A Review of the Performance Properties of Geopolymer Pavement-Quality Concrete
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this paper, the authors present a broad review on the performance properties of geopolymer pavement concrete. A few geopolymer properties are described and compared to OPC concrete. Generally speaking, this is an interesting review. Below are my comments.
1. The abstract could be shorter without too much already well known information, especially the content before 'Although there are many research studies on application of geopolymer concrete as construction material, there are very few review studies on the application of geopolymer concrete in concrete pavements.'
2. In section 2.2.7, granulated phosphorous slag (GPS) should be added. Studies show its impact on physical, mechanical and microstructural characteristics of class F fly ash geopolymer. Denser microstructure was observed by adding GPS.
3. The paper should include a short discussion on the difference between geopolymer and alkali-activated materials. Current scientific community considers low-Ca alkali-activated materials as geopolymers. N-A-S-H is the major binding materials. Please check the recently published papers. (e.g., A state-of-the-art review of crushed urban waste glass used in OPC and AAMs (geopolymer): Progress and challenges)
4. Review should be extended to geopolymer (AAMs) design considering the wide range of raw materials. Recent studies have shown thermodynamic modeling can help the design of geopolymer. The study of phase assemblages of alkali-activate materials in CaO-SiO2-Al2O3 systems can contribute to management of reaction products and designing of precursors.
5. The raw materials of geopolymer sourced from different places have very different reactivity. The could be a limitation of geopolymer.
6. The review of drying shrinkage does not include any concerns, but in the last section 'future recommendations', it mentioned drying shrinkage should be studied extensively. Please also check thermal expansion.
Author Response
Comment 1: The abstract could be shorter without too much already well known information, especially the content before 'Although there are many research studies on application of geopolymer concrete as construction material, there are very few review studies on the application of geopolymer concrete in concrete pavements.'
Response 1:
Thank you for your comment. We have reduced the content in the abstract and modified accordingly. We have removed the know information in the first paragraph of the abstract. Following is the revised paragraph in the introduction.
The construction of concrete pavements has increased due to its better durability, lifespan, and lower maintenance cost. However, this has resulted in the increased consumption of Portland cement, which is one of the major contributors of carbon emissions. As a consequence, the research on alternative binders such as geopolymer concrete has increased in the recent times. There are several research studies that investigate the feasibility of geopolymer concrete as construction material, with limited studies exploring its application in concrete pavements
Comment 2: In section 2.2.7, granulated phosphorous slag (GPS) should be added. Studies show its impact on physical, mechanical and microstructural characteristics of class F fly ash geopolymer. Denser microstructure was observed by adding GPS.
Response 2: Another type of slag which increases the strength of geopolymer concrete is granulated phosphorous slag (GPS). GPS is produced from the yellow phosphorous manufacturing process through electric furnace method. The research shows that the addition of GPS up to 50% influences the physical, mechanical and microstructural properties of fly ash based geopolymer concrete. The compressive strength of the class F fly ash based geopolymer concrete has increased with the increase of GPS content up to 50% which indicates that GPS is a good resource from the waste [72].
[72] Wang, Y.; Xiao, R.; Hu, W.; Jiang, X.; Zhang, X.; Huang, B. Effect of granulated phosphorus slag on physical, mechanical and microstructural characteristics of Class F fly ash based geopolymer. Constr. Build. Mater. 2021, 291, 123287
Comment 3: The paper should include a short discussion on the difference between geopolymer and alkali-activated materials. Current scientific community considers low-Ca alkali-activated materials as geopolymers. N-A-S-H is the major binding materials. Please check the recently published papers. (e.g., A state-of-the-art review of crushed urban waste glass used in OPC and AAMs (geopolymer): Progress and challenges)
Response 3:
Thank you for the comment. We have added a paragraph which discuss about difference between geopolymer and alkali-activated material. Following paragraph is included in the revised manuscript.
Another interesting field is alkali-activation, which is different from geopolymerization. Alkali-activated materials have shown potential for sustainable binding material due to lower embodied energy [8]. Slag is used in alkali-activation, and the glassy substance in such system depolymerize due to action of alkaline environment, resulting in the formation of calcium di-silicate hydrate (CSH) [9]. In case of Ca-rich alkali-activated material, the major binding phase is calcium (sodium) aluminate silicate hydrate (C-(N-)A-S-H), while in case of Ca-low system the major binding phase is sodium-aluminate-silicate-hydrate (N-A-S-H) [10]. Therefore, low calcium alkali-activated materials are called geopolymer.
[8] Xiao, R.; Huang, B.; Zhou, H.; Ma, Y.; Jiang, X. A state-of-the-art review of crushed urban waste glass used in OPC and AAMs (geopolymer): Progress and challenges. Clean. Mater. 2022, 4, 100083.
[9] Paiste, P.; Liira, M.; Heinmaa, I.; Vahur, S.; Kirsimäe, K. Alkali activated construction materials: Assessing the alternative use for oil shale processing solid wastes. Constr. Build. Mater. 2016, 122, 458–464.
[10] Xiao, R.; Jiang, X.I.; Zhang, M.; Polaczyk, P.; Huang, B. Analytical investigation of phase assemblages of alkali-activated materials in CaO-SiO2-Al2O3 systems: The management of reaction products and designing of precursors. Mater. Des. 2020, 194, 108975.
Commnet 4: Review should be extended to geopolymer (AAMs) design considering the wide range of raw materials. Recent studies have shown thermodynamic modeling can help the design of geopolymer. The study of phase assemblages of alkali-activate materials in CaO-SiO2-Al2O3 systems can contribute to management of reaction products and designing of precursors.
Response 4: Thank you for the comment. The main scope of this review was to discuss the articles published on geopolymer pavement quality concrete. Moreover, the authors stressed on the properties relevant to the performance of the concrete pavements. While we agree that thermodynamic modelling can help the design of geopolymer concrete, the authors feel that it would be relevant to discuss such interesting subject in a different paper, in order to avoid the discussion to digress from the main topic
Commnet 5: The raw materials of geopolymer sourced from different places have very different reactivity. The could be a limitation of geopolymer
Response 5: Thank you for the comment. Yes, we agree with this comment. However, the use fly ash and slag has shown promising results with consistent properties. It is also important to characterize the different raw materials to understand its reactivity potential in geopolymer concrete
Comment 6: The review of drying shrinkage does not include any concerns, but in the last section 'future recommendations', it mentioned drying shrinkage should be studied extensively. Please also check thermal expansion.
Response 6: Thank you for the comment. Yes, it is true that, generally, geopolymer concrete demonstrate lower drying shrinkage when compared to conventional concrete. However, very few studies have explored this property in the past. Moreover, for pavements it is important to study the drying shrinkage of concrete, since it will influence its performance. Therefore, the authors have given significance to study the drying shrinkage of geopolymer concrete, along with other relevant properties as part of future recommendations
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article is relevant because it summarizes and analyzes the possibilities of various by-products to be used in geopolymers. Geopolymers, in turn, are an innovative direction in biomaterials, when an inorganic polymer structure is created with the help of activators.
Some notes:
- Its recommend in mentioning fly ash, which originates as a by-product of burning biomass/wood, which is often used in countries rich in wood resources, such as Finland, Baltic countries. Green thinking is based on reducing fossil fuels, such as coal consumption for energy production.
- Regarding red mud and fly ash, it could be pointed out that their storage can significantly worsen the surrounding environment by leaching into the soil. It would also be good to mention that the environmental criteria (LCA) and the environmental cost criteria (LCCA), which numerically show the environmental and economic effects, for example, referring to DOI: 10.1080/14680629.2021.1900899
- Conclusions should include meaningful numerical values.
- Future directions should include a numerical assessment of the environmental impact, which could make this product more attractive to an industry that is constrained by CO2 quotas.
Author Response
Comment 1: Its recommend in mentioning fly ash, which originates as a by-product of burning biomass/wood, which is often used in countries rich in wood resources, such as Finland, Baltic countries. Green thinking is based on reducing fossil fuels, such as coal consumption for energy production.
Response 1: Thank you for your comment. We have discussed the origin of fly ash as per the suggestions. Following sentence is included in the revised manuscript;
It is a fine powder generated in thermal power plants during the combustion of coal, commonly referred to as coal fly ash. However, in countries with abundant wood resources, the combustion of wood or biomass generates wood ash or biomass fly ash.
Comment 2: Regarding red mud and fly ash, it could be pointed out that their storage can significantly worsen the surrounding environment by leaching into the soil.
Response 2: Thank you for your comment. We have included discussions regarding harmful effects of red mud and fly ash storage on the environment. Following paragraphs with correct citations have been included in the revised manuscript.
For fly ash;
Due to its fine nature, it has become a challenge for the industries with respect to its handling and disposal. Moreover, the storage of fly ash poses environmental issues due to secondary dusting [30]. The secondary dust from fly ash storage can be a source of water and air pollution. Further, fly ash can contain heavy metals and toxic substances which can enter the environment and impact the human health and the ecosystem.
For red mud;
The storage of untreated red mud can cause serious challenges, due to its strong alkalinity. In a disastrous event, the strong alkaline red mud effluents can enter the ground, contaminating the soil and the groundwater [52]. Further, it can affect the growth of crops and pose health hazard to human beings.
[30] Obraniak, A.; Gluba, T.; Ławińska, K.; Derbiszewski, B. Minimisation of environmental effects related with storing fly ash from combustion of hard coal. Environ. Prot. Eng. 2018, 44, 177–189.
[52] Wang, M.; Liu, X. Applications of red mud as an environmental remediation material: A review. J. Hazard. Mater. 2021, 408, 124420.
Commnet 3: It would also be good to mention that the environmental criteria (LCA) and the environmental cost criteria (LCCA), which numerically show the environmental and economic effects, for example, referring to DOI: 10.1080/14680629.2021.1900899
Response 3: Thank you for your comment. We have added a discussion related to LCA and LCCA by citing relevant papers as per the suggestions. Following point has been included in the future recommendations of the revised manuscript;
Research shows the environmental benefits of various precursors in construction materials [143, 144], and there is great scope to explore its benefits in geopolymer pavement quality concrete. A systematic approach such as life cycle analysis (LCA) or life cycle cost analysis (LCCA) can provide valuable insights in the products life, environmental and financial impacts. Additionally, it can assist in assessing the sustainability of product by identifying areas of improvements and minimizing environmental burdens.
[143] Lima, M.S.S.; Hajibabaei, M.; Thives, L.P.; Haritonovs, V.; Buttgereit, A.; Queiroz, C.; Gschösser, F. Environmental potentials of asphalt mixtures fabricated with red mud and fly ash. Road Mater. Pavement Des. 2021, 22, S690–S701.
[144] Kurda, R.; de Brito, J.; Silvestre, J.D. A comparative study of the mechanical and life cycle assessment of high content fly ash and recycled aggregate concrete. J. Build. Eng. 2020, 29, 101173.
Comment 4: Conclusions should include meaningful numerical values.
Response 4:Thank you for your comment. The authors have included a separate section to present and discuss the numerical findings of the research papers. Please refer to section 6 and table 3 of the paper which provide all the details
Comment 5:Future directions should include a numerical assessment of the environmental impact, which could make this product more attractive to an industry that is constrained by CO2 quotas.
Response 5: Thank you for your comment. Based on the suggestions, we have added the discussion on assessing the environmental impact of geopolymer pavement quality concrete as part of the future recommendation. Please refer to the last bullet point of the future recommendations in the revised manuscript
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe paper has been revised and should be good for publication.
