Abstract
Sponge gourd (Luffa cylindrica L.) fiber-reinforced cement composites were developed and analyzed. Dried sponge gourd fruit’s fibrous vascular system forms a natural 3D network that can reinforce matrices in composite materials, diverting cracks along the complex array of 3D interfaces between the fibers and the cementitious matrix. To avoid fiber deterioration, the cement paste was modified by incorporating pozzolanic materials. The fibers were mechanically characterized by tensile testing of strips of the 3D natural fiber array and of single fibers extracted from the array. The fibers had an average tensile strength of 140 MPa and an average Young’s modulus up to 28 GPa. Image analysis showed that the fiber spatial distribution inside the 3D network was random. The modified cement paste was characterized by its workability (flow table test) and mechanical behavior (compression and three-point bending tests), with average results of 430 mm, 62.7 MPa, and 6.2 MPa, respectively. Under bending, the cement matrix collapsed after the first crack. The sponge gourd-cement composite manufactured with 1 wt% of fibers showed an average flexural strength of 9.2 MPa (approximately 50% greater than the unreinforced matrix). Importantly, the composite also presented a limited deflection-hardening behavior. These results support sponge gourd’s possible use as reinforcement in cement matrix composites.
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Development and Analysis of Sponge Gourd (Luffa cylindrica L.) Fiber-reinforced Cement Composites
Victor A. Querido,a José Roberto M. d’Almeida,a,* and Flávio A. Silva b
Sponge gourd (Luffa cylindrica L.) fiber-reinforced cement composites were developed and analyzed. Dried sponge gourd fruit’s fibrous vascular system forms a natural 3D network that can reinforce matrices in composite materials, diverting cracks along the complex array of 3D interfaces between the fibers and the cementitious matrix. To avoid fiber deterioration, the cement paste was modified by incorporating pozzolanic materials. The fibers were mechanically characterized by tensile testing of strips of the 3D natural fiber array and of single fibers extracted from the array. The fibers had an average tensile strength of 140 MPa and an average Young’s modulus up to 28 GPa. Image analysis showed that the fiber spatial distribution inside the 3D network was random. The modified cement paste was characterized by its workability (flow table test) and mechanical behavior (compression and three-point bending tests), with average results of 430 mm, 62.7 MPa, and 6.2 MPa, respectively. Under bending, the cement matrix collapsed after the first crack. The sponge gourd-cement composite manufactured with 1 wt% of fibers showed an average flexural strength of 9.2 MPa (approximately 50% greater than the unreinforced matrix). Importantly, the composite also presented a limited deflection-hardening behavior. These results support sponge gourd’s possible use as reinforcement in cement matrix composites.
Keywords: Lignocellulosic fibers; Sponge gourd; Mechanical properties; Cement composites
Contact information: a: Department of Chemical and Materials Engineering, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente, 225, Gávea, 22451-900, Rio de Janeiro, Brazil; b: Department of Civil and Environmental Engineering, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente, 225, Gávea, 22451-900, Rio de Janeiro, Brazil;
* Corresponding author: dalmeida@puc-rio.br
INTRODUCTION
Lignocellulosic fibers have attracted considerable interest as reinforcements for cementitious matrices, especially in developing countries, because these fibers have many advantages. For example, lignocellulosic fibers are readily available in many places around the globe at low cost. Moreover, they allow energy savings in their processing and are environmentally friendly, because they are biodegradable (Agopyan 1988). According to Swamy (1990), using these fibers to reinforce slabs, roof tiles, and prefabricated materials, among other uses, helps to improve the infrastructure of developing countries.
In fact, lignocellulosic fibers can be used to produce asbestos-free fiber cement boards and other cement structures with suitable mechanical properties. For example, the tensile and flexural strength of concrete is increased by the incorporation of kenaf fibers (Elsaid et al. 2011). Similar behavior was observed when coconut fiber was incorporated into a cement paste (Kwan et al. 2014). The fibers inhibited crack initiation and propagation and acted as a stress-transfer bridge after the first crack had formed, thus avoiding brittle fractures. This is an important characteristic because multiple cracking is the desired failure mode of fiber-reinforced cement composites, as toughness can be greatly enhanced (Naaman and Reinhardt 1996). With multiple cracks, the reinforced cement will present a quasi-strain hardening behavior instead of a brittle one (Li and Stang 2004). Silva et al. (2009) presented the potential use of continuous natural reinforcement to obtain a cement composite system with multiple-cracking behavior. This composite is able to bridge and arrest cracks, leading to high mechanical performance and energy absorption capacity. Sisal fibers can naturally present mechanical bond components (Silva et al. 2009) due to different fiber morphologies, which contribute to improved mechanical behavior. Nevertheless, the durability of natural fiber cement composites remains an issue, and proper attention should be given to the matrix design (Melo Filho et al. 2013).
In this respect, the fibrous vascular system of the dried fruit of sponge gourd (Luffa cylindrica L.) presents some interesting structural characteristics for reinforcing cement matrices. As shown in Fig. 1a, the dried sponge has a 3D array of continuous fibers forming a natural mat-like material. This 3D structure has been shown to be suitable to detour advancing cracks and to enhance the toughness of polymer matrix composites (Boynard and d’Almeida 2000). The random distribution of fibers in this material can also, in principle, reduce manufacturing costs, because fiber preparation before incorporation into the matrix will be simpler when using this natural mat. However, to the knowledge of the authors, luffa fibers have not been used to reinforce cement or concrete composites. There is also a lack of information about the mechanical properties of the fibers themselves, although luffa fibers are being used in applications where their mechanical performance is of prime interest, such as reinforcement in composites (Boynard and d’Almeida 2000; Kocak et al. 2015) or as scaffolds for tissues (Alshaaer et al. 2017).
Therefore, the objectives of this study were to characterize the tensile mechanical behavior of luffa fibers and to verify the feasibility of their use to reinforce cement matrices, describing the mechanical behavior of luffa-reinforced cement composites.
EXPERIMENTAL
The sponge gourds used in this study were obtained from a farm in the county of Itápolis (21° 35’ 45” S, 48° 48’ 46” W), São Paulo state, Brazil. Because the sponges had varied dimensions, including their cross sections, they were opened in the middle and pressed to obtain a lamina-like material with a fairly constant thickness (Fig. 1b).
Fig. 1. The sponge, (a) before and (b) after being opened
Fig. 2. Tensile testing of (a) the sponge gourd strip, (b) unbranched fibers, and (c) fibers with knots
The sponges were characterized with respect to their mechanical, morphological, and structural characteristics. Tensile tests were performed both on the sponge as a whole (using strips cut from the pressed lamina-like material) and also on individual fibers extracted from the sponges. Fifteen strips (500 mm long and 25.4 mm wide) were tested with a gauge length of 300 mm, under a displacement control rate of 0.5 mm/min. Figure 2a shows a test specimen used.
The single fibers to be tested were extracted after the sponges were opened but prior to mechanical pressing. These fibers were divided into two groups: one consisting of rectilinear and unbranched filaments and another with filaments that presented knots from branches (Fig. 2b and 2c). Ten fibers were tested per group to determine whether the presence of branches can impair the mechanical strength of the fibers. The tensile tests were performed following the recommendations of the ASTM C1557 (2014) standard. A gauge length of 20 mm was used, and the tests were performed at a displacement rate of 0.1 mm/min. The displacement was measured using a linear variable differential transformer (LVDT), as shown in Fig. 2b and 2c.
Although it is recognized in the literature that the fibers of the sponge gourd’s vascular system are randomly oriented, a measure of fiber distribution has not yet been reported, to the authors’ knowledge. Therefore, an analysis of the fibers’ spatial distribution was performed using the plugin OrientationJ, available with the free software ImageJ/Fiji (National Institutes of Health, Bethesda, MD, USA). Images of the sponge gourd surfaces were acquired using a stereoscopic microscope (Nikon SMZ800N, Nikon Corporation, Tokyo, Japan) and were analyzed. Quantitative and qualitative measurements could be made using the software. Qualitatively, colors could be attributed to specific orientations computed by the program for each pixel of the image and correlated with angles, ranging from -90° to +90° (Fig. 3).
Quantitatively, the measurement of the orientations could be made using a discolored image, in which delimitations were made to analyze the orientations within a certain location in the image.
Fig. 3. Color palette that correlates colors with fiber orientation
Figure 4 shows an example of how the analysis was performed, highlighting three of the areas analyzed. The ellipses delimit the areas for spatial analysis, in which two parameters are evaluated: the preferred orientation of the fibers within the specific area and the coherence with which the fibers’ angles adhere to the measured orientation. The coherence varies between 0 and 1; a value closer to 1 (100%) indicates a strong fiber coherence to the preferred orientation in the region analyzed.
Fig. 4. Example of area delimitations to evaluate the spatial distributions of the fibers within the 3D mat of sponge gourd
The fibers were also characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Scanning electron microscopy was performed to observe the morphologies of the fibers and to evaluate their cross sections. Evaluation of lignocellulosic fibers’ cross sections can be a source of error when the tensile mechanical properties of fibers are evaluated, as detailed in previous studies (Souza and d’Almeida 2014; Diaz et al. 2016). Direct measurement of the cross section by SEM has proven to be a good way to reduce errors (Diaz et al. 2016). The SEM analysis (JEOL JSM-6510 LV, JEOL, Tokyo, Japan) was performed on gold sputtered fibers using secondary electrons imaging at a beam voltage of 10 kV. X-ray diffraction was performed to determine the degree of crystallinity of the fibers. This is an important parameter when dealing with lignocellulosic fibers, because greater fiber crystallinity corresponds to greater chemical and thermal stability of the fiber. The analysis was performed from 5° to 80°, with steps of 0.02°/s, using CuKα (λ = 1.5406 Å) radiation at 40 kV and 30 mA.
The degree of crystallinity was calculated using the deconvolution method of the crystalline peaks and the amorphous halo (Diaz et al. 2016). With this method, the crystalline and amorphous areas were calculated, and then the degree of crystallinity was determined using Eq. 1 (Tserki et al. 2005),
(1)
where DC is the degree of crystallinity, Ac is the summation of the areas below the crystalline peaks, and Aa corresponds to the area of amorphous halo.