Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript investigates the corrosion behavior of AlMg3.5 alloy (referenced from the Studsvik R2 reactor) embedded in concrete containing varying amounts of Fly Ash (FA) as a partial replacement for Ordinary Portland Cement (OPC). The study aims to evaluate the feasibility of using FA concrete to mitigate hydrogen generation and corrosion rates in the context of radioactive waste repositories. The topic is of significant interest to the nuclear waste management community. The authors employ a multi-method approach, including mass loss measurements, pH modeling (PHREEQC), micro-computed tomography (µCT), ATR-FTIR, and SEM-EDX. The finding that FA replacement reduces corrosion rates is valuable. However, the manuscript requires Major Revision before it can be considered for publication. There are notable inconsistencies in the interpretation of spectroscopic data (specifically regarding crystallinity), a lack of experimental pH validation for the simulations, and insufficient discussion on how the high water-to-cement ratio affects the microstructural benefits of Fly Ash.
(1) The study relies heavily on PHREEQC modeling (Figure 4) to assert that FA concrete reduces the pore solution pH to ~11.36 after 6 months. However, the manuscript lacks experimental validation of the pore solution pH (e.g., pore water expression or Ca(OH)₂ saturation analysis). Simulations are theoretical; without experimental confirmation, the claim that the pH drop is the sole or primary driver for corrosion inhibition remains weak. The authors should provide experimental pH data or discuss the limitations of the model in this specific matrix.
(2) In the Conclusions (Line 294), the authors state: "ATR-FTIR spectra showed more significant bands in FA concrete which could indicate higher crystallinity of oxidation products." This interpretation contradicts the data presented in Figure 7. The spectrum for the FA sample (red line) exhibits a lower signal-to-noise ratio and broader peaks compared to the OPC sample (black line). Broad peaks typically indicate lower crystallinity or amorphous phases, not higher. The authors must correct this interpretation. If the peaks are less distinct, it suggests the corrosion products in the FA matrix are less crystalline, which should be discussed in the context of the lower pH environment.
(3) The use of a w/c ratio of 0.7 is justified as a method to "facilitate corrosion continuation" (Line 100). However, this is significantly higher than the w/c ratios used in real repository concrete (typically < 0.45). Fly Ash is known to improve durability primarily by densifying the microstructure (filler effect and pozzolanic reaction). In a w/c 0.7 mix, the porosity is already very high, potentially masking the microstructural densification benefits of Fly Ash. The authors should discuss whether the observed reduction in corrosion is purely chemical (pH effect) or if the physical barrier effect of FA is suppressed by the high w/c ratio.
(4) The corrosion rates are calculated based on the initial geometric surface area (Line 136). As seen in Figure 3 (OPC sample), the corrosion is highly localized (pitting). Calculating corrosion rates based on geometric area for pitting corrosion significantly underestimates the actual penetration rate and local kinetics. The authors should acknowledge this limitation more prominently or, if possible, estimate a roughness factor or pit density to provide a more realistic range for the corrosion rate.
(5) Figure 6 provides a qualitative visualization of gas channels and cracking in OPC. However, the analysis lacks quantitative data. The authors should provide quantitative metrics, such as the volume of the void space around the sample, crack width distribution, or total porosity changes. Furthermore, µCT images for the FA samples (especially the 45% replacement) should be provided to visually confirm the suppression of hydrogen-induced cracking compared to the OPC reference.
(6) In the discussion of the FA sample (Figure 9), the authors note that Ca and Si are enriched in the same phases, attributing this to C-S-H formation. However, Class F Fly Ash is a silico-aluminous material. How can the authors definitively distinguish between unreacted Fly Ash particles, secondary C-S-H gel, and potential aluminum-silicate corrosion products (e.g., zeolitic phases) which often form in alkali-activated systems? A more rigorous analysis of the Ca/Si and Al/Si ratios is needed to support the claim of C-S-H formation vs. unreacted precursor.
(7) The identification of corrosion products relies heavily on FTIR. The peak at ~1000 cm⁻¹ is assigned to Si-O vibrations of C-S-H (Line 260). However, in a system with corroding Aluminum and Fly Ash, this peak could also correspond to aluminum-silicate species or aluminates. The authors should consider if XRD data (even if amorphous humps are present) or a more detailed deconvolution of the FTIR spectra could help distinguish between simple aluminum hydroxides and complex aluminosilicates.
(8) The results indicate that FA promotes more uniform corrosion, while OPC promotes pitting (Figure 3). The discussion should elaborate on the electrochemical mechanism behind this. Does the lower pH in the FA mix prevent the formation of a stable passive film, leading to active dissolution (uniform corrosion), whereas the high pH in OPC allows for passivation that is locally broken down (pitting)? Clarifying this mechanism would strengthen the paper significantly.
(9) While FA reduces the initial pH and corrosion rate, FA concrete is generally more susceptible to carbonation. In a repository scenario, deep carbonation could eventually lower the pH below the passivation threshold or alter the chemical stability of the waste form. The authors should briefly discuss the long-term trade-off: does the benefit of lower initial pH outweigh the risk of faster carbonation ingress over hundreds of years?
(10) Abstract: The sentence structure "AlMg3.5 alloy... compared to ordinary Portland cement concrete" is grammatically ambiguous. It should be rephrased to clearly state that the corrosion of the alloy in FA concrete is being compared to that in OPC. 
(11) The font size in the legends of Figure 5 is too small. Additionally, the use of green and red lines may be difficult to distinguish for color-blind readers or in black-and-white print; consider using different line styles (dashed vs. solid) or markers.
(12) Ensure consistent formatting for all references (e.g., capitalization of titles).

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Title: Corrosion 2. AlMg3.5 alloy under simulated cementitious repository conditions in Fly Ash concrete compared to ordinary Portland cement concrete

This manuscript studied the influence of replacing ordinary Portland cement I with 15, 30 and 45% of fly ash on the corrosion rate of a 2. AlMg3.5 alloy embedded in concrete, through the evaluation of mass loss, mCT, ATR-FTIR, and SEM/EDS.

The Introduction presents the relevant topics with appropriate detail, and the references are appropriate and relevant to support the sentences. The Conclusions summarize the concrete findings of the experimental work; however, Materials and Methods (section 2.) and Results (section 3.) should be improved:

1. Section 2. Lines 125 to 130. The modelling of pH evolution is not described in sufficient detail to allow the reader to follow the procedure. More details should be provided about the model used (for example, what do the authors mean by “It was produced as a kinetic system evaluating carbonation over a 6-month period including a simulation as a buffering capacity through a Portlandite and C-S-H buffer phase”?).
2. Section 3. Model: pH evolution and mineralogical alterations. Lines 172 to 176. Figure 4 presents the simulated pH results as a function of time for OPC concrete and for FA concrete (30%). More details should be provided regarding the lack of results for concrete with 15% and 45% of FA and on the specific findings for “mineralogical alterations”.
3. Section 3. Corrosion rates. Lines 180 to 189 and Figure 5. Please explain why the 3-month results for OPC are missing, as well as the 1 and 3-month results for FA concrete (15%) and FA concrete (45%).
4. Section 3. Micro computed tomography. Lines 194 to 197. The criteria used for sampling should be presented, as well as the results obtained for the other conditions under study.
5. Section 3. Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy. Lines 200 to 206. The results obtained for the other conditions under study should also be presented, or the criteria used to select only 6 months of exposure in 45 % FA concrete and OPC concrete should be explained.
6. Section 3. Scanning Electron Microscopy with Energy Dispersive X-ray analysis. Line 201 to 229. The criteria used for sampling should be presented.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed my main concerns, and the manuscript has been sufficiently improved to warrant acceptance.