Low-Carbon Composite Based on MOC, Silica Sand and Ground Porcelain Insulator Waste
, Milena Pavlíková 1, Martina Záleská 1, Michal Lojka 2, Anna-Marie Lauermannová 2, Ondřej Jankovský 2 and Zbyšek Pavlík 1,*Round 1
Reviewer 1 Report
The need to find viable and environmentally acceptable alternatives to Portland cement is currently a critical issue. The speed of the urbanisation of our planet and the resulting amount of construction that all too often just churns out more of the same, based upon the use of Portland cement and steel, has to be addressed immediately. Your investigation of and research into this issue is commendable, seeking as you do alternative materials that use natural resources to manufacture and assess their capabilities. Although knowledge of Magnesium oxychloride cement (MOC) as an alternative non-hydraulic binder to Portland cement has been known since the nineteenth century, as you refer to in your paper, very little research has been carried out into the merits or viability of the substance - until now. Your investigation into MOC, the very detailed research that you have carried out and the results and recommendations you have come up with are commendable.
You set out to evaluate composites of MOC with laboratory processed porcelain waste as a partial replacement for natural sand, and analysed a number of samples using a broad spectrum of analytical methodologies to determine their chemical and phase composition, their thermal behaviour and their mechanical properties. The detailed write up of the broad range of methodologies you engaged with reads well, has a logical and convincing narrative, and is ably supported by a number of relevant and informative figures and tables.
The conclusions that emerge from your research are potentially world-changing. The fact that MOC represents a potentially viable alternative to Portland cement-based materials, as it is a low carbon binder and thus does not negatively effect the quality of the environment by depleting natural resources or by generating excessive greenhouse gasses is a huge step forward for the construction of our urban futures. The fact that the properties of MOC could be modified still further suggest that, as you say, more research on the topic should be carried out.
This is an excellent paper that presents and reports on ground-breaking research that can make a significant contribution to the future quality of our built environment.
Author Response
Thank you very much for positive review of our paper. Your evaluation of our paper and formulation of potential benefits of presented research for the quality of future built environment is highly appreciated.
We have checked and corrected English language and style throughout the manuscript and based on iThenticate software results removed most of the found similarities with previously published papers.
Reviewer 2 Report
The authors believe that the raw materials used require less CO2, but if the use of waste is a recovery, the quantities to produce MgCl2 nH2O, are not given and an overall assessment must be made to justify the hypotheses.
The word composite should not be used without evidence. A composite material must exhibit a specific stress-strain curve with dissipative behaviour. The word mixture is more approriate.
Obviously porcelain wastes are made of mullite, residual grains of quartz and amorphous silicate. Fig. 1 is therefore useless. I don’t see the interest of Fig. 2.
I’m sure the decimals given in the tables don’t make sense. Error bars must be determined and no decimals greater than uncertainty should be given. The assignments of the IR spectra must be improved. In addition, the spectra must be normalized. However, the data collected using FTIR-ATR is very sensitive to the grain size and the quality of contact with the diamond or Ge crystal. This explains the different intensities. Only the absolute intensities recorded on powder mulls or KBr/ICs pellets can be compared quantitatively.The 1021 cm-1 peak cannot be due to the Si-O or Al-O modes of mullite. It is the Si-O mode of magnesium silicate hydrate H2MgO4Si, a molecular compound (https://webbook.nist.gov/cgi/inchi?ID=B6004664&Mask=80), not Mg silicate. The modes of ionocovalent (alumino)silicates are much broader, see the spectrum of mullite (e.g. J. Mat. Sci 24, 1989, 3011). The 1607 cm-1 peak is also too narrow to be that of water
The strong 3390 cm-1 mode arises from water and the narrow peaks between 3500 and 3700 from O-H modes of hydroxides groups. The spectra are strongly perturbed near 3600 and 1600 cm-1. Is the automatic correction of water vapor carried out by the software (this point should have been indicated in the experimental part).
The following sentence is wrong: The bending and rocking vibrations of H-O-H in H2O can be observed as a series of absorption bands around 2000 cm-1. Features in this range results from (Mg)carbonate overtones. This part must be completely rewritten. The conclusions of the IR study should be used to discuss TGA. Why the authors do not show the TGA above 900°C in order to assess the amount of carbonates.
Why not compare with standard cements? If the mechanical (compressive and tensile) strength is too different from ‘standard’ cements, the interest of the work is very low.
Author Response
The authors believe that the raw materials used require less CO2, but if the use of waste is a recovery, the quantities to produce MgCl2 nH2O, are not given and an overall assessment must be made to justify the hypotheses.
REPLY: We thank reviewer 2 for review of our paper. We believe we addressed all your comments and suggestions in the revised manuscript. We hope the made changes significantly increased the overall quality of the manuscript and thus the presentation of our research. We would like to point out, other two reviewers rated out paper as excellent reporting on “ground-breaking research that can make a significant contribution to the future quality of our built environment”.
The particular comments and suggestions are responded below:
Porcelain waste was used as a partial substitution of silica sand. It means the CO2 already emitted within the production of porcelain insulators can be potentially split in the overall assessment of carbon footprint of porcelain waste. On the other hand, the porcelain waste can be considered as almost CO2 zero material, except the energy consumed for its milling. MgO-based cements are considered low-carbon materials, thanks to their CO2 sequestration ability and low calcination temperature (the typical calcination temperatures to produce light-burn MgO are 900–1050 °CC [R1, R2]. It was reported for MOC-based materials that they are able to offset the CO2 emissions during the carbonation, and the final net emitted CO2 linked with the whole life cycle of MOC is, therefore, 40–50% lower than that associated with Portland cement manufacturing [R3]. The text was revised accordingly.
R1. Wang, L.; Chen, L.; Provis, J.L.; Tsang, D.C.W.; Poon, Ch.S. Accelerated carbonation of reactive MgO and Portland cement blends under flowing CO2 gas. Cem. Concr. Compos. 2020, 106, 103489.
R2. Unluer, C. Investigating the Carbonation and Mechanical Performance of Reactive MgO Cement Based Concrete Mixes. Nano Hybrids Compos. 2018, 19, 23–33.
R3. Tang, S.; Hu, Y.; Ren, W.; Yu, P.; Huang, Q.; Qi, X.; Li, Y.; Chen, E. Modeling on the hydration and leaching of eco-friendly magnesium oxychloride cement paste at the microscale. Constr. Build. Mater. 2019, 204, 684–690.
The word composite should not be used without evidence. A composite material must exhibit a specific stress-strain curve with dissipative behaviour. The word mixture is more appropriate.
REPLY: In research of building materials, term composite is used for materials made of two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure, differentiating composites from mixtures and solid solutions. The typical engineered composite materials are: concrete, reinforced polymers, ceramic matric composites, plasters, etc. We believe the term composite is proper for our material as it is formed from binder – MOC, two types of fillers and water. The combination of raw materials and their properties made possible to produce versatile MOC-based composite.
Obviously porcelain wastes are made of mullite, residual grains of quartz and amorphous silicate. Fig. 1 is therefore useless. I don’t see the interest of Fig. 2.
REPLY: As the information on the mineral and the phase composition of porcelain waste was not provided by its producer, we conducted XRD analysis and identified main crystalline phases and observed amorphous background. We suppose, the diffraction pattern of porcelain waste is interesting for the readers and make the presentation of our research complete. Figure 2 provides information on the microstructure and particle size of porcelain waste. It gives also information on the distribution of main constituents of the porcelain matrix. We believe this information is interesting for readers and should be presented. We decided to move Figure 1 and Figure 2 to the Supporting information file (now as Figure S1 and Figure S2).
I’m sure the decimals given in the tables don’t make sense. Error bars must be determined and no decimals greater than uncertainty should be given.
REPLY: We agree with you and reduced or removed the decimals from Tables 4, 7, and 8. In case of mechanical parameters, the standard EN 1015-11 prescribes to round the strength values to 0.1 MPa. Therefore the data were left in their original form. Table 7 was modified, as the expanded combined uncertainties of the particular hygric parameters are newly introduced.
The assignments of the IR spectra must be improved. In addition, the spectra must be normalized. However, the data collected using FTIR-ATR is very sensitive to the grain size and the quality of contact with the diamond or Ge crystal. This explains the different intensities. Only the absolute intensities recorded on powder mulls or KBr/ICs pellets can be compared quantitatively.
REPLY: The data were normalized for ATR instrument and additionally the KBr pellets were prepared for the quantitative analysis. It was clearly visible the differences between the Mg(OH)2 amounts. The results from this measurement correspond to data obtained with ATR.
The 1021 cm-1 peak cannot be due to the Si-O or Al-O modes of mullite. It is the Si-O mode of magnesium silicate hydrate H2MgO4Si, a molecular compound (https://webbook.nist.gov/cgi/inchi?ID=B6004664&Mask=80), not Mg silicate. The modes of ionocovalent (alumino)silicates are much broader, see the spectrum of mullite (e.g. J. Mat. Sci 24, 1989, 3011). The 1607 cm-1 peak is also too narrow to be that of water.
REPLY: The FT-IR analysis of ceramic waste shows the intensive mode at 1079 cm-1, which is characteristic of the amorphous silica. In the case of composites, this peak is lower and more intensive is the hydrated product. Thank You for your opinion better will be formulation the „Si-O mode of magnesium silicate hydrate“. The text was revised as suggested.
The strong 3390 cm-1 mode arises from water and the narrow peaks between 3500 and 3700 from O-H modes of hydroxides groups. The spectra are strongly perturbed near 3600 and 1600 cm-1. Is the automatic correction of water vapor carried out by the software (this point should have been indicated in the experimental part).
REPLY: The automatic correction was used for the final spectra presentation. The measured data were analyzed as a raw data firstly, without any automatic correction. This information was newly provided in the description of experimental methods.
The following sentence is wrong: The bending and rocking vibrations of H-O-H in H2O can be observed as a series of absorption bands around 2000 cm-1. Features in this range results from (Mg) carbonate overtones. This part must be completely rewritten. The conclusions of the IR study should be used to discuss TGA.
REPLY: This spectral range includes a lot of absorption bands come from the bending and rocking vibrations of O-H in H2O and (Mg(OH)2) and also carbonates overtones. But the absorption bands intensities are low and were not analyzed in detail. The manuscript was modified accordingly.
Why the authors do not show the TGA above 900°C in order to assess the amount of carbonates.
REPLY: In the case of MOC-based materials, the carbonation led to the formation of chlorartinite - Mg2(CO3)(OH)Cl.n H2O. The process was described previously e.g. in Appl. Sci. 2020, 10, 2254; doi:10.3390/app10072254. This phase (if present) will decompose at approx. 350°C, which was previously confirmed by STA-MS. For this reason, we believe that above 800° there are present only MgO and PW in our samples.
Why not compare with standard cements? If the mechanical (compressive and tensile) strength is too different from ‘standard’ cements, the interest of the work is very low.
REPLY: We newly added the comparison of compressive and flexural strength and their ratio with ordinary PC concrete. Two new references were added and the text was revised accordingly.
Finally, we have checked and corrected the English language and style throughout the manuscript and based on iThenticate software results removed most of the found similarities with previously published papers.
Reviewer 3 Report
The paper can be accepted at present form.
Author Response
Thank you very much for positive review of our paper. Your evaluation is highly appreciated. We have checked and corrected English language and style throughout the manuscript and based on iThenticate software removed most of the found similarities with previously published papers.
Round 2
Reviewer 2 Report
All points have been clarified
