A Comprehensive Review on Effects of Material Composition, Mix Design, and Mixing Regimes on Rheology of 3D-Printed Geopolymer Concrete

1. INTRODUCTION

In this new era of Industry 4.0, three-dimensional (3D) printing or additive manufacturing has attracted a lot of interest in recent years from both the building industry and the scientific community around the globe. [1]. This technique involves the deposition of materials in a layer-by-layer fashion to create the desired structure with a gantry-based 3D printer or an advanced robotic arm from a digitally designed model via a geometric code (G-code) generated from computer-aided design and slicing software [2]. Additive manufacturing technology, which is already used in the automobile [3], medical [4, 5], and aerospace [6] sectors, is yet in a stage of infancy in its contribution to the industrialization and digitalization of the construction sector [7, 8]. Many additive manufacturing technologies, including extrusion-based [9] and powder-bed-based [10-15], have been cultivated in the previous decade for 3D printing concrete. Building components with complicated geometry as per industry demand can be manufactured offsite utilizing powder-bed-based methods without costly formwork and assembled on-site. Hence, most of the research has focused on this method of 3D printing concrete [16]. 3D printing in concrete is a technologically advanced method holding potential benefits such as rapid construction, reduced labor expenditure, architectural freedom, high precision, formwork-free construction, and lesser construction waste generation [9, 17-24]. With technological advances like topology optimization, 3D printing can be a sustainable technology, providing economic and environmental benefits [25-28]. Due to its distinctive benefits, including enhanced production on demand and construction safety over traditional construction processes, 3D printing has recently expanded its application to the building and construction sectors. It appears to be the trend-setter, providing tremendous opportunities to revolutionize the construction industry in the coming years [29-35].

Extrusion-based concrete 3D printing is fundamentally characterized by the ingredients (raw materials) and ingredient properties and their design mix to fulfill the rheological essentials of printability, i.e., pumpability, extrudability, and buildability of the mixture [29, 36-43]. The mix design poses one of the major challenges for 3D printing in concrete [44]. When extruded, owing to the absence of formwork, the mixture must be able to hold itself and succeeding layers while still flowing freely throughout the pumping system [45]. Fast-setting yet adequate bonding mixtures are best suited for concrete 3D printing, which may be achieved by incorporating admixtures [46, 47]. The use of Ordinary Portland Cement (OPC) [48, 49] or Supplementary Cementitious Materials (SCMs) or industrial by-products [50, 51] such as Fly Ash (FA), Ground Granulated Blast-Furnace Slag (GGBS), metakaolin, Silica Fume (SF), etc., has been reported as a primary binder in the mix.

The continuous production of cement has led to massive carbon dioxide emissions in the atmosphere, resulting in global warming, which has a pernicious effect on the environment. Geopolymer is a promising long-term substitute for OPC binders. The formulation of the geopolymer mix contains aluminosilicate sources such as GGBS, FA, etc., which are industrial waste, thus beneficial in solving their storage and disposal issues [52, 53]. The utilization of geopolymers provides a sustainable solution to deal with this issue because cement is not required in its preparation, resulting in a cleaner environment by reducing carbon footprints [54-57]. Furthermore, geopolymers possess attractive rheological properties such as high thixotropy, which is demanded and desirable in the case of 3D printing of concrete [58, 59]. Studies comparing geopolymers to OPC for 3D printing of concrete have been limited despite the advantages noted above. Generally, a solid aluminosilicate precursor and a liquid alkaline activator (usually sodium (Na) or potassium (K) - based) make up the two-part geopolymer combination [60, 61]. For commercial and large-scale use, alkaline activator solutions that are viscous and corrosive are eventually difficult to manage [62]. Geopolymer’s rheological characteristics are also intricate and highly reliant on the amount and nature of alkaline activators and the alkali/silicate ratio, which is difficult to regulate in actuality [63]. Thus, the use of small doses of solid activators (known as “one-part geopolymer” or “just-add-water geopolymer”) [64] rather than huge amounts of liquid activators in 3D-Printed Geopolymer Concrete (3DPGC) should be investigated to utilize the benefits of one-part geopolymer, as very few attempts have been made in this area [65, 66].

Traditional methods of reinforcing concrete, on the other hand, either do not work with 3D printing or completely negate its advantages, and quite a few experiments have been carried out by experts in this direction [43, 47, 67-74]. Scientists have also examined the inclusion of fibers in the 3D printing ink to enhance ductility and flexural performance [75-77].

This article describes the rheology of 3DPGC related to the design mix and the effect of geopolymer formulation (one-part and two-part) in terms of material composition, mix design, and mixing regimes on the rheological and structural characteristics. In addition, the use of various reinforcing elements and their effects on the properties of 3DPGC are discussed, along with the research gaps for future investigations.

2. RHEOLOGY OF 3DPGC

Two fundamental metrics that characterize the rheological properties of fluids are yield stress and plastic viscosity. The yield stress is caused by the adherence and contact roughness between the particles in the slurry, which reflects the slurry's difficulty in surmounting frictional resistance and generating plastic flow [78]. The static yield stress denotes the greatest stress necessary for the substance to flow from its resting state, whereas the dynamic stress represents the minimum stress necessary to sustain flow [79]. The plastic viscosity is the attribute by which the slurry's internal structure impedes its flow. Plastic viscosity reflects slurry cohesiveness. Slurry will be more stable when sheared if its plastic viscosity is higher [78]. Plastic viscosity expresses shear stress growth with shear rate [79]. Viscosity recovery demonstrates the material's ability to recover across various shear rates. In contrast, the structural build-up rate serves as a guide for the material's stiffening rate before its final setting [80]. Due to the material's thixotropy, there is a distinction between static and dynamic stresses. Thixotropy refers to a reversible, isothermal, time-dependent decrease in viscosity when a fluid is subjected to increased shear stress or shear rate [61]. The material must be flowable and extrudable via a nozzle, adhere to the preceding layer, and keep its form under the growing stress induced by successive layer deposition to realize the acceptable quality of 3DPGC. The printability (pumpability and extrudability) and buildability of the printable ink characterize the quality of 3DPGC, thus relating them to the rheological properties of 3DPGC.

2.1. Printability (Pumpability and Extrudability)

In the printing process, pumpability refers to a print mix's ability to be pumped to the printing device under pressure and maintain its initial qualities. An easily transportable substance (typically by pumping) is required for 3D printing in concrete. Yet, an extruded material that is reasonably rigid is required to guarantee that the deposited filaments must preserve their shape. Print mix material's capacity to extrude smoothly as a continuous filament at the print head with an unimpeded material flow can be termed extrudability. Extrudability is dependent on several factors, including the nozzle design (size and shape), the mode of extrusion (such as screw or ram), the rate of extrusion, and, perhaps most intriguingly, the rheology of the material [21, 81-86].

3. MATERIAL COMPOSITION AND MIX DESIGN OF 3DPGC

Fig. (1) presents the various aluminosilicate precursors, alkali activators, additives, and reinforcing elements employed in the preparation of geopolymers for 3D printing applications, as reported in the literature.

The geopolymer material composition and preparation, including types of aluminosilicate precursors, alkali activators and their molar ratios, aggregates, additives, reinforcing elements, and curing conditions, are summarized in Table 1. The rheological and physical properties, such as workability, yield stress, viscosity, setting time, thixotropy, shape retention, print quality, buildability, and porosity are discussed.

4. MIXING REGIME FOR PREPARATION OF 3DPGC

The presented data in Table 2 outline various mixing regimes for the preparation of geopolymer mixtures for 3DPGC, as reported in the literature. The analysis illustrates a diverse range of methodologies, including both one-part and two-part mixing approaches, with distinct stages and equipment usage. For two-part geopolymers, mixing durations and sequences vary across studies, with dry mixing of binders and aggregates being a common initial step. The addition of fibers, thixotropic modifiers, and activators is often a gradual process to ensure uniform dispersion.

Notably, the hobart mixer and planetary mixer are frequently employed for two-part mixing, each contributing to specific advantages such as improved extrusion rheology or prevention of fiber bundles. In one-part geopolymers, water is introduced along with thixotropic thickeners, and the mix is stirred at varying speeds for optimal dispersion. The use of Hobart mixers in the case of one-part geopolymers is prominent, ensuring thorough mixing of binders, aggregates, activators, and additives.

In certain instances, a resting period is introduced after the initial mixing stages to improve workability. Quality control is emphasized through separate mixing regimes for each stage, enhancing the overall uniformity of 3DPGC. Challenges, such as a drop in workability during mixing stages, are addressed with specific solutions, such as halting stirring to clean the mixer. The use of additives like VMA is highlighted for improving extrusion rheology, emphasizing the importance of careful admixture incorporation.

Attempting to apply the multi-level material design [147] to 3DPGC suggests that rheology is primarily influenced by the nature of the precursors and admixtures used, which will affect pumpability and buildability and, eventually, the structural characteristics of 3DPGC. Incomplete mixing or a shorter mixing duration required may result in unreacted ingredient particles in the mix, affecting the pore structure, rheology, and mechanical properties of 3DPGC. It is vital to avoid agglomeration and confirm the uniform distribution of fibers when used in 3DPGC since it can result in choking or blockage at the print head, the formation of cold joints, or adversely affect flexural and inter-layer bond strengths. Typical mixing regimes adopted for the preparation of geopolymer mixtures (one-part and two-part) for 3DPGC are presented in Table 2. The binders are usually mixed dry to give a homogeneous mix. The addition of fibers and aggregates is done either before or after the addition of water to the mix. The time of mixing of ingredients also varies in different studies.

The diverse mixing regimes in Table 2 indicate the adaptability of geopolymer mixtures for 3DPGC to different methodologies. The choice of mixing equipment, sequence, and duration play pivotal roles in achieving optimal rheological properties, preventing issues like fiber agglomeration, and ensuring uniform distribution of additives. This comprehensive review underscores the importance of tailoring mixing regimes to specific requirements, contributing to the broader understanding and advancement of 3DPGC.

5. INFLUENCE OF REINFORCING ELEMENTS ON 3DPGC

According to Table 3, fiber insertion has a considerable effect on flexural, tensile, and interlayer bond strengths, although its effect on compressive strength is less pronounced due to the intrinsic anisotropy of 3DPGC specimens. Introducing fibers (hooked-end steel 1%, PP 0.5%) during stacking can modify bond strength [75]. The layer matrix may not be the same for fiber distribution in paste. An uneven surface may prevent layer adhesion, especially if steel fibers are utilized, which is not encouraged [75]. The maximum strength in the perpendicular direction increased from 22 MPa to 36 MPa when the volume of PP fibers was increased from 0% to 0.25 vol.% [77]. Because of the higher porosity and stiffness of the mix [149-151], the compressive and inter-layer bond strengths decreased beyond 0.25% fiber addition. Using 0.25% fibers increased flexural strength by 17-34%, but lowered interlayer bond strength because of the tightening of the matrix [16]. During flexural tests, the bottom layer failed in tension, not in shear [16]. Longitudinal orientation had the highest mean compressive strength, followed by perpendicular and lateral. PVA fibers caused the least loss in interlayer bond strength, while PBO fibers improved flexural strength. Using the three-point bending test, the 15 N/mm² strength in flexure showed a suitable printing speed, good layer adhesion, and fiber alignment, which offset the detrimental effect of voids [51]. The 3D-printed samples had fibers oriented along the print direction [110, 152]. Fibers only helped fracture bridging [153] in lateral and perpendicular directions, not longitudinal (along the printing path). 3DPGC compression strength was anisotropic based on loading direction, irrespective of layer count. The number of printed layers affects the flexural behavior of 3D-printed specimens in four-point bending tests [110]. Two-layered specimens demonstrated increased first crack strength and rupture modulus. One-layered specimens have a higher deflection capacity than two-layered specimens [110].

In 3D printing, fresh concrete is subject to shrinkage cracking since no formwork is used. Microwave heating can promote hydration (self-desiccation) and moisture loss, leading to crack formation [115]. To preserve mechanical performance and durability, 1% micro PVA fibers (6 mm long, 26 μm diameter) are employed. Fibers affect 3DPGC's porosity and shrinkage [32]. The inclusion of fibers in the mix was assumed to have reduced micropores [98]. An FA-based geopolymer mix with 0.25% 4 mm-long microglass fibers reduced shrinkage and distortion in the plastic state in a similar way [76]. As the nano-clay particles dispersed and the fibers bridged cracks, the mixture's mechanical performance, shape retention, and buildability increased greatly, as evidenced by its outstanding microstructural properties and lowest pores anisotropy [139].

A previous study [103] examined two production procedures (cast and injected samples—3D printing simulations), curing duration (7 and 28 days), and composites (short fiber-reinforced geopolymers and plain samples). Flax fiber injection specimens illustrated maximum flexural strength after 28 days, up to 36% higher than casting specimens [103]. Due to the high dose, carbon fiber specimens' flexural strength was not improved. Flax-fibered specimens outperformed carbon-fibered ones [103]. Reinforcement was inserted into the geopolymer matrix using a method that allowed steel cable to enter the extruded filament seamlessly [67]. 8 mm PVA fibers were utilized in the matrix to create interlocking between the fibers and prevent cable slippage. In the early stages of tensile stress, short fibers can bridge microcracks; however, in the latter stages of crack propagation, longer fibers begin to function. The hybrid reinforcement technique increased geopolymer composites' flexibility by up to 290% relative to the control specimen. The first fracture load and fracture toughness of hybrid reinforced geopolymer composite specimens reinforced with cable increase as the diameter of the steel cable increases [67]. Two incline-crossed and concentric path printing setups were employed to make the samples [102]. Each filament has a micro cable in the center. The use of PP fibers prevents shrinking [154, 155]. An 8-fold and a 70-fold increase in flexural strength and deflection resistance over unreinforced geopolymer composites were found in the incline-crossing printing configuration [102]. The strength of micro-cables under a variety of loading conditions was evaluated using both continuous and simultaneous strengthening approaches [68]. 7 shares and 19 strands per share comprised the micro-cables. The specimens were created using a concentric route and two crosshatch zigzag designs. A pull-out test was utilized to assess the bonding properties. The tensile behavior was mostly a result of the cable reinforcing designs.

In summary, the meticulous exploration of reinforcing elements in 3DPGC underscores the critical interplay between fiber incorporation and material performance. Key findings highlight the significant influence of fibers on flexural, tensile, and interlayer bond strengths. Notably, the anisotropic nature of 3DPGC, coupled with the intricate relationship between fiber volume and mechanical properties, highlights the need for precise engineering in material design. Optimizing the volume of PP fibers showcases a substantial increase in perpendicular strength, yet careful consideration is required to avoid compromising compressive and interlayer bond strengths beyond a certain threshold. The effects of different fiber types, such as the minimal loss in interlayer bond strength with PVA fibers and improved flexural strength with PBO fibers, emphasize the importance of tailored material selection. The exploration extends to the realm of micro-reinforcement strategies, introducing hybrid approaches with steel cables and varied fiber lengths. This innovation improves flexibility and fracture toughness, paving the way for advanced applications in construction. The intricate balance between short and long fibers in tension stages highlights the dynamic nature of the reinforcing mechanisms at play.

Furthermore, the study delves into the challenges associated with porosity, decreased interlayer bond strength, and workability degradation in the presence of fibers. Flexural strength increases with fiber incorporation over 0.25%, but workability of the mix degrades due to porosity and decreased interlayer bond strength. Further, different fiber types have different effects on workability of the mix. To improve the micro-reinforcement impact, specific pretreatment measures are required. As a result, polymer-based cables should be studied to increase corrosion resistance in comparison to steel cables.

In conclusion, this section not only contributes to the nuanced understanding of 3DPGC behavior under diverse reinforcing elements but also charts a course for future advancements. The delicate optimization of material composition, coupled with innovative reinforcement strategies, propels the scientific community toward the development of 3DPGC with enhanced precision, durability, and application versatility.

The impact of geopolymer material composition and mix design on the rheological and structural behaviors of 3DPGC, as discussed comprehensively in Section 4, is summarized in Table 4.

The comprehensive analysis presented in Table 4 illuminates the intricate relationship between geopolymer material composition, mix design, and resulting rheological and structural properties of 3DPGC. Key insights can be gleaned from the summarized influence of various components on properties crucial for successful 3D printing.

Components such as FA, GGBS, and metakaolin exhibit varying impacts on workability, yield stress, and viscosity. FA enhances workability but decreases yield stress, while GGBS and metakaolin show contrasting effects. Alkali activators, including Na-based and K-based activators, play a crucial role, with sodium-based activators generally improving workability and compressive strength, but potassium-based activators enhance yield stress and shape retention. SiO₂/M₂O (M = Na or K) considerably influences multiple properties, contributing to improved yield stress, viscosity, and shape retention. The aggregate-to-binder ratio and aggregate size impacts properties like workability, yield stress, and porosity, with a careful balance required for optimal performance.

Various additives, including nano-clay, actigel, cellulose, and nanographene platelets, display distinct effects on thixotropy, print quality, and yield stress. The introduction of fibers and steel micro-cables influences multiple parameters, with a general trend toward improving yield stress, compressive strength, and buildability, while impacting workability and print quality.

The findings underscore the delicate balance needed in geopolymer composition for 3DPGC, with different components contributing synergistically or adversely to specific properties. SiO₂/M₂O (M = Na or K) emerges as a critical parameter influencing both rheological and structural aspects, highlighting the need for precise control over alkali activator composition. The study lays the foundation for tailored geopolymer formulations, enabling the optimization of workability, structural strength, and print quality in 3DPGC. In conclusion, this systematic exploration not only advances our understanding of the intricate interplay within geopolymer compositions but also provides a scientific roadmap for formulating 3DPGC with superior performance characteristics. The nuanced insights garnered from this analysis pave the way for informed material design strategies in the burgeoning field of 3D-printed construction materials.

CONCLUSION

3D printing technology offers several benefits over traditional construction methods in the field of building applications. The quest for a sustainable approach to replace conventional concrete has been a major issue due to its environmental impact. An environmentally friendly and more sustainable alternative to typical OPC-based concrete is geopolymer, which is made from industrial waste. The strong mechanical qualities and improved durability of geopolymers allow for a reduction in carbon footprint. Despite the advantages of geopolymer over OPC, the research conducted into using geopolymer in 3D printing is scarce.

The rheology requisites in terms of printability (pumpability and extrudability) and buildability, which distinguish 3DPGC from conventional concrete, are discussed. Some typical mixing regimes adopted for the preparation of geopolymer mixtures (two-part and one-part) for 3DPGC are reported. The printability, buildability, and structural characteristics are greatly dependent on the material design of the geopolymer mixture used for 3D printing. An in-depth review of the influence of mix design parameters of geopolymer formulation, such as aluminosilicate source materials, alkali activators, admixtures, aggregates, reinforcing elements, and curing conditions, on the properties of 3DPGC is systematically presented.

In conclusion, the detailed exploration of the rheological and structural aspects of 3DPGC provides a comprehensive understanding of the complex interplay between material composition, mix design, and processing parameters. The study delves into fundamental metrics like yield stress and plastic viscosity, emphasizing their significance in characterizing the flow behavior of the geopolymer slurry. Thixotropy, a time-dependent decrease in viscosity under increased shear stress, is identified as a crucial property for ensuring flowability and extrudability required for successful 3D printing.

The investigation extends to the critical parameters of printability (pumpability and extrudability) and buildability, shedding light on the challenges associated with layer deposition, structural integrity, and the prevention of failure due to gravity-induced stresses. The review systematically examines the material composition and mix design of 3DPGC, emphasizing the influence of aluminosilicate precursors, alkali activators, additives, aggregates, and curing conditions on properties such as workability, yield stress, viscosity, and structural build-up rate.

The exploration of different mixing regimes for the preparation of geopolymer mixtures demonstrates the role of equipment, sequence, and duration in achieving optimal rheological properties. The diverse methodologies, including one-part and two-part mixing approaches, underscore the adaptability of geopolymer mixtures to various methodologies, emphasizing the need for tailored mixing regimes to meet specific 3DPGC requirements.

Finally, the in-depth analysis of reinforcing elements provides valuable insights into the effects of fibers and steel micro-cables on flexural, tensile, and interlayer bond strengths. The section illustrates the delicate balance required in material design to optimize reinforcement while addressing challenges such as porosity, decreased interlayer bond strength, and workability degradation.

This systematic and comprehensive review not only advances the current understanding of 3DPGC but also serves as a valuable resource for researchers and practitioners seeking to optimize geopolymer formulations for 3D printing applications. The insights provided pave the way for future advancements, informed material design strategies, and continued evolution of 3D-printed construction materials toward enhanced precision, durability, and sustainability.

CHALLENGES AND RESEARCH OPPORTUNITIES

Based on this comprehensive review, research areas have been outlined and suggested for future studies, as follows:

• Precursor and activator effects:
  • Challenge: Investigating the effects of precursor type, activator type, concentration, molar ratios, and chemical admixtures on 3DPGC properties is complex due to the numerous variables involved.
  • Opportunity: Developing a systematic and standardized testing methodology to comprehensively study the multifaceted influences of different precursor and activator combinations on the properties of 3DPGC.
• Optimal mixing regime:
  • Challenge: Determining the optimal mixing regime for 3DPGC is challenging due to the varied nature of the geopolymer formulation and its impact on the final product's characteristics.
  • Opportunity: Conducting extensive experimental and computational studies to identify the most effective mixing methods for achieving desirable properties in 3DPGC, considering factors such as mixing speed, duration, and order of addition.
• Diversification of aluminosilicate precursors:
  • Challenge: Limited investigations have explored unconventional aluminosilicate precursors for 3DPGC, hindering the understanding of the full spectrum of available materials.
  • Opportunity: Evaluating the use of alternative aluminosilicate sources such as red mud, granite powder, marble dust, rice husk ash, construction and demolition wastes, and mine tailings to expand the range of sustainable materials for 3DPGC.
• User-friendly activators:
  • Challenge: Current activator solutions are corrosive and hazardous, posing safety concerns. Exploring user-friendly alternatives is necessary.
  • Opportunity: Research and develop solid activators or one-part geopolymer systems that are safer, easier to handle, and have the potential for large-scale application in 3DPGC.
• Interaction of admixtures:
  • Challenge: Understanding the intricate interactions between accelerators, retarders, thixotropy modifiers, and VMAs in 3DPGC formulations is crucial for optimizing printability and buildability.
  • Opportunity: Performing in-depth studies and simulations to unravel the synergistic or antagonistic effects of various admixtures, providing insights for fine-tuning their concentrations in 3DPGC mixtures.
• Rapid setting and hardening materials:
  • Challenge: Identifying and incorporating rapid setting and hardening materials while maintaining desired properties are a complex task.
  • Opportunity: Exploring novel materials and formulations that enhance the speed of setting and hardening in 3DPGC, ensuring compatibility with the 3D printing process.
• Sustainable aggregates:
  • Challenge: The high cost associated with fine aggregates in 3DPGC necessitates research on sustainable alternatives without compromising performance.
  • Opportunity: Investigating the partial or complete replacement of fine sand with geopolymer aggregates or other sustainable materials, assessing both economic and environmental aspects.
• Incorporation of organic fibers:
  • Challenge: Understanding the impact of organic fibers on 3DPGC characteristics and optimizing their use are a relatively unexplored area.
  • Opportunity: Evaluating the influence of different types and concentrations of organic fibers on the mechanical properties and printability of 3DPGC, aiming for enhanced performance.
• Durability and microstructure analysis:
  • Challenge: Long-term performance prediction of 3DPGC and comprehensive microstructure analysis pose challenges owing to the complex interactions within the material.
  • Opportunity: Doing extensive research on the durability aspects of 3DPGC, including exposure testing, and employing advanced imaging techniques for a detailed microstructure analysis to understand the material's behavior over time.
• Curing conditions and real-field applications:
  • Challenge: Optimizing curing conditions, especially ambient curing, for 3DPGC in real-field construction applications is challenging.
  • Opportunity: Examining the effects of ambient curing on the hardened qualities of 3DPGC and developing practical strategies for extrusion-based printing simultaneously with heat curing in real-field scenarios.
• Sustainability and life cycle assessment:
  • Challenge: Assessing the sustainability and conducting life cycle assessments of 3DPGC involves considering a wide range of environmental factors.
  • Opportunity: Developing a comprehensive framework for sustainability assessment and life cycle analysis of 3DPGC, considering factors such as raw material extraction, production, transportation, and end-of-life considerations.
• Topology optimization for material efficiency:
  • Challenge: Implementing topology optimization methods for 3DPGC to minimize material consumption requires a balance between structural integrity and material efficiency.
  • Opportunity: Exploring advanced optimization algorithms and design methodologies to achieve the maximum advantages of 3D printing, focusing on reducing material consumption without compromising structural performance.
• Machine learning and numerical modeling:
  • Challenge: Predicting and controlling material composition parameters for 3DPGC prior to the printing process are intricate and demand advanced modeling techniques.
  • Opportunity: Integrating machine learning and numerical modeling methods to develop predictive models for 3DPGC, aiding in the control of mix design and rheological and structural properties and facilitating real-field applications.

These challenges and research opportunities collectively aim to advance the understanding and application of 3DPGC in the construction sector. Researchers and practitioners can address these issues to contribute to the development of more efficient, sustainable, and widely applicable 3D printing technologies for construction materials.

AUTHORS’ CONTRIBUTIONS

Conceptualization, P.B., A.B. and S.S.; methodology, P.B., A.B. and S.S.; validation, A.B. and S.S.; formal analysis, A.B.; investigation, P.B., A.B. and S.S.; resources, A.B.; writing—original draft preparation, P.B., A.B. and S.S.; writing—review and editing, A.B.; project administration, A.B.; All authors have read and agreed to the published version of the manuscript.

REFERENCES