Influence of Blast Furnace Slag on Pore Structure and Transport Characteristics in Low-Calcium Fly-Ash-Based Geopolymer Concrete

Round 1

Reviewer 1 Report

This  manuscript  tries  to  present  mechanical  and  microstructural  characteristics  of  fly  ash/slag-based  geopolymers.  The  paper  is  recommended  for  publication;  however,  a  minor  revision  is  requested  addressing  the  following  concerns:

1.  it  is  recommended  to  eliminate  the  term  of  cement-free  from  the  title.

2.  Introduction:  It  is  strongly  recommending  to  revise  the  "Introduction"  part  in  the  light  of  addressing  the  characterization  of  slag-based  geopolymers. 

3.  Adding a microstructural characterization (XRD test) of fly ash and slag is recommended.

4. Consider the sources of the fly ash and slag.

5. Replace the term of AAFS by geopolymer.

6. Recheck  the  reference  format  carefully  to  make  sure  it  is  consistent  with  the  journal.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

This paper performed an experimental study on the cement-free low-calcium fly ash-based geopolymer concrete. The effects of the slag incorporation were investigated on the strength, pore structure, and transport properties of the AAFS with various levels of fly ash replacements with slag. The replacement ratios of FA to SG 86 were 0, 10, 20, and 30%.

However, some aspects require clarification and modification, as detailed in the specific comments below.

1- The article needs grammatical and syntax improvements. The linguist level of this article does not meet the requirements.

2- Title

The title is probably the most crucial section of the whole paper. The title immediately clues the reader and the reviewers into what your point is and why it’s important. The title is not informative, specific, and understandable. The authors should modify the title.

3- Abstract

The abstract requires significant revision to improve the quality of the manuscript.

4- Introduction

4-1- In the introduction section, the literature review part is generally not well organized. The introduction needs to be revised.

4-2- The theoretical, analytical, and standard approaches should be discussed.

4-3- You are suggested to look into the following recent article as well:

https://doi.org/10.1016/j.conbuildmat.2021.125874

5- The following paragraphs (sentences) are unclear:

5-1- “Gas adsorption and desorption are the most widely used method for determining the surface area of powders and the porous materials' pre-size distribution. The Belsorp mini II surface area and pore size analyzer from Bel Company were used in this regard. Before performing the N2-adsorption/desorption test, the specimens were vacuum dried before degassing. The experiment was conducted at -195.85 °C by analyzing the nitrogen adsorp tion/desorption isotherms of around 2 g of AAFS pastes. The alkali liquid/binder and wa- ter/binder ratio of AAFS paste samples were kept equal to the corresponding concrete specimens.”

5-2- “Furthermore, ESR has been reported as a favorable way to evaluate chloride ion in gression and rebar corrosion in cement-based materials. The ESR value of 12 kΩ.cm has been reported as the limit for corrosion in internal steel reinforcing bars [16]. However, low ESR values of AAFS specimens might be due to the existence of ions such as Na in pore solution [16]. It is recommended to do further studies on ESR and rebar corrosion in alkali-activated materials. Moreover, Lee et al. revealed that slag addition significantly decreased the chloride ion penetration depth due to the denser and more tortuous matrix of the AAFS compared to AAF [26]. The ESR results were plotted versus compressive strengths, and the correlation between them was calculated. As shown in Figure 15, a strong correlation was observed between ESR and compressive strength..”

5-3- “Aligizaki and Hanif et al. reported pore size classification for the cement-based materials [66]. The pore size ranging between 2.5-10 nm was defined as small capillary pores (gel pores). The pores in the range of 10-50 nm were defined as medium capillary pores, whereas the pores sized between 50-10 micro were defined as large capillary pores. Table 6 displays the volume fraction of gel pores and capillary pores in the specimens. As shown in Table 3 and Figure 9, the total pore volume did not vary conspicuously with slag addition; however, an obvious increase in gel pores was observed, which related to the pore network refinement. The volume fraction of gel pores of the GP10, GP20, and GP30 increased 33.3 %, 175%, and 216.7%, respectively, compared with the GP0. These results revealed that large N-A-S-H capillary pores altered to the finer N - C-A-S-H gel pores. Furthermore, the highest increase in the gel pore volume fraction was related to the GP20. This finding is in line with compressive strength results in which GP20 showed the highest strength enhancement.”

6- Research significance

The purpose of the study is not mentioned in the abstract and introduction. What is the purpose of the study and the contribution of the results to the literature? Also, the significance of the study must be described in a separate section after the introduction.

7- The authors need to support the following sentences with appropriate references.

7-1- “Based on the SEM observations and pore structure analysis, adding slag to the GP0 led  to more geopolymeric products, which have finer pores, connecting geopolymeric products.”

7-2- “The decrease in WVP with increasing the SG could be related to the porosity, tortuosity, cracks of the gel, and pore size distribution.”

8- The authors only hardly discuss results. Please discuss your results more deeply.

9- Conclusion

Limited new knowledge can be found in the conclusion part. The conclusion section needs to be re-written.

10- Figures

The descriptions of axis and legends in different figures are not of the same size fonts. Furthermore, authors can enhance the quality of the figures.

11- The format of some references should be checked.

Minor editing of English language required

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The research topic is popular, and a lot of work has been done. In the experimental study of Cement-free low-calcium fly ash-based geopolymer concrete, this paper does provide some help. While there are still many problems. The authors are suggested to further revise the manuscript. Some suggestions:

1. The organization of the abstract is relatively rough. Avoid using abbreviations directly, such as N-A-S-H.

2. The statement in the section of Introduction is very fragmented. The content of this part should be close to the research points of this paper. Please reorganize it.

3. Please revise the errors on lines 94 and 100.

4. Please improve the clarity of the picture.

5. Please provide the device diagram of Unconfined Compression Test.

6. Numbers in Table 3 are not aligned, please correct.

7. Please add error bars to all data figures.

8. The conclusions are too general. Please summarize them point by point according to the display order in the text.

Author Response

please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The manuscript has been well revised, and it is recommended to be accepted for publication.