Review Reports - Optimizing the Use of Fly Ash as Partial Replacement of Fine Aggregate and Cement in Portland Cement Concrete Mixes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

2. Materials and Experimental Program
Significant differences in oxide ranges. Do the authors have any comment on that?
What values were used?

A photo of the ash would be useful to confirm the 'grey' colour referred to.

Any detail on the source of the coal and how Class C and F are being produced there?
How was the ask sourced and divided into Class C & F.

Line 109 - Error! Reference source not found. No discussion on Fig. 1.

What is the justification/reasoning for the replacement levels chosen?

Has the Cement Type I & have a standard associated with it?

What are the design criteria for the concrete mix in Table 2 - strength, etc.

21 MPa at 28 days is a poor concrete. I would have expected 35+ MPa.

'Stone' may be used in the US but I don't understand what it means here.

Are there different aggregate sizes?

Bottom of Fig. 2 & Fig. 3 are cut-off.

A discussion on why the concrete and cements tests were undertaken including the data they will provide and where in the literature has this shown to be adequate to demonstrate the performance and behaviour.

3.1 Workability
There are relatively minor differences in the workability but the applications suggestions range from pumpable concrete to durability-focused applications. What are the standards required for each of these applications to quantify this statement?

3.2 Strength
21MPa is a very low strength target for concrete. The control samples strength achieved is also relatively low.
Again, some quantification on these results and their suitability is required.
I'm surprised 56 day strengths are not taken, which is common for SCMs in concrete due to their slow strength gain.
Not sure what the trendlines Fig 5(b) are adding to the results. The points made could be done using a bar-chart, like Fig 5(a)

The use of 'we', 'you'(line 248) etc. should be removed.

The use of electrical resistivity (lines 253-258) should be moved to the introduction.

Line 276-277 - Previous literature should be referred to and cited to support the claims.

3.2.2 Cement replacement
The authors state that pozzolanic behaviour is responsible for the increase in strength & durability of the blended cements. There is plenty of literature supporting this and it should be referred to here. XRD would have shown this.

Lines 342-345 - can the authors refer to previous work to support this conclusion?

References
The references throughout are nearly all pre-2020. Are there more up to date references available to bring this work to the state of the art?

Author Response

Comment 1:

Materials and Experimental Program
Significant differences in oxide ranges. Do the authors have any comment on that?
What values were used?

Response 1: The oxide levels were not determined specifically for the local fly-ash (FA) used in this study. Values in Table 1 were extracted from literature and a reference has been mentioned there.

Comment 2: A photo of the ash would be useful to confirm the 'grey' colour referred to.

Response 2: This description has been removed as it is not a scientific characterization of FA.

Comment 3: Any detail on the source of the coal and how Class C and F are being produced there?
How was the ask sourced and divided into Class C & F.

Response 3: We added “The fly ash (FA) sample was collected from a local Georgia Power coal plant (Plant Bowen) located in Bartow County, Georgia.”

Comment 4: Line 109 - Error! Reference source not found. No discussion on Fig. 1.

Response 4: Mechanical sieving was used for the coarse‐grained portion and hydrometer analysis was used for the fine-grained portion of the material for grain size distribution, in accordance with ASTM D2487‐06, ASTM D422, D1140 and AASHTO T88 and ASTM D7928‐17. No other gradation analysis was provided as D₆₀, D₃₀, D₁₀, etc. for FA cannot be determined from Figure 1.

Comment 5: What is the justification/reasoning for the replacement levels chosen?

Response 5: We chose these replacement rates arbitrarily to see the variation of strength as well as based on other studies.

Comment 6: Has the Cement Type I & have a standard associated with it?

Response 6: It is ASTM C150-07 Standard Specification for Portland Cement

Comment 7: What are the design criteria for the concrete mix in Table 2 - strength, etc.

Response 7: we have already posted the target levels for the key design variables in the table and updated based on the suggestions by the reviewer.

Comment 8: 21 MPa at 28 days is a poor concrete. I would have expected 35+ MPa.

Response 8: This mix design is for concrete pavement in Georgia, which aims at 21 MPa (3000 psi) as the 28-day compressive strength.

Comment 9: 'Stone' may be used in the US but I don't understand what it means here.

Response 9: We did not use the stone; we used coarse aggregate instead. It has been changed to Coarse Aggregate.

Comment 10: Are there different aggregate sizes?

Response 10: There are two aggregate stockpiles (fine and coarse aggregates) used for the PCC mixtures

Comment 11: Bottom of Fig. 2 & Fig. 3 are cut-off.

Response 11: Figures have been corrected.

Comment 12: A discussion on why the concrete and cements tests were undertaken including the data they will provide and where in the literature has this shown to be adequate to demonstrate the performance and behaviour.

Response 12: Thank you for the comment, we updated the Experimental Program with the following paragraph to support the flowcharts:

The selection of laboratory tests was guided by their relevance in evaluating both the fresh and hardened performance of concrete containing fly ash. The slump measurement was employed to assess workability, which directly influences concrete’s pumpability, placement, and finishing characteristics—properties critical in construction applications, particularly where fly ash can significantly modify flow behavior. Compressive strength indicates the key structural performance and durability potential, and is extensively used to quantify the effectiveness of pozzolanic reactions in blended cementitious systems. Electrical resistivity is effective in measuring concrete’s resistance to chloride ion penetration and assessing the characteristics of pore structure. Adopted in this study, surface resistivity is a tool to evaluate durability of concrete incorporating FA

Comment 13: 3.1 Workability
There are relatively minor differences in the workability but the applications suggestions range from pumpable concrete to durability-focused applications. What are the standards required for each of these applications to quantify this statement?

Response 13: Thank you for your insightful observation. You are correct that our discussion touches on a range of applications—from pumpable concrete to durability-critical scenarios—without yet specifying the standards associated with each. We have revised the manuscript to include references to relevant standards that delineate performance benchmarks for these applications.

For pumpable concrete, workability is commonly evaluated using the slump test (ASTM C143), along with additional parameters like flowability and segregation resistance when applicable, following ASTM C1611 for self-consolidating concrete. Standards such as ACI 304.2R provide guidance on the requirements for pumping.

For durability-focused applications, key performance metrics include resistance to chloride ion penetration (ASTM C1202), freeze-thaw durability (ASTM C666), and sulfate attack resistance (ASTM C1012). These tests help quantify the long-term performance of concrete exposed to aggressive environments.

In the revised version, we have clarified that while the observed changes in workability are modest, the compositional adjustments still enable the material to meet these performance criteria for specialized applications, as per the cited standards. This contextualization ensures our conclusions are more grounded in practice-relevant metrics.

Comment 14: 3.2 Strength
21MPa is a very low strength target for concrete. The control samples strength achieved is also relatively low.

Response 14: This design was for concrete pavement in Georgia, where 21MPa (3000 psi) is the required strength.

Comment 15: Again, some quantification on these results and their suitability is required.
I'm surprised 56 day strengths are not taken, which is common for SCMs in concrete due to their slow strength gain.

Response 15: We appreciate the reviewer’s thoughtful suggestion. We agree that quantification is essential for validating the suitability of fly ash-modified concrete for specific applications. To address this, we have enhanced the discussion in the manuscript by linking observed performance metrics—such as slump values, compressive strengths, and surface resistivity—to relevant industry standards (e.g., ASTM C143, C1611, C1202, C666, C1012) that establish benchmarks for workability, strength, and durability. This added context helps clarify the practical applicability of our results across different use cases.

Regarding the omission of 56-day strength data, we acknowledge that extended curing periods are especially relevant when dealing with supplementary cementitious materials (SCMs) like fly ash, which are known for slower pozzolanic reactions and long-term strength development. In this preliminary study, we focused on standard 7-, 14-, and 28-day strengths to remain aligned with typical construction timelines and to capture early-to-mid-term performance. However, we fully agree that 56-day data would offer valuable insight—particularly in evaluating the long-term benefits of FA Type F. We have included this as a clear direction in the “Recommendations and Future Research” section, noting the need for extended curing evaluations to better understand strength evolution beyond 28 days.

Comment 15: Not sure what the trendlines Fig 5(b) are adding to the results. The points made could be done using a bar-chart, like Fig 5(a)

Response 15: Fig 5(b) has been removed, and discussion has been updated not to include Fig 5(b). Similarly, Figs. 6(b), 9(b), and 10(b) have been removed and the discussions have been updated not to include these Figures.

Comment 16: The use of 'we', 'you'(line 248) etc. should be removed.

Response 16: Those words have been replaced.

Comment 17: The use of electrical resistivity (lines 253-258) should be moved to the introduction.

Response 17: It has been moved to the Introduction.

Comment 18: Line 276-277 - Previous literature should be referred to and cited to support the claims.

Response 18: Thanks for the comment. The revision describes the observations with clear and well-structured sentences as follows:

Figure 8b illustrates the relationship between electrical resistivity and compressive strength for mixtures containing FA Type F across different curing periods. At 7 and 14 days, a slight inverse trend is observed—resistivity tends to decrease as strength increases. This may be attributed to early-age mixtures achieving higher strength despite still having relatively interconnected pores, which facilitate ionic movement and result in lower resistivity. Conversely, at 28 days, this trend reverses, with higher resistivity corresponding to greater strength, likely reflecting ongoing pozzolanic activity that refines the pore structure and enhances both strength and durability. The subtle shifts in trend lines across curing ages highlight that electrical resistivity does not always correlate directly with compressive strength, especially at early stages. These findings suggest that the influence of FA Type F on strength and durability is both time- and dosage-dependent, with delayed pozzolanic reactions becoming more prominent at later curing stages.

Comment 19: 3.2.2 Cement replacement

The authors state that pozzolanic behaviour is responsible for the increase in strength & durability of the blended cements. There is plenty of literature supporting this and it should be referred to here. XRD would have shown this.

Response 19: We appreciate the reviewer’s comment highlighting the need to substantiate the pozzolanic mechanisms discussed in our manuscript. In response, we have revised the relevant section to incorporate recent literature that directly supports the role of pozzolanic reactions in strength and durability enhancement.

“The pozzolanic behavior in blended cements significantly contributes to improvements in both strength and durability by reacting with calcium hydroxide to form additional calcium silicate hydrate (C–S–H) gel, which densifies the microstructure and enhances mechanical properties. This reaction not only increases the compressive strength compared to plain Portland cement but also improves durability by reducing porosity and permeability, leading to longer service life under aggressive environments. Furthermore, the incorporation of pozzolanic materials reduces the clinker content and associated CO₂ emissions, aligning with sustainability goals.

X-ray diffraction (XRD) analysis is a well-recognized technique to demonstrate pozzolanic activity by identifying the reduction of portlandite (Ca(OH)₂) peaks and the formation of new crystalline and amorphous phases such as C–S–H gel in blended cements. Studies have shown that blended cements containing pozzolanic additions like basalt powder exhibit decreased intensity of Ca(OH)₂ peaks after hydration, indicating consumption by pozzolanic reaction, alongside the appearance or increase of C–S–H and other mineral phases. Similarly, infrared spectroscopy complements these findings by detecting characteristic bands related to calcium hydroxide and C–S–H gel shifts, confirming the pozzolanic activity.”

Comment 20: Lines 342-345 - can the authors refer to previous work to support this conclusion?

Response 20: A proper reference has been added to the paragraph

Based on the comparative analysis (Table 4), FA Type C is better for early strength and higher durability (resistivity), making it ideal for fast-track construction and precast elements. FA Type F is better for long-term strength gain and improving workability, making it suitable for marine structures and high-performance concrete.

Comment 21: References
The references throughout are nearly all pre-2020. Are there more up to date references available to bring this work to the state of the art?

Response 21: Several post-2020 references have been added to revision

Reviewer 2 Report

Comments and Suggestions for Authors

Abstract (Page 1)
Comment: The abstract provides a good overview but lacks specificity regarding the key findings for durability and workability. For example, it mentions "comparable workability and durability" but does not quantify or specify the conditions under which FA Type F underperforms in workability for aggregate replacement.
Suggestion: Include specific results, such as the optimal replacement percentages (e.g., 10% for fine aggregate, 10-20% for cement) and clarify the workability issue with FA Type F (e.g., reduced slump at higher percentages). This will make the abstract more informative.
Introduction, Section 1 (Page 1-2)
Comment: The introduction cites the American Coal Ash Association (ACAA) data from 2016, which is outdated given the current date of May 2025. This may not reflect recent trends in fly ash production and utilization.
Suggestion: Update the reference [1] with more recent data (e.g., ACAA reports from 2023 or 2024, if available) or acknowledge the limitation of using older data and justify its relevance.
Introduction, Section 1 (Page 2)
Comment: The discussion on fly ash as a fine aggregate replacement cites conflicting findings from Gupta et al. [7] and Siddique [8] but does not explain why these differences occur (e.g., fly ash type, particle size, or mix design). This leaves the reader unclear about the context of your study.
Suggestion: Briefly discuss potential reasons for the conflicting results (e.g., differences in fly ash composition or testing conditions) to better position your study’s contribution in resolving these discrepancies.
Materials and Experimental Program, Section 2.1 (Page 3)
Comment: Table 1 lists the chemical composition ranges for FA Types C and F but does not specify the exact composition of the fly ash used in this study. This reduces the reproducibility of the experiments.
Suggestion: Provide the specific chemical composition (e.g., CaO, SiO₂ percentages) of the FA Types C and F samples used in this study, either in Table 1 or a new table, to enhance transparency and allow comparison with literature.
Materials and Experimental Program, Section 2.2 (Page 4)
Comment: The concrete mixture design (Table 2) does not mention the water-cement ratio, which is critical for understanding the mix’s performance, especially since fly ash affects hydration.
Suggestion: Include the water-cement ratio in Table 2 or in the text and discuss how it was controlled across batches to ensure consistency, particularly for mixes with high FA content.
Results and Discussions, Section 3.1 (Page 6-7)
Comment: Figure 4 shows slump values, but the text does not quantify the slump changes (e.g., percentage increase/decrease compared to the control). This makes it difficult to assess the practical significance of workability changes.
Suggestion: Add quantitative comparisons of slump values (e.g., “FA Type C at 15% increased slump by X% compared to control”) in the text to complement Figure 4 and improve clarity.
Results and Discussions, Section 3.2.1 (Page 7-9)
Comment: The discussion on compressive strength for fine aggregate replacement (Figures 5 and 6) is thorough, but the polynomial equations in Figures 5b and 6b are mentioned without explaining their practical utility or statistical significance.
Suggestion: Clarify the purpose of the polynomial equations (e.g., for predictive modeling) and provide statistical metrics (e.g., p-values or confidence intervals) to support the reliability of the trend lines.
Results and Discussions, Section 3.2.1 (Page 10)
Comment: The discussion on resistivity (Figures 7 and 8) suggests a complex relationship between strength and resistivity but does not explore the underlying mechanisms (e.g., pore structure changes or ionic conductivity). This limits the depth of the analysis.
Suggestion: Incorporate insights from the referenced study [13] on FTIR and SEM analyses to explain how FA Types C and F affect pore structure and conductivity, linking these to the observed resistivity trends.
Results and Discussions, Section 3.3.2 (Page 13)
Comment: Table 4 compares FA Types C and F for cement replacement but does not address the economic or environmental implications of choosing one over the other, despite the study’s emphasis on sustainability (Page 2).
Suggestion: Expand Table 4 or add a brief discussion in Section 3.3.2 to include considerations like cost savings or CO₂ reduction for FA Type C versus Type F, aligning with the sustainability goals mentioned in the introduction.
Conclusions, Section 4.1 (Page 16)
Comment: The conclusion states that no empirical correlation between surface resistivity and strength could be established but does not discuss why this differs from studies like Lübeck et al. [20] or Medeiros-Junior and Lima [24], which suggest such correlations. This omission weakens the conclusion.
Suggestion: Briefly explain possible reasons for the lack of correlation (e.g., specific FA properties, testing conditions, or microstructure variations) and compare with findings from references [20-24] to provide a more robust conclusion.

Author Response

Comment 1:

Materials and Experimental Program
Significant differences in oxide ranges. Do the authors have any comment on that?
What values were used?