A Comprehensive Review on Sustainable Natural Fiber in Cementitious Composites: The Date Palm Fiber Case

Abstract

The use of natural fibers in cementitious composites continue gaining acceptability and applicability due to the shortcomings and disadvantages of synthetic fiber; this is because natural fibers have advantages of sustainability, eco-friendliness, and economy. Biodegradable natural fibers, being low density and lightweight, with typical values of strength-to-weight ratio, aspect ratio, elastic modulus, and strength, may be competitive for substituting synthetic fibers such as glass and carbon. Indeed, natural fibers are mostly non-irritating for the skin and typically pose no troubles or issues for breathing, which is not the case with many synthetic fibers. Date palm fiber (DPF) is a natural fiber obtained as waste material from a date palm tree. In many countries, with large date production, DPF is easily available as a process by-product, with a low processing cost. Being sustainable and environmentally friendly, DPF is continuously gaining acceptability as fiber material in different composites such as concrete, mortar, gypsum composites, clay composites, and bricks. Based on the most available literature reviewed, DPF reinforced composites have been found to be a good insulation material, with higher thermal properties, thereby reducing energy consumption which consequently saves the running and maintenance cost of the building. DPF reinforced composites were reported to have higher energy absorption capacity, ductility, and bending resistance, leading to delaying crack propagation and preventing catastrophic failures of structures such as beams and slabs. Additionally, due to its lower density, DPF reinforced composites have the advantage for usage in areas prone to seismic effects, and when used for buildings, the overall weight of the building is expected to reduce hence reduction in foundation cost. The major setback of using DPF in composites is the reduction in the compressive strength of the composites and the durability performance of the composites. Therefore, for effective usage of DPF in composites to derive the maximum benefits, there is a need to devise a method of mitigating its negative effects on the compressive strength and durability performance of the Composites; this is a future study that needs to be explored for better performance of DPF in cementitious and other materials composites.

1. Introduction

Cementitious material such as concrete is generally a composite material consisting of aggregate (fine and coarse), and binder material (mostly cement) mixed with water to form a solid and hard matrix. The major advantage of concrete is its ability to be molded into various shapes, in addition to its high compressive strength to resist crushing loads; however, the major disadvantages or shortcomings of concrete are its high brittle nature, low tensile strength, and strain, low bending resistance; this makes concrete crack easily when subjected to tensile stresses. To overcome these composites may be considered to reinforce the concrete element. Fibers are used in concrete composites to further improve their strength but are limited by cost. Different types of reinforcing fibers have been used in concrete to improve its mechanical and physical properties, and are categorized as either natural or synthetic fibers [1,2,3]. The American Concrete Institute defined fiber-reinforced concrete as a composite material mainly consisting of a mixture of cement, fine aggregate, coarse aggregate, water, and short, discrete, and discontinuous fibers added which are uniformly dispersed. The most suitable fibers for concrete application are made from glass, steel, and organic polymers or plastics (synthetic fiber). Some naturally occurring fibers are also used in concrete which include asbestos fibers and cellulose or agricultural fibers such as jute, sisal, date palm mesh, etc. [4,5]. The history of using fibers in brittle composites can be dated back to about 3500 years ago, when a 57 m high building in Baghdad was constructed using straw fibers reinforced by sun-baked bricks. The use of asbestos fibers in cementitious composite can also be dated back to 100 years ago, while vegetable (cellulose) fibers dated back to 50 years ago [6,7]. The most recent types of fibers used in cementitious composites are artificial or synthetic fibers such as polypropylene fibers, steel fibers, and glass fibers, where their applications in cementitious composites began more than 30 years ago [6,7]. The main aim of using fibers in concrete is not to improve its compressive strength, and are not considered as an alternative to conventional reinforcement, but used as supporting or complimentary reinforcing methods [8]. Fibers are added to concrete to improve some of its properties, such as reducing plastic cracking of the freshly cast concrete, enhancing the bending and tensile strength of the concrete, improving the energy absorption and impact resistance of the concrete, reducing drying shrinkage, enhancing the durability performance of concrete, controlling, bridging and delaying the propagation of cracks thereby significantly enhancing post-cracking ductility [1,4,5,6,7,8,9].

Fibers used in cementitious composites are mainly classified based on how they are produced or obtained; they can be organic synthetic fibers, inorganic fibers, natural fibers, or recycled material fibers. The summary of fiber classification is presented in Figure 1.

The selection of the type of fiber to use in cementitious composites depends on many factors such as cost, availability, properties of concrete to improve, safety (hazardous nature of the fiber), etc. Inorganic fibers such as asbestos have good compatibility with cement matrix thereby improving the properties of concrete; however, due to its fibrous silicate content it is hazardous to health, where it leads to cancer; this resulted in many countries banning the use of asbestos as building materials [9,10]. Steel fibers come in different geometry, shape, and surface textures. Due to their high tensile strength and elasticity, they significantly improve the tensile strength and ductility of concrete; however, some shortcomings of using steel fibers in cement composites include its production is not environmentally sustainable, it is very expensive compared to other fibers, and they have high susceptibility to corrosion [9,11,12,13]. Glass fibers have less weight than steel fibers and are not susceptible to corrosion, but they are vulnerable to long term alkaline environment and aged with time, which reduces the mechanical properties and durability of the composite with time [9,14,15]. Synthetic fibers such as PP, PE, and PVA are less expensive compared to inorganic fibers but do not significantly improve the properties of the concrete. Additionally, they have weak or poor bonding ability with cement matrix, thereby leading to a decrease in compressive strength [9]. Natural fibers, on the other hand, have continued to gain acceptance for use in cementitious composites due to their environmental sustainability and friendliness, widespread availability, low process cost, recyclability, and non-toxicity, as well as their low density and weight and proper mechanical properties [9,16,17].

Based on the available literature reviewed, date palm fiber (DPF) is continuously gaining acceptability as a natural fiber in cementitious composite. The available studies showed that DPF significantly improved the tensile strength, flexural strength, ductility and energy absorption, thermal insulation, and impact resistance of cementitious composites; however, most studies reported a decrease in compressive strength when DPF is added. To date, there are review articles that focused mainly on the use of DPF as natural fiber in cementitious composites. Most of the available literature articles focused on natural or cellulose fibers in general. Therefore, this study comprehensively studied and reviewed the latest research on the use of DPF in cementitious composite; it also reviewed the ways in which the DPF is treated to improve the performance of the fiber-reinforced composite. Additionally, some of the ways used by different researchers to mitigate the negative effects of the DPF on the compressive strength of cementitious composite have been comprehensively studied and summarized; this study was limited to only one type of natural (cellulose) fiber, i.e., DPF. Therefore, in other to promote the use of cellulose (natural) fibers in concrete, this paper presents the current and up-to-date review of previous works on the use of a specific type of natural fiber in concrete, i.e., date palm mesh fiber reinforced concrete.

2. Natural Fibers

The use of natural fibers in cementitious composites continue gaining acceptability and applicability due to its advantages over other artificial and synthetic fiber; this is because natural fibers are more economical and environmentally friendly and sustainable. Additionally, their biodegradability, lightweight and lower density, higher strength-to-weight ratio, and non-toxicity give them some advantages when used in place of synthetic fibers such as glass and carbon fibers [18,19]. Furthermore, the production and processing of natural fibers involve less energy in comparison to other synthetic fibers. As a reference, the processing and production of jute natural finer, requires only about 7% of the energy needed to produce the same weight of polypropylene artificial fibers. Additionally, the production of 1 tons of polypropylene fiber generates about 3.7 tons of CO₂, while the jute fiber absorbs CO₂ [20]. Some of the major shortcomings of using natural fibers are its hydrophobic nature. Another problem related to the use of natural fibers in cement composites is its higher variations in properties, causing erratic cementitious materials properties [21,22]; these defects need modification for the fiber to be effectively used and enhance the properties of cementitious composites [19,23].

2.1. Types of Natural Fibers

Natural fibers are hair-like raw materials of different kinds obtained from various natural sources such as plant, vegetables, animals, and other mineral sources. After obtaining them, they are then formed into fabrics of non-woven type, then processed to threads, ropes, or filaments before using them as fiber materials in cementitious composites [24,25,26]. Figure 2 the shows classifications of natural fibers.

2.1.1. Animal Based Natural Fiber

Animal fibers are protein based natural fibers obtained from animal sources and have much limited usage in cementitious composites compared to other types of natural fibers [28,29,30]. Initially animal fibers were obtained from the fur and hair of mammals including horses, goats, and sheep wools. Other sources includes insect’s dried spittle and fluids, bird’s feathers [31]. A typical animal fiber from sheep wool is shown in Figure 3. There is not much literature on the utilization of animal fibers in cementitious composites.

Narayanan and Kumar [33] characterized wool fiber to produce reinforced composite materials using polyester resin as a base matrix; their findings showed that the compressive strength, impact strength, and hardness of the composite are significantly influenced by the natural fiber and resin composites, and they recommended the usage of the manufactured composite containing wool and glass fibers reinforced with resins for shelf applications.

Alyousef et al. [30] studied the effect of sheep wool fibers (SWF) and modified SWF (MSWF) on the properties of concrete; they added WWF at 0%, 0.5%, 1%, 1.5%, 2%, 3%, 4% and 6% by weight of cement, and prepared other mixes by adding MSWF at 0%, 0.5%, 1% and 1.5% by weight of cement. MWSF was obtained through pretreating the SWF in salty water for 24 h at a temperature range from 22 to 28 °C to improve bonding between the fiber and cement paste; their findings showed that the addition of both SWF and MSWF decreases the workability of the concrete. Similarly, the compressive strength of the concrete decreases with increase in SWF content, where the addition of between 0.5–6% SWF resulted to decrease in range of about 5.5–79.7%, 12.5–75.2%, 9.8–64.3%, 7.7–46.9% and 5.1–61.5%, at 7, 28, 90 and 180 days, respectively. The addition of MWSF also resulted to decrease in compressive strength, but the decrease was less pronounced compared to SWF, where for 0.5–1.5% MSWF addition, the reduction in strength were 7.7–18.8%, 7.8–18.5% and 0.6–15.1% at 7, 14, and 28 days, respectively. On the contrary, the splitting tensile and flexural strengths increased in all cases with the addition of both SWF and MSWF, with the modified fiber concrete having the highest strengths; they also reported improvement in the bonding and adhesion of the fiber with cement paste due to pretreatment which resulted to improved strengths.

Fiore et al. [28] investigated the effect of sheep wool fiber (SWF) as additive on the mechanical properties and thermal conductivity of mortar. Three different fiber lengths, 1 mm, 6 mm, and 20 mm, were used with fiber weight fractions of 46%, 23% and 13%, respectively; their findings showed that SWF, regardless of its content or length, improved the thermal insulation of the mortar. Furthermore, the mechanical properties including the compressive strength of the mortar decreased with the addition of SWF irrespective of the fiber length. The thermal conductivity of the mortar also decreased with the increment in SWF addition, where the shorter fibers have more influence on the reduction of the thermal conductivity compared to the longer ones; they finally concluded that the addition of 13% 6 mm length fiber produced the composite with the highest strength. In a similar study, Manivannan et al. [34] investigated the effects of varying percentages of sheep hair fiber (SHF) on the mechanical performance of sheep hair fiber reinforced polymer (SHFRP); they prepared the composite using weight variations of SHF at 20%, 30% and 40% by weight, and reported improvement in tensile strength and tensile modulus, flexural strength, and impact strength and of the composite with increase in SHF content. The optimum SHF in terms of tensile and flexural strengths was reported to be 40%, while for impact strength and tensile modulus the optimum SHF were 30% and 20%, respectively. Alyousef et al. [35] studied the effect of SWF on the mechanical properties of concrete. Before using the fiber, they pre-treated it by soaking in saline water for 24 h at ambient temperature to remove impurities, improve surface roughness of the fiber to enhance the bonding between the fiber and cement matrix; they added different dosages of the SWF in proportions of 0.5%, 1%, 1.5%, 2% and 2.5%, and subsequently reported that the concrete produced using the treated SWF had higher mechanical performance in comparison to the concrete with untreated SWF. Additionally, the addition of the treated SWF increased the tensile strength first crack and ultimate failure impact resistances of the concrete as shown in Figure 4a–c, respectively. The researchers attributed this improvement to the crack bridging effect of the fiber. On the contrary, the reported slight reduction in compressive strength with addition of the SWF as seen in Figure 4d, which they attributed to the reduction in density of the concrete due to addition of the fiber.

Other studies utilized animal based fibers not for cementitious composites but for other applications. Chen et al. [36] used chicken feather for development of poly lactide grafted composites. Sogawa et al. [37] used silk for the production of natural rubber and 3,4-Dihydroxyphenylalanine (DOPA)-Modified Silk/NR Composites. Brenner et al. [38] developed a bio-based polycondensation-type thermoset composites using poultry feather. Pan et al. [39] used spider silk fiber to develop supertough electro-tendons for transmitting actuation forces in robotic hands.

2.1.2. Plant Based Natural Fibers

Fibers obtained from agricultural products such as trees and vegetables are referred to as plant fibers; these fibers are classified based on their original sources i.e., part of the tree or vegetable. The fibers obtained from the skin or bast around the stem of the tree are referred to as bast fibers. Those obtained from the tree or vegetable leaves are referred to as leaf fibers. Others are seed fibers found from shell or seeds; grass fibers from grass plants; core fibers from plant stalks; root fibers from tuber or roots; fruit fibers from fruit structures [31,40]. Plant fibers are also referred to as cellulose or lignocellulose fibers due to their cellulose and lignin contents as the main chemical composition. Additionally, plant fibers contain hemicellulose and pectin. The hemicellulose is the main compound for thermal degradation and moisture absorption biodegradation of the fiber, while the ultraviolet degradation of the fiber is controlled by the lignin. Plant fibers are mainly used for construction purposes, automotive and aerospace interior designs and parts, and for the manufacture of sport equipment [27,41].

Table 1. Properties of some selected plant based natural fibers.

Origin	Fibre Name	Chemical Composition (wt %)					Diameter (µm)	Density (g/cm3)	Tensile Strength (MPa)	Tensile Modulus (GPa)	Elongation at Break (%)
		Cellulose	Hemicellulose	Lignin	Pectin	Wax
Tree	Softwood	30–60	20–30	21–37	-	-	20	-
	Hardwood	31–64	25–40	14–34	-	-	33	-	-	-	-
Leaf	Sisal	60.0–78.0	10.0–14.2	8.0–14.0	10.0	2.0		1.33–1.5	363–700	9.0–38	2.0–7.0
	Pineapple	70–84	-	5–12	-	-	20–80	1.44–1.56	413–1627	60–82	-
	Abaca	56–63	20–25	7–13	1	3	17–21.4	1.5	400–980	6.2–20	1.0–10
	Curauá	70.7–73.6	9.9	7.5–11.1	-	-	7–10	1.4	87–1150	11.8–96	1.3–4.9
	Banana [42]	53.5–67.6	10–28.6	5–15.4	-	-	12–30	1.3–1.5	355–500	12–33.8	1.5–9
	Corn husk	50–55	39.4	7.5	-	-	130–140	0.34	150–175	4.0–5.5	--
Seed	Coconut Coir	32–43.8	0.15–20	40–45.8	3–4	-	10–460	1.15–1.46	95–230	2.8–6	15–51.4
	Cotton	82.7–90.0	5.7	<2.0	<1.0	0.6	10–45	1.5–1.6	287–800	5.5–12.6	3–10
Bast	Jute	59.0–71.5	13.6–20.4	11.8–13.0	0.2–0.4	0.5	20–200	1.3–1.49	320–800	8–78	1–1.8
	Kenaf	31–72	20.3–21.5	8–19	3–5	-	-	1.24–1.4	223–930	14.5–53	1.5–2.7
	Flax	62.0–72.0	18.6–20.6	2.0–5.0	2.3	1.5–1.7	12–600	1.4–1.5	343–2000	27.6–103	1.2–3.3
	Hemp	68.0–74.4	15.0–22.4	3.7–10	0.9	0.8	25–500	1.4–1.5	270–900	23.5–90	1–3.5
	Baggase	32.0–55.2	16.8	19.0–5.3	-	-	10–300	1.25–1.3	222–290	17–27.1	1.1
	Coconut	36–43	0.15–0.25	41–45	3–4	-	100–450	-	131–175	4–13	-
	Corn cob	26.1	45.9	11.3	-	-	-	-	-	-	-
Grass	Bamboo	26.0–65.0	30.0	5.0–31.0	-	-	25–40	0.6–1.1	140–800	11–32	2.5–3.7
	Elephant Grass	-	-	-	-	-	<17	2.5–2.59	2000–3500	70–76	1.8–4.8

Sources: Yan et al. [43], Tang and Yan [44], Koolen et al. [45], Page et al. [46], Rocha and dos Santos Rosa [31], Alzeer and MacKenzie [47].

provides a summary of the composition and mechanical properties of some types of plant fibers and their origin.

2.2. Treatment and Modifications of Natural Fibers

Most natural fibers contain chemicals and impurities such as cellulose, lignin, hemicellulose, oil, and wax; this significantly affects their performance in terms of improving the mechanical properties and durability performance of cementitious composites [48]. Therefore, to derive the maximum benefits of natural fibers in concrete, researchers developed methods of treating them to remove the chemicals and impurities before applying in concrete and mortar. Several studies used different methods to treat the fibers before applying in cementitious composites. Pehanich et al. [49] applied some chemical treatments to the fibers and reported improved mechanical performance. Castellano et al. [50] and Abdelmouleh et al. [51] reduced the hydrophilic nature of the fibers using organofunctional silane coupling agents. Arsène et al. [52] used pyrolysis process to treat the fibers, and they reported improvement in the fiber strength by up to three times. Khelifa et al. [53] treated the natural fiber using different concentrations of NaOH at different times; they found that treating the fiber using 3% NaOH concentration for 3 h resulted to improvement of the mechanical properties of the fiber reinforced composites. Benaimeche et al. [54] and Vantadori et al. [55] submerged the natural fibers in water for 24 h at room temperature, after which they air dried it to reduce the hydrophilic nature of the fiber and prevent it from absorbing mixing water during casting. Ali-Boucetta et al. [56] carried out three different methods for treating natural fiber before applying in concrete. The methods were boiling, treatment using NaOH and using polymer surface method by applying linseed oil. Their findings showed that boiling the fiber for 3 h in water, treating using 3% NaOH concentration and surface treatment using 1.5% linseed oil/fiber ratio produced the optimum results in terms of setting time and tensile strength. The best improvement was obtained when treated using NaOH and least or non-improvement was when surface treated with linseed oil. Furthermore, NaOH treatment decreased the capillary water absorption of the fiber reinforced composites by 8.9% after 24 h while linseed oil treatment decreased it by 69.8% after 24 h. A summary of some methods used to treat different types of natural fibers is presented in

Table 2. Methods of treating different natural fibers in cementitious composites.

References	Natural Fiber	Method of Treatment	Outcome
Khan et al. [61]	Coconut Fibers	Soaked a hard skin (husk) in water and embedded the matured coconut to it; this allows decomposition of soft materials around the fibers before extracting the coconut fiber	No outcome reported
Kesikidou and Stefanidou [62]	Jute, Coconut, Kelp	Soaked the fibers in water for 24 h before adding to the composites	Reduced absorption of mixing water by the fibers
Castillo-Lara et al. [63]	Henequen fibers	NaOH aqueous solution at 2% concentration at 25 °C fir 1 hr at 550 rpm using mechanical stirrer. After which the fiber was washed with water and oven dried for 24 h	Improved tensile strength and tensile absorbed energy
Zhang et al. [64] and Marvila et al. [65]	Jute (Zhang), acai (Marvila)	Soaked in 5% NaOH solution for 24 h and then washed	Impurities, wax, and oil residues removed, improved roughness of the fiber
Boumaaza et al. [66]	Sisal, flax, and jute	Soaked the fibers in NaOH at different concentrations for 12 h, afterwards the fiber was washed, and oven dried for 2h at 40 °C	Improved flexural strength, modulus, and displacement
Zhou et al. [67]	Hemp	Soaked about 100 g of fiber into 2% Ca(OH)2 concentration for 14 h at 20 °C, after which the fiber was washed with water and air dried for 48 h at 20 °C	Enhanced surface roughness and adhesion, improved strength, and ductility
Grubeša et al. [68]	Hemp	Applied different treatments using 2.5% and 5% NaOH, and 2% Na2SO3 for 1 hr at 95 °C. After which the fiber was washed with water and oven-dried for 24 h at 70 °C	improved bending and mechanical performance, fire resistance, and lowered degradation temperature
Lazorenko et al. [69]	Flax tows fibers	Soaked fibers in 5% NaOH solution at 25 °C for 1 hr, and ultrasonicate for 0.5 hr. afterwards the fiber was washed with deionized water and then dried at 60 °C for 12 h	Reduced balling, enhanced flexural strength and bonding
de Azevedo et al. [70]	Curauá Fiber	Soaked in 5% NaOH for 0.5 h, then neutralized using HCl solution and washed with water	Enhances the durability and removes impurities in the fiber, improved mechanical compression and tensile strengths of the composite
Tragoonwichian et al. [71]	Sisal	Treated the fiber using 2%, 4% and 6% NaOH concentration for 1, 5 and 24 h. afterwards the fiber was used with water and oven dried at 110 °C for 2 h. The treated fiber was further soaked for 2 h in silane solution. After which the fiber was washed, and oven dried at 110 °C for 2 h	Change in morphology, hydrophilicity, and chemical groups of the finer. Improved fiber adhesion and improved mechanical strengths of the composites
Santos et al. [72]	Piassava Fibers	Used two methods for treatment: (1) stirred the fiber in hot water at 75 °C for up to 24 h, then air dried at 168 ± 2 h. (2) soaked the fiber in 2% NaOH or Ca(OH)2 solutions for up to 400 min, after which the fiber was washed and air dried for 48 h	Both methods removed impurities from the fiber. Treatment using alkaline solutions improved the properties of the fiber more

. From

Table 3. Properties of NaOH treated fiber-reinforced concrete.

References	Natural Fiber	Concentration of NaOH	Outcomes
Kim and Netravali [73]	Sisal	2% for 2 h	Improved tensile strength (21.5%), and Tensile modulus (35%)
Mwaikambo and Ansell [74]	Hemp	0.24% for 48 h	Improved tensile strength, elastic modulus and failure strain by 80.6%, 54.8% and 2.4%, respectively
Zhang et al. [75]	Sisal	5% for 2 h	Improved elastic modulus and tensile strength by 38% and 31%, respectively
Rout et al. [76]	Coir	2% at 300 °C for 1 hr	Improved flexural and tensile strengths by 15% and 26%, respectively
Sreekumar et al. [77]	Sisal	5% for 0.5 h	Improved tensile (36%) and flexural strengths (53%), and elastic modulus (21%)
Dong et al. [78]	Coir	2% at 70 °C for 1 hr	Improved flexural strength, tensile strength and elastic modulus by 25%, 5% and 1.5%, respectively
Mwaikambo and Ansell [79]	Sisal	0.16% for 48 h	Improved elastic modulus and tensile strength by 53% and 64%, respectively.
Tragoonwichian et al. [71]	Sisal	2%, 4% and 6% for 1 hr, 5 hr and 24 h	Decreased tensile strength by 10%. Enhanced flexural strength, flexural modulus and and elastic modulus by 11%, 10% and 42%, respectively

, it can be concluded that the use of alkaline solution i.e., sodium hydroxide (NaOH) is the most used method for treating different natural fibers; this is due to the fact in addition to the removal of chemicals and impurities, NaOH also increases the surface roughness of the fiber which results in enhancement of the bonding between the cement matrix and fiber and hence leading to improvement in mechanical properties of the composite in comparison to when the fiber is not treated [57,58]. Figure 5 shows a microstructural morphology of untreated and treated natural fiber (hemp) at different concentrations of NaOH, where the use of NaOH cleaned and makes the surface of the fiber rougher; however, alkali treatment has been found to reduce the tensile strength of the fiber as shown in Figure 6. Table 2 briefly summarizes the effect of treating natural fibers using NaOH on the properties of the fiber reinforced composites.

2.3. Plant-Based Natural Fibers in Cementitious Composites

Plant-based natural fibers have been used in cementitious composites such as concrete, mortar and geopolymer due to its advantages, availability, and less cost. Several studies reported improvement in concrete’s properties which includes compressive strength, tensile strength, flexural strength, impact resistance, energy absorption capacity, ductility, fracture toughness, reduction of propagation of crack growth and sizes with the addition of fibers, such as sisal fibers [80,81,82,83], coir and coconut fiber [80,84,85,86,87], jute fiber [88,89,90,91,92], basalt fiber [93,94,95,96], kenaf fiber [84,85,97,98], bamboo fiber [99,100,101,102,103,104,105], flax fiber [106,107,108], etc.

Asim et al. [109] studied the effects of natural fibers on the strengths and thermal insulation properties of concrete; they added four different fibers (sugar cane, jute, sisal, and coconut) in four different percentages i.e., 2.5%, 5%, 7.5%, and 10% by weight of cement. Their results findings showed that the thermal conductivity of the concrete decreased with a percentage increment of any type of fiber, with coconut fiber giving the optimum enhancement in thermal insulation with improvement ranging between 6.5–17.4%. In terms of thermal degradation at a temperature range between 30–250 °C, the coconut fiber has the maximum degradation in the composite with about 8.9% of the total mass, followed by jute, sugar cane and sisal; however, within the normal or practical temperature between 30–50 °C, sugar cane and jute fiber reinforced concretes have lower thermal degradation compared to the plain concrete in terms of ultimate compressive strength, addition of 2.5% coconut and jute fibers increased the strengths of the composite by 3.7% and 6.7%, respectively; however, for higher percentage addition of these fibers, and addition of the other fibers, the compressive strength decreased with an increase in fiber content.

Khan et al. [61] examined the effect of coconut fiber addition on the properties of silica fume modified concrete for road applications. The replaced cement with silica fume at 5%, 10%, 15% and 20% by weight; they added 5 cm length coconut fiber in proportion of 2% by weight of cementitious materials, and found that the addition of 2% coconut fiber increases the compressive strength, modulus of elasticity and total energy absorption by 19%, 29% and 31%, respectively, for concrete containing 15% silica fume. Similarly, the splitting tensile strength splitting tensile energy absorption and split-tensile toughness index improved by 20%, 29% and 4%, respectively, for concrete containing 15% silica fume; they further revealed that the thickness of the concrete pavement can be reduced by up to 8% when 15% silica fume is used in combination with 2% coconut fiber in the concrete.

Kesikidou and Stefanidou [62] added three natural fibers in cement and lime mortar. The fibers which include jute, coconut and kelp were added at 1.5% by volume of the mortar, each having 1 cm length. The flexural strength and fracture energy of the fiber reinforced cement mortar improved with addition of any type of the fiber. An increase in flexural strength by 28%, 24% and 16% was observed for kelp, coconut, and jute fibers, respectively. For lime mortar, the flexural strength of jute fiber composite improved three times more than the plain mortar. For coconut and kelp lime mortar, the flexural strength improved by 90% and 77%, respectively, compared to the plain lime mortar. Additionally, after crack formation and fracture, the fiber reinforced mortar still retained its shape stability. In terms of compressive strength, the addition of any type of the three fibers decreased the strength of the cement mortar by up to 15%, which was attributed to the excess porosity caused by the increased mixing water after it has evaporated. For lime mortar, the addition of kelp and jute fibers increased the compressive strength by almost four times, while, with the addition of coconut fiber, a 63% increase was observed. The increase in strength for the lime mortar was attributed to the fact that cellulose rich fibers have works better with lime than lignin-rich fibers.

Castillo-Lara et al. [63] investigated the effect of natural fiber addition in foamed concrete; they added natural fiber from henequen plant (henequen fiber) in proportions of 0.5%, 1% and 1.5% by volume, and reported increased compressive strength, tensile strength and plastic behavior and post cracking ductility of the foamed concrete with addition of the fiber. The plastic behavior and high energy absorption of the foamed concrete with addition of the fiber was attributed to the higher ductile behavior and the toughness of the fiber which gives it the ability to bridge crack and reduce brittleness of the concrete. Wongsa et al. [110] investigated the effect of natural fiber addition on the properties of high calcium fly ash geopolymer mortar, where they incorporated sisal and coconut fibers with varying proportions of 0%, 0.5%, 0.75%, and 1.0% volume fraction; they compared the results of the natural fiber addition with that of synthetic fiber (glass fiber) and plain geopolymer mortar. The addition of both natural fibers and glass fiber decreases the workability of the geopolymer mortar. The flow of geopolymer mortar containing sisal, coconut, and glass fiber ranges between 22–54%, 55–97% and 27–65%, respectively, when compared to the plain geopolymer with 132%; they attributed the decrease in workability with addition of fibers to the irregular stripes, porosity texture and rough surface of the fibers. Furthermore, the splitting tensile and flexural strengths of the geopolymer mortar improved with increase in addition of any of the fiber type, which was due to the higher elasticity and tensile strength of the fiber, and due to stress transfer between the specimen to the fiber through the geopolymer matrix interface. Natural fibers geopolymer exhibit higher flexural strength than glass fibers. The flexural strength of geopolymer with sisal and coconut fibers ranges between 5.3–6.6 MPa, while that with glass fiber range between 3.1–3.7 MPa against 3.1 MPa for plain geopolymer mortar. Okeola et al. [111] incorporated sisal fibers into concrete, where they added different proportions of 0.5%, 1.0%, 1.5% and 2% by weight of cement; they reported improvement in the splitting tensile strength and modulus of elasticity of concrete with addition of sisal fiber; however, they observed decrease in workability, compressive strength, and water absorption of the concrete with increase in sisal fiber addition

Khaleel et al. [112] examined the effect of addition of jute fibers on the mechanical. properties of masonry bricks; they applied a thin layer of mortar to all the surfaces of the masonry prisms, after which they applied jute fibers by hand pressing so that the jute fiber goes into the mortar, then, they applied another layer of mortar to the wrapped fiber to cover it completely. Then, they prepared other composite using different jute fiber thickness, and reported improvement in the compressive strength of the jute fiber bonded prisms by 13% and 28% for the two fiber thicknesses compared to the unreinforced composite. Additionally, the flexural strengths of the fiber composites measured in terms of load carrying capacity improved by about 3.54 and 5.5 times for the two jute fibers thicknesses compared to the unreinforced composite. Furthermore, the energy absorption capacity of the composite significantly improved with the addition of the jute fiber, where there is increase in shear bod strength by 96% compared to the unreinforced composites.

Razmi and Mirsayar [113] studied the fracture properties of jute fiber reinforced concrete; they added 20 mm length jute fibers in proportions of 0.1%, 0.3% and 0.5% by weight of cementitious materials, and reported improvement in fracture toughness with the addition of 0.1% jute fiber by up to 45% compared to plain concrete. Fiber content above 3%, however, does not improve the fracture toughness of the concrete. Additionally, the addition of jute fibers significantly improves the cracking resistance of the concrete by restraining crack growth. The addition of jute fiber increased the compressive, flexural, and splitting tensile strengths of the concrete. The compressive strength increased in the range of 10% to 37% with the addition of 0.1% to 0.5% jute fibers. The splitting tensile strength increased by 5%, 10% and 17% with the addition of 0.1%, 0.3% and 0.5% jute fibers, respectively, compared to the unreinforced concrete. While for flexural strength, there was an increment in the range of 5% to 10% with the addition of 0.1% to 0.5% jute fibers; they attributed the improvement in strengths to the ability of the fiber to retrain crack propagation and growth which resulted to reduced stress concentration due to crack tightening and hence improved strengths.

Zhou et al. [114] investigated the effect of kenaf fiber (KF) on the mechanical performance of high strength concrete; they prepared three concrete grades A, B and C using 0.25, 0.3 and 0.55 water-cement ratios, respectively. For each concrete grade they added KF in proportions of 0%, 1%, 1.5% and 2% by weight of cement; they reported decrease in compressive strength with addition of KF, which was more pronounced in the higher strength concrete (Grade A). There was a reduction in strength with addition of 1%, 1.5% and 2% KF by 19.5%, 30.9% and 46.2%, respectively for grade A concrete, 17.9%, 21% and 36.5%, respectively, for grade B, and 12.2%, 19.5% and 33.7%, respectively, for grade C. The decrease in compressive strength was attributed to the weak bonding and interfacial transition zone between the cement matrix and KF. On the contrary, the addition of KF increased the flexural strengths for all the concrete’s grades. The addition of 1%, 1.5% and 2% increased the flexural strength of grade A by 47.9%, 32% and 30.7%, respectively, for grade B by 66.8%, 56.4% and 48.3%, respectively, and grade C by 46.5%, 47.5% and 45.3%, respectively, compared to unreinforced concrete. The increase in flexural strength was attributed to the bridging effects of the laterally distributed fibers which resulted to slowing cracks development; they further reported improved ductility of the concrete with increased KF content which was more pronounced on the lower grade concrete (A). Additionally, there was significant enhancement of flexural toughness by 0.7–1.7 times more than that of the unreinforced concrete which increased with increment in fiber content.

Gupta and Kumar [115] Investigated the effect of coir fiber addition on the strength and abrasion resistance of nano silica modified concrete; they added different variations of coir fibers (0.25%, 0.5% and 0.75% by weight of fine aggregate) in concrete containing 2% and 3% nano silica with 15% fly ash as SCM, and reported decrease in the concrete’s slump with increase in fiber content. The addition of coir fiber up to 0.5% resulted to increment in compressive strength for all nano silica contents but was more pronounced on concrete containing 3% nano silica. In terms of abrasion resistance, it decreases with increment in coir fiber content for all percentages of nano silica, which implied that coir fiber was not able to fill the voids within the concrete matrix, thereby resulting to lower abrasion. Maier et al. [116] investigated the effect of bamboo fiber (BBF) on the mechanical performance and flexural behavior of fiber reinforced mortar; they added two sizes of BBF i.e., 500 µm and 300 µm in different proportion of 4%, 6% and 8% by volume of concrete. Before adding they treated the BBF by heating it in water for 72 h at 85 °C and then drying for 24 h at 80 °C to reduce the lignin content. After which they subject to BBF to alkaline treatment by stirring it in Ca(OH)₂ (lime) solution for 2 h and then dried for 24 h before applying to the concrete; they reported less decrease in compressive strength with addition of BBF, where 300 µm BBF addition reduced the strength by 7.8–19.9%, and 500 µm decreased strength by 9.1–27%. The reduction in strength was attributed to the lower mechanical properties of the BBF in addition to its aspect ratio influence. The splitting tensile strength also decreases with increase in addition of BBF, which was more pronounced with addition of 500 µm BBF. A reduction in the range of 6.9–31.9% was reported for the split tensile strength. Furthermore, the strain softening behavior, crack bridging effect, post cracking behavior and flexural toughness of the concrete improved with increase in addition of BBF.

Zhou [67] studied the effect of natural hemp fiber (NHF) treatment on the properties of fiber reinforced concrete; they treated the NHF using alkali Ca(OH)₂ before adding to the concrete, and prepared two fiber composites using the treated and untreated NHF of length 15 mm and 1% volume content. The findings revealed that treating the NHF increases its surface roughness which consequently enhanced the adhesion and interfacial bond between the fiber and cement matrix. Furthermore, there was increased in early strength at 7 and 14 days when treated NHF was added compared to untreated NHF. There was also significant improvement in ductile behavior, fracture toughness and energy absorption with addition of NHF, which was more pronounced when treated NHF was added. Figure 7 presents a summary of results for cement and lime mortar containing different natural fibers. Plain cement mortar (P-CM) and plain lime mortar (P-LM) were prepared without any fiber addition. Jute fiber cement mortar (JF-CM) and jute fiber lime mortar (JF-LM), coconut fiber cement (CF-CM) and coconut fiber lime mortar (CF-LM), kelp fiber cement mortar (KF-CM) and kelp-fiber lime mortar (KF-LM) mixes were prepared by adding 1.5% of the fibers by weight of cementitious material. From Figure 5a,b, the addition of any of the type of natural fiber improved the flexural strength and fracture energy for both cement and lime mortar series. On the contrary, from Figure 5c, the addition of any type of the natural fiber decreased the compressive strength of the cement mortar in comparison to the plain cement mortar. While for the lime mortar series, the addition of all the fiber types resulted to improvement in compressive strength [88].

3. Date Palm Fiber Reinforced Composites

3.1. Date Palm Tree

Date palm tree followed the botanical classification as: Kingdom of Plantae; Subkingdom of Tracheobionta; Superdivision of Spermatophyta; Division of Magnoliophyta; Class of Liliopsida; Subclass of Arecidae; Order of Arecales; Family of Arecaceae; Genus of Phoenix L.; Species of Phoenix dactylifera L [117]. Date palm tree belongs to the Angiosperms/monocotyledon (Aracaceae family) consisting of more than 200 genera and 200 species. The date palm tree falls into the phoenix (Coryphoideae phoeniceae) genera with approximately 14 species as shown in

Table 4. Date Palm Trees Species.

Species	Local Name	Location Found
Phoenix Atlantica	Cape Verde Island date palm	Cape Verde
Phoenix Canariensis	Canary Island date palm	Canary Island, Australia, Bermuda, Spain, Italy
Phoenix Acaulis	Stemless date palm	Northern India, Nepal, Bhutan
Phoenix Andamanensis	Andaman Island date palm	Myanmar
Phoenix Caespitosa	Date palm	Saudi Arabia, Oman, Somalia, Djibouti, Yemen
Phoenix Dactylifera L.	Date palm	Arabian Peninsula, California, USA, Pakistan, Spain, North Africa, West Africa, India, Mauritius, Fiji, Australia, Spain, Iran, India, Elsalvador
Phoenix Pusilla	Ceylon date palm	India, Sri Lanka
Phoenix Loureiroi	Mountain date palm	Himalayas, India, China, Indochina, Philippines
Phoenix Paludosa	Mangrove date palm	India, Andaman, Indochuna, Sumatra
Phoenix Reclinata	Senegal date palm	Arabian Peninsula, Africa, Comoros, Madagascar
Phoenix Roebelenii	Pygmy date palm	China, North Ido-China
Phoenix Rupicola	Cliff date palm	India, Bhutan, Andaman Islands
Phoenix Theophrasti	Cretan date palm	Turkey, Greek Islands
Phoenix Sylvestris	Indian date palm	Myanmar, Indian Subcontinent, Southern China

Source: Siddiq et al. [131], Eoin [132], and Al-Alawi et al. [121].

, with only the Phoenix dactylifera which means “finger bearing” cultivated for its fruit [118,119]. Date palm (phoenix dactylifera L) is one of the oldest trees grown mainly for fruit and is mostly cultivated in arid regions of North Africa, Middle East including the Persian Gulf Nations dated back to more than 3,500 years ago [120,121,122,123]. According to another findings, the history of date palm tree cultivation was dated back to more than 7000 years back [118]. Based on archaeological findings and data, the cultivation of date palm tree dated back to 5000–3000 BC near the Gulf of Persia and spread throughout the Middle East [124,125,126,127]. The date palm is one of the few trees that can tolerate high temperature, long period without rain (drought), and salinity; this is why the desert regions consider it as a symbol of life [127]. Date palm trees can withstand and grow in any climatic zone as it has a good resistance to bad climatic conditions. It can grow in regions with temperature ranging between –6 and 50 °C [118]. The average productive lifespan of a date palm tree is 40 to 50 years, with few reaching more than 100 years [128].

The date palm tree is generally dioecious, i.e., have different trees categorize as male (staminate) and female (pistillate). The pollen is produced by the male tree while the fruit is produced by the female palm tree, and the date fruit is produced through pollination which occurs naturally by wind action; however, for commercial production purposes, artificial pollination is normally introduced [119,129]. The main advantage of date palm tree is the fruit production. The date fruit is very sweet and is consumed in fresh, hardened, or processed state. It is a good source of energy to humans and animals when consumed. The average energy for a kilogram of fresh and dry dates are 1570 calories and 3000 calories, respectively. About two-third of date fruit weights is sugar and one-fifth is made of water. The remaining weight comprises minerals, protein, fat, vitamins, tannis and other components, which is why date fruit is beneficial for the human diet [118]. Other beneficial use of date palm to humans includes the production of baskets, mats, and ropes using the leaves. Some parts of the date palm trees are utilized for making paper, production of energy, toxic and heavy metals absorption [122,130]. Figure 8 shows a typical date palm tree and date palm fruit.

There are more than 120 million date palm trees globally, out of which about 67% of it are cultivated in the Middle East and North Africa [133]. Date palm is also found in countries such as India, Pakistan USA (California), Canary Islands, Mexico, southern Africa, South America [128,130].

Table 5. Date Palm Trees Species.

Rank	Country	Production (tons)	Rank	Country	Production (tons)
1	Egypt	1,603,762	11	Libya	174,583
2	Saudi Arabia	1,539,756	12	China	162,041
3	Iran	1.307,908	13	Morocco	129,562
4	Algeria	1,136,025	14	Kuwait	87,391
5	Iraq	639,315	15	Yemen	47,615
6	Pakistan	483,071	16	Israel	43,967
7	Sudan	438,700	17	USA	39,300
8	Oman	372,572	18	Turkey	38,535
9	United Arab Emirate	323,478	19	Qatar	29,404
10	Tunisia	288,700	20	Jordan	25,222

presents the list of 20 top countries in date palm fruit annual production as of 2019. Egypt was the world’s largest producer of date palm fruit followed by the kingdom of Saudi Arabia with production rate of about 15% of the total global date fruit. Furthermore, for Saudi Arabia alone, in the process of growing and harvesting the date palm fruit, about 75,000 tons of waste per year in the form of thorns, foliar and fronds were produced [134,135].

3.2. Date Palm Fiber

Date palm fiber (DPF) is one of the most readily available, cost effective, sustainable, and environmentally friendly natural fiber in many countries (cf. Table 5). The DPF are obtained from the large quantities of the waste generated from the date palm tree which are mostly disposed without proper utilization. DPF has a specific advantage of higher strength-to-cost ratio in comparison to other natural and synthetic fibers [136]. A typical date palm tree is normally trimmed annually to cut down the tree branches and fibers. For a single tree about 10 to 15 branches are removed and more than 20 kg of dry fiber and leaves are generated, which are mostly not properly recycled or utilized, despite the DPF having high content of cellulose, lignin, hemicellulose, and other compounds. [136,137,138,139,140,141,142]. In Saudi Arabia, more than 500,000 tons of date palm waste including the fiber are generated annually, and are mostly not properly utilized [142]. Out of these amounts, about 100,000 tons are waste generated from the date palm leaves and 15,000 tons from the DPF [140,143,144]. The DPF is obtained from the palm tree stem which is covered with a mesh of single fiber. A natural woven mat of intersected fibers of different diameters are formed by the fiber all-round the tree stems/trunk as shown in Figure 9a. The woven mat is conventionally used for making baskets and ropes in several countries after it has been removed from the tress and cleaned; however, only a small percentage of the fiber is used for these applications considering the global generation of the DPF [145,146]. A typical DPF after processing is shown in Figure 9b.

DPF contains high amount of cellulose within the lignin matrix. The cellulose and lignin contents of DPF are similar to that of hemp, sisal and coir fibers, buts its water absorption is less than that of the other fibers mentioned. The lower water absorption of the DPF in addition to its lower density compared to other plant based natural fibers, makes it suitable for use in automotive and polymeric composite applications [148,149].

Table 6. Chemical composition of date palm fiber.

Source Region	DPF Type	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Ash (%)	Wax (%)	Moisture Content (%)	Reference
-	Mesh	46	-	20	-		-	[148,150,151]
Algeria	Mesh	43 ± 2	8 ± 2	35 ± 5	1.2 ± 0.3		9–10	[56]
Morocco	Mesh	50.6 ± 1.3	8.1 ± 0.3	31.9 ± 1.3	6.8 ± 0.2	0.4 ± 0.1	9.1 ± 0.9	[152,153]
Morocco	Leaflets	29.7 ± 1.3	23.3 ± 1.2	11.6 ± 1.3	9.2 ± 0.4	6.8 ± 0.7	5.9 ± 0.7	[152,153]
Morocco	Leaf Sheath	34 ± 0.7	28.9 ± 1.8	18.2 ± 0.7	12.3 ± 0.2	0.5 ± 0.1	8.8 ± 1.1	[152,153]
Morocco	Rachis	39.8 ± 0.9	31.4 ± 3.2	14 ± 0.9	9.2 ± 0.1	1 ± 0.2	7.5 ± 1.4	[152,153]
Iran	Leaflet	40.21	12.8	32.2	10.54	-	-	[154]
Iran	Rachis	38.26	28.17	22.53	5.96	-	-	[154]
Morocco	Leaflet	54.75	15.3	20	1.75			[155]
Saudi Arabia	Leaflet	47.14	36.73	16.13	-	-	-	[156]
Saudi Arabia	Rachis	45.16	28.68	28.16	-	-	-	[156]
Saudi Arabia	Leaflet	46.41	25.89	-	7.91	-	-	[157]
Saudi Arabia	Frond base	40.74 ± 1.9	33.84 ± 2.7	25.45 ± 2.6	7.64 ± 2.0	-	-	[158]
	Leaflet	46.14 ± 2.4	24.59 ± 2.0	29.27 ± 3.3	3.40 ± 0.6	-	-

Presents the chemical composition of some selected date palm tree fibers based on the regions or areas they were obtained as reported by several researchers. Furthermore,

Table 7. Physical and Mechanical Properties of Date Palm Fiber.

Source Region	DPF Type	Length	Diameter (mm)	Spe. Density (kg/m3)	Condition	Tensile Strength (MPa)	Modulus of Elasticity (GPa)	Elongation (%)	Reference
Tunisia	Mesh	100	0.2–0.8	-	Dry	72.34 ± 18	3.15 ± 1.5	8.7 ± 2.2	[159]
					Wet	74.34 ± 15	2.3 ± 2	9.5 ± 2.5
Algeria	Mesh	100	0.1–0.8	1300–1450	Dry	170 ± 40	4.74 ± 2	416 ± 3	[55,160,161,162]
					Wet	175 ± 30	3.78 ± 2	17.4 ± 2	[160,162]
Algeria	Mesh	20	0.1–0.8	1300–1450	Dry	290 ± 20	5.25 ± 3	11 ± 2	[55,160,161,162]
					Wet	300 ± 20	3.55 ± 2	12 ± 2	[160,162]
Algeria	Mesh	100		1300–1450	Dry	88.15 ± 20	3.50 ± 1.2	11.1 ± 2.5	[160]
					Wet	90 ± 18	3.10 ± 1.5	12 ± 3	[160]
Algeria	Mesh	100	0.1–0.8	1300–1450	Dry	72.34 ± 18	3.15 ± 1.5	8.7 ± 2.2	[160]
					Wet	74.34 ± 15	2.30 ± 2	9.5 ± 2.5	[160]
Algeria	Mesh	100	0.1–0.8	1300–1450	Dry	71.15 ± 16	2.50 ± 1	7.5 ± 2.3	[160]
					Wet	73.19 ± 13	2.10 ± 1	8.5± 2.7	[160]
Qatar	Leaf	-	-	900	Dry	100	6.2	1.4	[163]
Algeria	Mesh	10–40	0.1–1	1089	Dry	107.36	2.57	4.50	[164,165]
Lebanon	Leaf	-	0.2–0.3	600–800 (bulk)		90–110	4.5–6.5	1.5–2	[60]
Morocco	Mesh		0.389	979	Dry	12.9 ± 0.5	1.26 ± 0.34	-	[152,153]
Morocco	Leaflets		0.374	1011	Dry	8.4 ± 2.7	0.93 ± 0.4	-	[152,153]
Morocco	Leaf Sheath		0.354	881	Dry	6.1 ± 1.5	0.43 ± 0.13	-	[152,153]
Morocco	Rachis		0.426	984	Dry	8.5 ± 2.9	0.93 ± 0.25	-	[152,153]

presents the chemical properties of the DPFs

3.3. Date Palm Fiber Utilization in Cementitious Composites

Date palm fiber (DPF) has been used in different cementitious composites, such as concrete and mortar as reported by several studies. Other DPF reinforced composites are gypsum and clay bricks composites.

3.3.1. DPF Reinforced Mortar

Benmansour et al. [166] studied the effect of DPF as an insulating building material in cement mortar; they prepared different cementitious composites using three different fiber sizes based on diameter. The DPF sizes were 3mm, 6mm and a combination of the 3mm and 6 mm diameters; they prepared different mortar mixes using DPF concentrations of 5%, 10%, 15%, 20%, 25% and 30% of the total weight of the composites. The findings showed that the density and thermal conductivity of the mortar decreases with increases in DPF contents; this decrement is more pronounced on the lower sized DPF. Similarly, the compressive strength of the mortar decreased with increase in DPF. At 5% DPF content, there was a decrease by 92%, 91.9% and 95% for 3 mm DPF, 6 mm DPF and their combinations, respectively; however, they achieved acceptable strengths at lower concentrations of DPF less or equal 15% which can be used for structural applications and improved thermal insulation. Boukhattem et al. [167] produced binder less board using DPF, and in another mix produced cement composite (mortar) using DPF with varying percentages from 0 to 51%; they washed the DPF with high pressure water to remove impurities and then oven dried it at 70 °C until it was completely dried. Their findings showed that DPF reduced the unit weight of the cement composite, where for 51% DPF addition, a reduction of 30% and 39% in fresh and hardened densities, respectively, were reported. Furthermore, DPF increased the water absorption and porosity of the mortar, where for 51% DPF addition an increment in porosity by 71% was recorded. Thermal conductivity of the mortar was also reported to decrease with addition of DPF, where a decrease by 42% and 70% at saturated and dry state, respectively, were reported. In a similar study, Boumhaout et al. [168] investigated the effect of DPF addition on the mechanical properties and thermal insulation of mortar; they varied the DPF from 0% to 51% by volume of the material, and washed the DPF using high pressure water and then sun-dried it for 48 h and then oven-dried it at 70 °C to remove impurities. The findings showed the ductility of the mortar increased with increment in DPF content as shown in Figure 10; however, the flexural and mechanical strengths of the mortar decrease with increment in DPF addition. A reduction of up to 63% and 81.5% were recorded for flexural and compressive strengths, respectively, at 51% DPF. Additionally, the thermal conductivity and diffusivity of the mortar decreases with increase in DPF addition. For 51% DPF addition, a reduction by 70% and 52% were reported for thermal and diffusivity conductivities, respectively. In a similar work, Haba et al. [169] studied the thermal and hygric properties of cement mortar reinforced using 15% DPF by weight as a new insulation building material; they also reported increased porosity and water absorption by up to 58% and 62%, respectively, with addition of 15% DPF. The thermal conductivity of the mortar was reported to decrease with the addition of the DPF. Their conclusions revealed that DPF concrete is highly permeable to water vapor due to its high porosity. Additionally, the DPF mortar was classified as type II based on its sorption isotherm curve which was characterized as a macro-porous adsorbent material. Lastly, they concluded that DPF mortar can be used as a thermal insulation material. Khelifa et al. [53] studied the effect of DPF volume, length, and treatment (NaOH concentration and immersion time) on the mechanical properties of mortar; they designed the experiments using response surface methodology (RSM), where they used the following variables: fiber content (1%, 3% and 5% volume), fiber length (5mm, 10mm and 15 mm), NaOH concentration (1%, 3% and 5%) and immersion time (2, 8 and 14 h). The concentrations of NaOH and immersion time were used for treating the fibers before applying into the mortar. After treating with NaOH, the DPF were immersed into 0.5% sulfuric acid and then washed with deionized water to achieve a neutral pH. Their findings showed that the bending stress and modulus, compressive strength and modulus increases or decreases depending on the combinations of the variables; they achieved the highest flexural strengths using 1% fiber with 5 mm length, when treated using 3% NaOH for 8 h; this showed increase in flexural stress and modulus of 27.5% and 18.9%, respectively, compared to the control mortar. The highest compressive strength was achieved using 1% fiber with 10 mm length treated using 5% NaOH for 8 h; this gave an increase by 46.6% and 36.3% for compressive stress and modulus, respectively, compared to the control mortar. On the contrary, the lowest flexural properties were obtained using 5% DPF of 15 mm length treated using 3% NaOH for 8 h, giving a reduction by 35% and 82.1% for bending stress and modulus, respectively. In terms of compressive properties, the lowest values were obtained using 5% DPF with 10 mm length treated using 3% NaOH solution for 2 h, achieving a reduction by 37.5% and 62.5% for compressive stress and modulus, respectively.

Vantadori et al. [55] and Benaimeche et al. [54] studied the effect of DPF on the flexural performance mortar; they added 0%, 2%, 4%, 6%, 8% and 10% by volume. The findings showed that DPF decreased the density, flexural strength, and fracture toughness of the mortar. Flexural strength decreased by 9%, 17% and 52% with the addition of 2%, 4% and 10% DPF, respectively; they attributed the decrease in flexural strength to high porosity resulting from fiber addition and lower modulus of elasticity of the fiber compared to the mortar. The fracture toughness decreased by 7%, 32% and 66% with the addition of 2%, 6% and 10% DPF, respectively; they attributed this decrease to the poor bonding between the fiber and cement matrix. On the contrary, they reported enhancement in ductility with the addition of fiber. There was an increase from 27% to 162% with the addition of 2% to 10% DPF. In similar research, Benaniba et al. [170] also investigated the effect of DPF on the mechanical properties of bio-composite mortar; they added concentrations of 6%, 12%, 18%, 24%, and 30% of DPF weight, and reported increase in water absorption with increased DPF content, where this increment to the hygroscopic nature of the fiber. The thermal conductivity of the mortar decreased with increment in fiber content, where addition of 24% DPF reduced the thermal conductivity by 92%. There was improvement in flexural strength with the addition of up to 12% DPF, where the optimum DPF content was 6%, and this improvement to the crack bridging effect of the fiber; however, the addition of DPF above 12% resulted to significant reduction in flexural strength. Additionally, the compressive strength of the mortar decreased with increment in addition of DPF; they attributed the decrease in strength to increase in porosity and poor fiber distribution in the cement matrix. Ali-Boucetta et al. [56] examined the effect of different treatment methods of DPF on the fresh and hardened properties of mortar; they applied three treatment methods namely NaOH, boiling water and linseed oil treatments to the DPF before adding 2% DPF by volume of sand. For the NaOH treatment, they used different concentrations of NaOH (1%, 3%, 6% and 9%). For boiling water treatment, they boiled the DPF at different boiling times (5 min, 1 h, 2 h and 3 h). For linseed oil treatment, they used different ratios of linseed oil/DPF (0.25, 0.50, 1 and 1.5) for the treatment. The water absorption of the DPF and the setting time of the DPF mortar were all reduced for all kind of treatment, with the linseed oil treatment method giving the most significant reductions. For water treatment, boiling for 1 h or more led to reduction in water absorption of the fiber, which was due to partial depolymerization and dissolution of lignin. For NaOH treatment below 9% concentration also reduced the water absorption of the fibers, where the lowest absorption was reported using 3% NaOH. For linseed oil treatment, the reduction in water absorption was proportional to the amount of oil added to the DPF. The tensile strength of the DPF was improved with boiling for 3 h and treatment using 3% NaOH; however, linseed treatment does not contribute to tensile strength improvement of DPF. Furthermore, the flexural strength and compressive strength of the mortar improved with fiber addition, and further improved with the fiber treatments. The flexural strength was enhanced at 28 days by 60.1% and 9% and compressive strength by 19.9% and 11% for NaOH and boiling water treatments, respectively, compared to mortar containing untreated DPF; however, linseed oil treatment decreased the flexural and compressive strengths strength of the DPF mortar; they attributed the improvement in flexural strength to the change in morphology, increased roughness, and fineness of the fiber due to treatment, which resulted to improved bonding between the cement matrix and fiber, hence improved strengths.

3.3.2. DPF Reinforced Concrete

Kriker et al. [160] investigated the effect of different DPF types on the physical and mechanical properties of concrete; they selected the best DPF from male palm, Deglette-Nour, Degla-Bida and Elguers palms for use in concrete, and prepared mixes using of the best DPF, where they added the fiber at 2% and 3% by volume. The fiber length used were 15 mm and 60 mm; they found that the male palm DPF has the highest tensile strength and elongation compared to the other fibers and hence was selected for producing the DPF reinforced concrete. From their result findings, the compressive strength at all age decreased with increase in fiber addition and length as shown in Figure 11; however, increase in fiber length and percentage enhanced the ductile behavior of the concrete. The flexural strength which was measured in terms of load-deflection also decreased with increase in fiber volume. At 28 days, the first crack load for 3–60% mm DPF concrete was lower than that of control concrete by about 65% as shown in Figure 12; they attributed this decrease to mediocre mechanical properties of the DPF and the fiber–cement paste interface strength adhesion, and recommended DPF to be treated before use in concrete.

Djoudi et al. [165] studied the effect of DPF on the thermal properties of gypsum (plaster concrete); they added DPF at dosages of 1%, 1.5% and 2% by volume using different fiber lengths of 2cm, 3cm and 4cm. The thermal conductivity increased with increase in the fiber length and decreased with increment in fiber volume due to escalation of void content in the cement matrix. The best insulating concrete material was achieved using 2% DPF with 2 cm length with reduction in thermal conductivity by about 40%. Additionally, the specific heat capacity increased while the thermal diffusivity of the concrete decreased with increment in fiber content and decrease in fiber length, which was attributed to the alveolar structural behavior of the DPF which was opposite to heat flow. Lastly, they reported based on microstructural evaluation that a good bonding existed between the hydrated gypsum crystals, calcite layers and the DPF cells. Alatshan et al. [171] investigated the effect of DPF as alternative to conventional fibers on the mechanical properties of concrete; they added the fibers at dosages of 0.5, 1, 1.5, 2 and 2.5% by mass. The fiber length added were 5cm, 6cm and 7cm; they reported a decrease in workability and compressive strength with increment in fiber dosage and length; however, they observed increase in compressive strength with addition of 0.5%-5cm DPF which was higher than that of the plain concrete. Additionally, they reported increase in flexural strength with addition of DPF and length; they attributed the increase in flexural strength to the tensile and ductile behavior of the fiber.

3.3.3. DPF Reinforced Gypsum Composites

Al-Rifaie and Al-Niami [172] reported the influence of DPF on the performance of gypsum as a low-cost construction material. The composites were produced using gypsum and DPF; they first premixed the gypsum with water then added DPF to the plastic slurry and mixed. After which they fabricated panels of sizes 500 × 500 × 20/25 mm using the composites for testing. The variables considered were water-to-gypsum ratio (w/g) and fiber content. The proportions for w/g were 40%, 60% and 80% while for DPF were 0%, 2%, 4%, 6%, 8% and 10%; they reported decreased workability with increment in DPF content in the composite. The maximum compressive strength was achieved with the composite containing 4% DPF, after which the strength reduces with increment in fiber content which they attributed to the lower density and strength of the fiber. DPF increases the plastic modulus of rupture and impact resistance of the composite. The highest improved property of the gypsum composite with addition of DPF is its impact strength with about 50% improvement. Braiek et al. [173] developed a gypsum/plaster composite with DPF for reduction in energy consumptions in buildings, where the DPF-gypsum boards can be used as substitute to plaster boards as building insulation materials; they studied the effect of the DPF on the thermal properties of the gypsum composites where they added different proportions of DPF of 5%, 10%, 15% and 20% by weight. Their findings showed that the density of the composite decreased with increment in DPF content, where a reduction of up to 44% was reported with the addition of 20% DPF, this gives the composite advantage of seismic resistance and lower handling cost. Additionally, the thermal conductivity, thermal diffusivity, thermal effusivity and capacity of the composites all decreases with increment in DPF content. With the addition of 20% DPF to the composites, there was a reduction by 61.5%, 39.58%, 50.5% and 36.22% for thermal conductivity, diffusivity, effusivity and capacity, respectively. Therefore, the recommended the addition of up to 20% DPF in gypsum composites for improved and better thermal properties for a sustainable building

Chikhi et al. [174] investigated the effect of DPF on the mechanical properties, thermal conductivity and water absorption of gypsum based composite material; they utilized hemihydrate gypsum (HG) for producing the composite using two different DPF sizes of 3 mm and 6 mm diameters added at proportions of 0%, 1.2%, 3%, 5%, 7%, 8% and 10% by weight. Their results showed the water absorption and saturation time of the composite at any time increased with increment in DPF content. Furthermore, Gypsum composites containing smaller sized DPF had higher water absorption compared to the larger sized DPF composite. In terms of mechanical properties, the compressive and flexural strengths of the composite decrease with increase in fiber content., with the 3mm-DPF composites having higher strengths compared to the 6mm-DPF composites. The addition of 1.2% DPF decreases the compressive strengths of the 3mm-DPF and 6mm-DPF composites by 58% and 45%, respectively, at 14 days, and 15% and 4% at 28 days, respectively. While with 1.2% DPF addition, the flexural strength decreased by 45% and 31% for the 3mm-DPF and 6mm-DPF composites, respectively, at 14 days, and by 33% and 26%, respectively, at 28 days. With regards to thermal properties, the addition of DPF to the gypsum composites decreases the thermal conductivity of the composites, where with the addition of 10% DPF, there was a reduction in thermal conductivity by 62% and 66% for 3mm-DPF and 6mm-DPF composites, respectively. Finally, they recommended that a good gypsum composites with high mechanical and thermal properties and for use for thermal insulation in building can be produced using 5% DPF [174]. In a similar study by Chikhi [175], he studied the effect of DPF addition on the elasticity and thermophysical properties of gypsum composites. He added two sizes of DPF (3mm and 6 mm diameter) in proportions of 0%, 1.2%, 3%, 5%, 7%, 8% and 10% by weight. The porosity of the gypsum composites was affected negatively with the addition of both sizes of DPF. with regards to the effect of DPF sizes on the porosity of the gypsum composite, there was no trend as shown in Figure 13. He attributed the increase in porosity of void formation at the interface regions between the gypsum matrix and fiber. He further reported increase in modulus of elasticity of the composite with increase in DPF content at 28 days, however at 14 days the modulus of elasticity decreases with increased DPF content. The stress–strain, ductile behavior and insulation properties of the gypsum composite improves with increment in fiber addition.

3.3.4. DPF Reinforced Clay Brick Composites

DPF have been used in the production of clay mortars for improved thermal conductivity and heat resistance due to its better thermal properties. Mekhermeche et al. [176] produced a brick using a combination of clay, sand and DPF; they varied the percentage of DPF between 0% to 3% and sand dune between 0 to 40% by weight of the materials. Their results revealed that the thermal properties of the clay brick improved with increase in the fiber and sand dune content; this was measured through reduction in specific heat capacity, thermal diffusivity and effusivity, and thermal conductivity. For 3% DPF addition, they reported a decrease by 6%, 57% and 6% for specific heat capacity, thermal conductivity, and density, respectively. In a related study, Hakkoum et al. [177], also produced clay brick using clay, sand dune and DPF, where they kept the sand dune at 30% and varied DPF between 0% to 3% by weight of material; they reported similar results i.e., improvement in thermal properties, however, they observe reduction in compressive and flexural strengths in the composites with increase in DPF addition. [178] also developed clay bricks using clay, sand and DPF; they varied the sand at 20% and 30% by weight of materials, and DPF at 0%, 1%, 2% and 3% by weight of Material, and reported a decrease in thermal conductivity and specific heat capacity with increment in DPF and sand content in the composite. In terms of thermal conductivity, for the clay composite containing 30% sand, they found a reduction by 7.04%, 18.3% and 32.4% for 1%, 2% and 3% DPF contents, respectively. While for the composite with 20% sand, they reported a decrease by 1.6%, 11% and 28.13% for 1%, 2% and 3% DPF contents, respectively. Furthermore, they reported increased thermal resistance of the composite with increment in DPF content. For the composite containing 20% sand, they reported increase by 2.9%, 5.9% and 16% for 1%, 2% and 3% DPF contents, respectively. While for the composite with 30% sand, they recorded an increment by 2.3%, 8.1% and 9.2% for 1%, 2% and 3% DPF contents, respectively.

4. Concluding Remarks

Recent studies and research work are continuously ongoing on the use of DPF as a natural fiber in different composites such as concrete, mortar, gypsum composites, clay composites etc. In this study, the available, and known to the Authors, literature on DPF have been reviewed, and the following conclusions were derived.

• DPF is one of the most available natural fibers globally, as the number of date palm trees exceeds 120 million, out of which about 67% of it are cultivated in the Middle East and North Africa; this makes DPF readily available as a waste material for use in Fiber-reinforced composites with no processing cost.
• Due to its availability, zero cost, very low processing cost, sustainable, and environmentally friendly, DPF is continuously gaining acceptability as fiber material usually as substitute to synthetic and other organic fibers.
• DPF has been used in different composites, such as concrete, mortar, gypsum composites, clay composites and bricks and have been found to significantly improved the mechanical properties such as tensile strength, flexural strength, and impact resistance.
• DPF have been found to be a good insulation material, when used in composites, such as mortar or gypsum they were reported to significantly improve the thermal properties of buildings thereby leading to efficient energy saving which consequently reduced the cost of running and maintaining the buildings, as the DPF is a waste material with no processing cost.
• DPF have been reported to significantly enhance the toughness, ductility and bending strengths of composites, such as concrete, through crack bridging effect, delaying crack growth and propagation and reduction in stiffness; this gives it an advantage for use in structures mostly subjected to bending loads such as beams and columns, where it helps in delaying crack propagation and preventing catastrophic failures.
• DPF has been reported to decrease the density of composites, such as concrete, mortar, gypsum, or clay; this gives it the advantage for usage in areas prone to seismic effects, and when the DPF composite is used for buildings, the overall weight of the building is expected to reduce hence reduction in foundation cost.
• However, studies had shown that the use of DPF in cementitious composites, such as concrete and mortar resulted to decrease in compressive strength, which is the major property of concern. Additionally, DPF have been reported to decrease the durability performance of composite by increasing porosity in the matrix; this happens mostly due to poor adhesion between the composite matrix and the DPF, and due to increased porosity in the composite caused by the fiber.
• Therefore, for DPF to be efficiently used in composites especially cementitious composites such as concrete, which is mostly used for structural applications, methods must be devised to diminish or lessen the negative effect of the DPF on the properties of the concrete. Some studies used different treatment methods to remove impurities and improve the roughness of the fiber for enhanced bonding, thus it slightly mitigated the strength loss in the composite due to DPF addition, but it is not yet very effective.
• Hence, future research should focus on developing ways to improve the mechanical and durability performance of DPF-reinforced composite through developing ways to lessen or mitigate the harmful effect of DPF on the cementitious composite’s performance for proper acceptability and utilization.