The Development of Fiber–Cement Flat Sheets by Young and Mature Coconut Fibers to Replace Asbestos for Eco-Friendly Products

Abstract

This study investigated young and mature coconut fibers as an asbestos replacement in fiber–cement flat sheets. The ratio of fiber content ranged from 5% to 9.5% in increments of 0.5% by weight of binder. Crushed rock dust (CRD) was also utilized in this study at a ratio of 50% as sand replacement. The results showed that the addition of young coconut fiber (YCF) and mature coconut fiber (MCF) in flat sheets increased with decreasing bulk density and thermal conductivity. The optimum fiber content was 6.5%–7% by weight of binder for two types of fiber with the highest modulus of rupture of 12–13 MPa. The modulus of rupture and density of fiber–cement flat sheets using YCF were higher than that of fiber–cement flat sheets using MCF, which was clarified by SEM results due to the denser structure of MCF. Moreover, the modulus of rupture was directly proportional to the modulus of elasticity in fiber–cement flat sheets.

1. Introduction

Asbestos is a fibrous material with remarkable technical characteristics that has been used in the manufacturing of numerous building materials since its discovery. It has two types—serpentine and amphiboles. Because of its sound absorption capabilities, strength, fire resistance, heat resistance, electrical resistance, chemical resistance, and low cost, asbestos was prominent in the late nineteenth century. Some of its applications have come onto the market, such as asbestos cement particles, asbestos yarn cord and fabric, asbestos joint and millboard, and water and sewerage pipes [1]. However, the movement of asbestos fibers through the atmosphere in plants can lead to a bad effect on human health. Breathing asbestos-containing air particles causes severe respiratory difficulties [2]. As a result, the development of raw materials for asbestos must be taken into account in order to protect public health and the environment.

Coconut is a significant food and industrial product found mostly in Thailand’s coastal areas due to its tropical environment. Coconut fiber may be extracted by hand or machine from the coconut husk and is readily accessible for use as a raw material in fiber–cement products due to its abundance, low cost, appropriate mechanical qualities, non-toxicity, and chemical reactivity. Coconut fiber (CF) is typically 350 mm long, 0.12–0.25 mm in diameter, and 1250 kg/m³ in density. Coconut fiber has the strongest lignin coating of any natural fiber, making it stronger than most others: only banana fiber has a higher tensile strength. It is also unusual in its resistance to microbial deterioration and salt water [3]. Researchers focus on composite materials reinforced with natural fibers, as these composites combine good mechanical properties with low density. These compounds offer several advantages, such as cost, availability of renewable natural resources, and biodegradability [4]. Lertwattanaruk et al. [5] produced fiber–cement boards (FCBs) using pretreated coconut coir and oil palm fibers at 5%, 10%, 15%, and 20% by weight. The results indicated that the addition of natural fibers led to a reduction in density, compressive strength, and flexural strength. However, the natural fibers improved sound insulation performance. Coconut fibers also outperformed oil palm fibers in both physical and mechanical properties in FCBs. Ramli et al. [6] investigated the physical and mechanical properties of concrete produced with shredded coconut fiber at volumes of 0.6% and 1.8%. The results showed a decrease in compressive strength as the fiber content increased. Coconut fiber–concrete exhibited higher water absorption compared to conventional concrete, suggesting that coconut fiber has potential for use in lightweight concrete. However, Bamigboye et al. [7] incorporated treated and untreated coconut fibers into concrete and subjected the concrete cubes to elevated temperatures of 250 °C and 150 °C for 2 h. The results indicated an increase in compressive strength compared to the control. The compressive strength improved by 3.88% with up to 0.5% fiber replacement. Beyond 0.5% fiber content, the compressive strength began to decrease. Amaguaña et al. [8] produced concrete reinforced with coconut fiber in proportions of 0.5%, 0.75%, and 1%. Concrete containing 0.75% fiber exhibited a significant reduction in crack propagation on the slab surfaces, as well as a decrease in crack depth caused by plastic shrinkage during the early stages of curing. Shcherban’ et al. [9] studied the improvement in mechanical characteristics in normal-weight concrete by varying the proportion of coconut fiber from 0% to 2.5% with increments of 0.25 wt.%. The obtained results revealed that the most appropriate proportion of coconut fibers was 1.75% with compressive strength of 40–50 MPa and axial tensile strength of 3–4.3 MPa. Coconut fibers in the range of 2–2.5 wt% can reduce strength properties. Varghese and Unnikrishnan [10] investigated the strength characteristics of coconut fiber-reinforced concrete and discovered that it enhanced mechanical properties such as compressive and tensile strength. Shear strength in particular increased by 25% to 30% for both 50 mm- and 75 mm-long fibers compared to reference concrete.

Based on the aforementioned studies, the use of young and mature coconut fibers in building products is considered sustainable and green. In this study, young and mature coconut fiber was used as an asbestos replacement in fiber–cement flat sheets from 5% to 9.5% in increments of 0.5% by weight of binder. Crushed rock dust was also utilized in this study at a ratio of 50% as sand replacement. Thanks to the use of coconut fiber in construction, the waste from agriculture and industry is reduced in Thailand and mitigates the use of asbestos so as to protect human health and the environment at low cost.

2. Materials and Methods

2.1. Materials

2.1.1. Binders

Ordinary Portland cement (OPC) type 1, manufactured by TPI Cement, Bangkok, Thailand, was used in this research, conforming to ASTM C150-07 [11]. The chemical and physical properties are listed in

Table 1. Chemical and physical compositions of ordinary Portland cement (OPC) type 1.

Chemical Composition (%)	OPC
Silicon dioxide (SiO2)	20.2
Aluminum oxide (Al2O3)	4.3
Ferric oxide (Fe2O3)	3.5
Calcium oxide (CaO)	63.7
Sulfur trioxide (SO3)	0.26
Magnesium oxide (MgO)	1.3
Sodium oxide (Na2O)	0.32
Potassium oxide (K2O)	2.62
Loss on ignition (LOI)	2.77
Physical properties
Specific gravity	3.15
Retained on a 325 sieve (% by weight)	20.1

. The main composition of OPC has high CaO content, up to 63.7%, with an LOI of 2.77%. The particles retained on a 325 sieve (45 μm) were 20.1% by weight.

2.1.2. Fine Aggregate

Fine natural aggregate (FNA) from river sand was used in this study. Crushed rock dust (CRD) from a limestone quarry in Saraburi Province was also utilized in the fiber–cement sheets. Figure 1 presents a visual comparison of CRD and sand with similar particle sizes, highlighting their distinct coloration. Both FNA and CRD had particle sizes less than 4.75 mm. The physical characteristics are listed in

Table 2. Physical characteristics of fine aggregate.

Physical Characteristics	Fine Coarse Aggregate (FNA)	Crushed Rock Dust (CRD)
Fineness modulus	3.2	3.68
Specific gravity	2.8	2.71
Absorption (%)	0.7	0.47
Unit weight kg/m3	1840	1695

.

2.1.3. Coconut Fibers

Young and mature coconut fibers are natural fibers obtained as by-product of agricultural enterprises. In this study, the coconut fibers were prepared using steam explosion technology by a company in Thailand. Figure 2 illustrates the geometry of the fibers; however, any incomplete elements in the figure should be revised to ensure clarity and proper interpretation. Young coconut fibers (YCF) were derived from young coconuts and were thicker and stronger compared to mature coconut fibers (MCF), which were obtained from mature coconuts. The higher moisture content of MCF contributed to its lower flexibility and strength. Conversely, YCF contained more hemicellulose, enhancing its flexibility and strength. A notable color difference was observed between the two fiber types, with YCF being darker due to its thicker and more robust fibers. For testing split tensile strength and moisture content, the fibers were transferred to the Faculty of Engineering at Rajamangala University of Technology Thanyaburi. The fibers were dried in an oven at 110 ± 5 °C until no moisture remained before being shredded using a fiber cutter machine and passed through a 1-inch sieve for use in fibrous cement board production. The physical characteristics of YCF and MCF are presented in

Table 3. Physical characteristics of coconut fibers.

Characteristics	Young Coconut Fiber	Mature Coconut Fiber
Split tensile strength (MPa)	148.22	141.50
Moisture content (%)	11.11	11.61
Elongation (%)	21.39	27.72
Diameter ($\mathsf{\mu }\mathrm{m}$)	208.52	116.48

. YCF exhibited a split tensile strength of 148.2 MPa, while MCF achieved 141.5 MPa. The moisture content of YCF and MCF was 11.11% and 11.61%, respectively. Additionally, YCF demonstrated an elongation of 21.39% and a diameter of 208.52 μm, whereas MCF had an elongation of 27.72% and a diameter of 116.48 μm.

2.2. Sample Preparation

Young and mature coconut fibers replaced asbestos to produce fiber–cement flat sheets using a water-to-binder ratio of 0.4. The ratio for the total mix of Portland cement–sand–crushed rock dust–water was 1:1:1:0.4. The addition of fibers ranged from 5% to 9.5% in increments of 0.5% by weight of binder. The mixing procedure shown in Figure 3. Firstly, the young and mature fibers were shredded for use as fiber reinforcement in the mixture (Figure 3a). Secondly, the coconut fiber was added and mixed along with binder and fine aggregate using a pan mixer with a capacity of 0.3 m³ (Figure 3b). Next, the fresh mixture of fiber–cement flat sheets was poured into the mold and compressed (Figure 3c). Finally, the flat fiber–cement sheets were placed on a flip platform and removed from the molds to cure the specimens (Figure 3d). The experimental tests included bulk density, modulus of rupture, thermal conductivity, and microstructure to compare the influence of using young and mature coconut fibers in fiber–cement flat sheet applications. For bulk density and modulus-of-rupture tests, specimens with a dimension of 250 × 250 × 15 mm were cast. Thermal conductivity was determined using specimens with dimensions of 300 × 300 × 15 mm. The mix proportions of fiber–cement flat sheets are listed in

Table 4. Mix proportion of fiber–cement flat sheets.

Mix Proportion	OPC (g)	FNA (g)	CRD (g)	YCF/MCF (g)	Water (g)
CF-50	1000	1000	1000	50	400
CF-55	1000	1000	1000	55	400
CF-60	1000	1000	1000	60	400
CF-65	1000	1000	1000	65	400
CF-70	1000	1000	1000	70	400
CF-75	1000	1000	1000	75	400
CF-80	1000	1000	1000	80	400
CF-85	1000	1000	1000	85	400
CF-90	1000	1000	1000	90	400
CF-95	1000	1000	1000	95	400

.

Three types of chemical accelerators—sodium silicate (Na₂SiO₃), designated SS mortar, aluminum sulfate (Al₂(SO₄)₃) designated AS mortar, and calcium chloride (CaCl₂), designated CC mortar—were used to support for the hardening of the specimens. These chemical compounds can significantly improve the hydration kinetics of cement, leading to faster setting and increased early strength. Applying ASTM C807-21 [12], this study determined the best accelerator for fiber–cement flat sheets. The proportion of accelerators used was approximately 0.05 wt% by weight of binder. As shown in Figure 4, for all mortar mixes at 120 min, significant differences in the depth of the Vicat needle were found between the CT (mortar without accelerators) and the mixes of SS mortar (with Na₂SiO₃), AS mortar (with Al₂(SO4)₃), and CC mortar (with CaCl₂). It is also worth noting that the hardening period of the SS mortar was faster than that of the AS and CC mortars. Thus, sodium silicate (Na₂SiO₃) was the optimum material for use as an accelerator for the fiber–cement flat sheets. Similar observation was found in the study of Salain, which confirmed that the CaCl₂ is good chemical accelerators to support for the hardening in concrete [13].

2.3. Testing Procedure

2.3.1. Bulk Density

The bulk density of fiber–cement flat sheets was conducted following the standard of ASTM C1185-08 [14]. Three specimens were measured and the average taken.

2.3.2. Modulus of Rupture

The modulus-of-rupture test was carried out using a universal tensile tester, model CY-6040A12/CHUN YEN (Taiwan), in accordance with ASTM C1185-08 [14] using a single compressor with simple specimen support. The loading was carried out at a rate of 6000 N/min. Each modulus-of-rupture sheet was 250 × 250 × 15 mm in size, and three samples of the modulus-of-rupture test for each concrete mix were utilized to calculate an average value, as shown in Figure 5. The formula for calculation of the modulus of rupture is presented in Equation (1). The modulus-of-rupture test was carried out using the universal tensile tester.

$$R_f={\displaystyle \frac{3PL}{b\ast d^2}}$$

(1)

Here, R_f is the modulus of rupture (MPa), P is the maximum load (N), L is the length of span (mm), b is the width of specimens (mm), and d is the average thickness (mm).

2.3.3. Elastic Modulus

The elastic modulus was determined in accordance with ASTM C469-10 [15]. This involved employing a compressometer linked to the specimens. Longitudinal deformation was assessed by subjecting the specimens to a 40% compression load for 28 and 90 days, then they were subsequently affixed to a load cell. The loading rate applied was 0.3 MPa/s. Data from the linear variable differential transformers (LVDTs) and the load cell were recorded using a data collection system.

2.3.4. Thermal Conductivity

A heat flow meter (HC-074-200) was used to measure the samples’ thermal conductivity (k) in accordance with ASTM C518-21 [16]. An instrument with a microprocessor that can test materials with conductivities between 0.005 and 0.800 W/m K was used. The samples’ thermal conductivity was calculated using Equation (2):

$$\mathrm{K}={\displaystyle \frac{Q_U+Q_L}{2}}\times {\displaystyle \frac{D}{∆T}}$$

(2)

where Q_U is the output of the upper heat flux transducer, Q_L is the output of the lower heat flux transducer, D is the thickness of the sample, and ΔT is the temperature difference between the surfaces of the sample.

2.3.5. Microstructure

Scanning electron microscopy (SEM) was used to analyze the effect of the different coconut fiber types on the microstructure of fiber–cement flat sheets. In order to prepare the samples for testing, samples were sliced into pieces of around 1–2 mm in size from fiber–cement flat sheet specimens and thinly coated with gold. Then, the morphology and mineralogy of the fiber–cement flat sheets was carried out. A 10 kV accelerating voltage were used for the SEM.

3. Results and Discussion

3.1. Bulk Density

Figure 6 shows the bulk density of fiber–cement flat sheets in terms of comparison between young coconut fiber (YCF) and mature coconut fiber (MCF) with 5% to 9.5% by weight of binder. The density of fiber–cement board mixed with YCF ranged from 1399 to 1882 kg/m³, while the density of fiber–cement board using MCF had a density of 1498 to 1993 kg/m³. The results showed that the density of fiber–cement board mixed with MCF was higher than that of fiber–cement board made with YCF. This can be explained by the formation of natural coconut fiber. Generally, the density of raw fiber from mature coconuts is higher than that of raw young coconuts due to the increased lignin content, which also makes them tougher and stiffer. Soumen and Chowdhury [17] indicated that the density of concrete mixed with coconut was lower than 1900 kg/m³. Furthermore, it can be observed that incorporating coconut fiber into the fiber–cement flat sheets at levels ranging from 5% to 9.5% weight of binder resulted in a decrease in bulk density, which was dependent on the volume of coconut fiber added. Lertwattanaruk and Suntijitto [3] revealed that the density of fiber–cement mortars decreased with increased coconut fiber content (from 2110 kg/m³ to 1800 kg/m³). They also demonstrated that the reduction in density resulted in an increase in porosity and water absorption in fiber–cement flat sheets mixed with coconut fiber compared to non-fiber–cement mortars.

3.2. Modulus of Rupture

Figure 7 depicts the modulus of rupture of fiber–cement flat sheets with the addition of YCF and MCF. In terms of YCF, the modulus of rupture of cement board increased when YCF content increased from 5% to 6.5% by weight of binder. However, the modulus of rupture tended to decline if the YCF exceeded 6.5% by weight of binder. The optimum modulus of rupture was approximately 12 MPa and the optimum YCF content 6.5%. For MCF, the optimum modulus of rupture was nearly 13 MPa and the optimum MCF content 7%. It was also observed that the strength decreased when the optimum MCF content was exceeded. Ali et al. [18] found that the addition of coconut fiber at 5% showed the best mechanical strength in concrete. Vidhya and Mahalakshmi [19] reported that incorporating coconut fiber into concrete, at proportions ranging from 5% to 10% by weight of the binder, resulted in optimal strength. It is worth noting that the fiber–cement flat sheets made with MCF had higher bulk density and modulus of rupture than the fiber–cement flat sheets made with YCF. This is due to the young coconut fiber increasing the porosity in the cement board.

3.3. Elastic Modulus

The relationship between the modulus of elasticity (E) and modulus of rupture (R_f) is shown in Figure 8. There was a good relationship with a high coefficient of correlation (R² = 0.71). The elastic modulus in this study ranged from 4.789 GPa to 11.801 GPa and the modulus of rupture ranged from 8.53 MPa to 12.96 MPa. In other words, the increase in modulus of rupture can lead to an increase in the modulus of elasticity. In a previous study, the same result was observed, with a good relationship between the two parameters [20,21]. A great modulus of elasticity enhances deformation capacity and indicates that the material can deform more flexibly under stress without cracking and can absorb and redistribute loads more effectively. The results also indicated that the elastic modulus was related to the modulus of rupture. In other words, the elastic modulus value increased when the modulus of rupture value increased. It can be noted that the coconut fibers were not significantly affected to the relation between E and R_f, but the high modulus of rupture led to a higher modulus of elasticity. The relationship between the two parameters is expressed by Equation (3):

E = 1.22R_f − 4.91, R² = 0.71

(3)

where E is the modulus of elasticity (GPa) and R_f the modulus of rupture (MPa).

3.4. Thermal Conductivity

Figure 9 shows the conductivity of fiber–cement flat sheets with the addition of YCF and MCF. The thermal conductivity of cement sheets decreased when the addition of YCF and MCF content increased over time. The conductivity coefficient of cement board made with YCF decreased from 0.334 W/mK to 0.224 W/mK with the addition of fiber from 5% to 9.5% by weight of binder. The thermal conductivity of fiber–cement flat sheets mixed with MCF was reduced from 0.354 W/mK to 0.245 W/mK with the addition of fiber ranging from 5% to 9.5% by weight of binder. Typically, thermal conductivity properties increase because of the decreasing unit weight of concrete [22].

Figure 10 and Figure 11 illustrate the relationship between the bulk density and thermal conductivity of fiber–cement flat sheets with YCF and MCF. There was a strong relationship between two parameters, with correlation coefficients (R²) of 0.90 and 0.84. The relation is expressed by Equations (4) and (5):

$$K=0.0002D-0.1033, R^2 = 0.90$$

(4)

$$K=0.0002D-0.0852, R^2 = 0.84$$

(5)

where K is thermal conductivity (W/mK) and D is bulk density (kg/m³).

3.5. Microstructure Results

As observed in Figure 12, the young coconut fiber (YCF) and mature coconut fiber (MCF) were long needle-like in shape at a magnification of 75 times (100 μm scale bar). The MCF also had a roughness surface more than YCF. The rough surface of coconut fiber leads to improved bonding within the cement matrix. Figure 13 shows the SEM of fiber–cement flat sheets made with YCF and MCF. The flat sheets made with MCF had denser microstructure than the flat sheets made with YCF. This also supports the modulus-of-rupture results, which indicated that fiber–cement flat sheets were better in terms of using the MCF. The microstructure results also supported the modulus-of-rupture findings, indicating that the fiber–cement flat sheets with MCF exhibited better performance in terms of modulus-of-rupture properties than the fiber–cement flat sheets using YCF.

4. Conclusions

This study investigated young and mature coconut fibers as asbestos replacements in fiber–cement flat sheets. Based on the results achieved, the following conclusions can be drawn.

(1)

Asbestos in flat sheets was replaced by young coconut and mature coconut, which represents innovative research on the application of natural fibers in construction.

(2)

Fiber–cement boards with YCF had densities of 1399–1882 kg/m³, while those with MCF ranged from 1498 to 1993 kg/m³. Adding coconut fiber (5–9.5%) reduced density and increased porosity. In terms of mechanical properties, the modulus of rupture peaked at 12–13 MPa for both fiber types with an optimum content of 6.5–7% by weight of binder.

(3)

A good relationship between the modulus of elasticity and modulus of rupture was found. The modulus of elasticity (E) and modulus of rupture (Rf) showed a strong correlation (R² = 0.71), with E ranging from 4.789 GPa to 11.801 GPa. A higher Rf led to a higher E, improving deformation capacity and load redistribution. Coconut fiber type had minimal impact on this relationship.

(4)

The thermal conductivity of fiber–cement sheets decreased with higher YCF (0.334 to 0.224 W/mK) and MCF (0.354 to 0.245 W/mK) content. Strong correlations between bulk density and thermal conductivity were observed.

(5)

The flat sheets made with mature coconut fiber had denser microstructure than the flat sheets made with young coconut fiber based on SEM results.