Introduction

Composites are materials used for very diverse applications including biomedicine [1], the civil construction sector [2] and aerospace [3]. These materials are formed from the union of two components in order to obtain a material with improved properties [4, 5]. The material in the majority phase is named as matrix, which can be produced using metallic [6, 7], ceramic [8, 9] or polymeric [10, 11], as shown in Fig. 1. It is usually a continuous and compact material responsible for the second phase named as dispersed phase [12].

Fig. 1
figure 1

Types of matrices used in composite materials

Full size image

The dispersed phase can be used through particles or fibers, being responsible for the main mechanical properties, such as strength and ductility, of the composite, being known as reinforcement [13, 14]. The matrix phase is responsible for shaping the material and distributing the stresses applied in it [5, 15]. A key feature for composite materials to perform well in service is the adherence between their phases [16]. If there is no adhesion, no composite formation occurs, as one material will be randomly dispersed within the other, without any solidarity between the mechanical efforts applied to the material [17, 18]. For this reason, a composite can be considered as formed by the sum of three factors: matrix phase + dispersed phase + adhesion.

In the specific case of cementitious composites, as the name suggests, the matrix is composed of cement-based materials, which can be obtained through the application of pastes, mortars or concrete [19, 20]. Since the concrete or mortar, itself is already a composite material, the behavior of cementitious composites is complex and can have a significant variation depending, depending on the type of fiber used as reinforcement. It is known that cement-based materials have excellent compression properties and fragile behavior [21, 22]. For this reason, the use of fibers is with the objective of improving the tensile strength and ductility [23, 24].

Historic

Although history shows that ancient people already dominated the technology of composites with natural fibers [25], as, for example, in the case of the Egyptians who used straws to reinforce clay bricks [21, 26], it was from the 1980s and 1990s that studies with vegetable fibers gained greater notoriety [27]. In the case of the application of fibers in general, it became popular in the 1960s, with the use of asbestos in cementitious matrices, this being the first cement-based composite used on a commercial scale worldwide [28]. However, after the identification of health problems related to the use of asbestos, a new type of cementitious composite appeared, using artificial fibers, such as steel and glass. The first products made using these artificial fibers were commercialized in the mid-1970s [28, 29].

Due to cost-related concerns, in the mid-1980s they started their first studies using vegetable fibers in concrete and mortar. Some of these studies deserve to be highlighted, due to their importance for the technology of cementitious composites. Swift and Smith [30] developed a model of the flexural behavior of composites based on cement reinforced with sisal fibers; Kulkarni et al. [31] evaluated the mechanical behavior of coconut fibers in cementitious composites; Kulkani et al. [32] studied the mechanical behavior of banana fibers applied in cement matrices; Mukherjee and Satyanarayana [33] evaluated the behavior of the stress x strain diagram for cementitious composites produced with sisal fibers; Morrissey et al. [27] studied the connection between the cementitious matrix and sisal fibers, creating a model of the adhesion between the two phases of the composite; Mukherjee and Satyanarayana [34] verified the mechanical influence of pineapple fibers in cementitious materials; Coutts and Warden [35] characterize concretes with incorporation of pineapple fibers, which is a type of pine fruit, through tests of tensile strength and breaking surface, proving the ductilization of the composite by the fiber. Coutts [36] studied the application of wood fibers to concrete, demonstrating an improvement in fracture toughness and a cost reduction of approximately 25% compared to artificial fibers. Akers and Studinka [37] evaluated the durability of cementitious composites with wood fibers, proving that increasing the adhesion between the phases of the material through appropriate treatments improves the durability parameters.

Other important studies continued the studies previously highlighted and improve the understanding of the behavior of this type of composite: Coutts and Ni [38] verified the behavior of cementitious composites produced from bamboo fiber treated by the autoclave process, improving the tensile strength of materials; Toledo Filho et al. [39] experimentally evaluated the compression behavior of mortars produced with the use of sisal and coconut fibers, observing that the use of fibers delay the propagation of cracks in the material, improving the ductility of the material. Quintero-Dávila et al. [40] have shown a significant improvement in ductility of cement past with bamboo fibers without chemical treatment and in ductility of cement past with coconut fiber waste [41]. Savastano and Agopyan [42] studied the transition zone in cementitious composites using mallow, sisal and coconut fibers, noting that the water/cement factor affects the transition zone, normally porous, cracked and rich in macrocrystals of calcium hydroxide. These characteristics are directly related to the fiber-matrix connection and the mechanical performance of the composite. All the works cited, although old, serve to highlight the beginning of studies of cementitious composites with vegetable fibers, where it is observed that the first fibers of this type studied were those of sisal, coconut, wood and bamboo.

Advantages and disadvantages

Fibers can be classified as natural, such as vegetable, mineral (asbestos) [43, 44] and siliceous (such as wollastonite) [45, 46]; or synthetic fibers, such as steel [47, 48], glass fibers [49, 50], carbon [51, 52] and polymeric fibers (nylon, polyester, PVA) [53, 54]. The main advantages of applying natural plant fibers are the availability of the material, as typically these materials are abundant in nature [55]. In addition, agro-industrial residues can be used, such as those from coconut, banana or sisal farms [56]. This type of fiber is environmentally friendly, as the production of the material does not emit polluting gases or consume mineral resources, in addition to the material being biodegradable [57]. Another benefit is the fact that the planting of this material is simple, not requiring very bold technologies, contributing to decrease the final price of this type of fiber, which is usually cheaper than synthetic ones [58, 59]. Another advantage that deserves to be highlighted is the density of the composite: as natural fibers are lighter, they produce less dense composites than synthetic fibers [60, 61].

The disadvantages can be presented as the fact that the properties of natural vegetable fibers are more heterogeneous than synthetic fibers. As they come from a controlled industrial process, synthetic fibers have well-defined properties and characteristics [62, 63]. Natural fibers, on the other hand, are subject to the weather and the climatic variation provided by nature, changing their properties depending on the geographic location, time or season of the year, and other environmental conditions. Another problem associated with vegetable fibers applied in cementitious composites is related to the durability of these materials [64, 65]. The fibers are chemically composed of organic materials, such as cellulose, lignin and hemicellulose, which are also the compounds responsible for the formation of microfibrils that reinforce the material matrix [66]. However, in an alkaline environment, as is the case with the cementitious matrix, these fibers undergo degradation due to chemical decomposition of lignin and hemicellulose, compromising the durability of the material [67, 68].

In this scenario, it is observed that some characteristics are essential for the understanding of cementitious composites with vegetable fibers. This work proposes a detail review of available literature and a critical discussion on the behavior of these materials, analyzing important properties of the fibers, such as critical length and optimum content, in addition to analyzing the characteristics of adhesion between the fiber and the matrix. In addition, some works that studied different natural fibers, such as coconut, sisal, pineapple, in cementitious matrices, will be explored.

Behavior of cementitious composites with natural vegetable fibers

In this topic, the principles and concepts related to the science of cementitious composites with vegetable fibers will be addressed. The mechanical, physical and geometric properties of the fibers, which alter the material properties, as well as the chemical properties related to the structure and composition of the fibers, that could affect the durability of the composite, will be discussed. The treatments carried out on the fibers that will improve the adhesion and durability of this class of material will also be analyzed. Economic study comparing natural and synthetic fibers will also be discussed in this section.

Geometric properties of vegetable fibers

The main geometric properties of fibers are length and diameter [69]. In general, these are variable, since natural fibers are produced through the direct action of nature, reducing the technological control of these properties [70]. Although the diameter of the fibers is an intrinsic characteristic of the material, its value is extremely irregular and not uniform in the same fiber [25]. Table 1 presents some geometric properties of natural fibers used in cementitious matrices.

Table 1 Geometric properties of natural vegetable fibers
Full size table

Some authors point out that there is a direct relationship between strength of the composite and the diameter of the fiber used, as highlighted by Tomczak et al. [90]. The authors carried out this analysis using 20-mm-long curauá fiber with diameters from 26 to 61 µm, as shown in Fig. 2. The authors observed that the increase in fiber diameter, in general, does not affect the breakage strain of the composite, which is around 4.5%, but reduces its breaking stress, that is, reduces its strength. In addition, the use of fibers with a smaller diameter favors an increase in the elasticity modulus of the material. This same behavior is observed by other authors in the study of pineapple [34], banana [32], sisal [33], coconut [31] and piassava [91].

Fig. 2
figure 2

Influence of the diameter of the curauá fiber on cementitious composites. Source: [90]

Full size image

It is necessary to compare the diameter values of the natural fibers highlighted in Table 1 with the artificial fibers. It is observed that the steel fibers have a diameter varying between 380 to 600 µm, highlighted by Huo et al. [92]. The natural fibers have a diameter ranging from 10 to 450 µm. It appears that some natural fibers have diameters smaller than steel fibers, indicating a potential for reducing the mechanical strength due to the discussion carried out in Fig. 2. However, this reduction in the diameter of the fibers may also indicate better workability properties of natural fibers when compared to steel fibers. Some natural fibers have diameters compatible with steel fibers. This is the case with sisal, coconut and piassava.

Evaluating the polymeric fibers, it is observed that the diameter varies between 24 and 40 µm as highlighted by Wang et al. [93], being in general thinner than the natural fibers. The glass fibers have a diameter of around 14 µm according to Bilisik and Ozdemir [94] and they are also thinner than natural fibers. It is known that steel fibers have a mechanical behavior superior to polymeric and glass fibers, which is related to several factors, among them the diameter of these materials. Thus, it is likely that the mechanical behavior of natural fibers in cementitious composites is an intermediate behavior between steel fibers and polymeric or glass fibers, which is very promising information to prove the feasibility of applying cementitious composites with natural fibers.

In the case of length, the fibers can be cut to present different values, with this geometric property being varied by the authors in several studies to define the critical length. The calculation of this property is possible by applying Eq. (1), through an assay called Pullout, outlined in Fig. 3 [95].

$${l}_{\mathrm{c}}=\frac{{\sigma }_{\mathrm{f}}*d}{2*{\tau }_{\mathrm{c}}}$$
(1)

where lc = critical length, σf = fiber tensile strength, d = diameter of the fiber, and τc = strength of the interface.

Fig. 3
figure 3

Scheme of the Pullout test

Full size image

This concept is defined as the shortest length necessary for the development of stresses in the fibers, equal to their strength, enabling mechanisms for transferring efforts between the fiber and the matrix [68, 96]. When the fiber length is less than the critical length, the anchoring is not sufficient to generate strength in the fibers interface, implying that the fibers do not act as reinforcement in the material. If the length of the fibers is equal to or greater than the critical length, the fiber may lock, which prevents its removal and increases the tension acting on the material, that is, the reinforcement is effective [97]. However, the use of very long fibers can negatively affect the strength of the composites, as fiber agglomeration can occur, compromising the matrix's wettability. Therefore, the ideal is to use natural vegetable fibers with a slightly higher than critical behavior.

Table 1 shows the critical length of natural fibers ranging from 10 mm for the curauá fiber to 40 mm for the flax, banana and pineapple fibers. The highlighted values are compatible with the values observed for polypropylene, polyolefin, polyacrylonitrile and polyvinyl alcohol fibers that vary between 11 and 42 mm, as highlighted by Wang et al. [93], Jeon et al. [98] and El-Newihu et al. [99]. Steel fibers, in turn, have a critical length varying between 12.7 and 30 mm according to Zhang et al. [100] and Huo et al. [92], slightly less than the critical length of the flax, banana and pineapple fibers, but compatible with the values observed for other natural fibers in Table 1. It is known that the critical length and the other geometric properties of the fibers affect various properties of cementitious composites, such as mechanical behavior and workability. As a result, the compatibility of the critical length values evaluated for natural fibers and other artificial fibers is beneficial from the point of view of the application of the fibers discussed in this article.

Another important concept related to geometric properties is the form factor or aspect ratio (A.R.) [75], which is defined as the relationship between the length of the fiber and the diameter obtained through a virtual circumference whose area would be equivalent to the cross section of the fiber [101, 102]. The length used to calculate the A.R. is that of the cell that makes up the fiber, and therefore, the aspect ratio is considered an intrinsic property of the fibers. If the A.R. of the fiber increases, the length of the fiber necessarily increased or a reduction in its diameter was caused. The main A.R. values are defined in Table 1. The importance of this parameter is that it can be used to standardize the geometric parameters of the fibers, that is, instead of varying the diameter and length in isolation, it is possible to vary both together using the A.R. value.

Most natural fibers have A.R. greater than 100, as is the case of sisal, jute, banana and bamboo highlighted in Table 1. Other fibers, such as flax, hemp and pineapple, have A.R. greater than 300, similar to fibers polymeric materials with A.R. varying between 288 and 667 [93]. Coconut and piassava fibers have A.R. between 35 and 55, similar to steel fibers whose A.R. value varies around 50 [100, 103]. That is, despite the fact that the A.R. information is very dispersed, natural fibers have properties compatibility with synthetic fibers.

Fiber orientation is another factor that causes direct impacts on the mechanical properties of composites, especially on long fibers [104]. Vegetable fibers, however, are generally classified as short fibers [105]. Furthermore, in cementitious matrices the fiber orientation is almost always random [106], as the material production process, usually molding, does not allow the use of fibers in a preferred direction. Therefore, there is no direct relationship between fiber orientation and mechanical properties for cementitious matrices.

Another important information related to the orientation of the fibers is the natural inclination of the fibers, defined by X-ray techniques and indicated by the microfibrillar angle of the fibers, illustrated in Table 1 [33]. This technique also allows to verify if the fibers are presented in the form of a helix, such as banana, or in a spiral form, such as coconut [32]. The shape of the fibers is important because the helix-shaped fibers tend to cluster in smaller percentages than the spiral-shaped fibers [34]. Therefore, the critical volume of these fibers’ changes depending on the shape and the microfibrillar angle. In general, the greater the microfibrillar angle, the less the fiber tends to agglomerate and the higher the fiber content that can be used in cementitious matrices without loss of mechanical properties. In addition, the microfibrillar angle is the slope of the chains that make up the fiber in relation to the microfiber axis and, therefore, is related to the deformation of the reinforcement when subjected to direct tensile stresses [107]. Indirectly, it is a measure of ductility of the material.

Another important characteristic from the geometric point of view is the irregularity in the shape of the fibers [70, 108], as already highlighted. In addition to varying length and diameter, the fibers have a roughness on their surface; this is, they are not entirely smooth [25]. Therefore, an important characterization of natural fibers is specific surface area. Even some treatments carried out on natural fibers increase the surface area, improving the adhesion with the matrix and the mechanical properties [109]. In other words, the greater the irregularity and roughness of the fibers, the better from a mechanical point of view because greater amounts of stress transfer bridges are formed [110].

Physical properties of vegetable fibers

The main physical properties of vegetable fibers are density, natural moisture content, water absorption and wettability, defined by the critical fiber volume. Regarding density, it is important to highlight that the use of natural fibers in cementitious composites generally reduces the weight of the material, as reported by several authors [71, 95, 96, 111, 112]. This is because the density of the cementitious matrix is approximately 2.16 g/cm3 [113], while the fiber density varies around 1.5 g/cm3, as seen in Table 2.

Table 2 Physical properties of natural vegetable fibers
Full size table

Comparing the density values of natural fibers, which vary from 0.50 to 1.50 g/cm3, as shown in Table 2, it is observed that these fibers are much lighter than steel fibers, whose density varies between 7.80 and 7.85 g/cm3 according to Zhang et al. [100] and Bilisik and Ozdemir [94]. They are also lighter than glass fibers, whose density is approximately 2.80 g/cm3 according to Bilisik and Ozdemir [94]. The density values are even compatible with those of polymeric fibers whose density varies between 0.91 and 1.30 g/cm3, highlighted by Jeon et al. [98] and Whang et al. [93]. These same authors point out that the main advantages of polymeric fibers are precisely their low density, and this characteristic is shared by natural fibers that are beneficial in this aspect.

Water absorption of the fibers is extremely high, as shown in Table 2. This makes the behavior of cementitious composites with natural fibers extremely problematic, due to two principles: first, as the fiber absorbs very excessive amounts of water, water may be lacking in the matrix cement for the cement hydration reactions to take place [137], and according to the high water absorption it can internally form a point of water accumulation that will make the composite weaker and consequently problematic [101, 138]. Several problems of durability presented by cementitious materials with vegetable fibers occur due to this problem. On the other hand, if the water absorption of the fibers was very low, another problem would occur because points of adherence of the efforts between matrix and reinforcement would not be formed [69, 139]. These points occur due to the suction that the fibers cause in the matrix paste, generating bridges caused mainly by the formation of ettringite crystals, whose appearance in the form of needles maintains a strong bond between the fiber and the matrix [140, 141].

A relevant fact in this analysis is the nature of the fibers that directly interferes with the absorption of water and the formation of the ettringite bridges. In general, the fibers can be fibrillar or tubular [142, 143], as shown in Fig. 4. The tubular fibers are cast on the inside and have a high adhesion to the matrix, superior to the fibrillar fibers that are massive, presenting less adhesion with the matrix and consequently lower mechanical strength [143]. This pattern, however, must be thoroughly analyzed due to the absorption of water, greater in the tubular fibers, which may cause problems of durability, regions of internal water concentration, or excessive suction of the matrix water may occur, compromising the hydration of the cement and the final strength of the composite [142, 144].

Fig. 4
figure 4

Aspect of the fibers: a tubular, example of the bagasse; b fibrillar, example of hemp. Source: [142]

Full size image

The highlighted behavior for natural fibers is different from other synthetic fibers that do not need to absorb water to form connection bridges with the cementitious matrix. This is the case of steel fibers, which have very low water absorption, and polymeric fibers, considered hydrophobic, with water absorption of practically 0% [99, 145]. Natural fibers, as shown in Table 2, present water absorption above 100%, reaching values of 360% in the case of curauá fiber and 400% in the case of banana fiber. This leads to the conclusion that the use of natural fibers without previous treatments is impracticable due to the problems highlighted in the previous paragraph.

Crystallinity is a property that indicates the degree of crystallization and amorphism of the fibers. It can be calculated by different techniques, such as X-ray diffraction (XRD), magnetic resonance spectrometry (NMR) or Fourier transform Infrared spectrometry (FTIR), with the use of X-ray diffraction being more common [115]. In the case of using this technique, the crystallinity index, which is given in%, can be calculated by equation (2).

$${\mathrm{CrI}}_{\mathrm{XRD}}=\frac{({I}_{\mathrm{c}}-{I}_{\mathrm{a}})}{{I}_{\mathrm{c}}}\times 100$$
(2)

where CrIXRD = XRD crystallinity index. Ic = intensity of the peak of the largest crystalline plane of cellulose, representing the crystalline region, the peak of 2θ = 22.6° is generally used [116, 119]. Ia = peak intensity of the most intense amorphous region for cellulose, the peak of 2θ = 19° is generally used [116, 119].

XRD crystallinity index is an important parameter to verify the efficiency of treatments on natural fibers, since an increase in this index improves the mechanical behavior of cementitious composites with natural fibers. This is because the increased crystallinity of natural fibers favors the increased adhesion between fiber and matrix, according to Ilyas et al. [146]. In addition, more crystalline natural fibers have greater atomic strength and greater wettability in the matrix, according to Pietak et al. [147]. That is why it is important to calculate the XRD crystallinity index in the study of treated natural fibers.

A final important physical property of fibers is wettability. In fact, this property is not only of the fiber, but of the composite formed, varying according to the type of matrix [147, 148]. The values, therefore, of wettability in mortar and concrete matrix vary significantly, depending on the composition and the proportion between binders, aggregates and additives used [149]. Wettability can be understood as the maximum fiber volume that can be absorbed by a matrix without loss of properties due to fiber agglomeration [150, 151]. It is an important concept that can be understood as the optimal fiber content that enhances the properties of the matrix.

In addition to wettability, which is the maximum recommended fiber content, another related concept is that of critical volume, defined as the minimum percentage of fibers that must be used for the reinforcement to be effective in the studied property [97]. That is, if a percentage of fibers below the critical volume is used, the amount is so small that it does not significantly benefit the properties studied.

Figure 5 presents the results of the study conducted by Marvila et al. [152], where the authors incorporated açaí fibers in the percentages of 0, 1.5, 3 and 5% in cementitious mortar matrices. The authors observed an increase in the compressive strength of mortars using 1% fiber, indicating that this percentage is already above the critical fiber volume. The highest values were observed with 3%, where the compressive strength was almost 8.5 MPa, much higher than 6.75 MPa observed for the reference composition. However, using 5% fiber, there was a drop in strength compared to the composition with 3%. This indicates that the wettability of the açaí fibers was compromised with the use of 5% of the material. This illustrates the importance of the concepts of wettability and critical volume in cementitious composites, since these two parameters must be investigated in research so that it is possible to define the optimum percentage of fibers in the material.

Fig. 5
figure 5

Compressive strength of mortars with açaí fiber. Source: [152]

Full size image

Mechanical properties of vegetable fibers

The main mechanical properties of vegetable fibers are tensile strength, modulus of elasticity and breakage deformation, as shown in Table 3. It is initially observed that the tensile strength of the fibers is extremely high, ranging from 43 to 79 MPa for the fiber from curauá to values from 413 to 1627 MPa in the case of pineapple fibers. Cement matrices, on the other hand, have extremely low tensile strength, with values around 1.1–1.34 MPa in the case of mortars, for example [137, 153, 154]. This information highlights the need to use natural fibers in mortars, as this is an ecologically correct and economically viable way to increase the strength of mortars.

Table 3 Mechanical properties of natural vegetable fibers
Full size table

The high tensile strength of the fibers is related to the reduced cross section of the material, which minimizes the possibility of the occurrence of an excessive amount of defects in the material composition. Some factors of natural fibers interfere with their tensile strength, such as the diameter of the fibers, as experimentally proven by Tomczak et al. [90] and other authors [31,32,33,34, 91]; the crystallinity of the material, where more crystalline fibers favor the increase in strength of the composite [146, 160]; and the cellulose content, which is responsible for the formation of microfibrils, responsible for the high strength of the material [139, 161]. The results of tensile strength in fibers are obtained by the Pullout test, as shown in Fig. 3.

When it comes to the fibers applied to the composites, the factors that interfere with the material's strength are: fiber content, which must be above the critical volume, but in a way that does not compromise the wettability of the matrix the critical length, representing the minimum length necessary for the transfer of forces between the fiber and the matrix [68, 96]; and the adhesion between fiber and matrix [17, 18], depending on factors such as water absorption, for example [142, 143].

Comparing the tensile strength of natural fibers with synthetic fibers, it is observed that steel fibers have a superior mechanical behavior with values around 1100–1600 MPa, highlighted by Zhang et al. [100]. The same occurs with glass fibers, with values in the order of 1500 MPa by Bilisik and Ozdemir [94]. Natural fibers, as highlighted in Table 3, have tensile strength values ranging from 100 to 900 MPa, with the exception of pineapple fibers with values above 1000 MPa, and of piassava fibers, less than 100 MPa. Although the values seem low, they are superior to polymeric fibers whose tensile strength varies between 400 and 650 MPa, highlighted by Han et al. [145] and Jeon et al. [98]. This indicates that, from a mechanical point of view, most natural fibers present an intermediate behavior between steel and glass fibers and polymeric fibers. The same pattern of behavior had been observed for the geometric and physical properties, highlighted in Tables 1 and 2, and prove the feasibility and versatility of the application of this type of fiber in cementitious composites.

In the case of the modulus of elasticity, it is observed that fibers with low modulus of elasticity, such as sisal, coconut and piassava, present a very low tension at the moment when the matrix breaks and transfers the effort to the fiber. On the other hand, fibers with greater elasticity modulus, such as flax, hemp and pineapple fibers, already present a high level of tension at the moment of the matrix rupture, which allows it to act as reinforcement immediately, in case its strength is not overcome [162, 163]. In other words, when the matrix breaks, fibers with less modulus of elasticity will not be mobilized instantly, there is an interval for them to start acting as reinforcement. On the other hand, fibers with a greater modulus of elasticity are mobilized immediately, however, as they are already subjected to a higher stress level, they may be closer to their breaking stress.

An important calculation related to the modulus of elasticity is presented by the mixtures rule, which can be used to estimate the density and modulus of elasticity of the formed composite [118, 164]. In the case of the modulus of elasticity, Eqs. (3) and (4) apply [165]:

$${{E}_{\mathrm{c}}}^{\mathrm{u}}={E}_{\mathrm{m}}*{V}_{\mathrm{m}}+{E}_{\mathrm{f}}*{V}_{\mathrm{f}}$$
(3)
$${{E}_{\mathrm{c}}}^{\mathrm{l}}=\frac{{E}_{\mathrm{m}}*{E}_{\mathrm{f}}}{({E}_{\mathrm{f}}*{V}_{\mathrm{m}}+{E}_{\mathrm{m}}*{V}_{\mathrm{f}})}$$
(4)

where Ecu = upper limit of the elastic modulus of the composite. Ecl = lower limit of the elastic modulus of the composite. Em = matrix elasticity modulus. Ef = fiber elastic modulus. Vm = volumetric fraction of the matrix. Vf = volumetric fraction of the fiber.

To exemplify the calculation of the modulus of elasticity by the mixtures rule, the following values are used: modulus of elasticity of the cement matrix of 13.27 GPa [89, 113]; elasticity modulus of the 40 GPa curauá fiber [89, 107, 166]; and volumetric fraction of 4.4% fiber and 95.6% matrix [89, 107]. These values lead to a modulus of elasticity lower than 13.67 GPa and higher than 14.45 GPa, compatible with the experimental values ​​obtained by the authors [107, 167], proving the applicability of the mixtures rule in this calculation.

Comparing the modulus of elasticity values of the natural fibers, highlighted in Table 2, with the artificial fibers, an intermediate behavior is again observed. Natural fibers, whose modulus of elasticity varies between 20 and 70 GPa for most fibers, with the exception of piassava, coconut and sisal fibers, are more rigid than polymeric fibers, whose modulus of elasticity is approximately 3.8–7 GPa [93, 145]; however, they are more flexible than steel fibers whose elastic modulus is 200 GPa [103]. There is again an intermediate behavior between steel and polymer fibers, similar to the results of tensile strength, density, diameter and aspect radio.

A final mechanical property that should be mentioned is the breakage strain, varying between 1 and 5% in the case of jute, hemp, bamboo, banana, sisal and curauá fibers, and higher values in the case of coconut fibers and piassava, can reach 20 or 40% strain. These values are important because the use of fibers in cementitious matrices improves the ductility of the material. It is known that cementitious materials exhibit fragile behavior, which is not safe with regard to structural applications [123, 168]. That is why it is important to apply materials that ductilize the material's behavior, such as plant fibers [169]. This fact is illustrated in Fig. 6a, extracted from the study by Hussain and Ali [123] presenting the load x displacement behavior of a concrete named as PC in the composition 1:3:2:0.7 (cement/sand/gravel/water) and a composite containing an increase of 5% jute fiber in relation to the cement mass, named as JFRC, the matrix being composed of the same composition as the PC sample. It is observed that there was a considerable increase in tensile strength, as the maximum load increased from approximately 5.6 kN to almost 13 kN, leading to an increase in strength from 1.8 to 4.1 MPa, according to the authors. In addition, there was a considerable increase in the ductility of the section, observed by the increase in deflection at rupture from 3.5 to 15 mm, as shown in Fig. 6a,). Furthermore, it is possible to observe in Fig. 6b) that while the rupture of the concrete without fibers (CP) was abrupt and without crack propagation, which characterizes a fragile behavior, the rupture of the concrete with jute fibers occurred with the propagation of cracks, characterizing the behavior as ductile.

Fig. 6
figure 6

a Load × displacement behavior for concrete with jute fibers; b propagation of cracks during loading. Source: [123].

Full size image

Chemical properties of vegetable fibers

The main components present in plant fibers are celluloses, lignin, hemicellulose and pectin, as seen in Table 4. These four compounds combined are responsible for more than 95% of the fiber composition. Of these compounds, the most important is, without a doubt, cellulose, responsible for the resistance of natural fibers responsible for the formation of microfibrils [139, 161]. In addition, the more crystalline the cellulose, the more resistant it tends to be [122, 170].

Table 4 Chemical properties of natural vegetable fibers
Full size table

From the morphological point of view, it can be considered that plant fibers are composed of elementary fibrils, divided into three distinct layers, as illustrated in Fig. 7 [161]. The first layer is the middle lamella composed of pectin, hemicellulose and lignin; the second layer is the primary cell wall, composed of hemicellulose and cellulose; the third layer, in the central part of the fiber, is formed by the secondary cell walls, subdivided into three portions and composed of cellulose in the fibrillar form, responsible for the strength of the material [139, 174]. The distinct structure of the cell wall gives excellent mechanical properties to natural fibers, with low density, while also leading to the low durability of natural fibers in the cement matrix, due to the low resistance to degradation of lignin and hemicellulose in alkaline environments [161, 175].

Fig. 7
figure 7

Structure of plant fibers. Source: [161]

Full size image

It is observed in the chemical composition of natural fibers the presence of impurities, such as waxes, ashes, nitrogenous matter and sugar [176, 177], in general fructose [152], which can cause negative effects in cementitious matrices [178]. This is because sugar is a retardant in the hardening of cement, causing difficulty in the hydration reaction and delaying the gain in strength of the material [152, 179]. In general, it is admitted that at 28 days the concrete and mortar reached a satisfactory level of strength; however, with the presence of impurities in the natural fibers this pattern can be obtained at much older ages [109, 180]. This hinders the performance of the material, causing the need to carry out treatments to eliminate the impurities present. These treatments will be discussed in topic 3.

Finally, Table 4 shows the natural moisture content of the fibers, varying from 8 to 18%. The moisture in the fibers occurs because they are plant cells in association. A precaution that must be taken is the drying of fibers in greenhouses, which should not be carried out at temperatures above 100 °C, because they can degrade the fibers due to the natural moisture present [131, 132].

Economic studies on the costs of natural fibers versus synthetic fibers

Several authors [137, 181, 182] point out that the use of natural fibers is more economically viable than the use of synthetic fibers in composites. This is because natural fibers are a renewable resource that can be obtained through agricultural wastes from the production of fruits and other products. As an example, the residue from the pineapple crown, studied by Azevedo et al. [178] and [181], is highlighted in Fig. 8.

Fig. 8
figure 8

Recycling of pineapple fibers from agro-industrial waste in cementitious composites. Source: [181]

Full size image

In addition to cost reduction, recycling of waste provides sustainable development for the construction sector. Other recycled natural vegetable fibers, for example, are curauá [183] and açai fiber [137, 184]. However, even with the use of recycled fibers, there is an economic cost associated with fiber consumption and the chemical treatments necessary to produce the medium. Estimated data indicate that about 0.11 USD/kg of fiber is used to reduce the particle size of the material [178, 184] and approximately 0.03 USD/kg [152, 184] to perform heat treatments, which results in 0.14 USD/kg for recycling of natural vegetable fibers initially treated with waste.

These values are much lower than those observed for the synthetic fibers highlighted below: 0.80 USD/kg for steel fibers; 0.75 USD kg for glass fibers; 0.89 USD/kg for polypropylene fibers according to Mastali et al. [185] and Hussain et al. [186]. Although vegetable fibers from the agricultural industry are used, taking into account all production steps to obtain the material, from planting and harvesting to treatments for application in composites, the observed values are lower. Under these conditions, Shahunur and Hasan [187] point out that jute fibers cost approximately 0.21 USD/kg and that coconut and banana fibers cost 0.30 USD kg. Sisal fibers, the most expensive studied by this author, cost 0.45 USD kg, representing a reduction of 43.75% of the price in relation to steel fibers. If recycled fibers are used, the reduction increases to 82.5%.

In this way, it is possible to prove the economic viability of natural vegetable fibers, especially when they are recycled from agro-industrial waste, when compared to synthetic fibers.

Applications of natural fibers in cementitious matrices

In this topic, important aspects will be addressed for the application of vegetable fibers in cementitious materials. Initially, the classification of the fibers will be presented, followed by some important characteristics about the treatments carried out on the material to enable the application in cement-based matrices. Finally, some relevant works will be presented that studied the application of different natural vegetable fibers in mortars and concrete.

Classification of vegetable fibers

The classes of fibers used in cementitious matrices, as presented in Introduction, are: artificial, such as steel, carbon and polymeric ones; or natural. In the case of natural fibers, as shown in Fig. 9, there is a subdivision that depends on the origin of the fiber, which can be mineral, such as asbestos, animal or vegetable, the focus of this work [82, 114].

Fig. 9
figure 9

Classification of natural fibers. Source: [188]

Full size image

In the specific case of vegetable fibers, there is a classification of the fibers according to the part of the vegetable in which it is removed: seed, such as cotton; fruit, such as coconut fiber; bast or stem, such as flax fibers, jute and kenaf; leaf or hard, such as pineapple and sisal fibers; wood, such as soft and hard fibers; stalk, where rice, oat, rye and barley fibers stand out; and finally, fibers extracted from grass or reeds, such as bamboo and bagasse [82, 111, 114, 188]. The main common characteristic of these fibers is the chemical composition based predominantly on cellulose, creating problems of durability in an alkaline medium, such as that of the cementitious matrix. Therefore, it is necessary to carry out treatments, as discussed in topic 3.2.

Vegetable fiber treatments

The degradation of natural fibers immersed in Portland cement occurs due to the highly alkaline medium, which dissolves the chemical bonds of the lignin and hemicellulose phases, weakening the fiber structure and decomposing the material's fibrils [66, 138]. This was confirmed in the study by Toledo et al. [189], where the authors subject sisal and coconut fibers to exposure in alkaline solutions of sodium hydroxide and calcium hydroxide. The authors evaluated the changes obtained in the tensile strength of the formed cementitious composites. It was observed that the composites subjected to sodium hydroxide solution maintained the tensile strength in 72.7% (sisal) and 60.9% (coconut) after exposure in 420 days of durability test. For composites subjected to calcium hydroxide, the tensile strength was completely lost after 300 days of exposure. This pattern of responses occurs due to the crystallization of lime in the pores of the fibers, degrading the material and reduces its durability. Given the above, there is a need to carry out appropriate treatments that improve the durability of the fibers.

Some authors point out that the best way to improve the durability of plant fibers is to subject the material to a treatment with NaOH solution [137, 152, 190]. Parre et al. [190], for example, performed the analysis of the improvement of treatments caused by solutions containing 1, 3, 5, 7 and 9% NaOH in banana fibers. The procedure used by the authors was to dry the fibers previously in an oven, then submitting the material in immersion with solutions with a concentration of 1, 3, 5, 7 and 9% NaOH for 24 hours at normal room temperature. The fibers were then cleaned with distilled water to remove any particles of NaOH on the surface of the fiber and then were dried for 2 days in sunlight and kept in a hot air oven at 90–100 °C for 24 h to remove any moisture content.

The authors verified through FTIR analysis that there was a reduction in the elongation of the O–H bond, due to the strengthening of the hydrogen bonds between the O–H groups of the cellulose and hemicellulose molecules. Thus, the authors concluded that the treatment with alkali improved the mechanical behavior of the fibers, due to the strengthening of the hydrogen bonding bonds. In addition, most components of lignin and hemicelluloses were removed from the treated fibers and the hydrophilic nature of the fiber changed to a hydrophobic behavior. Through morphological analysis conducted by SEM, the authors found that there was an increase in irregularities in the surface of the fibers, which is beneficial because it improves the adhesion between the fiber and matrix. In addition, the authors proved that there was a reduction in the amount of lignin and hemicellulose observed in natural fibers and treated fibers. Morphological analysis also allowed the authors to observe that the presence of waxes and other impurities was reduced with alkaline treatment.

Finally, the authors performed thermal analysis by TGA where they verified the thermal stability of the studied fibers, both natural and treated. As can be seen in Fig. 10, the fibers treated with 5% have greater thermal stability, which is observed by the loss of mass that for this composition was 68%. Natural fibers, for example, showed a loss of mass of 78%, while fibers treated with 1, 3, 7 and 9% showed a loss of 72, 76, 70 and 76%, respectively. This indicates that the treatment carried out with a 5% NaOH solution is the most effective since this thermal stability is related to the level of decomposed cellulose during the treatment. As this compound contributes to the strength of the fibers, the treatment that presents the least mass loss is the one that causes less impact on the cellulose content, but that reduces the lignin and hemicellulose contents, in addition to other impurities.

Fig. 10
figure 10

Thermal analysis of natural banana fiber and treated with NaOH solution with a concentration of 1, 3, 5, 7 and 9%. Source: [190]

Full size image

Other authors carried out similar analyses, but analyzing other fibers, such as Gu [191] who evaluated the treatment of natural coconut fibers and subjected to treatment with NaOH solution in a concentration of 2, 4, 6, 8 and 10%. The author carried out the morphology analysis of the fibers by SEM, verifying the increase of the fiber surface area and the removal of impurities such as waxes, ashes and sugars, as shown in Fig. 11, in addition to performing tensile strength tests in composites produced with fiber and polypropylene matrix. The results obtained by the author were 602.8 MPa for natural fibers and 729.3, 726.1, 738.9, 734.4 and 680 MPa for fibers treated with 2, 4, 6, 8 and 10% NaOH, respectively. The same results were obtained for pineapple [109, 129], bamboo [38, 121] and açaí fibers [137, 152].

Fig. 11
figure 11

Microscopy of coconut fibers: a natural; b treated with 4% NaOH solution. Source: [191]

Full size image

Thus, it is possible to observe that the performance of treatments with NaOH solution acts to increase the adhesion between the matrix and the fiber, since it increases the specific surface of the material, as observed in Fig.  11 [137, 152, 191]. Also, it reduces impurities present in the fiber, such as waxes, ashes and sugars [109, 114, 190]; improves the absorption properties of the fibers, as seen in Table 2, which are very high [114, 190]; and improves the individual mechanical properties of the fibers, due to the strengthening of the reactions that make up the cellulose that forms the fibrils, increasing the crystallinity of the fibers [114, 190]. In addition, it reduces the content of lignin and hemicellulose, responsible for the low durability of cementitious composites with vegetable fibers [121, 190] and improves the thermal stability of the fibers, also reducing the loss of mass of the material during eventual burns [190, 192]. In other words, the performance of treatments is essential to obtain a composite with good mechanical parameters and durability.

Behavior of cementitious composites with natural fibers after exposure to accelerated aging conditions

As highlighted in topic 3.2, the performance of chemical treatments on vegetable fibers allows the application of this material in cementitious matrices. In this topic, some works will be discussed that carried out a study of the behavior of cementitious composites with natural fibers after exposure to accelerated aging conditions, proving the information contained in “Vegetable fiber treatments” section.

Initially, the work of Azevedo et al. [181] evaluated the behavior of cementitious composites containing untreated pineapple fibers and those treated with NaOH solution. The authors expelled cementitious mortars to degradation cycles in salt spray using a salt spray chamber with a 5% NaCl solution. The authors observed that the compressive strength of the reference mortar, without the inclusion of fibers, decreased from 7.83 to 6.81 MPa after the degradation cycles, as shown in Fig. 12. On the other hand, cementitious composites with untreated fibers presented very low strength values in both evaluated conditions, both before and after the degradation cycle. This was justified by the authors due to the presence of impurities, such as waxes, ash and sugar, which inhibits the hardening of the cement.

Fig. 12
figure 12

Compressive strength of mortar before and after evaluation of saline exposure. Source: [181]

Full size image

After treatment, however, the authors observed an increase in the compressive strength of mortars, mainly using 2.5% fibers. The behavior after the degradation cycle was also satisfactory and superior to the reference mortar. This is because the substances attacked by the alkaline medium of the cementitious matrix are removed from the pineapple fibers with the treatment carried out, through cleaning in NaOH solution. Pineapple fibers also increase their crystallinity after treatment, in addition to increasing their specific surface area and making it more irregular, improving adhesion with the matrix. This provides a compressive strength of 8.81 MPa before the degradation cycle in composites containing 2.5% treated fibers. The strength drops to 8.02 MPa after the degradation cycle, a 9% percentage reduction in strength, proving the need to carry out treatments on vegetable fibers before their application in cementitious matrices.

Other important works that verified behavior of cementitious composites with natural fibers after exposure to accelerated aging conditions are highlighted below: Ceferino et al. [193] evaluated cementitious composites containing natural cellulosic pulp fibers exposed to accelerated carbonation (20% of CO2 concentration) and exposed to accelerated aging, through 200 immersion and drying cycles. The authors verified that the composites present good mechanical parameters, as long as the composites are cured in an appropriate way and the natural fibers properly treated and applied in the cementitious matrices.

Wei [194] evaluated the useful life span of cementitious composites containing sisal fibers in alkaline media. The author found that the use of 30% metakaolin clay in the cementitious matrix reduces the alkalinity present in the sisal fibers and reduces the degradation of the material in accelerated media due to the reduction of the concentration of OH- in the cementitious matrix. This generates an alternative means in the treatment of fibers, instead of performing previous treatments with alkaline solutions, materials that reduce the alkalinity of the cementitious medium without loss of strength can be used, as proved by Wei [194]. As a result, the loss of tensile strength of the aged sisal fiber has been mitigated and the useful life of the composite has been extended by 13.7 times.

Finally, it highlights the work of Bui et al. [195], where the authors verified the accelerated carbonation of cementitious composites containing coconut fibers. The authors concluded that the use of appropriate treatments in the fibers reduces the depth of carbonation of the mortars studied, subjected to accelerated carbonation in a cycle of 12 weeks at 4% of CO2 concentration, temperature of 20 °C and 65% of relative humidity. The feasibility of applying natural vegetable fibers in cementitious composites was purchased, provided that the fibers are correctly treated as highlighted in topics 3.3 and 3.4.

Cementitious matrix composites with natural fibers

In this section, relevant works, published in recent years, of natural vegetable fibers in cementitious materials, that is, mortars and concrete, will be highlighted. The purpose of this section is to present the latest research findings and demonstrate that the field of study for this type of composites is still immense and needs to be further explored.

Cementitious matrix composites with banana fibers

Akinyemi and Dai [196] studied the application of banana fibers treated in an alkaline solution with a concentration of 5% NaOH in cementitious mortars in the proportion 1:3:0.35:0.3 (cement/sand/water/additive for workability), using 1.5% of banana fibers in relation to the cement mass. The fibers were 95 µm in diameter, 4 mm in width and 13 mm in length, in addition to 1.3 g/cm3 of density. The authors performed tests of tensile strength in flexion, modulus of elasticity, microscopy by SEM, and complementary analyses by FTIR and TGA. As a conclusion, they observed that the fiber increases the strength of the mortar and ductilizes the section, due to the formation of adhesion points. The use of ash increases the strength of the mortar (mainly when used by 1.5%) because pozzolanic reactions occur mainly between the fiber and the matrix, as observed in the microscopy analysis, where hydrated cementitious compounds were detected within the fiber region.

Poongodi and Murthi [197] proposed the development of a lightweight and self-compacting concrete using aggregates based on coconut shell. The use of coconut instead of the classic coarse aggregate in 30%, 40% and 50%, cured at 28 and 90 days, reduced the strength compressive of material. Therefore, they proposed the inclusion of banana fibers in levels of 0, 0.25, 0.50, 0.75, 1.0, 1.25, 1.50% in volume of concrete in order to increase the strength of the material. The banana fiber used had the following properties: diameter between 0.15 and 0.30 mm, length between 15 and 30 mm, density from 1.29 to 1.32 g/cm3, elasticity module from 12 to 13.5 GPa, and tensile strength of 275–350 MPa. Density, impact resistance and compressive strength tests were performed, where it was possible to verify the correlation between these two tests, as highlighted by the bibliography (R2 = 0.94). The authors had raised a hypothesis that coconut could cause a greater reduction in resistance to impact than a reduction in compression, but this fact was invalidated by the experimental results. The density indicated that the concrete behaves as light (less than 1800 kg/m3). Slump flow test tests were also carried out, in which it was found that the fibers do not act in an undesirable manner in workability; though there was a reduction, it could be controlled. The best results were obtained as 1.25% coconut fiber, improving the compressive strength and also the impact resistance of the evaluated concretes. Above 1.25% of coconut fiber, the authors verified that the saturation point of the matrix is reached, due to the loss of wettability. Elbehiry et al. [198] studied banana fibers of different species to apply the material in the form of bars in concrete, aiming to improve the tensile strength properties, reinforcing the material and improving ductility. The fibers were incorporated in the form of bars, in beams of reduced size with dimensions of 200 × 250 × 1000 mm, being tested the flexion in three points. As a main conclusion, the authors observed that the use of banana fiber in the form of a bar increases the flexural strength of concrete by 25%, highlighting the possibility of using this material.

Cementitious matrix composites with sisal fibers

Sabarish et al. [199] studied the incorporation of sisal fibers in concrete using a percentage of 1.5% in cement mass through the study of reduced beam models. The authors observed that the use of fibers improved the properties of compressive and flexural strength, in addition to discussing considerably the breaking behavior of the studied concrete elements.

Klerk et al. [68] studied the durability of sisal fibers in cementitious matrices studying an alternative way of treating the fibers using acetic acid. Conventional treatments with alkaline NaOH solution were also carried out. In addition, fiber extraction tests were carried out to evaluate the influence of chemical treatment on fiber strength, on fiber-matrix interaction and also to determine the critical fiber length. The samples were tested in bending tests at 28 days to determine the strength of the composite. Additional aging tests by prolonged curing in water at 24 °C, hot water saturated with lime at 70 °C, and alternative wetting and drying cycles were performed. Aged samples were tested at 90 days to evaluate the fiber's durability. It was found that a combination of alkaline treatment and acetylation was the most effective treatment condition, followed by alkaline treatment at low concentrations of sodium hydroxide. At higher concentrations of sodium hydroxide, the authors observed a significant reduction in the strength of the composites. The authors' main conclusion was that the combined chemical treatment improves the durability of sisal fibers in concrete.

Bello et al. [200] studied the use of sisal fibers impregnated with resins in cementitious mortars. The results were compared to composites with sisal fibers without impregnation. Tensile strength tests and shear tests were evaluated. The authors observed that the composite presented an elastic-brittle behavior with greater tensile strength (239.7 MPa) compared to dry fiber yarns (209.1 MPa), due to a greater uniform distribution of stresses between the fibers. That is, the main conclusion of the authors was that the use of impregnation in the sisal fibers slightly increases the strength of the composite, improving the ductility of the material because there is a greater transfer of efforts between the matrix and the fiber, due to the greater adhesion between the fiber’s phases.

Cementitious matrix composites with jute fibers

Islam and Ahmed [81] evaluated the interference of jute fiber in the properties of the fresh and hardened concrete state. Jute fibers with two different lengths of 10 mm and 20 mm and four different volumes of 0.00%, 0.25%, 0.50% and 1.00% were added. The specimens were tested for compressive strength at 7, 28 and 90 days and tensile strength at 28 and 90 days. In addition, several statistical analyses were carried out to verify the effect of the volume and size of the jute fibers on the concrete properties. The experimental results revealed that the addition of 0.50% jute fiber had an adverse impact on the fresh properties of the concrete, mainly using 20 mm fibers. However, a lower dosage, with 0.25% jute fiber, did not show any influence on the fresh state properties of the studied concretes. The results obtained in the statistical analysis showed that the length and volume of the fiber had a positive influence on the properties of the hardened concrete at the ages of initial and prolonged curing, respectively. That is, the variation in the length of the fibers alters the strength in the early ages, but in the more advanced ages there is no statistical difference. In the case of variation in the volume of fibers, there is no change in the initial ages, but an impact is observed in the final ages of the concrete, where the use of 0.50% of fibers is not recommended, due to the decrease in the obtained strength.

Sultana et al. [201] carried out the study of the incorporation of jute fiber in reinforced concrete, evaluating the change in the properties of compressive strength and tensile strength. Three independent factors in the composites were evaluated, such as water–cement ratio (w/c), length and volume of the jute fiber, through mathematical modeling and experimental results. It was observed in both analyses that the optimal values were obtained with fibers of 6 mm in length, applied in a volume of 0.2% and with a w/c factor of 0.55. Comparing the experimental and modeling results, it was observed that the optimal conditions foreseen in the modeling analyses validated varied around 5% of the values predicted by the experimental analysis. In addition, it is concluded that the use of jute fibers in reinforced concrete is feasible.

Cementitious matrix composites with pineapple fibers

Azevedo et al. [178] used natural fibers and treated with 5% NaOH alkaline solution in cementitious mortars 1:1:6 (cement/lime/sand) in percentages of 0, 2.5, 5, 7.5 and 10%. The authors evaluated the following variables: consistency, water retention, incorporated air, mass density in the hardened state, mechanical strength, water absorption by immersion and capillarity, in addition to checking the influence of the fiber incorporation mode. It was observed that the treatment with NaOH solution and the percentage of natural pineapple fiber incorporated in up to 5% in relation to the cement mass form mortars with technological viability, since there was an improvement in the strength properties and the other evaluated properties were not too harmed.

Marvila et al. [202] carried out the incorporation of pineapple fibers in mortars in the 1:3 mass composition (cement/sand), incorporating percentages of 0, 2.5, 5 and 10% of pineapple fiber in relation to the mass of the cement. Tests of compressive strength and flexural tensile strength were carried out, in addition to workability tests. The results confirm that the mortar containing 10% significantly improves the mechanical properties, but causes a high loss in the workability properties. Therefore, the proportion that contains 5% of fibers is the most suitable, because in addition to improving the mechanical strength does not affect the workability properties of the material.

Cementitious matrix composites with coconut fibers

Syed et al. [203] studied the incorporation of long coconut fibers with 5 cm in length in concretes, using 0.6 and 1.2% of fibers in relation to the total volume of aggregates. Slump tests, compressive strength, flexural strength and impact strength were performed. The results obtained were satisfactory, mainly with regard to flexural and impact resistance, which were increased. In addition, the authors point out that there was an increase in the ductility of the material, as observed in the propagation of cracks during the rupture process.

Danso and Manu [204] evaluated the incorporation of coconut fibers in percentages of 0.2 to 0.8% in cement and lime mortars. Density, water absorption, compressive strength, tensile strength and SEM microscopy tests were performed. The highest strength values, exceeding even the reference mortar, were obtained using 0.2% coconut fiber, both for compression and for traction. Through microscopy analysis, it was observed that the use of 0.2% of fibers allowed the fiber to be covered by the cementitious matrix, increasing the adhesion between the two materials and consequently the obtained resistance values. An increase in the percentage of fibers, however, hindered the iteration between fiber and matrix due to the overlap of the fibers, that is, compromised the wettability of the formed composite.

Cementitious matrix composites with açaí fibers

Azevedo et al. [137] carried out a study on the application of açaí fibers in cementitious mortars in the proportion 1:1:6 (cement/lime/sand), incorporating percentages of 1.5%, 3% and 5%. The authors studied the properties of consistency, water retention, incorporated air, mechanical strength (compression and flexion), mass density (fresh and hardened state), capillary water absorption. The authors concluded that the use of açaí fibers treated with alkaline solution in a percentage of 3% can be used as a structural reinforcement in cementitious composites due to the increase in the observed properties.

Based on this study, Marvila et al. [152] performed durability tests on cementitious mortars containing açaí fiber in percentages of 1.5%, 3% and 5%, treated with alkaline NaOH solution. Degradation tests were performed by wetting and drying cycles, exposure to salt spray and thermal shock. The authors proved that mortars containing up to 3.0% of açaí fiber were reinforced with the incorporation, presenting an increase in compressive strength from 6.23 to 8.41 MPa, but mortars containing 5.0% suffered a drop in strength due to the excess of fibers, which compromised the wettability of the cement matrix. The durability results proved that the presence of fibers in a percentage of up to 3.0% created an adhesion and tension transfer bridge that reduced the drop in strength and the loss of mass of the material when subjected to durability in the proposed tests, complementing the results obtained by Azevedo et al. [137] and proving the feasibility of applying natural açaí fiber to cementitious matrices.

Conclusions

The main objective of this work was to highlight the concepts and applications of vegetable fibers in cementitious matrix composites. The main properties of plant fibers were highlighted, such as geometric, physical, mechanical properties and information about the chemical composition of this material. The main geometric properties are the critical length, the diameter of the fibers and the aspect ratio. The critical length is defined as the minimum anchorage length between the matrix and the reinforcement. The fiber diameter has as its main characteristic the inverse relation to the tensile strength. The aspect ratio can be obtained by dividing the length by the diameter of the cell that makes up the plant fiber. Other relevant geometric information is the microfibrillar angle that can indicate the geometry of the fibers, in the form of a helix or spiral.

With regard to physical properties, density stands out, where plant fibers present values lower than the matrix, reducing the final weight of the composite, and water absorption, extremely high for plant fibers, reaching values above 100%. Another property worth mentioning is crystallinity, related to the tensile strength of the fibers. Regarding the mechanical properties, the tensile strength stands out, high for fibers, thus improving tensile property in cementitious materials. Other relevant properties are the modulus of elasticity, calculated by the rule of mixtures, and the deformation by rupture. It is noteworthy that the deformation due to rupture of the fibers is greater than the cementitious matrix, contributing to the increase in ductility of the cementitious composite.

Regarding the chemical properties, it is observed that the vegetable fibers are characterized by being predominantly cellulose, lignin, hemicellulose and pectin, with smaller amounts of wax, ash and sugars, which must be removed from the vegetable fibers to enable the application in cementitious matrices. It is noteworthy that cellulose is the compound responsible for fiber resistance, while lignin and hemicellulose are responsible for the material's low durability. Therefore, it is necessary to carry out treatments, usually with alkaline solutions, to improve the adhesion of the matrix with the fiber, increase the crystallinity and reduce the water absorption of the fibers, reduce the lignin and hemicellulose contents and remove impurities, such as waxes, ashes and sugars, enabling the application of fibers in cementitious matrices.

In the work, they also highlighted some viable applications of vegetable fibers in cementitious matrices, such as banana, coconut, pineapple, sisal and jute fibers. The application of other fibers in cementitious matrices, such as açaí fiber, which already has been initially studied, and guaruman fiber, which has already been studied for reinforcement in polymeric matrices, is suggested as studies in future works. This fiber has an extremely low density, around 0.57 g/cm3, microfibrillar angle of approximately 7.8°, crystallinity index of 60%, elasticity modulus of 21 GPa and tensile strength of 614 MPa, according to the references highlighted in that article. In this way, the viability of applying this fiber in cementitious matrices is verified.

Thus, it is suggested as future works:

  • Application of açai fibers in cementitious matrices;

  • Application of guaruman fibers in cementitious matrices;

  • Application of other alkaline treatments, such as potassium and magnesium hydroxide, on vegetable fibers;

  • Evaluation of the interference of the main fiber treatment parameters, such as time of exposure to the solution, treatment temperature, need for agitation or not, amount of solution required per kg of vegetable fiber.

  • Standardization of vegetable fiber treatments for application in cementitious matrices.