1. Introduction
Natural fibers have been used in different applications for centuries. Their reuse in composite materials is increasing, as they are low cost, abundant, renewable and have many environmental and economic advantages over synthetic fibers. Natural fibers from plants include jute, hemp, dates, palm, banana spine, sugar cane bagasse and coconut fibers, etc.
Among these natural fibers, palm oil fibers as waste from the palm oil industry are continuously increasing around the world. This waste constitutes 90% of biomass waste available after oil processing activities [
1]. A palm tree, including oil and lignocellulosic materials, provides about 230 kg dry weight/year annually [
2]. Palm trees are planted on millions of hectares of land worldwide. Large-scale palm cultivation has environmental consequences due to the production of waste fibers. Every year, nearly 30 million tons of palm waste is generated in the world [
3]. Waste fibers are traditionally burned and sometimes left in fields to decay. The burning of fibers pollutes the environment, while on the other hand, the decay of fibers in soil is time-consuming and creates environmental troubles in the form of diseases in fresh plants. Given the large quantities generated, these wastes are a risk to the environment and urgently need to be recycled. Furthermore, availability, price, performance and biodegradable nature are among the favorable factors that promote the recycling of oil palm fibers as value-added products [
4]. The possibilities of recycling waste fibers in buildings and other materials are numerous and promising. Financial benefits could be generated from the fibrous waste coming from the palm oil production process as long as recycling takes place close to the production site. The cost of the raw material, i.e., waste fiber, would be null, and its transport for processing would remain limited.
Palm oil waste fibers can be used in building materials in the form of earth bricks, concrete and mortar. Natural fibers used in construction materials reduce CO2 emissions due to the partial replacement of natural resources.
Palm oil waste fibers are extracted from palm oil empty fruit bunches and palm fruits. Palm oil flower fibers (POFL) were obtained from empty fruit bunches, and palm oil fruit fibers (POFR) were obtained from palm fruit after oil extraction [
5], as shown in
Figure 1. Natural fiber extraction is usually performed by mechanical processing and retting [
6]. The physical, chemical, mechanical and thermal characteristics of fibers are important for their recycling in composite materials. Their physical properties include quantity, length, density, cross-section, water absorption, etc. The quantity of natural fibers in composite materials such as earth bricks ranges from 1% to 7% by mass. It varies with the type of natural fiber and composite material. A higher quantity of fibers decreases the bonding between fiber and matrix [
7]. Natural fibers of any size and shape can be used as additives in construction materials. However, the common length of fibers reported in the literature varies from 2 cm to 10 cm. Increasing the length of fibers increases the tensile strength of composite materials [
8,
9,
10,
11]. For concrete applications, the recommended average fiber length is 2.5 cm [
12].
The density of natural fibers is another important property. The density of natural fibers increases with the diameter of fibers and ranges from 0.65 to 1.4 g/cc. The low density of natural fibers makes them suitable for building materials [
13,
14,
15].
Natural fibers are hydrophilic materials. Absorption of water and the presence of humidity decrease the performance and shelf life of fibers in composite materials. Treatment of fiber reduces its water absorption capacity and increases its strength [
16].
The morphology of natural fibers shows that these fibers have a lumen (empty tubular structure)-based structure. With an increasing lumen area, the thickness of the wall around it decreases, which reduces the tensile strength of natural fibers.
The mechanical characteristics of natural fibers also have a considerable influence on the tensile strength of construction materials. The mechanical characteristics of fibers include tensile strength, tensile modulus, elongation to break and strain at failure [
17]. The tensile strength of palm oil flower fibers ranges from 21 to 400 MPa [
10,
14,
18]. Fibers with a higher tensile strength increase the tensile strength of composite materials and transform brittle failure into ductile failure.
The mechanical properties of fibers are influenced by their physical characteristics such as density and cross-section [
19]. The heterogeneous nature, diameter and length variation, morphology and chemical composition of fibers affect their tensile strength. In the case of technical fibers, the tensile strength of natural fibers depends on the diameter of the fiber bundle, which varies with the number of elementary fibers. The diameter of palm oil empty fruit fibers ranges from 150 to 700 μm [
13]. The area of natural fibers is estimated by different techniques such as scanning electron microscopy and Vernier caliper.
The chemical composition of fibers is another factor that affects their strength and characteristics. Fibers consist of cellulose, hemicellulose, lignin and pectin. Cellulose is a key element of natural fibers and is responsible for fiber tensile resistance. Natural fibers with higher cellulose content normally have higher tensile strength [
7,
20,
21,
22,
23].
Natural fibers have low thermal conductivity, which makes them suitable for composite materials used for construction [
24]. The addition of natural fibers improves the thermal, mechanical and acoustic characteristics of construction materials [
25,
26,
27,
28]. The addition of fibers increases the tensile strength and post-peak load-bearing capacity of construction materials [
12]. In the case of earth bricks, the compressive strength of bricks increases with the addition of natural fibers [
29]. Fibers are randomly distributed in composite materials. The uniform distribution of natural fibers in building materials is important to increase the tensile strength of these materials. The longitudinal distribution of fibers along the axis of bricks and mortars improves their tensile strength substantially. As fibers are lightweight, upward movement of fibers takes place during the compaction of building materials, which leads to fiber clusters. The choice of the compaction method is important to stop the movement of the fibers inside the matrix [
30]. Compaction of composite materials increases the bonding between fiber and matrix by removing voids. Fiber-reinforced composites have low density and are lightweight construction materials [
31].
The addition of fibers plays a key role in the improvement of thermal and hygroscopic properties as the humidity absorption/evaporation regulation. The addition of fibers to raw earth materials provides good hygroscopic and thermal behaviors with a low economic and environmental cost. The addition of natural fibers reduces shrinkage in building materials [
29] and improves their insulation [
6].
There are some drawbacks associated with the use of natural fibers in composite materials such as deterioration of the physical and mechanical characteristics of fibers with time due to humidity, weather and bacterial actions. The water absorption of fiber-based composite materials increases due to the hydrophilic nature of natural fibers. The presence of small voids between matrix and natural fiber reduces the effectiveness of composite materials. Chemical and physical treatments are commonly applied to increase the shelf life, strength, durability and thermal stability of fibers [
28,
32,
33]. Alkali treatment of fibers is common. Akali treatment of natural fibers increases their tensile strength and improves their thermal characteristics. Alkali treatment increases adhesion between fiber and matrix and reduces the affinity of fibers for water [
34,
35]. However, the process of fiber treatment is not eco-friendly and increases the cost, so we focused only on raw palm oil fibers without any treatment.
Many reviews have been conducted on natural fibers in general and a few on palm oil fibers (raw) [
5,
6]. These studies show the possible recycling of natural fibers in different applications such as polymers, construction materials, geotextiles, automobile parts, etc. However, this paper focuses only on the characteristics of palm oil fibers for their prospective use in construction materials.
The objective of this study is to investigate the physical, chemical, mechanical and thermal characteristics of palm oil flower and palm oil fruit fibers taken from Mexico. These characteristics include length, cross-section, chemical composition, tensile strength, thermal conductivity, etc. Detailed analysis of fiber characteristics helps to determine their suitability for recycling in construction materials such as crude bricks, concrete and mortar.