Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

The paper is well structured, and it presents a few potentially interesting points. Nevertheless, I have some doubts on aspects such as practical pilot industrial applications and statistical robustness.

I added few comments in the text and 

I would like the authors’ views on possible limitations:

• • Laboratory-Scale Testing: Experiments relied on laboratory-moulded cylindrical samples fired at a single temperature (950 °C), limiting direct extrapolation to industrial standard-format brick production14. What about possible industrial pilot-scale production? Do you think material size and temperature will make a difference? What about the economic impact?
• • Dependency on Pre-Calcined Ore: The manganese ore was pre-fired at 1200 °C before being added to the batch; the study did not determine whether raw, unfired ore could achieve the same strengthening effect during standard firing. Have you tried using raw materials? Do you think the firing temperature may produce different characteristics?
• • Small Sample Size for Correlations: While individual mix properties were evaluated across 5–6 specimens, correlation analysis was conducted on only four mix means, so only the density–water absorption relationship achieved formal statistical significance, and the dataset is still very limited.
• • Lack of Durability & Weathering Data: Frost resistance, long-term mechanical stability, and weathering under freeze–thaw conditions were not tested, leaving compliance with severe weathering standards (e.g., EN 771-1 categories F1/F2 or ASTM C62 grades MW/SW) W)

I understand that a few of these points might not be the main aims of this research, but the authors should at least include a few final remarks on the conclusions, as the suggested experimentation is intended for marketing.

Regards

 

Comments for author File: Comments.pdf

Author Response

Comment

The paper is well structured, and it presents a few potentially interesting points. Nevertheless, I have some doubts on aspects such as practical pilot industrial applications and statistical robustness.

Response

We thank the reviewer for this assessment. Both aspects are addressed below. Regarding statistical robustness, the whole data set was re-examined (Part A of this letter and Table R1).

Comment

I added few comments in the text.

Response

The reviewer file available to us contains the manuscript without annotations. We would be grateful if the comments in the text could be forwarded to us; we will address them promptly. All points raised in the report are answered below.

Comment

Laboratory-Scale Testing: Experiments relied on laboratory-moulded cylindrical samples fired at a single temperature (950 °C), limiting direct extrapolation to industrial standard-format brick production. What about possible industrial pilot-scale production? Do you think material size and temperature will make a difference? What about the economic impact?

Response

We agree. Specimen size and firing conditions are expected to matter: standard-format units fired in industrial kilns develop temperature gradients and follow longer firing cycles than small laboratory specimens, which may change the phase development and the uniformity of the color. The firing temperature also affects sintering and the stability of hausmannite, which is stable in air above approximately 890 °C and metastable below. A pilot-scale firing of standard-format units is therefore the next step before industrial use. The economic impact was not assessed; it depends mainly on the energy demand of ore calcination at 1200 °C, on transport and on the availability of the ash, and it requires a techno-economic assessment of the full cycle. These points are now stated explicitly.

Changes in the manuscript. Section 4, limitations paragraph (size, temperature gradients, firing cycle) and directions for further research (pilot-scale firing; techno-economic assessment including calcination at 1200 °C); Section 5, new final paragraph.

Comment

Dependency on Pre-Calcined Ore: The manganese ore was pre-fired at 1200 °C before being added to the batch; the study did not determine whether raw, unfired ore could achieve the same strengthening effect during standard firing. Have you tried using raw materials? Do you think the firing temperature may produce different characteristics?

Response

The as-received ore was not tested in this series. We expect different characteristics: the as-received ore consists of cryptomelane, braunite, calcite and quartz with trace hollandite. Without pre-calcination, these minerals would decompose and transform within the brick body during firing, close to the lower stability limit of hausmannite in air, and the CO₂ released by the carbonate would add porosity. In addition, the ore would be fired inside the body with restricted access of oxygen, unlike the calcination of the ore powder in air, so that the manganese oxides could be reduced further and bound as Mn²⁺ in silicates or in the glassy phase. Both the strength and the color may therefore differ from those obtained with the calcined ore. Comparative tests with as-received ore are planned.

Changes in the manuscript. Section 4: pre-calcination is described as an essential part of the formulation (paragraph on the possible explanation of the strength tendency) and the expected behavior of as-received ore is added to the limitations; Section 5, final paragraph; the title and abstract now specify the calcined ore.

Comment

Small Sample Size for Correlations: While individual mix properties were evaluated across 5–6 specimens, correlation analysis was conducted on only four mix means, so only the density–water absorption relationship achieved formal statistical significance, and the dataset is still very limited.

Response

We agree. The correlation analysis is now described as descriptive: the coefficients are computed over four mean values (two degrees of freedom), no correction for multiple comparisons was applied, and the composition variables are not independent. Only two coefficients formally reach p < 0.05 (clay content–water absorption, r = −0.96, p = 0.042, and density–water absorption, r = −0.98, p = 0.020), and the statement of the submitted version that density can serve as an operational criterion for property control was removed. We also correct the number of specimens: bulk density, linear shrinkage and mass loss were determined on five to six specimens per mix, but water absorption and compressive strength on three specimens per mix; the submitted version incorrectly stated five to six specimens for all tests.

Changes in the manuscript. Sections 2.2, 2.6, 3.5 and 4; Table 9 note; Section 5.

Comment

Lack of Durability & Weathering Data: Frost resistance, long-term mechanical stability, and weathering under freeze–thaw conditions were not tested, leaving compliance with severe weathering standards (e.g., EN 771-1 categories F1/F2 or ASTM C62 grades MW/SW).

Response

We agree. Frost resistance, long-term strength and weathering were not determined, so no EN 771-1 exposure category and no ASTM grade MW or SW can be assigned; this is now stated in Sections 3.4.2–3.4.4, 4 and 5. In checking this point, we found that the submitted version had assigned the categories F0 and F1 on the basis of water absorption. Since F0–F2 are freeze–thaw exposure categories established by testing, this assignment was removed.

Changes in the manuscript. Sections 3.4.2 and 3.4.3; Table 10 and its note; Section 4 (limitations and further research); Section 5.

Comment

I understand that a few of these points might not be the main aims of this research, but the authors should at least include a few final remarks on the conclusions, as the suggested experimentation is intended for marketing.

Response

A final paragraph has been added to the Conclusions. It states that the results refer to laboratory specimens with one ore dosage fired at one temperature and that pilot-scale firing of standard-format units, frost-resistance and durability testing, a comparison with as-received ore and a techno-economic assessment are required before industrial use.

Changes in the manuscript. Section 5, final paragraph.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The simultaneous evaluation of mechanical performance, phase composition, water absorption, density, shrinkage, and colour characteristics provides a useful basis for assessing the potential of the proposed material. In particular, the simultaneous strengthening and colouring effect associated with manganese-ore addition represents an interesting aspect of the work. The manuscript is generally well structured, and the experimental results are clearly presented. Nevertheless, several issues should be addressed before publication.

1. The proposed strengthening mechanism should be expressed more cautiously.
2. The conclusions concerning the beneficial effect of manganese ore are based on a single manganese-ore dosage (7%) and a single firing temperature (950 °C). Consequently, the study demonstrates the effectiveness of this particular formulation but does not establish an optimum manganese-ore content or firing condition. 
3. The role of pre-calcination of the manganese ore requires particular emphasis. 
4. Please ensure a consistent distinction between the colouring effect and its proposed physicochemical origin. 
5. The Introduction provides a good overview of ash-containing ceramics, manganese-bearing additives and the proposed role of Mn₃O₄. 

Author Response

Comment

The simultaneous evaluation of mechanical performance, phase composition, water absorption, density, shrinkage, and colour characteristics provides a useful basis for assessing the potential of the proposed material. In particular, the simultaneous strengthening and colouring effect associated with manganese-ore addition represents an interesting aspect of the work. The manuscript is generally well structured, and the experimental results are clearly presented.

Response

We thank the reviewer. After the revision, the strengthening effect is presented as a tendency (see Part A of this letter and the next response).

Comment

The proposed strengthening mechanism should be expressed more cautiously.

Response

We agree and have revised the corresponding statements. The strength difference between Mixes 4 and 3 is not statistically significant (p = 0.34) and is described as a tendency. The submitted version attributed the strengthening to nanoscale hausmannite together with the crystallization of mullite and akermanite. After re-refinement, mullite was not confirmed in the fired bodies, and melilite forms in all mixes, including the clay-only control, so these phases cannot explain the difference between Mixes 3 and 4; the hausmannite crystallites are about 113 nm in size rather than nanoscale. The nucleation of silicate phases on Mn₃O₄ particles is now presented as an unverified hypothesis, and the text notes that such a mechanism would also have to explain the absence of a strength gain in the ASW-free body.

Changes in the manuscript. Abstract; Highlights; Sections 3.2.4, 4 and 5.

Comment

The conclusions concerning the beneficial effect of manganese ore are based on a single manganese-ore dosage (7%) and a single firing temperature (950 °C). Consequently, the study demonstrates the effectiveness of this particular formulation but does not establish an optimum manganese-ore content or firing condition.

Response

We agree, and this is now stated explicitly: the results characterize the formulation studied and do not establish an optimum ore content or firing schedule. Variation of the ore content (5–15%) and of the firing temperature (950–1100 °C) is listed among the directions for further research.

Changes in the manuscript. Section 4, limitations paragraph and directions for further research; Section 5, final paragraph.

Comment

The role of pre-calcination of the manganese ore requires particular emphasis.

Response

We agree. Pre-calcination converts the manganese minerals of the ore into hausmannite before the ore enters the batch, so that the brick firing at 950 °C does not have to accomplish this transformation. This is now stated in the Discussion, and the calcined ore is specified in the title, abstract, highlights and conclusions. The limitations paragraph describes the expected behavior of as-received ore and notes the energy cost of calcination.

Changes in the manuscript. Title; Abstract; Section 2.1 (calcination procedure); Section 4; Section 5.

Comment

Please ensure a consistent distinction between the colouring effect and its proposed physicochemical origin.

Response

We have separated the two throughout. The colorimetric data quantify the darkening (L*, C*, h° and ΔE₀₀); its attribution to Mn₃O₄ is now described as an interpretation based on the XRD detection of hausmannite and on the optical properties of this phase reported in the literature, which was not verified by optical measurements on the fired bodies. The possible contribution of iron–manganese spinels is given as a possibility. The submitted version attributed the stronger darkening of Mix 4 compared with Mix 2 to Fe-bearing phases supplied by the ash; since the Fe₂O₃ contents of the clay and the ash are nearly equal, this explanation was removed, and the cause is stated as not established.

Changes in the manuscript. Section 1 (wording on the decorative effect reported in the literature); Section 3.6; Section 4.

Comment

The Introduction provides a good overview of ash-containing ceramics, manganese-bearing additives and the proposed role of Mn₃O₄.

Response

We thank the reviewer.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors The paper is very interesting. It covers a wide range of complex material study on fired clay bricks production process. This study examines whether a manganese ore can simultaneously restore strength and impart a decorative color to a brick body containing ash-slag waste. In my opinion the paper can be slightly improved as pointed above: - Please discuss the significant spread of results shown in the charts 5 and 6, how it influences the practical applications of the study? - Fig. 7 can have better resolution as well as Fig. 8 - Please clearly describe the main goal of the project considering practical application of the results.

Author Response

Comment

Please discuss the significant spread of results shown in the charts 5 and 6, how it influences the practical applications of the study?

Response

A paragraph on the scatter has been added. The strength scatter is large for Mixes 1 and 4 (coefficients of variation 39% and 28%) and moderate for Mixes 2 and 3 (12.8% and 13.5%), whereas density and water absorption vary much less. For practical application, the grades describe the mean performance and carry a considerable uncertainty: the lowest individual value of Mix 4 (8.76 MPa) is only slightly above the individual minimum of ASTM grade NW (8.6 MPa), and the lowest value of Mix 1 limits its GOST grade to M125. Reliable grading requires tests on a larger number of standard-format units, and serial production would require control of batch homogenization and forming. Figures 5 and 6 now show the individual specimens together with the mean ± standard deviation.

Changes in the manuscript. Section 3.3 (new paragraph after the reference to Figure 6); Figures 5 and 6; Section 4.

Comment

Fig. 7 can have better resolution as well as Fig. 8.

Response

Both figures have been redrawn. Figure 7 now has axis labels in words, and both figures are supplied at about 630–700 dpi at the printed width.

Changes in the manuscript. Figures 7 and 8.

Comment

Please clearly describe the main goal of the project considering practical application of the results.

Response

The practical aim is now stated in the last paragraph of the Introduction: to obtain a dark-colored brick body in which one fifth of the clay is replaced by ASW while the lowest strength grades of the regional, European and American standards are still met. The Discussion and Conclusions assess the results against this aim.

Changes in the manuscript. Section 1, last paragraph; Sections 4 and 5.

Author Response File: Author Response.pdf