# Degradation Kinetics and Durability Enhancement Strategies of Cellulosic Fiber-Reinforced Geopolymers and Cement Composites

Authors: Lulseged Belay Addis; Zenamarkos Bantie Sendekie; Neela Satheesh
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

The need to overcome the low energy-absorbing response and low tensile strength of geopolymers and cementitious materials has motivated researchers to develop fiber-reinforced composites. Interestingly, natural cellulosic fibers provide exciting prospects for advancements in bio-based materials compared to synthetic fibers. Incorporating fibers in such binders offer a crack limiting effect by creating multiple cracks than forming one major crack, usually leading to improved toughness and tensile strength of the composites. However, their sensitivity in alkaline media and environmental fluctuations deteriorates their durability and reinforcing capacity in composites. Therefore, exploring suitable mitigation strategies for such drawbacks of natural fibers based on previous research work and figuring out the existing gaps for further developments is extremely important. In this paper, cellulosic fiber degradation in geopolymer and cement-based matrices and the respective remedial actions have been reviewed based on a comprehensive analysis of the available literature. The influence of fiber modification and matrix treatment on the durability and strength characteristics of such composites is thoroughly discussed. Finally, recommendations and suggestions for further research in the field are presented.


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## Body

## 1. Introduction

Both geopolymer [1] and cement-based materials [2, 3] exhibit usually brittle failure modes and limited tensile strength properties. The relatively low tensile strength and fracture toughness of those materials can lead to early cracking under thermal or mechanical loads [4, 5]. The phenomenon further aggravates the sudden failure of this type of material on account of the growth and propagation of cracks under various loading conditions [5]. According to Khan et al. [6], fibers can prevent further propagation of cracks and may act as a sewing effect that appears to hold the matrix together, attributing a positive impact on the mechanical properties of the composite. Often, synthetic and natural fibers are incorporated into these brittle materials as a remedy to limit crack propagation, improve their fracture toughness, and enhance their post-cracking behavior [7–9], although the application of natural plant-derived fibers in geopolymers is a relatively new approach [5]. In the last few decades, vegetable fibers have gained increasing attention in comparison to their synthetic counterparts. This is due to economic advantages and being hazard-free to the environment even at the end of their useful life cycle [10], besides offering, under some conditions, comparable mechanical and physical properties to be used as reinforcement in composites [10–14]. Thus, the trend towards the exploitation of bio-based fibers as reinforcement for composites is an exciting prospect in line with the ongoing environmental consciousness and the need for the development of sustainable energy-efficient materials [8, 15, 16]. Many investigations have shown that the use of plant-derived fibers in geopolymers and cement-based materials as a reinforcement leads to improved post-cracking toughness, energy absorption properties, impact resistance, and flexural strength. Alomayri et al. [17] investigated the use of cotton fibers in fly ash-based geopolymer matrices and noticed that various energy-absorbing mechanisms, such as fiber pull-out, fiber-bridging, and fiber fracture, contributed to achieving a significant improvement in the fracture toughness of the geopolymer. The inclusion of an appropriate fiber content also allows good fiber dispersion throughout the matrix which in turn makes provision for a better transfer of load from the matrix to the fiber, giving rise to enhanced flexural strength of the geopolymer composites. It was noticed that further increasing the fiber content beyond a certain level deteriorates the mechanical properties via fiber agglomeration and crack initiation by void formation [17]. Fibers can bridge cracks, restrain their width, and delay the propagation of cracks so that they can improve the post-elastic energy absorption properties of composites [1]. Furthermore, da Silva Alves et al. [18] reported that the use of sisal fibers improved the toughness and modulus of elasticity of metakaolin-based geopolymer binders. Similar results have been reported by different research groups [19, 20] using geopolymer precursors as a binding agent. Analogous to geopolymers, cementitious matrices have shown a remarkable improvement in fracture toughness, crack resistance, and strength characteristics when reinforced with natural cellulosic fibers [21, 22]. With an appropriate mix design, the cement matrix cracking behavior becomes halted because of the fiber-bridging effect [23]. According to Li et al. [21], the energy absorption modes and failure patterns of cementitious composites depend on the magnitude of the impact energy. For higher impact energy, fiber delamination and fiber breakage are exclusively responsible while fiber debonding is associated with low impact energy. Li et al. [24] noticed that coir fiber–reinforced cement mortar specimens displayed greater deformation before rupture and offered improved loading support even after the onset of cracking. Furthermore, various authors [5, 17, 25–27] noticed that the fragile type failure mode of geopolymer binders and cementitious matrices could be changed to a quasi-ductile type performance with greater energy-absorbing power and better resistance to deformation by introducing cellulosic plant fibers. Even if such improvements in mechanical properties were achieved, the long-term service life of such lignocellulosic fibers in geopolymer and cementitious-based binders has remained a critical aspect for a broader application and wider acceptance of such composites. In the literature, although several important studies and promising results have been presented, there are gaps in finding compiled information related to this research area; to the authors knowledge, no review paper is available to date discussing the durability of natural plant fiber–reinforced geopolymer and cement-based composites. In complement to this, Camargo et al. [11] had reported a comprehensive review on the treatment methods for natural fibers and the associated improvements obtained in connection with fiber-matrix interface adhesion, which in turn makes provision for further improvements in mechanical strength characteristics of geopolymer and cement-based composites. However, durability aspects were not covered in that review. This paper, therefore, presents a critical assessment of current research studies focusing on fiber modification and matrix treatment strategies along with their effects on the durability of natural fiber-reinforced geopolymer and cement-based composites. This review paper comprises five main sections. The first section highlights the significance of this review article. Section two presents cellulosic fiber as reinforcement agents in polymer composites and their application areas. Degradation kinetics of polymeric composites and the various factors that cause the deterioration of cellulosic fibers within geopolymer and cementitious binders are discussed in section three. The mitigation strategies to minimize the loss of fiber reinforcing capacity are discussed in detail in the following section. Finally, in the last section, conclusions and remaining research challenges and opportunities are presented.


## 2. Significance of the Paper

This review article deals with the durability of lignocellulosic fibers that are incorporated in geopolymer and cement-based binders. It focuses on a critical assessment of the various factors such as physical, mechanical, biological, chemical attacks, or a combination of them that cause the deterioration of plant-derived cellulosic fibers within polymeric binders. The review considers a comprehensive analysis of the available literature on current research studies focusing on fiber modification and matrix treatment strategies and the corresponding effects on the durability and strength characteristics of such composite materials. The review article also identifies research gaps for further developments and advancements in the field.


## 3. Cellulose Fibers Used as Fillers for Geopolymers and Cementitious Matrices

The exploitation of natural-based materials such as cellulosic fibers as reinforcement agents in polymeric matrices dates back to ancient times. For instance, the Egyptians construct their houses by reinforcing mud with straw [14]. Later with the advent of man-made fibers such as carbon, Kevlar, aramid, and glass fibers, the application of natural fiber was greatly limited or even abandoned in the composite market [28, 29]. As a result, the synthesis of high-performance synthetic fiber–reinforced polymeric materials increased significantly. However, the treatment and removal of these materials at the end of their useful life cycle have turned into a problem [28]. The environmental pollution generated from the solid wastes of these materials has become a serious issue Thus, the growing interest in the use of biodegradable materials by end-users and the implementation of clean development mechanisms have prompted renewed interest in bio-based materials [29, 30]. As a result, the application of plant fibers in the manufacture of composites as reinforcement material is being widely implemented instead of synthetic fibers, mainly in the automobile sector, sports tech industry, aerospace engineering, and construction sectors [31–33]. This is mainly due to the natural fiber’s ability to deliver economic benefits, reduction of carbon footprint, and their ability to offer, in many cases, comparable mechanical properties to those of synthetic fibers [10, 12, 13]. Other important benefits include lightweight, low cost, abundant availability, easily recyclability, and less abrasiveness in tool wear during processing [34, 35]. The incorporation of fibers in polymeric binders as fillers can improve not only the tensile and flexural properties but also their fracture energy [36]. Besides these, fibers can act as the main source of strength while the matrix material glues the fiber together in order to provide the desired shape and to transfer stress to the reinforcing fibers [14]. In this respect, sisal, flax, kenaf, hemp, jute, pineapple, ramie, banana, coir, and sugar cane fibers are the most common cellulosic fibers that have been extensively incorporated in geopolymeric, cement-based, and plastic composites [16, 37–41]. For instance, coir and sugar cane fibers are used as a composite formation with Portland cement matrix [42]. Kenaf fiber is incorporated to produce high-strength cement composites [23]. Coir and sisal fibers are used to produce fly ash-based geopolymer composites [7]. Sisal and pineapple leaf fibers were successfully implemented in metakaolin-based geopolymeric binders [43]. Jute and sisal fibers have been efficiently implemented in cement-based matrices [44]. Hemp fiber is used to produce cement paste composites [45]. Flax woven fabrics have been incorporated into polyester resin to manufacture bioactive composites [46]. Flax fibers are used as reinforcement additives in fly ash and fired clay brick powder–based geopolymer binders [47]. Therefore, it can be concluded that cellulosic fiber composites can be applied in pretty much every field of life.


## 4. Degradation Kinetics of Vegetable Fiber-Reinforced Polymer Composites

### 4.1. General Considerations

The durability of cellulosic fiber–reinforced polymer composites is a measure of its resistance to any process of degradation. The deterioration might have external causes (like weathering action, alkaline attack, etc.) and/or internal ones (such as fiber-matrix interface zone compatibility and volumetric changes of the fiber) [42, 48]. Thus, cellulosic fibers become embrittled at various degrees of degradation when immersed in cement/aluminosilicate precursors as the fibers are exposed to both internal and external causes of deterioration. It may be degraded either due to physical, biological, mechanical, chemical attacks, or a combination of them. Under the influence of key environmental factors, like ultraviolet radiation, temperature, air, moisture, mechanical stresses, chemical agents, and microorganism activities, cellulosic fiber-reinforced composites show a higher risk of deterioration, particularly when exposed to outdoor applications. This is more related to the properties of natural fibers which are vulnerable to biodegradation. Plant fiber–reinforced polymer composites can be decomposed to CO2 and H2O when exposed to weathering [14, 49, 50]. Another important fact about the degradation of plant-based fiber-reinforced polymer composites is emanated from the main chemical composition of the fiber itself which is vulnerable to biodegradation and the presence of voids in it [51]. Plant fibers typically comprise cellulose, hemicellulose, and lignin as the main components. Other components include wax, pectin, oil, and various water-soluble constituents [52]. Cellulose is a network of fine threadlike crystalline micro-fibrils embedded in a matrix of amorphous phases (such as hemicellulose and lignin) that offers structural support to the fiber [32, 53, 54]. According to Bhatia et al. [55], the highly complicated arrangement of bundles of cellulose micro-fibrils gives rise to cellulose fibers more resistant to chemical and biological attacks. Even though it is degradable to chemicals to some extent, cellulose is highly resistant to hydrolysis, oxidizing agents, and strong alkalis [56]. On the other hand, hemicellulose is highly hydrophilic, can readily hydrolyze in weak acids and bases, easily degrade when exposed to thermal and biological attacks [57, 58], and is highly flammable [50, 56]. Lignin is hydrophobic and highly resistant to most microorganism attacks and acid hydrolysis but susceptible to ultraviolet degradation [57, 59]. The presence of voids in plant fibers also promotes the absorption and penetration of more moisture into fiber cells from the surrounding environments, facilitating the degradation of polymer composites. For instance, according to the study conducted by Manfredi et al. [60], flax fiber exhibits a relatively higher thermal stability than jute and sisal fiber, as flax contains a lower amount of lignin. Methacanon et al. [58] studied the properties of reed, sisal, water hyacinth, and roselle fibers for geotextiles application. It was noticed that reed and water hyacinth fibers contain higher hemicellulose content, leading to thermal degradation at low temperatures and absorbing more moisture as compared to roselle and sisal fibers. As mentioned previously, one of the prominent challenges in using natural fibers as filler material in polymer composites is their inherent hydrophilic properties. According to Azwa et al. [56] and Väisänen et al. [51], water penetrates the fiber cells and forms a hydrogen bond which in turn creates inferior interfacial compounding with the matrix material when exposed to moisture. The degradation process started to occur as a result of the development of micro-cracks due to the stress developed via frequent swelling and shrinkage of cellulosic fibers. In addition, Methacanon et al. [58] and Chen et al. [61] explained that high moisture absorption by the natural fiber also creates a favorable situation for biodegradation (microbial attack) of the polymer composites. When more water is absorbed by the fiber cells, bound water starts to increase while free water declines catalyzing the leaching of water-soluble constituents from fibers. As a result, ultimate fiber debonding and delamination from the matrix material is experienced. As per Joseph et al. [62], the development of osmotic pressure at the fiber surface has initiated fiber-matrix detachment. Moisture absorption phenomena of materials follow either Fickian or non-Fickian behavior depending on whether the surrounding temperature is low or high, respectively [62]. According to the authors, the non- Fickian moisture absorption process has induced substantial hygro-thermal stresses in polymer composites. The moisture uptake of composites has become accelerated with temperature and their saturation time is highly reduced since temperature promotes the diffusion process significantly, in effect causing the peeling and surface decomposition of composites [56, 62]. Besides this, the moisture uptake capacity of natural fibers is also associated with the relative humidity (RH) of air. In this regard, Methacanon et al. [58] has studied the moisture absorption properties of sisal, reed, water hyacinth, and roselle fibers at a different relative humidity of the air. The authors observed that the moisture absorption property of the fibers increases with increasing RH. For instance, water hyacinth absorbs moisture almost 8 times higher at 97% RH than that it absorbs at 75% RH. Other fibers show a 4% increase in moisture absorption when the RH has been increased to 97%. Another important challenging issue that prohibits the use of natural fibers as potential reinforcement is their limited thermal stability. Most natural fiber has a limited processing temperature which is restricted to around 200°C [51]. Exposure of natural fiber-reinforced polymer composites in direct sunlight also causes a significant challenge impeding their application in structural composites. Ultraviolet (UV) radiation causes the breakdown of covalent bonds in organic polymer composites, leading to color fading, weight loss, yellowing, mechanical strength reduction, surface roughening, and so on [63, 64]. The color change observed is an indication of the chemical changes occurring on the surface of the composites that gives a clear picture of the degree of deterioration [56]. A study performed by Joseph et al. [62] on the degradation behavior of sisal fiber–reinforced polypropylene composites confirmed that the tensile strength of the composite deteriorates with increasing UV radiation exposure. This is attributed to chain scission and degradation via photo-oxidative reactions prompted by UV radiation. In addition to this, since the cementitious binder itself is a source of alkaline media with a pH value exceeding 13 [65, 66], it induces loss of fiber reinforcing capacity in different ways. The high alkaline environment generated within the binder during composite processing in both fresh and hardened states causes the dissolution of lignin and hemicellulose encrusting the fibers with mineral deposits, in consequence weakening the strength of the fiber and initiating micro-cracks in the paste matrices [67]. As one would expect, the initiation of matrix micro-cracking is another root instigating composite degradation [65].The leaching of hemicellulose and lignin depletes their reinforcing capacity that needs to be addressed first before approving the use of vegetable fibers in structural composites. Moreover, construction materials may be exposed to different harmful environments; for instance, in marine areas and offshore, the seawater may contain a high amount of sulfates and chlorides [68], and the effluents generated by human activities and their reaction may induce harmful effects to materials. Thus, material durability can decline severely by the interaction with such aggressive environments. It is therefore important to consider the long-term service life and degradation behavior of natural fibers before recommending them to be used as reinforcement material.


### 4.2. Alkaline Degradation of Cellulosic Fibers

Several studies have found that lignocellulosic fibers are highly susceptible to various degrees of decomposition when exposed to an alkaline environment. Mukhopadhyay and Khatana [2] reported the alkaline proneness of hemicellulose and lignin. According to the authors [2], there are two possible mechanisms for the alkaline decomposition of natural plant fibers: (i) a “peeling off effect” wherein the end groups of the natural fiber are continuously liberated due to the reaction between OH− groups and reducing end group and (ii) the alkaline degradation of hemicellulose and lignin. The strength reduction attributable to the dissolution of hemicellulose and lignin and the associated degradation effects have been discussed in-depth by Yan et al. [16]. Gram [65] studied the durability of natural fiber–reinforced ordinary Portland cement composites, extensively explaining the mechanisms of alkaline degradation of cellulosic fiber and their associated losses in tensile strength and toughness within a cementitious composite, taking sisal fiber as reinforcing material. The strong alkaline matrix pore solution causes the dissolution of the fundamental components of the sisal fibers, such as hemicellulose and lignin, and alkaline hydrolysis of cellulose molecules. As a result, the molecular chain of the fiber is damaged thereby leading to a lower tensile strength of cellulose-cement composite material. The delignification of the fiber via the alkaline cement pore water destroys the main links between the individual fibers instigating the disintegration of long molecular chains into fragmented single cells.Tolêdo Filho et al. [66] investigated the durability of vegetable fiber (sisal and coconut)–reinforced cement-based mortar composites by assessing their flexural properties and concluded that these fibers are poorly resistant to the alkaline solution that comes from the cementitious matrix. The fracture strength and toughness of the composite decreased due to the decomposition of fibers in the alkaline cement. The same research group [69] explored the durability of coir and sisal fibers by exposing them to alkaline media (calcium and sodium hydroxide). They found that sisal and coir fiber could retain 72.7% and 60.9% of the original strength, respectively, after 420 days of immersion in sodium hydroxide solution. When immersed in calcium hydroxide solution (Ca(OH)2), sisal and coir fibers could retain only 33.7% and 58.7% of their strength, respectively, after 210 days and lost their original strength completely after 300 days. The more significant alkaline attack on the fibers by Ca(OH)2 solution might be attributed to the crystallization of lime within the fiber capillaries.The alkaline degradation mechanisms of cellulosic fibers in cementitious matrices were explained in detail by Wei and Meyer [4]. They summarized the degradation mechanism in four basic stages: (i) the decomposition of lignin and part of hemicellulose, creating more exposure to both fresh and hardened solid phase of the cement matrix; (ii) decomposition of intramolecular hydrogen bonding and the residual hemicellulose, contributing to a reduction in the integrity and stability of the vegetable fiber cell walls; (iii) the dispersion and degradation of cellulose fibers by the alkaline cementitious matrix; and (iv) finally, the degradation of cellulose micro-fibrils, which ends in the complete degradation of the natural plant fiber.Moreover, the durability of various vegetable fiber–reinforced cement composites was investigated by Ramakrishna and Sundararajan [70] under alternate drying and wetting cycles and continuous immersion in different media (freshwater, saturated lime, and NaOH solution) for 60 days. All fibers have shown a significant reduction in their chemical composition and strength characteristics in all investigated media. According to the authors, the loss in the strength of the corroded fibers was due to the alkaline attack and microbiological actions, respectively, upon the use of alkaline solution and water as an aging effect. A substantial reduction in tensile strength and elongation of date palm fibers was observed by Kriker et al. [71] over a six-month time duration when exposed to strong alkaline media (pH varying between 12.5 and 12.95). The reduction in strength becomes more severe in Ca(OH)2 solution than in NaOH and, as expected, for a prolonged immersion time. According to the researchers, the calcium hydroxide (CH) attack mechanism is most likely uniform and diffused, unlike the attack by NaOH solution, which is localized to the fiber pores and cells. This effect is detected in SEM images, as illustrated in Figure 1 [71]. It reflects a uniform deposition of portlandite on the surfaces of the fiber when exposed to a 12.5 pH Ca(OH)2 solution and the degradation is confined to only around the fiber pores that leaves a hole very similar to a bird’s nest when exposed to NaOH solution (12.5 pH), resulting in the deterioration of the strength and elasticity of the fiber.Figure 1
SEM images of date palm fibers: (a) immersed in Ca(OH)2 solution for six months; (b) immersed in NaOH solution for six months at two magnifications [71]. Reproduced with permission from Elsevier.
(a)(b)


### 4.3. Degradation of Cellulosic Fibers Caused by Mineralization

The impregnation of fiber cell walls and open pores by ionic species like Ca, Mg, Al, and Si during the hydration process is defined as fiber mineralization [72]. Gram [65] investigated that the crystallization of lime in the lumen cells and void spaces of the vegetable fiber due to the migration of cement hydration products (Ca(OH)2) decreases the strength and flexibility of the fibers. As a result, the fiber becomes markedly embrittled, eventually leading to the loss of its reinforcing capacity and to a deleterious effect on the durability of the composites as a whole. Similar results were reported by Tolêdo Filho et al. [66] indicating that fibers Petrify with hydration products, C-S-H- gels, triggering fiber mineralization. According to Wei and Meyer [4], besides the alkaline hydrolysis of the major chemical constituent parts, fiber cell wall mineralization is an important route that leads to loss of fiber reinforcing strength and strain capacity. The infiltration of CH products into fiber lumen and middle lamella increases with an increasing water absorption rate of the fiber, accelerating fiber mineralization and embrittlement behavior. This effect depends on the alkalinity of the matrix (Ca(OH)2 amount) and the concentration of Ca2+ in the pore solution. According to Tolêdo Filho et al. [69], crystallization of lime in the lumen, fiber cavities, fiber cell walls, and middle lamella brings a reduction in flexibility and deformation capacity of the fiber. Findings obtained by Bentur and Akers [73] have shown that the mineralization of cellulose fibers in cement-based composites is the dominant cause of fiber embrittlement. The fiber cores become filled with cement hydration products after natural and accelerated aging in a CO2-rich environment. Thus, fibers lose their flexibility and become rigid, resulting in a decrease in their reinforcing capability and composite toughness [73].


### 4.4. Degradation of Cellulosic Fibers Caused by Weathering Change

For outdoor applications, the degradation rate of natural fiber-reinforced composites becomes significant against external meteorological conditions, largely in the presence of moisture. Moisture in natural plant fiber–reinforced composites promotes the penetration of ultraviolet light and water into the fiber cells, catalyzing both biological and photodegradation of the composites apart from the formation of micro-cracks in the matrix [57]. According to Espert et al. [74], the diffusion of moisture into natural fiber–reinforced composites is governed by three different mechanisms. They are: (i) the diffusion of water molecules into the micro-gaps of the polymer chain; (ii) the transport of water into the interface region by capillary action, caused by poor wetting during composite processing; and (iii) the delivery via micro-cracks in the matrix, arisen from continuous swelling and shrinkage of fibers. This phenomenon was explained in detail by Ghavami [75], wherein water absorption of bamboo fibers causes a significant dimensional variability of the fibers that can create micro/macro cracks in composite structures. When the reinforcing fiber absorbs water, it swells and becomes enlarged pushing the binder/concrete away and shrinks back to almost its initial size leaving a void space around during curing, inducing cracks and reducing the service life of the material, as detailed in Figure 2 [75].Figure 2
Water absorption behavior of bamboo in concrete reinforcements: (a) bamboo in fresh concrete; (b) bamboo during curing of concrete; (c) bamboo after cured concrete [75]. Reproduced with permission from Elsevier.
(a)(b)(c)Agopyan et al. [76] also reported the severe degradation of composites caused by interfacial damages attributed to cyclic volume changes of highly polarized cellulosic fibers within cementitious matrices. Alomayri et al. [19] investigated the effect of water absorption of cotton fibers in low calcium fly ash geopolymer composites. It was noticed that the poor resistance to water absorption of cotton fiber deteriorates the fiber-matrix interface creating fiber debonding and delamination, causing ultimate loss of structural integrity. The SEM image of fracture surfaces also demonstrated damaged cotton micro-fibrils, giving rise to loss of the reinforcing capacity of the fiber. Correspondingly, sisal and coir fiber-reinforced cement composites aged with water have shown a strength reduction of 17 to 23% after 420 days of exposure, likely due to microbiological actions [69]. Furthermore, the fibers’ strengthening capacity reduces and fibers become more embrittled in alternate weathering conditions, as this process encourages the movement of ions such as OH− and Ca2+ into the fiber pores. The justification for this behavior is that fiber capillary pores are frequently filled and emptied with alkaline pore solution in cement pastes as the environmental conditions change routinely, accelerating the process of fiber mineralization.In addition to this effect, Gram [65] also observed the influence of external environmental conditions on the degradation behavior of fibers by investigating alternate moistening and drying out of sisal fiber-reinforced cement specimens. This activity determines the movement of the new alkaline pore water to reach the cellulosic fiber and to facilitate the removal of washed-out products (like lignin, hemicellulose, and neutralized matrix pore water solutions) away from the fiber cells. It was found that the weathering change replenished the movement of water which in turn facilitates the penetration of OH ions into the voids of the fiber frequently, causing the dissolution and leaching of more lignin and hemicellulose from the fiber cells [65]. It was then concluded that weathering change causes fiber degradation and accelerates the embrittlement of the composite materials with increasing the rate of the drying/wetting cycles. Similarly, Roma [67] investigated the effect of aging under external weathering change in sisal and eucalyptus fiber–reinforced cement composites. The toughness of the composites falls to between 53 and 68% when compared to the nonaged counterparts after four months of exposure. Composite water absorption (about 30% by mass) related to the presence of high porosity contributed to a significant role in this reduction. Alshaaer et al. [77] studied a preliminary aging of luffa fiber–reinforced metakaolin-based geopolymer composites for 20 months by applying dry and water immersion techniques. The authors noticed a compressive strength reduction of the composites from 31 MPa in dry conditions to 18 MPa after water immersion. The deterioration in strength after immersion in water was reported to be due to the leaching of cations like sodium in the pore solution produced by the metakaolin binders.Despite the above findings, it has been also reported in the literature that some natural vegetable fibers did not show any significant change in their mechanical properties as well as their durability when used as strengthening material in aluminosilicate and cementitious-based composites, which needs further investigation.For instance, Lima et al. [78] studied the durability of bamboo fiber–reinforced concrete composites by considering the changes in tensile strength and Young’s modulus of the fiber. The samples were exposed to wetting-drying cycles (each cycle lasted for 24 h) using calcium hydroxide (Ca(OH)2) solution and water. The bamboo fibers showed no significant variation in strength properties after various cycles of aging, demonstrating the durability of bamboo fibers in aggressive conditions. According to the authors, either the bamboo fiber microstructure itself and/or the higher specific surface area/perimeter ratio offered by the bamboo fibers makes the migration of lime into bamboo fiber lumens very difficult, thus making them durable in damaging environments. John et al. [72] explored the durability of coir fiber (2% by volume) reinforced blast furnace slag geopolymer composites. The reinforcing fiber was obtained from the internal and external walls of a 12-year-old house. Upon the determination of the lignin content of the fiber, they found no significant variation between the aged and new coir fibers, confirming the durability of coir fiber–reinforced slag-based geopolymer composites. SEM observations also supported that coir fibers removed from a 12-year-old house sample were undamaged. Similarly, Batista dos Santos et al. [79] found a minor drop in the flexural strength of sisal fiber–reinforced geopolymer composites (sludge precursors generated in water treatment plants) when exposed to accelerated aging conditions, representing good durability to wetting-drying cycles.


## 4.1. General Considerations

The durability of cellulosic fiber–reinforced polymer composites is a measure of its resistance to any process of degradation. The deterioration might have external causes (like weathering action, alkaline attack, etc.) and/or internal ones (such as fiber-matrix interface zone compatibility and volumetric changes of the fiber) [42, 48]. Thus, cellulosic fibers become embrittled at various degrees of degradation when immersed in cement/aluminosilicate precursors as the fibers are exposed to both internal and external causes of deterioration. It may be degraded either due to physical, biological, mechanical, chemical attacks, or a combination of them. Under the influence of key environmental factors, like ultraviolet radiation, temperature, air, moisture, mechanical stresses, chemical agents, and microorganism activities, cellulosic fiber-reinforced composites show a higher risk of deterioration, particularly when exposed to outdoor applications. This is more related to the properties of natural fibers which are vulnerable to biodegradation. Plant fiber–reinforced polymer composites can be decomposed to CO2 and H2O when exposed to weathering [14, 49, 50]. Another important fact about the degradation of plant-based fiber-reinforced polymer composites is emanated from the main chemical composition of the fiber itself which is vulnerable to biodegradation and the presence of voids in it [51]. Plant fibers typically comprise cellulose, hemicellulose, and lignin as the main components. Other components include wax, pectin, oil, and various water-soluble constituents [52]. Cellulose is a network of fine threadlike crystalline micro-fibrils embedded in a matrix of amorphous phases (such as hemicellulose and lignin) that offers structural support to the fiber [32, 53, 54]. According to Bhatia et al. [55], the highly complicated arrangement of bundles of cellulose micro-fibrils gives rise to cellulose fibers more resistant to chemical and biological attacks. Even though it is degradable to chemicals to some extent, cellulose is highly resistant to hydrolysis, oxidizing agents, and strong alkalis [56]. On the other hand, hemicellulose is highly hydrophilic, can readily hydrolyze in weak acids and bases, easily degrade when exposed to thermal and biological attacks [57, 58], and is highly flammable [50, 56]. Lignin is hydrophobic and highly resistant to most microorganism attacks and acid hydrolysis but susceptible to ultraviolet degradation [57, 59]. The presence of voids in plant fibers also promotes the absorption and penetration of more moisture into fiber cells from the surrounding environments, facilitating the degradation of polymer composites. For instance, according to the study conducted by Manfredi et al. [60], flax fiber exhibits a relatively higher thermal stability than jute and sisal fiber, as flax contains a lower amount of lignin. Methacanon et al. [58] studied the properties of reed, sisal, water hyacinth, and roselle fibers for geotextiles application. It was noticed that reed and water hyacinth fibers contain higher hemicellulose content, leading to thermal degradation at low temperatures and absorbing more moisture as compared to roselle and sisal fibers. As mentioned previously, one of the prominent challenges in using natural fibers as filler material in polymer composites is their inherent hydrophilic properties. According to Azwa et al. [56] and Väisänen et al. [51], water penetrates the fiber cells and forms a hydrogen bond which in turn creates inferior interfacial compounding with the matrix material when exposed to moisture. The degradation process started to occur as a result of the development of micro-cracks due to the stress developed via frequent swelling and shrinkage of cellulosic fibers. In addition, Methacanon et al. [58] and Chen et al. [61] explained that high moisture absorption by the natural fiber also creates a favorable situation for biodegradation (microbial attack) of the polymer composites. When more water is absorbed by the fiber cells, bound water starts to increase while free water declines catalyzing the leaching of water-soluble constituents from fibers. As a result, ultimate fiber debonding and delamination from the matrix material is experienced. As per Joseph et al. [62], the development of osmotic pressure at the fiber surface has initiated fiber-matrix detachment. Moisture absorption phenomena of materials follow either Fickian or non-Fickian behavior depending on whether the surrounding temperature is low or high, respectively [62]. According to the authors, the non- Fickian moisture absorption process has induced substantial hygro-thermal stresses in polymer composites. The moisture uptake of composites has become accelerated with temperature and their saturation time is highly reduced since temperature promotes the diffusion process significantly, in effect causing the peeling and surface decomposition of composites [56, 62]. Besides this, the moisture uptake capacity of natural fibers is also associated with the relative humidity (RH) of air. In this regard, Methacanon et al. [58] has studied the moisture absorption properties of sisal, reed, water hyacinth, and roselle fibers at a different relative humidity of the air. The authors observed that the moisture absorption property of the fibers increases with increasing RH. For instance, water hyacinth absorbs moisture almost 8 times higher at 97% RH than that it absorbs at 75% RH. Other fibers show a 4% increase in moisture absorption when the RH has been increased to 97%. Another important challenging issue that prohibits the use of natural fibers as potential reinforcement is their limited thermal stability. Most natural fiber has a limited processing temperature which is restricted to around 200°C [51]. Exposure of natural fiber-reinforced polymer composites in direct sunlight also causes a significant challenge impeding their application in structural composites. Ultraviolet (UV) radiation causes the breakdown of covalent bonds in organic polymer composites, leading to color fading, weight loss, yellowing, mechanical strength reduction, surface roughening, and so on [63, 64]. The color change observed is an indication of the chemical changes occurring on the surface of the composites that gives a clear picture of the degree of deterioration [56]. A study performed by Joseph et al. [62] on the degradation behavior of sisal fiber–reinforced polypropylene composites confirmed that the tensile strength of the composite deteriorates with increasing UV radiation exposure. This is attributed to chain scission and degradation via photo-oxidative reactions prompted by UV radiation. In addition to this, since the cementitious binder itself is a source of alkaline media with a pH value exceeding 13 [65, 66], it induces loss of fiber reinforcing capacity in different ways. The high alkaline environment generated within the binder during composite processing in both fresh and hardened states causes the dissolution of lignin and hemicellulose encrusting the fibers with mineral deposits, in consequence weakening the strength of the fiber and initiating micro-cracks in the paste matrices [67]. As one would expect, the initiation of matrix micro-cracking is another root instigating composite degradation [65].The leaching of hemicellulose and lignin depletes their reinforcing capacity that needs to be addressed first before approving the use of vegetable fibers in structural composites. Moreover, construction materials may be exposed to different harmful environments; for instance, in marine areas and offshore, the seawater may contain a high amount of sulfates and chlorides [68], and the effluents generated by human activities and their reaction may induce harmful effects to materials. Thus, material durability can decline severely by the interaction with such aggressive environments. It is therefore important to consider the long-term service life and degradation behavior of natural fibers before recommending them to be used as reinforcement material.


## 4.2. Alkaline Degradation of Cellulosic Fibers

Several studies have found that lignocellulosic fibers are highly susceptible to various degrees of decomposition when exposed to an alkaline environment. Mukhopadhyay and Khatana [2] reported the alkaline proneness of hemicellulose and lignin. According to the authors [2], there are two possible mechanisms for the alkaline decomposition of natural plant fibers: (i) a “peeling off effect” wherein the end groups of the natural fiber are continuously liberated due to the reaction between OH− groups and reducing end group and (ii) the alkaline degradation of hemicellulose and lignin. The strength reduction attributable to the dissolution of hemicellulose and lignin and the associated degradation effects have been discussed in-depth by Yan et al. [16]. Gram [65] studied the durability of natural fiber–reinforced ordinary Portland cement composites, extensively explaining the mechanisms of alkaline degradation of cellulosic fiber and their associated losses in tensile strength and toughness within a cementitious composite, taking sisal fiber as reinforcing material. The strong alkaline matrix pore solution causes the dissolution of the fundamental components of the sisal fibers, such as hemicellulose and lignin, and alkaline hydrolysis of cellulose molecules. As a result, the molecular chain of the fiber is damaged thereby leading to a lower tensile strength of cellulose-cement composite material. The delignification of the fiber via the alkaline cement pore water destroys the main links between the individual fibers instigating the disintegration of long molecular chains into fragmented single cells.Tolêdo Filho et al. [66] investigated the durability of vegetable fiber (sisal and coconut)–reinforced cement-based mortar composites by assessing their flexural properties and concluded that these fibers are poorly resistant to the alkaline solution that comes from the cementitious matrix. The fracture strength and toughness of the composite decreased due to the decomposition of fibers in the alkaline cement. The same research group [69] explored the durability of coir and sisal fibers by exposing them to alkaline media (calcium and sodium hydroxide). They found that sisal and coir fiber could retain 72.7% and 60.9% of the original strength, respectively, after 420 days of immersion in sodium hydroxide solution. When immersed in calcium hydroxide solution (Ca(OH)2), sisal and coir fibers could retain only 33.7% and 58.7% of their strength, respectively, after 210 days and lost their original strength completely after 300 days. The more significant alkaline attack on the fibers by Ca(OH)2 solution might be attributed to the crystallization of lime within the fiber capillaries.The alkaline degradation mechanisms of cellulosic fibers in cementitious matrices were explained in detail by Wei and Meyer [4]. They summarized the degradation mechanism in four basic stages: (i) the decomposition of lignin and part of hemicellulose, creating more exposure to both fresh and hardened solid phase of the cement matrix; (ii) decomposition of intramolecular hydrogen bonding and the residual hemicellulose, contributing to a reduction in the integrity and stability of the vegetable fiber cell walls; (iii) the dispersion and degradation of cellulose fibers by the alkaline cementitious matrix; and (iv) finally, the degradation of cellulose micro-fibrils, which ends in the complete degradation of the natural plant fiber.Moreover, the durability of various vegetable fiber–reinforced cement composites was investigated by Ramakrishna and Sundararajan [70] under alternate drying and wetting cycles and continuous immersion in different media (freshwater, saturated lime, and NaOH solution) for 60 days. All fibers have shown a significant reduction in their chemical composition and strength characteristics in all investigated media. According to the authors, the loss in the strength of the corroded fibers was due to the alkaline attack and microbiological actions, respectively, upon the use of alkaline solution and water as an aging effect. A substantial reduction in tensile strength and elongation of date palm fibers was observed by Kriker et al. [71] over a six-month time duration when exposed to strong alkaline media (pH varying between 12.5 and 12.95). The reduction in strength becomes more severe in Ca(OH)2 solution than in NaOH and, as expected, for a prolonged immersion time. According to the researchers, the calcium hydroxide (CH) attack mechanism is most likely uniform and diffused, unlike the attack by NaOH solution, which is localized to the fiber pores and cells. This effect is detected in SEM images, as illustrated in Figure 1 [71]. It reflects a uniform deposition of portlandite on the surfaces of the fiber when exposed to a 12.5 pH Ca(OH)2 solution and the degradation is confined to only around the fiber pores that leaves a hole very similar to a bird’s nest when exposed to NaOH solution (12.5 pH), resulting in the deterioration of the strength and elasticity of the fiber.Figure 1
SEM images of date palm fibers: (a) immersed in Ca(OH)2 solution for six months; (b) immersed in NaOH solution for six months at two magnifications [71]. Reproduced with permission from Elsevier.
(a)(b)


## 4.3. Degradation of Cellulosic Fibers Caused by Mineralization

The impregnation of fiber cell walls and open pores by ionic species like Ca, Mg, Al, and Si during the hydration process is defined as fiber mineralization [72]. Gram [65] investigated that the crystallization of lime in the lumen cells and void spaces of the vegetable fiber due to the migration of cement hydration products (Ca(OH)2) decreases the strength and flexibility of the fibers. As a result, the fiber becomes markedly embrittled, eventually leading to the loss of its reinforcing capacity and to a deleterious effect on the durability of the composites as a whole. Similar results were reported by Tolêdo Filho et al. [66] indicating that fibers Petrify with hydration products, C-S-H- gels, triggering fiber mineralization. According to Wei and Meyer [4], besides the alkaline hydrolysis of the major chemical constituent parts, fiber cell wall mineralization is an important route that leads to loss of fiber reinforcing strength and strain capacity. The infiltration of CH products into fiber lumen and middle lamella increases with an increasing water absorption rate of the fiber, accelerating fiber mineralization and embrittlement behavior. This effect depends on the alkalinity of the matrix (Ca(OH)2 amount) and the concentration of Ca2+ in the pore solution. According to Tolêdo Filho et al. [69], crystallization of lime in the lumen, fiber cavities, fiber cell walls, and middle lamella brings a reduction in flexibility and deformation capacity of the fiber. Findings obtained by Bentur and Akers [73] have shown that the mineralization of cellulose fibers in cement-based composites is the dominant cause of fiber embrittlement. The fiber cores become filled with cement hydration products after natural and accelerated aging in a CO2-rich environment. Thus, fibers lose their flexibility and become rigid, resulting in a decrease in their reinforcing capability and composite toughness [73].


## 4.4. Degradation of Cellulosic Fibers Caused by Weathering Change

For outdoor applications, the degradation rate of natural fiber-reinforced composites becomes significant against external meteorological conditions, largely in the presence of moisture. Moisture in natural plant fiber–reinforced composites promotes the penetration of ultraviolet light and water into the fiber cells, catalyzing both biological and photodegradation of the composites apart from the formation of micro-cracks in the matrix [57]. According to Espert et al. [74], the diffusion of moisture into natural fiber–reinforced composites is governed by three different mechanisms. They are: (i) the diffusion of water molecules into the micro-gaps of the polymer chain; (ii) the transport of water into the interface region by capillary action, caused by poor wetting during composite processing; and (iii) the delivery via micro-cracks in the matrix, arisen from continuous swelling and shrinkage of fibers. This phenomenon was explained in detail by Ghavami [75], wherein water absorption of bamboo fibers causes a significant dimensional variability of the fibers that can create micro/macro cracks in composite structures. When the reinforcing fiber absorbs water, it swells and becomes enlarged pushing the binder/concrete away and shrinks back to almost its initial size leaving a void space around during curing, inducing cracks and reducing the service life of the material, as detailed in Figure 2 [75].Figure 2
Water absorption behavior of bamboo in concrete reinforcements: (a) bamboo in fresh concrete; (b) bamboo during curing of concrete; (c) bamboo after cured concrete [75]. Reproduced with permission from Elsevier.
(a)(b)(c)Agopyan et al. [76] also reported the severe degradation of composites caused by interfacial damages attributed to cyclic volume changes of highly polarized cellulosic fibers within cementitious matrices. Alomayri et al. [19] investigated the effect of water absorption of cotton fibers in low calcium fly ash geopolymer composites. It was noticed that the poor resistance to water absorption of cotton fiber deteriorates the fiber-matrix interface creating fiber debonding and delamination, causing ultimate loss of structural integrity. The SEM image of fracture surfaces also demonstrated damaged cotton micro-fibrils, giving rise to loss of the reinforcing capacity of the fiber. Correspondingly, sisal and coir fiber-reinforced cement composites aged with water have shown a strength reduction of 17 to 23% after 420 days of exposure, likely due to microbiological actions [69]. Furthermore, the fibers’ strengthening capacity reduces and fibers become more embrittled in alternate weathering conditions, as this process encourages the movement of ions such as OH− and Ca2+ into the fiber pores. The justification for this behavior is that fiber capillary pores are frequently filled and emptied with alkaline pore solution in cement pastes as the environmental conditions change routinely, accelerating the process of fiber mineralization.In addition to this effect, Gram [65] also observed the influence of external environmental conditions on the degradation behavior of fibers by investigating alternate moistening and drying out of sisal fiber-reinforced cement specimens. This activity determines the movement of the new alkaline pore water to reach the cellulosic fiber and to facilitate the removal of washed-out products (like lignin, hemicellulose, and neutralized matrix pore water solutions) away from the fiber cells. It was found that the weathering change replenished the movement of water which in turn facilitates the penetration of OH ions into the voids of the fiber frequently, causing the dissolution and leaching of more lignin and hemicellulose from the fiber cells [65]. It was then concluded that weathering change causes fiber degradation and accelerates the embrittlement of the composite materials with increasing the rate of the drying/wetting cycles. Similarly, Roma [67] investigated the effect of aging under external weathering change in sisal and eucalyptus fiber–reinforced cement composites. The toughness of the composites falls to between 53 and 68% when compared to the nonaged counterparts after four months of exposure. Composite water absorption (about 30% by mass) related to the presence of high porosity contributed to a significant role in this reduction. Alshaaer et al. [77] studied a preliminary aging of luffa fiber–reinforced metakaolin-based geopolymer composites for 20 months by applying dry and water immersion techniques. The authors noticed a compressive strength reduction of the composites from 31 MPa in dry conditions to 18 MPa after water immersion. The deterioration in strength after immersion in water was reported to be due to the leaching of cations like sodium in the pore solution produced by the metakaolin binders.Despite the above findings, it has been also reported in the literature that some natural vegetable fibers did not show any significant change in their mechanical properties as well as their durability when used as strengthening material in aluminosilicate and cementitious-based composites, which needs further investigation.For instance, Lima et al. [78] studied the durability of bamboo fiber–reinforced concrete composites by considering the changes in tensile strength and Young’s modulus of the fiber. The samples were exposed to wetting-drying cycles (each cycle lasted for 24 h) using calcium hydroxide (Ca(OH)2) solution and water. The bamboo fibers showed no significant variation in strength properties after various cycles of aging, demonstrating the durability of bamboo fibers in aggressive conditions. According to the authors, either the bamboo fiber microstructure itself and/or the higher specific surface area/perimeter ratio offered by the bamboo fibers makes the migration of lime into bamboo fiber lumens very difficult, thus making them durable in damaging environments. John et al. [72] explored the durability of coir fiber (2% by volume) reinforced blast furnace slag geopolymer composites. The reinforcing fiber was obtained from the internal and external walls of a 12-year-old house. Upon the determination of the lignin content of the fiber, they found no significant variation between the aged and new coir fibers, confirming the durability of coir fiber–reinforced slag-based geopolymer composites. SEM observations also supported that coir fibers removed from a 12-year-old house sample were undamaged. Similarly, Batista dos Santos et al. [79] found a minor drop in the flexural strength of sisal fiber–reinforced geopolymer composites (sludge precursors generated in water treatment plants) when exposed to accelerated aging conditions, representing good durability to wetting-drying cycles.


## 5. Durability Enhancement Schemes/Strategies

There are two broad approaches for enhancing the durability of natural fiber–reinforced geopolymer or cementitious-based binders. One group refers to matrix modification methods and the other relates to fiber treatment approaches.


### 5.1. Matrix Treatment

#### 5.1.1. Carbonation of the Cementitious Matrix

To improve the service life of natural plant fibers, one of the most efficient techniques is to reduce the alkalinity nature of the pore water within the matrix. This could be possible through the process called carbonation. It allows the reaction of alkaline hydroxides (may be calcium, sodium, or potassium) with diffused carbon dioxide into the binders, producing carbonates. As described before in this paper, the pH of the alkaline pore water in cementitious binders is about/exceeding 13 due to the formation of Ca(OH)2 by the hydration process of OPC (ordinary Portland cement). Thus, one of the techniques proposed to reduce the matrix alkalinity is by diffusing CO2 into the fresh cementitious matrix in the laboratory to simulate the actual phenomenon that could occur for an extended period of time. According to Hobbs [80], the CO2 diffusing into the paste matrices reacts with the hydration products of cement forming calcium carbonate (CaCO3). As a result, the amount of calcium hydroxide depletes gradually and the pore solution becomes less alkaline, hence the pH of the alkaline pore water drops to a value of 8. According to the report compiled by Torgal and Jalali [81], plant fibers can sustain their strength characteristics and flexibility without showing any degradation behavior in an alkaline environment up to a pH value of 9. As indicated by Gram [65], the decomposition of natural fibers in cement-based composites can be avoided or delayed by reducing the alkaline pore water to a value below 12 to 12.5. It was also noted that sisal fibers can be negatively affected when the pH value exceeds 12 [65].The effect of accelerated carbonation on the mechanical and physical properties of vegetable (sisal) fiber–reinforced cementitious roofing tiles was examined by Tonoli et al. [82]. After 480 days of test in the laboratory, it was found that the toughness of the roofing tiles had increased by 80% compared to specimens not exposed to the carbonation process, while the porosity together with the water absorption property of the composite were reduced due to the application of accelerated carbonation. The enhancement in the mechanical performance demonstrated that the fibers could retain their strength in the inorganic binder. Results suggest that the use of accelerated carbonation is an efficient mechanism to lower the aggressive environment (Ca(OH)2) to CaCO3, thus protecting the lignocellulosic fibers from being decomposed. The authors confirmed the disappearance of portlandite (Ca(OH)2) and the formation of more calcite (CaCO3) by performing X-ray diffraction tests on specimens exposed to three different aging conditions, as displayed in Figure 3 [82].Figure 3
X-ray diffraction pattern of roofing tiles: (condition (a) nonaged tiles; (condition (b) after 100 aging cycles; (condition (c) fast carbonated tiles after 100 aging cycles according to Tonoli et al. [82]. Reproduced with permission from Elsevier.
(a)(b)(c)The successful adsorption of carbon dioxide (CO2) by the hydration products of the cementitious matrix favors the formation of a greater amount of calcite (CaCO3) and the reduction of OH ions that exist in the cement composites.Similarly, Soroushian et al. [83] investigated the longevity of cellulosic fiber (softwood kraft pulp) reinforced Portland cement-based composites using accelerated carbon dioxide curing techniques under different accelerated aging effects. Accelerated aging [56] was developed in a laboratory aging chamber under controlled conditions that simulated the actual real-time damaging effects observed in materials when exposed to open-air conditions for a prolonged duration. The increased durability of the composites can be attributed to the reduction in the capillary porosity, the general drop in the alkaline content upon carbonation, the formation of stable CaCO3, and the enhancement in the interfacial bonding between the fiber and the matrix. Furthermore, carbonation leads to the filling of fiber open pores and lumens with reaction products contributing to improved adhesion between the fiber and the matrix so that the strength and rigidity of the composites will be enhanced. Moreover, the effect of carbonation of a cement matrix in a CO2-rich environment on sisal and coir fiber–reinforced composites was analyzed by Tolêdo Filho et al. [66]. They have shown that early curing of the fresh composite material in a CO2-rich environment increases the depth of carbonation, creating a more durable material under various aging conditions. Equally, early age accelerated carbonation of eucalyptus pulp reinforced cement composites has shown better mechanical performance after exposure to aging [84]. The carbonated composite revealed good fiber-matrix adhesion, lower capillary pores, and a higher density after 200 and 400 cycles of accelerated aging and one-year natural weathering. This behavior is related to the densification of the matrix by the precipitation of more dense reaction products of CaCO3 than Ca(OH)2. The better mechanical performance obtained after various aging conditions indicates thus the suitability of the carbonation process for improving the durability of natural fiber cement-matrix composites.


#### 5.1.2. Adding Pozzolanic Compounds

Many researchers proposed that the use of pozzolanic additives like blast furnace slag, rice husk ash, fly ash, silica fume, or metakaolin in Portland cement can reduce or eliminate the alkalinity in the cementitious paste [16, 81, 85]. Amorphous silica reacts with calcium hydroxide during the hydration process of cement, producing a stable hydrated salt of calcium silicate. Then, the hydrated calcium silicate salt facilitates the removal of CaOH2 by transforming it into a calcium-silicate-hydrate (C-S-H) gel, thus increasing the durability of fiber-reinforced cementitious composites. The basic reaction is presented as [85]:(1)3Ca+++2H2SiO42−+2OH−+2H2O⟶Ca3H2Si2O7OH2.3H2OGram [65] used supplementary materials to mitigate the embrittlement characteristics of natural fiber–reinforced cement-based concrete composites. Fiber’s resistance to degradation by the alkaline pore water was enhanced by replacing ordinary Portland cement with 40 to 50 in weight % silica fume. The results demonstrated that adding up to 50% of silica fume reduces the pore water alkalinity significantly to a level at which it is hard to dissolve lignin and hemicellulose. Due to this effect, the fiber degradation was reduced and the material exhibited enhanced durability.Likewise, the effect of partial replacement of Portland cement by metakaolin, slag, fly ash, silica fume, and volcanic ash in kraft pulp fiber-reinforced cement composites was investigated by Mohr et al. [86]. The composites did not show any sign of degradation in mechanical strength and toughness characteristics after exposure to 25 wet/dry cycles, displaying that kraft pulp fiber maintains its reinforcing capacity. It was thus demonstrated that the addition of auxiliary binders reduces the concentration of the alkaline content and improves the fiber’s resistance to decomposition.Toledo Filho et al. [87] also studied the durability of sisal fiber–reinforced mortar laminates by replacing cement with 50% calcined clay (by weight), as works of the literature suggest that 50% replacement of cement by weight with calcined clay produces CH-free matrices [88]. The specimens were subjected to wetting/drying cycles to evaluate the durability of the composites. After 100 wetting/drying aging cycles, the composite exhibited a similar post-cracking toughness and bending strength to that of the un-aged reference while a complete degradation of the fibers was observed in the unmodified matrices. Micrograph observations, as presented in Figures 4 and 5, confirm that cementitious matrices free from CH guarantee the development of durable vegetable fiber–reinforced composites even in aggressive situations.Figure 4
Micrographs of aged Portland cement specimens: (a) when subjected to 25 wetting and drying cycles; (b) cavities in the matrix due to sisal fibers breaking and pull-out; (c) mineralized fiber inside the specimen [87]. Reproduced with permission from Elsevier.
(a)(b)(c)Figure 5
Micrographs of calcium hydroxide-free sisal-reinforced specimens: (a) specimens subjected to 25 wetting and drying cycles; (b) zoom-in sisal fiber subjected to 25 cycles; (c) and (d) close-up view of the sisal fiber cells in specimens subjected to 50 cycles of wetting/drying [87]. Reproduced with permission from Elsevier.Similar investigations were performed by Silva et al. [89]. The calcium hydroxide-free composites obtained by the addition of metakaolin and calcined waste crushed clay brick presented a significant improvement in ultimate tensile strength (13.95 MPa) when compared to the Portland cement (9.24 MPa)–based composites. The composite toughness also showed twice higher values than that of the cement matrix. The authors further determined the durability of the produced composites via accelerated aging using the hot water (60°C) immersion technique [89]. The calcium hydroxide-free composites did not suffer any mechanical degradation after 6 months of immersion. The result obtained indicates a promising solution for the potential application of vegetable fibers in the construction sector.Tolêdo Filho et al. [66] employed the use of sisal and coir fibers in cement binders. The fibers were immersed in slurried silica fume before incorporating them into the matrix targeting the production of a low pH alkaline zone around the fiber. As discussed above, immersion of the vegetable fibers in silica fume slurry can be regarded as an effective means to slow down the embrittlement behavior of the composites. During immersion, the capillary pores of the fiber are filled with silica fume microparticles causing the elimination of the alkaline calcium hydroxide and promoting the formation of C-S-H gels [90]. Thus, composites reinforced with surface-modified natural fibers in silica fume have shown improved durability.


#### 5.1.3. Use of Matrix Pore Sealants and Fiber Impregnation with Water Repellent Agents

In the presence of moisture and temperature fluctuations, the movement of alkaline pore solution to the fiber cells and the removal of washed-out components (both the decomposed fiber component and the neutralized alkaline pore water) might be reduced or even halted by sealing the matrix pore system. Although there is no extensive research available in the literature in this regard, the approach can be very effective to slow down the embrittlement process of the fiber. In this context, Gram [65] studied the sealing of matrix pore systems in sisal fiber–reinforced concretes by admixing polymer microparticles, small balls of wax, and stearates. Admixing wax beads to a fresh mortar and applying heat when it gets hardened makes the wax to melt and flow out, thus sealing the matrix pore system effectively. The result obtained was not encouraging in the sense that the pore system sealed by polymer and stearate powder did not prevent the degradation of the fiber, although it was capable of achieving a finer and hydrophobic matrix pore system. However promising results were observed in a pore system sealed by wax beds. Similarly, Tolêdo Filho et al. [66] reported that mortar pore sealing using montan wax and colophony reduces the water absorption properties of sisal fiber–reinforced cement composites but failed to protect the fiber from becoming brittle. Fiber degradation in an alkaline environment might also be protected/delayed by impregnating the fiber with blocking agents that can react with a certain part of the fiber to build compounds that are hard to dissolve by the alkaline pore solution and form water repellent coatings. Gram [65] studied the effect of sisal fiber treatment with different blocking agents (such as barium nitrate, magnesium sulfate, sodium sulphide, etc.). However, results showed that the fiber had lost its tensile strength after 14 days of exposure to the cementitious pore water and none of the used agents protected the fiber from being degraded. Also, the application of a protective impregnation of sisal fibers using various water repellent agents has been investigated. All the treatments failed to prevent the fiber from deterioration but stearic acid and barium nitrate (formine) were shown to play a significant role in delaying the degradation process of the composites [65]. Sisal fiber impregnation with organic compounds obtained from timber was studied by Canovas et al. [91]. According to the authors, matrix pore sealing with tannin, colophony, and montan wax reduced the water absorption property and relative porosity of sisal fiber-reinforced cement mortar composites, indicating promising results in protecting the fiber from the penetration of water but not halting fiber embrittlement process completely. The same research group [92] investigated the possible way of preventing sisal fibers deterioration in cementitious matrices by fiber impregnation with water repellent agents. The application of natural resins like colophony, clove oil, and tannin (dissolved in turpentine, xilene, or alcohol) can reduce the water absorption property of the sisal fiber by more than 55% compared to the unimpregnated fiber. Moreover, the authors have given more emphasis on the selection of impregnating agents (such as their nature and structure, viscosity, and solubility) as vegetable fibers are heterogeneous and the various components of the fiber respond differently to different types of chemicals. In this regard, it was recommended that organic compounds are better waterproofing properties than inorganic ones, as organic compounds are insoluble in water and form a good attachment to fiber cells than inorganic ones [92]. The application of water repellent agents such as Negrolin in bamboo fibers also leads to improved bond strength between bamboo fibers and concrete beams [93]. The treatment of bamboo fibers with Negrolin only allowed 4% of water penetration into the fiber cells after 96 h exposure to drinking water, displaying a significant reduction in the water absorption phenomenon of the fiber. More studies are required to further assess the benefit of using water repellent agents and matrix pore sealants with the final goal of preserving fiber integrity and improving the fiber-matrix interface.


### 5.2. Fiber Treatment

Plant-based fiber-reinforced composites can be used in building and construction industries owing to their lightweight, relative low cost, biodegradability, abundance in nature, and promising physical and mechanical properties [32, 94, 95]. However, the major barrier associated with their application is their moisture absorption property [32, 96]. The presence of strongly polarized hydroxyl (OH) groups is the intrinsic drawback to the successful usage of plant-based fibers as reinforcing agents in cementitious and geopolymer matrices [11]. The hygroscopic nature of cellulose fibers causes inferior fiber-matrix adhesion potentially creating a separated zone at the interface region. The gap developed between fiber and matrix, in turn, hampers the transfer of load from the matrix to the fiber. Moreover, the swelling and shrinkage of these fibers leave void spaces, which represent a source of dimensional instability and the cause for the initiation and propagation of cracks as well. Several studies [11, 75, 76] have reported that the swelling and shrinkage of lignocellulosic fibers in composites promote the formation of high porosity, detachment of fibers, and initiation of fine micro-cracks in the matrix around the fiber, as shown in Figure 6. The occurrence of these phenomena also facilitates the biodegradation of fibers by microbiological actions [70]. Eventually, the fiber’s main chemical constituents are degraded, leading to microstructural damage of the composites and thereby damaging their durability.Figure 6
SEM image of a coir fiber–reinforced cement composite showing the effect of water absorption [76]. Reproduced with permission from Elsevier.Therefore, fiber modification via different physical and chemical treatments is another strategy for the durability enhancement of composites. Both treatments, physical and chemical, are essentially targeted to improve the fiber-matrix interfacial bonding, making the fibers more water repellent and less sensitive to the alkaline pore solution. A previous review paper has discussed treatment strategies for natural fibers [11]; the topic is treated briefly in the next sections, with a focus on the effect of fiber treatment on composite durability.


#### 5.2.1. Hornification of Cellulosic Fibers

Hornification is a convenient technique to minimize the water absorption properties of plant fiber cells by altering the polymeric structure of the fiber. The water repellent properties of plant-derived fibers can be enhanced by applying drying and rewetting of the fibers repeatedly, a process called hornification [44]. It is a phenomenon that can be quantified in terms of the percentage reduction in water retention value of the lignocellulosic fibers after subjecting them to alternate drying and wetting cycles. Generally, the water retention value (WRV) of the fiber is reduced while increasing the number of wetting-drying cycles. The process leads to a higher dimensional stability of the fibers against environmental changes, lower WRV, and it reduces the tendency of portlandite to crystallize on the open spaces and lumen of the fibers [97–99]. There is also a stiffening effect and close packing in cellulosic materials upon structural changes following repetitive wet-dry aging cycles [99]. Ferreira et al. [44] explained how hornification modifies the chemical properties and morphology of natural cellulosic fibers that could result in a strong positive effect on the fiber-matrix interfacial interaction and mechanical properties in cementitious matrices reinforced with jute, sisal, and curaua fibers. The hornification was done by immersing the fibers in water at a temperature of 22°C for 3 h and subsequent drying at 80°C in a furnace for 16 h; the process of wetting-drying cycles has been repeated 5 to 10 times. It was verified via SEM observation that the fiber cross-sectional area, as well as the size of the lumen openings, were reduced after continuous wetting-drying cycles. This might be due to an excessive fatigue effect developed during the hornification causing shrinkage of the cellulose fiber as a consequence of the formation of hydrogen bonds, promoting cross-linked bond structures that strengthen the structure of the lignocellulosic fiber cells. As a result, the water retention value of the fibers decreased after each cycle alternation. Besides, the FTIR spectra of the hornified fibers indicated an increase in the intensity of the peak at 3400 cm−1, confirming the formation of new hydrogen bonds with the water molecules. Moreover, the modified fibers showed a higher intensity of the carboxyl (COOH) groups at 1750 cm−1 wavelength, as seen in Figure 7, indicating the increase in the hydrophobicity of the lignin, promoting better interaction amongst the major constituents of the fiber (lignin, cellulose, and hemicellulose) resulting in a stronger material. The influence of the treatment on the composition of the fibers was further supported by the shift of the differential scanning calorimetry (DSC) curve of the hornified fibers to a higher enthalpy of decomposition than the untreated one [44]. Indeed, the authors concluded that the modifications of morphology, physical properties, and chemical composition of the fiber have played a significant role in achieving higher dimensional stability, improved fiber-matrix bonds, and enhanced mechanical properties.Figure 7
FTIR spectra of natural fibers, and fibers after 5 and 10 cycles of wetting and drying cycles [44]. Reproduced with permission from Elsevier.A similar trend was observed by the same research group [100] while studying the effect of hornification on sisal fiber and cement matrix bonding. In another investigation, Ferreira et al. [101] evaluated the effect of sisal fiber hornification on Portland cement–based composites. Fibers submitted to ten times wetting-drying cycles showed improved fiber-matrix interfacial bond and frictional bond strength by about 40 and 50%, respectively, when compared with the untreated fibers. The higher interfacial bond created contributed to a significant improvement in the toughness and energy absorption property of the composites, according to the capacity of the fibers to bridge cracks for enhanced durability. Claramunt et al. [98] investigated the effect of hornification of softwood kraft pulp and cotton linter fibers to reinforce cement mortars on its durability and mechanical performance after subjecting the fibers to aging (4 wet-dry cycles). The flexural strengths of the aged composites were improved by about 13% (for kraft pulp) and 21% (for cotton linter) and the compressive strengths by about 20% (for kraft pulp) and 10% (for cotton linter) in comparison with the untreated fibers. In addition, cotton linters showed lower permeability ensuring lower decomposition of the fibers and higher environmental resistance in the aged composites. On the other hand, the presence of pits at the surface of kraft fibers promoted the degradation of the fibers, causing less resistance in their composites as compared to the cotton linter fiber-containing composites. Other studies have also demonstrated that hornificated fibers significantly improved the interfacial shear strength between the fiber and the cementitious matrix [102]. According to Lima et al. [102], sisal fibers submitted to ten alternation wetting-drying cycles presented a 55% increase in the average interfacial bond strength which allows the production of composites with better mechanical properties. Moreover, Ferreira and co-workers [103] achieved an increase of up to 40% fiber-matrix interfacial bond and 50% in frictional bond strength with hornification of sisal fibers. Although a reduction in the elastic modulus is usually observed, the process improves the deformation capacity and the rupture strength of the fiber due to the direct traction at the fiber-matrix interface. Hornified sisal fibers further show a reduction in the water absorption capacity and become more stable in their cross-sectional variability against the variation in humidity [103].


#### 5.2.2. Fiber Modification via Chemical Treatments

Because of the variations in chemical constituents of natural plant fibers, for various reasons mentioned earlier, it is evident that their reinforcing capacity in geopolymer and cement-based composites vary significantly. In this regard, Ye et al. [104] studied the influence of different contents of cellulose, hemicellulose, and lignin on the properties of metakaolin-based geopolymers. It was observed that the degree of geopolymerization decreases with increasing the hemicellulose content. The alkaline attack of hemicellulose produces carboxylic acid that neutralizes the alkaline environment, causing a reduction in the geopolymerization reaction. As a result, this effect deteriorates the flexural and compressive strength of the composites that display a characteristic porous morphology and brittle failure mode. FTIR spectra show peaks at around 1610 cm−1 in samples containing 20% pure hemicellulose, which are due to the fact that carboxylate anion (COO−) starts to ionize for the formation of carboxylic acid [104]. This result shows the degradation of hemicellulose by alkaline media containing a mixture of 10 M NaOH solution and Na2SiO3. The mixing ratio of the two solutions was kept fixed by maintaining the SiO2/Na2O molar ratio of 1.33. On the contrary, increasing the fiber’s cellulose content enables them to achieve a denser microstructure that exhibits ductile failure mode characteristics, displaying enhanced tensile and compressive strength with a good fiber–matrix interface adhesion. In other studies, Klinke et al. [105] confirmed that higher carboxylic acid degradation products of hemicellulose and lignin are due to the alkaline wet oxidation pretreatment of wheat straw, resulting in a higher cellulose recovery (96%) as solid fractions. Consistent conclusions were also reported by Calvini et al. [106]. Gram [65] concluded that the degradation of plant-derived fibers in cementitious matrices is due to the high alkaline environment that dissolves the hemicellulose and lignin components of the fiber structure.In addition to these results, surface modification of lignocellulosic fibers using chemical treatments is beneficial. In particular, alkaline treatment of cellulosic fibers is essential not only for improving their adhesion to the matrix but also for removing debris and impurities at the surfaces of the fiber that prohibit its interaction with the binder material. It is one of the well-known techniques for reducing the embrittlement tendency of natural cellulosic fiber–reinforced composites [81]. Available literature suggests that mercerization of natural fibers gives rise to changes in the composition, morphology, and surface topography of fibers by removing hemicellulose, lignin, and other extractives, facilitating the adhesion between the fiber and the binding adhesives [11, 32]. Mwaikambo and Ansell [107] reported that alkali treatment of cellulosic vegetable fiber changes the fibers’ topography to a clean and rough surface, besides increasing the crystallinity index. A cleaner and rougher fiber offers more hydroxyl (OH) groups at the peripheral region that encourages bonding reaction with the cementitious binder material and mechanical interlocking, leading to improved performance composites. However, the authors remarked that the usage of appropriate alkaline concentration is essential, because otherwise the crystallite packing of the fibers may be reduced that may cause a reduction in the performance of the composites [107]:(2)Cell–OH+NaOH⟶Cell–O−Na++H2O+surfaceimpuritiesConcerning the alkalization of cellulosic vegetable fibers, Yan et al. [108] treated coir fiber with 5 wt.% NaOH solution (at 20°C for 30 min) to reinforce Portland cement–based concretes. They observed that alkali-treated coir fiber show clearer and rougher surfaces that improve substantially the mechanical strength characteristics compared to the untreated fibers. The influence of alkali (0.5 wt/vol% NaOH solution for 24 h) treatment of jute weave fibers in metakaolin-based geopolymer matrices has been analyzed by Sankar and Kriven [109]. A significant improvement in tensile strength (from 8.8 to 14.5 MPa) was observed due to the alkaline treatment of the jute fibers. This result could be attributed to the removal of the hemicellulose by the alkaline treatment leaving behind a rough surface fiber that promotes the frictional interlocking between the fiber and the geopolymer matrices. In other studies, Sedan et al. [45] treated hemp fiber with 6 wt.% NaOH solution for 48 h at room temperature to formulate cement-based composites. Compared to the plain cement pastes, the incorporation of alkali-treated hemp fibers noticeably improved the flexural strength of the composites by about 94%.Zhou et al. [110] reported on the use of pretreated hemp fibers in aqueous alkaline solution (2 wt.% Ca(OH)2 for 14 h at 20°C) to improve the mechanical properties and durability of concrete composites. After 28 days of curing, the tensile and compressive strengths, as well as the fracture toughness of the treated hemp fiber–reinforced composites were reported as 16.9, 10, and 7 to 13% higher, respectively, than the untreated fiber reinforcements. The authors noticed that the failure mode of the modified fiber-reinforced composite showed more ductile characteristics, indicating that the composites could be highly durable and resistant to long-term cracking. Beyond this, quantitative analysis such as surface roughness of the hemp fiber and resistance to fiber pull-out also supported the improvements in mechanical properties of the treated fiber-reinforced concrete composites [110]. Ozerkan et al. [111] investigated the effect of alkali treatment on the mechanical performance and durability of date palm fibers in cement mortar composites. The authors performed a durability test by measuring the sulfate resistance of specimens following ASTM C 1012 procedures in 4-month time duration. The fibers were treated with 2% NaOH and 0.173% Ca(OH)2 solution for an hour. Results showed that fibers treated with calcium hydroxide solution exhibited better tensile and flexural strength and stiffness compared to the NaOH-treated fiber reinforcements. The treatment offered also improved durability of the composites against sulfate attacks with the inclusion of 1 to 2% treated date palm fibers. Correspondingly, Jo et al. [112] studied the mechanical properties of cement mortar composites reinforced with jute fiber. The jute fiber was treated with 450 ml of 0.5% NaOH solution for 24 h aimed at improving strength characteristics and service life of the composites. The authors concluded that both raw and treated jute fibers augmented the overall mechanical performance of the cement composites due to the inhibition of crack propagation by the fibers’ crack bridging effect. It was observed that the use of alkali-treated jute fiber improves the compressive and flexural strength of the composites from 34.5 to 36.2 MPa and from 7.4 to 10.4 MPa, respectively, compared to the control mortar (unreinforced) samples. It was also noticed that treated fiber mortar composites improved the fracture toughness and post-cracking resistance behavior by 1.5 and 22.5 times, respectively, as compared to that of the control mortar. The raw jute fibers were able to improve those properties of the composites by only 1.4 and 20.4 times, respectively. Comparing to the untreated jute fiber, a slight improvement in alkali-treated jute fiber reinforcement was observed, which might be justified by the favorable fiber-matrix compatibility and enhanced fiber-matrix bonding achieved [112].Likewise, Lazorenko et al. [113] investigated the effect of successive alkali (5% aqueous NaOH) and ultrasonic treatment of flax tows on low calcium fly ash-based geopolymer composites. The amorphous components of the fiber were removed successfully via the treatment resulting in a 35% improvement in the flexural tensile strength of the geopolymer composites. The defibrillation of fiber bundles into fine micro-fibrils was shown to cause an increase in the fiber-matrix contact area, which explains the improved mechanical characteristics of the composites. Chen et al. [114] treated sweet sorghum fiber in a 2 M NaOH aqueous solution for 12 h before using it as a reinforcement material in fly ash-based (class F) geopolymer paste composites. The treatment was shown to yield rough surface fibers with pits encouraging mechanical interlocking between the fiber and the matrix. Moreover, the treatment improved the mechanical (splitting tensile and flexural) strength and fracture toughness. The transition from brittle to ductile failure mode observed in the splitting tensile test of the geopolymer composite confirmed the improvements in mechanical properties via the inclusion of treated sweet sorghum fibers.Another study [115] has shown that the flexural strength of 6 wt.% NaOH-treated flax fiber-reinforced cement composites was higher than that of the untreated ones. The treatment avoids the fiber-matrix debonding caused by fiber shrinkage during curing. In general, the incorporation of alkali-treated flax fibers causes a change in the rheological settings of the paste affecting positively the mechanical performance of the composites. Alshaaer et al. [77] studied the adhesive interaction between cellulosic luffa cylindrical fibers and metakaolin-based geopolymer binders. The high alkaline environment formed by a combination of sodium silicate and 6 M NaOH solution was shown to activate the geopolymerization reaction. Without the application of the alkali-pretreatment of the luffa fibers, the strong alkaline setting created for the reaction helped to extract the fiber’s hemicellulose and lignin constituents so that it produced a rough surface morphology with smaller fiber diameter, promoting strong bonding between the fiber and the binder. As a result, a significant improvement in compressive and flexural strength of the composite, 2.4 times and 4.4 times increase, respectively, has been observed, compared to the pure matrix [77]. The reduction in fiber diameter caused by the breakdown of fiber bundles into fine filaments favored greater wettability of the fiber within the metakaolin matrix that impairs fiber debonding. The authors observed crystal clusters of fiber-like or whisker morphology and cubic shape fine crystals grown in the produced geopolymer composite specimens, as shown in Figure 8 [77]. The development of such crystals was reported to deliver a local crack healing effect in the geopolymer composites.Figure 8
SEM images of crystal growth: (a) fibrous—A and B; (b) cubic–C in luffa cylindrical fiber-reinforced geopolymer composites [77]. Reproduced with permission from Elsevier.
(a)(b)(c)However, there are some contradictory research reports wherein the alkaline treatment of natural plant fibers does not show any improvement for use in geopolymer binders. For example, Sá Ribeiro et al. [116] evaluated the behavior of metakaolin-based geopolymer composites reinforced with bamboo fibers. The flexural strength of the composites was shown to remain the same for fibers treated with alkali and water. This means that further research has to be done for addressing the not completely understood mechanisms involved in the alkali treatment of natural fibers and the resulting effects on the composite mechanical properties. A summary of alkali treatment of natural fibers and their effects on the mechanical properties of polymer composites are presented in Table 1.Table 1
Effect of alkaline treatment of fibers on mechanical properties of composites.
Fiber typeAlkaline solutionTreatment timeMatrixCharacterized mechanical properties and their percentage improvementsRef.Specimen typeCompressive strength (MPa)Tensile strength (MPa)Flexural strength (MPa)Flexural toughness (MPa)Energy absorption (N.m)Hemp6 wt.% NaOH48 hCementCP——4.9——[45]UHFR——6.8 (39%)——THFRC——9.5 (94%)——Coir5 wt.% NaOH30 minCementPC22.4—9.81.232.7[108]UCFRC23.8 (6.25%)—11.2 (14.2%)6.34 (424%)54.5 (66.7%)TCFRC24 (7.1%)—11.9 (21.4%)6.64 (449%)58.4 (78.6%)EpoxyUCFRE—20.234.6——TCFRE23.8 (17.8%)40.4 (16.7)——Jute0.5% NaOH24 hMetakaolinUJFRC—8.8———[109]TJFRC—14.5 (64.8%)———Hemp2 wt.% Ca(OH)214 hCementUHFRC19.322.71———[110]THFRC21.24 (10%)3.17 (16.9%)———Jute0.5% NaOH24 hCementCCM34.5—7.4——[112]TJFRM36.2 (5%)—10.4 (40.5%)——Sweet sorghum2 M NaOH12 hfly ashG0—2.53.7——[114]TSSRC—3.4 (36%)5.5 (48.6%)——Kenaf5 wt.% NaOH2 hMetakaolinG032.5—6.98[117]KFGC30.2 (7%)—8.5 (21.8%)——TKFGC34.1 (4.9%)—9.21 (31.9%)——Sisal10 M NaOH1 hPPUSFRPPC—29-——[118]TSFRPPC—31 (6.9%)-——Date palm5 M alkali24 hMetakaolinG0——3.5——[119]TFRGC——12.5 (257%)——Note. Percentage improvements of measured mechanical properties are given in brackets; CP–cement paste; UHFRCP/THFRCP–untreated and treated hemp fiber–reinforced cement paste; PC–plane cement: UCFRC/TCFRC–untreated/treated coir fiber–reinforced concrete; UCFRE/TCFRE–untreated/treated coir fiber–reinforced epoxy; UJFRC/TJFRC–untreated/treated jute fiber-reinforced composites; UHFRC/THFRC–untreated and treated hemp fiber–reinforced concrete; CCM–controlled cement mortar; TJFRM–treated jute fiber–reinforced mortar; G0–pure geopolymer specimen; TSSRC– treated sweet sorghum reinforced composite; UKFGC/TKFGC–untreated/treated kenaf fiber–reinforced geopolymer composite; PP–polypropylene; USFRPPC/TSFRPPC–untreated/treated sisal fiber–reinforced PP composite; TFRGC–treated fiber-reinforced geopolymer composite.The use of silane treatment was found to be an effective means of modifying lignocellulosic fibers and making them more hydrophobic by reducing hydroxyl groups on the fiber surfaces, besides increasing the wetting behavior between fiber and matrix, which enhances the interfacial adhesion [95]. It was explained that when cellulosic fibers are treated with a silane coupling agent, a reaction with water molecules occurs forming two distinct functional groups: silanol (Si-OH) group at one end and alcohol (CH3OH) group at the other side [120, 121]. The silanol reacts with hydroxyl (OH) groups of the cellulose fiber producing a covalent bond (hydrogen bonding) while the alcohol reacts with the matrix, promoting a strong interaction between the fiber and the matrix, providing a bridging effect at the interface region.According to the report compiled by Huda et al. [122], silane treatment offers the maximum bonding between fiber and matrix by increasing the surface area of the fibers and the degree of cross-linking, which leads to improved composite performance and durability. Li et al. [24] modified the surfaces of coir fibers with 2-ethylhexanol coupling agent in cement-based composites. The use of 2-ethylhexanol coupling agent emulsifies and removes the natural protecting waxes and oily substances on surfaces of the fiber creating more capillarities, which permits more cement paste to penetrate the coir fibers producing a strong bonding with the cementitious matrix. The authors reported that treated coir fiber–reinforced cement mortar exhibited 20% overall improvement in flexural properties and ductility in comparison to the untreated counterparts, leading to enhanced performance of the composites [24]. Tonoli et al. [123] studied cement-based composites reinforced with surface-modified eucalyptus cellulose pulp fibers. The fibers were treated with silane (methacryloxypropyltri-methoxysilane and aminopropyltri-ethoxysilane) using a silane to eucalyptus cellulose pulp fiber weight ratio of 6% to explore the influence on the mechanical and microstructural characteristics of the composites for enhanced durability. Even though both treatments enhanced the modulus of elasticity and the resistance to fracture in comparison to the untreated fiber-reinforced composites, aminopropyltri-ethoxysilane treatment did not avoid fiber mineralization. Fibers modified with methacryloxypropyltri-methoxysilane showed improved performance by protecting the lumen from being filled with cement hydration products [123].Similarly, Bilba and Arsene [124] focused on the use of silane solutions (such as alkyltrialkoxysilane (S1) and dialkyldialkoxysilane (S2)) to treat sugar cane bagasse fibers for promoting the fiber-matrix interface interaction and composite durability. The silane coating solution was varied from 0.5 to 8% by volume. The fiber showed an increase in dimension in all directions after being coated with 6% silane for 2 h. Observation using an optical microscope demonstrated that there was an increase in fiber dimensions after coating with 6% silane solution, offering greater fiber sites for bond interaction with the cementitious matrix. This trend was attributed to the improvement in water absorption resistance of the composites via better interface contact and bonding. The treatment changed the fiber texture to a rougher surface with striations when using S1 and granular when using S2, as presented in Figure 9, besides the densification achieved [124]. The hygroscopic nature of the fiber was shown to decrease greatly in both treatments, although S2 demonstrated a more water repellent effect.Figure 9
SEM micrographs of bagasse fiber: (a) untreated fiber; (b) fibers treated with alkyltrialkoxysilane; (c) fibers treated with dialkyldialkoxysilane [124]. Reproduced with permission from Elsevier.
(a)(b)(c)


#### 5.2.3. Thermal Treatments of Fibers

Although there are few studies available in the literature, heat treatment has been reported as another alternative technique to modify natural plant fibers for strengthening fiber-matrix interfaces. For example, Bilba and Arsene [124] pyrolyzed bagasse fibers at 200°C for 2 h in a nitrogen atmosphere for their use in cement-based binders. The pyrolysis process reduces hemicellulose, some volatiles, and extractives in addition to the reduction of OH groups from the fiber surfaces. This treatment makes the fiber less hydrophilic and promotes enhanced adhesion to the matrix material, leading to improved composites’ resistance to aging. Wei and Meyer [125] applied thermal treatments on sisal fibers to improve the durability and degradation resistance of the fiber in cement concrete composite. The performance of the composite sample was evaluated by exposing them to wetting-drying cycles to accelerate aging and by measuring the resulting strength properties of the specimens. They found that thermal treatment of sisal fiber in a ventilated oven at 150°C for 8 h improves the durability of the composite by increasing the compressive and splitting tensile strength by 31.1% and 36.5%, respectively. The increased durability can be associated with the improved crystallinity of cellulose (since the amount of crystalline cellulose determines the strength of the fiber) [56]. However, in other studies [126], it was reported that heat treatment of eucalyptus fibers did not improve fiber-matrix interface properties.


## 5.1. Matrix Treatment

### 5.1.1. Carbonation of the Cementitious Matrix

To improve the service life of natural plant fibers, one of the most efficient techniques is to reduce the alkalinity nature of the pore water within the matrix. This could be possible through the process called carbonation. It allows the reaction of alkaline hydroxides (may be calcium, sodium, or potassium) with diffused carbon dioxide into the binders, producing carbonates. As described before in this paper, the pH of the alkaline pore water in cementitious binders is about/exceeding 13 due to the formation of Ca(OH)2 by the hydration process of OPC (ordinary Portland cement). Thus, one of the techniques proposed to reduce the matrix alkalinity is by diffusing CO2 into the fresh cementitious matrix in the laboratory to simulate the actual phenomenon that could occur for an extended period of time. According to Hobbs [80], the CO2 diffusing into the paste matrices reacts with the hydration products of cement forming calcium carbonate (CaCO3). As a result, the amount of calcium hydroxide depletes gradually and the pore solution becomes less alkaline, hence the pH of the alkaline pore water drops to a value of 8. According to the report compiled by Torgal and Jalali [81], plant fibers can sustain their strength characteristics and flexibility without showing any degradation behavior in an alkaline environment up to a pH value of 9. As indicated by Gram [65], the decomposition of natural fibers in cement-based composites can be avoided or delayed by reducing the alkaline pore water to a value below 12 to 12.5. It was also noted that sisal fibers can be negatively affected when the pH value exceeds 12 [65].The effect of accelerated carbonation on the mechanical and physical properties of vegetable (sisal) fiber–reinforced cementitious roofing tiles was examined by Tonoli et al. [82]. After 480 days of test in the laboratory, it was found that the toughness of the roofing tiles had increased by 80% compared to specimens not exposed to the carbonation process, while the porosity together with the water absorption property of the composite were reduced due to the application of accelerated carbonation. The enhancement in the mechanical performance demonstrated that the fibers could retain their strength in the inorganic binder. Results suggest that the use of accelerated carbonation is an efficient mechanism to lower the aggressive environment (Ca(OH)2) to CaCO3, thus protecting the lignocellulosic fibers from being decomposed. The authors confirmed the disappearance of portlandite (Ca(OH)2) and the formation of more calcite (CaCO3) by performing X-ray diffraction tests on specimens exposed to three different aging conditions, as displayed in Figure 3 [82].Figure 3
X-ray diffraction pattern of roofing tiles: (condition (a) nonaged tiles; (condition (b) after 100 aging cycles; (condition (c) fast carbonated tiles after 100 aging cycles according to Tonoli et al. [82]. Reproduced with permission from Elsevier.
(a)(b)(c)The successful adsorption of carbon dioxide (CO2) by the hydration products of the cementitious matrix favors the formation of a greater amount of calcite (CaCO3) and the reduction of OH ions that exist in the cement composites.Similarly, Soroushian et al. [83] investigated the longevity of cellulosic fiber (softwood kraft pulp) reinforced Portland cement-based composites using accelerated carbon dioxide curing techniques under different accelerated aging effects. Accelerated aging [56] was developed in a laboratory aging chamber under controlled conditions that simulated the actual real-time damaging effects observed in materials when exposed to open-air conditions for a prolonged duration. The increased durability of the composites can be attributed to the reduction in the capillary porosity, the general drop in the alkaline content upon carbonation, the formation of stable CaCO3, and the enhancement in the interfacial bonding between the fiber and the matrix. Furthermore, carbonation leads to the filling of fiber open pores and lumens with reaction products contributing to improved adhesion between the fiber and the matrix so that the strength and rigidity of the composites will be enhanced. Moreover, the effect of carbonation of a cement matrix in a CO2-rich environment on sisal and coir fiber–reinforced composites was analyzed by Tolêdo Filho et al. [66]. They have shown that early curing of the fresh composite material in a CO2-rich environment increases the depth of carbonation, creating a more durable material under various aging conditions. Equally, early age accelerated carbonation of eucalyptus pulp reinforced cement composites has shown better mechanical performance after exposure to aging [84]. The carbonated composite revealed good fiber-matrix adhesion, lower capillary pores, and a higher density after 200 and 400 cycles of accelerated aging and one-year natural weathering. This behavior is related to the densification of the matrix by the precipitation of more dense reaction products of CaCO3 than Ca(OH)2. The better mechanical performance obtained after various aging conditions indicates thus the suitability of the carbonation process for improving the durability of natural fiber cement-matrix composites.


### 5.1.2. Adding Pozzolanic Compounds

Many researchers proposed that the use of pozzolanic additives like blast furnace slag, rice husk ash, fly ash, silica fume, or metakaolin in Portland cement can reduce or eliminate the alkalinity in the cementitious paste [16, 81, 85]. Amorphous silica reacts with calcium hydroxide during the hydration process of cement, producing a stable hydrated salt of calcium silicate. Then, the hydrated calcium silicate salt facilitates the removal of CaOH2 by transforming it into a calcium-silicate-hydrate (C-S-H) gel, thus increasing the durability of fiber-reinforced cementitious composites. The basic reaction is presented as [85]:(1)3Ca+++2H2SiO42−+2OH−+2H2O⟶Ca3H2Si2O7OH2.3H2OGram [65] used supplementary materials to mitigate the embrittlement characteristics of natural fiber–reinforced cement-based concrete composites. Fiber’s resistance to degradation by the alkaline pore water was enhanced by replacing ordinary Portland cement with 40 to 50 in weight % silica fume. The results demonstrated that adding up to 50% of silica fume reduces the pore water alkalinity significantly to a level at which it is hard to dissolve lignin and hemicellulose. Due to this effect, the fiber degradation was reduced and the material exhibited enhanced durability.Likewise, the effect of partial replacement of Portland cement by metakaolin, slag, fly ash, silica fume, and volcanic ash in kraft pulp fiber-reinforced cement composites was investigated by Mohr et al. [86]. The composites did not show any sign of degradation in mechanical strength and toughness characteristics after exposure to 25 wet/dry cycles, displaying that kraft pulp fiber maintains its reinforcing capacity. It was thus demonstrated that the addition of auxiliary binders reduces the concentration of the alkaline content and improves the fiber’s resistance to decomposition.Toledo Filho et al. [87] also studied the durability of sisal fiber–reinforced mortar laminates by replacing cement with 50% calcined clay (by weight), as works of the literature suggest that 50% replacement of cement by weight with calcined clay produces CH-free matrices [88]. The specimens were subjected to wetting/drying cycles to evaluate the durability of the composites. After 100 wetting/drying aging cycles, the composite exhibited a similar post-cracking toughness and bending strength to that of the un-aged reference while a complete degradation of the fibers was observed in the unmodified matrices. Micrograph observations, as presented in Figures 4 and 5, confirm that cementitious matrices free from CH guarantee the development of durable vegetable fiber–reinforced composites even in aggressive situations.Figure 4
Micrographs of aged Portland cement specimens: (a) when subjected to 25 wetting and drying cycles; (b) cavities in the matrix due to sisal fibers breaking and pull-out; (c) mineralized fiber inside the specimen [87]. Reproduced with permission from Elsevier.
(a)(b)(c)Figure 5
Micrographs of calcium hydroxide-free sisal-reinforced specimens: (a) specimens subjected to 25 wetting and drying cycles; (b) zoom-in sisal fiber subjected to 25 cycles; (c) and (d) close-up view of the sisal fiber cells in specimens subjected to 50 cycles of wetting/drying [87]. Reproduced with permission from Elsevier.Similar investigations were performed by Silva et al. [89]. The calcium hydroxide-free composites obtained by the addition of metakaolin and calcined waste crushed clay brick presented a significant improvement in ultimate tensile strength (13.95 MPa) when compared to the Portland cement (9.24 MPa)–based composites. The composite toughness also showed twice higher values than that of the cement matrix. The authors further determined the durability of the produced composites via accelerated aging using the hot water (60°C) immersion technique [89]. The calcium hydroxide-free composites did not suffer any mechanical degradation after 6 months of immersion. The result obtained indicates a promising solution for the potential application of vegetable fibers in the construction sector.Tolêdo Filho et al. [66] employed the use of sisal and coir fibers in cement binders. The fibers were immersed in slurried silica fume before incorporating them into the matrix targeting the production of a low pH alkaline zone around the fiber. As discussed above, immersion of the vegetable fibers in silica fume slurry can be regarded as an effective means to slow down the embrittlement behavior of the composites. During immersion, the capillary pores of the fiber are filled with silica fume microparticles causing the elimination of the alkaline calcium hydroxide and promoting the formation of C-S-H gels [90]. Thus, composites reinforced with surface-modified natural fibers in silica fume have shown improved durability.


### 5.1.3. Use of Matrix Pore Sealants and Fiber Impregnation with Water Repellent Agents

In the presence of moisture and temperature fluctuations, the movement of alkaline pore solution to the fiber cells and the removal of washed-out components (both the decomposed fiber component and the neutralized alkaline pore water) might be reduced or even halted by sealing the matrix pore system. Although there is no extensive research available in the literature in this regard, the approach can be very effective to slow down the embrittlement process of the fiber. In this context, Gram [65] studied the sealing of matrix pore systems in sisal fiber–reinforced concretes by admixing polymer microparticles, small balls of wax, and stearates. Admixing wax beads to a fresh mortar and applying heat when it gets hardened makes the wax to melt and flow out, thus sealing the matrix pore system effectively. The result obtained was not encouraging in the sense that the pore system sealed by polymer and stearate powder did not prevent the degradation of the fiber, although it was capable of achieving a finer and hydrophobic matrix pore system. However promising results were observed in a pore system sealed by wax beds. Similarly, Tolêdo Filho et al. [66] reported that mortar pore sealing using montan wax and colophony reduces the water absorption properties of sisal fiber–reinforced cement composites but failed to protect the fiber from becoming brittle. Fiber degradation in an alkaline environment might also be protected/delayed by impregnating the fiber with blocking agents that can react with a certain part of the fiber to build compounds that are hard to dissolve by the alkaline pore solution and form water repellent coatings. Gram [65] studied the effect of sisal fiber treatment with different blocking agents (such as barium nitrate, magnesium sulfate, sodium sulphide, etc.). However, results showed that the fiber had lost its tensile strength after 14 days of exposure to the cementitious pore water and none of the used agents protected the fiber from being degraded. Also, the application of a protective impregnation of sisal fibers using various water repellent agents has been investigated. All the treatments failed to prevent the fiber from deterioration but stearic acid and barium nitrate (formine) were shown to play a significant role in delaying the degradation process of the composites [65]. Sisal fiber impregnation with organic compounds obtained from timber was studied by Canovas et al. [91]. According to the authors, matrix pore sealing with tannin, colophony, and montan wax reduced the water absorption property and relative porosity of sisal fiber-reinforced cement mortar composites, indicating promising results in protecting the fiber from the penetration of water but not halting fiber embrittlement process completely. The same research group [92] investigated the possible way of preventing sisal fibers deterioration in cementitious matrices by fiber impregnation with water repellent agents. The application of natural resins like colophony, clove oil, and tannin (dissolved in turpentine, xilene, or alcohol) can reduce the water absorption property of the sisal fiber by more than 55% compared to the unimpregnated fiber. Moreover, the authors have given more emphasis on the selection of impregnating agents (such as their nature and structure, viscosity, and solubility) as vegetable fibers are heterogeneous and the various components of the fiber respond differently to different types of chemicals. In this regard, it was recommended that organic compounds are better waterproofing properties than inorganic ones, as organic compounds are insoluble in water and form a good attachment to fiber cells than inorganic ones [92]. The application of water repellent agents such as Negrolin in bamboo fibers also leads to improved bond strength between bamboo fibers and concrete beams [93]. The treatment of bamboo fibers with Negrolin only allowed 4% of water penetration into the fiber cells after 96 h exposure to drinking water, displaying a significant reduction in the water absorption phenomenon of the fiber. More studies are required to further assess the benefit of using water repellent agents and matrix pore sealants with the final goal of preserving fiber integrity and improving the fiber-matrix interface.


## 5.1.1. Carbonation of the Cementitious Matrix

To improve the service life of natural plant fibers, one of the most efficient techniques is to reduce the alkalinity nature of the pore water within the matrix. This could be possible through the process called carbonation. It allows the reaction of alkaline hydroxides (may be calcium, sodium, or potassium) with diffused carbon dioxide into the binders, producing carbonates. As described before in this paper, the pH of the alkaline pore water in cementitious binders is about/exceeding 13 due to the formation of Ca(OH)2 by the hydration process of OPC (ordinary Portland cement). Thus, one of the techniques proposed to reduce the matrix alkalinity is by diffusing CO2 into the fresh cementitious matrix in the laboratory to simulate the actual phenomenon that could occur for an extended period of time. According to Hobbs [80], the CO2 diffusing into the paste matrices reacts with the hydration products of cement forming calcium carbonate (CaCO3). As a result, the amount of calcium hydroxide depletes gradually and the pore solution becomes less alkaline, hence the pH of the alkaline pore water drops to a value of 8. According to the report compiled by Torgal and Jalali [81], plant fibers can sustain their strength characteristics and flexibility without showing any degradation behavior in an alkaline environment up to a pH value of 9. As indicated by Gram [65], the decomposition of natural fibers in cement-based composites can be avoided or delayed by reducing the alkaline pore water to a value below 12 to 12.5. It was also noted that sisal fibers can be negatively affected when the pH value exceeds 12 [65].The effect of accelerated carbonation on the mechanical and physical properties of vegetable (sisal) fiber–reinforced cementitious roofing tiles was examined by Tonoli et al. [82]. After 480 days of test in the laboratory, it was found that the toughness of the roofing tiles had increased by 80% compared to specimens not exposed to the carbonation process, while the porosity together with the water absorption property of the composite were reduced due to the application of accelerated carbonation. The enhancement in the mechanical performance demonstrated that the fibers could retain their strength in the inorganic binder. Results suggest that the use of accelerated carbonation is an efficient mechanism to lower the aggressive environment (Ca(OH)2) to CaCO3, thus protecting the lignocellulosic fibers from being decomposed. The authors confirmed the disappearance of portlandite (Ca(OH)2) and the formation of more calcite (CaCO3) by performing X-ray diffraction tests on specimens exposed to three different aging conditions, as displayed in Figure 3 [82].Figure 3
X-ray diffraction pattern of roofing tiles: (condition (a) nonaged tiles; (condition (b) after 100 aging cycles; (condition (c) fast carbonated tiles after 100 aging cycles according to Tonoli et al. [82]. Reproduced with permission from Elsevier.
(a)(b)(c)The successful adsorption of carbon dioxide (CO2) by the hydration products of the cementitious matrix favors the formation of a greater amount of calcite (CaCO3) and the reduction of OH ions that exist in the cement composites.Similarly, Soroushian et al. [83] investigated the longevity of cellulosic fiber (softwood kraft pulp) reinforced Portland cement-based composites using accelerated carbon dioxide curing techniques under different accelerated aging effects. Accelerated aging [56] was developed in a laboratory aging chamber under controlled conditions that simulated the actual real-time damaging effects observed in materials when exposed to open-air conditions for a prolonged duration. The increased durability of the composites can be attributed to the reduction in the capillary porosity, the general drop in the alkaline content upon carbonation, the formation of stable CaCO3, and the enhancement in the interfacial bonding between the fiber and the matrix. Furthermore, carbonation leads to the filling of fiber open pores and lumens with reaction products contributing to improved adhesion between the fiber and the matrix so that the strength and rigidity of the composites will be enhanced. Moreover, the effect of carbonation of a cement matrix in a CO2-rich environment on sisal and coir fiber–reinforced composites was analyzed by Tolêdo Filho et al. [66]. They have shown that early curing of the fresh composite material in a CO2-rich environment increases the depth of carbonation, creating a more durable material under various aging conditions. Equally, early age accelerated carbonation of eucalyptus pulp reinforced cement composites has shown better mechanical performance after exposure to aging [84]. The carbonated composite revealed good fiber-matrix adhesion, lower capillary pores, and a higher density after 200 and 400 cycles of accelerated aging and one-year natural weathering. This behavior is related to the densification of the matrix by the precipitation of more dense reaction products of CaCO3 than Ca(OH)2. The better mechanical performance obtained after various aging conditions indicates thus the suitability of the carbonation process for improving the durability of natural fiber cement-matrix composites.


## 5.1.2. Adding Pozzolanic Compounds

Many researchers proposed that the use of pozzolanic additives like blast furnace slag, rice husk ash, fly ash, silica fume, or metakaolin in Portland cement can reduce or eliminate the alkalinity in the cementitious paste [16, 81, 85]. Amorphous silica reacts with calcium hydroxide during the hydration process of cement, producing a stable hydrated salt of calcium silicate. Then, the hydrated calcium silicate salt facilitates the removal of CaOH2 by transforming it into a calcium-silicate-hydrate (C-S-H) gel, thus increasing the durability of fiber-reinforced cementitious composites. The basic reaction is presented as [85]:(1)3Ca+++2H2SiO42−+2OH−+2H2O⟶Ca3H2Si2O7OH2.3H2OGram [65] used supplementary materials to mitigate the embrittlement characteristics of natural fiber–reinforced cement-based concrete composites. Fiber’s resistance to degradation by the alkaline pore water was enhanced by replacing ordinary Portland cement with 40 to 50 in weight % silica fume. The results demonstrated that adding up to 50% of silica fume reduces the pore water alkalinity significantly to a level at which it is hard to dissolve lignin and hemicellulose. Due to this effect, the fiber degradation was reduced and the material exhibited enhanced durability.Likewise, the effect of partial replacement of Portland cement by metakaolin, slag, fly ash, silica fume, and volcanic ash in kraft pulp fiber-reinforced cement composites was investigated by Mohr et al. [86]. The composites did not show any sign of degradation in mechanical strength and toughness characteristics after exposure to 25 wet/dry cycles, displaying that kraft pulp fiber maintains its reinforcing capacity. It was thus demonstrated that the addition of auxiliary binders reduces the concentration of the alkaline content and improves the fiber’s resistance to decomposition.Toledo Filho et al. [87] also studied the durability of sisal fiber–reinforced mortar laminates by replacing cement with 50% calcined clay (by weight), as works of the literature suggest that 50% replacement of cement by weight with calcined clay produces CH-free matrices [88]. The specimens were subjected to wetting/drying cycles to evaluate the durability of the composites. After 100 wetting/drying aging cycles, the composite exhibited a similar post-cracking toughness and bending strength to that of the un-aged reference while a complete degradation of the fibers was observed in the unmodified matrices. Micrograph observations, as presented in Figures 4 and 5, confirm that cementitious matrices free from CH guarantee the development of durable vegetable fiber–reinforced composites even in aggressive situations.Figure 4
Micrographs of aged Portland cement specimens: (a) when subjected to 25 wetting and drying cycles; (b) cavities in the matrix due to sisal fibers breaking and pull-out; (c) mineralized fiber inside the specimen [87]. Reproduced with permission from Elsevier.
(a)(b)(c)Figure 5
Micrographs of calcium hydroxide-free sisal-reinforced specimens: (a) specimens subjected to 25 wetting and drying cycles; (b) zoom-in sisal fiber subjected to 25 cycles; (c) and (d) close-up view of the sisal fiber cells in specimens subjected to 50 cycles of wetting/drying [87]. Reproduced with permission from Elsevier.Similar investigations were performed by Silva et al. [89]. The calcium hydroxide-free composites obtained by the addition of metakaolin and calcined waste crushed clay brick presented a significant improvement in ultimate tensile strength (13.95 MPa) when compared to the Portland cement (9.24 MPa)–based composites. The composite toughness also showed twice higher values than that of the cement matrix. The authors further determined the durability of the produced composites via accelerated aging using the hot water (60°C) immersion technique [89]. The calcium hydroxide-free composites did not suffer any mechanical degradation after 6 months of immersion. The result obtained indicates a promising solution for the potential application of vegetable fibers in the construction sector.Tolêdo Filho et al. [66] employed the use of sisal and coir fibers in cement binders. The fibers were immersed in slurried silica fume before incorporating them into the matrix targeting the production of a low pH alkaline zone around the fiber. As discussed above, immersion of the vegetable fibers in silica fume slurry can be regarded as an effective means to slow down the embrittlement behavior of the composites. During immersion, the capillary pores of the fiber are filled with silica fume microparticles causing the elimination of the alkaline calcium hydroxide and promoting the formation of C-S-H gels [90]. Thus, composites reinforced with surface-modified natural fibers in silica fume have shown improved durability.


## 5.1.3. Use of Matrix Pore Sealants and Fiber Impregnation with Water Repellent Agents

In the presence of moisture and temperature fluctuations, the movement of alkaline pore solution to the fiber cells and the removal of washed-out components (both the decomposed fiber component and the neutralized alkaline pore water) might be reduced or even halted by sealing the matrix pore system. Although there is no extensive research available in the literature in this regard, the approach can be very effective to slow down the embrittlement process of the fiber. In this context, Gram [65] studied the sealing of matrix pore systems in sisal fiber–reinforced concretes by admixing polymer microparticles, small balls of wax, and stearates. Admixing wax beads to a fresh mortar and applying heat when it gets hardened makes the wax to melt and flow out, thus sealing the matrix pore system effectively. The result obtained was not encouraging in the sense that the pore system sealed by polymer and stearate powder did not prevent the degradation of the fiber, although it was capable of achieving a finer and hydrophobic matrix pore system. However promising results were observed in a pore system sealed by wax beds. Similarly, Tolêdo Filho et al. [66] reported that mortar pore sealing using montan wax and colophony reduces the water absorption properties of sisal fiber–reinforced cement composites but failed to protect the fiber from becoming brittle. Fiber degradation in an alkaline environment might also be protected/delayed by impregnating the fiber with blocking agents that can react with a certain part of the fiber to build compounds that are hard to dissolve by the alkaline pore solution and form water repellent coatings. Gram [65] studied the effect of sisal fiber treatment with different blocking agents (such as barium nitrate, magnesium sulfate, sodium sulphide, etc.). However, results showed that the fiber had lost its tensile strength after 14 days of exposure to the cementitious pore water and none of the used agents protected the fiber from being degraded. Also, the application of a protective impregnation of sisal fibers using various water repellent agents has been investigated. All the treatments failed to prevent the fiber from deterioration but stearic acid and barium nitrate (formine) were shown to play a significant role in delaying the degradation process of the composites [65]. Sisal fiber impregnation with organic compounds obtained from timber was studied by Canovas et al. [91]. According to the authors, matrix pore sealing with tannin, colophony, and montan wax reduced the water absorption property and relative porosity of sisal fiber-reinforced cement mortar composites, indicating promising results in protecting the fiber from the penetration of water but not halting fiber embrittlement process completely. The same research group [92] investigated the possible way of preventing sisal fibers deterioration in cementitious matrices by fiber impregnation with water repellent agents. The application of natural resins like colophony, clove oil, and tannin (dissolved in turpentine, xilene, or alcohol) can reduce the water absorption property of the sisal fiber by more than 55% compared to the unimpregnated fiber. Moreover, the authors have given more emphasis on the selection of impregnating agents (such as their nature and structure, viscosity, and solubility) as vegetable fibers are heterogeneous and the various components of the fiber respond differently to different types of chemicals. In this regard, it was recommended that organic compounds are better waterproofing properties than inorganic ones, as organic compounds are insoluble in water and form a good attachment to fiber cells than inorganic ones [92]. The application of water repellent agents such as Negrolin in bamboo fibers also leads to improved bond strength between bamboo fibers and concrete beams [93]. The treatment of bamboo fibers with Negrolin only allowed 4% of water penetration into the fiber cells after 96 h exposure to drinking water, displaying a significant reduction in the water absorption phenomenon of the fiber. More studies are required to further assess the benefit of using water repellent agents and matrix pore sealants with the final goal of preserving fiber integrity and improving the fiber-matrix interface.


## 5.2. Fiber Treatment

Plant-based fiber-reinforced composites can be used in building and construction industries owing to their lightweight, relative low cost, biodegradability, abundance in nature, and promising physical and mechanical properties [32, 94, 95]. However, the major barrier associated with their application is their moisture absorption property [32, 96]. The presence of strongly polarized hydroxyl (OH) groups is the intrinsic drawback to the successful usage of plant-based fibers as reinforcing agents in cementitious and geopolymer matrices [11]. The hygroscopic nature of cellulose fibers causes inferior fiber-matrix adhesion potentially creating a separated zone at the interface region. The gap developed between fiber and matrix, in turn, hampers the transfer of load from the matrix to the fiber. Moreover, the swelling and shrinkage of these fibers leave void spaces, which represent a source of dimensional instability and the cause for the initiation and propagation of cracks as well. Several studies [11, 75, 76] have reported that the swelling and shrinkage of lignocellulosic fibers in composites promote the formation of high porosity, detachment of fibers, and initiation of fine micro-cracks in the matrix around the fiber, as shown in Figure 6. The occurrence of these phenomena also facilitates the biodegradation of fibers by microbiological actions [70]. Eventually, the fiber’s main chemical constituents are degraded, leading to microstructural damage of the composites and thereby damaging their durability.Figure 6
SEM image of a coir fiber–reinforced cement composite showing the effect of water absorption [76]. Reproduced with permission from Elsevier.Therefore, fiber modification via different physical and chemical treatments is another strategy for the durability enhancement of composites. Both treatments, physical and chemical, are essentially targeted to improve the fiber-matrix interfacial bonding, making the fibers more water repellent and less sensitive to the alkaline pore solution. A previous review paper has discussed treatment strategies for natural fibers [11]; the topic is treated briefly in the next sections, with a focus on the effect of fiber treatment on composite durability.


### 5.2.1. Hornification of Cellulosic Fibers

Hornification is a convenient technique to minimize the water absorption properties of plant fiber cells by altering the polymeric structure of the fiber. The water repellent properties of plant-derived fibers can be enhanced by applying drying and rewetting of the fibers repeatedly, a process called hornification [44]. It is a phenomenon that can be quantified in terms of the percentage reduction in water retention value of the lignocellulosic fibers after subjecting them to alternate drying and wetting cycles. Generally, the water retention value (WRV) of the fiber is reduced while increasing the number of wetting-drying cycles. The process leads to a higher dimensional stability of the fibers against environmental changes, lower WRV, and it reduces the tendency of portlandite to crystallize on the open spaces and lumen of the fibers [97–99]. There is also a stiffening effect and close packing in cellulosic materials upon structural changes following repetitive wet-dry aging cycles [99]. Ferreira et al. [44] explained how hornification modifies the chemical properties and morphology of natural cellulosic fibers that could result in a strong positive effect on the fiber-matrix interfacial interaction and mechanical properties in cementitious matrices reinforced with jute, sisal, and curaua fibers. The hornification was done by immersing the fibers in water at a temperature of 22°C for 3 h and subsequent drying at 80°C in a furnace for 16 h; the process of wetting-drying cycles has been repeated 5 to 10 times. It was verified via SEM observation that the fiber cross-sectional area, as well as the size of the lumen openings, were reduced after continuous wetting-drying cycles. This might be due to an excessive fatigue effect developed during the hornification causing shrinkage of the cellulose fiber as a consequence of the formation of hydrogen bonds, promoting cross-linked bond structures that strengthen the structure of the lignocellulosic fiber cells. As a result, the water retention value of the fibers decreased after each cycle alternation. Besides, the FTIR spectra of the hornified fibers indicated an increase in the intensity of the peak at 3400 cm−1, confirming the formation of new hydrogen bonds with the water molecules. Moreover, the modified fibers showed a higher intensity of the carboxyl (COOH) groups at 1750 cm−1 wavelength, as seen in Figure 7, indicating the increase in the hydrophobicity of the lignin, promoting better interaction amongst the major constituents of the fiber (lignin, cellulose, and hemicellulose) resulting in a stronger material. The influence of the treatment on the composition of the fibers was further supported by the shift of the differential scanning calorimetry (DSC) curve of the hornified fibers to a higher enthalpy of decomposition than the untreated one [44]. Indeed, the authors concluded that the modifications of morphology, physical properties, and chemical composition of the fiber have played a significant role in achieving higher dimensional stability, improved fiber-matrix bonds, and enhanced mechanical properties.Figure 7
FTIR spectra of natural fibers, and fibers after 5 and 10 cycles of wetting and drying cycles [44]. Reproduced with permission from Elsevier.A similar trend was observed by the same research group [100] while studying the effect of hornification on sisal fiber and cement matrix bonding. In another investigation, Ferreira et al. [101] evaluated the effect of sisal fiber hornification on Portland cement–based composites. Fibers submitted to ten times wetting-drying cycles showed improved fiber-matrix interfacial bond and frictional bond strength by about 40 and 50%, respectively, when compared with the untreated fibers. The higher interfacial bond created contributed to a significant improvement in the toughness and energy absorption property of the composites, according to the capacity of the fibers to bridge cracks for enhanced durability. Claramunt et al. [98] investigated the effect of hornification of softwood kraft pulp and cotton linter fibers to reinforce cement mortars on its durability and mechanical performance after subjecting the fibers to aging (4 wet-dry cycles). The flexural strengths of the aged composites were improved by about 13% (for kraft pulp) and 21% (for cotton linter) and the compressive strengths by about 20% (for kraft pulp) and 10% (for cotton linter) in comparison with the untreated fibers. In addition, cotton linters showed lower permeability ensuring lower decomposition of the fibers and higher environmental resistance in the aged composites. On the other hand, the presence of pits at the surface of kraft fibers promoted the degradation of the fibers, causing less resistance in their composites as compared to the cotton linter fiber-containing composites. Other studies have also demonstrated that hornificated fibers significantly improved the interfacial shear strength between the fiber and the cementitious matrix [102]. According to Lima et al. [102], sisal fibers submitted to ten alternation wetting-drying cycles presented a 55% increase in the average interfacial bond strength which allows the production of composites with better mechanical properties. Moreover, Ferreira and co-workers [103] achieved an increase of up to 40% fiber-matrix interfacial bond and 50% in frictional bond strength with hornification of sisal fibers. Although a reduction in the elastic modulus is usually observed, the process improves the deformation capacity and the rupture strength of the fiber due to the direct traction at the fiber-matrix interface. Hornified sisal fibers further show a reduction in the water absorption capacity and become more stable in their cross-sectional variability against the variation in humidity [103].


### 5.2.2. Fiber Modification via Chemical Treatments

Because of the variations in chemical constituents of natural plant fibers, for various reasons mentioned earlier, it is evident that their reinforcing capacity in geopolymer and cement-based composites vary significantly. In this regard, Ye et al. [104] studied the influence of different contents of cellulose, hemicellulose, and lignin on the properties of metakaolin-based geopolymers. It was observed that the degree of geopolymerization decreases with increasing the hemicellulose content. The alkaline attack of hemicellulose produces carboxylic acid that neutralizes the alkaline environment, causing a reduction in the geopolymerization reaction. As a result, this effect deteriorates the flexural and compressive strength of the composites that display a characteristic porous morphology and brittle failure mode. FTIR spectra show peaks at around 1610 cm−1 in samples containing 20% pure hemicellulose, which are due to the fact that carboxylate anion (COO−) starts to ionize for the formation of carboxylic acid [104]. This result shows the degradation of hemicellulose by alkaline media containing a mixture of 10 M NaOH solution and Na2SiO3. The mixing ratio of the two solutions was kept fixed by maintaining the SiO2/Na2O molar ratio of 1.33. On the contrary, increasing the fiber’s cellulose content enables them to achieve a denser microstructure that exhibits ductile failure mode characteristics, displaying enhanced tensile and compressive strength with a good fiber–matrix interface adhesion. In other studies, Klinke et al. [105] confirmed that higher carboxylic acid degradation products of hemicellulose and lignin are due to the alkaline wet oxidation pretreatment of wheat straw, resulting in a higher cellulose recovery (96%) as solid fractions. Consistent conclusions were also reported by Calvini et al. [106]. Gram [65] concluded that the degradation of plant-derived fibers in cementitious matrices is due to the high alkaline environment that dissolves the hemicellulose and lignin components of the fiber structure.In addition to these results, surface modification of lignocellulosic fibers using chemical treatments is beneficial. In particular, alkaline treatment of cellulosic fibers is essential not only for improving their adhesion to the matrix but also for removing debris and impurities at the surfaces of the fiber that prohibit its interaction with the binder material. It is one of the well-known techniques for reducing the embrittlement tendency of natural cellulosic fiber–reinforced composites [81]. Available literature suggests that mercerization of natural fibers gives rise to changes in the composition, morphology, and surface topography of fibers by removing hemicellulose, lignin, and other extractives, facilitating the adhesion between the fiber and the binding adhesives [11, 32]. Mwaikambo and Ansell [107] reported that alkali treatment of cellulosic vegetable fiber changes the fibers’ topography to a clean and rough surface, besides increasing the crystallinity index. A cleaner and rougher fiber offers more hydroxyl (OH) groups at the peripheral region that encourages bonding reaction with the cementitious binder material and mechanical interlocking, leading to improved performance composites. However, the authors remarked that the usage of appropriate alkaline concentration is essential, because otherwise the crystallite packing of the fibers may be reduced that may cause a reduction in the performance of the composites [107]:(2)Cell–OH+NaOH⟶Cell–O−Na++H2O+surfaceimpuritiesConcerning the alkalization of cellulosic vegetable fibers, Yan et al. [108] treated coir fiber with 5 wt.% NaOH solution (at 20°C for 30 min) to reinforce Portland cement–based concretes. They observed that alkali-treated coir fiber show clearer and rougher surfaces that improve substantially the mechanical strength characteristics compared to the untreated fibers. The influence of alkali (0.5 wt/vol% NaOH solution for 24 h) treatment of jute weave fibers in metakaolin-based geopolymer matrices has been analyzed by Sankar and Kriven [109]. A significant improvement in tensile strength (from 8.8 to 14.5 MPa) was observed due to the alkaline treatment of the jute fibers. This result could be attributed to the removal of the hemicellulose by the alkaline treatment leaving behind a rough surface fiber that promotes the frictional interlocking between the fiber and the geopolymer matrices. In other studies, Sedan et al. [45] treated hemp fiber with 6 wt.% NaOH solution for 48 h at room temperature to formulate cement-based composites. Compared to the plain cement pastes, the incorporation of alkali-treated hemp fibers noticeably improved the flexural strength of the composites by about 94%.Zhou et al. [110] reported on the use of pretreated hemp fibers in aqueous alkaline solution (2 wt.% Ca(OH)2 for 14 h at 20°C) to improve the mechanical properties and durability of concrete composites. After 28 days of curing, the tensile and compressive strengths, as well as the fracture toughness of the treated hemp fiber–reinforced composites were reported as 16.9, 10, and 7 to 13% higher, respectively, than the untreated fiber reinforcements. The authors noticed that the failure mode of the modified fiber-reinforced composite showed more ductile characteristics, indicating that the composites could be highly durable and resistant to long-term cracking. Beyond this, quantitative analysis such as surface roughness of the hemp fiber and resistance to fiber pull-out also supported the improvements in mechanical properties of the treated fiber-reinforced concrete composites [110]. Ozerkan et al. [111] investigated the effect of alkali treatment on the mechanical performance and durability of date palm fibers in cement mortar composites. The authors performed a durability test by measuring the sulfate resistance of specimens following ASTM C 1012 procedures in 4-month time duration. The fibers were treated with 2% NaOH and 0.173% Ca(OH)2 solution for an hour. Results showed that fibers treated with calcium hydroxide solution exhibited better tensile and flexural strength and stiffness compared to the NaOH-treated fiber reinforcements. The treatment offered also improved durability of the composites against sulfate attacks with the inclusion of 1 to 2% treated date palm fibers. Correspondingly, Jo et al. [112] studied the mechanical properties of cement mortar composites reinforced with jute fiber. The jute fiber was treated with 450 ml of 0.5% NaOH solution for 24 h aimed at improving strength characteristics and service life of the composites. The authors concluded that both raw and treated jute fibers augmented the overall mechanical performance of the cement composites due to the inhibition of crack propagation by the fibers’ crack bridging effect. It was observed that the use of alkali-treated jute fiber improves the compressive and flexural strength of the composites from 34.5 to 36.2 MPa and from 7.4 to 10.4 MPa, respectively, compared to the control mortar (unreinforced) samples. It was also noticed that treated fiber mortar composites improved the fracture toughness and post-cracking resistance behavior by 1.5 and 22.5 times, respectively, as compared to that of the control mortar. The raw jute fibers were able to improve those properties of the composites by only 1.4 and 20.4 times, respectively. Comparing to the untreated jute fiber, a slight improvement in alkali-treated jute fiber reinforcement was observed, which might be justified by the favorable fiber-matrix compatibility and enhanced fiber-matrix bonding achieved [112].Likewise, Lazorenko et al. [113] investigated the effect of successive alkali (5% aqueous NaOH) and ultrasonic treatment of flax tows on low calcium fly ash-based geopolymer composites. The amorphous components of the fiber were removed successfully via the treatment resulting in a 35% improvement in the flexural tensile strength of the geopolymer composites. The defibrillation of fiber bundles into fine micro-fibrils was shown to cause an increase in the fiber-matrix contact area, which explains the improved mechanical characteristics of the composites. Chen et al. [114] treated sweet sorghum fiber in a 2 M NaOH aqueous solution for 12 h before using it as a reinforcement material in fly ash-based (class F) geopolymer paste composites. The treatment was shown to yield rough surface fibers with pits encouraging mechanical interlocking between the fiber and the matrix. Moreover, the treatment improved the mechanical (splitting tensile and flexural) strength and fracture toughness. The transition from brittle to ductile failure mode observed in the splitting tensile test of the geopolymer composite confirmed the improvements in mechanical properties via the inclusion of treated sweet sorghum fibers.Another study [115] has shown that the flexural strength of 6 wt.% NaOH-treated flax fiber-reinforced cement composites was higher than that of the untreated ones. The treatment avoids the fiber-matrix debonding caused by fiber shrinkage during curing. In general, the incorporation of alkali-treated flax fibers causes a change in the rheological settings of the paste affecting positively the mechanical performance of the composites. Alshaaer et al. [77] studied the adhesive interaction between cellulosic luffa cylindrical fibers and metakaolin-based geopolymer binders. The high alkaline environment formed by a combination of sodium silicate and 6 M NaOH solution was shown to activate the geopolymerization reaction. Without the application of the alkali-pretreatment of the luffa fibers, the strong alkaline setting created for the reaction helped to extract the fiber’s hemicellulose and lignin constituents so that it produced a rough surface morphology with smaller fiber diameter, promoting strong bonding between the fiber and the binder. As a result, a significant improvement in compressive and flexural strength of the composite, 2.4 times and 4.4 times increase, respectively, has been observed, compared to the pure matrix [77]. The reduction in fiber diameter caused by the breakdown of fiber bundles into fine filaments favored greater wettability of the fiber within the metakaolin matrix that impairs fiber debonding. The authors observed crystal clusters of fiber-like or whisker morphology and cubic shape fine crystals grown in the produced geopolymer composite specimens, as shown in Figure 8 [77]. The development of such crystals was reported to deliver a local crack healing effect in the geopolymer composites.Figure 8
SEM images of crystal growth: (a) fibrous—A and B; (b) cubic–C in luffa cylindrical fiber-reinforced geopolymer composites [77]. Reproduced with permission from Elsevier.
(a)(b)(c)However, there are some contradictory research reports wherein the alkaline treatment of natural plant fibers does not show any improvement for use in geopolymer binders. For example, Sá Ribeiro et al. [116] evaluated the behavior of metakaolin-based geopolymer composites reinforced with bamboo fibers. The flexural strength of the composites was shown to remain the same for fibers treated with alkali and water. This means that further research has to be done for addressing the not completely understood mechanisms involved in the alkali treatment of natural fibers and the resulting effects on the composite mechanical properties. A summary of alkali treatment of natural fibers and their effects on the mechanical properties of polymer composites are presented in Table 1.Table 1
Effect of alkaline treatment of fibers on mechanical properties of composites.
Fiber typeAlkaline solutionTreatment timeMatrixCharacterized mechanical properties and their percentage improvementsRef.Specimen typeCompressive strength (MPa)Tensile strength (MPa)Flexural strength (MPa)Flexural toughness (MPa)Energy absorption (N.m)Hemp6 wt.% NaOH48 hCementCP——4.9——[45]UHFR——6.8 (39%)——THFRC——9.5 (94%)——Coir5 wt.% NaOH30 minCementPC22.4—9.81.232.7[108]UCFRC23.8 (6.25%)—11.2 (14.2%)6.34 (424%)54.5 (66.7%)TCFRC24 (7.1%)—11.9 (21.4%)6.64 (449%)58.4 (78.6%)EpoxyUCFRE—20.234.6——TCFRE23.8 (17.8%)40.4 (16.7)——Jute0.5% NaOH24 hMetakaolinUJFRC—8.8———[109]TJFRC—14.5 (64.8%)———Hemp2 wt.% Ca(OH)214 hCementUHFRC19.322.71———[110]THFRC21.24 (10%)3.17 (16.9%)———Jute0.5% NaOH24 hCementCCM34.5—7.4——[112]TJFRM36.2 (5%)—10.4 (40.5%)——Sweet sorghum2 M NaOH12 hfly ashG0—2.53.7——[114]TSSRC—3.4 (36%)5.5 (48.6%)——Kenaf5 wt.% NaOH2 hMetakaolinG032.5—6.98[117]KFGC30.2 (7%)—8.5 (21.8%)——TKFGC34.1 (4.9%)—9.21 (31.9%)——Sisal10 M NaOH1 hPPUSFRPPC—29-——[118]TSFRPPC—31 (6.9%)-——Date palm5 M alkali24 hMetakaolinG0——3.5——[119]TFRGC——12.5 (257%)——Note. Percentage improvements of measured mechanical properties are given in brackets; CP–cement paste; UHFRCP/THFRCP–untreated and treated hemp fiber–reinforced cement paste; PC–plane cement: UCFRC/TCFRC–untreated/treated coir fiber–reinforced concrete; UCFRE/TCFRE–untreated/treated coir fiber–reinforced epoxy; UJFRC/TJFRC–untreated/treated jute fiber-reinforced composites; UHFRC/THFRC–untreated and treated hemp fiber–reinforced concrete; CCM–controlled cement mortar; TJFRM–treated jute fiber–reinforced mortar; G0–pure geopolymer specimen; TSSRC– treated sweet sorghum reinforced composite; UKFGC/TKFGC–untreated/treated kenaf fiber–reinforced geopolymer composite; PP–polypropylene; USFRPPC/TSFRPPC–untreated/treated sisal fiber–reinforced PP composite; TFRGC–treated fiber-reinforced geopolymer composite.The use of silane treatment was found to be an effective means of modifying lignocellulosic fibers and making them more hydrophobic by reducing hydroxyl groups on the fiber surfaces, besides increasing the wetting behavior between fiber and matrix, which enhances the interfacial adhesion [95]. It was explained that when cellulosic fibers are treated with a silane coupling agent, a reaction with water molecules occurs forming two distinct functional groups: silanol (Si-OH) group at one end and alcohol (CH3OH) group at the other side [120, 121]. The silanol reacts with hydroxyl (OH) groups of the cellulose fiber producing a covalent bond (hydrogen bonding) while the alcohol reacts with the matrix, promoting a strong interaction between the fiber and the matrix, providing a bridging effect at the interface region.According to the report compiled by Huda et al. [122], silane treatment offers the maximum bonding between fiber and matrix by increasing the surface area of the fibers and the degree of cross-linking, which leads to improved composite performance and durability. Li et al. [24] modified the surfaces of coir fibers with 2-ethylhexanol coupling agent in cement-based composites. The use of 2-ethylhexanol coupling agent emulsifies and removes the natural protecting waxes and oily substances on surfaces of the fiber creating more capillarities, which permits more cement paste to penetrate the coir fibers producing a strong bonding with the cementitious matrix. The authors reported that treated coir fiber–reinforced cement mortar exhibited 20% overall improvement in flexural properties and ductility in comparison to the untreated counterparts, leading to enhanced performance of the composites [24]. Tonoli et al. [123] studied cement-based composites reinforced with surface-modified eucalyptus cellulose pulp fibers. The fibers were treated with silane (methacryloxypropyltri-methoxysilane and aminopropyltri-ethoxysilane) using a silane to eucalyptus cellulose pulp fiber weight ratio of 6% to explore the influence on the mechanical and microstructural characteristics of the composites for enhanced durability. Even though both treatments enhanced the modulus of elasticity and the resistance to fracture in comparison to the untreated fiber-reinforced composites, aminopropyltri-ethoxysilane treatment did not avoid fiber mineralization. Fibers modified with methacryloxypropyltri-methoxysilane showed improved performance by protecting the lumen from being filled with cement hydration products [123].Similarly, Bilba and Arsene [124] focused on the use of silane solutions (such as alkyltrialkoxysilane (S1) and dialkyldialkoxysilane (S2)) to treat sugar cane bagasse fibers for promoting the fiber-matrix interface interaction and composite durability. The silane coating solution was varied from 0.5 to 8% by volume. The fiber showed an increase in dimension in all directions after being coated with 6% silane for 2 h. Observation using an optical microscope demonstrated that there was an increase in fiber dimensions after coating with 6% silane solution, offering greater fiber sites for bond interaction with the cementitious matrix. This trend was attributed to the improvement in water absorption resistance of the composites via better interface contact and bonding. The treatment changed the fiber texture to a rougher surface with striations when using S1 and granular when using S2, as presented in Figure 9, besides the densification achieved [124]. The hygroscopic nature of the fiber was shown to decrease greatly in both treatments, although S2 demonstrated a more water repellent effect.Figure 9
SEM micrographs of bagasse fiber: (a) untreated fiber; (b) fibers treated with alkyltrialkoxysilane; (c) fibers treated with dialkyldialkoxysilane [124]. Reproduced with permission from Elsevier.
(a)(b)(c)


### 5.2.3. Thermal Treatments of Fibers

Although there are few studies available in the literature, heat treatment has been reported as another alternative technique to modify natural plant fibers for strengthening fiber-matrix interfaces. For example, Bilba and Arsene [124] pyrolyzed bagasse fibers at 200°C for 2 h in a nitrogen atmosphere for their use in cement-based binders. The pyrolysis process reduces hemicellulose, some volatiles, and extractives in addition to the reduction of OH groups from the fiber surfaces. This treatment makes the fiber less hydrophilic and promotes enhanced adhesion to the matrix material, leading to improved composites’ resistance to aging. Wei and Meyer [125] applied thermal treatments on sisal fibers to improve the durability and degradation resistance of the fiber in cement concrete composite. The performance of the composite sample was evaluated by exposing them to wetting-drying cycles to accelerate aging and by measuring the resulting strength properties of the specimens. They found that thermal treatment of sisal fiber in a ventilated oven at 150°C for 8 h improves the durability of the composite by increasing the compressive and splitting tensile strength by 31.1% and 36.5%, respectively. The increased durability can be associated with the improved crystallinity of cellulose (since the amount of crystalline cellulose determines the strength of the fiber) [56]. However, in other studies [126], it was reported that heat treatment of eucalyptus fibers did not improve fiber-matrix interface properties.


## 5.2.1. Hornification of Cellulosic Fibers

Hornification is a convenient technique to minimize the water absorption properties of plant fiber cells by altering the polymeric structure of the fiber. The water repellent properties of plant-derived fibers can be enhanced by applying drying and rewetting of the fibers repeatedly, a process called hornification [44]. It is a phenomenon that can be quantified in terms of the percentage reduction in water retention value of the lignocellulosic fibers after subjecting them to alternate drying and wetting cycles. Generally, the water retention value (WRV) of the fiber is reduced while increasing the number of wetting-drying cycles. The process leads to a higher dimensional stability of the fibers against environmental changes, lower WRV, and it reduces the tendency of portlandite to crystallize on the open spaces and lumen of the fibers [97–99]. There is also a stiffening effect and close packing in cellulosic materials upon structural changes following repetitive wet-dry aging cycles [99]. Ferreira et al. [44] explained how hornification modifies the chemical properties and morphology of natural cellulosic fibers that could result in a strong positive effect on the fiber-matrix interfacial interaction and mechanical properties in cementitious matrices reinforced with jute, sisal, and curaua fibers. The hornification was done by immersing the fibers in water at a temperature of 22°C for 3 h and subsequent drying at 80°C in a furnace for 16 h; the process of wetting-drying cycles has been repeated 5 to 10 times. It was verified via SEM observation that the fiber cross-sectional area, as well as the size of the lumen openings, were reduced after continuous wetting-drying cycles. This might be due to an excessive fatigue effect developed during the hornification causing shrinkage of the cellulose fiber as a consequence of the formation of hydrogen bonds, promoting cross-linked bond structures that strengthen the structure of the lignocellulosic fiber cells. As a result, the water retention value of the fibers decreased after each cycle alternation. Besides, the FTIR spectra of the hornified fibers indicated an increase in the intensity of the peak at 3400 cm−1, confirming the formation of new hydrogen bonds with the water molecules. Moreover, the modified fibers showed a higher intensity of the carboxyl (COOH) groups at 1750 cm−1 wavelength, as seen in Figure 7, indicating the increase in the hydrophobicity of the lignin, promoting better interaction amongst the major constituents of the fiber (lignin, cellulose, and hemicellulose) resulting in a stronger material. The influence of the treatment on the composition of the fibers was further supported by the shift of the differential scanning calorimetry (DSC) curve of the hornified fibers to a higher enthalpy of decomposition than the untreated one [44]. Indeed, the authors concluded that the modifications of morphology, physical properties, and chemical composition of the fiber have played a significant role in achieving higher dimensional stability, improved fiber-matrix bonds, and enhanced mechanical properties.Figure 7
FTIR spectra of natural fibers, and fibers after 5 and 10 cycles of wetting and drying cycles [44]. Reproduced with permission from Elsevier.A similar trend was observed by the same research group [100] while studying the effect of hornification on sisal fiber and cement matrix bonding. In another investigation, Ferreira et al. [101] evaluated the effect of sisal fiber hornification on Portland cement–based composites. Fibers submitted to ten times wetting-drying cycles showed improved fiber-matrix interfacial bond and frictional bond strength by about 40 and 50%, respectively, when compared with the untreated fibers. The higher interfacial bond created contributed to a significant improvement in the toughness and energy absorption property of the composites, according to the capacity of the fibers to bridge cracks for enhanced durability. Claramunt et al. [98] investigated the effect of hornification of softwood kraft pulp and cotton linter fibers to reinforce cement mortars on its durability and mechanical performance after subjecting the fibers to aging (4 wet-dry cycles). The flexural strengths of the aged composites were improved by about 13% (for kraft pulp) and 21% (for cotton linter) and the compressive strengths by about 20% (for kraft pulp) and 10% (for cotton linter) in comparison with the untreated fibers. In addition, cotton linters showed lower permeability ensuring lower decomposition of the fibers and higher environmental resistance in the aged composites. On the other hand, the presence of pits at the surface of kraft fibers promoted the degradation of the fibers, causing less resistance in their composites as compared to the cotton linter fiber-containing composites. Other studies have also demonstrated that hornificated fibers significantly improved the interfacial shear strength between the fiber and the cementitious matrix [102]. According to Lima et al. [102], sisal fibers submitted to ten alternation wetting-drying cycles presented a 55% increase in the average interfacial bond strength which allows the production of composites with better mechanical properties. Moreover, Ferreira and co-workers [103] achieved an increase of up to 40% fiber-matrix interfacial bond and 50% in frictional bond strength with hornification of sisal fibers. Although a reduction in the elastic modulus is usually observed, the process improves the deformation capacity and the rupture strength of the fiber due to the direct traction at the fiber-matrix interface. Hornified sisal fibers further show a reduction in the water absorption capacity and become more stable in their cross-sectional variability against the variation in humidity [103].


## 5.2.2. Fiber Modification via Chemical Treatments

Because of the variations in chemical constituents of natural plant fibers, for various reasons mentioned earlier, it is evident that their reinforcing capacity in geopolymer and cement-based composites vary significantly. In this regard, Ye et al. [104] studied the influence of different contents of cellulose, hemicellulose, and lignin on the properties of metakaolin-based geopolymers. It was observed that the degree of geopolymerization decreases with increasing the hemicellulose content. The alkaline attack of hemicellulose produces carboxylic acid that neutralizes the alkaline environment, causing a reduction in the geopolymerization reaction. As a result, this effect deteriorates the flexural and compressive strength of the composites that display a characteristic porous morphology and brittle failure mode. FTIR spectra show peaks at around 1610 cm−1 in samples containing 20% pure hemicellulose, which are due to the fact that carboxylate anion (COO−) starts to ionize for the formation of carboxylic acid [104]. This result shows the degradation of hemicellulose by alkaline media containing a mixture of 10 M NaOH solution and Na2SiO3. The mixing ratio of the two solutions was kept fixed by maintaining the SiO2/Na2O molar ratio of 1.33. On the contrary, increasing the fiber’s cellulose content enables them to achieve a denser microstructure that exhibits ductile failure mode characteristics, displaying enhanced tensile and compressive strength with a good fiber–matrix interface adhesion. In other studies, Klinke et al. [105] confirmed that higher carboxylic acid degradation products of hemicellulose and lignin are due to the alkaline wet oxidation pretreatment of wheat straw, resulting in a higher cellulose recovery (96%) as solid fractions. Consistent conclusions were also reported by Calvini et al. [106]. Gram [65] concluded that the degradation of plant-derived fibers in cementitious matrices is due to the high alkaline environment that dissolves the hemicellulose and lignin components of the fiber structure.In addition to these results, surface modification of lignocellulosic fibers using chemical treatments is beneficial. In particular, alkaline treatment of cellulosic fibers is essential not only for improving their adhesion to the matrix but also for removing debris and impurities at the surfaces of the fiber that prohibit its interaction with the binder material. It is one of the well-known techniques for reducing the embrittlement tendency of natural cellulosic fiber–reinforced composites [81]. Available literature suggests that mercerization of natural fibers gives rise to changes in the composition, morphology, and surface topography of fibers by removing hemicellulose, lignin, and other extractives, facilitating the adhesion between the fiber and the binding adhesives [11, 32]. Mwaikambo and Ansell [107] reported that alkali treatment of cellulosic vegetable fiber changes the fibers’ topography to a clean and rough surface, besides increasing the crystallinity index. A cleaner and rougher fiber offers more hydroxyl (OH) groups at the peripheral region that encourages bonding reaction with the cementitious binder material and mechanical interlocking, leading to improved performance composites. However, the authors remarked that the usage of appropriate alkaline concentration is essential, because otherwise the crystallite packing of the fibers may be reduced that may cause a reduction in the performance of the composites [107]:(2)Cell–OH+NaOH⟶Cell–O−Na++H2O+surfaceimpuritiesConcerning the alkalization of cellulosic vegetable fibers, Yan et al. [108] treated coir fiber with 5 wt.% NaOH solution (at 20°C for 30 min) to reinforce Portland cement–based concretes. They observed that alkali-treated coir fiber show clearer and rougher surfaces that improve substantially the mechanical strength characteristics compared to the untreated fibers. The influence of alkali (0.5 wt/vol% NaOH solution for 24 h) treatment of jute weave fibers in metakaolin-based geopolymer matrices has been analyzed by Sankar and Kriven [109]. A significant improvement in tensile strength (from 8.8 to 14.5 MPa) was observed due to the alkaline treatment of the jute fibers. This result could be attributed to the removal of the hemicellulose by the alkaline treatment leaving behind a rough surface fiber that promotes the frictional interlocking between the fiber and the geopolymer matrices. In other studies, Sedan et al. [45] treated hemp fiber with 6 wt.% NaOH solution for 48 h at room temperature to formulate cement-based composites. Compared to the plain cement pastes, the incorporation of alkali-treated hemp fibers noticeably improved the flexural strength of the composites by about 94%.Zhou et al. [110] reported on the use of pretreated hemp fibers in aqueous alkaline solution (2 wt.% Ca(OH)2 for 14 h at 20°C) to improve the mechanical properties and durability of concrete composites. After 28 days of curing, the tensile and compressive strengths, as well as the fracture toughness of the treated hemp fiber–reinforced composites were reported as 16.9, 10, and 7 to 13% higher, respectively, than the untreated fiber reinforcements. The authors noticed that the failure mode of the modified fiber-reinforced composite showed more ductile characteristics, indicating that the composites could be highly durable and resistant to long-term cracking. Beyond this, quantitative analysis such as surface roughness of the hemp fiber and resistance to fiber pull-out also supported the improvements in mechanical properties of the treated fiber-reinforced concrete composites [110]. Ozerkan et al. [111] investigated the effect of alkali treatment on the mechanical performance and durability of date palm fibers in cement mortar composites. The authors performed a durability test by measuring the sulfate resistance of specimens following ASTM C 1012 procedures in 4-month time duration. The fibers were treated with 2% NaOH and 0.173% Ca(OH)2 solution for an hour. Results showed that fibers treated with calcium hydroxide solution exhibited better tensile and flexural strength and stiffness compared to the NaOH-treated fiber reinforcements. The treatment offered also improved durability of the composites against sulfate attacks with the inclusion of 1 to 2% treated date palm fibers. Correspondingly, Jo et al. [112] studied the mechanical properties of cement mortar composites reinforced with jute fiber. The jute fiber was treated with 450 ml of 0.5% NaOH solution for 24 h aimed at improving strength characteristics and service life of the composites. The authors concluded that both raw and treated jute fibers augmented the overall mechanical performance of the cement composites due to the inhibition of crack propagation by the fibers’ crack bridging effect. It was observed that the use of alkali-treated jute fiber improves the compressive and flexural strength of the composites from 34.5 to 36.2 MPa and from 7.4 to 10.4 MPa, respectively, compared to the control mortar (unreinforced) samples. It was also noticed that treated fiber mortar composites improved the fracture toughness and post-cracking resistance behavior by 1.5 and 22.5 times, respectively, as compared to that of the control mortar. The raw jute fibers were able to improve those properties of the composites by only 1.4 and 20.4 times, respectively. Comparing to the untreated jute fiber, a slight improvement in alkali-treated jute fiber reinforcement was observed, which might be justified by the favorable fiber-matrix compatibility and enhanced fiber-matrix bonding achieved [112].Likewise, Lazorenko et al. [113] investigated the effect of successive alkali (5% aqueous NaOH) and ultrasonic treatment of flax tows on low calcium fly ash-based geopolymer composites. The amorphous components of the fiber were removed successfully via the treatment resulting in a 35% improvement in the flexural tensile strength of the geopolymer composites. The defibrillation of fiber bundles into fine micro-fibrils was shown to cause an increase in the fiber-matrix contact area, which explains the improved mechanical characteristics of the composites. Chen et al. [114] treated sweet sorghum fiber in a 2 M NaOH aqueous solution for 12 h before using it as a reinforcement material in fly ash-based (class F) geopolymer paste composites. The treatment was shown to yield rough surface fibers with pits encouraging mechanical interlocking between the fiber and the matrix. Moreover, the treatment improved the mechanical (splitting tensile and flexural) strength and fracture toughness. The transition from brittle to ductile failure mode observed in the splitting tensile test of the geopolymer composite confirmed the improvements in mechanical properties via the inclusion of treated sweet sorghum fibers.Another study [115] has shown that the flexural strength of 6 wt.% NaOH-treated flax fiber-reinforced cement composites was higher than that of the untreated ones. The treatment avoids the fiber-matrix debonding caused by fiber shrinkage during curing. In general, the incorporation of alkali-treated flax fibers causes a change in the rheological settings of the paste affecting positively the mechanical performance of the composites. Alshaaer et al. [77] studied the adhesive interaction between cellulosic luffa cylindrical fibers and metakaolin-based geopolymer binders. The high alkaline environment formed by a combination of sodium silicate and 6 M NaOH solution was shown to activate the geopolymerization reaction. Without the application of the alkali-pretreatment of the luffa fibers, the strong alkaline setting created for the reaction helped to extract the fiber’s hemicellulose and lignin constituents so that it produced a rough surface morphology with smaller fiber diameter, promoting strong bonding between the fiber and the binder. As a result, a significant improvement in compressive and flexural strength of the composite, 2.4 times and 4.4 times increase, respectively, has been observed, compared to the pure matrix [77]. The reduction in fiber diameter caused by the breakdown of fiber bundles into fine filaments favored greater wettability of the fiber within the metakaolin matrix that impairs fiber debonding. The authors observed crystal clusters of fiber-like or whisker morphology and cubic shape fine crystals grown in the produced geopolymer composite specimens, as shown in Figure 8 [77]. The development of such crystals was reported to deliver a local crack healing effect in the geopolymer composites.Figure 8
SEM images of crystal growth: (a) fibrous—A and B; (b) cubic–C in luffa cylindrical fiber-reinforced geopolymer composites [77]. Reproduced with permission from Elsevier.
(a)(b)(c)However, there are some contradictory research reports wherein the alkaline treatment of natural plant fibers does not show any improvement for use in geopolymer binders. For example, Sá Ribeiro et al. [116] evaluated the behavior of metakaolin-based geopolymer composites reinforced with bamboo fibers. The flexural strength of the composites was shown to remain the same for fibers treated with alkali and water. This means that further research has to be done for addressing the not completely understood mechanisms involved in the alkali treatment of natural fibers and the resulting effects on the composite mechanical properties. A summary of alkali treatment of natural fibers and their effects on the mechanical properties of polymer composites are presented in Table 1.Table 1
Effect of alkaline treatment of fibers on mechanical properties of composites.
Fiber typeAlkaline solutionTreatment timeMatrixCharacterized mechanical properties and their percentage improvementsRef.Specimen typeCompressive strength (MPa)Tensile strength (MPa)Flexural strength (MPa)Flexural toughness (MPa)Energy absorption (N.m)Hemp6 wt.% NaOH48 hCementCP——4.9——[45]UHFR——6.8 (39%)——THFRC——9.5 (94%)——Coir5 wt.% NaOH30 minCementPC22.4—9.81.232.7[108]UCFRC23.8 (6.25%)—11.2 (14.2%)6.34 (424%)54.5 (66.7%)TCFRC24 (7.1%)—11.9 (21.4%)6.64 (449%)58.4 (78.6%)EpoxyUCFRE—20.234.6——TCFRE23.8 (17.8%)40.4 (16.7)——Jute0.5% NaOH24 hMetakaolinUJFRC—8.8———[109]TJFRC—14.5 (64.8%)———Hemp2 wt.% Ca(OH)214 hCementUHFRC19.322.71———[110]THFRC21.24 (10%)3.17 (16.9%)———Jute0.5% NaOH24 hCementCCM34.5—7.4——[112]TJFRM36.2 (5%)—10.4 (40.5%)——Sweet sorghum2 M NaOH12 hfly ashG0—2.53.7——[114]TSSRC—3.4 (36%)5.5 (48.6%)——Kenaf5 wt.% NaOH2 hMetakaolinG032.5—6.98[117]KFGC30.2 (7%)—8.5 (21.8%)——TKFGC34.1 (4.9%)—9.21 (31.9%)——Sisal10 M NaOH1 hPPUSFRPPC—29-——[118]TSFRPPC—31 (6.9%)-——Date palm5 M alkali24 hMetakaolinG0——3.5——[119]TFRGC——12.5 (257%)——Note. Percentage improvements of measured mechanical properties are given in brackets; CP–cement paste; UHFRCP/THFRCP–untreated and treated hemp fiber–reinforced cement paste; PC–plane cement: UCFRC/TCFRC–untreated/treated coir fiber–reinforced concrete; UCFRE/TCFRE–untreated/treated coir fiber–reinforced epoxy; UJFRC/TJFRC–untreated/treated jute fiber-reinforced composites; UHFRC/THFRC–untreated and treated hemp fiber–reinforced concrete; CCM–controlled cement mortar; TJFRM–treated jute fiber–reinforced mortar; G0–pure geopolymer specimen; TSSRC– treated sweet sorghum reinforced composite; UKFGC/TKFGC–untreated/treated kenaf fiber–reinforced geopolymer composite; PP–polypropylene; USFRPPC/TSFRPPC–untreated/treated sisal fiber–reinforced PP composite; TFRGC–treated fiber-reinforced geopolymer composite.The use of silane treatment was found to be an effective means of modifying lignocellulosic fibers and making them more hydrophobic by reducing hydroxyl groups on the fiber surfaces, besides increasing the wetting behavior between fiber and matrix, which enhances the interfacial adhesion [95]. It was explained that when cellulosic fibers are treated with a silane coupling agent, a reaction with water molecules occurs forming two distinct functional groups: silanol (Si-OH) group at one end and alcohol (CH3OH) group at the other side [120, 121]. The silanol reacts with hydroxyl (OH) groups of the cellulose fiber producing a covalent bond (hydrogen bonding) while the alcohol reacts with the matrix, promoting a strong interaction between the fiber and the matrix, providing a bridging effect at the interface region.According to the report compiled by Huda et al. [122], silane treatment offers the maximum bonding between fiber and matrix by increasing the surface area of the fibers and the degree of cross-linking, which leads to improved composite performance and durability. Li et al. [24] modified the surfaces of coir fibers with 2-ethylhexanol coupling agent in cement-based composites. The use of 2-ethylhexanol coupling agent emulsifies and removes the natural protecting waxes and oily substances on surfaces of the fiber creating more capillarities, which permits more cement paste to penetrate the coir fibers producing a strong bonding with the cementitious matrix. The authors reported that treated coir fiber–reinforced cement mortar exhibited 20% overall improvement in flexural properties and ductility in comparison to the untreated counterparts, leading to enhanced performance of the composites [24]. Tonoli et al. [123] studied cement-based composites reinforced with surface-modified eucalyptus cellulose pulp fibers. The fibers were treated with silane (methacryloxypropyltri-methoxysilane and aminopropyltri-ethoxysilane) using a silane to eucalyptus cellulose pulp fiber weight ratio of 6% to explore the influence on the mechanical and microstructural characteristics of the composites for enhanced durability. Even though both treatments enhanced the modulus of elasticity and the resistance to fracture in comparison to the untreated fiber-reinforced composites, aminopropyltri-ethoxysilane treatment did not avoid fiber mineralization. Fibers modified with methacryloxypropyltri-methoxysilane showed improved performance by protecting the lumen from being filled with cement hydration products [123].Similarly, Bilba and Arsene [124] focused on the use of silane solutions (such as alkyltrialkoxysilane (S1) and dialkyldialkoxysilane (S2)) to treat sugar cane bagasse fibers for promoting the fiber-matrix interface interaction and composite durability. The silane coating solution was varied from 0.5 to 8% by volume. The fiber showed an increase in dimension in all directions after being coated with 6% silane for 2 h. Observation using an optical microscope demonstrated that there was an increase in fiber dimensions after coating with 6% silane solution, offering greater fiber sites for bond interaction with the cementitious matrix. This trend was attributed to the improvement in water absorption resistance of the composites via better interface contact and bonding. The treatment changed the fiber texture to a rougher surface with striations when using S1 and granular when using S2, as presented in Figure 9, besides the densification achieved [124]. The hygroscopic nature of the fiber was shown to decrease greatly in both treatments, although S2 demonstrated a more water repellent effect.Figure 9
SEM micrographs of bagasse fiber: (a) untreated fiber; (b) fibers treated with alkyltrialkoxysilane; (c) fibers treated with dialkyldialkoxysilane [124]. Reproduced with permission from Elsevier.
(a)(b)(c)


## 5.2.3. Thermal Treatments of Fibers

Although there are few studies available in the literature, heat treatment has been reported as another alternative technique to modify natural plant fibers for strengthening fiber-matrix interfaces. For example, Bilba and Arsene [124] pyrolyzed bagasse fibers at 200°C for 2 h in a nitrogen atmosphere for their use in cement-based binders. The pyrolysis process reduces hemicellulose, some volatiles, and extractives in addition to the reduction of OH groups from the fiber surfaces. This treatment makes the fiber less hydrophilic and promotes enhanced adhesion to the matrix material, leading to improved composites’ resistance to aging. Wei and Meyer [125] applied thermal treatments on sisal fibers to improve the durability and degradation resistance of the fiber in cement concrete composite. The performance of the composite sample was evaluated by exposing them to wetting-drying cycles to accelerate aging and by measuring the resulting strength properties of the specimens. They found that thermal treatment of sisal fiber in a ventilated oven at 150°C for 8 h improves the durability of the composite by increasing the compressive and splitting tensile strength by 31.1% and 36.5%, respectively. The increased durability can be associated with the improved crystallinity of cellulose (since the amount of crystalline cellulose determines the strength of the fiber) [56]. However, in other studies [126], it was reported that heat treatment of eucalyptus fibers did not improve fiber-matrix interface properties.


## 6. Conclusions and Future Recommendations

This paper has presented a review of previous research works focusing on factors that cause the degradation of plant fiber–reinforced geopolymer and cement-based composites and the various countermeasures that have been put forward for durability enhancement of the composites. Alkaline hydrolysis of lignin and hemicellulose, fiber mineralization particular to only cementitious matrices, and external climatic changes are the main reasons identified for the deterioration of cellulosic fibers in such composites.The degradation of fibers becomes worse in Ca(OH)2 than in NaOH solutions due to the crystallization of lime in the fiber cells, besides the alkaline attack. The alkaline pore water dissolves the main constituents of the fibers, particularly lignin and hemicellulose. This situation becomes more aggressive and leads to complete loss of fiber microfibril integrity in the presence of external weather fluctuations (i.e., the variation in the relative air humidity and temperature) by creating favorable conditions for the removal of leached-out fiber components and restocking alkaline pore solution into the fiber cells frequently. Besides, the expansion and shrinkage of fibers associated with environmental changes induce the formation of fine micro-cracks. The growth and coalescence of these fine micro-cracks lead to micro and macro-cracks, resulting in total loss of fiber strength properties and the loss of the long-term service life of composites. Thus, reducing the alkalinity of the pore water is considered one of the most practical and efficient ways to slow down the degradation of plant fibers and to improve composite durability.In cement-based binders, the addition of pozzolanic materials (like silica fume, metakaolin, rice husk ash, etc.) reduces the alkalinity of the pore water to a greater extent so that fibers can sustain their properties for a longer time and become less brittle within the binding agent. Similarly, early age carbonation of cementitious composites not only reduces the alkalinity of the matrix by transforming the alkaline hydroxide into carbonates but also prevents the effect of fiber mineralization by increasing the depth of carbonation, promoting the reinforcing capacity of the fiber for a prolonged service duration. Moreover, fiber modification via different physical and chemical treatments can enhance the fiber-matrix adhesion by avoiding water-filled space in the transition region, which subsequently improves the service life of the composites. In these conditions, the fiber retains a water repellent nature; as a result, it maintains its dimensional stability so that the initiation of micro-cracking is greatly reduced and the fibers become hardly sensitive to the alkaline attack.However, due to wide variation in the chemical composition of cellulosic fibers, the development of a standard treatment procedure for fibers to be incorporated in given geopolymer or cement-based binders to yield improved mechanical strength and durable composites is a complex task demanding further investigation. In fact, developing the correct amount of mixture proportions, in particular to fiber content to binder ratio, is extremely important to obtain dense and uniform microstructural characteristics, leading to achieving better service life composites with the required strength attributes. This approach should reduce the porosity, shrinkage, and water absorption properties of the composites. Moreover, the available literature data for cellulosic fiber–reinforced geopolymer composites, in particular, is usually related to microstructural properties and strength aspects; further research is needed to elucidate time-dependent degradation phenomena as this binding material has the potential to replace cement-based matrix composites from the perspective of energy and environment-related issues. More importantly, since the development of cellulosic fiber–reinforced geopolymer composites is in an initial stage, the issue calls for more research efforts.Furthermore, it was reported that some fibers such as bamboo in a cement-based matrix and coir and sisal in slag-based geopolymer binders are unaffected when exposed to wetting/drying cycles in (Ca(OH)2) solution and water, and various accelerated aging conditions, respectively; no significant variation in strength and composition of the fiber has been observed, demonstrating their durability. To some extent, conflicting results are also reported about the effect of alkaline pretreatment of fibers in geopolymer composites. Such discrepancies limit the development of standardized design codes for establishing the right fiber treatment schemes; thus, a detailed analysis of the specific fiber-matrix interaction is needed in each case by probing the chemical reactivity at the interfaces.



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*Source: 1981755-2022-08-12.xml*