Review Reports

Round 1

Reviewer 1 Report

The article presents the study of estimating the economic feasibility of four new recycling-based geopolymer concrete mixes.

In general, the text is clear and easy to follow. The references are in line with the article's topic. The Introduction is sufficient and the quality of the figures is good. Maybe it is not a typical scientific article, but it raised important problems. The article may be very interesting for the readers of the journal. My only remark is the untypical order of sections in the article. Why section "Materials and methods" is the last one? And it is preceded by "Discussion"?

Author Response

Thank you for the comments!

The manuscript follows the instructions of the journal on titling and order of sections:

https://www.mdpi.com/journal/recycling/instructions#preparation

Therefore the section "Materials and methods" is the last one, and it is preceded by "Discussion".

Reviewer 2 Report

This article focuses on  estimating the cost prices of four different concretes with different material compositions and carbon footprints, considering the raw material price variability and the potential impact of carbon emissions regulation (carbon price). The detailed comments are as follows:

1. Abstract is well-written.
2. The use of word "low-carbon concrete" and "geopolymer concrete" is confusing. Low-carbon concretes include geopolymer and other type of concretes. If article is limited to geopolymer, replace the word "low-carbon concrete" with "geopolymer concrete". Otherwise, use the word low-carbon concrete instead of geopolymer concrete.
3. Introduction section is weak to establish the novelty of this study.
4. Authors should discuss the limitations of this study clearly. There are a lot of materials used in the preparation of low carbon concretes/geopolymer concrete. Clearly, authors have not discussed all the materials and associated costs. Therefore, it is important for the reader to know the limitation of this work.

Author Response

Thank you for your valuable comments.

1. Thank you.
2. The title of the manuscript is revised. The new title is “Estimating the cost-competitiveness of recycling-based geopolymer concretes”. Also in abstract it is now expressed more clearly that the focus is on geopolymer concretes: The sentence “This study presents a case study estimating the cost prices of four different concretes…” has been changed to “This study presents a case study estimating the cost prices of four different geopolymer concretes…” The term “low-carbon concretes” is still used e.g. in section titles, as supplementary cementitious materials are also discussed
3. The novelty value of this study is now expressed more explicitly in the introduction: “Even though former literature on geopolymer concretes discusses the costs of manufacturing to some extent, the approach for determining the cost estimates has not been discussed in detail. In a case project, one of the goals was to develop new business related to recycling-based geopolymer concrete. It turned out that a resource-efficient and transparent cost assessment, also taking into account the possible effects of carbon taxation, is a necessary means for decision support in such a situation.”

 

4. In the introduction, the possible raw materials for geopolymer production are already discussed:

“Side stream-based binder precursors can be divided into already recognized materials in cement standard, such as silica fume, coal fly ash, and blast furnace slag. Additionally, there are other slags and ashes suitable for geopolymer purpose but are unrecognized in cement standard. Examples are bauxite residual from alumina production—also known as red mud [24]—and waste from mining and processing copper ore [25]. Furthermore, some consumer wastes and separate fractions of demolition waste, such as scrap glass [26], ceramic waste [15], and mineral wool [27], have shown potential for geopolymer purpose. There are studies that have replaced commercial alkali activators with recycling-based material [28, 29]; however, this has not been done on an industrial scale.”

 

The following remark has been added to the “Results” section:

“The price search focused on materials that were used in geopolymer concrete mixes in the case project, or in typical geopolymer concrete mixes presented in the literature. It needs to be noted that all possible geopolymer concrete raw materials are not sold on open markets, so the price information is not available.

 

For example, in the case project the utilization of various local ashes and slags as aggregate was considered. Due to disposal fees, these side streams have a negative price in principle, but the required processing (e.g. drying and sieving) would significantly increase their cost price. As large part of the processing costs consist of fixed costs, it is difficult to estimate the potential unit price for the processed material. Instead, a reference price was used, which is the price of the nearest commercial equivalent. This must be understood as a lowest possible price (range) for the material considered.”

 

The limitation mentioned is now discussed more profoundly in the “Discussion”: 

“…Secondly, with the approach described, it is not possible to accurately estimate the cost prices of material mixes for which the raw materials are not generally available for buying. However, in these cases, the market prices of industrially produced side streams can be used as a reference value for approximating which price of a side stream (requiring similar processing etc.) could be expected to reach. Based on the target price of the material mix, target values can be set for the recycling-based raw materials that are not yet commercially produced.”

Reviewer 3 Report

It is a good piece of paper with an interesting topic for reading. A informative background of AAM and the influence the cost of materials and potential acts of carbon taxes on promoting the use of low-carbon materials, has been provided. The manuscript is in a good shaped and can be considered for acceptance with minor revision. Comments below are provided for the authors.

 

1. Line 26 - 27, the construction industry in total should account for a higher carbon emission than 5%. Please revise the number referring to a updated source.
2. Table 3, it is understandable to use "3d printable" as a requirement for all five mixes. But I think compressive strength should be at least presented here so as to compare different mixes under the same rubrics.
3. Why mix 4 in Table 3 used so little amount of activator? This is also one of the reason why we need to see how it works in compressive strength.
4. For the initial cost price estimation, the prices summarized from Alibaba is useful. But we have to know that the transportation cost and the corresponding environmental impacts will greatly impact if foreign sources are used. Therefore, I would recommend to add more discussion on this aspect.

Author Response

Thank you for your valuable comments.

1. The first sentence in introduction is changed from “The construction industry generates approximately 5% of all carbon dioxide emissions worldwide…” to “The concrete production generates approximately 8.6 % of all carbon dioxide emissions worldwide…”.  The information source was updated to a newer one.
2. Thank you for this very relevant point. The compressive strengths of the materials are provided in table 4.
3. ^
4. The method choice is discussed now in more detail in “Materials and Methods” section, and it has been added that:

“If the production of concrete material is considered in a particular location, the freights of raw materials must be of course considered in the cost estimate. However, one strategy in the geopolymer business is to import technology to geographical areas where the availability of critical side streams is good [76], so in that sense it is not a problem that for some materials price information is limited on certain areas (in this case China and India). It is useful to get quickly a rough cost estimate, that can be refined e.g. according to specific geographical areas.”

In section “2.3 The impact of carbon pricing on cost prices” it has been added that: “The LCA was conducted assuming local material sourcing. If e.g. fly ash was transported between continents, emissions from transportation should also be considered in the calculation.”

Also, in the “Results” section, refining the price search has already been explained:

“This is a simplified preliminary cost price comparison, but it can be used in determining mixes that are by default clearly more expensive than the basic OPC mix. For example, the use of potassium-based alkali activator, high activator proportion, and silica fume or silica sand anticipate a high-cost price for the mix. In the literature, numerous described mixes are designed for laboratory purposes and are not cost-optimized.

The comparison can be refined to account for the location where the mix is to be prepared. Potential freight can be added to the price estimates, and suppliers can be removed and added according to the possible delivery area. Activators’ price ranges can also be narrowed by specifying the required activator characteristics in more detail. In the case project, for example, a higher price and/or freight would have to be applied to coal fly ash, which would clearly increase the price of Mix 4.”

Reviewer 4 Report

The paper attempted to estimate the cost-competitiveness of recycling-based low-carbon concretes. The effort put into the research is commendable. However, some concerns are presented below which must be attended to before the paper can be considered for acceptance.

1) Pg. 2, Line 59-60: It will be useful to refer to an amount from a particular period before mentioning the drop and projection to 2030.

2) Pg. 2, Line 83-84: Use subscripts for the numbers in the chemical formula.

3) The paper does not have a section on the conclusion. This must be added and the major findings of the paper shown.

4) I am worried that the authors only depended on Alibaba.com as the source of their information. There should have been a proper market survey and the results statistically analysed before conclusions can be drawn. The authors need to explain why this was not done.

5) While the title of the paper presents an investigation into low-carbon concretes, the authors seem to have narrowed down on geopolymer concrete. What about the other low-carbon concrete mentioned in the introductory section of this paper? Perhaps, the authors can consider revising the topic to capture what was truly researched.

 

Author Response

Thank you for your valuable comments.

1. The sentence “…the amount of CO₂ emissions from cement production has dropped to 667 kg/t (2017)…” has been changed to “…the amount of CO₂ emissions from cement production has dropped from 783 kg/t (1990) to 667 kg/t (2017)…”

 

2. Subscripts are now used for the numbers in the chemical formula.

 

3. The manuscript follows the instructions of the journal on titling and order of sections:

https://www.mdpi.com/journal/recycling/instructions#preparation

According to the policy of the journal, “Conclusions” section is not mandatory, “but can be added to the manuscript if the discussion is unusually long or complex”. On our judgement, the discussion section is quite plain, so we did not add a separate section for conclusions. However, if it is still considered necessary by reviewer, this revision can be done.

4. The reasoning behind the choice of collecting data from Alibaba.com is now presented in the beginning of “Materials and Methods” section:

 

“In this study, the focus is on experimenting an easy and repeatable method for material mix cost price determination. Therefore, the price search focuses on Aliababa.com, the largest business to business marketplace in the world. Electronical commerce can be expected to become more widespread and centralized, and therefore the applicability of the approach can be expected to improve in the future. Market price information from other sources than this type service is laborious to collect, the information becomes rapidly obsolete, and its local utilization is limited in any case.

If the production of (geopolymer) concrete is considered in a particular location, the freights of raw materials must be of course considered in the cost estimate. However, one strategy in the geopolymer business is to import technology to geographical areas where the availability of critical side streams is good [76], so in that sense it is not a problem, even if for some materials the price information is concentrated in a certain areas (in this case China and India). It is useful to get quickly a rough cost estimate, that can be refined e.g. according to specific geographical areas.”

 

5. The title of the manuscript is revised. The new title is “Estimating the cost-competitiveness of recycling-based geopolymer concretes”. Also in abstract it is now expressed more clearly that the focus is on geopolymer concretes: The sentence “This study presents a case study estimating the cost prices of four different concretes…” has been changed to “This study presents a case study estimating the cost prices of four different geopolymer concretes…” The term “low-carbon concretes” is still used e.g. in section titles, as supplementary cementitious materials are also discussed.

Round 2

Reviewer 2 Report

Authors have revised the article as per comments.

Author Response

Thank you for the review. Some minor language corrections have been made after review round 2.

Reviewer 4 Report

Necessary corrections have been made, but the grammar needs minor improvement

Line 26: "The production of concrete" not " The concrete production ... " Or you can delete the "The" and say "Concrete production ..." 

Line 247: Should read " ..... in the case of this project, the utilization ... "

Line 299: Revise the "table 4" to "Table 4"

Line 417: "Could be expected to reach." This is an incomplete statement. Reach what?

Author Response

Thank you for your comments. The following amendments have been made:

Line 26: " The concrete production ... " has been changed to "Concrete production ..." 

Line 247: The sentence has been changed to: " ... in the case of this project, the utilization ... "

Line 299: "table 4" has been revised to "Table 4"

Line 417: “…which price of a side stream (requiring similar processing etc.) could be expected to reach.” has been changed to “…which price of a side stream (requiring similar processing etc.) could be expected to have when commercially produced.”