# Impact of hemp shiv content and binder type on the thermal and mechanical properties of geopolymer composites

Hussein Nasreddine, Nicolas Dujardin, Assia Djerbi, Thouraya Salem, Laurent Gautron

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Hussein Nasreddine, Nicolas Dujardin, Assia Djerbi, Thouraya Salem, Laurent Gautron. Impact of hemp shiv content and binder type on the thermal and mechanical properties of geopolymer composites. Construction and Building Materials, 2025, 476, pp.141297. 10.1016/j.conbuildmat.2025.141297. hal-05078906

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Submitted on 22 May 2025

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# Impact of hemp shiv content and binder type on the thermal and mechanical properties of geopolymer composites

Hussein Nasreddine<sup>a,b,*</sup> <sup> ID </sup>, Nicolas Dujardin<sup>c, ID</sup>, Assia Djerbi<sup>b</sup>, Thouraya Salem<sup>b</sup>,
Laurent Gautron<sup>a, ID</sup>

## ARTICLE INFO

### Keywords:

Hemp shiv

Geopolymer composite

Thermal properties

Microstructural properties

Metakaolin

Fly ash

Biomass ash

## ABSTRACT

This study investigates the impact of incorporating hemp shiv (HS) fibers into geopolymer composites formulated with metakaolin (MK), coal combustion fly ash (FA1), and biomass fly ash (FA2) on their thermal and mechanical properties. HS content was varied (0, 200, 250, 300, and 350 wt% of the binder) to assess its effect on dry density, porosity, thermal conductivity, and microstructure. Results show that increasing HS content significantly increased porosity, leading to reduced density and thermal conductivity across all binder types. FA2-based composites exhibited the highest porosity (68.6%) and the lowest thermal conductivity (0.068 W.m⁻¹.K⁻¹) at 350 wt% HS, reinforcing their suitability for thermal insulation applications. SEM analysis revealed substantial interfacial voids and cracking, particularly in FA-based composites, contributing to enhanced insulation properties but reducing compressive strength. Incomplete geopolymerization was observed in high-fiber-content composites, likely due to the absorption of activator solution by HS fibers, which affected matrix cohesion. Despite these structural changes, all composites remained within NF EN 998-1 standards for rendering and plastering mortars, with FA2-based composites demonstrating the greatest potential for thermal insulation applications, contributing to sustainability in construction.

## 1. Introduction

One of the most crucial and widely utilized materials in the construction sector is Ordinary Portland Cement (OPC) [1]. However, the substantial CO₂ emissions associated with cement production pose a significant environmental concern [2]. With the rising demand for cement driven by population growth and urbanization, cement production is projected to increase from 2.8 billion tons annually to 4 billion tons by 2050 [3]. This necessitates the development of greener alternatives to replace cement as a binder material for concrete production. As a result, "Geopolymer Concrete" which uses an alumina-silica-rich binder with an alkali solution to make concrete, was discovered by Davidovits [4]. The environmental benefits of geopolymers are particularly noteworthy; they offer a sustainable alternative to conventional cement, significantly reducing carbon emissions [5].

Geopolymer (GP) is a synthetic aluminosilicate cement created by combining a solid aluminosilicate precursor with an alkaline aqueous solution [6]. The mechanism of geopolymerization involves several

sequential steps: initially, silicate and aluminate species are dissolved as monomers from solid raw materials. These monomers then undergo oligomerization in solution, followed by the polymerization of oligomers and monomers to create aluminosilicate polymers. This process leads to the gelation of aluminosilicate polymers, which is essential for the setting and hardening of the material. Consequently, the initial alkaline dissolution of precursor particles is a critical step, as it releases aluminate and silicate species into the solution, facilitating the formation of the geopolymer matrix through the polymerization process [7,8]. GPs possess mechanical properties that surpass those of OPC while maintaining a significantly lower environmental footprint by incorporating various inorganic waste materials as primary components [9]. Additionally, they are known for their durability, fire resistance, and enhanced performance in various construction applications [10–12]. Notably, GPs also exhibit promising thermo-mechanical performance, making them suitable for structural applications that demand both mechanical integrity and enhanced thermal resistance. Studies have explored their potential in masonry retrofitting, where their low thermal

<footnote><sup>a</sup> Univ Gustave Eiffel, LGE, Marne-la-Vallée F-77454, France</footnote>

<footnote><sup>b</sup> Univ Gustave Eiffel, Cerema, UMR MCD, Marne-la-Vallée F-77454, France</footnote>

<footnote><sup>c</sup> Univ Paris Est Creteil, CERTES, Créteil F-94010, France</footnote>

<footnote>* Corresponding author at: Univ Gustave Eiffel, LGE, Marne-la-Vallée F-77454, France.</footnote>

<footnote>E-mail address: hussein.nasreddine@univ-eiffel.fr (H. Nasreddine).</footnote>
conductivity combined with sufficient mechanical strength provides a unified solution for improving energy efficiency and structural resilience in buildings [13]. Longo et al. [13] demonstrated that geopolymer-based materials could address both energy efficiency and seismic retrofitting challenges in masonry structures, further highlighting their potential in sustainable and high performance construction.

In addition to commonly used materials like metakaolin (MK), fly ash (FA), and ground granulated blast furnace slag, various industrial waste streams containing reactive silica and alumina can be used as geo-polymerization precursors [9]. Each of these materials contributes unique properties to the resulting geopolymer matrix. FA and MK, in particular, have been extensively studied for their effectiveness in producing strong and durable geopolymer composites. For instance, studies have shown that FA-based geopolymers can achieve high compressive strengths and excellent thermal stability [14], while MK-based geopolymers are valued for their high reactivity and ability to form a robust aluminosilicate network [15].

In recent years, the incorporation of natural fibers into geopolymer composites has gained significant attention due to their ability to enhance thermal insulation properties. The introduction of plant-based fibers such as wood fibers, hemp shiv (HS), and miscanthus fibers increases porosity within the matrix, effectively reducing thermal conductivity and improving energy efficiency in buildings. Several studies have confirmed these benefits; for instance, Ranjithkumar et al. [16] demonstrated that incorporating Phoenix fibers (1, 2, 3, 4 wt%) into FA-based geopolymer composites significantly reduced thermal conductivity, with a 25 % reduction observed at 4 wt% fiber content. Similarly, Furtos et al. [17] found that 35 wt% of wood fibers resulted in a thermal conductivity of 0.23 $W.m^{-1}.K^{-1}$ in FA-based geopolymers, while Nasreddine et al. [18] demonstrated that incorporating 50 wt% of miscanthus fibers into MK-based geopolymers achieved a thermal conductivity of 0.21 $W.m^{-1}.K^{-1}$.

However, despite these advancements, prior studies have primarily focused on lower fiber contents, typically not exceeding 50 wt%, and have predominantly examined a single binder type rather than making comparative assessments. The effect of significantly higher fiber incorporation (beyond 50 wt%) on thermal insulation properties, particularly in biomass fly ash-based geopolymers, has not been extensively studied.

Despite the extensive research on FA and MK-based geopolymers, there remains a notable gap in comparing these two types of geopolymer in terms of their thermal properties, especially when reinforced with natural fibers such as HS. Most studies on fiber-reinforced geopolymers have only investigated a single type of binder, limiting the understanding of how different binders interact with high fiber content.

Furthermore, research on the use of agricultural residue biomass fly ash in geopolymer composites is limited. Biomass fly ash, derived from the combustion of agricultural residues such as corn cob, wheat straw, and rice husk, is rich in silica and alumina, making it a potential candidate for geopolymer production. However, the incorporation of biomass fly ash in geopolymers, especially when reinforced with natural fibers like HS, has not been extensively studied, particularly concerning thermal insulation properties. Studies by Olatoyan et al. [19], Ates et al. [3], and Archer de Carvalho et al. [9] have highlighted the potential of biomass fly ash as a sustainable alternative to traditional fly ash, noting improvements in concrete properties and reductions in environmental impact.

This study uniquely explores the use of higher fiber contents and biomass fly ash in geopolymer composites, addressing a gap in the existing literature. By comparing the thermal properties of FA-based geopolymer, MK-based geopolymer, and biomass FA-based geopolymer reinforced with varying contents of HS fibers, it aims to provide a comprehensive understanding of the thermal behavior of these composites, which is crucial for their application in thermal insulation. Additionally, investigating the use of biomass fly ash in geopolymers promotes waste valorization and aligns with global sustainability efforts in the construction industry. Furthermore, this research seeks to provide

alternative binder options based on the availability of materials in different regions, catering to countries with abundant fly ash, significant metakaolin resources, and regions that produce biomass from agricultural residues. The ultimate goal is to identify the most optimal binder type and fiber content for enhancing thermal insulation properties while maintaining mechanical strength within NF EN 998-1 standard [20]. This standard specifies the compressive strength for hardened rendering and plastering used for thermal insulation purposes, with requirements ranging from 0.4 MPa to 5 MPa, depending on specific cases classified as CS I and CS II [18].

To achieve this, varying percentages of HS fiber incorporation by weight of binder, including 200, 250, 300, and 350 wt% for metakaolin, fly ash, and biomass fly ash composites. By systematically assessing the performance of these composites, the most optimal binder type and fiber content can be identified in terms of enhancing thermal insulation properties while maintaining good mechanical strength. This research seeks to fill the existing knowledge gap and provide a foundation for the development of high-performance, eco-friendly thermal insulators, promoting the use of locally available materials and enhancing sustainability in the construction industry.

## 2. Materials and methods

### 2.1. Materials

The primary materials utilized in this study comprised hemp shiv (HS), metakaolin (MK), two types of fly ash (FA1 and FA2), and a sodium silicate solution. HS were obtained from the French company ECOHA-BITAT, as illustrated in Fig. 1. The dimensions of fibers retained on each sieve were determined using a caliper. The fibers were used as received from the suppliers without any chemical pre-treatment to maintain their natural characteristics and sustainability, aligning with the study's aim to develop eco-friendly building materials. This approach also allows for the evaluation of the fibers' inherent interactions with the geopolymer matrix. To stabilize the mass of the fibers and ensure accurate evaluation of their properties, they were dried in an oven at 40°C for 9 days. To formulate the geopolymer matrix, three distinct binders were used, each chosen to assess their individual effects on the composite's performance. MK was sourced from the French ARGECO company. Fly ash FA1 and FA2 were both obtained from SURSCHISTE. FA1 is composed entirely of 100 % fly ash from combusted coal, while FA2 is a mixed ash, consisting of 70 % fly ash from combusted coal and 30 % from combusted biomass (agricultural residue). Table 1 and Fig. 2 detail the chemical compositions and particle size distribution of the geopolymer binders, MK, FA1, and FA2, with the chemical composition evaluated by the manufacturers using X-ray fluorescence (XRF) in accordance with NF EN 196-2, and the particle size distribution determined using laser granulometry. In the

<figure><img src="image_4.png" /><figcaption>Fig. 1. Particle shape of HS.</figcaption></figure>
<table><caption>**Table 1**
Chemical composition of binders.</caption><thead><tr><th>Binder</th><th>SiO<sub>2</sub> (%)</th><th>Al<sub>2</sub>O<sub>3</sub> (%)</th><th>Fe<sub>2</sub>O<sub>3</sub> (%)</th><th>CaO (%)</th><th>MgO (%)</th><th>Na<sub>2</sub>O (%)</th><th>K<sub>2</sub>O (%)</th><th>TiO<sub>2</sub> (%)</th><th>SO<sub>3</sub> (%)</th><th>LOI (loss on ignition) (%)</th></tr></thead><tbody><tr><td>MK</td><td>67.10</td><td>28.80</td><td>2.60</td><td>1.10</td><td>7.20</td><td>0.01</td><td>0.12</td><td>1.40</td><td>-</td><td>0.84</td></tr><tr><td>FA1</td><td>55.46</td><td>23.30</td><td>6.16</td><td>4.83</td><td>2.50</td><td>1.60</td><td>2.17</td><td>1.38</td><td>0.80</td><td>1.52</td></tr><tr><td>FA2</td><td>48.69</td><td>22.03</td><td>7.41</td><td>8.96</td><td>2.80</td><td>1.70</td><td>3.07</td><td>1.21</td><td>2.41</td><td>1.47</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Particle size distribution of binders.</figcaption></figure>

alkaline solution context, sodium silicate (Na59) from WOLLNER®, Germany, was used, with its detailed chemical composition presented in Table 2.

## 2.2. Preparation of geopolymer composite

Table 3 illustrates the production of 15 fiber-reinforced geopolymer composite (FRGC) specimens, each incorporating distinct types of binders and varying HS content. The percentages of HS added to the samples were determined relative to the weight of the binder used, with concentrations of 200, 250, 300, and 350 wt%, selected based on previous findings and experimental considerations. A previous study by Nasreddine et al. [21] demonstrated that adding 100 wt% HS resulted in a thermal conductivity of 0.14 W.m⁻¹.K⁻¹. Therefore, the fiber content in this study was increased beyond this level to further enhance thermal insulation and potentially achieve a thermal conductivity below 0.1 W. m⁻¹.K⁻¹. The upper limit of 350 wt% was established due to the inability of the binder and alkaline solution to sufficiently bind the fibers, as the low density and high volume of HS fibers resulted in excessive voids and poor cohesion within the composite structure. Additionally, for comparative analysis, a reference sample devoid of any fiber additions was prepared for each binder type. The samples are labeled as XB, where 'X' represents the fiber content and 'B' denotes the type of binder, such as MK, FA1, or FA2, corresponding to the designations outlined in Table 3.

The mix design for the geopolymer composites was defined based on the inherent chemical composition of each binder type. For MK-based composites, the ratios of SiO₂/Al₂O₃ and Na₂O/H₂O were calculated as 4.24 and 0.30, respectively. In the case of FA1-based composites,

<table><caption>**Table 3**
Mixture formulations for the production of FRGC.</caption><thead><tr><th>Sample</th><th>MK (g)</th><th>FA1 (g)</th><th>FA2 (g)</th><th>Na₂SiO₃ (g)</th><th>Fiber content (g)</th><th>L/S</th></tr></thead><tbody><tr><td>0MK</td><td>100</td><td>-</td><td>-</td><td>200</td><td>-</td><td>2</td></tr><tr><td>0FA1</td><td>-</td><td>100</td><td>-</td><td>200</td><td>-</td><td>2</td></tr><tr><td>0FA2</td><td>-</td><td>-</td><td>100</td><td>200</td><td>-</td><td>2</td></tr><tr><td>200MK</td><td>100</td><td>-</td><td>-</td><td>200</td><td>200</td><td>2</td></tr><tr><td>250MK</td><td>100</td><td>-</td><td>-</td><td>200</td><td>250</td><td>2</td></tr><tr><td>300MK</td><td>100</td><td>-</td><td>-</td><td>200</td><td>300</td><td>2</td></tr><tr><td>350MK</td><td>100</td><td>-</td><td>-</td><td>200</td><td>350</td><td>2</td></tr><tr><td>200FA1</td><td>-</td><td>100</td><td>-</td><td>200</td><td>200</td><td>2</td></tr><tr><td>250FA1</td><td>-</td><td>100</td><td>-</td><td>200</td><td>250</td><td>2</td></tr><tr><td>300FA1</td><td>-</td><td>100</td><td>-</td><td>200</td><td>300</td><td>2</td></tr><tr><td>350FA1</td><td>-</td><td>100</td><td>-</td><td>200</td><td>350</td><td>2</td></tr><tr><td>200FA2</td><td>-</td><td>-</td><td>100</td><td>200</td><td>200</td><td>2</td></tr><tr><td>250FA2</td><td>-</td><td>-</td><td>100</td><td>200</td><td>250</td><td>2</td></tr><tr><td>300FA2</td><td>-</td><td>-</td><td>100</td><td>200</td><td>300</td><td>2</td></tr><tr><td>350FA2</td><td>0</td><td>0</td><td>100</td><td>200</td><td>350</td><td>2</td></tr></tbody></table>

these ratios were 4.74 and 0.30, while for FA2-based composites, they were 4.71 and 0.30. The Na₂O/H₂O ratio remained constant across all formulations due to the use of the same alkaline solution composition, while variations in the SiO₂/Al₂O₃ ratio resulted solely from the different intrinsic SiO₂ and Al₂O₃ contents in each binder. These ratios provide insight into the chemical composition of the geopolymer system and can serve as a reference for future studies involving different precursor materials or alkaline solutions. The selected parameters across all composites were essential for ensuring optimal geopolymerization and balancing the material's mechanical and thermal properties [22].

<table><caption>**Table 2**
Chemical composition of sodium silicate.</caption><thead><tr><th>Type of solution</th><th>Na₂O/SiO₂</th><th>SiO₂ (%)</th><th>Na₂O (%)</th><th>Molar ratio</th><th>Dry content (%)</th><th>Density (g/cm³)</th><th>Viscosity (kg/(m.s))</th></tr></thead><tbody><tr><td>Sodium silicate</td><td>0.59</td><td>27.5</td><td>16.74</td><td>1.70</td><td>44</td><td>1.55</td><td>0.45</td></tr></tbody></table>
The geopolymer composites were formulated with an alkaline solution to metakaolin ratio (L/S) of two. The mixing process began with the addition of sodium silicate to metakaolin in a Hobart mixer, initially at a low speed of 100 revolutions per minute (rpm) for 3 min, then at a high speed of 300 rpm for 2 min. Subsequently, the fibers were slowly added to the mixture rather than introduced all at once, reducing the tendency for fiber clumping, and were incorporated into the matrix through mixing for 3 min at 100 rpm, followed by 1.5 min at 300 rpm. To ensure uniform mixing, the inside walls of the mixing container were scraped regularly to reincorporate any material adhering to the bowl into the mixture. Each mixture was then molded into 14 cylindrical plastic molds, measuring 50 mm in diameter and 50 mm in height, and vibrated for 3 min to eliminate air bubbles. Following this, the specimens were covered with plastic film and kept for five days at room temperature, where the relative humidity was controlled at 50 % and the temperature was maintained at 20°C. After this period, the demolding process was carried out. Subsequently, the specimens were dried in an electric oven for 9 days at 40°C until their masses reached stability. Following their removal from the oven, they were sealed in plastic bags until they reached the desired age.

## 2.3. Characterization methods

### 2.3.1. Particle size analysis of HS

A sieve analysis approach was employed to analyze the particle size distribution of HS, following the RILEM TC 236-BBM [23].

### 2.3.2. Dry density of HS and geopolymer composite

In this study, the bulk density of HS was determined following the RILEM TC 236-BBM method. In contrast, the apparent density was accurately measured employing the Archimedes method, as elaborated in [24].

For the geopolymer composite samples, density measurements were conducted subsequent to mass stabilization. This involved the precise measurement of the diameter, height, and weight of the geopolymer composite to calculate its dry density.

### 2.3.3. Water absorption of HS

The water absorption capacity of HS was determined through immersion in tap water for varying durations (1, 15, 240, and 2880 min), followed by the assessment of the mass of the saturated fibers, in accordance with RILEM TC 236-BBM.

### 2.3.4. pH of geopolymer composite

To assess the pH variations in geopolymer composites reinforced with different binder types (MK, FA1, FA2) and varying HS contents, pH measurements were performed on hardened samples 28 days post-fabrication, ensuring thorough curing and mass stabilization. For each formulation, three samples were prepared to enhance the reliability of the results.

The procedure involved crushing the hardened geopolymer composites into smaller particles and mixing them with distilled water at a 10:1 liquid-to-solid ratio. The mixture was continuously agitated for 16 h to ensure thorough interaction between the solid particles and the liquid. Following this, the solution was extracted, and pH measurements were conducted using a calibrated pH electrode.

### 2.3.5. Compressive strength of geopolymer composite

In accordance with the NF EN 196-1 [25], cylindrical samples of geopolymer composite were systematically subjected to compression testing at specific time intervals, specifically on days 14 and 28, using a hydraulic press (Controls Pilot4). The testing protocol required a continuous application of compressive force at a rate of 2400 N/s until structural failure occurred. For each formulation, three cylindrical specimens, each with a diameter of 50 mm and a height of 50 mm, were tested to determine their compressive strength.

### 2.3.6. Thermal properties of HS and geopolymer composite

The thermal properties, including conductivity, diffusivity, and heat capacity, of HS and FRGC, were evaluated using the Hot Disk Thermal Constants Analyzer (TPS2500, Hot Disk AB, Gothenburg, Sweden). These measurements were conducted at a controlled temperature of 23 ± 1°C. The employed hot disk technique, detailed in reference [26], is a transient and non-destructive method. This method uses a nickel flat sensor, approximately 10 µm thick, encapsulated between two layers of Kapton measuring 25.4 µm each, serving both as a heat source and a temperature sensor.

For FRGC thermal property assessments, specimens were prepared with a diameter of 50 mm and a thickness of 22 mm. Each formulation underwent three independent measurements to ensure reliability, and the mean value was recorded in the results. A comparable methodology was employed for HS. The experimental setup involved positioning the sensor probe amidst the fibers, with tests conducted under both compacted and non-compact conditions to assess differences in thermal behavior.

### 2.3.7. Water absorption by capillarity

The sorptivity test was conducted to measure the rate of water penetration by capillary suction into the geopolymer matrix. The test was performed according to the NF 1015-18 [27]. Cylindrical samples with dimensions of 50 mm in height and 50 mm in diameter were used. The samples were oven-dried at 60°C for 24 h and then allowed to cool for 24 h at room temperature. To ensure that water absorption occurred only through the intended surface, the lateral sides of the samples were sealed with an impermeable tape. The initial mass of each sample was recorded before the test. Subsequently, the samples were partially immersed in water to a depth of 10 mm. The mass of the samples was measured at different time intervals to monitor the rate of water absorption.

### 2.3.8. Microstructural characterization of geopolymer composite and HS

The pore structure of the geopolymer matrix reinforced with HS was characterized by measuring open porosity and pore size distribution. Mercury Intrusion Porosimetry (MIP) was employed to investigate the open porosity and pore size distribution in sections of the fiber-free geopolymer matrix. To maintain the integrity of the porous structure, the samples were subjected to freeze-drying at -57°C and 0.07 mbar prior to analysis. MIP assessments were performed using a Micromeritics Autopore IV 9520 Porosimeter. During these analyses, mercury was incrementally introduced under pressures escalating to 414 MPa, enabling precise measurement of the porosity within the samples.

At the 28-day mark, microstructural analysis employed the Zeiss Auriga 40 Scanning Electron Microscope (SEM). SEM specimens, obtained from broken chips, were dried, embedded in epoxy resin, and polished for optimal SEM imaging and chemical analysis. The Zeiss Auriga 40 SEM, equipped with various detectors, including Secondary Electron Detectors (SED), Backscatter Detectors (BSD), and Energy-Dispersive X-ray Spectroscopy (EDS) detectors, facilitated comprehensive imaging and analysis under different vacuum conditions. Additionally, SEM analysis of the morphology of HS was conducted using the FEI Quanta 400 instrument, operating at 12.5 kV for detailed observation of microstructural characteristics.

## 3. Results and discussion

### 3.1. Characterization of HS

#### 3.1.1. Particle size analysis

Fig. 3 illustrates the cumulative percentage of particles passing through the sieves as a function of fiber diameter, as determined by the sieving method. The mass retained on each sieve was meticulously recorded, illustrating that HS fibers predominantly fall within the 0.125–4 mm diameter range. A considerable amount of fibers were
<figure><img src="image_6.png" /><figcaption>Fig. 3. Particle size distribution of HS.</figcaption></figure>

found to be retained on sieves with openings between 0.5 and 4 mm, underscoring their primary size range.

However, since the sieving method primarily categorizes fibers based on diameter rather than length, additional measurements were conducted using a caliper to determine fiber lengths. These measurements revealed lengths varying from 0.8 to 32 mm (see Table 4), confirming the presence of a wide spectrum of particle sizes within the HS. By incorporating both diameter and length measurements, this approach provides a more comprehensive characterization of the fiber dimensions, ensuring a clearer understanding of their size distribution.

### 3.1.2. Morphology

The SEM images, as depicted in Fig. 4, reveal distinct surface characteristics of HS. The top and bottom surfaces of HS exhibit two parallel linear structures. One of these structures is relatively smooth, as shown in Fig. 4(a), while the other presents a rough surface with a honeycomb-like porous structure, likely resulting from the shredding process (see Fig. 4(b)). These cavities, organized in a capillary form, contribute significantly to the thermal insulation properties of the reinforcement specimens. The observed microstructure of HS aligns with the findings of Zerrouki et al. [28].

HS has a large average pore diameter (10–75 µm), which suggests a high level of porosity within the stem. The external surface of these stems exhibits a denser structure, contrasting with the internal porosity. This combination of internal porosity and external density in HS is advantageous for applications as lightweight aggregates.

Additionally, the examination of HS reveals irregular shapes and the presence of impurities such as waxes, oils, pectins, proteins, and dust. These impurities can significantly influence the physical properties of the fibers and, consequently, their performance in specific applications. For instance, waxes and oils on the fiber surface can impact adhesion properties, potentially reducing the bonding strength within the composite material. This can lead to weaker structural integrity, emphasizing the importance of thorough processing and treatment of HS before their utilization in construction or insulation materials.

<table><caption>**Table 4**

Physical properties of HS.</caption><thead><tr><th>Properties</th><th>HS</th></tr></thead><tbody><tr><td>Median particle length (mm)</td><td>0.8–32</td></tr><tr><td>Median particle width (mm)</td><td>0.05–5</td></tr><tr><td>Median particle thickness (mm)</td><td>0.02–1.8</td></tr><tr><td>Aspect ratio (AR)</td><td>6.4–16</td></tr></tbody></table>

### 3.1.3. Densities and porosity

The following data were obtained from the experiment to estimate the density of HS (see Table 5): the bulk density without compaction was 104 ± 2.71 kg/m³, whereas the bulk density after compaction was 143 ± 2.09 kg/m³. Based on these data, it is clear that compaction has a significant impact on the bulk density of HS, as the compacted samples increased by roughly 27 % when compared to the non-compacted samples. The density and porosity data presented in Table 5 are consistent with the results reported by Ratsimbazafy et al. [29], which show that HS have a bulk density ranging from 70 to 163 kg/m³ and inter-particle porosity from 55 % to 61 %. This alignment indicates that the properties of the HS used are within the expected range reported in the literature. Using the Archimedes method, the density results were correlated with the compacted bulk density. Hence, HS display a porous structure due to the high percentages of inter-particle porosity. This characteristic could have implications on the material's compressive strength, workability, and thermal properties.

### 3.1.4. Water absorption

Fig. 5 displays the water absorption rates of HS over 48 h, showing a significant absorption rate of 350 %, consistent with findings by Ratsimbazafy et al. [29]. This highlights HS's potential in geopolymer development, contrasting with Ordinary Portland Cement's focus on water absorption. The result presented in Fig. 5 focuses exclusively on the water absorption characteristics of HS, revealing an initial rate of absorption (IRA) of 218 %. The parameter indicating the IRA significantly reflects the porosity of HS and forecasts its influence on the durability and mechanical characteristics of FRGC. The elevated IRA hints at an extensive porous structure within HS, potentially affecting the composite's performance in terms of strength, thermal insulation, and environmental resistance.

## 3.2. Geopolymer composite properties

### 3.2.1. Dry density

The dry densities of the composites incorporating different binder types and various HS content are presented in Fig. 6. For the reference samples without added fibers, the dry density measured 1286 kg/m³ for OFA2, which is the lowest among the binders tested. The reduced dry density of the FA2 binder can be attributed to several factors: FA2 includes 30 % biomass ash, which typically has a lower density compared to coal combustion ash, and has an intrinsic density of 2100 kg/m³, lower than that of FA1 (2370 kg/m³) and MK (2600 kg/m³).

The chemical composition of the binders also plays a significant role in the observed trends. MK binder, with its high silica (SiO₂) and
<figure><img src="image_7.png" /><figcaption>Fig. 4. SEM images of HS: (a) Smooth linear surface structure, and (b) Rough, honeycomb-like porous structure.</figcaption></figure>

<table><caption>**Table 5**

Physical parameters of HS.</caption><thead><tr><th>Physical parameters</th><th>HS</th></tr></thead><tbody><tr><td>Bulk density (without compaction) (kg/m³)</td><td>104 ± 2.71</td></tr><tr><td>Bulk density (with compaction) (kg/m³)</td><td>143 ± 2.09</td></tr><tr><td>Apparent density (Archimedes) (kg/m³)</td><td>163 ± 2.81</td></tr><tr><td>Inter-particle porosity (calculation) (%)</td><td>61 ± 1.32</td></tr></tbody></table>

alumina (Al₂O₃) contents, promotes a higher degree of geo-polymerization, resulting in a denser and more compact matrix [30]. Conversely, FA2 binder, which has lower concentrations of these oxides, leads to a less dense matrix and contributes to the overall lower dry density observed. FA1 exhibits intermediate properties due to its moderate SiO₂ and Al₂O₃ contents, resulting in correspondingly intermediate dry densities.

As the HS content increased, a notable decrease in dry density was observed across all binder types. For example, the addition of 350 wt% HS led to the most significant reduction in dry density among all binder types, with a reduction of 81.7% for FA2 compared to the reference sample (0FA2). This trend of decreasing dry density with increasing fiber content was consistent for the other binders as well. Composites with MK and FA1 also showed substantial reductions in dry density as the fiber content increased. Specifically, MK composites with 350 wt% HS showed a reduction to 265 kg/m³, while FA1 composites with the

same fiber content showed a reduction to 259 kg/m³.

The significant decrease in dry density with increasing fiber content is primarily due to the introduction of natural fibers into the geopolymer binder, which directly affects the density and porosity of the composites [31]. The inclusion of HS enhances the overall porosity of the geopolymer composite, resulting in a decrease in density. This effect is primarily due to weak bonding between the fibers and the matrix, which generates voids around the natural fibers [32]. Furthermore, the fibers can clump together during mixing, capturing water-filled spaces that later transform into voids as they dry.

This trend is consistent with findings in other studies; for example, a study conducted by Narattha et al. [33] demonstrated that incorporating different fiber contents (0.05, 0.10, 0.15, 0.20 mass ratio) of HS into FA-based geopolymer composites led to decreased dry density due to increased porosity.

### 3.2.2. pH

Fig. 7 shows the pH of geopolymer composites for different binder types and varying HS contents, revealing a clear trend of decreasing pH with increasing fiber content. This trend has significant implications for the mechanical properties, particularly the compressive strength of the composites. Typically, a pH range of 13.0–14.0 is optimal for maximizing geopolymer compressive strength [34].

For the reference samples without added fibers, the pH values were 13.2 for 0MK, 13.1 for 0FA1, and 13.0 for 0FA2. These baseline pH

<figure><img src="image_8.png" /><figcaption>Fig. 5. Water absorption characteristics of HS.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 6. Effect of binder type and HS content on the dry density of FRGC.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 7. pH levels of geopolymer composites with different binder types and varying HS contents.</figcaption></figure>

values indicate that the MK-based composites have the highest initial pH, followed by FA1 and FA2-based composites. The high pH levels in these binders are indicative of their alkaline nature, essential for the geopolymerization process. The differences in initial pH can be attributed to the intrinsic properties of the aluminosilicate precursors, including their amorphous fraction and the availability of Al and Si cations (see Table 1), which govern the geopolymerization process and influence the long-term chemical stability of the matrix [35]. The higher initial pH of MK-based composites aligns with previous studies that highlight metakaolin's strong alkaline nature and reactivity in geo-polymer systems [36].

As the HS content increased, a notable decrease in pH was observed across all binder types. For MK-based composites, the pH values decreased from 11.8 for 200MK to 11.2 for 350MK. Similarly, FA1 and FA2-based composites showed reductions from 11.7 to 11.1 and 11.7–10.9, respectively, over the same range of fiber content. These trends were also confirmed by Nasreddine et al. [21], who observed that increasing fiber content in geopolymer composites led to a progressive decrease in pH due to the decomposition of organic components such as lignin and hemicellulose, releasing carboxylic acids that lower the system's pH.

The introduction of natural fibers, such as HS, into the geopolymer matrix increases the overall porosity and introduces organic components that can decompose and release carboxylic acids [32]. This gradual decrease in pH with higher fiber content is indicative of the increased acid release from fiber decomposition. Lower pH values can slow down or inhibit the formation of the geopolymeric network, resulting in a less

cohesive matrix. The impact of decreasing pH on geopolymerization is well-documented, as reduced alkalinity can hinder the dissolution of aluminosilicate precursors, thereby limiting the formation of geo-polymeric bonds [37]. Nasreddine et al. [21] also highlighted that lower pH levels in fiber-reinforced geopolymer composites could hinder the geopolymerization process, leading to a less cohesive structure and a potential decline in compressive strength.

Interestingly, among the fiber-reinforced samples, FA2-based composites consistently exhibited the lowest pH values for each fiber content level. This suggests that FA2 is more susceptible to pH reduction due to its higher biomass ash content, which may contribute to greater organic acid release. Biomass ash typically contains more organic material compared to coal fly ash, which can decompose and produce acidic byproducts. Studies have shown that the presence of organic acids in biomass ash can significantly lower the pH of the composite matrix [38]. This increased acidity can negatively impact the geopolymerization process, leading to a weaker matrix and lower compressive strength. The higher porosity of FA2-based composites also allows for increased moisture retention, which can further accelerate fiber decomposition and contribute to pH reduction [39].

In contrast, MK-based composites maintained relatively higher pH levels, likely due to the more stable alkaline nature of metakaolin [7]. Metakaolin, a highly reactive pozzolanic material, contributes to a stable and highly alkaline environment conducive to efficient geopolymerization. The SiO₂ and Al₂O₃ content in metakaolin provides a robust framework for the formation of geopolymeric bonds, maintaining a higher pH even with the addition of fibers. This stability in pH ensures
a more consistent geopolymerization process and, consequently, higher compressive strength [40].

These findings align with existing literature, which highlights the critical role of pH in determining the mechanical properties of geo-polymer composites. For instance, a study by Duan et al. [41] found that the addition of wood sawdust to geopolymer paste resulted in a lowered pH, thereby decelerating the geopolymerization process and ultimately leading to a decrease in compressive strength. The addition of natural fibers, while beneficial for increasing porosity and potentially improving insulation properties, introduces challenges in maintaining optimal pH levels for geopolymerization [41].

### 3.2.3. Compressive strength

The compressive strength of the geopolymer composites was evaluated at days 14 and 28 for various binder types and HS contents. The results, illustrated in Fig. 8, indicate significant variations in compressive strength depending on the type of binder used and the amount of fiber incorporated.

For the reference samples without added fibers, the compressive strengths at day 28 were 7.1 MPa for 0MK, 6.1 MPa for 0FA1, and 5.3 MPa for 0FA2. These values reflect the intrinsic properties of the binders. MK-based composites demonstrated the highest compressive strength, which can be attributed to their higher SiO₂ and Al₂O₃ contents (96%) compared to FA1 (78%) and FA2 (70%). These oxides promote a greater degree of geopolymerization and result in a denser, more cohesive matrix [30]. Furthermore, the reactivity of the binders, as indicated by their amorphous content (refer to Table 1), plays a critical role in determining compressive strength. The higher amorphous content and larger specific surface area of MK enable more effective geo-polymerization, forming a robust aluminosilicate network (Si–O–Al) and enhancing compressive strength. Supporting this, Chen et al. [42] demonstrated that metakaolin’s larger surface area enhances its reactivity, contributing to higher compressive strength in geopolymer formulations.

In contrast, the FA-based geopolymer samples exhibited lower compressive strengths at day 28, with 0FA1 and 0FA2 reaching 6.1 MPa and 5.3 MPa, respectively. This lower performance is attributed to the lower SiO₂ + Al₂O₃ content and reduced reactivity of the fly ash binders. Additionally, FA2’s higher biomass ash content introduces porosity, further reducing the matrix’s load-bearing capacity. This increased porosity is partially attributed to the release of gases during the geopolymerization process, which can be linked to the decomposition of organic residues present in FA2. This phenomenon was particularly evident during mixing and molding, where bubbling was observed on the surface of fresh samples.

The addition of HS significantly impacted the compressive strength of the composites across all binder types. For MK-based composites, increasing HS content resulted in a marked decrease in compressive strength at day 28, with values dropping from 1.6 MPa at 200 wt% HS to 0.8 MPa at 350 wt% HS. A similar trend was observed for FA1 and FA2 binders, where the compressive strengths at day 28 decreased from 1.2 MPa to 0.7 MPa for FA1, and from 1.1 MPa to 0.6 MPa for FA2, as the fiber content increased from 200 wt% to 350 wt%.

This reduction in compressive strength with increasing fiber content can be attributed to several factors. The introduction of natural fibers into the geopolymer matrix increases the overall porosity, as evidenced by the corresponding decrease in dry density. Weak bonding between the fibers and the matrix generates voids around the fibers, compromising the structural integrity and load-bearing capacity of the composites [43]. Furthermore, the agglomeration of fibers during mixing can trap water-filled spaces, which transform into voids upon drying, further reducing the effective load-bearing area [44].

Interestingly, despite the general decrease in compressive strength with higher fiber content, the trend was consistent across all binder types, suggesting a fundamental interaction between the geopolymer matrix and the fibers. MK-based composites, while showing the highest compressive strength among the fiber-reinforced samples, still followed the same pattern of strength reduction as FA1 and FA2 composites.

The effect of decreasing pH on compressive strength is significant. As the pH decreases, the geopolymerization process becomes less efficient, leading to a matrix with more voids and weaker bonding between particles [41]. This is reflected in the reduced compressive strength observed with higher fiber content [40]. For instance, the compressive strength of MK-based composites at day 28 dropped from 1.6 MPa at 200MK to 0.8 MPa at 350MK. Similar trends were observed for FA1 and FA2-based composites.

Comparing the binders, it is notable that FA2-based composites, which had the lowest dry densities, also exhibited the lowest compressive strengths among the fiber-reinforced samples. This aligns with the understanding that lower dry density, associated with higher porosity and weaker matrix-fiber bonding, results in reduced mechanical performance. On the other hand, MK-based composites maintained relatively higher compressive strengths, which can be linked to their denser microstructure and better fiber-matrix interaction.

These findings are consistent with existing literature [45–47], where the addition of natural fibers to geopolymer composites has been shown to increase porosity and decrease compressive strength. For example, a study by Nasreddine et al. [18] demonstrated that incorporating different fiber contents (10, 15, 20, 30 wt%) of miscanthus fibers into MK-based geopolymer composites led to decreased compressive

<figure><img src="image_11.png" /><figcaption>Fig. 8. Compressive strength of geopolymer composites with different binder types and HS content at 14 and 28 days.</figcaption></figure>
strength due to increased porosity.

Furthermore, the compressive strengths measured in this study surpass the NF EN 998-1 [20] standards for lightweight and general-purpose mortars, which range from 0.4 to 5 MPa. These results are consistent with prior studies comparing compressive strength against established standards [48,49], verifying the appropriateness of the FRGC for uses such as block and plastering, where these strength levels are deemed sufficient.

### 3.2.4. Thermal conductivity

Fig. 9 illustrates the thermal conductivity of geopolymer composites as a function of dry density, evaluated for different binder types and varying HS contents, revealing significant insights into the material properties influenced by binder composition and fiber addition. The small standard deviations in thermal conductivity, approximately 0.001, highlight the precision and consistency of these measurements. For the reference samples without added fibers, the thermal conductivities were 0.491 $W.m^{-1}.K^{-1}$ for 0MK, 0.433 $W.m^{-1}.K^{-1}$ for 0FA1, and 0.395 $W.m^{-1}.K^{-1}$ for 0FA2. These values indicate that MK-based composites exhibit the highest thermal conductivity among the binders tested, likely due to the higher intrinsic density and better inter-particle connectivity in the MK matrix. Conversely, FA2 composites showed the lowest thermal conductivity, which correlates with their lower intrinsic density and higher biomass ash content, introducing more porosity into the matrix [50]. This was also observed in other studies comparing FA-based geopolymer to MK-based geopolymer; for instance, Chen et al. [42] demonstrated that MK-based composites exhibited higher thermal conductivity due to their denser microstructure, better particle packing, and lower porosity.

The addition of HS significantly impacted the thermal conductivity of the composites across all binder types. As the fiber content increased, a marked decrease in thermal conductivity was observed. For instance, MK-based composites showed a reduction from 0.104 $W.m^{-1}.K^{-1}$ at 200 wt% HS to 0.087 $W.m^{-1}.K^{-1}$ at 350 wt% HS. Similarly, FA1 and FA2 binders exhibited substantial reductions, with FA1 decreasing from 0.102 $W.m^{-1}.K^{-1}$ to 0.081 $W.m^{-1}.K^{-1}$ and FA2 from 0.101 $W.m^{-1}.K^{-1}$ to 0.068 $W.m^{-1}.K^{-1}$ over the same fiber content range.

This reduction in thermal conductivity with increasing fiber content can be attributed to several factors. The introduction of natural fibers into the geopolymer matrix increases the overall porosity, as corroborated by the corresponding decrease in dry density. The increased porosity disrupts the heat flow pathways, thereby reducing the

material's ability to conduct heat [51]. The weak bonding between the fibers and the matrix creates additional voids, further enhancing the insulating properties of the composites [51,52]. Moreover, the agglomeration of fibers during mixing can trap air, which is a poor conductor of heat, thereby lowering the thermal conductivity [53]. The thermal conductivity of the HS itself, which is 0.04 $W.m^{-1}.K^{-1}$, obtained at a bulk density of 104 kg/m$^3$ (without compaction), further supports the significant reduction in thermal conductivity observed with increasing fiber content.

The reduction in thermal conductivity was further analyzed in terms of the relative decrease compared to fiber-free geopolymer specimens. Compared to 0MK (0.491 $W.m^{-1}.K^{-1}$), the 350MK composite exhibited an 82% reduction in thermal conductivity. Similarly, FA1-based composites showed an 81% decrease (from 0.433 $W.m^{-1}.K^{-1}$ to 0.081 $W.m^{-1}.K^{-1}$), while FA2-based composites recorded the highest reduction of 83% (from 0.395 $W.m^{-1}.K^{-1}$ to 0.068 $W.m^{-1}.K^{-1}$). This substantial decrease is attributed to the disruption of thermal conduction pathways caused by fiber-induced porosity and the inherently low thermal conductivity of HS.

Interestingly, despite the general decrease in thermal conductivity with higher fiber content, the trend was consistent across all binder types, indicating a fundamental impact of fiber incorporation on the thermal properties of geopolymer composites. Among the fiber-reinforced samples, FA2-based composites consistently demonstrated the lowest thermal conductivity values. This can be linked to their lowest dry density and highest porosity, making them the most effective thermal insulators among the tested binders.

To ensure a more accurate comparison that accounts for density variations across different studies, Fig. 10 presents the normalized thermal conductivity ($\lambda/\lambda_0$), where $\lambda_0$ represents the thermal conductivity of the fiber-free reference sample (pure geopolymer matrix). This normalization provides a fairer comparison of the relative reduction in thermal conductivity due to fiber incorporation, considering differences in binder composition, fiber type, and matrix density. By evaluating $\lambda/\lambda_0$ as a function of density, the intrinsic thermal insulation efficiency of fiber-reinforced composites can be more effectively analyzed while mitigating the influence of varying material densities across studies.

The results indicate that the FA2-based composite with 350 wt% HS exhibits the lowest normalized thermal conductivity, reinforcing its effectiveness as a thermal insulator. Additionally, the binder type significantly influences thermal performance, with FA2-based composites demonstrating superior insulation properties due to their lower

<figure><img src="image_12.png" /><figcaption>Fig. 9. Thermal conductivity of geopolymer composites as a function of dry density with varying binder types and HS contents.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 10. Comparative analysis of density as a function of normalized thermal conductivity in geopolymer paste composites reinforced with various natural fibers. FA2-HS, MK-HS, and FA1-HS (this study) correspond to a fiber content of 350 wt%. (FA-WF: fly ash-wood Fiber, LS-BF: laterite soil-bagasse fiber, FA-HS: fly ash-hemp shiv, MK-MF: metakaolin-miscanthus fiber, MK-HS: metakaolin-hemp shiv, FA1-HS: fly ash 1-hemp shiv, FA2-HS: biomass fly Ash-hemp shiv) [17,18,33,55].</figcaption></figure>

matrix density and higher porosity. When compared to previous studies utilizing different natural fibers, it is evident that fiber type plays a crucial role in thermal behavior, influenced by intrinsic thermal conductivity, cellular structure, and fiber-matrix interactions.

These findings emphasize the role of HS fibers in significantly reducing thermal conductivity and highlight the potential of biomass fly ash as a promising binder for thermal insulation applications. This study not only fills an existing research gap but also provides valuable insights for developing sustainable, high-performance geopolymer composites. The comparison with previous studies demonstrates the advancements achieved in this research, particularly in achieving lower thermal conductivity with higher fiber content, thereby promoting sustainability in construction materials.

In summary, the thermal conductivity values and dry densities measured in this study align with the NF EN 1745:2020 standard requirements [54]. Specifically, masonry and rendering mortars with densities between 200 and 300 kg/m³ are expected to have thermal conductivities below 0.081–0.094 W.m⁻¹.K⁻¹. The formulations that meet these criteria include 250MK, 300MK, 350MK, 250FA1, 300FA1, 350FA1, 250FA2, 300FA2, and 350FA2, demonstrating their suitability for applications requiring specific thermal properties.

### 3.2.5. Thermal diffusivity and specific heat capacity

Table 6 shows that adding HS significantly impacts both the thermal diffusivity and specific heat capacity of the composites. Generally, an increase in fiber content results in a notable decrease in thermal diffusivity and an increase in specific heat capacity (calculated using Eq. (1)). For example, the thermal diffusivity of MK-based composites decreases from 0.460 mm².s⁻¹ for 0MK to 0.125 mm².s⁻¹ for 350MK, while the specific heat capacity increases from 740 J.kg⁻¹.K⁻¹ to 2626 J.kg⁻¹.K⁻¹ over the same range of fiber content. This trend is consistent across all binder types, indicating a fundamental alteration in the thermal properties due to fiber addition.

$$a = \frac{\lambda}{\rho \times c_p} \qquad (1)$$

Where, $a$ (mm².s⁻¹) is the thermal diffusivity, $\lambda$ (W.m⁻¹.K⁻¹) is thermal conductivity, $\rho$ (kg.m⁻³) is the density, and $c_p$ (J.kg⁻¹.K⁻¹) is the specific heat capacity.

The decrease in thermal diffusivity with increasing fiber content can be attributed to the increased porosity introduced by the HS, which

<caption>Table 6</caption>

Thermal properties of geopolymer composites with varying HS content and different binder types.

<table><thead><tr><th>Sample</th><th>ρ (kg.m<sup>-3</sup>)</th><th>λ (W.m<sup>-1</sup>.K<sup>-1</sup>)</th><th>a (mm<sup>2</sup>.s<sup>-1</sup>)</th><th>c<sub>p</sub> (J.kg<sup>-1</sup>.K<sup>-1</sup>)</th></tr></thead><tbody><tr><td>0MK</td><td>1442</td><td>0.491</td><td>0.460</td><td>740</td></tr><tr><td>0FA1</td><td>1397</td><td>0.433</td><td>0.420</td><td>738</td></tr><tr><td>0FA2</td><td>1286</td><td>0.395</td><td>0.410</td><td>749</td></tr><tr><td>200MK</td><td>350</td><td>0.104</td><td>0.168</td><td>1769</td></tr><tr><td>250MK</td><td>313</td><td>0.098</td><td>0.156</td><td>2007</td></tr><tr><td>300MK</td><td>284</td><td>0.093</td><td>0.136</td><td>2411</td></tr><tr><td>350MK</td><td>265</td><td>0.087</td><td>0.125</td><td>2626</td></tr><tr><td>200FA1</td><td>333</td><td>0.102</td><td>0.153</td><td>2007</td></tr><tr><td>250FA1</td><td>308</td><td>0.097</td><td>0.150</td><td>2103</td></tr><tr><td>300FA1</td><td>277</td><td>0.091</td><td>0.147</td><td>2239</td></tr><tr><td>350FA1</td><td>259</td><td>0.081</td><td>0.111</td><td>2814</td></tr><tr><td>200FA2</td><td>320</td><td>0.101</td><td>0.135</td><td>2347</td></tr><tr><td>250FA2</td><td>301</td><td>0.095</td><td>0.122</td><td>2600</td></tr><tr><td>300FA2</td><td>273</td><td>0.088</td><td>0.119</td><td>2700</td></tr><tr><td>350FA2</td><td>235</td><td>0.068</td><td>0.081</td><td>3588</td></tr></tbody></table>

disrupts the heat flow pathways. The introduction of fibers creates more air-filled voids, which are poor conductors of heat, thereby reducing the overall thermal diffusivity of the composites [56]. This is corroborated by the observed decrease in dry density with higher fiber content, as seen in the values presented in Table 6. Lower dry densities are typically associated with higher porosity, further supporting the explanation of reduced thermal diffusivity [57].

Conversely, the increase in specific heat capacity with higher fiber content can be explained by the higher heat absorption capability of the fibers compared to the binder matrix. HS have a lower thermal conductivity but a higher capacity to store heat, which translates to a higher specific heat capacity for the composite material. This enhanced heat storage capacity makes the composites more effective in applications requiring thermal insulation. This trend aligns with findings from Nasreddine et al. [18], who demonstrated that the addition of miscanthus fibers to MK-based geopolymers reduced the thermal diffusivity from 0.43 mm².s⁻¹ in the reference sample (0 wt% fiber) to 0.15 mm².s⁻¹ at 50 wt% fiber content. Additionally, the specific heat capacity increased from 725 J.kg⁻¹.K⁻¹ in the reference sample to 1542 J.kg⁻¹.K⁻¹ at 50 wt% fiber content.

When comparing the different binders, the effect of the binder type on thermal diffusivity and specific heat capacity is also evident. MK-based composites generally exhibit higher thermal diffusivity and
lower specific heat capacity compared to FA1 and FA2-based composites. For instance, the thermal diffusivity for the reference samples without fibers is 0.460 mm².s⁻¹ for 0MK, 0.420 mm².s⁻¹ for 0FA1, and 0.410 mm².s⁻¹ for 0FA2. Correspondingly, the specific heat capacity for these samples is 740 J.kg⁻¹.K⁻¹ for 0MK, 738 J.kg⁻¹.K⁻¹ for 0FA1, and 749 J.kg⁻¹.K⁻¹ for 0FA2. These differences can be linked to the intrinsic properties of the binders, with MK providing a denser and more cohesive matrix, thereby facilitating better heat transfer but storing less heat.

FA2-based composites consistently show the lowest thermal diffusivity and highest specific heat capacity among the tested binders. For example, at 350 wt% HS fiber content, FA2-based composites exhibit a thermal diffusivity of 0.081 mm².s⁻¹ and a specific heat capacity of 3588 J.kg⁻¹.K⁻¹. This can be attributed to the presence of biomass ash content in FA2, which enhance heat storage capacity but reduce thermal diffusivity due to increased porosity and lower intrinsic density.

### 3.2.6. Water absorption by capillarity

The results from the water absorption tests highlight the significant impact of HS content and binder type on the capillary absorption behavior of geopolymer composites. As illustrated in Fig. 11 and summarized in Table 7, the incorporation of HS leads to a substantial increase in water absorption, particularly as fiber content rises. For instance, the 350MK sample exhibits a maximum water absorption of 1119 mg/cm², compared to just 442 mg/cm² for the 0MK reference sample, underscoring the pivotal role of fiber addition in modifying the composite's microstructure.

This increase in water absorption is directly linked to the rise in total porosity observed with higher fiber content. The HS, due to their porous nature, disrupt the continuity of the geopolymer matrix, creating additional void spaces that facilitate greater water ingress. The total water absorption percentage also mirrors this trend, with values reaching as high as 187 % for the 350FA2 composite, compared to just 4.83 % for the 0MK sample. Such significant increases not only highlight the porous and hydrophilic nature of the fibers but also the transformation of the material's overall structure due to fiber integration.

The water absorption behavior follows a typical Fickian diffusion pattern, where the rate of water uptake is rapid during the initial stages and then slows as the material approaches saturation [58]. This behavior is particularly pronounced in composites with higher HS content, where the increased porosity and fiber-induced channels accelerate

<caption>**Table 7**</caption>

Capillary water absorption coefficient (k) and total water absorption (%) of geopolymer composites with varying binder types and HS contents.

<table><thead><tr><th>Sample</th><th>k (mg.cm<sup>-2</sup>.s<sup>-0.5</sup>)</th><th>Total water absorption (%)</th></tr></thead><tbody><tr><td>0MK</td><td>0.95</td><td>4.83</td></tr><tr><td>200MK</td><td>1.19</td><td>110.70</td></tr><tr><td>250MK</td><td>1.85</td><td>133.14</td></tr><tr><td>300MK</td><td>1.90</td><td>143.78</td></tr><tr><td>350MK</td><td>2.01</td><td>162.27</td></tr><tr><td>0FA1</td><td>0.89</td><td>5.33</td></tr><tr><td>200FA1</td><td>1.81</td><td>113.37</td></tr><tr><td>250FA1</td><td>2.09</td><td>144.79</td></tr><tr><td>300FA1</td><td>2.12</td><td>160.60</td></tr><tr><td>350FA1</td><td>2.20</td><td>172.09</td></tr><tr><td>0FA2</td><td>1.55</td><td>8.52</td></tr><tr><td>200FA2</td><td>2.28</td><td>138.15</td></tr><tr><td>250FA2</td><td>2.34</td><td>149.86</td></tr><tr><td>300FA2</td><td>2.47</td><td>173.51</td></tr><tr><td>350FA2</td><td>2.61</td><td>187.17</td></tr></tbody></table>

initial absorption but eventually stabilize as the material's capacity to absorb water is reached.

Supporting these findings, studies have shown that when natural fiber-reinforced composites are exposed to moisture, the hydrophilic nature of fibers, such as cotton (as demonstrated by Alomayri et al. [58]), causes them to absorb water and swell, leading to micro-cracking within the composite matrix. In the case of HS, which contain high levels of cellulose, the absorption of additional water exacerbates the development of swelling stresses [59]. This can result in composite failure as the micro-cracks expand, enhancing capillary action and transport of moisture through these cracks [58]. The capillary mechanism facilitates the flow of water molecules along fiber-matrix interfaces and diffusion through the bulk matrix, leading to de-bonding at the interface, which further contributes to the material's water absorption and potential degradation [60].

The influence of binder type on water absorption is equally notable. MK-based composites generally demonstrate lower water absorption rates compared to FA-based (FA1 and FA2) composites. The denser microstructure and lower total porosity of MK-based composites restrict water penetration, resulting in lower absorption values. Conversely, FA2-based composites, characterized by higher porosity and lower intrinsic density, exhibit the highest water absorption, reinforcing the

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Capillary water absorption test for geopolymer composites with varying binder types and HS contents.</figcaption></figure>
idea that binder composition critically affects the composite's ability to manage moisture.

These findings have significant implications for the use of these composites in thermal insulation applications. While the increased water absorption due to higher fiber content could be seen as a drawback, it also contributes to the hygroscopic nature of the material, which can enhance thermal stability. The ability of these composites to absorb and retain water without releasing it easily allows the absorbed water within the fibers to act as a thermal buffer, improving the insulating properties of the material under varying environmental conditions. However, the potential for water ingress and its associated effects on durability must be considered. Although no visible degradation was observed in the samples after water immersion, the long-term stability of these composites in wet environments may require further investigation. Protective measures, such as surface treatments or coatings, may be necessary to mitigate water ingress and ensure the durability of these materials in practical applications.

### 3.2.7. Microstructural characteristics

#### 3.2.7.1. Mercury Intrusion Porosimeter (MIP).
Fig. 12 presents the Mercury Intrusion Porosimeter (MIP) results, illustrating the total porosity and pore size distribution of various geopolymer composites. The graph categorizes the porosity into four pore size ranges: micro pores (d < 10 nm), small to medium capillary pores (10 nm < d < 100 nm), large capillary pores (100 nm < d < 10 µm), and macro pores (d > 10 µm). The analysis highlights the impact of different binder types and the incorporation of HS on the porosity and microstructure of the composites.

The reference samples without added fibers (OMK, 0FA1, and 0FA2) demonstrate varying total porosity levels. The MK-based sample (OMK) exhibits the lowest total porosity at 28.6 %, followed by FA1 (32.5 %) and FA2 (37.8 %). The lower porosity in the MK-based sample can be attributed to the higher density and better particle packing of the metakaolin matrix, resulting in fewer void spaces. This can be attributed to the amorphous nature and high reactivity of metakaolin particles, which have a large specific surface area, facilitating a more compact and less porous structure. Conversely, the FA2-based sample shows higher porosity due to its lower intrinsic density and higher content of biomass ash, which introduces more voids into the matrix.

The incorporation of HS significantly increases the total porosity of the composites. For example, the total porosity of MK-based composites increases from 28.6 % for the OMK sample to 59.0 % for the 350MK sample.

A similar trend is observed for FA1 and FA2-based composites, with total porosity increasing to 64.4 % for 350FA1 and 68.6 % for 350FA2, respectively. This increase in porosity is primarily due to the introduction of natural fibers, which disrupt the continuity of the geopolymer matrix, creating additional voids and pore spaces. This trend aligns with findings in the literature. For instance, Wongsa et al. [61] observed an increase in the porosity of geopolymer composites from 27.1 % to 28.2 % as the coconut fiber content increased from 0 wt% to 100 wt%. Similarly, Alomayri et al. [31] reported that increasing the cotton fiber content from 0 wt% to 1 wt% led to an increase in porosity from 20 % to 30 %. The addition of plant-based aggregates, characterized by their inherently porous structure, increases inter-granular porosity within the composite matrix [29].

Beyond total porosity, a detailed analysis of pore size distribution highlights an increase in large capillary pores and macro pores as the dominant pore families influenced by fiber addition, while micro pores remained negligible. This shift in pore dynamics can be attributed to the physical characteristics of the fibers size, shape, intrinsic porosity, and density which facilitate the formation of larger pore structures within the matrix. In MK-based composites, the percentage of macro pores (d > 10 µm) increases from 11.3 % in the OMK sample to 17.7 % in the 350MK sample. Similarly, the FA2-based composites show the highest increase in macro porosity, with the percentage of macro pores rising from 12.2 % in the 0FA2 sample to 18.4 % in the 350FA2 sample. This is consistent with the lower intrinsic density and higher biomass ash content of FA2, which contribute to a more porous matrix.

The morphology of the HS, characterized by their rough surface and honeycomb-like porous structure, plays a crucial role in influencing the porosity of the composites. The fibers' ability to create large void spaces and disrupt the matrix continuity results in higher macro and large capillary porosity, enhancing the thermal insulation properties of the composites. These fibers introduce more large capillary pores (100 nm < d < 10 µm), which are beneficial for thermal insulation applications due to their ability to trap air and reduce thermal conductivity [18,53].

The MIP results correlate well with the dry density measurements. As the fiber content increases, the dry density of the composites decreases, which is directly linked to the increase in total porosity (see Fig. 13). The higher porosity leads to a reduction in the material's overall density, as observed in the dry density values. For example, the FA2-based samples show a decrease in dry density from 1286 kg/m³ for 0FA2 to 235 kg/m³ for 350FA2, while MK and FA1 follow similar trends.

The shift in pore distribution contributes to the observed reduction in both mechanical strength and thermal conductivity, primarily due to the

<figure><img src="image_15.png" /><figcaption>Fig. 12. MIP results showing total porosity and pore size distribution of geopolymer composites with various binder types and HS fiber contents.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Correlation between dry density and total porosity of geopolymer composites with varying HS fiber content and different binder types (derived from Mercury Intrusion Porosimetry (MIP) analysis).</figcaption></figure>

proliferation of large capillary pores and macro pores. These pores influence the geopolymer's gas transportation, mechanical strength, and thermal properties [62]. Excessive fiber addition leads to the formation of large capillary and macro pores, which in turn reduces compressive strength, as noted in section 3.2.3.

The incorporation of HS fibers in geopolymer composites serves a similar role to foam addition by increasing porosity, reducing density, and enhancing thermal insulation while compromising mechanical strength. Foam concrete achieves this through a uniform distribution of air voids, whereas HS fibers introduce irregular voids due to fiber-matrix debonding and interfacial gaps. The porosity in fiber-reinforced geopolymers arises from three sources: granular-matrix porosity from imperfect packing, intra-matrix porosity from microscopic voids within the binder, and intra-granular porosity within the fibers themselves due to liquid absorption. Unlike foamed geopolymers, where insulation relies primarily on air entrapment, HS fibers also contribute through their intrinsically low thermal conductivity. Several studies demonstrated that optimizing pore size and distribution in foamed geopolymers can achieve comparable thermal performance [63–65]. However, HS addition offers the advantage of using renewable bio-based materials, supporting sustainability in geopolymer applications.

### 3.2.7.2. Scanning Electron Microscopy analysis (SEM).
The 350FA2 formulation exhibited superior thermal insulation properties among the tested composites. SEM analysis provided detailed insights into its microstructure, revealing both compositional characteristics of the binders and the effects of HS incorporation on the composite structure.

Fig. 14 illustrates a characteristic view of the 350FA2 composite, revealing significant interfacial gaps between the HS fibers and the geopolymer matrix. These voids, formed due to weak fiber-matrix adhesion, play a crucial role in the reduction of thermal conductivity by disrupting heat transfer pathways and increasing thermal resistance. The introduction of HS fibers inherently generates additional porosity, the volume of which increases with higher fiber content.

The mechanical performance degradation observed in HS-reinforced geopolymer composites is closely linked to microstructural defects. Fig. 15 highlights extensive cracking and matrix decohesion, particularly in FA2-based samples, which exhibit the lowest compressive strength among the tested composites, although it remains within the range required by NF EN 998-1 standards for rendering, plastering, and masonry mortars. The observed crack propagation suggests localized stress concentration points, where weak interfacial bonding fails to effectively transfer applied loads, contributing to early failure under compression.

Moreover, the high HS content introduces another challenge of reduced availability of the alkaline activator. HS fibers, due to their

<figure><img src="image_17.png" /><figcaption>**Fig. 14.** SEM image of the 350FA2 sample, illustrating the separation of HS from the matrix.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 15.** SEM image of the 350FA2 composite matrix, revealing extensive cracking and fracturing within the material.</figcaption></figure>
porous nature, absorb significant amounts of the activator solution, creating a local deficit in the matrix, which hinders complete geo-polymerization. The SEM image in Fig. 15 further confirms this, showing highly fractured zones even in areas without direct fiber presence. This observation suggests that incomplete geopolymerization is not solely induced by fiber addition but also results from insufficient matrix cohesion, leading to generalized cracking and a fragmented micro-structure. These findings align with Nasreddine et al. [18], which highlight that high fiber content can disrupt matrix formation by absorbing the alkaline solution, ultimately reducing the degree of geopolymerization.

While thermal conductivity reduction is primarily linked to increased porosity, additional factors contribute to this trend. The intrinsic low thermal conductivity of HS fibers (0.04 W·m⁻¹·K⁻¹) limits heat transfer within the matrix. Additionally, the random orientation of fibers further disrupts thermal conduction, while voids at fiber-matrix interfaces, as evident in SEM images, introduce air pockets that act as thermal barriers, further enhancing insulation performance.

Another indicator of incomplete geopolymerization is the presence of unreacted or minimally reacted grains within the matrix. The 350FA2 composite demonstrates a diverse and rich mineralogical profile. Unlike MK-based geopolymers, quartz is notably absent in FA-based formula-tions, including FA1 and FA2 composites. In FA2, the precursor composition is characterized by high calcium content, along with sig-nificant potassium and sodium concentrations (refer to Table 1).

The high calcium content (nearly 9 % by mass) in FA2 results in the formation of a calcium-rich aluminosilicate mineral, grossular, with minor magnesium and iron substitutions in the calcium sites. The chemical formula derived from EDX microanalysis (Fig. 16(a)) is as follows: (Ca₀.73, Mg₀.18, Fe₀.09)Al₂Si₃O₁₂. The formation of grossular suggests that the high calcium content in FA2 contributes to an alter-native geopolymeric network with lower strength compared to MK-based matrices, which benefit from quartz and mullite as reinforce-ment phases. While calcium-based mineral phases contribute to strength development in geopolymers, their impact on mechanical properties can vary depending on their distribution and interaction within the matrix. Some studies suggest that an excessive presence of calcium can lead to a heterogeneous structure and potential microcrack formation, which may influence the composite's toughness and failure behavior [66,67]. Further investigation would be required to quantify the exact role of these phases in the brittleness of FA2-based composites.

Furthermore, the moderate potassium content (slightly over 3 % by mass) in FA2 leads to the formation of several potassium-based minerals in the 350FA2 composite. These minor phases are present in smaller proportions compared to grossular and include: (i) kalsilite, with a composition of (K₀.88, Na₀.12)AlSiO₄ (Fig. 16(b)), (ii) leucite, with a

composition of (K₀.74, Na₀.26)AlSi₂O₆, and (iii) microcline, with a composition of (K₀.54, Na₀.46)AlSi₃O₈. Notably, these three minerals exhibit alkali deficits relative to their expected compositions, which could be attributed to the difficulty of analyzing alkali elements in the electron microscope, potentially resulting in their loss under the electron beam. The presence of potassium-based minerals suggests that the geopolymerization process in FA2-based composites may favor the formation of crystalline phases rather than a fully developed amorphous geopolymeric gel, potentially reducing mechanical integrity.

Calcium and potassium minerals observed in the 350FA2 composite material are expected to contribute to the mechanical strength of the HS-reinforced geopolymer. These minerals allow the composite to exhibit mechanical properties suitable for applications as mortars, as specified by the NF EN 998-1 standard. However, their presence is limited in the material, and they exhibit Mohs hardness values ranging from 5.5 to 6.5, which are lower than that of quartz (greater than 7). Notably, these minerals were also identified in other FA-based samples, both without fibers (0FA1, 0FA2) and with fibers (up to the maximum fiber content in 350FA1). In these samples, the compositions of the potassium minerals vary significantly, incorporating notable amounts of divalent cations such as Ca²⁺, Mg²⁺, or Fe²⁺. The presence of these softer mineral phases, combined with an incomplete geopolymerization process in high-fiber-content FA2-based composites, may contribute to their reduced compressive strength.

The substantial quantities of quartz present in the MK-based samples largely account for the relatively high mechanical performance of these materials, even with the addition of HS fibers. Another critical source of mechanical strength in MK-based geopolymers is the strong cohesion and interpenetration between the fibers and the geopolymer matrix. This phenomenon is clearly illustrated in Fig. 17, which contrasts sharply with the lack of cohesion observed between the fibers and matrix in the FA-based composites.

In addition to the abundant quartz observed in the MK-based samples (as previously noted in other studies [18,21]), another mineral identified in these composites is noteworthy: a mineral with the composition Al₂Si₃O₉, which could correspond to a silica-rich mullite with an Al₂O₃/SiO₂ ratio of 1:3 (referred to as mullite 3:1). The mullite detected in the MK-based composites of this study exhibits a composition similar to mullite incorporated into FA-based geopolymers in a previous study [68], with 18.12 wt% Al and 26.79 wt% Si in the present research compared to 19.05 wt% Al and 28.02 wt% Si in their work. Mullite is commonly used as a reinforcement material due to its refractory properties, relatively low thermal conductivity, and its mechanical strengthening capabilities, significantly enhancing compressive strength. Its Mohs hardness ranges broadly from 6.0 to 7.5. Therefore, the mullite identified in the MK-based geopolymer composites produced

<figure><img src="image_19.png" /><figcaption>Fig. 16. Chemical microanalysis of minerals observed in the 350FA2 composite material. (a) EDX spectrum of a grossular grain; (b) EDX spectrum of a kalsilite grain.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>Fig. 17. SEM image of the 200MK composite, showing strong cohesion between the matrix and the fibers.</figcaption></figure>

in this study may serve as a mechanical reinforcement, mitigating the reduction in compressive strength caused by the incorporation of HS.

It is noteworthy that the mineralogy of the geopolymer composites in this study varies depending on the precursor type used. Quartz is a characteristic mineral in MK-based geopolymers, while it is absent in FA-based composites. These latter composites exhibit a specific mineralogy dictated by their initial composition, with significant calcium and potassium content promoting the formation of minerals based on these elements. The same minerals are observed in composites made from both types of fly ash (FA1 and FA2), irrespective of fiber content (0–350 wt%). In contrast, the fiber content appears to influence the mineralogy of MK-based composites. At relatively low HS contents (up to 100 wt%), the observed minerals include kaolinite and a dehydrated zeolite [21]. However, in this study, at higher HS fiber contents (up to 350 wt%), a mullite-like structure is predominantly observed. The presence of mullite in MK-based composites suggests that the geopolymerization process is more complete in these samples [69], providing additional mechanical stability and contributing to the higher compressive strength observed compared to FA2-based composites.

These minerals observed in the composites of this study play a definitive role in providing mechanical reinforcement, as corroborated by previous studies ([18,21]). This reinforces the potential for these materials to meet the requirements for mortar applications under the NF EN 998–1 standard.

## 4. Conclusion

This study has provided a comprehensive analysis of the effects of incorporating varying amounts of hemp shiv (HS) fibers into geopolymer composites made from different binders, including fly ash (FA1), metakaolin (MK), and biomass fly ash (FA2). The key findings can be summarized as follows:

* The incorporation of HS fibers significantly increased porosity while reducing dry density across all binder types. FA2-based composites exhibited the highest porosity (68.6 %) and the lowest density (235 kg/m³), enhancing thermal insulation properties.

* Increasing fiber content reduced the pH of the composites, with the most significant decrease observed in FA2-based systems, where a drop from 13.0 to 10.9 was recorded. While the decomposition of lignin and hemicellulose in HS fibers contributed to pH reduction across all binder types, FA2-based composites exhibited a more pronounced decrease due to the higher organic content present in biomass fly ash. The decomposition of residual organic matter within FA2 likely released additional acidic byproducts, amplifying the reduction in alkalinity.

* While HS fibers improved thermal properties, they also increased porosity and weakened fiber-matrix bonding, reducing compressive strength. However, all composites remained within the NF EN 998–1 requirements for rendering and plastering mortars.

* A strong correlation was observed between dry density and thermal conductivity, with HS incorporation significantly lowering thermal conductivity due to increased porosity. FA2-based composites achieved the lowest thermal conductivity (0.068 W.m⁻¹.K⁻¹ at 350 wt% HS), attributed to their lower bulk density and higher biomass ash content, which enhanced pore formation and insulation capacity.

* Higher fiber content led to increased water absorption (up to 187 % for 350FA2) and shifted pore size distribution towards larger pores, which enhanced insulation capacity but required consideration for moisture protection.

* SEM observations and analysis of FA-based composites revealed a pronounced decohesion between the fibers and the matrix, as well as generalized cracking within the matrix. These features likely contribute to the enhanced thermal insulation performance of the final composites. The mineral phases retained within the matrix of both FA-based and MK-based geopolymers also play a crucial role in maintaining compressive strength at a sufficient level, enabling their application as rendering and plastering materials for thermal insulation purposes, in compliance with NF EN 998–1 standards.

These findings highlight the potential of HS in enhancing the thermal insulation properties of geopolymer composites, making them suitable for energy-efficient construction applications. However, the trade-off between improved thermal properties and reduced mechanical strength necessitates careful optimization of fiber content and binder type, making it essential for future research to refine the mix design to achieve a better balance between thermal insulation and mechanical performance. Additionally, investigating the long-term durability of these composites under environmental conditions, including mold growth and water ingress, would provide valuable insights for practical applications. Given the strong absorption capacity of HS fibers, further studies should explore the effects of pre-wetting the fibers before mixing to minimize excessive activator absorption, which could otherwise impact geopolymerization and composite performance. However, the choice of pre-wetting solution and soaking duration must be carefully optimized to avoid fiber clumping and ensure proper dispersion. Finally, life-cycle assessments are crucial to quantify the environmental benefits of these materials and promote their adoption in the construction industry.

## Funding

Univerité Gustave Eiffel.

## CRediT authorship contribution statement

**Gautron Laurent:** Writing – review & editing, Visualization, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Salem Thouraya:** Writing – review & editing, Visualization, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Djerbi Assia:** Writing – review & editing, Visualization, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Dujardin Nicolas:** Writing – review & editing, Visualization, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Nasreddine Hussein:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
# Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

# Data Availability

Data will be made available on request.

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