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23 October 2025

Bio-Enhanced Geopolymer Composites: Microstructural and Mechanical Insights from Sisal and Palm Fiber Reinforcement

,
and
1
LMPGI, High School of Technology, Hassan II University of Casablanca, B.P 8012 Oasis, Casablanca 20200, Morocco
2
CEDoc, National High School of Electricity and Mechanics (ENSEM), Hassan II University of Casablanca, B.P 8118 Oasis, Casablanca 20200, Morocco
3
LPMAT, Faculty of Sciences Ain Chock, Hassan II University of Casablanca, B.P 5366 Maârif, Casablanca 20100, Morocco
*
Author to whom correspondence should be addressed.
Constr. Mater.2025, 5(4), 77;https://doi.org/10.3390/constrmater5040077
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Abstract

This study investigates the fact that reinforcing geopolymers with natural fibers provides a practical way to improve their strength and durability. Offering environmental benefits compared to Portland cement, their mechanical performance still presents challenges. The particularity of this study lies in the pretreatment of natural fibers to limit their degradation within the alkaline geopolymer matrix. It also explores the effect of their length and content on matrix geopolymer. XRD (X-ray diffraction) analysis confirmed the crystalline structure of the geopolymer gels, unaffected by fiber inclusion. SEM (Scanning Electron Microscopy) observations showed a decrease or even disappearance of mineralization in treated sisal and palm fibers within the matrix, along with some partial detachment of the fibers. Optimal compressive strength was achieved using metakaolin and GGBS (Ground Granulated Blast-furnace slag). Incorporating 4% short palm fibers enhanced flexural strength, while long sisal fibers led to a 30% increase in flexural strength compared to short fibers, representing a 10.7% overall improvement. However, current geopolymer systems still face challenges such as low flexural strength and brittleness, which this study overcomes by incorporating processed natural fibers as sustainable reinforcements with optimal content.

1. Introduction

Recent research has explored the reinforcement of geopolymer matrices with various types of fibers to improve mechanical strength and durability [,]. However, the majority of these studies have concentrated on synthetic fibers such as steel, glass, or polypropylene, leaving a noticeable gap regarding the use of natural fibers. The increasing interest in sustainable building materials has led researchers to explore the use of natural fibers as reinforcement in geopolymer composites. Natural fibers stand out due to their biodegradability, renewability, and abundant availability at relatively low cost, especially in regions where agricultural residues are plentiful [,]. In particular, key issues such as fiber dispersion within the matrix, fiber–binder interfacial adhesion, and long-term performance in alkaline environments have not been fully addressed. In this context, the present work seeks to contribute new insights by examining the behavior of metakaolin-based geopolymer mortars reinforced with sisal and palm fibers. This approach not only promotes the use of renewable materials but also aims to enhance sustainability in construction through bio-based composite systems. Recent studies have focused on improving the mechanical performance and durability of geopolymer composites by incorporating natural fibers and optimizing matrix compositions. It showed that modifying fiber surfaces improves the fiber–matrix interface, leading to better crack resistance and load distribution []. These findings build upon and extend earlier work such as that of Alomayri (2013), reflecting the rapid progress in this field [].
The careful selection of construction materials is a critical factor in ensuring the success of a project and is instrumental in promoting environmental sustainability and the development of energy-efficient systems [,,]. Traditional cement-based binders such as Portland cement, lime, and blended hydraulic cements have been widely used in construction, whereas geopolymer binders typically synthesized from aluminosilicate sources like fly ash or metakaolin activated by alkaline solutions are gaining attention for their environmental benefits [,]. These innovative materials facilitate the activation of natural resources and industrial residues using various alkaline solutions, which may include blast furnace slag and metakaolin. The process of activating solid raw materials through these means results in the creation of two distinct categories of products: hydrated calcium silicate gel and hydrated aluminum-alkaline silicate gel, recognized as geopolymers []. Geopolymer has shown considerable potential as a sustainable alternative to Ordinary Portland Cement, with desirable technical characteristics for different types of applications [,,]. The mode of hardening has an impact on the geopolymer setting and hardening. The higher temperatures (60 °C) significantly accelerate the geopolymerization process, allowing compressive strengths to reach values between 30 and 50 MPa and flexural strengths between 4 and 7 MPa within 24 h []. Incorporating reinforcing fibers into the matrix might help to overcome these issues. Therefore, it is noted in the post-peak region of the stress–strain curve, the behavior of alkalized mortars can evolve towards a quasi-ductile behavior with an improved energy absorption capacity []. As in cement-based mortars, the reinforcement of these types of geopolymers consists of synthetic fibers (steel, plastic, glass fibers, etc.) []. But in recent years, and given the environmental cost of synthetic fibers, the quest for bio-sourced alternatives has interested researchers. Natural fibers (flax, sisal, hemp, cotton, jute, etc.) are progressively used as a potential substitute for synthetic fibers in geopolymer matrices []. The reasons are multiple: low price, worldwide availability, and good mechanical properties [,,]. Research has shown that geopolymers based on silica fume, bagasse ash, and basalt fiber showed better overall performance in several test scenarios []. Nevertheless, in an alkaline medium (matrix), the natural fibers are exposed to attacks that cause the dissolution of lignin and hemicellulose. On the other hand, the shifting of calcium hydroxide to the walls of the natural fibers causes the mineralization of the fibers while their hydrophilic character causes their swelling [,,]. For better durability, it is necessary to mitigate the degradation and deformation of natural fibers by protecting them with a specific coating []. Merta et al. [] studied the correlation between porosity and the mechanical properties of the material. The compressive strength can decrease by up to 30% depending on fiber content and carbonation effects. The results, reported by Merta et al., indicate that when the fiber dosage increases by 0.7% by weight, the compressive strength decreases by 24%. This was explained by the clumping of the fibers leading to poor dispersion in the mortar, and the hydrophobic nature of the fibers changed the water/binder ratio []. In a study conducted by Alomayri et al. in 2013 [], it was demonstrated that the strength decreased with the inclusion of cotton fibers in fly ash-based geopolymer beyond an optimal content (1.5 wt.%), reaching values as low as 3.2 MPa. A similar pattern in flexural strength was observed by Luo et al. in 2020 []. At fiber dosages equivalent to 2% of the fly ash mass, there was a notable 52% increase in flexural strength. Additionally, they also contributed the compressive strength to a higher reaction of Si–Al in the process of polymerization and stoving []. The dense and compact structure of the geopolymer samples is responsible for the increase in compressive strength [,].
The novelty of this work lies in the specific treatment applied to the fibers to enhance fiber-matrix bonding and thereby improve the composite’s mechanical performance and resistance to cracking. This approach contributes to the development of sustainable and mechanically robust construction materials. This article will detail, with experimental results, the effect of each of these elements on the characteristics of activated binders reinforced with natural fibers. The final mechanical characteristics of the mortar can be affected by factors such as fiber length, dosage, and their distribution within the matrix.

2. Materials and Methods

Métakaolin M1200S (MK) from IMERYS (Paris) France was used as a precursor for all geopolymer samples in this study and showed that it was composed of quartz, Illite (I), and microline (M), conforming to the results reported by Garcia-Lodeiro in 2015 [] (Table 1). It is an aluminosilicate with a semi-crystalline structure, in addition to the amorphous phase represented by a halo centered on 2θ = 20°–30° (Figure 1).
Table 1. Chemical composition of raw materials MK, GGBS, and desert sand by X-ray fluorescence (XRF).
Figure 1. X-ray diffraction of raw metakaolin (MK1200) and raw blast furnace slag (GGBS).
Blast furnace slag (GGBS) from ECOCEM (Dunkerque) France: The analysis by X-ray diffraction (XRD) showed the diffraction pattern had the amorphous hump at 15°–40° (Figure 1). The composition was 43.9% of CaO, 37.6% of Silicate, and 10.26% of Alumine (Table 1).
Desert sand from (Zagora) southern Morocco: This consists of 51.11% of SiO2, 17.76% of CaO, and 14.7% of Al2O3 by mass (Table 1). Sisal fiber and palm fiber were extracted using scrapers. The fibrous cells are linked to each other by the middle lamella fibers and were treated and cut; long fibers were 160 mm and short fibers were 10 mm (IMA 1). Several studies have stated that the best alkaline solution of sodium hydroxide (NaOH) solution is 8 Mol with deionized water [,].

2.1. Processing

2.1.1. Preparation of Fiber

The palm and sisal fibers (diameter ranged between 155 μm and 230 μm) were washed with water and oven-dried for 24 h to remove traces of water. Afterwards, it was immersed in paraffin oil for 72 h and wiped to remove excess paraffin oil before use. This oil-coating treatment is essential to decrease water absorption by the fibers and protect the natural fiber of the alkali-activated composite [].

2.1.2. Elaboration of the Mortar Geopolymer

The silica/alumina ratios SiO2/Al2O3, and alkali/alumina ratios Na2O/Al2O3 of each composition, metakaolin and sand (MA) and metakaolin and blast furnace slag (MB), are mentioned in the following (Table 2). They are important factors defining the density, open porosity, microstructure and strength of geopolymer mortars.
Table 2. The ratios SiO2/Al2O3 and Na2O/Al2O3 of each paste sample.
We mixed metakaolin with 2% by mass of activator. The next step was the resting period (30 s). Next, we added desert sand and fiber to the blender and kneaded the mixture for 90 s. We applied the same process of mixing metakaolin and blast furnace slag (GGBS) with palm fiber (Table 3). Due to its low density, the fibers floated above the other components during mixing, which is why the mixing speed was reduced for the fiber-reinforced samples, particularly for the samples with 8% by mass of fibers. It gave what is called a “bullet effect”, resulting in a low dispersion of fibers in the paste.
Table 3. Mixing design for geopolymer samples.
The different types of mortar pastes were placed in a standard prismatic mold of 40 × 40 × 160 mm3 according to the (NF EN 1097-6:2022) [], for 24 h before being removed from the mold. They were left to dry in the oven for 3 days at 50 °C.

2.2. Characterization Testing

2.2.1. Structural Analysis

A comprehensive set of analytical techniques was employed to assess the structural, morphological, and thermal properties of the geopolymer samples:
X-Ray Diffraction (XRD): Mineralogical compositions were identified using a Siemens D-501 diffractometer—Munich (Germany). Data were collected within a 2θ range of 20° to 80°, with a step size of 0.0167° and a count time of 18 s per step under ambient conditions.
Scanning Electron Microscopy (SEM): The microstructural features and fiber–matrix interfacial interactions were examined using a FEIFEG 450 SEM—Shanghai (China), providing detailed insights into morphology and fiber adhesion.
Thermogravimetric Analysis (TGA): Thermal degradation behavior and phase transitions were evaluated using a Shimadzu DTG-60 Simultaneous TG-DTA instrument analyzer (Tokyo, Japan). Samples were subjected to heating from 22 °C to 1000 °C at a constant rate of 5 °C/min.
Hydrostatic Method (NF EN 1097-6:2022): The apparent density (ρ, in kg/m3) and accessible porosity (n, in %) of the samples were determined through hydrostatic weighing.

2.2.2. Mechanical Properties

Flexural Strength Test: The evolution of flexural tensile strength was monitored over time using prismatic specimens (40 × 40 × 160 mm3) and prepared in accordance with NF EN 1097-6:2022. Specimens were subjected to flexural testing after a curing regime consisting of 3 days of oven treatment followed by 28 days of air exposure (Graph 1).
Constrmater 05 00077 gr001
Graph 1. Images of geopolymer based on metakaolin and blast furnace slag samples before (a) and after flexural strength without fiber (b) and with sisal fiber (c).
Compressive Strength Test: Compressive strength was assessed on the broken halves of each prismatic specimen post-flexural testing. The mean compressive strength value was calculated from the two resulting fragments for each sample.

3. Results

3.1. Structural Analysis (X-Ray Diffraction)

X-ray diffraction (XRD) patterns exhibit a broad hump centered at approximately 30° (2θ), indicating the predominantly amorphous nature of the reaction products formed after alkaline activation (Figure 2). This feature reflects the dissolution of the initial crystalline phases and the formation of amorphous aluminosilicate gels, typical of geopolymer systems. In the paste combining metakaolin and ground granulated blast furnace slag (MK + GGBS), a notable reduction in the intensity of the crystalline peak is observed compared to the raw materials, confirming the transformation towards a disordered structure. This microstructural change is associated with the formation of calcium-rich gels, which contribute to pore filling and matrix densification. This development has a direct impact on mechanical performance, particularly on flexural strength. This result contributes to generating amorphous products and as a result refining the pores of the matrix []. The presence of GGBS in the mixtures leads to an increase in the maximum intensity of C-S-H (hydrated calcium silicate). The formation of Ca(OH)2 and CaCO3 can enhance the mechanical strength and chemical durability of the geopolymer by contributing to matrix densification and reducing microcracking, as supported by previous studies [,,]. In Figure 3, MA1, MA2, and MA3 patterns show that there is no notable decrease in the intensity of the crystalline peaks, especially those associated with quartz and kaolinite, compared to the raw materials. This indicates that the partial dissolution of the original phases and the formation of amorphous reaction products after alkali activation were not successful, which may have had an impact on the mechanical strength of the geopolymer, as shown in the following results. On the other hand, the results of samples MB1, MB2 and MB3 show that the presence of an amorphous halo alone does not necessarily confirm the formation of a geopolymer gel, nor does it imply an improvement in mechanical performance. As highlighted in previous studies [,], a complete understanding of the gel phase and its contribution to material properties requires additional characterization methods, such as thermogravimetric analysis (TGA), which can help to better identify the nature and proportion of amorphous components.
Figure 2. X-ray diffraction of different samples of metakaolin and blast furnace slag.
Figure 3. X-ray Diffraction of different samples of geopolymers based on metakaolin and sand.

3.2. Morphological Test (SEM)

The SEM micrographs provided in Figure 4 offer a comprehensive view of the geopolymers, both in their unreinforced state and when reinforced with natural fibers. What becomes immediately evident is the distinct difference in morphology between the two types of samples, namely MB1 and MA1.
Figure 4. SEM of all different samples of geopolymers based on MK and sand (MA1; MA2; MA3) and samples of geopolymers based on MK and slag (MB1; MB2; MB3) with and without natural fibers.
The SEM micrographs provided in Figure 4 offer a comprehensive view of the geopolymer, both in its unreinforced state and when reinforced with natural fibers. What becomes immediately evident is the distinct difference in morphology between the two types of samples, namely MB1 and MA1. In the case of samples MA1 and MA2, they exhibit a granular structure consisting of fine particles. These fine particles are indicative of lamellar metakaolin particles, a characteristic feature associated with metakaolin-based geopolymers.
Samples (MB1, MB2, MB3) had a glassy appearance, which contributes to the microstructure enhancing compressive strength by promoting matrix densification, as reported by Provis and van Deventer (2007) []. This difference in microstructures will influence physical and mechanical properties such as compressive strength. On the other hand, the fiber content has greatly impacted the porosity of the geopolymers. Thus, the hydrophilicity of the fibers and their mineralization in an alkaline medium led, in addition to reinforcement/matrix detachments, to significant morphological deformations, which affected the porosity of the fiber geopolymers. Although the fibers were treated with paraffin to reduce water absorption, minor imperfections in the coating can still allow localized water absorption and partial mineralization, which influences the porosity and morphology of the fiber-reinforced geopolymers. The long sisal fibers appear well aligned and coherently integrated into the matrix, suggesting good fiber-matrix adhesion that may contribute to efficient load transfer. In contrast, image MB3, corresponding to a higher content (8 wt.%) of palm fibers, shows visible detachment from the matrix. This behavior suggests a possible saturation effect, where excessive fiber addition leads to poor dispersion and weak interfacial bonding, potentially compromising the mechanical integrity of the composite.
SEM observations were further investigated to highlight the fiber mineralization phenomena observed in samples incorporating natural fibers. In particular, the presence of calcium hydroxide (Ca(OH)2), released upon GGBS activation, is believed to play a central role in promoting mineral deposition on the fiber surface, enhancing chemical bonding at the fiber/matrix interface. This mineralization contributes to a denser interfacial area, thereby improving mechanical interlocking and reducing fiber pullout. Moreover, fiber detachment, especially at higher fiber dosages, has been discussed in correlation with increased porosity. It is suggested that excessive fiber content may disrupt matrix continuity, leading to the formation of interfacial voids and weakened bonding zones.

3.3. Thermal Analysis (TGA)

TGA curves revealed a slight mass loss at the beginning of the test, over a temperature range of approximately 40 to 200 °C, due to the dehydration of free water (Rahier, Skaf, Davidovits) [,]. It has also been noted that water evaporation causes shrinkage and does not induce adverse stresses in the materials. In Figure 5a, the slightly lower mass losses observed for MA2 and MA3 compared to MA1 can be attributed to two factors. First, poor fiber coating may allow some residual water absorption and localized mineralization in the matrix. Second, the increased development of the geopolymer gel in MA2 and MA3 likely leads to a denser microstructure that better retains water, thus reducing its release during thermal analysis. This interpretation is consistent with previous studies on fiber-reinforced geopolymers (Provis and Van Deventer, 2007) []. The temperature range between 800 °C and 900 °C shows the occurrence of a chemical reaction, carbonation, which occurs in the presence of Ca(OH)2. Nevertheless, in the case of samples MB1, MB2, and MB3 (Figure 5b), at elevated temperatures ranging from 300 °C to 450 °C, water is released from the Si-OH bonds. The mass loss remained relatively low at 500 °C, indicating their thermal stability due to the incorporation of blast furnace slag. A peak observed between 700 °C and 750 °C is attributed to the reactions of alkali with amorphous silica, accounting for 70%–75% of the mass loss. This phenomenon is related to dehydroxylation and represents the complete evaporation of the chemically bound water and the OH− hydroxyl group [].
Figure 5. (a) ATG thermogram curves of different types of geopolymer based on MK and sand with and without long natural fibers. (b) ATG thermogram curves of different types of geopolymer based on MK and slag with and without court natural fibers.

3.4. Physical Properties

Figure 6a,b shows the variation in bulk density and water-accessible porosity of different geopolymer specimens when adding different types and contents of treated fibers (sisal and palm). These results show that the apparent density of the raw matrix MB1 is greater than the matrix MA1. And clearly, we can see that it decreases with the incorporation of natural fibers regardless of the matrix. It can be explained by the low density of the palm fibers compared to the matrix. Increasing palm fiber content did not have an impact on density, but the porosity has increased considerably. In addition, the type of fiber impacted the porosity even if we worked with the same content. Indeed, the palm fiber showed an increase in the porosity of MA3 compared to sisal fiber MA2. We explained this by the aspect ratio of the fibers. As the ratio increases, it can also increase the volume fraction of the matrix porosity []. The hydrophilicity of natural fibers and the morphology change in the presence of water; they swell and shrink when the matrix hardens. Consequently, a detachment occurs at the fiber/matrix contact surface, generating pores [].
Figure 6. (a) Water-accessible porosity (%) of different types of geopolymer based on MK with and without natural fibers. (b) Absorption density (kg/m3) of different types of geopolymer based on MK with and without natural fibers.
At the same time, the increase in fiber content has a major impact on porosity [,]. This is explained by the fiber/matrix detachment phenomena, which increase significantly. In addition, the increase in pores of the mortars leads to a decrease in their density. According to our experimental study, the natural fiber type factor influences the density due to the difference in fiber density and composition, but its impact on the porosity of the geopolymer fiber is limited regardless of the fiber type [].

3.5. Mechanical Properties

Figure 7 shows the results obtained from the mechanical tests, which confirm that the MK and sand-based samples did not exceed 5.4 MPa in compression. On the other hand, the GGBS and MK-based samples showed relevant results (35 MPa). Indeed, the effect of silica content on the mechanical performance of metakaolin and sand-based geopolymers had shown that the compressive strength decreased with an increase in SiO2 content [] compared to GGBS samples containing a high CaO content, which made the matrix more compact and improved its compressive strength []. By adding the long fibers (160 mm) (sisal and palm) in the MA2 and MA3 samples, the strength remains slightly similar to that of the raw samples. However, the sand-based geopolymers only reached 5.4 MPa. The results show that the compressive strength is mainly determined by the matrix strength and much less by the effect of the fibers themselves []. It is also shown that the fiber type has no impact on the results.
Figure 7. Compressive and flexural strength of different types of geopolymer with and without natural fiber.
On the other hand, the slight decrease in compressive strength between fiber and non-fiber samples had been explained in previous research (Bahja et al. 2024) []. This is because the agglomerates are in the form of balls formed during mixing. This can lead to poor fiber distribution, especially during the short setting time of the geopolymer. The agglomeration of the fibers and their dispersion in the geopolymer results in a fundamental correlation between the porosity, density, and compressive strength of the material [], as in the case of cement composites reinforced with natural fibers [,]. The same results were observed by increasing the date palm fiber content by 4 and 8%. On the other hand, the best results were obtained with GGBS-based geopolymers (35 MPa). This is due to the generated amorphous matrix structure, which refines the structure and increases its compressive strength []. The flexural strength of unreinforced geopolymers is lower than that of fiber-reinforced geopolymers. Indeed, the increases in flexural strength with the incorporation of short palm fibers with 4% of tenor (MB1 and MB2) ranged from 13.4 MPa to 16.5 MPa. On the other hand, sisal long fibers did not react in the same way, with a slight decrease compared to the short fiber sample (10.4 MPa). This shows that the fiber type plays a key role in determining the flexural strength, which is due to the mechanical and physical properties of the fiber itself []. Karumanchi et al. (2022) [] noted a 52% increase in flexural strength at 2 wt.% cotton fiber content, beyond which performance plateaued. These results suggest that moderate fiber content optimizes the reinforcing effect, while excessive loading may lead to matrix bunching and discontinuities.
Furthermore, it is worth noting that a matrix with higher density and compactness, such as MB, was able to maintain its flexural strengths even at elevated fiber dosages (reaching 4% by mass).

4. Discussion

The pretreatment of sisal and palm fibers was beneficial in minimizing mineralization and water absorption, which directly impacted the geopolymerization of the specimens. Thus, the optimal content and length can increase the final results of the geopolymer. The optimal composition identified corresponds to a sisal fiber content of 4% by weight, which balances mechanical strength and porosity. Future research should focus on long-term durability tests, including frost resistance and exposure to aggressive environments, to fully evaluate the practical applications of these bio-geopolymer composites. This study demonstrates the potential of incorporating natural fibers into geopolymer mortars to improve flexural properties while maintaining adequate compressive strength. The main results reveal that fiber pretreatment, particularly oil coating, is crucial for preserving their integrity and optimizing material performance. The results of this study highlight the potential of geopolymer-based composites as sustainable construction materials. The optimal combination of materials and processing parameters was identified. This study provides a basis for future research and development in this field.

5. Conclusions

The highlights of this study are as follows:
  • Granulated blast furnace slag (GGBS)-based geopolymers exhibit higher compressive strength than sand-based geopolymers, which is attributed to the denser and more compact microstructure induced by the calcium content of GGBS.
  • The influence of natural fibers, such as palm and sisal, on compressive strength is minimal, as matrix strength primarily determines overall performance. However, fibers affect material porosity: longer fibers improve flexural strength, particularly in GGBS-based matrices. Shorter fibers, on the other hand, increase porosity and reduce mechanical properties due to poor distribution.
These results highlight the importance of matrix composition and fiber characteristics in optimizing the mechanical properties of geopolymer-based materials. Future studies should include long-term durability tests, such as wet–dry and freeze–thaw cycles, to better assess the material’s performance over time. Fiber dispersion remains a crucial aspect to address; further studies using image analysis and statistical distribution techniques are planned to provide a more quantitative understanding.

Author Contributions

All authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. B.B.: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing—Original Draft, Writing—Review and Editing, Visualization, Project administration. A.T.: Conceptualization, Methodology, Validation, Resources, Writing—Review and Editing, Supervision. L.E.H.O.: Validation, Formal analysis, Data Curation, Review and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

This research work has no conflicts of interest.

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