Comparative Life Cycle Assessment of Lightweight Aggregates Made from Waste—Applying the Circular Economy

Round 1

Reviewer 1 Report

Thank you for giving me the opportunity to review this article. Below my remarks:

• Numbering and headers have errors.
• Separate author names in the Author list.
• The title is too generalist and does not allow to identify exactly the content of the article.
• The abstract is too brief and does not fully explain the content of the work. It talks about 5 scenarios that are actually four, the results are not clear, etc.
• The type of lightweight aggregate and the field of application is unclear. Natural aggregates, which can also be used from recycling, have mechanical and physical properties for certain applications. Therefore, the substitution by other lightweight aggregates must make clear what the application is and in which type of blend or clay, specifying its characterization. The term "ceramic" appears for the first time in the "results" section, leaving everything very confusing regarding the scope of application and the process studied. Ceramics require baking and that greatly increases energy consumption and increases environmental impacts, so it should be compared in the study, if the mixture is baked.
• The characterization of the four wastes, not only in proportions but also in physical, chemical and mechanical properties, is essential as a previous step and this section does not appear in this article and is relegated to another article that is not yet published, so it makes it incomplete.
• Could you explain the meaning of the paragraphs in page 3, lines 115 to 129?. Also the paragraph on page 5, lines 181 to 183.
• In the Introduction section, the overwiew is limited. There are few references and similar previous works on the subject. The test procedure and methodology is not developed.
• Review the text because there are typos and repetitions.
• The content of Figure 1 is not understood. It's too generalist. It had to be specific to each waste.
• It is recommended to put Figure 2 in the form of a table. Don't you have real images of the wastes?
• Table 1: indicate source and there should be a correspondence with the phases of Figure 1. It is considered the "Extraction plant" when it is supposed to be biological waste. I understand that it is different from the "manufacturing plant" field. The columns need to be adjust.
• Table 2: Indicate source. I recommend colours in the graphics.
• Figure 3 does not look good or understandable. Define before this figure what the different “scenarios” are.
• “Figures 4 and 5 show the environmental benefits of the mixture variants compared individually with the traditional clay mixture they modify”. What are the characteristics of the original mixture and the impacts?
• Table 3: Indicate source and adjust columns.
• I believe that the article shows a first advance or approach of what could be a life cycle analysis study, but it needs to be more exhaustive in the determination of the functional unit, the characterization of the samples, the test procedure and the methodology.

Thank you

Author Response

Dear editor and reviewers,

We have found all the comments useful and constructive and, indeed, they have helped us to clarify our ideas and improve the presentation of our work. We have addressed the reviewers' comments and the manuscript has been corrected accordingly.

All comments have been answered in bold and the changes implemented with the change control in the manuscript. We have corrected all the points suggested by the reviewers.

We hope that all modifications will be to your liking and will meet your expectations. Thank you very much. Sincerely yours,

Carmen Martínez García, PhD Corresponding autor

 

FIRST REVIEWER

Comments and Suggestions for Authors

Thank you for giving me the opportunity to review this article.

Below my remarks:

• Numbering and headers have errors.

The headings have been revised and corrected.

 

• Separate author names in the Author list.

Authors' names are separated. This is how they usually sign.

 

• The title is too generalist and does not allow to identify exactly the content of the article.

Following the reviewer's recommendation, the title of the article has been replaced by the following:

Comparative Life Cycle Assessment of lightweight aggregates made from waste: applying the Circular Economy

 

• The abstract is too brief and does not fully explain the content of the work. It talks about 5 scenarios that are actually four, the results are not clear, etc.

The summary has been rewritten to include the recommendations made by the reviewer.

 

• The type of lightweight aggregate and the field of application is unclear. Natural aggregates, which can also be used from recycling, have mechanical and physical properties for certain applications. Therefore, the substitution by other lightweight aggregates must make clear what the application is and in which type of blend or clay, specifying its characterization. The term "ceramic" appears for the first time in the "results" section, leaving everything very confusing regarding the scope of application and the process studied. Ceramics require baking and that greatly increases energy consumption and increases environmental impacts, so it should be compared in the study, if the mixture is baked.

The use of lightweight aggregates made from expanded clay is an important ally in the manufacture of lightweight concrete. Thanks to their ability to alter their technological properties, such as expansion, they are the best substitute for traditional recycled aggregates due to the porosity they incorporate. Their use to lighten slender concrete structures, together with the low load they transmit to beams and columns when used as a substrate in green roofs, make them ideal for such functions. In addition, its porous structure allows water absorption and subsequent release, maintaining a constant flow of nutrients for plants and vegetation. Their widespread use in hydroponic cultivation, thanks to this function and their low weight, makes them ideal for vertical cultivation, a more productive alternative to the vast land occupations involved in traditional agriculture.

 

• The characterization of the four wastes, not only in proportions but also in physical, chemical and mechanical properties, is essential as a previous step and this section does not appear in this article and is relegated to another article that is not yet published, so it makes it incomplete.

The article referred to has passed the first review and we are awaiting the final reply from the journal. On the other hand, the technological properties of the waste have been taken into account in the elaboration of the life cycle inventory, given that its exothermic capacity is a great ally in reducing the energy costs associated with the sintering of the material.

 

• Could you explain the meaning of the paragraphs in page 3, lines 115 to 129? Also the paragraph on page 5, lines 181 to 183

With regard to the paragraph on page 3, lines 115 to 129, the manuscript submitted to the reviewers was compared with the one that was sent, and this content does not appear in the original. This must be a failure during the submission or layout process. the indicated text has been removed from the manuscript.

With regard to the paragraph on page 5, lines 181 to 183, it is indicated that the scientific article in which the materials that are the subject of the comparative study in this article are developed is in the process of publication. It should be noted that it has already passed the review phase and we are awaiting the final reply from the journal.

• In the Introduction section, the overwiew is limited. There are few references and similar previous works on the subject. The test procedure and methodology is not developed.

The introduction has been improved to take account of the indications made by the reviewer. References have been included, albeit with limitations, as there is little literature on comparative LCA studies on construction materials. In terms of methodology and procedure, improvements have been made in section 2 of the methodology.

• Review the text because there are typos and repetitions

The text has been revised.

 

• The content of Figure 1 is not understood. It's too generalist. It had to be specific to each waste.

Figure 1 contains the stages considered within the system boundaries under study, one of the indispensable requirements to be defined in an LCA. The figure has been edited to incorporate the recommended information.

 

• It is recommended to put Figure 2 in the form of a table. Don't you have real images of the wastes?

A diagram of the sintering conditions has been created to facilitate the reading and understanding of the scenarios considered in the study. The images in Figure 2 are from the actual waste used.

 

• Table 1: indicate source and there should be a correspondence with the phases of Figure 1. It is considered the "Extraction plant" when it is supposed to be biological waste. I understand that it is different from the "manufacturing plant" field. The columns need to be adjust.

Table 1 is prepared with data from the Life Cycle Inventory for each of the scenarios considered, obtained from experimental results and the Ecoinvent database. The "Extraction plant" process corresponds to the infrastructure of the clay extraction plant. The columns have been misaligned due to the initial assembly. The misalignment has been fixed.

 

• Table 2: Indicate source. I recommend colours in the graphics.

Table 2 is based on the data obtained from the calculation programme. The use of the chromatic range in grey is intended to facilitate reading in printed media.

 

• Figure 3 does not look good or understandable. Define before this figure what the different “scenarios” are.

The wide range of results for scenario 0 implies the use of a graph like the one in Figure 3. This figure represents the total contribution to the impact as a percentage for each of the stages considered at the system boundaries. A short excerpt has been added in the text for ease of understanding (). The scenarios are defined in section 2.1.2 and in Figure 2.

 

• “Figures 4 and 5 show the environmental benefits of the mixture variants compared individually with the traditional clay mixture they modify”. What are the characteristics of the original mixture and the impacts?

The characteristics of the original mixture and the impacts are specified in section 3.1.

 

• Table 3: Indicate source and adjust columns.

Table 3 is compiled with the data obtained from the calculation programme. It has been checked that the columns have the appropriate dimension in the final version.

 

• I believe that the article shows a first advance or approach of what could be a life cycle analysis study, but it needs to be more exhaustive in the determination of the functional unit, the characterization of the samples, the test procedure and the methodology.

Thank you for your comments, which have enabled us to improve our work. The descriptions in the various sections have been refined to make them easier to understand.

 

 

SECOND REVIEWER

Comments and Suggestions for Authors

The topic presented in this work is really interesting. However, several challenges are required:

I analyze the single sections:

• Abstract has inappropriate structure. I suggest to answer the following aspects: - general context - novelty of the work - methodology used (describe briefly the main methods or treatments applied) - main results and related interpretations.

The summary has been rewritten to include the recommendations made by the reviewer.

 

• Introduction: This section should briefly place the study in a wide context and emphasize why it is relevant carrying out the analysis. It should define the purpose of the work and its significance. In this perspective, this section is too succinct and fails to effectively point out the relevance of your contribution towards the existing literature. For example, the authors could discuss also social aspects for LCA.

The recommendations of the reviewer have been taken into account, placing the study within the context of the Circular Economy and highlighting the relevance and environmental advantages of carrying out this type of comparative study. The objective of the work and the importance of this type of work have also been clearly included in the introduction, given the few scientific publications that exist on comparative LCA studies, hence the relevance of the contribution. Finally, it should be noted that the authors have also reflected in this section the social aspects related to LCA, in accordance with the bibliography recommended by the reviewer.

 

• Moreover, in this section the authors presents interesting information. However, does not succeed to frame the circular economy concept within relevant literature. Scientific literature is almost absent. This section need to be reinforced: You could for example emphasize the role of circular economy also looking at social aspects. I would suggest to use this section to discuss about the relevance of waste materials for the application of circular economy principle. The circular economy approach has the goal to make better use of resources/materials through reuse, recycling and recovery, and also to minimise the energy and environmental impact of resource extraction and processing. Bio-based industry and bioenergy are paramount:

The recommendations suggested by the reviewer in the introduction section have been taken into account, emphasising the role of the Circular Economy, increasing the scientific literature on the subject and reflecting the social aspects related to LCA, thanks to the consultation of the recommended bibliography.

 

• Materials and methods: I found this section very important for the readability of the paper. However, the research methodology seems underdeveloped. Methods should be described in detail. I think the research procedure could be much more clearly described by means of a diagram also highlighting its potential and limit.

An attempt has been made to improve the methodology section in accordance with the reviewer's indications.

Author Response File: Author Response.docx

Reviewer 2 Report

The topic presented in this work is really interesting. However, several challenges are required:

 

I analyze the single sections:

Abstract has inappropriate structure. I suggest to answer the following aspects: - general context - novelty of the work - methodology used (describe briefly the main methods or treatments applied) - main results and related interpretations.

Introduction: This section should briefly place the study in a wide context and emphasize why it is relevant carrying out the analysis. It should define the purpose of the work and its significance. In this perspective, this section is too succinct and fails to effectively point out the relevance of your contribution towards the existing literature. For example, the authors could discuss also social aspects for LCA.

See

• Social Life Cycle Approach as a Tool for Promoting the Market Uptake of Bio-Based Products from a Consumer Perspective 
• Life Cycle Assessment: a review of the methodology and its application to sustainability

Moreover, in this section the authors presents interesting information. However, does not succeed to frame the circular economy concpet within relevant literature. Scientific literature is almost absent. This section need to be reinforced: You could for example emphasize the role of circular economy also looking at social aspects. I would suggest to use this section to discuss about the relevance of waste materials for the application of circular economy principle. The circular economy approach has the goal to make better use of resources/materials through reuse, recycling and recovery, and also to minimise the energy and environmental impact of resource extraction and processing. Bio-based industry and bioenergy are paramount:

Please see:

https://doi.org/10.1016/j.erss.2021.102238

https://doi.org/10.1016/j.jclepro.2019.117868

https://doi.org/10.1016/j.scitotenv.2021.149605

https://doi.org/10.1016/j.renene.2020.12.034

 

Materials and methods: I found this section very important for the readability of the paper. However, the research methodology seems underdeveloped. Methods should be described in detail. I think the research procedure could be much more clearly described by means of a diagram also highlighting its potential and limit.

Author Response

Dear editor and reviewers,

We have found all the comments useful and constructive and, indeed, they have helped us to clarify our ideas and improve the presentation of our work. We have addressed the reviewers' comments and the manuscript has been corrected accordingly.

All comments have been answered in bold and the changes implemented with the change control in the manuscript. We have corrected all the points suggested by the reviewers.

We hope that all modifications will be to your liking and will meet your expectations. Thank you very much. Sincerely yours,

Carmen Martínez García, PhD Corresponding autor

 

FIRST REVIEWER

Comments and Suggestions for Authors

Thank you for giving me the opportunity to review this article.

Below my remarks:

• Numbering and headers have errors.

The headings have been revised and corrected.

 

• Separate author names in the Author list.

Authors' names are separated. This is how they usually sign.

 

• The title is too generalist and does not allow to identify exactly the content of the article.

Following the reviewer's recommendation, the title of the article has been replaced by the following:

Comparative Life Cycle Assessment of lightweight aggregates made from waste: applying the Circular Economy

 

• The abstract is too brief and does not fully explain the content of the work. It talks about 5 scenarios that are actually four, the results are not clear, etc.

The summary has been rewritten to include the recommendations made by the reviewer.

 

• The type of lightweight aggregate and the field of application is unclear. Natural aggregates, which can also be used from recycling, have mechanical and physical properties for certain applications. Therefore, the substitution by other lightweight aggregates must make clear what the application is and in which type of blend or clay, specifying its characterization. The term "ceramic" appears for the first time in the "results" section, leaving everything very confusing regarding the scope of application and the process studied. Ceramics require baking and that greatly increases energy consumption and increases environmental impacts, so it should be compared in the study, if the mixture is baked.

The use of lightweight aggregates made from expanded clay is an important ally in the manufacture of lightweight concrete. Thanks to their ability to alter their technological properties, such as expansion, they are the best substitute for traditional recycled aggregates due to the porosity they incorporate. Their use to lighten slender concrete structures, together with the low load they transmit to beams and columns when used as a substrate in green roofs, make them ideal for such functions. In addition, its porous structure allows water absorption and subsequent release, maintaining a constant flow of nutrients for plants and vegetation. Their widespread use in hydroponic cultivation, thanks to this function and their low weight, makes them ideal for vertical cultivation, a more productive alternative to the vast land occupations involved in traditional agriculture.

 

• The characterization of the four wastes, not only in proportions but also in physical, chemical and mechanical properties, is essential as a previous step and this section does not appear in this article and is relegated to another article that is not yet published, so it makes it incomplete.

The article referred to has passed the first review and we are awaiting the final reply from the journal. On the other hand, the technological properties of the waste have been taken into account in the elaboration of the life cycle inventory, given that its exothermic capacity is a great ally in reducing the energy costs associated with the sintering of the material.

 

• Could you explain the meaning of the paragraphs in page 3, lines 115 to 129? Also the paragraph on page 5, lines 181 to 183

With regard to the paragraph on page 3, lines 115 to 129, the manuscript submitted to the reviewers was compared with the one that was sent, and this content does not appear in the original. This must be a failure during the submission or layout process. the indicated text has been removed from the manuscript.

With regard to the paragraph on page 5, lines 181 to 183, it is indicated that the scientific article in which the materials that are the subject of the comparative study in this article are developed is in the process of publication. It should be noted that it has already passed the review phase and we are awaiting the final reply from the journal.

• In the Introduction section, the overwiew is limited. There are few references and similar previous works on the subject. The test procedure and methodology is not developed.

The introduction has been improved to take account of the indications made by the reviewer. References have been included, albeit with limitations, as there is little literature on comparative LCA studies on construction materials. In terms of methodology and procedure, improvements have been made in section 2 of the methodology.

• Review the text because there are typos and repetitions

The text has been revised.

 

• The content of Figure 1 is not understood. It's too generalist. It had to be specific to each waste.

Figure 1 contains the stages considered within the system boundaries under study, one of the indispensable requirements to be defined in an LCA. The figure has been edited to incorporate the recommended information.

 

• It is recommended to put Figure 2 in the form of a table. Don't you have real images of the wastes?

A diagram of the sintering conditions has been created to facilitate the reading and understanding of the scenarios considered in the study. The images in Figure 2 are from the actual waste used.

 

• Table 1: indicate source and there should be a correspondence with the phases of Figure 1. It is considered the "Extraction plant" when it is supposed to be biological waste. I understand that it is different from the "manufacturing plant" field. The columns need to be adjust.

Table 1 is prepared with data from the Life Cycle Inventory for each of the scenarios considered, obtained from experimental results and the Ecoinvent database. The "Extraction plant" process corresponds to the infrastructure of the clay extraction plant. The columns have been misaligned due to the initial assembly. The misalignment has been fixed.

 

• Table 2: Indicate source. I recommend colours in the graphics.

Table 2 is based on the data obtained from the calculation programme. The use of the chromatic range in grey is intended to facilitate reading in printed media.

 

• Figure 3 does not look good or understandable. Define before this figure what the different “scenarios” are.

The wide range of results for scenario 0 implies the use of a graph like the one in Figure 3. This figure represents the total contribution to the impact as a percentage for each of the stages considered at the system boundaries. A short excerpt has been added in the text for ease of understanding (). The scenarios are defined in section 2.1.2 and in Figure 2.

 

• “Figures 4 and 5 show the environmental benefits of the mixture variants compared individually with the traditional clay mixture they modify”. What are the characteristics of the original mixture and the impacts?

The characteristics of the original mixture and the impacts are specified in section 3.1.

 

• Table 3: Indicate source and adjust columns.

Table 3 is compiled with the data obtained from the calculation programme. It has been checked that the columns have the appropriate dimension in the final version.

 

• I believe that the article shows a first advance or approach of what could be a life cycle analysis study, but it needs to be more exhaustive in the determination of the functional unit, the characterization of the samples, the test procedure and the methodology.

Thank you for your comments, which have enabled us to improve our work. The descriptions in the various sections have been refined to make them easier to understand.

 

 

SECOND REVIEWER

Comments and Suggestions for Authors

The topic presented in this work is really interesting. However, several challenges are required:

I analyze the single sections:

• Abstract has inappropriate structure. I suggest to answer the following aspects: - general context - novelty of the work - methodology used (describe briefly the main methods or treatments applied) - main results and related interpretations.

The summary has been rewritten to include the recommendations made by the reviewer.

 

• Introduction: This section should briefly place the study in a wide context and emphasize why it is relevant carrying out the analysis. It should define the purpose of the work and its significance. In this perspective, this section is too succinct and fails to effectively point out the relevance of your contribution towards the existing literature. For example, the authors could discuss also social aspects for LCA.

The recommendations of the reviewer have been taken into account, placing the study within the context of the Circular Economy and highlighting the relevance and environmental advantages of carrying out this type of comparative study. The objective of the work and the importance of this type of work have also been clearly included in the introduction, given the few scientific publications that exist on comparative LCA studies, hence the relevance of the contribution. Finally, it should be noted that the authors have also reflected in this section the social aspects related to LCA, in accordance with the bibliography recommended by the reviewer.

 

• Moreover, in this section the authors presents interesting information. However, does not succeed to frame the circular economy concept within relevant literature. Scientific literature is almost absent. This section need to be reinforced: You could for example emphasize the role of circular economy also looking at social aspects. I would suggest to use this section to discuss about the relevance of waste materials for the application of circular economy principle. The circular economy approach has the goal to make better use of resources/materials through reuse, recycling and recovery, and also to minimise the energy and environmental impact of resource extraction and processing. Bio-based industry and bioenergy are paramount:

The recommendations suggested by the reviewer in the introduction section have been taken into account, emphasising the role of the Circular Economy, increasing the scientific literature on the subject and reflecting the social aspects related to LCA, thanks to the consultation of the recommended bibliography.

 

• Materials and methods: I found this section very important for the readability of the paper. However, the research methodology seems underdeveloped. Methods should be described in detail. I think the research procedure could be much more clearly described by means of a diagram also highlighting its potential and limit.

An attempt has been made to improve the methodology section in accordance with the reviewer's indications.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I thank the authors for the new version, however I believe that there is a main consideration for the publication of this manuscript like the scientific data necessary to understand the research process described in it is pending publication in another journal. It makes that this manuscript is incomplete due to lack of the reference.

In addition,  there are still some errors and lack of information such as:

• Numbering still has errors from page 10
• I would eliminate the colon in the title
• Separate author names in the Author list according template: Firstname Lastname, ….
• Section 2.1.1. Objective and scope definition does not refer to the scope of this article referring again to the article to be published.
• Section 2.1.2 or 3.6 have not changed substantially from the previous version. It is not considered an improvement
• Figure 3 does not still look good neither is understandable

Thank you

Author Response

FIRST REVIEWER

Comments and Suggestions for Authors

I thank the authors for the new version, however I believe that there is a main consideration for the publication of this manuscript like the scientific data necessary to understand the research process described in it is pending publication in another journal. It makes that this manuscript is incomplete due to lack of the reference.

We understand the doubts raised by the reviewer in relation to the scientific data pending publication. After a thorough analysis of the subject, we have come to the conclusion that the results presented in the unpublished work refer to the technological, mechanical and microstructural characterization of the manufactured aggregates, and not to the LCA that is the subject of this work. The data that are really needed for the comparative LCA study are the percentages of clay substitution by residue, the sintering temperatures and times and the calorific values of the added residues. These data are shown in Tables 1 and 2 which have been incorporated in a new section added to the article in which a description of the starting raw materials and the manufacturing process followed to obtain the lightweight aggregates is given (section 2.1. Manufacture of LWAs from wastes). In this way, all the information necessary to carry out the study that is the subject of this work has been incorporated into the text. We thank the reviewer for his comments, which have allowed us to improve this part of the text that could lead to some confusion. We have also removed all the parts of the text in which reference was made to the unpublished article.

In addition, there are still some errors and lack of information such as:

• Numbering still has errors from page 10

He is indeed right. Thank you very much for your comment. It has been corrected

• I would eliminate the colon in the title

The two title points have been removed

Separate author names in the Author list according template: Firstname Lastname, ….

The authors' names and surnames have been separated according to the format of the template

• Section 2.1.1. Objective and scope definition does not refer to the scope of this article referring again to the article to be published

In this section, which has now become 2.2.1, the specific objective of this work has been clarified.

 

• Section 2.1.2 or 3.6 have not changed substantially from the previous version. It is not considered an improvement

The sections indicated by the reviewer have been improved. In addition, the addition of section 2.1 is expected to be clarifying and give consistency to the paper.

• Figure 3 does not still look good neither is understandable

The figure has been replaced by a higher resolution figure with a new design which we hope will make it easier to understand.

Author Response File: Author Response.pdf

Reviewer 2 Report

There are two central issue that need to be solved. 

The first one is about the novelty of your contribution. What does it add to the current literature? You should elaborate further.

 

The second one is about the reference to CE which should be expanded also looking at the literature. More references are needed. I report some here for your convenience:

The circular economy approach has the goal to make better use of resources/materials through reuse, recycling and recovery, and also to minimise the energy and environmental impact of resource extraction and processing. Bio-based industry and bioenergy are paramount:

Please see:

https://doi.org/10.1016/j.erss.2021.102238

https://doi.org/10.1016/j.jclepro.2019.117868

https://doi.org/10.1016/j.scitotenv.2021.149605

https://doi.org/10.1016/j.renene.2020.12.034

 

Author Response

SECOND REVIEWER

Comments and Suggestions for Authors

There are two central issue that need to be solved. 

The first one is about the novelty of your contribution. What does it add to the current literature? You should elaborate further.

The novelty of this work to the currently published literature on LCA lies in two aspects. The first is that there is little literature on LCA of building materials. A review of the literature shows that there is a larger volume of LCA studies that address the building as a whole, i.e., once the construction is in use, but there are few that focus on the manufacturing process of the building material.

Secondly, it should be noted that if there are already few studies published on the LCA of building materials, there are even fewer studies available in which comparative studies are carried out to assess the environmental benefits of building materials made from waste compared to traditional materials. This is, in our opinion, the main novelty of the present work.

The second one is about the reference to CE which should be expanded also looking at the literature. More references are needed.

The authors would like to thank the reviewer for his commentary and recommended bibliography. Work has been done to improve the vision of the Circular Economy that appears in the article, in order to try to convey to the reader a more accurate picture of what it represents and how it can be and is being applied to very diverse productive areas.

 

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

The manuscript is much improved