Effect of Immersion Time in Chloride Solution on the Properties of Structural Rebar Embedded in Alkali-Activated Slag Concrete
Round 1
Reviewer 1 Report (New Reviewer)
It is a very well written paper about the performance of cement samples during 9 years. The performance was monitored by EIS experiments, as well as chloride measurements, and X-ray analysis.
The paper gives more data about a topic that has been studied in literature. Unfortunately, the paper gives no a new interpretation or original way to analyze the results.
Author Response
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Reviewer 2 Report (New Reviewer)
This is a nice manuscript on the electrochemical monitoring of concrete corrosion, and it is well written in both English and science. The authors used EIS to have a prolonged measurement (up to 9 years) on their concrete sample, and various effective & important physical quantities (e.g., Cl permeability, pore volume, migration coeff.), based on which many insightful conclusions about corrosion products, pore-size change, internal chemical effects/change (e.g., Cl fixer), and their temporal variations have been naturally derived.
This manuscript can be accepted for publication as it.
BTW, the XRD results in Figure 8 have also impressed me, because it should be a nice evidence for a fundamental issue on the relative stability between Magnetite (Fe3O4) and Hematite (Fe2O3). Which one is more stable under an electrochemical condition? It has been a controversial point for decades, and recent accurate first-principles calculations, together with the validation by decades of evidences, show that Fe3O4 is obviously more stable than Fe2O3 (npj Mater Degrad 2019, 3, 26). This is also proved by the XRD measurements here during the 9-year period, and also consistent with the measurement on the weathering products on both ancient Earth and Mars, i.e., natural experiments during Billions of years (Nature Commun 2018, 9, 634; Sedimentology 2022, 69, 2371).
Author Response
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Reviewer 3 Report (New Reviewer)
This paper investigated the corrosion resistance of steels in alkali-activated slag concrete under long-term chloride environment. Electrochemical impedance spectroscopy (EIS) was used to monitor the corrosion status of steels in different periods, and the permeable pore volume and chloride permeability of alkali-activated slag concrete were studied simultaneously. The analysis and discussion of the test results are specific. However, the experimental method of this article is relatively simple and lacks some necessary test. For example, the influence of hydration products of alkali-active slag on concrete pores and chloride penetration has been mentioned many times in this paper, but no test results of hydration products in this paper have been found to support the author's opinion. Therefore, I really think this paper should be carefully revised. Some other comments are:
1. The introduction part lacks the elaboration of the research status, research gaps and innovation points, which should be supplemented.
2. The "Results and Discussion" section can be divided into small chapters to facilitate readers to better understand the research content.
3. The information about the type of steel used in the study should be added to the title.
4. Some key conclusions with quantitative data should be added to the abstract.
5. The reason why impedance values of the high frequency region at 7.5 years were higher than those at 6 years was not discussed sufficiently. Please elaborate on the mechanism more comprehensively and deeply.
6. In table 1, the value of R2 at 9 years is higher than that at 7.5 years. Please explain it.
7. The writing needs thorough proofreading as there are several mistakes throughout the paper. For example, Page 4 line 146, “are” is missing before “related”. Page 7 line 214, it should be “Figure 8”.
Author Response
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Reviewer 4 Report (New Reviewer)
Please see the comments.
Comments for author File: Comments.pdf
Author Response
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Round 2
Reviewer 3 Report (New Reviewer)
The authors have addressed all the issues raised by the reviewer.
Reviewer 4 Report (New Reviewer)
The manuscript was well revised.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
1. Table 2, all the Physical quantities should be in italics.
2.Figure 3, the phase angle should plotted as dot, not line, because EIS is an measurement at certain frequency.
3. There is a mistake in Figure 3, it is not -deg, it is deg.
4. A recent critical review published by Macdonald Digby in the Journal of Materials Science and Technology (2022, volume 112), should be discussed.
Author Response
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Reviewer 2 Report
The content of the manuscript under consideration is regarded adequate for the journal, although REVISION is required, especially with regards to the following issues:
1. The cites in the journal must be correctly matching the calls in the body of the manuscript, which clearly is not the case at least for those calls in Section 3. For instance, the CNLS acronym and procedure cannot be found in reference 17 (line 96), and the reference 21 does not seem to fit in the context of line 116.
In addition, also in the first half of section 3, the citations are placed as to bring results and/or observations from other works, instead of showing that they may agree with previous observations.
2. A major error occurs in line 88 in regards to the frequency range of EIS measurements, clearly not matching what is plotted in Figure 3.
3. Figure 3 should be splitted into 2 separate graphs depicting the impedance amplitude and phase respectively. More important, the lines in those plots should be the calculated functions obtained by using the parameters in Table 1, as this procedure will allow to directly observe the quality of the fits when the symbols show the experimental data.
Additional minor observations:
1. Use the same symbols for the various parameters whether included in a figure (Figure 4), table (Table 1) and the body of the manuscript. As for now, sometimes they are in subscripts, low case, etc. For example, the various presentations for Rs.
2. The sentence in lines 134-136 is not clear.
3. The verb in line 166 is quite strange, as the chloride ions are able to "be looking for..."
4. line 185 - It maybe a promoter rather than "corrosion generator". Corrosion occurs (although slower) even in the absence of chloride ions.
5. line 215, Use superscripts for the charge of ions, and subscript for stoichiometric indexes.
6. Figure 7 - Units are missing in the axes. And units should be added next to the labels included in the figure, that correspond to years. NOTICE that decimals must be separated by a stop instead of a comma.
7. Line 283 - The volume is missing
Author Response
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Reviewer 3 Report
The authors present a manuscript intended to describe the corrosion behaviour of steel rebars embedded in an alkali-activated slag concrete during a long testing period of 9 years. Although the title suggests an intensive use of EIS in the study, its use is minimal. Therefore, one can say that the paper is limited to some results obtained from applying the ASTM C1202 standard but without any justification for its applicability to this type of concrete.
The experimental design is inadequate for the large frequency range used in impedance (0.1 MHz to 1 mHz, not 10 kHz as stated in line 88) because the working electrode surface is not uniformly accessible for all the frequencies and can lead to erroneous R2 values in table 1. Moreover, the position of the reference electrode is inadequate to obtain Rs values coherent with the evolution of the system resistivity. This is perhaps why the impedance and the chloride penetration data do not match after seven years.
The assignation made in lines 127-33 to the parameters in Fig. 4 is unclear. R1Y1 corresponds to the concrete cover or the steel interface?. The sentence “The evolution of the impedance occurs in the first 28 days” is based on experimental data?.
In line 101, the authors talk about “flattened semicircles”, but no Nyquist plots are provided.
Those are the main aspects that, in my opinion, make the manuscript not suitable for publication.
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Author Response
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Round 2
Reviewer 2 Report
The comments and suggestions have been satisfactorily addressed in most cases, and only some minor errors require additional action. They are namely,
(1) In figure 3b, the ellipse employed to indicate the frequency range included in Semicircle 2 is wrongly labeled. A time constant RC (that leads to a capacitive semicircle in a Nyquist plot) includes a region with capacitive (phase < -45º) and resistive (phase > -45º). In Figure 3b, one time constant is found below 100 Hz, and all the values above this frequency are contributing to the second time constant.
(2) In Figure 7, either in the legend or inside the box in the graph, the time values 0-9 should be described (including units).
Author Response
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Reviewer 3 Report
In Fig 3b, the assignation of "semicircles" is incorrect. Moreover, at phase angle = 0, there is no capacitive behaviour.
I am sorry, but the sentence "The EIS technique was used because it does not consider the size of the reinforcing rod (working electrode) nor the depth within the concrete."
Author Response
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Round 3
Reviewer 3 Report
The authors did not make relevant improvements in the manuscript; thus, my advice remains unchanged.
Some examples funding my advice are as follows:
a) I insist the experimental set-up is inadequate to follow the pore structure's development because of the uneven electric field distribution between the working and counter electrodes. This fundamental aspect was not addressed by the authors.
b) Lines 90-91. The frequency range is 10^6 to 10^-3. However, Fig. 3 shows 10^5 as the high-frequency limit.
c) Fig. 3, the phase angle should be negative values in the Y axis.
d) In line 101 (results section), the authors state, “In addition, the EIS test allows the diffusion coefficient to be calculated and the degree of porosity of material to be related”. However, their model does not contain a diffusion-related element.
e) In line 107, the authors state, “The inflection defined at high frequencies is associated with the porosity process”. The porosity is not a process.
f) Further, in line 108the authors state, “…the one determined at low frequencies with the aluminosilicate mineral compounds' adsorption process on the steel surface”. Such an adsorption process is not demonstrated. Moreover, ascribing this time constant to that phenomenon is incongruent with the necessary charge transfer occurring in the low-frequency domain.
g) In lines 133-138, the lack of clarity in the assignation of time constants limits the claimed applicability of the model to follow the development of the porous structure.
h) In conclusion, the title is not in line with the paper's content because only the second part reports relevant data.
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