Evaluating the Influence of Alfa Fiber Morphology on the Thermo-Mechanical Performance of Plaster-Based Composites and Exploring the Cost–Environmental Effects of Fiber Content

Abstract

The construction industry’s escalating energy demands and greenhouse gas emissions underscore the need for sustainable, high-performance building materials. This study investigates the incorporation of locally sourced alfa fibers (AFs) into plaster-based composites to enhance thermal insulation, reduce environmental impact, and lower production costs. Three distinct AF morphologies—small (<5 mm), medium (10 ± 5 mm), and large (20 ± 5 mm)—were incorporated at fixed mass ratios, and their effects on key material properties were systematically evaluated. The results indicate that integrating AFs into plaster reduces composite density by up to 16.5%, improves thermal characteristics—lowering thermal conductivity and diffusivity by up to 52%—and diminishes both CO₂ emissions and production costs. The addition of fibers also enhances flexural strength (up to 40%) through a fiber bridging mechanism that mitigates crack propagation, although a general decline in compressive strength was observed. Notably, composites containing medium and large fibers achieved significantly lower densities (~1050 kg/m³) and superior thermal insulation (~0.25 W/mK) compared with those with small fibers, with the largest fibers delivering the greatest thermal performance at the expense of compressive strength. Overall, these findings highlight the potential of AF–plaster composites as environmentally responsible, high-performance building materials, while emphasizing the need to carefully balance mechanical trade-offs for structural applications.

1. Introduction

Globally, buildings account for a significant share of total energy consumption. Morocco, renowned for its rich architectural heritage, faces similar challenges as its building sector increasingly demands energy, thereby elevating concerns over environmental impact and rising operational costs [1,2,3]. Traditional construction materials, while widely available and adaptable, often lack the thermal insulation required to meet modern energy-efficiency standards [4,5]. This limitation is particularly critical in regions like Morocco, where pronounced temperature fluctuations demand materials capable of stabilizing indoor climates without excessive energy use.

In response to these challenges, the demand for sustainable and eco-friendly materials has spurred considerable interest in natural fiber-reinforced composites [6,7,8,9]. These composites offer a promising alternative to synthetic reinforcements by combining environmental benefits with competitive mechanical and thermal properties. Among natural fibers, alfa grass (Stipa tenacissima)—also known as esparto grass (or Halfa in certain Arabic regions) [10]—has gained attention due to its abundance, low cost, and favorable mechanical characteristics. Alfa fibers are particularly attractive for applications in construction [11], insulation [12,13,14], and lightweight composites [15], where thermal conductivity, mechanical strength, and environmental impact are critical considerations.

Alfa grass is a perennial plant native to the western Mediterranean region and is particularly abundant in North Africa. For instance, Algeria, Tunisia, Libya, and Morocco are major producers—with Algeria covering approximately 4 million hectares, Tunisia 1.5 million hectares, Libya around 350 thousand hectares, and Morocco roughly 3 million hectares—wherein Morocco and Algeria together account for over 80% of global alfa grass resources [16,17]. Belonging to the Poaceae family, alfa grass is characterized by long, tough, fibrous leaves that can reach up to 1 m in length [18]. Thriving in arid and semi-arid environments [19], the plant is well adapted to the climatic conditions of Morocco. Chemically, alfa fibers consist primarily of cellulose (40–50%), hemicellulose (20–30%), and lignin (15–20%) [20,21]. This composition endows the fibers with high tensile strength, low density, and excellent thermal insulation properties, while also rendering them biodegradable, renewable, and environmentally friendly—qualities that align well with the growing demand for sustainable construction materials.

In this context, numerous researchers have explored bio-composites based on cement and gypsum matrices incorporating bio-sourced aggregates [4]. Studies by Althoey et al. [22], Çomak et al. [23], Ahmad et al. [24], Suárez et al. [25], and Sawsen et al. [26] have examined the incorporation of various vegetal fibers—including date palm, hemp, coconut, fique, and flax—into plaster and cement matrices. Their findings demonstrated significant improvements in flexural strength and thermal performance with increasing fiber concentration. These enhancements are attributed to the fibers’ high aspect ratio, low density, and natural porosity, which reinforce the matrix by bridging microcracks and improving stress distribution while also reducing thermal conductivity.

Even though these studies have provided valuable insights into the mechanical and thermal properties of natural fiber-reinforced composites, they have largely overlooked the impact of fiber morphology—such as length, cutting, and grinding—on material performance. For instance, most studies have focused on fiber content rather than fiber size, despite evidence that fiber morphology plays a critical role in determining mechanical strength, thermal insulation, and workability [27,28]. Furthermore, the environmental and economic implications of using natural fibers in composites have not been thoroughly explored.

Alfa fibers, in particular, have been the subject of several studies due to their unique properties and potential applications. Early research by Garrouria et al. [29] highlighted the high tensile strength and low density of alfa fibers, making them suitable for lightweight composites. More recently, Sakami et al. [30] reported that incorporating alfa fibers into cement-based materials significantly reduces thermal conductivity. In terms of mechanical performance, T. Messas [31] found that while alfa fibers can enhance both compressive and flexural strength, excessive fiber content may lead to a reduction in compressive strength. Similarly, Ajouguim et al. [27,28] emphasized the role of fiber length, noting that shorter, ground fibers improve workability whereas longer fibers tend to enhance mechanical properties.

Despite the growing interest in alfa fibers, few studies have systematically investigated the influence of fiber morphology—such as length, cutting, and grinding—on both the thermal and mechanical properties of composites. Previous research, notably by Ajouguim et al. [27,28,32], has predominantly focused on fiber-reinforced cement mortars, examining parameters such as workability, hydration kinetics, water-accessible porosity, and compressive and flexural strengths. In these studies, various cutting lengths (10, 20, and 30 mm) and ground fibers (shorter than 2 mm) were evaluated with fiber contents ranging from 1% to 8%, but the primary emphasis was on mechanical performance rather than thermal behavior.

Consequently, the influence of fiber morphology on thermal performance—critical for insulation—remains underexplored. Furthermore, the environmental and economic implications of incorporating alfa fibers, including their carbon sequestration potential and cost benefits, have not been fully assessed. Notably, previous research has largely focused on mortar systems, whereas our study targets plaster-based composites. Given the distinct composition and behavior of plaster compared with mortar, evaluating the impact of fiber morphology on plaster is essential for developing sustainable building materials tailored for interior applications. These gaps may limit the large-scale industrial adoption of alfa fiber composites.

This study aims to bridge these gaps by evaluating the impact of alfa fiber morphology on thermal conductivity and compressive and flexural strength in plaster-based composites. Three fiber size ranges were investigated—small (<5 mm), medium (10 ± 5 mm), and large (20 ± 5 mm)—which are closely related to those reported in previous research, to determine their effects on material performance. Thermophysical properties are assessed using advanced techniques such as the Hot Disk method, while mechanical characteristics are evaluated through uniaxial compression and flexural tests. Additionally, the environmental and economic benefits of the composites are estimated, including their potential to reduce production cost and carbon emissions. By identifying the optimal fiber sizes and understanding their corresponding effects on plaster performance, this study seeks to support the development of energy-efficient, environmentally responsible construction materials, stimulate local green economic development, and potentially create new employment opportunities in the sector.

2. Materials and Methods

2.1. Raw Materials and Preparation Process

2.1.1. Local Sourcing and Sustainability Considerations

All raw materials were sourced from the eastern Moroccan region to promote local green construction practices and reduce environmental impact. Utilizing regionally available materials supports the local economy, minimizes transportation-related greenhouse gas emissions, and fosters the development of sustainable bio-composites.

2.1.2. Plaster

The primary binder in this study is a commercially available high-grade hemihydrate gypsum plaster (GYPSE RYAD) procured from a factory in Zaio, Nador, in northeastern Morocco. This plaster, composed of both α- and β-hemihydrates, has been characterized as having superior quality [33]. Its microstructure, presented in Figure 1, displays the typical morphology associated with high-grade hemihydrate plaster. Moreover, its compliance with EN 13279-2 [34] guarantees a consistent quality and performance, thereby supporting reproducibility despite inherent local variations.

2.1.3. Alfa Fibers (AF)

Alfa grass (Stipa tenacissima L.) is abundant in the eastern Moroccan region, covering extensive areas around the city of Jerada. Its fibers are naturally rich in cellulose—approximately 40% by weight [20]—a feature known to enhance thermal properties in bio-composite materials. Due to their high cellulose content, low cost, and ready availability, alfa fibers represent a promising bio-aggregate for improving the thermal insulation properties of plaster-based composites.

2.1.4. Preliminary Characterization Tests

Prior to composite fabrication, the raw materials underwent X-ray diffraction (XRD) analysis using a SHIMADZU (Columbia, MD, USA) XRD-6000 diffractometer. The scans were conducted at a rate of 2°/min over a 2θ range of 10–80°. For the plaster, the XRD pattern (Figure 2a) revealed the presence of calcium sulfate dihydrate and calcite carbonate, along with quartz—likely introduced during manufacturing to enhance mechanical strength [35]. In contrast, the XRD pattern for the alfa fibers (Figure 2b) indicated a predominantly amorphous phase with characteristic cellulosic peaks around 17°, 23°, 35°, and 44° 2θ, corresponding to the (110), (200), (004), and (400) planes commonly observed in cellulosic materials [36,37,38]. Given that cellulose positively influences thermal performance [39], these findings underscore the potential of AF as an effective thermal modifier in plaster-based composites.

2.1.5. Sample Preparation

Alfa fibers were manually cut into three size categories—large (20 ± 5 mm), medium (10 ± 5 mm), and small (<5 mm). To ensure homogeneous fiber dispersion and minimize agglomeration, the fibers were first dry-mixed with the plaster for 2 min, as recommended in the literature [15,33]. This initial step allowed for a uniform distribution of fibers in the dry state. Next, water was added gradually while continuing to mix for an additional 5 min. This progressive wet mixing process was carefully performed to prevent the alfa fibers from absorbing excessive water and to achieve a consistent, workable plaster–AF (PAF) paste. After thorough mixing, the paste was cast into 50 × 50 × 50 mm³ cubes and 40 × 40 × 160 mm³ rectangular prism specimens for mechanical testing. Additionally, cylindrical samples (20 mm in height and 60 mm in diameter) were prepared for thermal evaluations. The composition of the mixtures is summarized in

Table 1. Composition of the PAF specimens.

Sample Designation	AF		Plaster (wt%)		Water-to-Binder Ratio
	wt%	kg/m3	wt%	kg/m3
P	0	0	100	1150	0.5
PAFS2	2	5.45	98	1085.96	0.5
PAFS4	4	10.38	96	1028.13	0.5
PAFS6	6	14.85	94	975.65	0.5
PAFM2	2	545	98	1085.96	0.5
PAFM4	4	10.38	96	1028.13	0.5
PAFM6	6	14.85	94	975.65	0.5
PAFL2	2	5.45	98	1085.96	0.5
PAFL4	4	10.38	96	1028.13	0.5
PAFL6	6	14.85	94	975.65	0.5

Note: The symbols S, M, and L refer to the fiber length (small, medium, or large).

, and the preparation methodology is illustrated in Figure 3.

2.2. Characterization Techniques

2.2.1. Thermophysical Properties

Thermal conductivity and diffusivity were measured using the Hot Disk method, as per ISO 22007-2 [40], recognized for its rapid and precise evaluation of material thermal properties. In this test, a Hot Disk sensor (model 5501, with a radius of 6.4 mm, Gothenburg, Sweden) was placed between two identical composite specimens. A controlled power input—ranging from 80 mW to 50 mW—was applied for 80 s, during which the sensor simultaneously heated the specimens and recorded the resulting temperature rise. The thermal conductivity and diffusivity values were subsequently calculated using the Hot Disk Thermal Constants Analyzer software (v7.3.5), ensuring reliable and reproducible measurements.

The density of each composite was determined by measuring the mass-to-volume ratio of the prepared specimens. With the known values of thermal conductivity (λ), thermal diffusivity (α), and density (ρ), the specific heat capacity ($c_p$) was computed using the relationship:

$$c_p=\lambda /\alpha \rho$$

(1)

2.2.2. Mechanical Properties

Although PAF composites are primarily intended for non-structural applications, evaluating their mechanical performance is critical to achieving a suitable thermo-mechanical balance. Both compressive and flexural strengths were examined:

Compressive Strength:

Cubic specimens (50 × 50 × 50 mm³) were subjected to uniaxial compression using a testing machine operating at a constant loading rate. The maximum load ($F_c$) recorded at specimen failure was used to calculate the compressive strength (${\sigma }_c$):

$${\sigma }_c=F_c/A$$

(2)

where $A$ is the initial cross-sectional area of the specimen. This test was conducted in accordance with ASTM C109 [41] to ensure comparability and reliability of results.

Flexural Strength:

Rectangular prism specimens (40 × 40 × 160 mm³) were tested, conforming to ASTM C348 [42], in three-point bending using the same uniaxial testing apparatus equipped with appropriate supports and loading noses. A uniform loading rate was applied until failure. The maximum load ($F_f$) sustained by the beam prior to fracture was used to determine the flexural strength (${\sigma }_f$):

$${\sigma }_f=3F_{f}L/2b{d}^2$$

(3)

Here, $L$ is the span length between the supports, and $b$ and $d$ are the width and depth of the specimen, respectively.

These standardized testing procedures, as illustrated in Figure 4, provide a comprehensive evaluation of the mechanical integrity of the composites. They enable the identification of optimal fiber sizes and composite formulations that balance enhanced thermal properties with acceptable mechanical performance.

3. Results and Discussion

3.1. Density

Figure 5 illustrates the density (ρ) of the composites as a function of AF content. As expected, increasing the fiber content leads to a reduction in density because the density of alfa fibers is considerably lower than that of the plaster matrix. Moreover, fiber morphology significantly influences this property. Composites containing medium $({\rho }_M)$ and large $\left({\rho }_L\right)$ fibers exhibit a greater reduction in density compared with those with small fibers $\left({\rho }_S\right)$. For instance, at a 6 wt% AF content, ${\rho }_L$ decreased by 16.5%, from 1246 kg/m³ in the reference sample to 1043 kg/m³, while ${\rho }_M$ and ${\rho }_S$ achieved 1061 and 1147 kg/m³, respectively.

This trend aligns with previous studies on fiber-reinforced composites. For instance, Ajouguim et al. [28] demonstrated that longer fibers create more porosity, leading to a more pronounced density reduction. Another study by El Hammouti et al. [43] examined the integration of wheat straws in plaster matrix. The findings mirror ours, as the density reduction presents a linear trend achieving a 14% decrease for a concentration of 4% of straws.

3.2. Thermal Conductivity and Diffusivity

The thermal conductivity (λ) of the composites is given in Figure 6, where ${\lambda }_S$, ${\lambda }_M$, and ${\lambda }_L$ correspond to PAF_S, PAF_M, and PAF_L, respectively. The graph reveals that fiber length significantly impacts the thermal conductivity. The thermal conductivity of composites reinforced with small fibers decreased linearly with each AF increment up to 6 wt%, going from 0.48 to 0.36 W/mK. In contrast, both ${\lambda }_M$, and ${\lambda }_L$ exhibit a steep linear decline up to 4 wt% AF content, achieving 0.27 W/mK. However, between 4 and 6 wt%, the decrease in ${\lambda }_M$ levels off (changing by less than 0.01 W/mK), whereas ${\lambda }_L$ continues to decline—albeit at a reduced rate—down to 0.22 W/mK. Overall, at 6 wt% AF, large fibers exhibit the greatest impact on insulating capacity, reducing thermal conductivity by 52.8%, followed by medium (45.5%) and small fibers (24.2%).

Figure 7 shows the thermal diffusivity (α) of the composites, labeled α_S, α_M, and α_L for PAF_S, PAF_M, and PAF_L, respectively. All three mixtures start at similar values around 0.46 mm²/s with no added fibers (0 wt%). As the alfa fiber (AF) content increases, α decreases linearly for all composites, but the magnitude of this reduction varies with fiber length.

The rate of reduction of α_S is relatively modest, from about 0.46 to 0.39 mm²/s at 6 wt% AF—a drop of roughly 13%. For medium fiber incorporation the slope is steeper than for small fibers, with α_M dropping from around 0.46 to near 0.32 mm²/s. This translates to a more pronounced reduction of about 29% by 6 wt% AF. The large fibers exhibit the most significant decrease in thermal diffusivity, from about 0.46 to 0.23 mm²/s, indicating a reduction of 50%.

This pattern demonstrates that larger fiber dimensions more effectively hinder heat propagation. This can be explained by the fact that larger fibers, due to their greater size, introduce more substantial voids within the composite matrix. These voids are filled with air—a poor conductor of heat—which significantly lowers both thermal conductivity and diffusivity. In contrast, small fibers pack more uniformly, creating fewer and smaller voids, which results in a comparatively moderate reduction in thermal properties.

In support of our findings, Charai et al. [44] reported reductions of approximately 23% in thermal conductivity and 24% in thermal diffusivity for composites with 8 wt% fine AF, which is consistent with the trends observed for our PAF_S composites.

The specific heat capacity $\left(c_p\right)$ was calculated using Equation (1), based on the measured λ, α, and ρ values. As summarized in

Table 2. Specific heat capacity of the reference and PAF composites at 6 wt% AF content.

Composites	P	PAFS6	PAFM6	PAFL6
Specific heat capacity (J/kgK)	833	792	750	916

, the best-performing composite—PAF_L6—exhibited the lowest diffusivity and density, which more than compensated for the decrease in conductivity, resulting in an approximate 10% increase in $c_p$. This increase in specific heat capacity implies enhanced heat storage and thermal lag, potentially contributing to improved thermal comfort. Other notable composites, such as PAF_S2 and PAF_L2, showed $c_p$ values of approximately 844 J/kgK.

3.3. Compression and Flexural Strengths

Although compressive strength is less critical for plaster’s non-structural applications, it must still meet industry standards. Figure 8 shows that the compressive strength $\left({\sigma }_c\right)$ of the plaster decreases with increasing AF content. This decline follows an exponential decay trend across all fiber morphologies, which is expected since vegetal fibers inherently possess lower strength than conventional construction materials. Notably, larger fibers lead to a more pronounced reduction in ${\sigma }_c$ due to the increased porosity they introduce. For example, while composites with 2 wt% AF and PAF_S4 maintain ${\sigma }_c$ values above the 2 MPa threshold recommended for plaster-based applications [45], composites with higher fiber contents—especially those with larger fibers—fall below this limit. Consequently, while some formulations remain suitable for typical plaster applications, others are more appropriate for non-load-bearing uses [46].

Similar trends have been reported in previous studies [6,47]. Horma et al. [48], for instance, investigated peanut shell (PS)-reinforced concrete and found that both increased biomass content and larger PS aggregates significantly reduced compressive strength, primarily due to the formation of large pores.

Flexural strength $\left({\sigma }_f\right)$ is particularly important for plaster-based materials used in roofing or other horizontal surfaces subject to bending forces. The incorporation of AFs is intended to improve ${\sigma }_f$ by promoting a fiber bridging effect that mitigates crack propagation. As shown in Figure 9, the relationship between AF content, fiber morphology, and flexural strength is nonlinear, suggesting the interplay of multiple mechanisms.

At 2 wt% AF, all fiber sizes exhibit their highest ${\sigma }_f$ values, indicating that this concentration optimally promotes fiber bridging. When the AF content increases to 4 wt%, both small and large fibers display a noticeable decline in ${\sigma }_f$; however, medium fibers maintain comparatively higher strength. This “sweet spot” for medium fibers may result from their balanced dimensions, which prevent the formation of dense fiber clusters while avoiding the creation of large voids in the plaster matrix. At 6 wt% AF, small and large fibers show a partial recovery in ${\sigma }_f$, likely because the increased fiber dosage forms a more robust reinforcement fiber network (Figure 10) that counteracts the negative effects of added porosity. Notably, ${\sigma }_f$ falls below 4 MPa in only one formulation (PAF_S4), whereas other composites either match or exceed the strength of the reference plaster (P). Among all mixes, PAF_L2 achieves the highest flexural strength at 6.45 MPa—representing a 40% improvement over the reference. Moreover, the thermally best-performing composites (PAF_M6 and PAF_L6) also show slight increases in ${\sigma }_f$, from 4.58 MPa to 4.71 MPa and 4.86 MPa, respectively.

The enhanced flexural strength is primarily attributed to the fiber bridging effect, wherein the fibers effectively span and arrest crack propagation, thereby improving the composite’s tensile response. However, this mechanism does not contribute to compressive strength; in fact, the increased porosity that facilitates fiber bridging under bending can create weak spots under compressive loading. This trade-off highlights the importance of optimizing fiber size to balance improved flexural performance against reductions in compressive strength. The fiber bridging effect has been extensively documented in studies involving Diss fibers and grass straws [49,50]. In addition, Ajouguim et al. [28] observed that while increased AF size and content generally reduce compressive strength, low percentages (around 1% by volume) can improve flexural strength before higher fiber dosages lead to a decline.

3.4. Practical Implications

The incorporation of alfa fibers (AF) into plaster-based composites leads to systematic improvements in thermal properties while influencing mechanical performance. Our study shows that at a 2 wt% AF incorporation, all composites meet the mechanical thresholds required for most plaster applications—specifically, a compressive strength above 2 MPa and a flexural strength above 1 MPa. Among these, PAF_L2 exhibits the best overall performance by combining high thermal efficiency, the greatest flexural strength, and the lowest density. Consequently, PAF_L2 is recommended for load-bearing plaster applications where a balanced thermo-mechanical performance is needed.

In contrast, at higher AF contents (4 wt% and 6 wt%), the significant reduction in compressive strength suggests that these composites are better suited for non-load-bearing applications such as interior partitions, wall finishes, or insulating layers. Among these formulations, PAF_L6 emerges as the most energy-efficient and lightweight material, achieving more than a 50% reduction in both thermal conductivity and diffusivity, along with a 10% increase in specific heat capacity—albeit with lower compressive strength.

Table 3. Measured properties of the recommended composites and the reference.

Composite	ρ (kg/m3)	λ (W/mK)	α (mm2/s)	${\mathit{\sigma }}_{\mathit{c}}$(MPa)	${\mathit{\sigma }}_{\mathit{f}}$(MPa)
P	1246 ± 24	0.476 ± 0.019	0.458 ± 0.028	7.22 ± 0.81	4.57 ± 0.49
PAFL2	1167 ± 21	0.390 ± 0.023	0.396 ± 0.020	2.69 ± 0.22	6.45 ± 0.62
PAFL6	1043 ± 15	0.225 ± 0.012	0.236 ± 0.017	1.06 ± 0.14	4.86 ± 0.45

displays the measured properties of the recommended composites for general (PAF_L2) and insulating (PAF_L6) applications, along with their respective standard deviations, compared with the reference sample (P).

While these findings provide clear guidance for immediate practical applications, further work is needed to enhance the compressive strength of higher AF content composites without sacrificing thermal performance. Future research could focus on processing optimizations—such as surface treatments to improve fiber–matrix bonding, pre-soaking fibers to reduce water absorption, and the use of appropriate admixtures—to minimize void formation and improve compaction [51,52]. Such modifications would aim to achieve a more favorable balance between mechanical strength and thermal insulation, thereby expanding the range of potential applications for these sustainable composites.

4. CO₂ Emissions and Cost Analysis

4.1. Carbon Emissions Analysis

The environmental impact of construction materials has become increasingly critical, driven by the need to reduce carbon emissions, conserve resources, and promote sustainable development. Traditional materials—such as synthetic fibers and conventional insulation—often have a high environmental footprint due to energy-intensive production and non-biodegradable waste. In contrast, natural fibers like alfa offer a promising alternative by combining low environmental impact with competitive performance.

The environmental advantages of alfa fibers stem from their biological origin. Alfa grass mitigates climate change through photosynthesis, absorbing CO₂ from the atmosphere according to the following reaction [53]:

6 CO₂ + 6 H₂O (+ light energy) → C₆H₁₂O₆ + 6 O₂

(4)

To evaluate the impact of alfa fibers on the overall carbon footprint of plaster-based composites, the total CO₂ emissions (xCO₂) for each formulation were calculated by considering the contributions from both plaster and alfa fibers [54]:

xCO₂ = CO_2-Plaster × M_Plaster + CO_2-Alfa × M_Alfa

(5)

Here, CO_2-Plaster and CO_2-Alfa are the emission factors (kg CO₂ per kg of material), and M_Plaster and M_Alfa denote the masses (kg/m³) of plaster and alfa fibers, respectively (see Table 4).

Given the local availability of alfa fibers, emissions related to their extraction, processing (CO_2-prod), and transportation (CO_2-trans) were considered negligible. Instead, the environmental benefit of alfa fibers is captured by their CO₂ sequestration during plant growth. Based on previous studies [55], the CO₂ sequestered can be estimated as:

CO_2-Alfa ≈−CO_2-Absorbed = − (%C/100) × (M_CO2/M_C);   CO_2-prod + CO_2-trans ≈ 0

(6)

where %C is the carbon content of the fibers (70% [15,56]), M_CO2 is the molar mass of CO₂ (44 g/mol), and M_C is the molar mass of carbon (12 g/mol).

Substituting Equation (3) into (2), we obtain:

xCO₂ = CO_2-Plaster × M_Plaster − (%C/100) × (M_CO2/M_C) × M_Alfa

(7)

Figure 11 displays the carbon emissions for various composite formulations. The addition of alfa fibers results in significant CO₂ emission reductions compared to pure plaster. Specifically, composites with 2%, 4%, and 6% alfa fibers show reductions of approximately 15%, 30%, and 40%, respectively.

Furthermore, similar sustainable material substitutions have shown promising results. For instance, Bahmani et al. [57] demonstrated that replacing silica sand with steel slag in ultra-high-performance concrete (UHPC) enhanced mechanical properties while reducing environmental impact. These findings support our approach of incorporating natural alfa fibers to achieve both improved performance and a lower carbon footprint.

Our findings also align with those of Oladikpo et al. [55] who investigated the carbon footprint of flax fiber-reinforced gypsum boards. The authors reported in this study that incorporating 3% flax fibers reduced the carbon footprint of gypsum boards by 29%. Moreover, the carbon footprint of gypsum boards with 10% flax fibers was reported to be nearly zero, highlighting the potential of natural fiber-reinforced composites to reduce the environmental impact of construction materials.

This reduction is mainly attributed to the low carbon intensity and minimal processing requirements of alfa fibers, along with their inherent carbon sequestration capability during growth. Consequently, higher fiber contents enhance the environmental benefits while simultaneously improving mechanical and thermal properties.

4.2. Cost Analysis

The economic viability of alfa fiber-reinforced plaster composites was evaluated using a simple cost model. The total material cost xCost is calculated as:

xCost = Cost_Plaster × M_Plaster + Cost_Alfa × M_Alfa

(8)

Here, Cost_Plaster and Cost_Alfa are the costs (in USD per kg) of plaster and alfa fibers, and M_Plaster and M_Alfa are their respective masses (kg/m³) in the composite (see Table 4).

Table 4. Unit costs and CO₂ emission factors.

Materials	CO2 Emission (kg·CO2/kg) [58]	Disposal Cost (USD/kg) [16,59]
Plaster	0.1402	0.128
Alfa fibers	−2.56	0.00403

Table 4. Unit costs and CO₂ emission factors.

Materials	CO2 Emission (kg·CO2/kg) [58]	Disposal Cost (USD/kg) [16,59]
Plaster	0.1402	0.128
Alfa fibers	−2.56	0.00403

Figure 12 illustrates the total costs for different composite formulations. Incorporating alfa fibers leads to significant cost reductions—up to 16% compared with traditional plaster. Specifically, composites with 2%, 4%, and 6% alfa fibers reduce costs by approximately 6%, 11%, and 16%, respectively. This trend is due to the low cost and minimal processing requirements of alfa fibers, which enhance both the economic and environmental viability of the composites.

It is important to note that fiber size also affects cost and emissions. For example, small fibers (<5 mm) require additional grinding, increasing both cost and environmental impact, whereas large fibers (20 ± 5 mm) are less costly to produce and generate lower emissions due to minimal processing.

Overall, the cost-effectiveness and environmental benefits of alfa fiber-reinforced composites make them an attractive alternative to traditional construction materials, supporting sustainable and economically viable building practices.

4.3. Limitation and Future Directions:

It is important to note that our current cost and CO₂ emissions analysis is based solely on the mass fractions of plaster and alfa fibers and does not explicitly account for the effects of fiber morphology. Although different fiber sizes lead to variations in composite density (with larger fibers creating more pores), our calculations do not capture these nuances on a per volume basis. Future work should focus on developing a volume-normalized model that incorporates fiber morphology parameters, providing a more detailed life-cycle assessment (LCA) and cost analysis of these composites.

5. Conclusions

This study demonstrates that incorporating locally sourced alfa fibers into plaster-based composites can effectively enhance their thermo-mechanical performance, reduce environmental impact, and lower production costs. The integration of alfa fibers resulted in a notable reduction in composite density, thermal conductivity, and thermal diffusivity, while also enhancing flexural strength through a fiber bridging mechanism. In particular, formulations containing large alfa fibers (PAF_L) exhibited the most significant improvements, with PAF_L6 achieving a 40% reduction in CO₂ emissions, a 16% decrease in cost, over 50% lower thermal conductivity and diffusivity, a 6% increase in flexural strength, a 16.5% reduction in density, and a 10% increase in specific heat capacity.

Based on these findings, PAF_L2 is recommended for most plaster applications due to its balanced performance, ensuring adequate mechanical integrity and improved thermal properties. Conversely, PAF_L6 is ideal for non-load-bearing applications, where its superior environmental and cost benefits, coupled with excellent thermal insulation, are particularly advantageous.

Overall, this research underscores the potential of alfa fiber-reinforced plaster composites as sustainable alternatives to conventional materials, supporting both environmental conservation and economic efficiency. Future work should focus on optimizing processing techniques to further improve the compressive strength of these composites and to assess their long-term durability under real-world conditions.