Recent Progress and Methodology for the Characterization of Layer-Effects of Extrusion-Based 3D-Printed Concrete

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is interesting and timely work. Some comments:
1. The abstract is overall easy to follow.
2. This paper presents a comprehensive review of recent advances in characterizing layer effects of extrusion-based 3D-printed concrete.  
3. Please expand the discussion to include recent developments in processing techniques and material requirements for adapting 3DPC to extreme environments, such as cold regions, high-altitude areas, and off-planet infrastructure. 
4. Please summarize cited data through original figures and tables, therefore improving readability and data interpretation.
5. What are the future perspectives?

Author Response

This is interesting and timely work. Some comments:

1. The abstract is overall easy to follow.

Response: Thank you very much for your positive feedback on the abstract of our paper.

 

2. This paper presents a comprehensive review of recent advances in characterizing layer effects of extrusion-based 3D-printed concrete.

Response: Thank you for your positive feedback.

 

3. Please expand the discussion to include recent developments in processing techniques and material requirements for adapting 3DPC to extreme environments, such as cold regions, high-altitude areas, and off-planet infrastructure.

Response: We have adopted your recommendations and expanded the discussion to include recent developments in processing technologies and material requirements to accommodate 3DPC applications in extreme environments, such as cold regions, high altitudes, and off-planet infrastructure. The following is an excerpt of the newly added content in the manuscript, for example: “In recent years, three-dimensional printed concrete (3DPC) has gained increasing attention as an emerging technology in the construction industry. It holds the potential to revolutionize building practices, particularly in extreme environments such as polar regions, high altitudes, cold climates, and off-planet infrastructure that demand exceptional thermal insulation.”

 

4. Please summarize cited data through original figures and tables, therefore improving readability and data interpretation.

Response: We appreciate your recommendation to summarize the cited data using original figures and tables to enhance readability and data interpretation. We have created additional visual representations, In the original manuscript, we have successfully cited some of the points you raised. Specifically, see lines 96-97, line 110, lines 134-135, line 181, lines 265-266, etc., of the submitted manuscript.

 

5. What are the future perspectives?

Response: Future research should prioritize the development of robust mix designs and optimized printing strategies that simultaneously ensure printability, interlayer bonding quality, mechanical performance, and thermal insulation efficiency. The establishment of standardized testing protocols for interlayer characterization, together with advanced inline and real-time monitoring techniques, is necessary to improve process reliability and reproducibility. Furthermore, greater emphasis should be placed on the integration of sustainable materials, including recycled aggregates and supplementary cementitious materials. Ultimately, a holistic optimization of material composition, printing processes, and environmental control will be essential for enabling large-scale, durable, and energy-efficient 3DPC construction. For details, please refer to lines 1264-1274 of the submitted manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

General thought:

The article presents the influence of various agents on the properties of 3DPC. It incorporates a vast number of different agents, which is generally nicely presented.

Remark 1: The article jumps from one thought to another too quickly. Quote: "Overall, this paper aims to be a valuable resource for researchers and industry professionals working to advance the field". This statement requires more in-depth approach to describing the effect of each factor. The article might not pose as a complete state-of-art or review, but it just call for a more extensive description of the influence of various factors. Providing the data from only a single article to back up a thesis might be misleading.

Editorial:

Remark 2: General: Figure captions aren't typically ended with a period. In the Summary bold the sections like "Factors governing interlayer properties" for clarity and highlight.

Remark 3 - Figures

Figure 1:

1.  In the left column, the commas are improper, spaces before colons, word orientation -> Orientation (check point nr 3 here)
2. Middle column - print->printed;
3. Env. column - If you write everything startin with a capital letter then Relative should also be done like that.

Figure 2:

1. Caption: a and b are in bold, rest is not

 

Section 2.1.1.

Remark 4: Line 103: Needle-shaped structures in cement matrix are generally associated to the formation of ettringite, which is visible in the early stages of hydration of CSA. Calcium carbonate is typically visible as rhombs of calcite. Cannot provide specific references here, please find in online. Please check if I understood the sentence correctly.

Remark 5: Summary of the section:

I would highly recommend inserting a table in the summary with the list of agents and their influence on parameters (e.g. arrow up mean increases etc.). Or, which might be also helpful, do it at the end of the artcile for all of the mentioned agents.

Remark 6: Line 148-149: "Specifically, the interlayer properties of 3D printed concrete (3DPC) are influenced to varying degrees by multiple factors [...]" is an extensively general thought and can be said about anything, please rephrase.

Section 2.1.2.

Remark 7: Line 160: typically increase in the volume of coarse aggregate is correlated to the increase of porosity, as the finer grains can fit into empty voids. This is why it is important to confront the results of a single study with other (See Section 3 of your manuscript).

Section 2.2

Remartk 8: Line: 330 - in this line we hve nozzle exit size, in the Figure 1 nozzle orifice, it can be shortened throughout the article to nozzle size, which is generally understandable for a reader from this field.

Remark 9: Every agent is described as "has a significant impact" or is"key factor" try to provide small variation in the phrasing.

Section 2.3

Remark 10: Line 415-416 - which fibers shrink due to the water loss in concrete to such a degree that provide interfacial defects? Do steel fibers or artificial fibers shrink this much?

Remark 11: Line 441-442 "[...] performance of 3D-printed fiber-reinforced concrete (3DPC)." The 3DPC was earlier defined as ordinary 3D printed concrete (Line 12), so the abbreviation should be either (3DFRPC) or removed.

Remark 12: Line 857: What is the thickness of the voids?

Remark 13: Section 5.1 - as in Section 2, please provide more than a single reference to a statement

Remark 14: I would highly recommend a summary statement at the end about problems, further approach etc.

Comments on the Quality of English Language

The language is generally fine, some slip-ups, will be fine after small tweaks.

Author Response

General thought:

The article presents the influence of various agents on the properties of 3DPC. It incorporates a vast number of different agents, which is generally nicely presented.

Response: Thank you for your comments. We have addressed your comments and carefully proofread the manuscript to eliminate any potential misunderstandings and improve the language.

Remark 1: The article jumps from one thought to another too quickly. Quote: "Overall, this paper aims to be a valuable resource for researchers and industry professionals working to advance the field". This statement requires more in-depth approach to describing the effect of each factor. The article might not pose as a complete state-of-art or review, but it just call for a more extensive description of the influence of various factors. Providing the data from only a single article to back up a thesis might be misleading.

Response: Thank you for the comments and suggestions. We have revised the statement accordingly in introduction (Line 60-65) and in the following paper to avoid misleading.

Editorial:

Remark 2: General: Figure captions aren't typically ended with a period. In the Summary bold the sections like "Factors governing interlayer properties" for clarity and highlight.

Response: We have removed the periods at the end of each caption to conform to the standard formatting. Additionally, we agree that highlighting the sections in the Summary, such as "Factors governing interlayer properties," would improve clarity. We have revised the manuscript to bold these sections for better emphasis and readability.

Remark 3 - Figures

Figure 1:

1. In the left column, the commas are improper, spaces before colons, word orientation -> Orientation (check point nr 3 here)
2. Middle column - print->printed;
3. Env. column - If you write everything startin with a capital letter then Relative should also be done like that.

Figure 2:

1. Caption: a and b are in bold, rest is not

Response: We have made revisions to the content of Fig. 1 based on the issues you raised. Regarding the Fig. 2 caption, our response is that the bold formatting of 'a' and 'b' follows the official template requirements provided by the journal. We sincerely appreciate your valuable suggestions once again.

Section 2.1.1.

Remark 4: Line 103: Needle-shaped structures in cement matrix are generally associated to the formation of ettringite, which is visible in the early stages of hydration of CSA. Calcium carbonate is typically visible as rhombs of calcite. Cannot provide specific references here, please find in online. Please check if I understood the sentence correctly.

Response: We sincerely appreciate the reviewer's meticulous examination and professional correction regarding the microstructural characterization. It is correct that the needle-shaped crystals observed in cement-based systems are indeed characteristic of ettringite formation, particularly during the early hydration stages of calcium sulfoaluminate (CSA) cement, rather than calcium carbonate. In addition, calcium carbonate typically manifests as rhombohedral calcite crystals. We have carefully revised the relevant sentence in the manuscript  (Line 116-120) to accurately distinguish between these distinct crystalline morphologies and their corresponding chemical compositions, now correctly attributing the needle-shaped structures to ettringite formation. The above content is mainly sourced from the following references: https://doi.org/10.1016/j.jobe.2025.114079 , https://doi.org/10.3390/jmse13061076 , https://doi.org/10.3390/molecules30132759 , https://doi.org/10.3390/ma17215216 , among others.

Remark 5: Summary of the section:

I would highly recommend inserting a table in the summary with the list of agents and their influence on parameters (e.g. arrow up mean increases etc.). Or, which might be also helpful, do it at the end of the article for all of the mentioned agents.

Response: We sincerely thank you for your valuable suggestions. Based on your comments, we have organized the various agents mentioned in the manuscript and their effects on 3DPC concrete into a table, which has been placed in the corresponding section of the manuscript. See Table 1 for the newly added content.

Table 1. Summary of the Effects of Different Materials on 3DPC Performance.

Main Types	Specific type	Primary function
Additives and Admixtures	Fluid-retaining polycarboxylate superplasticizers (FR-PC)	maintain high interlayer bond strength
	highly dispersive polycarboxylate superplasticizers (HD-PC)	weaken the interlayer bond performance
	moisture-retaining types SAP	improve the early strength and interlayer adhesion
	CO2-activated interfacial enhancer (CIE)	improve the interlayer strength
	protein-based foaming agents	enhance the interlayer bonding strength
	Magnesium oxide (MgO) at a dosage of 6%	enhance the thixotropy and compressive strength
	slag into metakaolin-based geopolymers	enhance interlayer bond strength
	Polyacrylamide (PAM)	improve the interlayer bond strength
	anionic polyacrylamide (APAM)	reduce interlayer durability and shear bond strength
	attapulgite	reduces interlayer bond strength and durability
Aggregates	recycled coarse aggregates (RCA)	enhances the interlayer shear strength
	recycled plastic eco-aggregates (Resin8)	increase interlayer porosity and weaken interlayer bond strength
	recycled lightweight aggregates	weakens the bond strength at the interlayer interface
	rice husk	enhances interlayer bond strength
Fibers	polypropylene (PP) fibers	improve the interlayer bond properties
	polyoxymethylene (POM) fibers	improve compressive and flexural performance
	modified PET (MPET) fibers	improve the interlayer splitting tensile strength
	polyvinyl alcohol (PVA) fibers	enhance interlayer toughness
	bamboo fibers	improves the interlayer flexural strength
	plant fibers (e.g., coconut shell and flax)	enhance interlayer flexural strength
	0.2 wt% cellulose nanofibrils (CNF) with 1 vol% PE fibers and 0.5 vol% steel fibers	enhance the interlayer bond strength
	steel fibers - polyethylene (PE) fibers	improve the interlayer splitting tensile strength
	hydroxypropyl methylcellulose (HPMC) - micro steel fibers	weaken the fiber–matrix bond

Remark 6: Line 148-149: "Specifically, the interlayer properties of 3D printed concrete (3DPC) are influenced to varying degrees by multiple factors [...]" is an extensively general thought and can be said about anything, please rephrase.

Response: We agree. The expression in this part of the original text was indeed too vague. We have made the necessary revisions to this part of the manuscript, and the revised content is as follows: “The interlayer properties of 3D-printed concrete (3DPC), such as bonding strength, mechanical anisotropy, and durability, are significantly influenced by the material's composition. Key factors include the types and dosages of admixtures—such as superplasticizers, internal curing agents, reinforcing fibers, and foaming agents—as well as the incorporation ratios of oxides and geopolymers. Additionally, the chemical nature of supplementary additives plays a crucial role in controlling hydration kinetics, interlayer adhesion, and overall structural performance.”

Section 2.1.2.

Remark 7: Line 160: typically increase in the volume of coarse aggregate is correlated to the increase of porosity, as the finer grains can fit into empty voids. This is why it is important to confront the results of a single study with other (See Section 3 of your manuscript).

Response: Thank you very much for your valuable feedback on our manuscript. Your question is truly thought-provoking, especially the point about "comparing the results of a single study with those of other studies." We will pay more attention to this aspect in our future research.

Section 2.2

Remartk 8: Line: 330 - in this line we have nozzle exit size, in the Figure 1 nozzle orifice, it can be shortened throughout the article to nozzle size, which is generally understandable for a reader from this field.

Response: We have standardized the expression of the nozzle size in both the manuscript text and images to help readers in this field better understand the content.

Remark 9: Every agent is described as "has a significant impact" or is"key factor" try to provide small variation in the phrasing.

Response: We have proofread the manuscript and provide more variations in the phrasing.

Section 2.3

Remark 10: Line 415-416 - which fibers shrink due to the water loss in concrete to such a degree that provide interfacial defects? Do steel fibers or artificial fibers shrink this much?

Response:  Thank you very much for your comment. Upon verification, we have rephrased the statement. The rephrased content is " At the same time, the fibers shrink due to water loss and are directionally aligned, thereby promoting the formation of interfacial defects.", which can be seen in lines 439-441 of the currently submitted manuscript. The intention conveyed in this part of the manuscript is that the fibers shrink due to water loss and exhibit directional alignment, thereby promoting the formation of interfacial defects.

Remark 11: Line 441-442 "[...] performance of 3D-printed fiber-reinforced concrete (3DPC)." The 3DPC was earlier defined as ordinary 3D printed concrete (Line 12), so the abbreviation should be either (3DFRPC) or removed.

Response: We have removed the expression "(3DPC)" from the relevant section of the manuscript.

Remark 12: Line 857: What is the thickness of the voids?

Response: The thickness of interlayer voids in 3D-printed concrete refers to the vertical distance between adjacent printed layers resulting from the layer-by-layer deposition process, which represents the gap or discontinuity created at the interface between successive concrete layers due to insufficient interlayer bonding, bleeding, or inadequate compaction during the printing process. Alternatively, it can be defined as the height of the air gap or weak zone formed between two consecutive layers of extruded concrete material, typically ranging from 0.1 to 4 mm depending on printing parameters such as nozzle height, layer height setting, material rheology, and time interval between layers. The submitted manuscript has supplemented the description of this content. For specific details, please refer to lines 950-952 of the currently submitted manuscript.

Remark 13: Section 5.1 - as in Section 2, please provide more than a single reference to a statement

Response: We have added additional references in this section to further enrich and strengthen the supporting evidence for our arguments.

Remark 14: I would highly recommend a summary statement at the end about problems, further approach etc.

Response: We highly appreciate your suggestions regarding the “Future Outlook”. We have included the perspectives in the conclusion section, where the text reads “Outlook and future research direction: future research should prioritize the development of robust mix designs and optimized printing strategies that simultaneously ensure printability, interlayer bonding quality, mechanical performance, and thermal insulation efficiency. The establishment of standardized testing protocols for interlayer characterization, together with advanced inline and real-time monitoring techniques, is necessary to improve process reliability and reproducibility. Furthermore, greater emphasis should be placed on the integration of sustainable materials, including recycled aggregates and supplementary cementitious materials. Ultimately, a holistic optimization of material composition, printing processes, and environmental control will be essential for enabling large-scale, durable, and energy-efficient 3DPC construction.” emphasizing key research directions, including the development of robust hybrid designs, optimized printing strategies, standardized testing protocols, and the integration of sustainable materials.

Thanks for reviewer's comments and suggestions. We would like to express our great appreciation again for giving us the opportunity to revise our manuscript.