose porous structure. Therefore, the carbonization reaction generates calcium carbonate to block the pores and thus hinders the further carbonization reaction.

8. The study attributes this to silicon depolymerization at high alkalinity, but could adsorption-desorption hysteresis loops provide more insight into whether pore connectivity limits CO₂ uptake?

Response: In the ongoing experiments, we will focus on the pore volume, pore size and the contribution of different pore size ranges before and after CO₂ curing to the CO₂ uptake of C-S-H under different alkalinity and calcium-silicon ratio conditions. At the same time, in the analysis process, the pore connectivity and the degree of pore bending will also be taken into account. Finally, a model of pore structure and CO₂ uptake will be established to clearly and systematically explain the correlation between C-S-H pore structure and CO₂ uptake.

9. The study focuses on CO₂ uptake within 6 hours, but does not address long-term kinetics. Since C-S-H carbonation is a diffusion-controlled process, does CO₂ uptake continue at a decreasing rate over time? Including kinetic modeling would provide deeper insights.

Response: In the subsequent experiments, C-S-H will be cured with different CO₂ curing time and different CO₂ concentration to explore the hydration kinetics, carbonation kinetics and CO₂ uptake rate of different C-S-H seeds under different CO₂ curing time and CO₂ concentration.

 

Thanks again for your constructive comments on this article!

 

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

I have analyzed the manuscript "Factors of effecting synthesized C-S-H on CO2 uptake: initial alkalinity and Ca/Si" and identified the following weaknesses:

 

• The introduction presents a lot of technical information (e.g. types of chemical reactions, behavior of different mineral phases), but does not organize it clearly enough. Also, the introduction provides a lot of details about the background of the research, but the objectives are not clearly expressed.
• Several analytical techniques are mentioned (SEM, XRD, BET, NMR), but it is not sufficiently well explained why each of them was chosen in relation to the objectives of the study. A comparison with other alternative methods would be useful.
• Although the experimental data are presented somewhat well, the correlations between the analyzed parameters and the observed effects are not always clearly explained. For example, the relationship between pore diameter and CO₂ uptake could be better argued
• In some sections, the relationships between variables are not explained clearly enough. For example, the study states that “CO₂ uptake is correlated with the Ca/Si ratio”, but does not clearly explain why this correlation exists.
• Some figures (e.g. Figures 3, 5, 9) could use clearer explanations to highlight key findings. Also, the legends sometimes lack information that would help interpret the results.
• The study cites previous work but does not compare it in sufficient detail with its own results. For example, it states that “these results are consistent with previous studies”, but does not explain how they are similar or different. Adding a more developed discussion section would improve the validity of the conclusions.
• The conclusions are presented in a technical way, but without clarifying the practical implications of the research. It would be useful to mention how these findings can be applied in industry or future research.

Comments on the Quality of English Language

The english language is good

Author Response

Responses to reviewer

Dear reviewer,

Thank you very much for your huge contribution on the review of our paper. The comments and suggestions are very valuable and constructive comments for improving the quality of our work. Therefore, all your suggestions have been adopted and the related revisions have been made point by point in the corresponding positions. All the changes are marked with yellow background in the revised manuscript. All your queries have been answered properly point by point as blew:

1. The introduction presents a lot of technical information (e.g. types of chemical reactions, behavior of different mineral phases), but does not organize it clearly enough. Also, the introduction provides a lot of details about the background of the research, but the objectives are not clearly expressed.

Response: The introduction part has been modified, and the unnecessary research background information has been streamlined to highlight the research focus of this paper.

2. Several analytical techniques are mentioned (SEM, XRD, BET, NMR), but it is not sufficiently well explained why each of them was chosen in relation to the objectives of the study. A comparison with other alternative methods would be useful

Response: The test method used in this paper is a more conventional test method in the field of materials science. At the same time, in the introduction part, the purpose of selecting each test method is described in detail. The use of SEM to observe the microstructure of CSH is mainly due to the magnification of SEM is 10-500000 times, which is enough to accurately characterize the morphology of the synthesized CSH. Compared with TEM, which can also characterize the microstructure, SEM has the advantages of simple operation and high cost performance, and is widely used by material researchers.

The main reason for choosing XRD is that XRD can test the composition and crystallinity of CSH, and can clearly characterize the phase composition of the sample to detect whether the material is pure. At the same time, it can also calculate the crystallinity of CSH to reveal the influence of different alkalinity and calcium silicon ratio on the crystallinity of CSH.

BET is a test method commonly used in materials science to characterize the pore structure of the material. Compared with the MIP test method, BET can more accurately characterize the pore structure of the powder sample, and the synthesized CSH is bulky and granular after drying and has the characteristics of easy powdering. Therefore, the pore structure of the synthesized CSH was characterized by BET.

²⁹Si MAS NMR has been widely used in the field of cement-based materials because it can characterize the linkage mode of silicon-oxygen tetrahedron in CSH. At the same time, the chain length and degree of polymerization of CSH can be obtained by calculating the proportion of Qⁿ. In this paper, the decalcification degree of CSH with different seed crystals was characterized by comparing the difference of Qⁿ proportion and chain length in CSH before and after CO₂ curing.

Inkson B J. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for materials characterization[M]//Materials characterization using nondestructive evaluation (NDE) methods. Woodhead publishing, 2016: 17-43.https://doi.org/10.1016/B978-0-08-100040-3.00002-X

Pan G , Bao B , Wang Y .Evolution Law of Average Chain Length of C-S-H during the Carbonation Process[J].Journal of Building Materials, 2016. https://doi.org/10.3969/j.issn.1007-9629.2016.03.003.

3. Although the experimental data are presented somewhat well, the correlations between the analyzed parameters and the observed effects are not always clearly explained. For example, the relationship between pore diameter and CO₂ uptake could be better argued

Response: A linear fitting of the CO₂ uptake and the average pore diameter ( shown in Figure 10 ) has been performed, allowing the reader to more intuitively and clearly understand the correlation between the CO₂ uptake and the average pore diameter.

4. In some sections, the relationships between variables are not explained clearly enough. For example, the study states that “CO₂ uptake is correlated with the Ca/Si ratio”, but does not clearly explain why this correlation exists

Response: The correlation between CO₂ uptake and Ca/Si ratio was further explained in Sections 3.2 and 4.2.

5. Some figures (e.g. Figures 3, 5, 9) could use clearer explanations to highlight key findings. Also, the legends sometimes lack information that would help interpret the results.

Response: The related mechanisms of Fig.3, Fig.5 and Fig.9 have been further elaborated in order to show the key results more clearly.

6. The study cites previous work but does not compare it in sufficient detail with its own results. For example, it states that “these results are consistent with previous studies”, but does not explain how they are similar or different. Adding a more developed discussion section would improve the validity of the conclusions.

Response: Further analysis has been made on the similarities and differences between the results shown in the references in Sections 3.1, 3.2 and 4.2 and this paper.

7. The conclusions are presented in a technical way, but without clarifying the practical implications of the research. It would be useful to mention how these findings can be applied in industry or future research.

Response: The conclusion part has been modified to add the practical significance of this study ：The research results can provide a theoretical reference for optimizing the C-S-H seed crystal with excellent carbon fixation ability as an admixture to greatly improve the carbon fixation ability of cement-based materials while reducing the energy consumption and CO₂ emission of cement production.

 

Thanks again for your constructive comments on this article!

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

ALL comments have been eliminated.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors responded to the comments made and improved the paper accordingly.

Comments on the Quality of English Language

The english language is good(