**4. Application in reinforced mortars by natural fibers as cementitious material**

An innovative solution to reduce the negative impact on the environment is the production of structural reinforced composites from these natural fibers. Section 4 introduced an alternative binder to improve the durability of these plant fibers in an alkaline environment of cementitious composite. CSA cement was used in coconut fiber-reinforced mortar to totally replace the traditional cement. This section is divided into 2 parts including the *mechanical properties of fiber-reinforced mortar*, and *carbonation resistance of fiber-reinforced mortar*. In each part, the comparison between unreinforced- and reinforced mortars is presented.

Among the natural fibers, coconut fiber is considered is a potential candidate for reinforcement in cement matrix due to its most ductile and energy absorbent properties compared to other plant fibers. Incorporation of natural fibers into cementitious composite could, therefore, constitute an alternative solution to waste management and contribute to the development of reinforced mortars by improving comfort performance in buildings. Besides, it is necessary to use alternative binders to improve the durability of these plant fibers into composites and reduce the negative impacts on the environment. In this section, the new formulations of mortar are proposed, in which the Portland cement is totally replaced by calcium sulfoaluminate cement (CSA cement). CSA cement, consisting of nearly 55% of calcium sulfoaluminate, could be considered as a clean, green and alternative binder due to its environmentally-friendly features [31]. Additionally, CSA cement contains a lower alkali content compared to

PC, i.e., the pH ranges of CSA cement and PC are 10–11 and 12–13, respectively. As a consequence, the lower pH value of CSA cement can also lead to the less natural degradation rate of the fibers in the alkaline environment of the cementitious matrix.

### **4.1 Mechanical properties of fibers-reinforced mortars**

The mechanical properties of fibers-reinforced mortars depend on various parameters such as intrinsic properties of fibers, fiber contents, fiber distribution, fiber orientation, interfacial transition zone (ITZ), i.e., fibers and cementitious matrix adhesion. The decrease in the compressive strength of mortars with increasing fiber content is observed. A part of the explanation is that the pectin, ash, and other impurities are included in the fiber component, inducing the reduction of the bond between fibers and cementitious matrix. Additionally, the higher air content and porosity, relative to the increase in fiber content, involve a decrease in compressive strength. The combination method of coconut fibers and CSA cement in mortar significantly increased flexural strength of mortar, up to approximately 17%, which meets the desired mechanical performance since fibers are used as reinforcement. However, at the higher content of fibers (≥3% by mass of cement), the flexural strength starts suffering a slight decrease due to much more fibers being in the restricted area of the brittle cementitious phase, which leads to the significant cumulative effects on the strength of the material. In addition, frictional energy losses considerably in the wake of pulling out of fibers due to the debonding at the interface, which is partly responsible for the failure.

**Figure 17** shows the typical evolution graphs of the force applied as a function of the displacement at mid-span of the specimen for unreinforced and 2% fiber-reinforced mortars. To clarify the understanding of the different periods of crack initiation and propagation in bending, five particular points corresponding to five load steps are noticed for reinforced mortar. Point A is at the end of the non-linear elasticity period (so A is also at the first of the linear period). This point shows how the normal displacement evolves in the elastic period during the flexural test. Crack has not occurred in this step, although the load reaches 55% of the maximum load. In the next step, point B represents the displacement in the linear part of the curve and corresponds to the point where crack starts appearing at the load of 85% the maximum. It should be noted that the formation and development of cracks also depend on the characteristics of supporting

**Figure 17.** *Typical curves of behavior in 3 points bending of mortars [32].*

(two) and loading (one) rollers of the flexural test. If one of them is capable of tilting or sliding slightly, a uniform distribution of the load over the width of the specimen is well applied. And thus, this induces the appearance of a single crack. Otherwise, multi-cracks would have occurred, and flexural behavior will be affected if all supporting rollers cannot freely rotate. Therefore, the scatter of cracks is observed on the cross-section of the sample in this case [33]. In the third step, point C corresponds to the peak of the force-displacement curve, i.e., the maximum of the flexural load. As the sample partly suddenly fails, point C′ is reached to introduce the residual force. The load reaches the maximum load, and some fibers begin pulling out from the cementitious matrix and then slip inside the mortar, as clearly shown by the drop from point C to point C′. The period from point D to point E is along the residual force step which mobilizes the shear resistance of fibers. This step describes a nearly constant load period while the bending displacement continues increasing due to the remaining fibers. The crack initiates at the base, i.e., an opposite plane to the applied load, of the sample and propagates toward the direction of loading in the wake of the appearance of the initial crack. In this stage, the contribution of fiber to preventing brittle fracture suddenly is shown clearly. Additionally, resisting fragmentation is observed as there is no spalling at the surface of the specimen due to the bridging effect of the fiber distribution. For control mortar, the bridging effect could not be observed. The sample shows a sudden drop at about 80% of the maximum applied force. The strain development of the control mortar is characterized by a non-linear elastic part followed by a nearly linear behavior before sudden failure occurs (fragile behavior). The single crack appears at the base of the samples on which it is believed to have the maximum bending moment and no shear load. The reinforced mortars show that a progressive load decrease is likely associated with a progressive rupture of the fiber-matrix interface and then limits a brittle fracture. The addition of fibers into mortar has remarkable effects on the cracking behavior of mortar. Fiber acts as a crack-arrester since the presence of fibers could contribute to preventing brittle fracture suddenly after the first crack appears. Also, the bridging effect of the fiber distribution induces a decrease in the crack width and length compared to the control sample at the same level of loading. The enhancement of toughness and preventing the development of cracks inside reinforced mortars are the most important contributions of fibers.

### **4.2 Carbonation resistance of fibers-reinforced mortars**

In terms of durability, the usage of CSA cement with low alkali content could lead to a significant decrease in carbonation resistance owing to the lower content of CaO compared to conventional cement. Additionally, several previous studies [34, 35] also pointed out the negative effects of the Ca/Si ratio on the carbonation resistance performance. They believed that a rapid carbonation degree was acquired in consequence of the rapid decalcification of calcium silicate hydrate gel (CSH) at the higher ratio of Ca/Si. Additionally, the formation of carbonation products that result from the decomposition of ettringite, which is the principal phase of CSA cement, and contributes to boosting the carbonation depth in mortar specimens. This observation also proves that the dense microstructure formed by ettringite has negligible effects on the carbonation resistance of the CSA cement-based matrix. Besides, incorporating fibers could improve the carbonation rate due to the high air content (the fibers act as channels and entrain air), encouraging CO2 penetration happened could be easier [36].

The various effects of carbonation on the performances of mortar were obtained. In detail, the compressive strength increased by approximately 9 and 33% for

### *Recycling of Tropical Natural Fibers in Building Materials DOI: http://dx.doi.org/10.5772/intechopen.102999*

conventional cement-based mortars incorporating and no fibers, respectively. In contrast, the carbonation process could induce a slight decrease by 1–3% in compressive strength of CSA specimens with and without fibers, respectively. The pore structure of composite, which acquires significant changes after accelerated carbonation, is partly responsible for these results. It should be noted that a higher carbonation depth is found in CSA specimens. Therefore, the relationship between carbonation resistance and mechanical strength seems to be significantly dependent on the binder type used in composite [37]. Carbonation-induced strengths of mortar are various due to the cumulated effect of fibers incorporated. Mechanical behavior, hence, could not be a substantial factor in deciding the carbonation resistance of the cementitious composite.

The process of carbonation also induces a slight decrease in the thermal resistance ability of the matrix. For instance, non-carbonated zones have a strong ability to resist temperature than others in carbonation. In detail, at elevated temperature (~900°C), the carbonated area lost up to 14% of its mass. Meanwhile, the mass loss value of the non-carbonated area is below 10%. This observation is the result of CaCO3 formation during the carbonation process. This compound is thermally decomposed at a temperature higher than 650°C. Otherwise, the calcium-carbonated filler, which is generated in conventional cement production, is the main phase decomposed at this temperature.

Mechanical properties of composite materials need to be assured considering the environmental vulnerability. Generally, exposure in wetting and drying cycles has strong effects on the mechanical properties of samples due to the repetition of the negative environment on the interfacial bonding between fibers and cementitious matrix. After the sample is exposed to wetting and drying cycles, compressive strength is the most critical factor in assessing the performance of composite materials [38]. The wetting and drying repetition has adverse effects on the mechanical performance of mortar, regardless of the number of fibers, and reduces both compressive and flexural strengths. Generally, losses in mechanical properties of CSA-based mortars were higher than that of PC-based mortars. However, it should be noted that the maximum compressive strength was observed after one cycle since complete hydration of cement was reached due to the addition of water during the wetting process. In the next cycles, due to the formation of crystallized hydrate products [39], more micro-cracks appeared gradually inside the mortar structure and induced a decrease in compressive strength. Both strength and deformation of mortar samples decreased at the higher level of porosity and the higher number of cycles. The loss of strength was observed when fibers were incorporated into the mortar. More pores in fine aggregate mortar appear due to adding coconut fibers, which creates a convenient environment for the deep penetration of ambient air and water. The change in mechanical strength with predicted tendency was governed by the porosity, the number of cycles and fiber content as well, i.e., the higher fiber content, the higher porosity, the higher number of wetting and drying cycles, the lower mechanical strength.

In conclusion, for natural fibers reinforced composite to become widely used construction materials, consistent and predictable results need to be obtained. To achieve these outcomes, further studies are required on these composite performances by testing and modeling, which are necessary to help the application of this material for the building materials widely. These outcomes might contribute to environmental benefits and sustainable development of the construction industries in the future.
