Molecular Dynamics Simulation of CNT Reinforced Cement: A Step Toward Sustainable Construction

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper reports the MD simulation results in CNT's role in cementitious materials. 

The central hypothesis is to test the CNT concentrations and determine whether surface modification of CNT can improve the mechanical properties. The paper mainly discussed the chemical bond interface between cementitious materials. The following recommendation should be considered in the revised version.

1. From a practical perspective, do you consider the correlation between the cement-to-CNT weight ratio and the concentration ratio? How can the concentration of CNTs be interpreted in the real mix design (CNT concentration with respect to the weight of cement)?

2. Is the porosity constant regardless of the concentration of CNTs? The CNTs can also contribute to the filling effect. Is there any variation in porosity to the change in CNT concentration?

3. In the experimental results, the higher concentration degraded the mechanical properties. Are there any insights to explain the reason for the degradation of mechanical properties in higher concentrations of CNTs in cementitious materials?

4. As per the MD simulation, the higher the CNT concentration increases, the higher the elastic modulus is achieved. Much prior research also reported the opposite trend when the concentration exceeds a certain threshold. Could you elaborate more regarding this aspect?

5. More references may be helpful for future researchers by including the mechanical prediction models in serious study. The mechanical bonding between cementitious materials is another important factor in explaining the flexural strength. Actually, cracks are mitigated by the bridging effect of CNTs across cracks. This might be out of the scope of this study, but it would be informative if the authors included additional references. 

Author Response

We would like to express our sincere gratitude to the reviewers for their insightful comments and valuable suggestions to improve the manuscript. We greatly appreciate the time and effort they dedicated to reviewing our work. In response to their feedback, we have carefully addressed all of their suggestions, and the corresponding changes have been highlighted in yellow for easy reference.

Reviewer 1

The paper reports the MD simulation results in CNT's role in cementitious materials. 

The central hypothesis is to test the CNT concentrations and determine whether surface modification of CNT can improve the mechanical properties. The paper mainly discussed the chemical bond interface between cementitious materials. The following recommendation should be considered in the revised version.

1. From a practical perspective, do you consider the correlation between the cement-to-CNT weight ratio and the concentration ratio? How can the concentration of CNTs be interpreted in the real mix design (CNT concentration with respect to the weight of cement)?

We have indicated the CNT concentration, expressed as a weight percentage, in the right column of Table 1 (values in parentheses). The concentrations used in experimental studies usually range from 0.5% to 3%.In our study, with the models used, the concentration does not vary significantly between them. Therefore, we have chosen to rewrite some parts of the text to base our analysis on the number of CNTs rather than their concentration. An interesting conclusion that emerges is that the key factor in improving mechanical properties appears to be the number of carboxyl groups present in the model. A greater number of CNTs with a higher degree of functionalization improves interfacial interaction and, consequently, the mechanical properties. Since, at the experimental level, a high CNT percentage increases material costs and can lead to CNT agglomeration, degrading mechanical properties, the optimal approach might be to use small amounts of CNTs but with a high degree of functionalization.

We have addressed this matter in the abstract, in lines 236-245, as well as in the conclusions (lines 391-395).

2. Is the porosity constant regardless of the concentration of CNTs? The CNTs can also contribute to the filling effect. Is there any variation in porosity to the change in CNT concentration? In real systems, if the concentration of CNTs increases and agglomeration occurs, an increase in porosity is observed.

We have not directly studied the effect of porosity, as this would require the use of larger models or even coarse-grained simulations over much longer timescales. To approximate the impact of porosity on mechanical properties, we have relied on the Knudsen and Helmuth model, which considers two levels of porosity (high and low, equation 3). With our current calculations, we can only evaluate the mechanical properties in the absence of porosity and apply the results from the Knudsen and Helmuth equation for further analysis. In conclusion, to study the effect of CNT concentration on porosity, other types of calculations would be required, which would entail a higher computational cost.

We have added information regarding this in the conclusions and in lines 315-318 and in the conclusions section (lines 401-405).

3. In the experimental results, the higher concentration degraded the mechanical properties. Are there any insights to explain the reason for the degradation of mechanical properties in higher concentrations of CNTs in cementitious materials?

In real systems, increasing the concentration of carbon nanotubes in a cement matrix can degrade mechanical properties due to agglomeration, increased porosity, disruption of cement hydration, weak interfacial bonding, and poor workability. These effects create structural defects and reduce the efficiency of CNT reinforcement. However, our calculations cannot account for most of these factors, as they rely on idealized models. Despite these limitations, such models allow us to verify that increasing the number of interaction points between CNTs and the cement matrix—through greater functionalization—can enhance cement strength by improving interfacial bonding. At an experimental level, the goal would be to find the optimal working conditions that allow the use of the appropriate amount of functionalized CNTs, while avoiding the negative factors mentioned earlier. By fine-tuning these variables, it would be possible to enhance the reinforcing effect of the CNTs without compromising the overall mechanical properties of the cement. As explained

As we explained in response to your first question, we can reduce the amount of CNTs by increasing their degree of functionalization.

4. As per the MD simulation, the higher the CNT concentration increases, the higher the elastic modulus is achieved. Much prior research also reported the opposite trend when the concentration exceeds a certain threshold. Could you elaborate more regarding this aspect?

The same explanation applies as for the previous question. The most studied cause is usually the agglomeration of CNTs and their poor dispersion in the cement matrix.

5. More references may be helpful for future researchers by including the mechanical prediction models in serious study. The mechanical bonding between cementitious materials is another important factor in explaining the flexural strength. Actually, cracks are mitigated by the bridging effect of CNTs across cracks. This might be out of the scope of this study, but it would be informative if the authors included additional references. 

We have added new references (references 15-25) and modified the text to analyze in more detail the experimental results regarding the improvement of cement properties, including the crack bridging effect (references 26-29).  Lines 60-81.

We have also conducted a more thorough analysis of the results obtained through molecular dynamics simulations by other authors (references 36-41), lines 108-136. Some predictions of the mechanical properties made through simulations are also presented in references 47-54, 56, 63, 67, and 69.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Editor

I am grateful to you for giving me the honor of reviewing of this research “Molecular dynamics simulation of CNT reinforced cement: a step towards
sustainable construction” with Manuscript ID: sustainability-3470163

This research paper presents an interesting subject, but it needs some MAJOR modifications keeping in view the following suggestions:

1. The abstract is not well organized. The authors should rephrase the abstract more clearly and display some numerical data with some future recommendations.
2. The introduction lacks much of the previous literature related to CNTs. Authors should cover this part adequately with at least 10 references. Please insert the following reference “Effect of elevated temperature and cooling regimes on the compressive strength, microstructure and radiation attenuation of fly ash–cement composites modified with miscellaneous nanoparticles” & “Ceramic waste as an efficient material for enhancing the fire resistance and mechanical properties of hardened Portland cement pastes”
3. The innovation in this work is not well presented. The authors should present the innovation in a more attractive way at the end of the introduction.
4. Materials and Methods section, Real samples/images for CNTs, modified CNTs, Tobermorite, dispersion solution, mixing, methodology must be displayed in a in block diagram to increase the credibility of the work
5. Please insert XRD-patterns of starting materials (CNTs, modified CNTs, Tobermorite )
6. Please insert HR-TEM CNTs, modified CNTs, and Tobermorite.
7. Some languages should be polished; grammatical correction is required.
8. The conclusion is long. It is recommended to formulate it in defined points.

Comments on the Quality of English Language

it must be improved 

Author Response

We would like to express our sincere gratitude to the reviewers for their insightful comments and valuable suggestions to improve the manuscript. We greatly appreciate the time and effort they dedicated to reviewing our work. In response to their feedback, we have carefully addressed all of their suggestions, and the corresponding changes have been highlighted in yellow for easy reference.

Rewiewer 2

This research paper presents an interesting subject, but it needs some MAJOR modifications keeping in view the following suggestions:

1. The abstract is not well organized. The authors should rephrase the abstract more clearly and display some numerical data with some future recommendations.

The abstract has been modified.

2. The introduction lacks much of the previous literature related to CNTs. Authors should cover this part adequately with at least 10 references. Please insert the following reference “Effect of elevated temperature and cooling regimes on the compressive strength, microstructure and radiation attenuation of fly ash–cement composites modified with miscellaneous nanoparticles” & “Ceramic waste as an efficient material for enhancing the fire resistance and mechanical properties of hardened Portland cement pastes”

We have added the two new references suggested by the author (references 9 and 10).

We have made an exhaustive revision of the literature, both in terms of experimental results (references 15-35, lines 60-81)) and simulations (references 36-41, lines 108-136), detailing some of the findings reported by other authors

3. The innovation in this work is not well presented. The authors should present the innovation in a more attractive way at the end of the introduction.

Our primary objective is to identify the characteristics that CNTs must possess to enhance the mechanical properties of cement. Given that excessive CNT concentrations can lead to aggregation, which compromises mechanical properties and increases costs, this research aims to identify the key CNT features that optimize cement reinforcement while minimizing CNT content.

We have restructured the text in different sections of the manuscript (lines 142-152, lines 236-245 and lines 391-395) to clarify our objective and demonstrate how it can be achieved.

4. Materials and Methods section, Real samples/images for CNTs, modified CNTs, Tobermorite, dispersion solution, mixing, methodology must be displayed in a in block diagram to increase the credibility of the work

Please insert XRD-patterns of starting materials (CNTs, modified CNTs, Tobermorite )

Please insert HR-TEM CNTs, modified CNTs, and Tobermorite.

Dear Reviewer, 

We are unable to include images of experimental data, as our work focuses on simulation and we do not have the data you requested. We believe the literature review is comprehensive and covers a wide range of experimental aspects related to CNTs, allowing interested readers to access such data by consulting the referenced sources.

5. Some languages should be polished; grammatical correction is required.

The language has been reviewed and polished.

6. The conclusion is long. It is recommended to formulate it in defined points.

The conclusion has been modified.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Pg 2, lines 90-92, MD simulation not well justified with the current study. It needs more recent references and well justification why MD simulation is required for CNT modified asphalt characterizations. 
How the homogeneous mixing of CNT was assured in the study, please clarify.

Fig2 and Fig4: Are those figures drawn from any experimental investigation? If no, please cite proper references.

Need more extensive laboratory investigation like moisture damage on CNT and MD simulation.

The conclusion (and abstract) part should include some numerical findings. How much percentage CNT is the optimum to be mixed with asphlat? How a practicing engineer will implement this findings in the field?

Also, please attach the detailed data set to verify your claims with the paper.

Comments on the Quality of English Language

Thwe whole work needs to be edited with a native english speaker/language specialist.

Line 367 and 368 need improvement. Please throughly check your work and improve english. Same with line 370-371.

Author Response

We would like to express our sincere gratitude to the reviewers for their insightful comments and valuable suggestions to improve the manuscript. We greatly appreciate the time and effort they dedicated to reviewing our work. In response to their feedback, we have carefully addressed all of their suggestions, and the corresponding changes have been highlighted in yellow for easy reference.

Reviewer 3

Pg 2, lines 90-92, MD simulation not well justified with the current study. It needs more recent references and well justification why MD simulation is required for CNT modified asphalt characterizations.

We have restructured the text in different sections of the manuscript (lines 142-152, lines 236-245 and lines 391-395) to clarify our objective and demonstrate how it can be achieved usinh MD dynamics.

We have added more references, including both experimental results (references 15-29, lines 60-81) and simulations references (36-41 , (lines 108-136), 47-54, 56, 63, 67, 69) analyzing the contributions of each type of study and how simulation results complement experimental findings.

How the homogeneous mixing of CNT was assured in the study, please clarify.

We are not sure we fully understand the reviewer's question. We believe that by "mixing," he is referring to the process of creating homogeneous equilibrium structures during the simulation process.

When creating the model using the Amorphous Cell module, we constructed multiple structures and selected those in which the CNTs were more dispersed within the matrix. Structural equilibrium is achieved during the simulation process itself, which, in some cases, lasts up to 2000 ps. The mechanical properties are calculated from the final frames, which represent the equilibrated system.

We have clarified how cells with better mixing were selected in lines. 172-174.

Fig2 and Fig4: Are those figures drawn from any experimental investigation? If no, please cite proper references.

All the work we have done is based on simulations; there are no experimental results. Figures 2 and 4 were created using the software employed and based on the results obtained.

Need more extensive laboratory investigation like moisture damage on CNT and MD simulation.

Dear Reviewer, 

We are unable to include experimental data, as our work focuses on simulation and we do not have the data you requested. We believe the literature review is comprehensive and covers a wide range of experimental aspects related to CNTs, allowing interested readers to access such data by consulting the referenced sources.

The conclusion (and abstract) part should include some numerical findings. How much percentage CNT is the optimum to be mixed with asphlat? How a practicing engineer will implement this findings in the field?

We have modified the text, providing practical recommendations for the application of these results in cement reinforcement (lines 236-245).

The specific quantities that should be used from a practical standpoint depend on many factors, such as the type of CNT (single or multi-walled), its length, diameter, the water-to-cement ratio, or the type of functional groups, and cannot be predicted from simulation calculations. The reality is much more complex than our models. Simulations allow us to predict certain trends. To get closer to reality, coarse-grained simulations could be used, which lose atomic-level detail but overcome some of the limitations of all-atom models. We have added this comment in the conclusions (lines 401-406).

Also, please attach the detailed data set to verify your claims with the paper.

We have attached a compressed file containing the trajectories of all the systems. The property values are extracted from these trajectories. The software handles this process, and the values presented in the article are obtained. Interested readers can work with these trajectories.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

1.  In this paper, molecular dynamics (MD) simulations are used to study the role of CNT in cementitious materials. does MD simulation fully reflect the macroscopic mechanical behavior in real engineering? Are there certain microscopic mechanisms that are overlooked in MD simulations?
   2.  Although CNTs perform well in the laboratory, they are expensive to produce. Is there a way to reduce the production cost of CNTs to make them more economical for real projects?
   3. The article mentions that CNTs reduce the porosity of the material. Are there other factors that also play an important role in reducing the porosity of a material?
   4. In addition to carboxyl functionalization, are there other functional groups that can be tried to further enhance the interaction of CNT with the gelling material?

Comments on the Quality of English Language

English could be improved to express research more clearly.

Author Response

We would like to express our sincere gratitude to the reviewers for their insightful comments and valuable suggestions to improve the manuscript. We greatly appreciate the time and effort they dedicated to reviewing our work. In response to their feedback, we have carefully addressed all of their suggestions, and the corresponding changes have been highlighted in yellow for easy reference.

In this paper, molecular dynamics (MD) simulations are used to study the role of CNT in cementitious materials. does MD simulation fully reflect the macroscopic mechanical behavior in real engineering? Are there certain microscopic mechanisms that are overlooked in MD simulations?

All-atom molecular dynamics (MD) simulations face limitations when computing the mechanical properties of cement-CNT composites due to their small time and length scales, which restrict the simulation of long-term behaviors and large system sizes. Environmental factors like temperature, humidity, and the curing process are often simplified or neglected, limiting the realism of the simulations. Additionally, MD overlooks non-equilibrium processes such as cement hydration, quantum effects, and the evolution of microcracks, as well as long-term behaviors like creep and viscoelasticity. These limitations mean that while MD provides valuable atomic-level insights, it cannot fully represent the complexity of real-world cement-CNT composites. Coarse-grained molecular dynamics can address some of the limitations of all-atom MD simulations by simplifying the representation of atoms into larger interaction sites, effectively reducing the computational cost and allowing for the simulation of larger systems and longer time scales. This approach is particularly useful for studying macroscopic material properties and long-term behaviors such as crack propagation, phase transitions, and large-scale structural evolution in cement-CNT composites. While it sacrifices some atomic-level detail, it can capture essential features of the material’s mechanical properties, providing a more practical tool for simulating complex systems over extended times and larger spatial domains. However, it still overlooks some atomic-scale phenomena, and the accuracy depends on how well the coarse-grained models are parameterized to reflect the true behavior of the material.

We have added a paragraph regarding this in the conclusions section (lines 401-406).

2. 
   Although CNTs perform well in the laboratory, they are expensive to produce. Is there a way to reduce the production cost of CNTs to make them more economical for real projects?

One option would be to work with multi-walled CNTs (MWCNTs), which are cheaper than single-walled CNTs, or to use lower-purity CNTs that are still sufficient for applications like composite reinforcement. Another approach is to optimize synthesis methods. Many researchers have found that growing CNTs using metallic catalysts and various supports, including plastic-based supports (https://doi.org/10.1038/s41598-023-29578-w), can help lower production costs.

In our study, we observe that the degree of functionalization is a key factor, as it enhances adhesion at the CNT–tobermorite interface. We also find that by keeping the CNT percentage constant while increasing the surface density of functional groups, the mechanical properties improve. Therefore, a high degree of functionalization would allow for a lower CNT content while still enhancing performance, ultimately reducing costs.

We have addressed this matter in the abstract, in lines 236-245, as well as in the conclusions (lines 391-395).

3. The article mentions that CNTs reduce the porosity of the material. Are there other factors that also play an important role in reducing the porosity of a material?

In addition to using nanomaterials, such as CNTs, to reduce the porosity of cement, other effective strategies include optimizing the water-to-cement ratio, incorporating supplementary cementitious materials like silica fume and fly ash, and using superplasticizers.However, all these treatments are conducted at an experimental level and are beyond the scope of our study.

4. In addition to carboxyl functionalization, are there other functional groups that can be tried to further enhance the interaction of CNT with the gelling material?

In addition to carboxyl (-COOH) groups, the most commonly used functional groups for improving CNT-cement adhesion are hydroxyl (-OH), amine (-NH₂), sulfonic (-SO₃H), and silane (-Si(OR)₃) groups. Hydroxyl and amine groups enhance hydrogen bonding with the C-S-H gel (the same interaction mechanism as the carboxyl groups), while sulfonic groups improve dispersion and interaction with calcium ions, and silane groups create strong covalent bonds with C-S-H.

See, for example, https://doi.org/10.1016/j.jcis.2008.04.023 or https://doi.org/10.1016/j.conbuildmat.2020.120500, which involves graphene instead of CNTs.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Good luck!

Comments on the Quality of English Language

Need some improvement with english language.

Reviewer 4 Report

Comments and Suggestions for Authors

No.

Comments on the Quality of English Language

No.