Soil–Cement Mixtures with Fiber Reinforcement in 3D Printing: Challenges and Opportunities for Sustainable Construction

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript entitled “Soil-Cement Mixtures with Fiber Reinforcement in 3D Printing: Challenges and Opportunities for Sustainable Construction” presents an integrative literature review focused on the role of fiber reinforcement in extrusion-based additive manufacturing (AM) using soil-cement matrices. The topic is timely and relevant, particularly in light of increasing global pressure to decarbonize the construction sector and to develop low-cost, resource-efficient building technologies. The manuscript attempts to bridge materials science, rheology, and sustainability assessment through a multi-level evidence framework. Overall, the work demonstrates substantial effort and a broad survey of the literature; however, several conceptual, methodological, and structural aspects require clarification and strengthening before the manuscript can be considered for publication.

The introduction effectively contextualizes the environmental burden of conventional construction and positions soil - cement mixtures as a lower-carbon alternative to ordinary Portland cement-based concrete. The discussion on the potential of additive manufacturing to reduce material waste and construction time is well aligned with current trends in digital construction. The authors correctly identify brittleness, low tensile strength, and anisotropy as principal barriers to the structural use of 3D-printed soil–cement. The motivation for fiber reinforcement is therefore technically justified. However, although the manuscript claims to be an “integrative literature review,” it does not clearly define the review methodology. There is no explicit description of search strategy, inclusion/exclusion criteria, time window, database selection, or quality assessment procedure. Without a transparent review protocol (e.g., PRISMA-type structure or at least a structured narrative methodology), it is difficult to assess completeness and reproducibility. For a review paper, this omission is a major methodological weakness.

The section dedicated to fiber parameters is comprehensive in scope. The classification into natural, inorganic, and synthetic fibers is appropriate, and the discussion of fiber geometry, dosage, aspect ratio, and orientation provides useful technical detail. The manuscript demonstrates a solid understanding of rheology - mechanics trade-offs, particularly regarding extrudability versus mechanical enhancement. The identification of nozzle clogging thresholds and the influence of fiber length and diameter on flow resistance is well reasoned. Nevertheless, the treatment remains largely descriptive. Quantitative comparisons are occasionally mentioned (e.g., increases in viscosity of 30–50%), but the review does not synthesize data across studies in a systematic way. A meta-analytical comparison or at least normalized plots comparing strength gains versus fiber dosage would significantly strengthen the technical contribution.

The analysis of fresh-state properties is one of the stronger sections of the manuscript. The structured breakdown into extrudability, flowability, buildability, and setting time aligns with established frameworks in 3D concrete printing research. The use of rheological parameters such as static yield stress (τₛ), dynamic yield stress (τd), and plastic viscosity (μ) reflects appropriate technical depth. The discussion of thixotropy and structural build-up (including 3ITT testing and performance indicators such as G′/τf) demonstrates familiarity with advanced rheological characterization. However, the manuscript would benefit from clearer differentiation between soil-dominated systems and conventional cement-based mortars. In several instances, concepts from 3D concrete printing are transferred to soil–cement without sufficiently discussing differences in clay mineralogy, particle packing, or capillary-driven cohesion. While the authors acknowledge this limitation in the multi-level framework, the technical consequences of these differences are not fully developed.

The hardened-state section appropriately addresses anisotropy, interlayer bonding, and mechanical performance. The discussion of interlayer desiccation, reduced-permeability skins, and cold joints is technically sound. The reported strength reductions (12 - 55%) under parallel loading are consistent with the broader additive manufacturing literature. The explanation of crack-bridging mechanisms and strain-hardening effects induced by fibers is accurate and clearly presented. Nevertheless, the compressive strength values reported for stabilized earth (0.98 - 2.43 MPa) appear relatively low for structural applications. The manuscript would benefit from a clearer discussion of whether these strengths meet minimum regulatory thresholds for load-bearing elements in different jurisdictions. Additionally, the interaction between fiber alignment and anisotropy is discussed qualitatively but lacks quantitative orientation metrics (e.g., orientation factors derived from image analysis).

The treatment of shrinkage and crack control is adequate, particularly the emphasis on drying gradients and warping due to high surface-to-volume ratios. The reference to ASTM standards adapted for printed specimens is appropriate. However, the manuscript does not critically assess whether these standards are fully applicable to layered, anisotropic geometries. A more rigorous discussion of how standard test geometries may misrepresent real printed components would enhance the critical dimension of the review.

The durability section identifies relevant degradation mechanisms, including freeze–thaw damage, capillarity, and alkaline degradation of natural fibers. The acknowledgement that as-printed surface roughness influences moisture retention is insightful. However, the review repeatedly identifies the absence of long-term durability data and standardized testing, yet it stops short of proposing concrete experimental protocols or durability indices tailored to 3D-printed soil - cement. Given the stated aim of outlining a “research roadmap,” this section should be more prescriptive.

The multi-level evidence framework (Levels A, B, and C) is conceptually interesting and represents an attempt to systematize fragmented literature. Distinguishing between direct 3DP soil - cement studies, traditional soil - cement analogues, and 3D-printed concrete analogues is logically sound. This triangulation approach is a notable strength of the manuscript. Nevertheless, the framework remains largely narrative. It would benefit from a more formalized matrix that explicitly indicates which mechanical, rheological, and durability parameters are transferable across levels and which are not. In its current form, the framework risks being perceived as a descriptive categorization rather than an analytical tool.

From a sustainability perspective, the manuscript emphasizes circular economy principles and reduced carbon emissions. However, despite repeatedly identifying the lack of life cycle assessments (LCAs), the authors do not provide even approximate embodied carbon comparisons or scenario-based estimations. For a paper strongly framed around sustainability, the absence of quantitative environmental benchmarking weakens the argument. At minimum, the authors could synthesize available embodied carbon values for soil - cement versus conventional concrete and discuss how fiber type (natural vs. synthetic) shifts the environmental balance.

Structurally, the manuscript is generally well organized, and the tables summarizing fiber types and mechanical performance are useful. However, some redundancy exists, particularly in the repeated discussion of printability trade-offs across sections. Language quality is generally good, though certain passages are verbose and could be condensed for clarity. Minor typographical inconsistencies (e.g., notation formatting and occasional grammatical slips) should be corrected during revision.

In conclusion, the manuscript addresses a highly relevant and emerging topic and demonstrates strong familiarity with the interdisciplinary literature connecting soil stabilization, fiber reinforcement, and additive manufacturing. Its principal strengths lie in the comprehensive coverage of fiber typologies and the structured discussion of fresh-state rheology and anisotropy. However, significant improvements are required in methodological transparency, quantitative synthesis, and environmental analysis. The review would benefit from a clearly defined literature review methodology, more systematic cross-study comparison of mechanical and rheological performance, deeper critical evaluation of transferability between soil - cement and concrete-based AM systems, and incorporation of quantitative sustainability metrics. With these revisions, the manuscript could make a meaningful contribution to the consolidation of fiber-reinforced soil - cement as a viable material system for sustainable additive manufacturing in construction.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript provides a timely and extensive integrative literature review examining the integration of fiber reinforcement within soil-cement mixtures for extrusion-based 3D printing. The authors construct a multi-level evidence framework that bridges fresh-state rheology with hardened-state mechanical performance, offering valuable insights into optimal fiber dosages. While the concept is promising, substantial revisions are needed to strengthen the scientific foundation, clarify ambiguities, and provide more comprehensive results. The authors are strongly encouraged to rigorously address the specific comments below.

1. In Section 2.2.3, the authors highlight the three-interval thixotropy test as a pivotal method for monitoring structural build-up. The lack of standard shear rates and resting durations leads to high testing variability across different literature sources. The text should issue a clear call to action for international standardization bodies to define uniform rotational rheometry parameters specifically calibrated for extrusion-based earth construction.
2. In Section 2.3.1, mechanical anisotropy is correctly identified as a critical structural vulnerability. However, the manuscript fails to address the specific micro-mechanical phenomenon of interface-parallel cracking, which acts as the predominant failure mechanism when structures are subjected to complex multidirectional loads. The text should reference recent empirical studies indicating that lowering the layer height, combined with incorporating specific interlocking microfibers, can drastically reduce the anisotropy coefficient.
3. In Section 2.3.1,while discussing the orientation of fibers parallel to the printing path, the manuscript neglects the influence of flowability on the internal pore structure. Articulating how optimized flowability mitigates pore-induced anisotropy provides a crucial theoretical bridge between fresh-state rheology and hardened-state fracture resistance.
4. In Section 2.3.3, the evaluation of material durability under aggressive environmental exposure severely neglects freeze-thaw cycling, which constitutes one of the most destructive degradation pathways for capillary-rich soil-cement matrices in cold climates.
5. To provide a broader and more up-to-date perspective, please consider incorporating the following recent publications into the literature review: DOI: 10.1016/j.conbuildmat.2025.145063. Mechanical, hydration process, and microstructural properties of eco-friendly UHDC incorporating recycled concrete powder[J]. Construction and Building Materials, 2026, 507: 145063

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This review systematically examines the fresh- and hardened-state performance of fiber-reinforced soil–cement mixtures for extrusion-based 3D printing, highlighting their mechanical enhancement mechanisms, rheological constraints, sustainability potential, and remaining methodological and durability challenges. Ky review comments are listed below.

1. Lines 84–100: The research gap and specific contribution of this review are not sufficiently articulated, and the manuscript should more clearly distinguish how its multi-level framework advances beyond prior reviews on 3D printed soil-cement or fiber-reinforced systems.

2. Lines 48–70: While the benefits of fibers are described conceptually, the discussion lacks deeper synthesis of quantitative trends, and a clearer comparative evaluation across fiber families would strengthen the scientific rigor of the review.

3. The section on mechanical strength and anisotropy would benefit from integration of recent studies on bond performance and interlayer behavior, such as Bond performance between FRP bars and 3D-printed high-performance concrete, which provide valuable insights into interface-controlled failure. The review should incorporate findings from Mechanical and microstructural characterization of interlayer bonding in multi-material 3D-Printed concrete, especially regarding microstructural evidence of interlaminate weakness and its implications for soil–cement systems.

4. The discussion on anisotropy and fiber alignment could be strengthened by referencing “3D-printed functionally graded concrete plates: Concept and bending behavior,” which offers useful understanding of directional reinforcement strategies and structural performance.

5. The critique of non-standardized testing methods is valid, but the review should propose more concrete, actionable recommendations for unified rheological and interlayer bond characterization protocols.

6. The sustainability discussion remains largely qualitative, and the authors should provide a more critical comparison of life-cycle impacts between natural and synthetic fibers, including embodied energy and long-term durability trade-offs.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for your honest consideration of my suggestions and comments. I recommend the article for publication in the journal.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have properly addressed the comments, and the manuscript has been revised comprehensively. I recommend acceptance of this paper in its current form.