Utilization of Construction and Demolition Waste in Concrete as Cement and Aggregate Substitute: A Comprehensive Study on Microstructure, Performance, and Sustainability

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents a comprehensive study on the utilization of construction and demolition wastes (CDWs) in concrete as cement and aggregate substitutes, focusing on microstructure, performance, and sustainability. While the research addresses an important environmental challenge, several scientific and methodological concerns warrant attention to strengthen the validity and clarity of the findings. Below are detailed comments and suggestions for improvement:

1. Line 20-21: The statement that the 28-day compressive strength reached 46.7 MPa with a 24.9% increase requires clarification. The baseline strength for comparison is not explicitly provided, making it difficult to assess the significance of this improvement. Please specify the reference concrete's 28-day compressive strength to allow for a clear evaluation of the reported enhancement.
2. Line 33-34: The claim that urbanization in China produces approximately 1.5 billion tons of CDWs per year is cited broadly ([1-3]). However, the wide range of references suggests variability in estimates. Provide a more precise source or average value based on the cited studies to enhance credibility, or justify the use of this specific figure.
3. Line 107-108: The particle size and chemical composition of CDW admixtures are reported, but the grinding process (e.g., equipment, duration, or particle size distribution) lacks detail. Elaborate on the grinding methodology to ensure reproducibility, as particle size significantly affects concrete performance.
4. Line 120-124: Table 2 lists the properties of CDW aggregates, including water absorption values (e.g., 17.5% for brick-based fine aggregate). These high values suggest potential issues with workability, yet the impact on mix design (e.g., additional water compensation) is not discussed. Address how these properties were accounted for in the mix proportions to maintain consistency.
5. Line 189-194: The workability results indicate a significant reduction in slump and slump flow (80% and 64%, respectively) with a 60%/40% brick-based/concrete-based aggregate ratio. The explanation attributes this to higher water absorption, but no quantitative adjustment (e.g., water-to-cement ratio or superplasticizer dosage) is provided. Include details on how the mix was optimized to mitigate this effect.
6. Line 204-205: The peak compressive strength (46.7 MPa) at a 20%/80% brick-based/concrete-based aggregate ratio is highlighted, but the underlying mechanism (e.g., densification or pozzolanic reaction) is not substantiated with microstructural evidence. Supplement this with SEM or pore structure data from the relevant curing age to support the claim.
7. Line 244-245: The workability analysis of the matrix with varying binder compositions shows a complex trend (slump increasing then decreasing). The lack of a clear explanation for the peak slump at 40% brick-based CDW admixture (230 mm) undermines the discussion. Provide a hypothesis, possibly linked to particle packing or superplasticizer interaction, to rationalize this behavior.
8. Line 165-169: The cost and CO₂ emission analysis relies on literature values (Tables 4 and 5) without validation against the specific experimental conditions. Given the variability in regional material costs and emission factors, conduct a sensitivity analysis or justify the applicability of these values to the study’s context.
9. Line 73-76: The introduction of large-scale CDW utilization is proposed, yet the discussion of prior studies (e.g., Yang et al. [22]) does not address potential trade-offs, such as durability concerns noted by Fernández-Ledesma et al. [24]. Include a balanced discussion of these trade-offs to provide a more robust assessment of large-scale applicability.
10. Line 154-156: The compressive strength testing method specifies a loading rate of 0.4 MPa/s, but the number of specimens tested per mix (three) is insufficient for statistical reliability given the variability in recycled materials. Consider increasing the sample size or performing statistical analysis (e.g., standard deviation) to enhance the robustness of the results.

Author Response

Response: Thanks for your comments and careful work, we have modified the related content according to your comments, and the details are as follows:

Comment 1: The statement that the 28-day compressive strength reached 46.7 MPa with a 24.9% increase requires clarification. The baseline strength for comparison is not explicitly provided, making it difficult to assess the significance of this improvement. Please specify the reference concrete's 28-day compressive strength to allow for a clear evaluation of the reported enhancement.

Response: Thanks for your comment and careful work, we have provided the baseline strength in the abstract for comparison. Compared to the dosage of concrete-CDW aggregates was 100%, the 28-day compressive strength at the optimal relative proportion (20% brick-CDW and 80% concrete-CDW aggregate) showed a clear increase of 24.9%. Many thanks for your kind work and comment.

Comment 2: The claim that urbanization in China produces approximately 1.5 billion tons of CDWs per year is cited broadly ([1-3]). However, the wide range of references suggests variability in estimates. Provide a more precise source or average value based on the cited studies to enhance credibility, or justify the use of this specific figure.

Response: Many thanks for your comment and suggestion. We estimated the annual production of CDWs in China using a calculation model from a relevant study [1-4]. In this study, CDWs were categorized into demolition waste and construction waste, The specific calculations are as follows:

1. Demolition waste production = Demolition area × Waste production factor per unit area, the demolition area is estimated as 10% of the new built area in that year, the waste production factor per unit area for demolition was obtained from Chinese Construction Manual [1].
2. Construction waste production = Construction area × Waste production factor per unit area. the waste production factor for construction was roughly estimated at 5% of the building materials consumption.

In recent years, the calculated CDW generation was about 1.3-1.5 billion tons/year. Thanks for your warm work.

[1]  Y. Zuo, Research of Chinese construction waste resource utilization and recommendation. V Beijing University of Civil Engineering and Architecture, 2015.

[2] A.S. Fernanda, A.S. Flavio, Recycled aggregates from construction and demolition waste towards an application on structural concrete: A review. J. Build. Eng. 52 (2022) 104452.

[3] H. Wu, R. Hu, D. Yang, Z. Ma, Micro-macro characterizations of mortar containing construction waste fines as replacement of cement and sand: a comparative study, Constr. Build. Mater. 383 (2023) 131328.

[4] R. Sabrin, M. Shahjalal, H.A.E. Bachu, M.M.L. Habib, T. Jerin, A.M. Billah, Recycling of different industrial wastes as supplement of cement for sustainable production of mortar, J. Build. Eng. (2024) 108765.

Comment 3: The particle size and chemical composition of CDW admixtures are reported, but the grinding process (e.g., equipment, duration, or particle size distribution) lacks detail. Elaborate on the grinding methodology to ensure reproducibility, as particle size significantly affects concrete performance.

Response: Many thanks for your comment and work, we have provided the specific ball milling method, duration, and the model of the ball mill used for producing the CDW admixtures, ensuring the reproducibility of the preparation process. Many thanks for your kind work and comment.

Comment 4: Table 2 lists the properties of CDW aggregates, including water absorption values (e.g., 17.5% for brick-CDW fine aggregate). These high values suggest potential issues with workability, yet the impact on mix design (e.g., additional water compensation) is not discussed. Address how these properties were accounted for in the mix proportions to maintain consistency.

Response: Thank you for your comment and warm work, we investigated the influence of the different relative proportion of brick-CDW and concrete-CDW aggregates on the concrete of performance, under the condition of a constant water content and superplasticizer content. the brick-CDW/concrete-CDW ratio for aggregate was fixed at 20%/80%, as this proportion demonstrated optimal compressive strength, the workability of recycled concrete changed slightly. In subsequent experiments, we found that the dosage of brick-CDW and concrete-CDW admixture was lower, the slump and slump flow of concrete increased in section 3.2. So, no additional water was introduced, ensuring the same workability of the concrete mixtures. Many thanks for your kind work and comment.

Comment 5: The workability results indicate a significant reduction in slump and slump flow (80% and 64%, respectively) with a 60%/40% brick-CDW/concrete-CDW aggregate ratio. The explanation attributes this to higher water absorption, but no quantitative adjustment (e.g., water-to-cement ratio or superplasticizer dosage) is provided. Include details on how the mix was optimized to mitigate this effect.

Response: Many thanks for your kind work and comment, we investigated the effect of the relative proportion of concrete-CDW aggregate and brick-CDW aggregate on the workability of concrete under the condition of constant water, the relative proportion of brick-CDW aggregate increased, the workability of recycled concrete mixture decreased, the superplasticizer and water-to-cement ratio of the concrete mixture was adjusted; the slump and slump flow could increase. Notably, the brick-CDW/concrete-CDW ratio for aggregate was fixed at 20%/80%, as this proportion demonstrated optimal compressive strength, the workability of recycled concrete changed slightly. In subsequent experiments, we found that the dosage of brick-CDW and concrete-CDW admixture was lower, the slump and slump flow of concrete increased in section 3.2, the workability of concrete was optimized. Many thanks for your kind suggestion and comment.

Comment 6: The peak compressive strength (46.7 MPa) at a 20%/80% brick-based/concrete-based aggregate ratio is highlighted, but the underlying mechanism (e.g., densification or pozzolanic reaction) is not substantiated with microstructural evidence. Supplement this with SEM or pore structure data from the relevant curing age to support the claim.

Response: Thanks for your comment and careful work, we have addressed the reason behind the strength enhancement from brick-CDW from the perspectives of pozzolanic reactivity and paste densification, The physical effect of the lower actual water-to-binder ratio and the chemical effect of the pozzolanic reaction synergistically led to increase the 3-day, 7-day, and 28-day compressive strength in section 3.1. Furthermore, the reasons for the strength improvement are further corroborated by SEM analysis within the section on the microstructure of recycled concrete (Section 3.3). When the volume ratio of brick-CDW aggregate to concrete-CDW aggregate reached to 20%/80%, the aggregate absorbed the surrounding free water, the water-to-binder ratio of the paste decreased, a water film could not easily form around the aggregate, and the amorphous siliceous and aluminous phases from the brick-CDW aggregate underwent the secondary hydration (pozzolanic) reaction with Ca(OH)₂ from the cement paste, a large amount of hydration products appeared around the aggregate, the densification of the paste increased, bonding the brick-CDW aggregate and the paste tightly, so the 28-day compressive strength of recycled concrete containing brick-CDW aggregate can increase. Many thanks for your kind work and comment. Many thanks for your kind work and comment.

Comment 7: The workability analysis of the matrix with varying binder compositions shows a complex trend (slump increasing then decreasing). The lack of a clear explanation for the peak slump at 40% brick-based CDW admixture (230 mm) undermines the discussion. Provide a hypothesis, possibly linked to particle packing or superplasticizer interaction, to rationalize this behavior.

Response: Many thanks for your comment and work, we have added a hypothesis in the relative paragraph, which explains that the slump and slump flow reached the peak values at the dosage of both the brick-based and concrete-based CDW admixtures was 20%, respectively. This can be attributed to the particle packing effect of brick-CDW and concrete-CDW admixtures with the help of superplasticizer [32], which occupied the spaces between cement particles, thereby reducing the water demand of the binders, consequently, the workability of recycled concrete improved under a constant water content. Thanks for your warm work.

Comment 8: The cost and CO₂ emission analysis relies on literature values (Tables 4 and 5) without validation against the specific experimental conditions. Given the variability in regional material costs and emission factors, conduct a sensitivity analysis or justify the applicability of these values to the study’s context.

Response: We sincerely thank the reviewer for this insightful and constructive comment, we have conducted a comprehensive regional variability analysis, which now serves as a robust validation of our assessment. The revisions are detailed in section 3.5 of the revised manuscript. Specifically, our approach was as follows:

We collected the costs and CO₂ emission of the most recent and representative raw material factors from four major regions in China, the costs and CO₂ emission were shown in Table 7 and Table 8. This data reflects a wide spectrum of real conditions. We also recalculated the total cost and CO₂ emission of our concrete mixtures using the data of these four cities systematically. The results, summarized in the new Figures 12 and 13, consistently demonstrate that the concrete mixtures incorporating CDW admixtures maintain the advantages of cost and CO₂ emission. To further reinforce this point, we have included the new Figure 14, which visually depicts the range of cost savings and CO₂ reduction percentages achieved across all four regions, the excellent performance of the recycled concrete is primarily governed by the reduction of cement content, this is a universally beneficial factor regardless of regional price or emission fluctuations. Furthermore, we conducted an sensitivity analysis focused on the minimum cost and carbon emissions, the regional differences have a negligible impact on the cost of the recycled concrete, this result implied the regional influence on the carbon footprint of concrete, primarily attributable to differences in the energy consumption and transportation involved in CDW management, which lead to variations in CO₂ emissions of the recycled concrete. And the sensitivity analysis of recycled concrete containing CDW admixture regarding the benefit of CDW utilization remains widely applicable. Many thanks for your kind suggestion and comment.

Comment 9: The introduction of large-scale CDW utilization is proposed, yet the discussion of prior studies (e.g., Yang et al. [22]) does not address potential trade-offs, such as durability concerns noted by Fernández-Ledesma et al. [24]. Include a balanced discussion of these trade-offs to provide a more robust assessment of large-scale applicability.

Response: We sincerely thank the reviewer for this insightful comment regarding the need for a balanced discussion on the trade-offs of large-scale CDW utilization. We agree that this perspective is crucial for a robust assessment, we have revised the respective paragraph in the introduction. When the large-scale CDW incorporated concrete, the mechanical performance of concrete deteriorated, the durability also declined in the introduction. For example, Alexandridou et al. [27] fabricated a kind of recycled concrete containing 75% recycled aggregate, the 28-day compressive strength decreased by 37%, the water absorption, frost and carbonation resistance of the recycled concrete mixtures exhibited lower performance than reference concrete. Based on this result, we evaluated the mechanical properties of concrete varied with the different CDW content, linked the durability of concrete with the different CDW content, and further durability tests will be undertaken in our subsequent research. Many thanks for your kind suggestion and comment.

[27] C. Alexandridou, G. N. Angelopoulos, F. A. Coutelieris, Mechanical and durability performance of concrete produced with recycled aggregates from Greek construction and demolition waste plants, J. Clean. Prod. 176 (2018) 745-757.

Comment 10: The compressive strength testing method specifies a loading rate of 0.4 MPa/s, but the number of specimens tested per mix (three) is insufficient for statistical reliability given the variability in recycled materials. Consider increasing the sample size or performing statistical analysis (e.g., standard deviation) to enhance the robustness of the results.

Response: Thanks for your comment and careful work, we tested three identical specimens for each group in accordance with the Chinese Standard GB/T 50081. Furthermore, error bars representing the standard deviation are included in the compressive strength results (Figure 6). The standard deviation within 15%, complies with the acceptance criteria of the Chinese Standard GB/T 50081. Thanks for your warm work.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents a comprehensive investigation into the utilization of construction and demolition waste (CDW) as substitutes for both aggregate and cement in concrete, aiming to overcome the technical barriers to its large-scale application. The systematic evaluation of workability, hardened properties, microstructure, and sustainability offers a valuable contribution to the field. To strengthen the manuscript, the following points require attention:
1. It is recommended to include detailed pre-treatment parameters (e.g., crushing, screening, and grinding processes) for the CDW-derived aggregates and supplementary cementitious materials.
2. The design basis for the ternary binder system, particularly the reasoning for the cement content range of 40% to 100%, should be succinctly explained.
3. A deeper discussion on the mechanism behind the strength enhancement observed with brick-based CDW is suggested, potentially linking it to the pozzolanic activity of the material.
4. The legibility of several figures (e.g., Figures 1, 4, 9, 10) needs improvement by increasing the font size of the labels and annotations.

 

Author Response

Response: Thanks for your comments and careful work, we have modified the related content according to your comments, and the details are as follows:

Comment 1: It is recommended to include detailed pre-treatment parameters (e.g., crushing, screening, and grinding processes) for the CDW-derived aggregates and supplementary cementitious materials.

Response: We are so sorry for that, we have provided the detailed pre-treatment parameters (crushing, screening, and grinding procedures) for both the CDW aggregates and admixtures in the relevant sections of the manuscript. Many thanks for your kind suggestion and comment.

Comment 2: The design basis for the ternary binder system, particularly the reasoning for the cement content range of 40% to 100%, should be succinctly explained.

Response: Many thanks for your comment and work, we have provided a succinct explanation of the design basis for the ternary binder system in the respective paragraphs, including the reasoning for selecting the cement content range of 40% to 100%. Our study employed the simplex centroid design, seven distinct concretes containing brick-CDW and concrete-CDW admixture were prepared, corresponding to the three vertices, three edge midpoints, and the centroid of the ternary diagram. This data was subsequently fitted using Design-Expert software to develop a predictive model for estimating properties at other mixture proportions. We incorporated concrete-CDW and brick-CDW admixtures into recycled concrete at replacement levels ranging from 0% to 60%. x1+x2+x3=100%, so the cement content ranged from 40% to 100%. Thanks for your warm and careful work.

Comment 3: A deeper discussion on the mechanism behind the strength enhancement observed with brick- CDW is suggested, potentially linking it to the pozzolanic activity of the material.

Response: Many thanks for your comment and suggestion, we have thoroughly revised section 3.2 of the manuscript to provide a more in-depth discussion on the mechanism, with a specific focus on the pozzolanic activity of the brick-CDW. we have elaborated on the pozzolanic reaction mechanism, explaining that the amorphous siliceous and aluminous phases in the brick-CDW react with the portlandite (Ca(OH)₂) liberated from cement hydration. We have clarified that this secondary reaction generates additional C-S-(A)-H gel, which is pivotal for refining the pore structure and strengthening the interfacial transition zone (ITZ), thereby leading to the observed increase in compressive strength. we are grateful for this valuable suggestion.

Comment 4: The legibility of several figures (e.g., Figures 1, 4, 9, 10) needs improvement by increasing the font size of the labels and annotations.

Response: Many thanks for your comment and suggestion, The font sizes of all labels and annotations in the figures throughout the manuscript have been increased for better readability. Many thanks for your kind work and comment.

Reviewer 3 Report

Comments and Suggestions for Authors

This study aims to explore the feasibility and effects of CDW (construction and demolition waste) as a substitute material in concrete. However, the paper does not clearly indicate the innovation of this study in comparison to existing literature. A more detailed comparison with previous research is recommended.

In the materials and experimental methods section, although the author provides the mixing proportions and experimental procedures, there is a lack of discussion on the repeatability of the experiments.

The paper mentions that the substitution ratio of CDW significantly affects the strength of concrete, but how can these ratios be optimized for achieving the best performance? Has the author considered conducting further experiments to explore other possible optimal combinations?

The paper analyzes the microstructure of CDW concrete using SEM and XRD, but there is a lack of in-depth discussion on how these microstructural changes affect the macroscopic performance. It is suggested to further explain the specific impact of these microstructural changes on the performance of concrete.

In the microstructure analysis section, it is recommended that the author conduct further quantitative analysis, such as using image analysis software to measure features like porosity and crack distribution in SEM images, and quantitatively compare the mineral composition changes based on XRD spectra. This will provide a more accurate description of the impact of CDW on the microstructure of concrete.

The clarity and information presentation of some figures, such as Figure 1, still leave room for improvement. It is suggested that the author increase the resolution of the figures.

Currently, the experiments only test the strength and performance at 28 days. Has the author considered conducting long-term performance tests (e.g., at 90 days or 1 year) on these concrete samples?

Comments on the Quality of English Language

This study aims to explore the feasibility and effects of CDW (construction and demolition waste) as a substitute material in concrete. However, the paper does not clearly indicate the innovation of this study in comparison to existing literature. A more detailed comparison with previous research is recommended.

In the materials and experimental methods section, although the author provides the mixing proportions and experimental procedures, there is a lack of discussion on the repeatability of the experiments.

The paper mentions that the substitution ratio of CDW significantly affects the strength of concrete, but how can these ratios be optimized for achieving the best performance? Has the author considered conducting further experiments to explore other possible optimal combinations?

The paper analyzes the microstructure of CDW concrete using SEM and XRD, but there is a lack of in-depth discussion on how these microstructural changes affect the macroscopic performance. It is suggested to further explain the specific impact of these microstructural changes on the performance of concrete.

In the microstructure analysis section, it is recommended that the author conduct further quantitative analysis, such as using image analysis software to measure features like porosity and crack distribution in SEM images, and quantitatively compare the mineral composition changes based on XRD spectra. This will provide a more accurate description of the impact of CDW on the microstructure of concrete.

The clarity and information presentation of some figures, such as Figure 1, still leave room for improvement. It is suggested that the author increase the resolution of the figures.

Currently, the experiments only test the strength and performance at 28 days. Has the author considered conducting long-term performance tests (e.g., at 90 days or 1 year) on these concrete samples?

Author Response

Response: Thanks for your comments and careful work, we have modified the related content according to your comments, and the details are as follows:

Comment 1: This study aims to explore the feasibility and effects of CDW (construction and demolition waste) as a substitute material in concrete. However, the paper does not clearly indicate the innovation of this study in comparison to existing literature. A more detailed comparison with previous research is recommended.

Response: Many thanks for your comment and work, we have indicated innovation of this study in introduction of manuscript in comparison to existing literature, While the use of CDW in concrete has been widely studied, previous research has predominantly focused on single-component substitution of CDWs and lacked systematic optimization strategies for reconciling large scale utilization with performance retention. Our study specifically addressed this gap by simultaneously incorporating concrete-CDW and brick-CDW at high replacement levels and optimizing their relative proportions as aggregate and admixture systematically. This dual-component optimization approach was beneficial to achieving large-scale CDW utilization and maintaining the performance of recycled concrete. Many thanks for your kind work and comment.

Comment 2: In the materials and experimental methods section, although the author provides the mixing proportions and experimental procedures, there is a lack of discussion on the repeatability of the experiments.

Response: Thank you for your comment and warm work, we characterized the CDW aggregates and admixtures many times until their properties stabilized. During concrete preparation, we first pre-mixed all solid raw materials uniformly for two minutes before introducing water and superplasticizer, followed by a final three-minute mixing stage, the performance of the recycled concrete was measured in strict compliance with the Chinese standards. This rigorous testing process ensures the reliability and reproducibility of the experimental outcomes. Our future work will involve the industrial-scale verification of the optimized mix design through a pilot test to assess its performance under actual production conditions. Thanks for your warm and careful work.

Comment 3: The paper mentions that the substitution ratio of CDW significantly affects the strength of concrete, but how can these ratios be optimized for achieving the best performance? Has the author considered conducting further experiments to explore other possible optimal combinations?

Response: Thank you for your comment and warm work, we have identified the best performance by varying the relative proportions, as documented in the manuscript. When concrete-CDW and brick-CDW were simultaneously used as aggregates with 100% replacement of natural aggregates, the relative volume proportion of brick-based and concrete-CDW aggregate reached to 20%/80%, the highest compressive strength reached 46.7 MPa. This finding provides a valuable reference for further experiments of CDW as aggregate in concrete formulations. When concrete-CDWs and brick-CDWs were simultaneously used as mineral admixture with 60% replacement of cement. Compared with concrete-CDW admixture, brick-CDW admixture incorporated into concrete, the compressive strength of concrete was higher. This result provides crucial guidance for the further experiments to explore other possible optimal combinations. Many thanks for your comment.

Comment 4: The paper analyzes the microstructure of CDW concrete using SEM and XRD, but there is a lack of in-depth discussion on how these microstructural changes affect the macroscopic performance. It is suggested to further explain the specific impact of these microstructural changes on the performance of concrete.

Response: Thanks for your comment, we have explained the reason for the 28-day compressive strength of concrete containing brick-based CDW aggregate increased. Based on SEM analysis, a significant amount of fibrous and flocculent hydration products appeared around brick-based CDW aggregate, bonding the brick-CDW aggregate and the paste tightly, so the 28-day compressive strength of concrete containing brick-based CDW aggregate can increase. Based on XRD analysis, we have provided further clarification on the decrease of the 28-day compressive strength of concrete containing concrete-based and brick-based CDW admixtures, the XRD results indicated a pronounced increase of the quartz and calcite crystallinity, along with a reduction in the Ca(OH)₂ diffraction peak in the paste containing the two types of CDW admixtures. Therefore, concrete-based and brick-based CDW admixture incorporated into the paste, the hydration reaction of the paste weakened, on one hand the hydration product decreased, the bonding capacity of the paste reduced, on the other hand the unhydrated residual water increased, the porosity of the paste increased after the residual water evaporating, These two reasons resulted in the 28-day compressive strength of concrete decreased. Many thanks for your kind work and comment.

Comment 5: In the microstructure analysis section, it is recommended that the author conduct further quantitative analysis, such as using image analysis software to measure features like porosity and crack distribution in SEM images, and quantitatively compare the mineral composition changes based on XRD spectra. This will provide a more accurate description of the impact of CDW on the microstructure of concrete.

Response: Many thanks for your comment and work, we acknowledged that the pores and cracks of the paste observed may potentially be attributed to the sample preparation process during extraction, thus are likely incidental. Therefore, these features may not be suitable for quantitative assessment of pore structure in the paste. A comprehensive analysis of the pore structure of the paste containing CDW was characterized by mercury intrusion porosimetry (MIP) in the subsequent section of this paper. Furthermore, in this study, thermogravimetric analysis was carefully employed to quantitatively evaluate the evolution of hydration products in the matrix containing brick-CDW and concrete-CDW CDW admixtures. The results clearly illustrated the variations of hydration products, thereby providing a more accurate description of the microstructure of concrete containing CDW. Many thanks for your comment.

Comment 6: The clarity and information presentation of some figures, such as Figure 1, still leave room for improvement. It is suggested that the author increase the resolution of the figures.

Response: We are an apologize for that. We have enhanced the readability of all figures by modifying and increasing the font sizes of the labels and annotations. Many thanks for your kind suggestion and comment.

Comment 7: Currently, the experiments only test the strength and performance at 28 days. Has the author considered conducting long-term performance tests (e.g., at 90 days or 1 year) on these concrete samples?

Response: Thanks for your warm and careful work, we tested the 56-day compressive strength, and observed that the 56-day compressive strength development rule of concrete containing CDW aggregates or admixtures closely mirrored the 28-day trend. Since the 28-day strength serves as the primary metric for evaluating concrete mechanical properties in Chinese standard, we focused the analysis and reporting on the 28-day compressive strength in this paper. Long-term performance tests will be carried out for further study. Many thanks for your kind work and comment.

Reviewer 4 Report

Comments and Suggestions for Authors

Conclusively, my suggestion for this article is to accept it after a minor revision. This article presents how to use the construction and demolition wastes in concrete. It conforms to the scope of Sustainability. However, I suggest the authors provide the following changes:

1. Figure 1 is too small. It is unclear. Readers need a more big one.
2. The title of Figure 2 is unclear. Please check it.
3. Adding a gap may be necessary to separate Figures 3(a)-3(b). They look like one figure.
4. Figure 4 is too small. It is unclear. Readers desire a more big one.
5. Adopt the SI unit in the whole article. Some other units such as psi may be changed.
6. Adding a gap may be necessary to separate Figures 9(a)-9(b). They look like one figure. Also, these two figures are too small.
7. The authors must present the advantages and disadvantages of using demolition wastes in concrete in the Conclusion section. 
8. The format of Reference 4, 7, 12 is inconsistent with other journal references. Please revise these references.

Author Response

Reviewer 4#:

Conclusively, my suggestion for this article is to accept it after a minor revision. This article presents how to use the construction and demolition wastes in concrete. It conforms to the scope of Sustainability. However, I suggest the authors provide the following changes:

Figure 1 is too small. It is unclear. Readers need a more big one.

The title of Figure 2 is unclear. Please check it.

Adding a gap may be necessary to separate Figures 3(a)-3(b). They look like one figure.

Figure 4 is too small. It is unclear. Readers desire a more big one.

Adopt the SI unit in the whole article. Some other units such as psi may be changed.

Adding a gap may be necessary to separate Figures 9(a)-9(b). They look like one figure. Also, these two figures are too small.

The authors must present the advantages and disadvantages of using demolition wastes in concrete in the Conclusion section.

The format of Reference 4, 7, 12 is inconsistent with other journal references. Please revise these references.

Response: Thank you for your comment and warm work, we have modified the related content according to your suggestion and comment. The details are as follows:

Comment 1: Figure 1 is too small. It is unclear. Readers need a more big one.

Response: We are an apologize for that. We have enhanced the readability of all figures by modifying and increasing the font sizes of the labels and annotations. Many thanks for your kind suggestion and comment.

Comment 2: The title of Figure 2 is unclear. Please check it.

Response: Many thanks for your comment and work, we have revised the caption of Figure 2 to provide a more detailed and clearer description of its content. Many thanks for your kind work and comment.

Comment 3: Adding a gap may be necessary to separate Figures 3(a)-3(b). They look like one figure.

Response: Many thanks for your comment and suggestion, we have added the separation between Figures 3(a) and 3(b) to facilitate easier distinction for the reader. Thanks for your warm work.

Comment 4: Figure 4 is too small. It is unclear. Readers desire a more big one.

Response: We are an apologize for that. We have enlarged all figures throughout the manuscript, including the font sizes within them, to facilitate reader comprehension. Thanks for your warm work.

Comment 5: Adopt the SI unit in the whole article. Some other units such as psi may be changed.

Response: We are sorry for the problem of the SI unit, we have revised all units in the manuscript to conform to the International System of Units (SI). Many thanks for your kind work and comment.

Comment 6: Adding a gap may be necessary to separate Figures 9(a)-9(b). They look like one figure. Also, these two figures are too small.

Response: Many thanks for your comment and suggestion, we have added a gap to separate Figures 9(a)-9(b) and Figures 9(c)-9(d), and we have enlarged all figures throughout the manuscript, including the font sizes within them, to facilitate reader comprehension. Many thanks for your kind work and comment.

Comment 7: The authors must present the advantages and disadvantages of using demolition wastes in concrete in the Conclusion section.

Response: We sincerely thank the reviewer for this valuable suggestion, we agree that a balanced discussion on the advantages and disadvantages is crucial for a comprehensive conclusion. The conclusion as follows:

When CDWs were employed as aggregates, the volume ratio of brick-CDW content exceeding 20%, the workability and compressive strength of recycled concrete declined, the volume ratio of brick-CDW aggregate to concrete-CDW aggregate was 20%/80%, yielding the highest compressive strength. When CDWs were used as admixtures, the workability of recycled concrete reduced at high incorporation levels. While the content of CDW admixture increased, the 28-day compressive strength generally decreased. Compared to concrete-CDW, brick-CDW admixture contributed to a high compressive strength.

Many thanks for your kind work and comment.

Comment 8: The format of Reference 4, 7, 12 is inconsistent with other journal references. Please revise these references.

Response: We are sorry for the problem of the format of reference, we have modified all references in manuscript, ensuring uniform reference format. Thanks for your warm work.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The revisions are thorough and satisfactory. I now support the publication of this paper.

Author Response

Many thanks for your suggestions and comments, we are truly encouraged by your kind and warm feedback and are delighted to hear that you now support the publication of our work. Your insightful comments throughout the review process were invaluable in helping us significantly improve the quality of our manuscript. We sincerely appreciate your time and expertise again.

Reviewer 3 Report

Comments and Suggestions for Authors

No comments.

Author Response

Many thanks for your suggestions and comments, we are truly encouraged by your kind and warm feedback and are delighted to hear that you now support the publication of our work. Your insightful comments throughout the review process were invaluable in helping us significantly improve the quality of our manuscript. We sincerely appreciate your time and expertise.