Abstract
This paper presents an optimization of the mechanical properties of cement–Posidonia composite by means of experimental characterization. We are interested in producing composite of cement reinforced by Posidonia raw fibers. The ratio of fibers and water-to-cement mass is varied to determine its effect on the mechanical properties of the cement–Posidonia composite, particularly, its resistance to fracture. Three point bending and compression tests were carried out to study the mechanical properties of the composite. Scanning electron microscopy was used to examine the surface of the tested samples. The experimental investigation shows an improvement of flexural strength for a ratio W/C equal to 0.5 and fiber content equal to 10 vol%.
Introduction
Although the use of natural fibers in construction applications dates back to centuries ago (roofs, surfaces for insulation), 1 only in the last few decades their application for the reinforcement of rigid matrices has been scientifically proven. In fact, thanks to their properties (mechanical, thermal, acoustical, etc.), these materials become widely used for reinforcement and insulation applications. Nevertheless, the development of such materials must take into account environmental considerations.
The thermal and acoustical insulation properties of these materials lead to substantial energy savings. 1 The reinforcement material must have good intrinsic properties (module of elasticity, geometry, etc.) and a compatibility with the cement matrix, while being cost effective.
Considerable industrial efforts have been made to exploit the flora rich in natural and marine fibers. Natural fibers are used as reinforcement in composites using cement matrix. Previous researches tended to determine the mechanical, thermal, and acoustical properties of cements reinforced by natural fibers such as hemp,2,3 flax, 4 bagasse, 5 coir, 6 sisal,7,8 etc.
The behavior of the material changes substantially following the fracture of the matrix.
Several factors can influence the properties of cements reinforced by natural fibers. They include the fiber type, geometry, fiber surface roughness, matrix properties, blending process, drying method, etc. 2
Posidonia oceanica, a marine biomass, which is lignocellulosic, renewable, light, and inexpensive fiber can be exploited to reinforce cement matrix and to consider new industrial application. In fact, the availability of large quantities of such fibers and the well-known mechanical properties are a general prerequisite for the successful use of these materials.
A review of the literature concerning Posidonia fibers shows a lack of technological investigations of these fibers, notably when used in cement composite.
The marine type of these fibers exists naturally in large quantities on the Mediterranean coastline. Posidonia fibers are likely to consolidate the cement materials with properties found in the classical fibers.
The objective of this paper is to show the promising properties of the cement composite. We investigated experimentally the effect of water-to-cement (W/C) ratio and the level to reinforcement by Posidonia fibers on the mechanical properties of cement using both three point bending and compression tests.
Introduction and characterization of Posidonia fibers
Raw materials
P. oceanica is a submarine plant found on beaches in the form of balls, which are agglomerates of fibers (Figure 1(a)). Posidonia balls were harvested in autumn from Monastir bay (Tunisia). They were mechanically crushed to extract fibers (Figure 1(b)). Then, these fibers were washed thoroughly with water to remove sand and impurity. Finally, they were dried in an oven at a temperature of 30℃ for 48 h and then stored.
(a) Posidonia oceanica balls, (b) P. oceanica fibers.
Scanning electron microscopy
ZEIXX field emission scanning electron microscope (SEM) was used for the examination of fiber and composite microstructure. For morphological and textural characterization of the P. oceanica fibers, external and transversal observations were carried out. For microscopic microstructure examination of the composites, cross-section along the longitudinal direction with different thickness was prepared.
Chemical composition
Natural fibers are biological structures composed mainly of cellulose, hemicellulose, and lignin. 9 In a smaller proportion, fibers contain extractable proteins and some inorganic compounds. 10
In order to determine the chemical composition of Posidonia fibers, a succession of extractions was carried out to isolate different substances contained in the material (waxes, pectin, hemicelluloses, and lignin). Protocols are presented in the literature.3,11–13 Fibers are treated by ethanol under agitation at 80℃ for 20 min to eliminate waxes. Then, they were treated twice with boiling water and twice with ammonium oxalate solution to remove pectin. Lignin is extracted in two steps in a mixture of chlorite of sodium and glacial acetic acid. Hemicelluloses are dissolved in a solution of potash and then in a sodium hydroxide solution. After extraction in soda, the cellulosic residue is estimated as follows
Preparation and characterization of Posidonia fiber–cement composites
Manufacturing composites
Portland cement type CEM II/A-L 32.5R was used to elaborate composites cement reinforced with raw Posidonia fibers with various contents between 5 and 20 vol %. Prismatic specimens with dimensions 40 × 40 × 160 mm3 were made from prepared mixture according to the AFNOR NF EN 196-1 standard. First, cement is blended with water. The W/C mass ratio is variable; we used the following ratios: 0.5, 0.6, 0.8, and 1. Then, the weighed quantity of reinforcement is added by supporting blending. After blending, the mixture is poured into a mold to make specimen. The specimen is kept at an ambient temperature for 28 days and then put in an oven at a temperature of 50℃ until a constant mass state is reached.
Mechanical characterization
Three point bending test
A three-point bending test was carried out using a LLOYD Instrument tensile test machine of capacity 5 kN. We used specimens 28 days old. A span of 140 mm and a deflection rate of 10 mm/min were used for all tests. At least 12 specimens were tested using a three-point bending configuration for each composite formulation.
The values of the load and the bending were simultaneously recorded. Assuming that the material is homogeneous, the normal stress or the flexural strength Scheme of the flexural testing.
Compression test
For the compression tests, we used a Shimadzu mechanical testing machine of capacity 30 kN according to the AFNOR EN 196-1 standard. Twenty-eight-day-old specimens were used (Figure 3). The cross head speed was set at 5 mm/min. The stress–strain curves obtained from the compressive tests were used to determine the compressive strength. At least 12 specimens were tested in compression for each composite formulation.
Scheme of the compression test.
Results and discussions
Chemical and morphological characterization
Chemical characterization
The total average length for the tested Posidonia fibers was 8.36 mm, the average diameter was 0.13 mm, and the density was 0.42 g/cm3. The principal constituent of Posidonia fibers is cellulose (48.39%). Other main constituents include hemicellulose (18.91%), pectin (4.78%), lignin (23.12%), and waxes (4.8%). Similar constituent ratios are also reported in the literature. 14 In this reference, Ncibi et al. have clearly proved that the natural P. oceanica fibers contain significant amount of cellulosic substance. Furthermore, Posidonia fibers have a high amount of lignin. Thus, they can be considered as a promising biological resource to improve fiber–matrix adhesion in composites applications.
SEM observation should be conducted in order to characterize the morphology of these fibers.
Morphological characterization
Figure 4 shows the typical morphologies of the Posidonia fiber. Raw Posidonia fiber, noted A, similar to all lignocellulosic fibers (Figure 4(a)), is composed of several fibrils or technical fibers (noted B) partially linked together by a weak pectin and lignin interphase.
Scanning electron micrographs of Posidonia fibers. (a) Bundles of fibrils, (b) transverse cross-section of one fiber, (c) fibril, and (d) longitudinal section of fiber.
Figure 4(b) displays transverse cross-sections of a number of fibrils indicating not only the differences in their size and shape, but also the existence of empty spaces in the form of a hollow (noted C).
The average thickness of the wall of these fibrils varies between 0.5 and 1 µm.
In Figure 4(c), a high magnification of one of the cross-sections shows some irregularities in section with elliptic shape. In the center, a hollow known as lumen where nutrients and water flow along the fiber.
Figure 4(d) shows the main features of Posidonia fibers. The fiber has a rough surface with many protrusions. It can be pointed out that these features are similar to those observed in earlier studies in the case of Piassava and Talipot fibers.15,16 Some authors reported that these protrusions should help the mechanical interlocking of the fiber with the matrix in the composite. 15
Properties of cement paste
To characterize the cement paste, we carried out three point bending and compression tests after 28 days of cure, for a selected set of
Figure 5 represents the variation of the mean value of flexural and compression strengths of cement paste with Effect of W/C ratio on the mechanical properties of cement paste using flexural and compressive tests.
Mechanical properties of cement–Posidonia fibers composites
Bulk density
The relationship between the bulk density and the fiber content depending on the W/C ratio is shown in Figure 6. The density of these composites decreases for a larger quantity of Posidonia fibers included in the matrix. This is due to the lower density of the fibers comparing to that of the cement paste and to the onset of bubbles of air by fibers during mixing. The same trend is observed, if we increase the W/C ratio, the porosity and water absorption increases.
Bulk density versus fiber content.
Flexural testing
A three-point bending test was carried out to examine the mechanical behavior of cement composites and to study the influence of fiber contents on mechanical properties.
Figure 7 shows typical flexural strength–strain curves for cement paste and a composite containing 20% volume of fibers. Compared with the cement paste, the addition of Posidonia fiber in the cementitious composite significantly improves the flexural strength and ductility. The composite has a much higher capacity to withstand the load and crack than pure cement.
Example of flexural strength–strain curves for cement paste and Posidonia–cement composite samples (W/C = 0.5).
Figure 7 shows two different types of curves. The first type consists in a flexural strength–strain curve of cement sample having a linear elastic region which leads to a premature fracture because of its fragile behavior in traction. The second curve which corresponds to a composite material presents two main regions:
Region 1: The first region is linear and resembles that of the cement sample. It seems that the matrix is supporting the main part of the applied load. This region ends with an optimum corresponding to the maximum flexural load with a slow propagation of crack. At this stage, fiber–matrix interface is affected. Indeed, the first macroscopic damage in the material can be easily observed on the sample (Figure 8). The picture shows that some of the fibers are debonded on both sides of the crack. We can also observe that some of the fibers link the two sides of the crack. Initially, the fibers in the composite are randomly oriented. During the bending test and as load increases, the fibers tend to become perpendicular to the direction of the load as it increases (Figure 9). This process leads to a pull-out phenomenon which disturbs the load transfer process.
3
Propagation of the crack in the composite during the three-point bending test. SEM micrographs of sample fracture surface of Posidonia–cement composite (5 vol %) after three-point bending test.
Region 2: When maximal flexural load is reached, the load decreases gradually with the propagation of the crack. At this stage, the presence of fibers in the composite modifies the behavior for the cement sample. Indeed, the content and length of fibers as well as their orientation are the main cause of the debonding process of the fibers at the cement–matrix interface. Therefore, no sudden failure is observed but a continuous decrease of the load is recorded (Figure 8).
Figure 10 shows the evolution of the mean flexural strength obtained from a three-point bending test for a fiber content varying from 0 to 20%. Flexural strength is shown for different W/C ratio. It can be observed that the highest flexural strength is obtained for a composite with a W/C ratio equal to 0.5. On the contrary, the flexural strength is minimal at a W/C ratio equal to 1. This is due to the lower density of this composite.
Increase of flexural strength with decreasing the W/C ratio for cement–Posidonia composite.
For a W/C ratio equal to 0.5, a steady increase of flexural strength is noticed by the addition of Posidonia fibers and reaches a maximum for 10% volume of fiber content. This increase can be explained by the rise of the number of fiber–matrix interfaces. Beyond this value, flexural strength decreases. This property has already been observed for composites with organic matrix. 17 Swamy and Mangat 18 showed that flexural strength of concrete fibers increases with fiber length and fiber content. Beyond a certain critical value, the flexural strength decreases quickly.
The flexural strength generally increases with fiber content until a state in which mixing becomes difficult. Thus, the homogenization of this mixture becomes difficult to the extent that fibers cannot be linked by the matrix and curling up occurs. This results in an increase of the porosity of the composite and lower fiber–matrix cohesion.
Naaman et al. 19 demonstrated that the efficiency of each fiber reduced for a larger fiber content. In fact, fracture strength is not being directly proportional to the rate of fibers, 20 even though some authors 18 proved the opposite in certain specific cases.
Compressive test
Compressive strength is one of the most important properties of cement composite materials. In a typical compression experiment, an axial force is applied to the rectangular specimen and the compressive strength is calculated from the value of the maximum force recorded.
Figure 11 presents the results of the mean compressive strength values using compression tests for fiber content from 5 to 20% volume. Finished compression tests show that the strength of cement samples is in the order of 7.04 MPa for a W/C ratio of 0.5. The compressive strength is improved by the addition of Posidonia fibers and reaches a maximum for 5% volume of fiber content. Beyond this value, the compressive strength decreases slightly when the fiber content is increased. This is due to the higher porosity of the mixture and consequently the reduction of fiber–matrix cohesion.
Evolution of the compressive strength of composite cement–Posidonia for a ratio W/C = 0.5.
Conclusion
The present paper shows that raw Posidonia fibers can be used in cementitious composites as a reinforcement material.
We investigated the effect of adding Posidonia fibers to cement matrix on the composite performance. This study has also confirmed that Posidonia fiber reinforced cementitious composites have better flexural strength, higher ductility, and lower density than conventional cementitious materials.
Better performances are achieved with the addition of a low percentage of Posidonia fibers in the cement matrix. It has also been found that optimal mechanical properties are achieved when 5–10% volume fraction Posidonia fibers are used for a ratio W/C = 0.5.
Footnotes
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Conflict of interest
None declared.