Strength, Chloride Ion Penetration, and Nanoscale Characteristics of Concrete Prepared with Nano-Silica Slurry Pre-Coated Recycled Aggregate

Round 1

Reviewer 1 Report

The paper investigated the compressive strength and microstructural analyses of the RAC added with NS slurry. However, the problem statement and discussions  are not properly written.

 

Detailed comments are given below:

1.       Introduction:

·         Line 30: please refine the 1.8 billion tons of CDW is global or regional production.

·         Line 85: The statement “there are no reports on the preparation of RAC by modifying RCA with NS slurry” is invalid as there is similar research article “Zhang et al. (2015) The modification effects of a nano-silica slurry on microstructure, strength, and strain development of recycled aggregate concrete applied in an enlarged structural test”, at which the article also utilized the nano silica slurry on the microstructure and mechanical properties of the recycled aggregate concrete. This showed a high overlapping of the scopes between both studies. Highly recommending the author(s) to revise the problem statements, and include this related reference in the Introduction, to emphasize which research gaps that the paper was trying to fill in.

·         Line 85: please rephrase the term “mechanical properties” as only compressive strengths were examined in this paper.

2.       Section 2.1:

·         Since NS is the main component of the study, can the author(s) explain on how the particle size of NS to be obtained? There was no methodology provided on the characterizations of the NS.

·         Chemical compositions of NS slurry are not provided.

3.       Section 2.2.1:

·         Any justifications on why 28-day and 91-day compressive strength age were chosen? Also, the difference between the 28-day and 91 day compressive strengths were not discussed at all in the Section 3.1 and thus, I failed to visualize the importance to find out the 91-day compressive strength.

·         Consider to rephrase the title for Section 2.2.1 from mechanical properties to compressive strength, as only compressive strengths were determined.

4.       Table 5: please elaborate on how the cost for the MRAC was obtained.

5.       Line 200: I agree that Figures 4d-f could show that the pores at the ITZ reduced compared to Figure 4a-c. However, how can the author(s) confirm that the compounds that wrapped around the RCA is NS slurry? It can be due to the author(s) who randomly selected an ITZ with a denser ITZ (with less pores), instead of NS slurry?

6.       Line 202: Please explain and justify the reaction between NS with calcium hydroxide to form C-S-H gel, preferably with the chemical reactions and reference citations. Also, as I commented earlier, the chemical compositions of the NS slurry is important to prove the feasibility of this reaction.

7.       Section 3.4: Please explain on how the classifications of the harmless-harmful pore sizes are made.

8.       Line 248: As commented earlier (on both SEM and line 202), the statement “NS on the surface of RCA provides nucleation sites for cement hydration, which causes more C-S-H gels to gather near RCA, strengthening the ITZ region of the RAC” remains doubtful unless the author(s) could prove that the NS slurry improved the ITZ by filling up the pores, similar comment is applicable to conclusion (3).

9.       Line 254, highly recommending the author(s) to rename the cost/strength (not to call it cost of concrete).

10.    Conclusion (5) is doubtful as I do not see any ‘significant’ improvement of Cp and CI based on Figure 8 or Section 3.6.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper with the title " Strength, chloride ion penetration, and nanoscale characteristics of concrete prepared with nano-silica slurry pre-coated recycled aggregate" is completely in line with the journal's themes and will undoubtedly be of interest to both academic and professional readers.

The paper is clearly written and organized on the form. However the conclusion is relatively long and must be shorten.

The approach of the mix design is highly is debatable and needs to be clarified by the authors. They would have three different concretes with three different effective water to cement ratios if they had employed dry aggregate without taking into account the absorbed water. Since various parameters have changed in this instance, comparisons are not realistic. Given the aggregates' varying capacity for water absorption, the problem is worse if the amount of water absorbed has been added to the mixing. The aggregates should have been presaturated before usage, but the wording was unclear about this.

For all these reasons the reviewer proposes a major revision

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

1. The response 2, on the problem statement/research gaps are not satisfactorily addressed. For instance, the author(s) only modify the statement "there are few reports on the preparation of RAC by modifying RCA with NS", and added one reference (Zhang et al. [18]), without elaboration on the research gaps of the past researches.

Furthermore, though the author(s) mentioned that "the effect of NS on the durability of RAC was not discussed", however, this only support one of the conclusions (chloride ion penetration). The author(s) failed to elaborate the research background and gaps in the literature, on why the other testing/analyses done in paper are essential (for instance, compressive strength, SEM, MIP, nanoindentation etc.)  

 

2. Response 5, I failed to find the TEM in the manuscript. 

3. Response 8: the calculations of the costs only covers the material costs. What about the fabrication costing, costs associated with the materials preparation, transportation? 

4. Response 12, the responses given in the response 12 is good and highly recommended to be included in manuscript. The discussions on all SEM, MIP, nanoindentation and reference citations are well integrated in the response and useful to explain the mechanism of adding NS slurry in the RAC, but I failed to find it in the manuscript.   

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The revised version can be accepted

Author Response

Thank you for arranging a timely review for our manuscript. Your critical comments and suggestions are very important for us to improve our work. Thank you for your recognition.