Evaluation of Workability Parameters of Straw-Reinforced Earth-Based Materials for Extrusion-Based 3D Printing

Abstract

Despite increasing interest in earth-based materials for additive manufacturing in construction, limited research has systematically investigated the workability parameters governing their suitability for extrusion-based 3D printing. This study evaluates the influence of wheat straw fiber content on the flowability, pumpability, extrudability, and buildability of earth-based mixtures for extrusion-based 3D printing applications. Four mixtures containing varying proportions of clay (76–100 wt%), wheat straw fiber (0–20 wt%), additives (0–4 wt%), and water were experimentally assessed and subsequently evaluated for compressive strength. Flowability was determined using modified flow tests; pumpability and extrudability were assessed through manual extrusion tools (triangular bag and grout gun), and buildability was evaluated using a modified compression test measuring green strength. Results showed that increasing the wheat straw fiber content from 5% to 20% reduced the flowability from 31.42% to 11.42%, despite corresponding increases in water content. The SP15 mixture (82% clay, 15% wheat straw fiber, 3% additive, and 35% water) exhibited the most balanced performance, achieving the lowest height reduction (17.14%) during buildability testing and demonstrating superior structural stability and compressive performance. The proposed methodology provides practical guidance for developing printable earth-based materials and supports the adoption of locally available, low-carbon materials in sustainable additive manufacturing for construction.

1. Introduction

As rapid urbanization continues globally, researchers and professionals in the Architecture, Engineering, and Construction industry are increasingly investigating automation to expedite the development of housing and infrastructure. Simultaneously, the construction sector is responsible for approximately 11% of global annual carbon emissions; this data is expected to rise over the next five decades. Notably, around 8% of these emissions stem directly from cement production, a critical component of concrete, which remains the most widely used building material worldwide [1]. To tackle the dual challenge of rapid urbanization and its environmental impact, innovative systems are required to integrate low-carbon building materials with scalable construction automation technologies. Expanding upon existing earthen construction technique research establishes a generalized framework for design and construction using 3D-printed earth as an emerging fabrication method [2].

Additive manufacturing (AM), commonly referred to as three-dimensional (3D) printing, has emerged as a transformative technology in the Architecture, Engineering and Construction (AEC) sector. Unlike conventional construction methods, extrusion-based AM enables automated fabrication directly from digital models, reducing material waste, labor requirements, and construction time while offering greater design freedom [3]. Recent developments have demonstrated the potential of AM to improve construction efficiency, optimize material placement, and support circular construction practices through the use of locally sourced and low-carbon materials [4].

Large-scale additive manufacturing is an expanding field of research and industry innovation. Across the globe, 3D-printed buildings have been constructed, with some already serving as inhabited structures [5]. All of these constructions use a combination of traditional construction methods and 3D-printed components composed of concrete, plastic, clay, earth, or other experimental combinations of materials. Extensive research has been undertaken to determine the material qualities of soils suitable for raw earthen construction, but, as Delgado [6] discovered in a global survey, current building codes around the world provide little direction for building with earth. In regions where earthen construction is regulated—such as New Zealand, Australia, Germany, and the U.S. state of New Mexico—building codes often recommend adding cement or other stabilizers to structural earthen elements to meet the minimum strength requirements. Studies have shown that particle size distribution, fiber content, and clay content can all have a major impact on the compressive strength of earthen mixtures, allowing for greater strength and durability without losing circularity [7]. Koutous recently proved that increasing the fiber content of compressed earth can improve the compressive strength to the same extent as stabilization with 1–2% cement [8]. Finding effective alternatives to cement stabilization for structural earthen materials is critical to maintaining their status as truly low-carbon, circular materials. Furthermore, cement-stabilized earth cannot be recycled directly into new construction materials.

Although cement-based printable materials currently dominate the construction sector, concerns regarding their embodied carbon have motivated researchers to investigate alternative materials. Earth-based materials have gained increasing attention because of their low environmental impact, local availability, recyclability, and compatibility with circular economy principles [9,10]. Recent studies have demonstrated that earth-based printable mixtures can provide satisfactory mechanical and rheological performance while significantly reducing environmental burdens compared with cement-rich mixtures [11].

Recent characterization studies have focused on the rheology, buildability, extrusion, and printability of mixtures. The impact of fresh-state rheological characteristics of a printed mix on extrusion and buildability was highlighted by V. Saruhan et al. [12]. Additionally, they have examined how the quantities of the material mix affect the rheology of a printable concrete mix. S. Muthukrishnan et al. [13] examined specific methods and mix selections to enhance the constructability of 3D-printed concrete. N. Roussel [14] conducted a thorough analysis of the rheological specifications for extrusion-based printable mixtures and came to the conclusion that these specifications vary based on the shape and procedure. S. Hou et al. [15] identified the performance prerequisites for printed mixes in terms of their rheological, toughened, and durability qualities. Tourtelot et al. [16] suggest a weighted pump test as a field-oriented test method to determine yield stress. However, the requirement for certain equipment makes it difficult to apply quickly and easily.

Recent investigations have highlighted the importance of fresh-state material properties in determining successful extrusion-based printing. Studies by Rocha et al., Faleschini et al., and Assunção et al. [4,9,17] demonstrated that flowability, extrudability, buildability, shape retention, and open time are among the most critical parameters governing the printability of earth-based mixtures. Furthermore, natural fiber reinforcement has emerged as a promising strategy for improving dimensional stability and reducing shrinkage cracking in earthen composites. However, excessive fiber incorporation may negatively affect extrusion quality due to increased internal friction and nozzle blockage. Consequently, achieving an appropriate balance between fiber content and workability remains a significant challenge in the development of printable earth-based materials [18,19].

Prototypes have been 3D-printed throughout the last decade by commercial product WASP and IAAC (Institute for Advanced Architecture) researchers, showing promising potential for advanced, high-performance digitally fabricated earth-building systems [20,21]. Despite recent advances, limited studies have systematically evaluated the combined influence of wheat straw fiber content on multiple workability parameters, including flowability, pumpability, extrudability, and buildability, using practical and field-applicable characterization methods. Furthermore, many existing studies rely on sophisticated rheological equipment, limiting their applicability in resource-constrained environments. Therefore, this study aims to experimentally evaluate earth-based mixtures reinforced with wheat straw fibers for extrusion-based 3D printing and identify an optimized composition capable of balancing printability and structural stability.

2. Research Methodology

The process to achieve the goals of the study began with the research and selection of potentially suitable earth-based combinations for printing. This section briefly includes following steps:

-

Preliminary materials selection;

-

In-depth information of the final material selection;

-

Preparing earth-based mortar in the laboratory with selected materials and adjusting the ratios;

-

Characterization tests of the earth-based mortar samples to determine and optimize the workability of the material as a printable mixture.

The process to achieve the objectives of this study began with a comprehensive research and selection phase focused on identifying potentially suitable earth-based mix combinations for printing applications. This initial stage involved a preliminary assessment aimed at selecting materials based on criteria such as availability, sustainability, mechanical properties, and compatibility with printing technologies (Figure 1). Following this, a more in-depth analysis was conducted on the final material choices (Figure 2), including a detailed evaluation of their physical, chemical, and mechanical characteristics to ensure their suitability as components in printable mixtures. Once the most promising materials were identified, various earth-based mortar mixtures were prepared in the laboratory by adjusting the proportions of each component. Special attention was given to balancing the amounts of water, binders, and aggregates in order to achieve the desired consistency and workability necessary for the printing process. Finally, a series of characterization tests were performed on the prepared mortar samples to evaluate key properties such as their mechanical strength, plasticity, drying rate, and dimensional stability. These tests enabled the determination and optimization of the mixture’s workability, ensuring that it was sufficiently fluid and cohesive for printing, while also guaranteeing the quality and durability of the final printed product. The overall experimental methodology adopted in this study is summarized in Figure 1. The workflow illustrates the material selection process, mixture formulation variables and their ranges, preliminary screening procedure, workability characterization methods, optimization process, and compressive strength assessment performed to identify the most suitable earth-based mixture for extrusion-based 3D printing applications.

2.1. Materials

At the beginning, the preliminary selection of the materials included sand, clay, and straw fibers (Figure 2) in different quantities. The preliminary material mixes were tested in the laboratory; the consistency was not suitable to be used for the printing as the goal was to achieve a combination with a high percentage of straw in the earth-based mortar sample. Therefore, the sample prepared with clay, sand and straw fiber showed weak bonding and failed the preliminary characterization test.

Figure 2. Materials used for sample preparation.

Figure 2. Materials used for sample preparation.

Kaolin clay is a fine-grained aluminosilicate (Al₂Si₂O₅(OH)₄) characterized by low plasticity, low swelling potential, high thermal stability, and relatively inert chemical behavior. These properties make it particularly suitable for earth-based mortars, as they help reduce shrinkage cracking and improve dimensional stability during drying. It was selected because of its worldwide availability as a mineral in the Earth’s crust; it is affordable and environmentally friendly in nature, which is an important factor for upscaling and furthering the results for different uses. The unpurified clay used in this study was a natural kaolinitic clay sourced from the region of Puglia, and has been used in all the earth-based mortar samples. Clay was received in the natural form, then transformed into powder form by manual hammering. While organic clay is flexible and easily transformed into various forms, it was easier to get the appropriate configuration. Kaolinite clay has a moderate swelling capacity and less shrinkage than other types of clay [22]. Furthermore, straw fiber was selected as the main eco-friendly material. Straw waste was collected from the nearby industry to limit transportation. Market-based maize starch as the additive was selected to be added in the mixture. Maize starch was selected as a natural and biodegradable additive to improve the cohesion, water retention, and workability of the earth-based mixtures while maintaining the sustainability objectives. Direct tap water (approximately 18–20 °C at the time of use) was used for preparation of the earth-based mortar samples. All the ratios and weight percentages (wt%) are mentioned in

Table 1. Experimental mixtures tested at the beginning: component ratios (the percentages are expressed in weight).

Samples	Clay wt%	Wheat Straw Fiber wt%	Additives wt%	Water wt%
SP0	100	0	0	22
SP3	96.5	2.5	1	25
SP5	94	5	1	28
SP7	90.5	7.5	2	30
SP10	88	10	2	31
SP12	84.5	12.5	3	33
SP15	82	15	3	35
SP17	78.5	17.5	4	37
SP20	76	20	4	39

.

2.2. Experimental Details

Since previous studies have suggested a link between fiber content and mechanical characteristics in relation to earthen materials, our research investigates the influence of straw fiber on the printability of the selected mixtures.

For the present study, a total of three characterization tests were carried out. Nine mixtures were initially developed (Table 1). While preparing initial samples, the mix of selected quantities showed a similar consistency due to the smaller amount of straw difference in the sample mixtures; therefore, the number of samples for final testing was reduced, and 4 samples were selected to further work with in order to perform workability tests (

Table 2. Final samples selected after the preliminary test of all samples.

Sample	Clay wt%	Wheat Straw Fiber wt%	Additive wt%	Water wt%
SP5	94	5	1	28
SP10	88	10	2	31
SP15	82	15	3	35
SP20	76	20	4	39

). The workability of the material was assessed in the first two characterization tests. Flowability was assessed first, followed by pumpability and extrudability. Specifically, the tests assess the cohesiveness, which includes the segregation properties and deformation properties when force is applied during the printing process, and the consistency, which measures the ease of flow [16,23]. Additionally, they enable the material to be rechecked right before printing.

The third characterization test, defined by [24], is the buildability test. It is a test to measure the strength of the sample mixtures right after 10 minutes of preparation, when it is still fresh. The term buildability describes a material’s capacity to maintain its extruded shape while allowing the formation of a layered structure. Together, these three tests evaluate the printability properties of samples when they are fresh. After carefully analyzing the workability of the printing mixture, the sample was also tested for compressive strength.

2.3. Sample Preparation Method

The earth-based mortar was prepared by manually mixing the clay powder with water, straw fibers, and additives in various proportions (see Table 2). Manual mixing was intentionally selected to simulate field-applicable and low-cost fabrication methods, consistent with the study objective of developing printable earth-based mixtures without reliance on specialized laboratory equipment. The ratios represent the weight percentages of each component. Each ingredient was weighed separately according to the sample requirements, then combined in a bowl and stirred by hand for one minute before adding any water. The chosen percentages were determined through a trial-and-error process.

Following dry mixing to achieve a homogeneous mixture, water was gradually added, and a mix was prepared. Efforts were made to achieve the consistency of a mixture that closely resembled that typically used in the printing process. It had been expected that increasing the water concentration would modify the mix’s rheology during and after extrusion. Therefore, it was critical to investigate the behavior of the mixture during the water addition process. The mixture was immediately used after mixing and preparing to avoid a change in state of sample mixtures. In this study, this examination provides a comprehensive understanding of the variation of the straw fiber content in the sample mixture to evaluate the performance of the mix during extrusion without causing damage or clogging in the nozzle.

For a compressive strength test, from the selected samples, one of the best performing samples was selected and prepared for the test. Rectangular prism specimens with dimensions of approximately 4 cm × 4 cm × 15 cm were prepared for compression testing. After preparing the samples, the load was applied using a universal testing machine (UTM). The load direction for the test was perpendicular to the 4cm x 15cm face of the block, following procedures adapted from IS 1725:1982 [25] (Soil-Based Blocks) and ASTM E2392/E2392M-10 [26] (Earthen Wall Systems). The load was increased at a constant rate until a visible failure occurred. The maximum load at failure was recorded and used for the compressive strength evaluation. Since the primary objective of this study was to assess the load-bearing capacity of the optimized mixture, displacement and strain measurements were not recorded during testing.

2.4. Flowability Test

The flowability test aims to define a material’s consistency for workability purposes using a modified flow test. According to ASTM C230/C230 M-21 [27], this test enables immediate final assessment prior to printing, which is not achievable with standardized mortar testing. Flow consistency is also referred to in the workability test for fresh-state mortar according to EN 1015-3 [28]. For this study, the standards are followed, and the process described by S. Rückrich et al. [29] has been applied. A cylindrical sample was obtained by manually extruding the material from a cylindrical mold (35 mm in height and 35 mm in diameter). The mortar mixture was poured into the mold and compacted to remove any air bubbles. In order to check the flow, the sample was impacted. It was positioned in the middle of a wooden plate (30 cm × 11 cm) (Figure 3), one side of which was raised to the sample’s starting height (35 mm) with the help of a metallic rod placed on the plate. Later, the plate was dropped and hit the surface (table) underneath. This was carried out 10 times for each sample, making sure that the wooden plate impacted the underlying surface in a uniform way. A caliper gauge was used to measure the material’s spread. The material’s flowability increases with the spread diameter. Before printing, the uniformity needed for pumpability and extrudability can be controlled by measuring the material’s flowability based on its spread. Flowability can be calculated in percentages by using the given equation, Equation (1), below [29]:

$$\mathrm{F}\mathrm{l}\mathrm{o}\mathrm{w} \%={\displaystyle \frac{\mathrm{s}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{d} \mathrm{d}\mathrm{i}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{r} \left(\mathrm{c}\mathrm{m}\right)-\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}{\mathrm{e}}^{\prime }\mathrm{s} \mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e} \mathrm{d}\mathrm{i}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{r} \left(\mathrm{c}\mathrm{m}\right)}{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}{\mathrm{e}}^{\prime }\mathrm{s} \mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e} \mathrm{d}\mathrm{i}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{r} \left(\mathrm{c}\mathrm{m}\right)}}\times 100$$

(1)

2.5. Extrudability and Pumpability Test

In order to comprehend the importance of the material pumping on the printing process, the quality of the printed element and its effect on the subsequent stage, it is necessary to have a holistic view of the process of the extrusion-based 3D print and the associated challenges. In additive manufacturing or 3D printing, pumping is the method most frequently deployed to carry freshly mixed material to the printer nozzle. It is important to prevent resource waste while preparing and attempting 3D printing. Therefore, the material mix should be evaluated to ensure it is printable without damaging the printer or other equipment associated with the 3D printing process. To evaluate pumpability and extrudability, two experimental methods were adopted, and two devices were used to simulate the printing process on a small scale in the laboratory. To determine the suitable consistency of samples, in the first phase, plastic bags with triangular shapes were used, and then and a grout gun (construction equipment) was employed. Both devices required small quantities of samples for extrusion with a cylindrical shape (1.5 cm diameter thick and 25 cm long) and had an opening like an extrusion nozzle with similar dimensions as those used in a WASP printer, ranging from 0.8 cm to 3 cm.

Triangular bag: Initially, for the experiments, triangular plastic bags (Figure 4a) available on the market were used to evaluate pumpability and extrudability. The tip of the triangular plastic bag was cut to size as per the extruder nozzle. Material prepared for the samples was manually filled and extruded through an opening; the same shear forces are involved in both testing and printing. It simulates the size of the nozzle and, being manually manipulated, it allows checking of the consistency and cohesion of the mixture. However, when the mix is less fluid, replicating a seamless printing process can be harder.

Grout gun: Another device that requires a small quantity of material, with similar nozzles for extrusion as those used in the WASP printer, was identified and selected during the experiment to ensure the proper replication of the printing process and improve the extrudability and pumpability in order to get the desired results. The stainless-steel mortar grout gun commonly used in construction and easily available in construction equipment stores was used (Figure 4b). The capacity of the grout gun is 1.3 L with a cylinder diameter of 6 cm and length of 45.5 cm. The grout gun proved to be a highly efficient device for simulating the extrusion that occurs with the large-scale printer [30]. The method for loading the gun with the mixture was easy to operate, quick, and prevented air from entering the cylindrical chamber. The gun was held vertically, and the chamber was filled manually, with one hand steadying the grout gun while the other poured in the mix, mimicking the 3D printer extrusion process. The simulated extruding of samples was performed at room temperature without any mechanical heating or drying requirements.

The test determines the Optimum Water Content (OWC), which is defined as the specific amount of water needed for the mixture to achieve the best balance between plasticity, cohesion, and extrudability [31,32]. If the water content is below this optimum, the mixture becomes too stiff and unpumpable; if it exceeds the optimum, the mixture becomes overly fluid, leading to segregation, reduced shape stability, and excessive deformation after extrusion [33]. Thus, identifying the OWC is essential for ensuring consistent material behavior during extrusion.

An ideal extrusion is obtained when the material can be efficiently and uniformly extruded into an established cylindrical line without separating the water. The mixture’s workability can be changed systematically by adding water, clay, and additives. The fluidity of the extrudable mixture can then be characterized using the flowability test (see Section 2.4. Flowability Test). Together, the flowability, pumpability, and extrudability tests can be used to evaluate the workability, which is essential as a starting point for examining the buildability.

2.6. Buildability Test

The buildability of printed materials is an important factor that influences the structural integrity and printability of extruded components [34]. Buildability is estimated by measuring the green strength of a material, defined as a material’s mechanical resistance in its fresh, uncured state, and provides critical information about the capacity to preserve geometric stability immediately following extrusion [35]. The green strength characterization was conducted using a modified compression test methodology to ensure reproducibility and accurate assessment of material buildability. Sample mixtures were prepared as described in Section 2.1 of the Materials, and extruded for the test as indicated in Section 2.2. All samples were tested within a 10 min window to minimize potential material degradation or structural changes. The samples were extruded to a consistent initial height and maintained at a controlled ambient temperature (22 ± 2 °C) and relative humidity (50 ± 5%).

The experimental setup was prepared in the laboratory, where the extruded samples were tested for buildability. The setup included a cubical polystyrene base, with 4 metal rods firmly inserted in it acting as guides for a flat thin metal plate (thickness 2 mm) where the load is finally applied (Figure 5). As a first step, the sample mixtures were extruded using the grout gun on the flat level surface of the cubical polystyrene base. Then, a thin metal plate was placed over the extruded sample material to prevent direct contact and minimize surface deformation. After that, the weight (1 kg) was placed gently over the sheet and maintained in position for a duration of 10 s. Immediately after the last step, the sample height was measured using a calibrated ruler. Height measurements were taken twice, before and after placing weight over it, for comparison. Buildability was quantified as the percentage height reduction compared to the initial sample height as given by:

$$\mathrm{H}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{r}\mathrm{e}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{\%}={\displaystyle \frac{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}-\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l} \mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}}\times 100\mathrm{\%}$$

(2)

This buildability characterization method provides a quantitative approach to evaluating the early-stage mechanical performance of 3D printable materials, offering critical insights into their potential printability and structural stability.

3. Results and Discussions

3.1. Flowability

In the context of 3D printing with an earth-based material mix, flowability is a crucial parameter that determines the ability of a material to flow during an extrusion process. For this study, a final four samples were tested to measure flowability to describe the consistency of these different sample mixtures. Initially, the amounts mentioned in Table 2 were used to measure flowability. Figure 6 shows the process of the flowability test.

The results (

Table 3. Water percentage and flow percentage before and after.

Sample	Water %	Flow %	Water +5%	Flow %
SP05	28	31.40	33	39.00
SP10	31	22.80	36	34.30
SP15	35	14.30	40	28.60
SP20	39	11.40	44	22.80

) of the flowability test demonstrated a progressive reduction in flow percentage as the wheat straw fiber content increased from 5% to 20%. The SP05 sample exhibited the highest flow percentage of 31.42%, while the SP20 sample showed the lowest flow percentage of 11.42%. The gradual increase in wheat straw fiber content significantly impacts the rheological properties of the earth-based mixture. As the fiber content increases, reduced flowability appears due to the increased fiber surface area leading to higher internal friction and the formation of fiber networks that limit material movement. Although the water content increases from 28% to 39% across the samples, the flow percentage decreases. Therefore, we repeated the entire process with a 5% increase in water (see Table 3) for each sample to examine the behavior as the water levels increase. The mixtures with a high amount of water and straw showed negative results, as shown in Figure 7. The fibers blocked up against each other and led to clogging inside both tools, and with increased water content, the yield stress became too low to form a stable shape.

The laboratory experiment results show a substantial link between the flowability, straw fiber, and water content for sample mixtures. As displayed in Figure 7, the flow percentage decreases with an increasing sample spread diameter. This inverse relationship indicates that a higher water content enhances material spread while simultaneously reducing flow resistance. At SP05, the flow percentage was 31.40%, decreasing to 11.40% at SP20 under initial conditions. As the water content was increased by 5%, the flowability improved, with the flow percentage (see Figure 8) rising to 39% at SP05 and 22.80% at SP20. This shows that moderate increases in water content improve material mobility, which is consistent with findings from fluidized granular materials and suspensions [36], where hydration levels have a direct impact on internal friction and flow characteristics.

In addition to water content, the presence of straw fibers has a considerable effect on flow dynamics. As the straw content increases, the flowability decreases due to the fibrous structure’s strengthening impact. Higher straw percentages increase physical interlocking and resistance to deformation, limiting the material spread ability. Findings show that at SP05–SP10, the influence of straw content is less noticeable because the material is sufficiently hydrated for flow to occur. However, with SP15–SP20, the effect of the increased straw percentage is more evident, resulting in a significant decrease in flowability. This behavior is due to the increased fiber–matrix contact, in which straw particles form a denser network that prevents mobility and fluid-like spreading.

The study demonstrates that both the water and straw content are important factors in determining material flowability. While water improves spread ability, higher straw content causes flow limits due to fiber interactions. Achieving optimal processing and performance requires a careful balance between hydration and reinforcement. In terms of application, the observed flow behavior has direct implications for construction materials, bio-composites, and slurry-based processes.

The data indicate that adjusting the water content is an effective method for improving the workability of straw-reinforced mixes, ensuring process efficiency while maintaining structural integrity.

3.2. Extrudability and Pumpability

The extrudability test for the earth-based materials was evaluated using the two tools described above. Samples were first extruded through a 1.5 cm triangular nozzle, corresponding to the printer’s inner nozzle diameter, and subsequently using a grout gun. The diameter size was chosen to represent the inner diameter of the nozzle in the printer configuration. After some practicing sessions, it was easy to produce layers (Figure 9) with consistent thickness, constant printing speeds, and in a straight line.

The extrusion tests using a grout gun and triangular bag demonstrated varying flowability and structural integrity across different sample compositions. As shown in Figure 10, the extruded samples display different morphological differences depending on the proportion of clay, straw fiber, and water content. SP05 resulted in a well-compacted extrusion with low distortion. The high clay percentage (94%) and low water content (28%) produced a solid, stable material that retained its shape after extrusion. However, minor cracking was found, most likely due to lesser fiber content and reduced flexibility. SP10 had slightly better fluidity than SP05, mainly due to the increased fiber and water content (10% and 31%, respectively). The material demonstrated gentle plasticity while maintaining adequate shape stability, indicating an improvement in extrudability.

SP15 showed smooth extrusion, although the presence of more fibers contributed to visible roughness and increased porosity, while the higher water content improved workability. SP20, with the highest fiber (20%) and water (39%) content, maintained geometric precision in the extrusion test but exhibited noteworthy cracks (Figure 11). The reduced clay content (76%) compromised structural firmness, and the surface texture appeared to be rough and less smooth.

Observations showed that the addition of wheat straw fiber had a direct impact on the rheology and extrudability of the earth-based combination. Increasing the fiber percentage improved internal reinforcement and reduced shrinkage, while increasing beyond 15% fiber content interrupted the smooth extrusion flow.

3.3. Buildability

During 3D printing, buildability is a crucial parameter that determines whether a material can maintain its intended geometry and structural integrity during and after the printing process. A lower height reduction percentage indicates better buildability, meaning the material can retain its shape under self-weight and external influences. Conversely, a higher height reduction percentage suggests a greater tendency for deformation or collapse. An initial attempt was made to check the layering of the mix with a triangular bag (Figure 10 and Figure 11). For the buildability assessment, the test method described in the experimental methodology section was followed, and the further height reduction percentage for each sample was calculated by using Equation (2), where the initial height before testing was 7 cm (Figure 12). The final heights measured after the test varied among the samples, leading to different reduction percentages. As small variations also appeared within each sample, depending on the position, the given subsequent values were determined as the mean of three readings (at the center and at the left and right sides of the sample). The results in

Table 4. Height reduction characterization.

Sample	Height Before Test (cm)	Height After Test (cm)—3 Trials	Mean After (cm)	Standard Deviation (SD) (cm)	Height Reduction (%)	Height Reduction ± SD (%)
SP05	7.0	5.3, 5.2, 5.4	5.3	0.10	24.29	24.29 ± 1.43
SP10	7.0	5.3, 5.4, 5.5	5.4	0.10	22.86	22.86 ± 1.43
SP15	7.0	5.9, 5.8, 5.7	5.8	0.10	17.14	17.14 ± 1.43
SP20	7.0	5.5, 5.6, 5.7	5.6	0.10	20.00	20.00 ± 1.43

summarize the average height after testing, standard deviation (SD), and the corresponding height reduction percentages with associated uncertainties, illustrated in Figure 12. The results showed that among the tested material samples, SP15 exhibited the lowest height reduction (17.14% ± 1.43%), indicating the best buildability and structural stability. This suggests that SP15 maintains its geometry more effectively, with a 15% fiber content, combined with 3% additive and 35% water, and provides an optimal balance of cohesion, viscosity, and internal reinforcement to support the printed layers without excessive slumping. However, increasing the wheat straw fiber content beyond 15% (as SP20 with 20% fiber) resulted in a slight increase in height reduction (20.00% ± 1.43%). This could be due to excessive fiber incorporation leading to non-uniform material flow and weaker bonding between layers, reducing structural stability.

On the other hand, SP05 showed the maximum height reduction (24.29% ± 1.43%), indicating the poorest buildability among the samples. This could be attributed to the higher material flowability, insufficient yield stress, or inadequate structural support, leading to excessive deformation post-extrusion. Similarly, SP10 (22.86%) exhibited intermediate levels of height reduction, although SP10’s height reduction was slightly lower than SP05, advising that inadequate fiber reinforcement contributes to increased deformation due to weaker interparticle bonding in the earth-based medium. The relatively low standard deviation values across all samples (±0.10 cm in height, translating to ±1.43% in reduction) reflect a high level of repeatability in the testing process (Figure 13). This consistency reinforces the reliability of the data and supports the observed trends in the mechanical response.

These variations in height reduction are likely influenced by differences in straw content; as the percentage of straw increased, the mixture exhibited a trend toward reduced compressibility, probably due to the reinforcing effect of the fibrous material. SP05, containing 5% straw, showed the highest height reduction, indicating that the mix was more easily deformed under compressive load. In contrast, SP15, with 15% straw, exhibited the lowest reduction, suggesting a stiffer, more resilient structure. The increased straw content contributes to internal resistance during compression by forming a denser, interlocked medium. The height reduction comparison after and before the buildability test is shown in Figure 14. This enhanced structural integrity becomes especially significant in 3D printing applications, where dimensional stability and load-bearing capacity are critical.

Therefore, the results showed that the addition of wheat straw fiber had a direct impact on the rheology and extrudability of the earth-based combination. Increasing the fiber percentage improved internal reinforcement and reduced shrinkage, while beyond 15% fiber content interrupted the smooth extrusion flow. The SP15 mixture (with 35% water) provided an optimal balance, ensuring both smooth extrusion and buildability. The results indicate that SP15 (82% clay, 15% wheat straw fiber, 3% additive, and 35% water) exhibited the best buildability, making it an optimal formulation for sustainable 3D printing applications.

Taken together, the buildability and extrudability tests (triangular bag and grout gun methods) provide an effective approach for establishing the mixture designs prior to the preparation of large quantities of building material. These tests are particularly valuable for defining composition ratios for printable mixtures and for conducting comparative investigations on mixture design, such as assessing the effects of additive formulation.

The results obtained in this study are consistent with previous research on earth-based and fiber-reinforced printable materials, which reported that fresh-state properties strongly control the printability of extrusion-based mixtures. Similar to the findings of Rocha et al. and Faleschini et al. [4,9], the present study confirms that water content, clay fraction, and fiber reinforcement directly influence flowability, extrudability, and buildability. The reduction in flowability observed with the increasing wheat straw fiber content can be attributed to higher internal friction and fiber–matrix interactions, as reported in studies on natural fiber-reinforced earthen composites [37]. However, moderate fiber incorporation improved shape retention and reduced deformation under loading, confirming the beneficial role of fibers in improving green stability. In this study, SP15 provided the most balanced performance, showing sufficient extrudability together with the lowest height reduction during buildability testing. This result agrees with previous observations that an optimum fiber content exists, beyond which excessive fiber addition may lead to clogging, rougher extrusion surfaces, and reduced mixture homogeneity. Therefore, the experimental findings support the conclusion that wheat straw fibers can improve the early-stage stability of printable earth-based mixtures when incorporated within an optimized range.

Beyond the experimental evaluation of individual mixtures, this study contributes to the emerging body of knowledge on earth-based additive manufacturing by providing a systematic framework for assessing the printability of fiber-reinforced earthen materials. The results demonstrate that printability is governed by a balance between material mobility and structural stability, both of which are strongly influenced by the interaction between fiber content and water demand. Increasing the wheat straw fiber content improved dimensional stability and buildability but simultaneously reduced flowability and extrusion quality due to increased internal friction within the mixture. These findings support the theoretical principle that an optimum workability window exists for earth-based printable materials, where sufficient flowability must be maintained without compromising shape retention and green strength.

The originality of this work lies in the combined evaluation of flowability, pumpability, extrudability, buildability, and compressive performance using simple and field-applicable testing procedures rather than specialized rheological equipment. The identification of SP15 as an optimized composition further provides practical guidance for the development of low-carbon, fiber-reinforced earth-based materials for additive manufacturing. From a construction science perspective, the study contributes to understanding the material-design relationships governing the printability of sustainable earthen composites and supports the broader adoption of locally sourced materials in digital construction technologies.

3.4. Compressive Strength of SP15

The compressive strength test of SP15 blocks (82% natural clay, 15% straw, and 3% starch additive) was carried out (Figure 15), and the results in

Table 5. Compressive strength results for SP15.

Sample No.	Peak Load (kN)	Peak Load (lbf)	Compressive Strength (kPa)	Compressive Strength (psi)
SP15-1	4.75	1067	792	115
SP15-2	3.82	858	637	92
SP15-3	4.15	933	692	100
Mean value			707	103

show that the average compressive strength of the SP15 samples was 707 kPa (103 psi). These results are within the range reported in the literature for adobe and natural clay and straw blocks (500–1000 kPa) [38], confirming that the selected mixture provides mechanical properties consistent with traditional earthen construction materials. The inclusion of 15% straw fibers was fundamental in minimizing shrinkage cracks during curing; however, straw typically reduces compressive strength due to its lower stiffness compared to clay. The presence of 3% starch likely compensated for this reduction by enhancing particle bonding, contributing to the observed strength values. These results align with the established standards (IS 1725, ASTM E2392, and the Adobe Code) [39] and literature values for adobe-like materials. This confirms the structural viability of the bio-based block.

Optical microscopy was employed to assess the binding effect and examine the interaction between the clay matrix and wheat straw fibers. As shown in Figure 16, the microstructural observations revealed a relatively homogeneous distribution of fibers within the clay matrix, with no evidence of significant fiber agglomeration. The images demonstrated good adhesion between the clay matrix and the straw fibers, suggesting effective interfacial bonding and mechanical interlocking between the two phases. Furthermore, the compact arrangement of the matrix and the absence of large voids indicate adequate cohesion within the composite structure. The observed fiber–matrix interaction may contribute to the improved dimensional stability and shape retention identified during the buildability tests. Although optical microscopy provides useful qualitative information regarding the internal structure of the composite, higher-resolution characterization techniques such as scanning electron microscopy (SEM) could provide further insight into the fiber–matrix interface and are recommended for future research.

4. Conclusions

This study investigated the printability characteristics of earth-based mixtures reinforced with wheat straw fibers for extrusion-based 3D printing applications. Through a series of flowability, pumpability, extrudability, buildability, and compressive strength assessments, the influence of fiber content on the fresh-state and early-age performance was evaluated. The main findings are summarized as follows:

• A systematic and reproducible framework was developed for evaluating the printability of earth-based materials using practical laboratory-scale characterization methods. The proposed methodology enables assessment of flowability, pumpability, extrudability, and buildability without requiring specialized rheological equipment.
• The results confirmed that printability is highly dependent on material composition. In particular, flowability, pumpability, extrudability, and buildability were strongly influenced by the interaction between the clay content, wheat straw fiber content, additive dosage, and water content.
• Increasing the wheat straw fiber content reduced flowability due to increased internal friction and fiber–matrix interactions. Although additional water partially compensated for this effect, excessive fiber and water contents resulted in reduced extrusion quality and nozzle clogging.
• Buildability performance improved with moderate fiber incorporation, demonstrating that an appropriate water–fiber ratio is essential to maintain dimensional stability while preserving adequate flowability during extrusion.
• A field-oriented mix-design methodology was introduced, enabling the development of printable earth-based formulations using simple and accessible testing procedures.
• Among the investigated mixtures, SP15 (82% clay, 15% wheat straw fiber, 3% additive, and 35% water) exhibited the most favorable balance between workability and mechanical performance. This mixture achieved the lowest buildability height reduction (17.14%), indicating superior shape retention and structural stability immediately after extrusion.
• While fiber reinforcement enhanced dimensional stability and reduced deformation, excessive fiber content negatively affected flowability and layer continuity, highlighting the need for an optimized fiber dosage in printable earth-based mixtures.
• The proposed methodology contributes to the standardization and validation of printable earth-based materials and supports their application as sustainable, low-carbon alternatives for large-scale additive manufacturing in construction.

Future research should investigate the influence of fiber length, fiber treatment techniques, and alternative natural reinforcements on printability and mechanical performance. Additional studies should also include microstructural characterization, long-term durability assessment, shrinkage behavior, and full-scale 3D printing validation to further support the practical implementation of earth-based materials in sustainable construction.