Use of Clay and Titanium Dioxide Nanoparticles in Mortar and Concrete—A State-of-the-Art Analysis

Round 1

Reviewer 1 Report

The topic was interesting and useful, while much more work should be added as a review paper. Some suggestions:

1. The Abstract should be very specific and concise. It needed to be improved. For example, “Both nanomaterials have been proved, by various studies, to increase the strength of the composite, when added, by themselves, in certain percentages.”, certain percentages? Why did not give the recommended dosage range directly?

2. There is no picture in the manuscript. This is very inappropriate. Pictures can help readers better understand the authors' point of view.

3. Please pay attention to some typos, such as “TiO2”.

4. The authors mixed the two different reference citation formats.

5. Missing section 3.1 and 3.7!

6. The structure of the paper was very strange, the section 4 Discussion was quite short. Generally speaking, the text shown in the Discussion part should be very rich.

7. The author mentioned the synergistic effect of TiO2 and NC in the Abstract, while, only the effect of the two materials acting alone was summarized in detail in the text.

8. The conclusions were not well organized, please try to summarize it point by point.

Author Response

1. The abstract was changed to include the suggestion of the reviewer.

2. Figure 1 inserted in the manuscript after receiving permissions from the corresponding author); Figure 2 inserted in the manuscript after receiving permissions from the corresponding author); Figure 3, section 2.5 was added to the text; Figure 4, section 3.5 was added to the manuscript

3. The typos related to character indices were corrected.
4. The agreed citation form by the journal was kept. All other forms of citation were removed and rewordings were done where necessary
5. Section 3 was renumbered so that it begins with 3.1 and ends with 3.6.

6. Since this is a review type of paper, we removed the discussions section and, instead, enriched the Conclusions section.

   Following the general trend of review manuscripts published lately, the Discussions section should refer to the own results of the authors in the general context of the state-of-the-art. This manuscript presents, in a concise manner, the findings available in the scientific literature and constitutes the starting point of a new research investigating the synergistic effect of TiO2 and NC in mortar and concrete.
7. The conclusions were rewritten to summarize the main findings of the study

Author Response File: Author Response.docx

Reviewer 2 Report

The paper as a whole is interesting. However, some comments are given below: They are either of clarifying nature or are intended for improvement.

The authors should clarify whether they intend to discuss clay or calcined clay or both.  Because, simple clay, even at nanoscale is not pozzolanic, whereas calcined clay may be pozzolanic. In the available literature, normally researchers differentiate clay from calcined clay. A clear demarcation would be better. While going through the document, it seems that nanoparticles (if not pozzolanic) act as nucleation sites for promoting hydration reactions.  

Abstract:

What are the other important properties that are improved? Kindly elaborate, e.g. workability, permeability, durability, etc. . Again what are the main variables mentioned in the abstract? Elaborate a little bit. Synergistic effects are mentioned in the abstract. However, after studying the whole document, the synergistic benefits of both nanomaterials are unclear. If this aspect is described for future studies, that should be clarified. The authors declare sustainable concrete with nanoparticles; however, there is no essence of sustainability in the abstract. Strength, durability, and sustainability are not synonyms. The authors have substantially discussed the effect of higher temperature throughout the document; however, it is nowhere mentioned in the abstract.

Introduction:

Again in the first paragraph, the authors are referring to sustainability. How the contents of the manuscript lead toward sustainability, are unclear. There is no direct link throughout the document.

The objectives mentioned in the last paragraph are two: The use of nano titania and clay is cos-effective and the available literature has conflicting information about their effect on the cementitious composites. In my view, these aspects are not elaborated well throughout the document. I will suggest making lines 952-955 the objectives of this study. Lines 952-955 better explain the objectives of the current study.

2.4.1 Scanning Electron Microscopy (SEM) Analysis: The literature review can be improved by inserting some relevant images.

2.4.3 X-Ray Power Diffraction (XRD) Analysis: The literature review can be reinforced with some relevant images.

Line 291: Which previous chapters?

Line 312 and Line 704: Correct the spellings of wass.

 Table 1: Which type of Montmorillonite was used by Shafabakhsha et al. (2020): Natural or calcined?

Table 2: What is an Organo-montmorillonite clay? Kindly elaborate somewhere.

What is the significance of Lines 410-411 under the compressive strength heading?

Line 421: Water curing is always superior to plastic warping? Isn’t? Is there any exception for nano-clay cement materials?

Line 431: How was CH consumed in a short duration of 7 days? Whereas secondary hydration is normally effective after 28 days.

2.5.3.1 Strength values at room temperature: It is not clear which nano clay is being employed.

Line 446-447: What is the reason for the impact of the w/c ratio?

Table 3: What nano clay, Wang is adding? It is missing?

2.5.3.2 Strength values at elevated temperatures: Kindly clarify the type of nano clay clearly.

2.6 Durability tests: This section lacks a table summarizing the impact of nano clay on the material.

Lines 594 and 595: Where these aspects are discussed in the manuscript?

Lines 605-611: Need a proper reference.

Table 5: Convert “3,36” to “3.36”. The type of Titania is missing.

Table 6: The type of Titania is missing in most cases.

Lines 947-949: The photocatalytic effect is not the topic of this study.

Conclusions: This section is dry. Rather “Discussion” section better explains the conclusions of the study. 

Author Response

1. 

The pozzolanic behavior of thermally and mechanically treated nanoclay is confirmed. Indeed, there is a debate regarding the pozzolanic potential of raw montmorillonite clay, which was used in several studies here discussed. However, in recent years, several studies were conducted which confirmed its pozzolanic potential. Papatzani et al. (2018) - developed a new method for determining the pozzolanic potential of nano-montmorillonite clay in its raw and organo-modified state. The carried out tests demonstrated that the inorganic nano-montmorillonite clay dispersions have a pronounced pozzolanic activity.

Moreover, the pozzolanic behavior of montmorillonite clay, i.e. MMT, is mentioned also in Mirgozar Langaroudi and Mohammadi (2018) - who used in its experiments natural hydrophilic MMT, and gives as reference for the pozzolanic potential of nano-raw MMT, the scientific article written by Chang et al. (2007). Irshidat and Al Saleh (2018) - who used in their studies hydrophilic nano-montmorillonite clay discussed the pozzolanic reaction observed between the nanoclay and free lime during the hydration process.

2. In the past decades, nanomaterials have become one of the focal points in the civil engineering research. When added to cement-based construction materials (e.g., concrete), it results in significant improvements in their strength and other important properties. However, the final mix characteristics depend on many variables that have to be taken into account. As such, there is no general consensus regarding the influence upon the original material of certain nano-sized additives, the optimum dosage or the synergistic effect of two or more nano-materials. This is also the case of titanium dioxide (TiO₂) and nano-clay (NC). The paper focuses on reporting the existing research data on the use of the above-mentioned materials when added to mortar and concrete. The collected data is summarized and presented in terms of strength and durability properties of cement mortar and concrete containing either TiO₂ or NC. Both nano-materials have been proved, by various studies, to increase the strength of the composite, at both room and elevated temperature, when added, by themselves, in 0.5%~12% for TiO₂ and 0.25%~6% for NC. It can be inferred that a combination of the two with the cementitious matrix can be beneficial and may lead to obtaining a new material with improved strength, elastic and durability properties that can be applied in the construction industry, with implications at the economic, social and environmental levels.

3. 

Although the word “sustainability” is not clearly present in the first paragraph, the information provided there may lead to this concept.

The reviewer in right, the analyzed data presented in the manuscript is not straight forward related to sustainability. On the other hand, the improvements of mechanical and durability properties of cement-based construction materials due to the addition of nanomaterials lead to smaller cross-section for elements, less maintenance / repairing / retrofitting works / less material consumption.

The manuscript was not intended to summarize all possible implications (beneficial) of using nano-materials in mortar and concrete as it would result in a very long and, maybe, difficult to read paper.

4. The last paragraph was modified to express more clearly the intention of the authors.

5. Figure 1 inserted in the manuscript after receiving permissions from the corresponding author)

6. Figure 2 inserted in the manuscript after receiving permissions from the corresponding author)

7. The chapters in question are 2.2 and 2.4. The “previous chapters” expression was deleted and replaced with a more specific reference - “Sections 2.2 and 2.4”.

8. The spelling was corrected.

9. Shafabakhsha et al. (2020) does not mention which type of montmorillonite was used in their experiments. The authors provide only information regarding the physical properties of the material (diameter, surface volume ratio, density and color). Taking into account the level at which the montmorillonite mixing was done, it can only be presumed that the authors used a natural hydrophilic montmorillonite clay. For clarifying purposes, in similar cases, a note has been added in the table (“type not specified”).

10. The organo-montmorillonite clay, or organo-modified montmorillonite clay, denoted also as OMMT, is obtained after an ammonium cation treatment of the hydrophilic montmorillonite clay. The purpose is to decrease the hydrophilicity of natural montmorillonite. Section 2.1 describes briefly the “organo-modified montmorillonite clay” term, while Section 2.4.1 elaborates on the different effects that natural montmorillonite and OMMT clays have on cement matrix.

11. Taking into account that the paragraph found at 409-413 is misleading, it has been removed. It was meant to make a connection between section 2.5.2 and 2.5.3.

12. 

According to the test results presented in Shafabakhsha et al. (2020) - reference [51] - water curing is always superior to plastic wrapping for nanoclay-modified cement composites. The smallest difference was obtained at 28 days, but as the curing period increased, this difference also increased.

The reason is that both cement and nanoclay particles require water to hydrate and promote CSH formation. When cured in water, they have all the water needed for the hydration process. When the curing is done by plastic wrapping, they depend only on the water added during the mixing stage. At some point the water will be consumed and the hydration stopped, along with the formation of CSH crystals.

13. Not all CH was consumed in those 7 days, but a large amount of it. Actually, as the fragment states, based on the Mirgozar Langaroudi and Mohammadi (2018) study, in the time interval 0-14 days of curing  the hydration process reached a maximum level. From then on, the compressive strength increment between the curing stages decreases, due to a decrease in hydration. The hydration process continues, even after 28 days, but at a much slower rate. To be noted that nanoclay addition increases the hydration rate compared to the control sample

14. Except for the cases in which the nanoclay type was mentioned, for the other ones, it was added “type not specified”. As discussed above regarding the Shafabakhsha et al. (2020) scientific paper, only assumptions can be made. To be noted that this is only the case for nano-montmorillonite clay

15. By comparing the 0.4 and 0.5 water to cement ratio, Wang (2017) emphasized the importance of adding a sufficient amount of water in the concrete mixing. The workability, consistency and the level of hydration are determining factors. Thus, an optimum combination between w/c and nanoclay percentage must be determined.

16. Wang (2017) uses nanoclay in its concrete specimens. However, the type of nanoclay is not specified within its work. The author listed instead some properties of the used nanoclay: color, the chemical composition, particle diameter, pH value, cation exchange capacity, moisture content and Portland suction. Based on the references provided by the author in the introduction section, it can be presumed that nano-metakaolin was used in the experiments. Another point backing up this presumption is that the nanoclay-modified concrete specimens were subjected to temperatures up to 1000⁰C. Therefore, a calcined nanoclay type would have been a better choice, thus preventing phase changes of nanoclay during temperature increase. 

17. The type of nanoclay was added in Table 4, according to the information presented in Tables 3 and 2.

18. Table 5 added in Section 2.6. Similarly, Table 10 is added in Section 3.5

19. Line 687 - The plastic shrinkage decrease is mentioned at lines 659-665, based on Lee et al. (2018). Line 688 - The extended reinforcement deterioration time is discussed at lines 666-670, based on the impressed voltage test results of Mirgozar Langaroudi and Mohammadi (2018).

20. The references regarding the use of titanium dioxide in the construction of the Jubilee Church in Rome Italy were added from the existing Reference section, namely the references [58-60]. A new reference was added for the Torre de Especialidades building indicated in the text.[61]

21. The typo was corrected. The type of titanium dioxide is not mentioned in most cited papers. However, the authors gave information regarding the chemical and/ or physical properties of the used titania. Taking into account that no specific mention was made regarding the type of the titania used only presumptions can be made based on the particle size, for example. According to Janczarek et al. (2022) rutile is stable only for a particle size greater than 35 nm. For instance, Nazari and Riahi (2010), in their experiments, used titania with an average particle size of 15 nm - probably anatase. Bragança et al. (2017) added in their mixture titania of 25-100 nm particle size - probably a mixture between anatase and rutile. To avoid misleading information, when the type of titanium dioxide nanoparticles is unknown, the phrase “type not specified” was added in the tables.

22. When the information about the type of titania used is lacking, the phrase “type not specified” was added in the table, together with an assumption regarding the type (if possible). It was based on the information given by the author/authors of the cited work and on some specific characteristics of each titania type.

23. The fragment regarding the photocatalytic effect was removed.

24. The conclusions were rewritten to summarize the main findings of the study.

25. The Discussion section was removed since this is intended to be a state-of-the-art review. The results from the scientific literature are discussed in the appropriate sections of the manuscript and therefore there was no need for another such section.

Author Response File: Author Response.docx

Reviewer 3 Report

The article has great relevance in the academic environment, however, to accept it the author must correct the following items

1) I propose that the author review the English version of the manuscript.

2) The summary should be written in the following order: aim, introduction, methodology, results, and conclusions.

3) Keywords are in lower case. Capitalize initials

4) My suggestion to the author is to write the state of the art at the end of the introduction.  Generally, it is one of the most important parts of the work, because it brings together the conclusions that another scientific research has reached on the subject. The state of the art highlights works by other authors describing the results of the influence of placing nanomaterials in concrete. Note that the author should describe whether there was an increase or decrease in resistance to compression, absorption, ...

5) References are relevant but there are obsolete items. I suggest replacing it with articles that have updated the references for a minimum of 5 years.

6) “Nanoclay is defined as a layered mineral silicate, which, due to its filler and pozzolanic characteristics succeeds in enhancing the properties of several types of materials like polymers or cement-based materials”. What are the properties?

7) “Previous studies [44] concluded that in case of Nanoclay, the sonication process has a positive effect on the final mechanical properties of the concrete samples”. What are the previous studies? Cite these studies in the text.

8) The phrase “considering that when using sand or aggregates” is confused. Sand is a fine aggregate and Brita is a coarse aggregate. The correct thing would be “considering that when using sand or gravel”.

9) What is the relationship between the improvement of the resistance to compression, traction, flexion of the material and the microstructure. It would be interesting to cite an author with an updated year proving this relationship in item 2.5.

10)  Explain in item 2.6, when the curing period increases, does the permeability coefficient decrease?

11) The first paragraph of item 3 put reference.

12)  In the discussion, the author put the citation and at the end the reference with the same author. Both cannot be put together.

13) In the sentence “However, the nanoscale particles have a filler effect on the matrix due to their reduced size. They can enter these voids and reconstruct the ITZ so that the weak areas are reduced in size and the development of microcracks is slowed down.

This phenomenon was observed by comparing the strength increment when adding TiO2 nanoparticles in the matrix, in cement mortar and concrete, respectively” put reference.

14)  In conclusion, it would be interesting for the author to mention the results of resistance to compression, flexion, ... with the placement of nanoparticles

Comments for author File: Comments.pdf

Author Response

1. English language has been thoroughly revised by the authors.

2. Abstract changed to reflect reviewer’s suggestions.

3. Following the manuscript template and by looking at other papers accepted and published in Coatings, all keywords are written without capitalizing the initials. Therefore, with the reviewer’s permission, we would like to keep the keywords written in lowercase.

4. The end of the Introduction section was rewritten to include some information from the main body of the manuscript in the form of a state-of-the-art.
5. The article cited 91 scientific works and 1 link.  From these, 51 are papers written between 2018 and 2022, 15 – papers written between 2013 and 2017, 14 – between 2008 and 2012 and 11 – between 2002 and 2007. In the case of nanoclay, there are 10 papers written between 2002 and 2007, which are cited at the beginning of Section 2, with the sole purpose of presenting shortly the history of nanoclay use in research. The research on both nanoclay and titanium dioxide addition mortar and/or concrete is dispersed in time, but the studies do not go to the next technology readiness level. Most of them, including the most recent ones, analyze cement paste samples. However, this article focuses on mortar and concrete. Moreover, our focus was on finding studies which use mainly nanoclay or titanium dioxide, without any other nanomaterial addition. Thus, if some studies were considered to have a great relevance on the subject or if they presented an interesting aspect, although they were written 10 years ago, they were added.
6. The phrase has been completed as follows: “[...] in enhancing the mechanical and durability properties of several types of materials “
7. The expression “Previous studies” has been replaced with “A previous study”, the reference being already given for this study. 
8. The word “aggregates” was replaced by “gravel”.

9. A denser microstructure, obtained by using supplementary cementitious materials and/or nao-materials, results in improved mechanical and durability properties as highlighted in 3 recent studies from the scientific literature. The following statement was inserted in the first paragraph of section 2.5. However, considering the reports from recent studies, the use of nano materials, nanoclay included, results in a denser microstructure of the material with net benefits in terms of strength and durability properties [51–53].

10. Two cases were discussed regarding the permeability coefficient. Chang et al. (2007) analyzed the permeability coefficient for a nano-montmorillonite-modified cement paste sample, for different curing periods. The final result outlined a decrease of the permeability coefficient as the curing period increases. Only at 0,8% nanoclay addition, the permeability coefficient increases up to approximately the same level as the control sample.

    On the other hand, Patel (2012) tests nano-metakaolin-modified concrete samples with the purpose of finding out the permeability coefficient for a single curing period. The author observed an increase of the permeability coefficient in the mortar sample, with the increase in nanoclay percentage.

    They are two different studies, with different parameters, and it is difficult to make a solid connection between them.

11. The references regarding the use of titanium dioxide in the construction of the Jubilee Church in Rome Italy were added from the existing Reference section, namely the references [58-61].

    A new reference was added for the Torre de Especialidades building indicated in the text
12. The agreed citation form by the journal was kept. All other forms of citation were removed and rewordings were done where necessary
13. This fragment is a conclusion of the authors based on the information provided by the results of the cited scientific papers, namely references [85-87].
14. The conclusions were rewritten to summarize the main findings of the study

Author Response File: Author Response.docx

Reviewer 4 Report

The paper was rigorously prepared! there is a lot of flow in the text that may be misleading. At time some phrase are uncompleted and some sections are presented as stand alone paper.

Also, the discussion in this document is not pertinent as the paper is proposed as à review!

Some observations are quoted below:

Line 66-67: the phrase does not end properly

Line 91: "A  more  detailed explanation  of  this  chemical  phenomenon  was given  in  [19]" if the phenomenon refer to is the  "Van der Waals forces" then this is not chemical but rather physical.

Line 244: XRD provide the mineralogical changes

Line 278-286: this introductive lines are not well justified here; this sentences are not of interest here and can be placed in the general introduction regarding the global paper objectives.

Line 291-293: Which previous chapter are the author referring to?

   333- when used in reinforced concrete elements.: "Although  nanoclay is not be able to completely prevent the occurrence of fractures, the increasing... when used in reinforced concrete elements. " not clear should be rewritten

410-411: "When tensioned or bent, the material should be provided with additional  reinforcement  for  taking  over  the  tensile  stresses,  because  concrete  /  mortar." something is missing!

415: "this chapter will follow..." ???

As a general question: What is the key interest for making a review on two nano fillers in the same paper? From the title the expectation is to have a review on composite associating both nanoclay and titation, but once inside, there is rather a juxtaposition of a work on nanoclay and titanium

Author Response

1. The Discussion section was removed, as suggested by the reviewer

2. The corresponding verb was added at the end of the sentence, as follows:

   “The use of nanoclay as a component of other materials started in the late 20^th century when researchers observed that, by using this nanomaterial, the properties of the new composite material improved [5-8].”

3. The reviewer in right, Van der Waals forces are of physical nature. We intended to refer to the chemical reaction but it was entirely misleading. We removed the word “chemical” from the manuscript. The underlying mechanisms are explained in more detail in the given reference.

4. The word “chemical” was deleted as it was misleading. The phrase became:

   “[...] XRD analysis provides an insight regarding the mineralogical changes that occur inside the material“

5. The lines were added in the Introduction section, together with similar statements from Section 3.5 pertaining to TiO2.

   However, they were also kept at the beginning of Section 2.5 to ensure a gradual transition from one section to another.
6. The chapters in question are 2.2 and 2.4. The “previous chapters” expression was deleted and replaced with a more specific reference - “Sections 2.2 and 2.4”.

7. The fragment has been modified as follows:

   Although nanoclay is not able to completely prevent the occurrence and propagation of fractures, the increase in the value of the flexural tensile strength of the composite leads to smaller and fewer cracks inside the material. Therefore, the reinforcement is protected from the ingression of chemical agents and humidity increasing the durability of reinforced concrete elements.
8. Taking into account that the paragraph found at 409-413 is misleading, it has been removed. It was meant only to make a connection between section 2.5.2 and 2.5.3.

9. The phrase has been modified as follows:

   “Considering that there are several studies that investigated the variation of the compressive strength depending on the temperature value, the analysis within Section 2.5.3 is pursuing two directions.”

10. The two nano fillers were chosen for a sigle review paper in view of the next stage of authors’ research, namely to investigate the combined, synergistic, effect of the two materials in mortar and concrete.

    As such, the gathered information would be of great help to devise the experimental plan. Moreover, the information pertaining to a single nano-material would not be enough to warrant a state-of-the art review.

Author Response File: Author Response.docx

Reviewer 5 Report

1. General

There only seems to be a summary of the mechanical properties.

However, this journal would focus on the coatings.

Would you also make a summary of some properties related to the coating function as the function of the additive amount or something?

2. Tables 4 and 8

It would be better to summarize the graph, not only the table.

Author Response

1. The journal focuses indeed on coatings, however, there is a special issue “Recent Progress in Sustainability and Durability of Concrete and Mortar Composites” to which the current paper was submitted. As presented by the editors, this issue does not have limitations in what concerns the subjects regarding the advances in concrete and mortar composites.

   Although the coating subject was not discussed in the current paper, it is indeed an interesting topic. There are, in fact, some studies where the nano-TiO₂ was coated with SiO₂, with the purpose of providing some pozzolanic properties to the inert TiO₂ particles. For instance, Han et al. (2017) - used this method for reinforcing reactive powder concrete, with positive results in mechanical properties and dispersion.

2. Figure 3 included to give an overall image of the reported results in Table 4. Figure 4 included to give an overall image of the reported results in Table 9 (former Table 8).

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript has been well revised, the authors are suggested to further increase the resolution of some figures, such as Figures 1, 2,etc. 

Author Response

Thank you for your time and effort in helping the authors improve the manuscript. Please see the attached file with our response to your suggestions and comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

the author made the changes requested by the evaluator, however a small revision of the article's English will be necessary to accept it.

Author Response

Thank you for your time and effort in helping the authors improve the manuscript. Please see the attached file with our response to your suggestions and comments.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments for author File: Comments.pdf

Author Response

Thank you for your time and effort in helping the authors improve the manuscript. Please see the attached file with our response to your suggestions and comments.

Author Response File: Author Response.pdf