Effect of Metakaolin and Biosilica on the Mechanical Properties of Cementitious Mortars

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. Some sentences are not expressed smoothly and sound somewhat colloquial, for example, “resulting in greater strength.” A thorough language polishing is recommended to meet SCI journal writing standards and to ensure accuracy and academic rigor.
2. Several figures in the manuscript are of low resolution, making it difficult to discern important details and affecting the reader’s ability to interpret the results. The authors are advised to provide higher-resolution original images.
3. The manuscript makes extensive reference to GOST standards, which may not be familiar to international readers. It is suggested that the corresponding international standards (e.g., ISO or EN) be provided in parentheses when first mentioned to improve accessibility.
4. Some references are duplicated (e.g., Sabir et al., 2001), and the citation style is inconsistent. The authors should carefully check the reference list, remove duplicates, and unify the formatting. In addition, more recent studies (2022–2025) should be included to highlight the novelty and relevance of the research.
5. The discussion of the mechanisms by which MK and BS synergistically enhance mortar performance is rather general and lacks in-depth microstructural explanations. The authors are encouraged to include additional characterization techniques to better support the mechanistic interpretations.
6. Some figures (e.g., Fig. 5 SEM) lack clear annotations of the observed features, which makes it difficult for readers to understand their relation to performance improvement. Detailed labels within the figures and more informative captions are recommended.
7. Certain results (e.g., mechanical properties at different replacement ratios) are only presented as raw data without sufficient comparison to existing studies. The discussion should be expanded to include comparisons with previous works on MK or BS used individually, thereby highlighting the advantages of their combined effect and the innovation of the present study.

Author Response

We would like to express our sincere gratitude to the reviewers for their constructive comments and valuable suggestions, which greatly helped to improve the quality and clarity of the manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

1- There are several inconsistencies that should be corrected: (1) Density reduction is described as “up to 15.3%,” yet Table 5 shows Control = 2.33 g/cm³ and the minimum reported density = 2.02 g/cm³ (≈13.3% reduction). Please reconcile the text, figure calculations (in kg/m³) and the table values. (2) In section 3.5 (Water absorption and density), the text states “water absorption decreased to 3.7%” for the 10MK+Mf+ULT case; Table 5 lists 9.8%—this needs correction. (3) The control paste setting time is stated as 140/305 min in Figure 6 narrative, but the cement property table earlier gives 80/365 min; clarify whether these are different mixes/tests and ensure consistency. (iv) Section numbering repeats “3.5” later for Compressive strength—should be 3.7. These edits will prevent reader confusion.

2- The DLS reporting needs clarification. Statements such as “peak at 392 nm (100%)” and “100s%” appear to be typographical/interpretive issues; DLS typically provides intensity/volume/number distributions and a polydispersity index (PDI), not “100% at a single peak.” Please specify the dispersion medium, refractive index model, scattering angle, PDI, and whether results represent intensity-weighted or number-weighted distributions. Because MK and BS are irregular/porous particles, corroborate DLS with laser diffraction (for broader ranges) and/or BET specific surface area; add representative SEM/TEM particle size histograms. This will solidify the link you later make between fineness and microstructural refinements.

3- The Results compellingly show strength and absorption benefits, but the sustainability motivation in the Introduction would be better connected to quantifiable environmental outcomes in the Results/Discussion. Consider either estimating clinker reduction–linked CO₂ savings for your best-performing mixes or, resources permitting, adding a brief cradle-to-gate LCA comparison.  

4- The SEM discussion identifies “needle-like and plate-shaped crystalline formations, possibly ettringite/AFt” and links micrographs to densification claims. Please provide scale bars and magnifications in all panels, plus EDS point scans/maps to substantiate phase identification. To support the porosity/refinement narrative associated with reduced water absorption, consider adding mercury intrusion porosimetry (MIP) or N₂ sorption (BJH) to report total porosity and pore size distribution. Correlating these metrics with absorption and strength would substantially strengthen the Results.

5- In Figure 7, consider plotting density and absorption in separate subplots (or a dual-axis chart with clear legends and units) and include error bars. In the text, where you discuss percentage reductions, specify the calculation basis (absolute vs relative). Also, reconcile the countervailing trends—bulk density reductions due to lower specific gravity of SCMs alongside improved microstructural densification—by explicitly distinguishing between material density (kg/m³) and porosity metrics. This will pre-empt misinterpretation.

6- For flexural/compressive strength (Figures 8–9), please: (1) provide load–deflection curves and failure mode photographs to evidence improvements in toughness/crack control, especially for mixes with BS+Mf±ULT; (2) normalize strengths to bulk density (strength-to-density index) to show performance efficiency; (3) discuss whether early-age gains (7 d) persist at later ages (56/90 d) and whether any strength regression occurs at higher MK (15%).

Author Response

We would like to express our sincere gratitude to the reviewers for their constructive comments and valuable suggestions, which greatly helped to improve the quality and clarity of the manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents an interesting and relevant study investigating the effects of metakaolin (MK) and biosilica (BS) as partial substitutes for cement in mortar. The topic aligns well with current research trends in sustainable and high-performance cementitious materials, and the experimental scope appears comprehensive. The study demonstrates technical rigor and provides detailed quantitative results. However, several aspects of the manuscript require improvement to enhance clarity, structure, and scientific interpretation. Specific comments and recommendations are provided below.

The abstract lacks a clear IMRAD (Introduction, Methods, Results, and Discussion) structure, which makes it difficult for readers to quickly grasp the study’s context, approach, and significance. While it presents detailed findings and numerical data, it does not begin with a concise statement of the research problem or objective, nor does it briefly describe the experimental design or methodology used to obtain the results.

Graphical abstract is clearly represented and describes the idea of the research.

Use abbreviations throughout the text. E.g. supplementary cementitious materials (P2., L.89), metakaolin (P2., L.90) etc.

Table 2 can be optimized so that column heading are in one row.

Can Ordinary Portland cement be described according to EN 196-1? E.g. CEMI 52.5 N (or R?)?

In Table 2, the Na2Oeq or Na2O and K2O should be expressed.  

Section 2.1.6. Water.

Please remove misleading information. Otherwise there could be the misunderstanding that salty or dirty water is used. Given limits are general requirements for water for concrete, most probable even for concrete without rebars.

3.5. Water absorption and density.

Results are represented through water absorption by weight. That could give wrong understanding about structure development and porosity as mortar tends to decrease density due to raw material properties. Open porosity expressed as water absorption by volume could indicate open pore structure properties.

Conclusions.

It is stated that “The partial replacement of cement with MK significantly accelerated the mortar setting time from 12% to 64.7% owing to its high pozzolanic reactivity and fine particle size.” However, it is not clear whether the impact is due to the mentioned reasons, or the mortar was just stiffer as fine material is incorporated. Was the consistency measured? The use of superplasticizer confirms this observation.

The manuscript addresses an important topic and provides valuable insights into the influence of metakaolin and biosilica on mortar performance. Nevertheless, the paper would benefit from several revisions to improve the clarity of presentation, strengthen the methodological and interpretative explanations, and ensure consistency with scientific writing standards. In particular, restructuring the abstract, refining data representation in tables, and clarifying certain methodological details will improve the overall readability and credibility of the work. After careful revision and incorporation of the suggested changes, the manuscript could make a meaningful contribution to the field of sustainable construction materials.

Author Response

We would like to express our sincere gratitude to the reviewers for their constructive comments and valuable suggestions, which greatly helped to improve the quality and clarity of the manuscript.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have successfully addressed all critical points raised in the previous review. The revisions enhance the clarity and robustness of the study's findings. 

Author Response

Thank you very much

Reviewer 3 Report

Comments and Suggestions for Authors

I appreciate the authors’ detailed and thoughtful responses to the comments provided in the previous review round. The revisions substantially improve the clarity, structure, and completeness of the manuscript.

Please pay attention to the figure formatting, so the axis and text are not overlapping (figures 6,8, and 9).

In fig.7., 8 and 9. - There is no need to represent data values with 0.01 MPa precision. Round it to 0.1 or 1 MPa.

Author Response

We would like to express our sincere gratitude to the reviewers for their constructive comments and valuable suggestions, which greatly helped to improve the quality and clarity of the manuscript.

 

Comment 1: Please pay attention to the figure formatting, so the axis and text are not overlapping (figures 6, 8, and 9).

 

Response: Thank you for the comment. The axes and text in Figures 6, 8, and 9 have been adjusted to avoid overlapping.

 

Comment 2: In fig. 7, 8, and 9. – There is no need to represent data values with 0.01 MPa precision. Round it to 0.1 or 1 MPa.

 

Response: Thank you for the comment. The represent data values in Figures 7, 8, and 9 have been marked with an accuracy of 0.1 MPa, and the data in Table 5 have been revised accordingly and marked with green color.