Effect of Adding Oil Palm (Elaeis guineensis Jacq.) Mesocarp Fibers to Cement Composites

Fiber characterization

Oil palm mesocarp residues, which contain fibers, were collected from a palm oil factory in Ucayali, Peru. The sieve size used to select the fibrous material with ambient moisture was N°8 (2830 µm). The selected material was immersed in water for 24 h to facilitate a mechanical pulping process at 10% consistency, using the 30 HP Bauer Disc Refiner with a disc diameter and a gap of 305 and 0.30 mm, respectively. The resulting fiber bundles are hereafter denoted as oil palm mesocarp fibers (OPMF).

The chemical composition of the OPMF is shown in Table 1. The following parameters were determined: holocellulose (Jayme-Wise’s method), Klason lignin (T 222 om-98; TAPPI, 1998), ash (T 211 om-93; TAPPI, 1993), ethanol extractives (T 204 cm-97; TAPPI, 1997), and water extractives (T 207 cm-99; TAPPI, 1999). The values of chemical constituents were found to be consistent with those reported by Momoh and Osofero (2020) .

Table 1: Chemical composition of the OPMF

Furthermore, the physical dimensions of the OPMF were obtained using a digital USB microscope (model XA24) (Table 2). In comparison with the values reported by Asyraf et al. (2022) for oil palm empty fruit bunch, the diameter of the OPMF was similar, but they were shorter. Consequently, the aspect ratio was notably lower.

Table 2: Physical dimensions of the OPMF

Fiber cement composite manufacturing

Prior to FCC production, the OPMF were mineralized in a calcium chloride (Cl₂Ca) solution at a concentration of 4% for a period of 15 min. Type I Portland cement paste (with a water/cement ratio of 0.4 w/w) was mixed with the OPMF at 0, 3, 6, and 9% by weight to produce the FCC. The resulting composite had a size of 350 x 350 x 10 mm and a final moisture content of 8.5% after three days of

Physical and mechanical characterization of the fiber cement composite

The following tests were conducted: moisture content (gravimetric method), density (DIN 52361; DIN, 1965a), water absorption (DIN 52364; DIN, 1965c), and porosity and volumetric swelling after 24 h of water immersion. Furthermore, the reduction in thickness was determined, following the application of a 39.23 MPa (400 Kg.cm^-2) load (DIN 53291; DIN, 1982), as well as the modulus of rupture (MOR), at a speed of 6.35 mm/min (0.25 inch/min) (DIN 52362; DIN, 1965b).

Scanning electron microscopy (SEM)

The interplay between cement and fibers was examined, as well as the internal structure of the FCC, using a scanning electron microscope (SEM; Thermo Scientific, Quantum 250, USA) with 12.5 and 15 kV acceleration voltages and 34X and 140X magnification. The samples were previously prepared and coated with a diluted gold solution.

Fourier transform infrared spectroscopic analysis

A Bruker Alpha II (Germany) Fourier transform infrared (FTIR) spectrometer was employed to examine the molecular vibrations of samples while applying the attenuated total reflection (ATR) method. The spectra were recorded via successive scans in the wavenumber range between 400 and 4000 cm^-1, with a resolution of 4 cm^-1. The analyzed samples were the control board (0% OPMF), the FCC containing 3, 6, and 9% OPMF, and the OPMF.

Data analysis

An analysis of variance (ANOVA) was performed to identify disparities between the mean values of the FFCs’ physical and mechanical properties. Afterwards, the Tukey test was conducted to determine the significance in the mean values, as indicated by different letters in each of the presented graph. The R programming language version 4.2.3 (RStudio Team, 2023) was used for data analysis.

Physical properties

The physical properties tested were density, moisture content, volumetric swelling, water absorption, and porosity. The results are shown in Figure 1. The highest density was found in the FCC with 3% OPMF (1.60 ± 0.05 g.cm^-3). The moisture content was not significantly different between FCCs, and the addition of 9% OPMF did not affect volumetric swelling or water absorption. The lowest porosity was found in the FCCs with 3% OPMF (25.13 ± 1.08%) and 6% OPMF (24.47 ± 0.52%).

Figure 1: Physical properties of FCCs with different OPMF proportions

Mechanical properties

The mechanical properties tested were thickness reduction and modulus of rupture (MOR). The results are shown in Figure 2.

Figure 2: Mechanical properties of FCCs with different OPMF proportions

The highest thickness reduction was found in the FCC with 9% OPMF (5.79 ± 1.18%). The MOR of the FCC with 3% OPMF (4.35 ± 0.65 MPa) was greater than those of the FCCs with 6% OPMF (2.98 ± 0.37 MPa) and 9% OPMF (3.19 ± 0.31 MPa), as well as similar to that of the control board (4.87 ± 1.30 MPa).

Scanning electron microscopy

The OPMFs’ random distribution in the cementitious matrix is shown in Figure 3.

Figure 3: SEM images: a) control board (0% OPMF); b) FCC containing 3% OPMF; c) FCC containing 6% OPMF; d) FCC containing 9% OPMF

Individual fibers and bundles of fibers are easily distinguishable in Figures 3b, 3c, and 3d in comparison with the control board (Figure 3a).

Figure 4: Internal structure of the FCC: a) an individual fiber and b) a bundle of fibers within the cementitious matrix

Figure 4 shows the internal structure of the FCC. A void distribution difference between an individual fiber (Figure 4a) and a bundle of fibers (Figure 4b) is observed.

Fourier-transform infrared spectroscopy

Figure 5 presents the FTIR spectra of the control board (0% OPMF), the FCCs containing 3, 6, and 9% OPMF, and the OPMF.

The FTIR graph shows vibrations in different band regions and similar patterns in the FTIR spectra.

Figure 5: FTIR spectra of the control board (0% OPMF), the FCCs containing 3, 6, and 9% OPMF, and the OPMF

Physical properties

The FCCs densities are reduced when the fiber proportion is increased. This indicates that adding higher OPMF contents tends to reduce the board density (Lertwattanaruk & Suntijitto, 2015; Adamu et al., 2022). The density of the composites is determined by the density and quantity of their components and their degree of compactness (voids/porosity) (Adamu et al., 2022). In this work, the control board (1.52 ± 0.02 g.cm^-3) exhibited a lower density than the FCC with 3% OPMF (1.60 ± 0.05 g.cm^-3). This is due to the presence of cavities, which is a characteristic of cementitious materials (Alencar et al., 2023), and to the increased number of voids resulting from the addition of fiber (Bellel & Bellel, 2023).

The similarity of the moisture contents in the FCCs can be explained by the fact that the fiber content was not enough to increase the samples’ moisture due to the hydrophobic nature of lignin (31.03%), which was not removed during the mechanical pulping process. The lowest moisture content corresponded to the control board (9.00 ± 0.55%), indicating that the addition of fibers increased the permeability of concrete materials, mainly due to their relatively high moisture absorption and hydrophilic behavior, which is attributed to the presence of hemicellulose and the porous nature of the materials (Komuraiah et al., 2014; Choi, 2022; Hamada et al., 2023).

The comparability of the volumetric swellings of FCC is contrary to that expected (the addition of OPMF should increase the swelling of the composite) (Adamu et al., 2022). This indicates that the FCC were not negatively affected by fiber with regard to swelling. The OPMFs’ 15.8 mm length could reduce heterogeneity in the samples and prevent surface cracking, which can lead to excessive water absorption and, subsequently, volumetric swelling (Cárdenas-Oscanoa et al., 2020; Córdova Contreras et al., 2020; Momoh & Osofero, 2020).

Water absorption is a physical property that is related to porosity; a lower water absorption increases the durability and stability of concrete (Jamshaid et al., 2022). The addition of 3% OPMF to cementitious composites has been generally found to reduce the water absorption of FCC (Jamshaid et al., 2022). However, other studies have yielded contradictory results (Kareche et al., 2020; Adamu et al., 2022).

The porosity at the interface increased as the fiber loading increased (Jamshaid et al., 2022; Bellel & Bellel, 2023). A possible explanation is that the interfacial transition zone (ITZ) of the FCC matrix, which is characterized by a high porosity, formed a gap (debonding) around the lignocellulosic fiber (Savastano et al., 1999; Ali et al., 2022). Furthermore, a higher porosity and a larger number of voids could influence strength properties, resulting in increased water absorption and reduced density (Cipra-Rodriguez et al., 2022; Fernando et al., 2023).

Mechanical properties

The increase in OPMF content tends to reduce the compressive strength of the materials, or to augment the thickness reduction (Lertwattanaruk & Suntijitto, 2015), as fibers typically exhibit a lower density than the matrix. The lowest value was achieved by the FCC with 3% OPMF, which agrees with other studies (Raut & Gómez, 2016; Alatshan et al., 2017; Adamu et al., 2022). The addition of 1 and 3% (by volume) of natural fibers in cement composites has been demonstrated to enhance the properties of cement mortars and concrete (Momoh & Osofero, 2020). Furthermore, an increased fiber loading fraction can deteriorate mechanical properties (Jamshaid et al., 2022).

The flexural behavior of the FCC depends on the type, length, diameter, aspect ratio, and texture of the fibers (Khorami & Ganjian, 2011). Nevertheless, the failure of a FCC is highly influenced by size and the cross-section of the lignocellulosic fiber, which is not uniform (Ishak et al., 2013). The length-to-diameter ratio (i.e., aspect ratio) of a fiber has significant effects on the properties of composites (Altez Basaldúa et al., 2020). Short oil palm fibers can be used as additive fillers in polymer composites to enhance mechanical properties (Arao et al., 2015). However, the aspect ratio of the fibers used in this study (48) was less than that observed in Asyraf et al. (2022) (155.5). Moreover, the addition of oil palm fiber has no effect on the chemical composition of the matrix, thus preventing any interference with the curing of the material (Bellel & Bellel, 2023).

Cement concrete without fiber is brittle and exhibits lower tensile strength, limited ductile properties, and limited resistance to cracking. The addition of fiber in concrete can arrest the propagation of microcracks while increasing its strength (Jamshaid et al., 2022). However, our results regarding mechanical properties showed equal or lower strength values when adding OPMF to FCC. The inferior performance of the FCCs containing 6 and 9% OPMF with respect to the control board may be attributed to the decomposition of the fibers’ lignin within the alkaline matrix (Savastano et al., 2005), promoting the presence of voids. It is worth highlighting that the hydroxyl groups available on the surface of the cellulose serve as the primary means through which fibers and cement bond together (Coutts, 2005). In addition, the homogeneous dispersion of fibers in the matrix is a crucial factor to develop FCC with well-balanced mechanical properties while incorporating plant fibers. Hence, the dispersion of fibers can be improved by increasing the water quantity to the optimal value (Fernando et al., 2023).

The physical and mechanical properties and the chemical components of OPMF may vary depending on factors such as the weather, soil conditions, fertilization, plant age, etc. (Bajuri et al., 2017). In this vein, the findings of each research study may be unique and therefore cannot be extrapolated to other work (Khorami & Ganjian, 2011). FCC can be employed in roofing, internal or external walls, façades, internal or external pavements, and flooring slabs as a sustainable construction material for the future (Khorami & Ganjian, 2011; Jamshaid et al., 2022). In this study, the optimal performance was achieved by FCC with 3% OPMF, which aligns with the findings of Abdalla et al. (2023) and Khorami & Ganjian (2011), who found a maximum of 4% to be the optimal replacement of natural fibers to enhance the properties of FCC in laboratory settings.

Scanning electron microscopy

The fibers and fiber bundles in the FCCs (Figures 3b to 3d) are readily discernible in the analyzed micro-images, unlike the case of the control board (Figure 3a). The presence of fibers and fiber bundles increases FCC porosity (Bellel & Bellel, 2023). Conversely, the observed cavities are characteristic of porous materials such as cement. However, the presence of gaps in the transition zone or in fiber tunnels could potentially render some points fragile or negatively alter the structure of the FCC (Alencar et al., 2023), which justifies the lower strength of the samples with 6 and 9% OPMF.

Figure 3 shows some fiber tunnels, which are the result of a rupture mechanism known as fiber pulling. Additionally, fiber breaking was observed in the majority of fibers (Khorami & Ganjian, 2011).

An adequate curing of the FCCs was observed, as indicated by the minimal number of microcracks in the internal structure (Figure 4). Furthermore, there were voids in the ITZ, which can be attributed to inadequate interfacial bonds, likely because of the heterogeneous fiber surface resulting from the mechanical pulping process (Feng et al., 2023). Fiber swelling tendencies and non-alkaline pre-treatments of OPMF have been found to attenuate interface efficiency in FCCs (De Souza Castoldi et al., 2023). The dissimilar void distribution between an individual fiber (Figure 4a) and a bundle of them (Figure 4b) is due to the fibers’ different diameters (Alatshan et al., 2017). A homogeneous dispersion of fiber is required to manufacture FCC with well-balanced mechanical properties (Fernando et al., 2023).

Fourier transform infrared spectroscopy

The FTIR graph illustrates the spectra of the FCCs and the OPMF, which was reported in a study on bamboo fiber-reinforced cement composite (Alencar et al., 2023). The differing vibration intensities (peaks) observed between spectra are dependent on the fiber content (Feng et al., 2023).

The 3708-3782 cm^-1 band is a result of the stretching vibration of OH groups, which originate from the hydroxyls present in the cellulose molecules, the primary means by which fibers and cement bond together, and the vibration of the hydrogen bridge, characteristic of intermolecular bonds between cellulose chains (Coutts, 2005; Feng et al., 2023; Jiang et al., 2023; Liu et al., 2023; Arango-Pérez et al., 2024). This band also reveals the presence of functional groups such as phenols, alcohols, and water in hemicellulose polymers, lignin, and polyphenols (Alencar et al., 2023).

The vibration in the 2357-2360 cm^-1 band is related to the presence of CO₂, which can easily adhere to the surface of lignocellulosic materials through the adsorption phenomenon (Arango-Perez et al., 2024).

The peaks in the 1594-1596 cm^-1 band are associated with the vibration of the acetyl and uronic ester bonds of hemicellulose and with ester linkage in carboxylic groups of lignin and/or hemicellulose. Furthermore, these peaks indicate the C-H deformation of lignin and the C=C bonds originating from its aromatic rings (Alencar et al., 2023; Ezugwu et al., 2023; Jiang et al., 2023; Arango-Pérez et al., 2024). The 1450 cm^-1 band indicates C-H deformation in cellulose/hemicellulose (Alencar et al., 2023).

The 1000 cm^-1 band can be attributed to deformations in the C-H and C-O couplings present in cellulose (Alencar et al., 2023; Ezugwu et al., 2023). The 865-867 cm^-1 region is associated with the stretching vibration of the Si-O bonds of the SiO4-4 tetrahedron of the C-S-H present in cementitious matrices (Alencar et al., 2023). Conversely, the peaks observed in the 800-900 cm^-1 band are attributed to C-OH deformation of the β-glucosidic bonds present in the hemicellulose of plant fibers (Alencar et al., 2023).