Performance and Durability of Cement Based Composites Reinforced ...

Materials and Manufacturing Processes, 22: 149–156, 2007 Copyright © Taylor & Francis Group, LLC ISSN: 1042-6914 print/1532-2475 online DOI: 10.1080/10426910601062065

Performance and Durability of Cement Based Composites Reinforced with Refined Sisal Pulp Gustavo Henrique Denzin Tonoli1 , Ana Paula Joaquim1 , Marie-Ange Arsène2 , Ketty Bilba2 , and Holmer Savastano Jr.1 1

Rural Construction Group, Department of Food Engineering, Faculdade de Zootecnia e Engenharia de Alimentos, Universidade de São Paulo, Pirassununga, São Paulo, Brazil 2 Laboratorie COVACHIMM, Departement de Chimie, Université des Antilles et de la Guyane, Guadeloupe This work evaluates the influence of three different intensities of refinement of sisal pulp and the effect of accelerated aging cycles on the behavior of composites. Fiber-cements were prepared by the slurry de-watering method and pressing as a crude simulation of the Hatschek process. Mechanical behavior of composites was evaluated by four-point bending test at 28 days, and after 50 and 100 wet/dry cycles. Refinement of pulp and aging have increased the strength and bulk density of composites and decreased the toughness and porosity. The best mechanical performance after aging was achieved for samples with highly refined fibers. Keywords Aging; Cellulose pulp; Fiber-cement; Porosity; Reinforcement; Sisal fiber; Wet/dry cycles.

One of the possible treatments to enhance mechanical performance of cellulosic pulp is the refinement process, which is carried out in the presence of water, usually by passing the suspension of pulp fibers through a refiner disc composed by a relatively narrow gap between the rotor and the stator [12, 13]. In the papermaking industry, refining or beating is an important stage in preparing the pulp aiming to improve the ability to form bonds among fibers. This is reached with the mechanical treatment of pulp due to the changing of the fiber structures and properties. The fiber treatment, a combination of flexing, shearing, rolling, and compressing, in the presence of water, has as the main function to plasticize it. This process, called internal fibrillation, starts with the removal of the remaining primary wall. The subsequent plasticization increases either wet flexibility or lateral conformability. It is generally assumed that the more highly plasticized fibers will compress more readily and, as a consequence, the area of inter-fiber and matrix-fiber contact will be greater [14]. Many other changes also take place during beating. The most important of these additional changes are fiber shortening and the generation of fines [14, 15], with fiber curling and weakening as secondary effects. These fibrillated and shorter fibers are responsible for the formation of a net inside the composite mixture with the consequent retention of cement particles during the dewatering stage of the manufacturing process. Enhanced fiber/matrix adhesion and mechanical performance can be achieved by increasing the fiber aspect ratio (specific surface area), by reduction of the fiber diameter and by production of a rough surface proportioning better mechanical anchorage [16]. This study was carried out in an attempt to produce a viable fiber-cement material using conventional sisal (Agave sisalana) Kraft pulp with three different degrees of refinement as the reinforcing fiber. The main objective was to evaluate the influence of refinement intensity of sisal pulp as the sole reinforcement on physical and mechanical performance of fiber-cement composites after aging cycles.

Introduction Sisal-fiber-reinforced gypsum and cement composites have been studied for applications as building materials [1–3] as the possible substitution of the asbestos-cement. These works are focused on fiber properties, fiber-matrix interface, and mechanical behavior of the composite in order to optimize behavior of fragile matrices with fibrous reinforcement for the achievement of thin components under flexural loadings. The utilization of natural fiber reinforced cement-based materials (NFRC) provides an alternative for cost-effective buildings [4] due to their initial low cost, adequate performance and durability. Wood and nonwood fibers have been studied because they offer many advantages: availability, simple processes for production of cementitious composites, renewability and recycling, nonhazardous nature and biodegradability [5, 6]. Sisal is one of the most widely used natural fibers, is easily cultivated, and provides short renewal time due to widespread grow [7]. Nearly 300,000 metric tons of sisal fibers are produced every year throughout the world [8]. Brazil and Tanzania are the main producers, with an amount of 191,000 and 23,000 metric tons, respectively, in 2004 [8]. The chemical composition of sisal has been reported by several researchers [9, 10]. According to these authors, cellulose content varies from 43–88%, lignin 3–9%, hemicelluloses 1–24%, waxes 2% and ashes 0.6–1.1% by mass, depending on the age of the plant. The success in a broad field of applications can be influenced by the choice of production method and by the ability to capture the optimum performance of the cellulose fibers [4]. It is also dependent on overcoming major concerns related to fiber degradation in alkaline media [11].

Received March 4, 2004; Accepted May 30, 2006 Address correspondence to Holmer Savastano Jr., Faculdade de Zootecnia e Engenharia de Alimentos, Universidade de São Paulo, P.O. Box 23, 13635-900 – Pirassununga, SP, Brazil; E-mail: [email protected]

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Experimental Fiber Preparation and Characterization Conventional sisal Kraft pulp was provided by Lwarcel Celulose e Papel, Brazil. Part of the unrefined pulp was submitted to a stirring process in water, only to provide fiber dispersion, maintaining the original CSF of 680 mL. Another part of the pulp with fibers concentration of 5.3 g/L in water was post-refined in a 300 mm disc refiner operating at current intensity of 50 amperes. Pulp was passed 10 and 15 times thru the refiner, for the achievement of CSF degrees of 220 mL and 20 mL, respectively. The Canadian Standard Freeness test (CSF) is a widely recognized standard measure of the drainage properties of pulp suspensions [17] and it relates well to the initial drainage rate of the wet pulp pad during the de-watering process. Low freeness values (less than 300 mL) are indicative of high degrees of external fibrillation and/or shortage of the fibers, leading to long drainage periods during the test. The CSF value of each pulp was determined in accordance with the correspondent Brazilian Standards (NBR 14344) [18]. The main physical attributes of the pulp were characterized by two procedures: particle size analyzer (Galai CIS-100 equipment) and optical microscopy. The analysis with Galai CIS-100 consists in the evaluation of the attributes of the whole fibrous material present in the pulp. Average length and width, coarseness, number of fibers per gram and fines content were analyzed and stored with the aid of the Wshape v.1.0 software. The optical microscopy permitted the evaluation of the entire fiber. Pulp was prepared by dispersion in water using a magnetic agitator and dilution to a 14 g/L concentration. Drops of each suspension were placed on a glass microscope slide. Microscopic morphological characterization was performed by image acquisition in equipment Olympus PV10-CB. Besides the average length and width of entire fibers, this analysis allows the evaluation of the lumen width and cell wall thickness. Composite Preparation Cement composite plates measuring 200 mm × 200 mm and reinforced by the sisal pulp were prepared in the laboratory using a slurry vacuum de-watering technique described in details by Savastano Jr., et al. [19]. Formulations (Table 1) were established based on previously published studies [20, 21]. Pulp was previously dispersed in water by mechanical stirring at 1,700 rpm during 1.0 h. The mixture formed with approximately 20%

Table 1.—Mix-design of fiber-cement composites. Raw material

Sisal pulp (CSF 680, 220, 20 mL) Portland cement (CP-IIE)b Carbonate fillerc

SS-680, SS-220, SS-20 (% by mass)

4.7a 78.8 16.5

Equivalent volume fraction = ∼4.5%. NBR 11578 [22]. Blast furnace slag = 6–34%; carbonate material = 0–10%. Specific surface area = 360 m2 /kg. c Specific surface area = 450 m2 /kg. a

b

of solids was stirred at 1,700 rpm for 20 min. The slurry was transferred to the evacuable casting box and the vacuum was applied (∼80 kPa gauge) until a solid surface formed. Three pads of each formulation were pressed simultaneously at 3.2 MPa for 5 min, then sealed wet in a plastic bag to cure at room temperature for two days and immersed in water for five days. Pads were cut wet into four 165 mm × 40 mm flexural test specimens using a diamond saw cooled with water. Specimen thickness was approximately 5 mm. Samples were allowed to air cure in an internal environment of 23 ± 2 C and 50 ± 5% of relative humidity prior to mechanical and physical testing. On completion of the air curing, specimens were tested at 28 days after production. Prior to mechanical test under saturated condition, specimens were immersed in water for 24 h to equilibrate. Wet/Dry Accelerated Aging Cycles This test aims to simulate natural aging with exposure to alternated cycles of wetting and drying. Specimens were successively immersed in water at 20 C ± 5 C during 18 h and exposed to the temperature of 60 C ± 5 C for 6 h in a ventilated oven as following standard EN 494 [23]. Each wet/dry procedure represents one cycle and it was performed for 50 and 100 times (50 and 100 cycles, respectively). Mechanical and Physical Characterization Tests Mechanical tests were performed in a materials testing machine Emic DL-30,000 equipped with 1 kN load cell. A four-point bending configuration was used to determine modulus of rupture (MOR), limit of proportionality (LOP), modulus of elasticity (MOE) and toughness values. A span of 135 mm and a deflection rate of 1.5 mm/min were adopted in the bending test. Equations (1) and (2) define MOR and MOE, respectively: P · Lv b · h2 P 276 · L3v · MOE = 3 1296 · b · h

MOR =

(1) (2)

where P is the maximum load, Lv is the major span between the supports, b and h are the specimen width and depth, respectively, is the deformation of the composite. Limit of proportionality (LOP) is described as the stress corresponding to the upper point of the linear portion of the stress–strain curve. Toughness [Eq. (3)] was defined as the energy absorbed during the flexural test and divided by the specimen crosssectional area. The absorbed energy was calculated by integration of the area below the load-deflection curve. Toughness =

absorbed energy b·h

(3)

where b and h are the specimen width and depth, respectively. Scanning electron microscopy (SEM) was used for the characterization of fiber-matrix interface on a fractured

CEMENT BASED COMPOSITE REINFORCED WITH REFINED SISAL PULP

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Table 2.—Microscopic morphological characterization of entire fibers.a Pulp CSF (mL)

680 220 20

Average length (mm)

Average width (µm)

Lumen (µm)

Thickness cell wall (µm)

26 ± 07 26 ± 06 23 ± 06

213 ± 49 210 ± 40 197 ± 41

95 ± 49 91 ± 40 90 ± 33

59 ± 11 60 ± 12 54 ± 11

a Pulp and Paper Laboratory, Department of Forestry Engineering of the Federal University of Viçosa, Brazil.

surface of specimens undergone mechanical tests. Samples were gold coated before being analyzed in a Zeiss LEO 440 microscope. Water absorption (WA), bulk density (BD), and apparent void volume (AVV) values were obtained from the average of ten specimens for each design, following procedures specified by ASTM C 948-81 [24]. Mercury intrusion porosimetry was performed using Micromeritics Poresizer 9320 (pressures of up to 200 MPa). Specimens were cut with a cubic side length of approximately 5 mm and completely dried prior to evaluation. The technique was adopted in order to evaluate the pore size distribution as it is usually applied in the characterization of cement-based materials [25, 26].

Figure 2.—Pulp submitted to intermediate refinement (CSF 220 mL): (a) 40× and (b) 1,000× (Pulp and Paper Laboratory, Department of Forestry Engineering of the Federal University of Viçosa, Brazil).

where yijk = observed mean for the refinement i, and aging j; = constant for all observations; Ri = fixed effect of refinement intensity i, for i = 1 (CSF 680 mL), 2 (CSF 220 mL) and 3 (CSF 20 mL); Aj = fixed effect of aging j, for j = 1 (0 cycles), 2 (50 cycles), and 3 (100 cycles); RAij = effect of interaction between refinement intensity i and aging j; eijk = random residuary effect associated with the observations on the refinement intensity i and aging j, with mean 0 and variance e2 .

Results and discussion Effect of Refinement on Fibers Morphological properties of the pulp (Table 2) were determined by optical microscopy. Only entire fibers were taken into account in this analysis with the purpose of evaluating the dimensions of undestroyed fibers. The reduction of the thickness of cell wall can be associated with the gradual opening of the external layer of the fiber, which increases for higher levels of refinement [14]. The fibers also acquire a ribbon-like shape which explains the diminution of lumen values in the refined pulp. Figures 1–3 show the aspect of pulps before and after refinement. The fibrillation observed in the refined pulps (Figs. 2 and 3) affects the retention of the cement particles. It also increases surface area of the fibers, reflecting directly in the performance of the composite material. The microscopy analysis has provided a good idea of the refinement effect in the pulp and how it affects the morphology of the entire fibers. However, it did not reflect the characteristics of the whole pulp, which includes the cut fibers and generation of fines. The characterization of the fibrous material, i.e., entire plus damaged fibers performed in Galai equipment gives a more precise idea of the actual morphology of the pulp. Results are shown in Table 3. Savastano, Jr., et al. [19] studied Kraft pulp from sisal field by-product. The pulp presented CSF 650 mL with fiber average length = 165 mm, average width = 135 m and aspect ratio = 122. The authors concluded that the unrefined sisal waste pulp was suitable for the reinforcement of cement based composite as affordable housing material.

Figure 1.—Unrefined pulp (CSF 680 mL): (a) 40× and (b) 1,000× (Pulp and Paper Laboratory, Department of Forestry Engineering of the Federal University of Viçosa, Brazil).

Figure 3.—Pulp after maximum refinement (CSF 20 mL): (a) 40× and (b) 1,000× (Pulp and Paper Laboratory, Department of Forestry Engineering of the Federal University of Viçosa, Brazil).

Statistical Analysis All the results of mechanical and physical tests were subjected to two-way analysis of variance with the design of cross-classification described in Eq. (4) in order to determine the significance of observed differences between sample means at the 95% confidence level ( = 005). yijk = + Ri + Aj + RAij + eijk

(4)

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G. H. D. TONOLI ET AL. Table 3.—Pulp and fiber physical propertiesa .

Pulp CSF (mL)

Average length (mm)

Average width (µm)

680 1.66 ± 0.02 22.2 ± 0.5 220 1.13 ± 0.05 18.7 ± 0.2 20 0.79 ± 0.01 20.0 ± 0.3

Aspect ratio

75 60 40

Coarseness (mg/100 m)

Fibrous material (106 fibers/g)

Fines content (%)

12.82 ± 0.12 4.69 ± 0.02 27.2 ± 0.2 9.99 ± 0.61 8.88 ± 0.93 40.6 ± 1.3 8.06 ± 0.10 15.64 ± 0.17 42.0 ± 0.7

a Pulp and Paper Laboratory, Department of Forestry Engineering of the Federal University of Viçosa, Brazil.

Comparison with values in Table 3 shows similar average length of the unrefined pulp. However, the average width was higher in the present work, which gives a lower aspect ratio. Table 3 depicts the drastic changes caused by the refinement to the universe of fibers present in the pulp. Average length was the most affected property, with a decrease of 52%, followed by the coarseness, with a reduction of 37%. Average width remained practically unchanged and fibrous material and fines content increased 233% and 54%, respectively. The distributions of length and width for entire fibers (microscopy method) and fibrous material (particle size analyzer) can be compared in Figs. 4 and 5. The reduction of the average length and of the coarseness is a consequence of the intensity of the refinement, which is also connected with the increase of fines content. The coarseness expresses the amount of mass by length unity and its decrease is due to the external layers of the fiber been removed by the mechanical treatment of the pulp. The definition of fines is related to the fibers with less than 75 µm of length [27] and consequently with almost no capacity of reinforcement of the matrix. According to Fig. 4, most of entire fibers are in the region of 2.0–3.0 mm for unrefined pulp (CSF 680 mL) or after intermediate refinement (CSF 220 mL). The peak is broadened and dislocated to the region of lower lengths (1.5–2.0 mm) for the higher refinement level (CSF 20 mL). Beating increases the portion of fibrous material with length of 0–1.5 mm by approximately 50% for the most refined

Figure 4.—Length distribution of entire fibers and fibrous material.

Figure 5.—Width distribution of entire fibers and fibrous material.

pulp (CSF 20 mL) with the consequent decrease of the portion with higher lengths. The width distribution (Fig. 5) is not affected by the refinement when entire fibers are considered. Curves are practically coincident and most of fibers are in the region from 15–25 µm. For the fibrous material, most of fibers lie in the 5–15 µm region which is increased by the refining procedures. Mechanical Properties of the Composite Effect of fiber refinement. The stress–strain curves of the composites containing sisal pulp with different degrees of refinement are shown in Fig. 6. Samples were submitted to 0, 50, and 100 wet/dry cycles. Cellulosic fibers are intrinsically stiff, and the refinement greatly improves their processability, which is necessary

Figure 6.—Stress–strain curves for composites with sisal pulp prepared with three different refinement intensities (CSF 20, 220 and 680 mL) and after (a) 0, (b) 50 and (c) 100 wet/dry cycles.

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Table 4.—Mechanical and physical properties of sisal fiber-cement composites, control sample (28 days of age) and after accelerated aging tests of 50 and 100 cycles. Fiber

CSF (mL)

Number of aging cycles

Sisal

680

0 50 100 0 50 100 0 50 100

220 20

MOR (MPa)

6.2 7.2 7.9 8.0 9.4 8.1 6.3 10.6 10.4

± ± ± ± ± ± ± ± ±

1.9 1.2 0.9 1.3 1.6 2.7 1.1 1.0 1.1

LOP (MPa)

3.4 4.2 6.6 6.0 7.0 6.8 5.4 5.8 7.3

± ± ± ± ± ± ± ± ±

1.3 0.4 2.5 1.2 2.6 2.9 0.7 1.0 2.2

MOE (GPa)

14.2 25.2 16.7 16.3 24.3 22.2 15.8 22.3 21.6

if the composite is to be successfully manufactured using the Hatschek production method [16]. The main effect of mechanical refinement on fiber structure is the fibrillation of the fiber surface, as reported in the literature [28, 29] and depicted in Figs. 1–3. The composites with unbeaten pulp and highly beaten pulp did not show a major difference in the MOR (Table 4) at 28 days (0 cycles). The unbeaten pulp presented filaments with no fibrillation and the mechanical anchoring seemed to be poor. For the composite with the most refined pulp, the shortening of filaments and the generation of high fines content (42.0% for CSF 20 mL, Table 3), which is approximately 50% greater than that of the unrefined pulp (CSF 680 mL), causes the low reinforcement capacity of the cellulosic pulp. The short fibers are unable to adequately contribute with the load transference to the fragile matrix. Although the pulp with intermediate degree of refinement (CSF 220 mL) had also presented an elevated fines content (40.6%) close to that of the most refined pulp, there was a significant increase in the value of MOR of the composite. This behavior can be attributed to an optimum level of adhesion and packing of fibers with the cement matrix for intermediate degrees of refinement and to the preservation of the aspect ratio around 60 (Fig. 7) of the fibers. This result confirms the necessity of avoiding excessive cutting and shortening [30, 31] during the refinement process. Toughness is often correlated to the length of reinforcing material. As the stress is transferred from the matrix to the fiber, debonding can take place at the interface and the fiber may be pulled out through the matrix, generating considerable frictional energy losses, which contribute to toughness [1]. Composites containing pulp with higher refinement degree (CSF 20 mL) presented a significant decrease of at least 92% (at 99% confidence level) in flexural toughness when compared to composites containing pulp with CSF 680 mL (Table 4) at 28 days of age. According to Soroushian and Marikunte [32], fibers with excessive bonding tend to rupture rather than pull out at the composite fracture surfaces, mitigating thus the desirable toughness characteristics associated with dissipation of energy. This suggests that the refinement softens the fiber, allowing it to adhere to the matrix and make intimate contact with surrounding particles [32–36]. The refined fiber-cement paste bond seems to be stronger than that of the unrefined fiber-cement paste bond as asserted by Mohr et al. [35].

± ± ± ± ± ± ± ± ±

2.0 8.4 2.2 2.2 8.0 7.8 1.8 4.1 7.7

Toughness (kJ/m2 )

1.43 0.22 0.41 0.89 0.14 0.11 0.12 0.15 0.76

± ± ± ± ± ± ± ± ±

0.68 0.14 0.38 0.39 0.05 0.09 0.04 0.02 0.27

WA (%)

20.5 14.8 14.8 19.7 15.5 10.5 20.3 15.7 14.4

± ± ± ± ± ± ± ± ±

2.0 1.0 1.3 0.9 1.3 0.9 0.7 1.0 1.1

BD (g/cm3 )

AVV (%)

± ± ± ± ± ± ± ± ±

33.1 ± 1.5 24.8 ± 1.2 24.5 ± 2.0 33.6 ± 0.8 27.3 ± 1.0 18.9 ± 1.0 33.9 ± 0.8 27.7 ± 1.6 24.0 ± 1.1

1.62 1.68 1.66 1.71 1.76 1.81 1.67 1.70 1.71

0.08 0.07 0.05 0.04 0.08 0.06 0.02 0.03 0.02

The excessive shortening of the refined pulp also contributes to the decay of the reinforcement capacity causing brittle rupture of composites. Consequently there is a progressive reduction of the strain to failure of the composite due to the increase of the refinement degree of the fiber as depicted by Fig. 6(a). The absence of fibrillation favors poor anchoring of the fiber to the matrix and considerable incidence of pull out. Similar results from the literature showed the same performance where unbeaten wood fiber composites exhibit greater toughness as compared to beaten fiber composites [16]. If the cellulosic fibers are fibrillated and the anchoring is improved, the fracture prevails over the pull out and toughness decreases for samples with 28 days of age (0 cycles). Effect of accelerated aging. The effect of aging can be observed in Table 4 and Fig. 6. The influence of refinement along the degradation process indicated that composites reinforced by fiber with higher intensities of refinement presented the best MOR performance. In general, MOR has increased with aging for all intensities of refinement.

Figure 7.—SEM of fracture surface of composite reinforced with refined sisal pulp CSF 220 mL at 28 days (250×). Arrow 1: pull-out. Arrows 2: strong adhesion and rupture.

154 The composite containing pulp with higher refinement has presented an increase of 65% in MOR, when comparing 100 cycle aged material with the control specimen (28 days). The difference in the MOR between composites with CSF 680 and 220 mL pulp was not statistically significant after 100 cycles. As discussed before for 28 days, the fibrillation accounts for the better mechanical anchoring of the fiber, with the consequent increase of the MOR. The aging effect associated to fibrillated fibers is illustrated in Fig. 8. In Fig. 8(a), short fibers appear to be intimately bonded to the cementitious matrix. Figure 8(b) shows that the external layer of the fiber was partially removed, confirming the optimized bonding between phases of the composite. Soroushian et al. [36] studied both refined virgin and recycled softwood Kraft fibers. In their investigation, micro-structural studies confirmed that the precipitation of cement hydration products within cellulose fiber cores (mineralization effect) and at interface zones is the key deterioration mechanism in composites. The flexural strength of composites, however, was not adversely influenced by the repeated cycles of soaking and drying. Mineralization of fibers and densification of interfacial zones caused tendencies toward increased strength and embrittlement with aging [36]. However, Agopyan et al. [37] reported the deterioration of load capacity of air-cured cement-based roofing tiles after five years under natural weathering in tropical regions. The capacity of reinforcement of coir fiber and cellulosic pulps suffered severe reduction, with the consequent decrease in the maximum load and specific energy of the composite (65% and 80%, respectively, in comparison with initial results). The decomposition of the hydration products of slag-cement matrix under carbonation process also affected the composite behavior. Additionally, the hygroscopic volume changes of vegetable fiber inside the cement matrix also contributed to microcracks and detachments between the two phases, causing the degradation of the transition zone. In the present investigation, toughness has greatly decreased with accelerated aging cycles, respectively to 29 and 12% of its original value, for composites with unrefined pulp and refined pulp at CSF 220 mL, respectively. The increase of fiber-matrix bonding explains the diminution of toughness because of lower energy dissipation due to fiber fracture. In the studies performed by Soroushian and

Figure 8.—SEM of fracture surface of composite reinforced with pulp refined at CSF 20 mL and after 100 wet/dry cycles. (a) 2,000× and (b) 3,000×.


Marikunte [32], 120 accelerated aging cycles also caused a drop of 77% in flexural toughness for specimens reinforced with unrefined pulp. Gram [38] also found significant losses in toughness with wet/dry cycling. The majority of these losses occurred by 12 cycles. Contrary to composites with CSF 680 and 220 mL pulps, composites with CSF 20 mL pulps presented an increase of 533% in toughness after 100 aging cycles. The improved contact surface area after refining contributes to the enhanced adhesion of the short fibers, despite the increase of the composite rigidity caused by a supposed mineralization or embrittlement of micro-fibrils after aging. This behavior was also reported by Mohr et al. [35], when toughness has increased by 2–32% after 25 wet/dry cycles in relation to 15 cycles. The authors also emphasized that after 5 cycles, composites with beaten fibers generally showed increased strength and toughness values when compared to unbeaten fiber composites. Limit of proportionality (LOP) represents the elastic region of the stress–strain curve and is related to the first crack strength of composites. At 28 days of age (0 cycles) the higher values of LOP for composites with refined pulps are indicative of improved fiber-matrix adherence and less susceptibility to plastic deformation. After 100 cycles, results of composites with fibers submitted to different intensities of refinement were statistically comparable. Aging seems to cause the increase of LOP for composites with unrefined and refined pulps, indicating the improvement of fiber-matrix adherence. Behavior of composites tested after 28 days was similar to those presented by Mohr et al. [35] after 78 days. However, after wet/dry cycles those authors found a decrease of approximately 50% of first crack strength and attributed this behavior to the embrittlement of fibers due to mineralization. Results of MOE were comparable for the different intensities of refinement. Aging provided the increase of the results after 50 cycles. The MOE suffered gradual decrease after 100 cycles but with values still, higher than at 28 days. This behavior could be associated with the increase of compressive strength [25]. Similar results were described elsewhere [19, 32, 36]. Physical properties After 28 days (0 cycles) the bulk density (BD) of the fiber cement increased due to the pulp refinement (Table 4). Higher values of BD were obtained for composites containing refined pulp at CSF 220 mL. These results can be attributed to the fibrillation generated by the refinement, which improves the packing of the composite. Additionally, refined fibers occupy less space due to the reduction of their inner volume (Table 2). Bulk density also increased significantly after cycles for both composites with unrefined and refined pulp, but differences were not relevant between 50 and 100 cycles. Soroushian and Marikunte [32] also observed densification for composites with unrefined softwood Kraft pulp (CSF 700 mL) after aging. The increase of BD with aging can be attributed to the formation of hydration products and their eventual carbonation around the cellulose fibers and inside of the lumen of fibers.


The physical properties were interrelated. The higher the BD, the lower both the WA and AVV. Water absorption and apparent void volume decreased significantly only after cycles of aging, for all the composites, with refined and unrefined pulp. Significant differences between 50 and 100 cycles were observed only for WA and AVV of composites with refined pulp. The technique of determining porosimetry by mercury intrusion was used to evaluate the size distribution of voids in the composites and helped with the understanding of the BD behavior. Figure 9 illustrates the performance of the composites submitted to the mercury intrusion porosimetry. Peaks in the zone of 800–1,000 nm are supposed to be related to cellulose fibers. Nita et al. [26] proposed that cellulose fibers are connected to the inclusion of pores with dimensions around 300 nm. Larger pores (dimension greater than 1,500 nm) can be observed when fibers are unbeaten (Fig. 9a). Pulp refinement causes the diminution of pore diameter in the zone of cellulose fiber-related voids, as evidenced by the dislocation of the peak that is in the region of 700–3,000 nm for composites with unrefined pulp, and in the region of 700–1,000 nm for composites with refined pulp. Beating process accentuates the ribbonlike format of the fiber and leads to its plasticization with the consequent improvement in the fiber-matrix bonding [14]. Mehta and Monteiro [25] evaluated mortars without fibers and identified capillary pores as those with dimension higher than 5 nm. This study with mortars showed similar mercury intrusion (∼0.05 mL/g) for pores around 60 nm. According to this, peaks of 5–100 nm in Fig. 9 are related to typical voids formed in hydration products and found in cementitious matrix regardless the presence of fibers. Aging effects were observed in composites with refined pulp (CSF 220 mL), mainly for the pores under 100 nm, which are related to the cementitious matrix (Fig. 9d).

Figure 9.—Mercury intrusion porosimetry of composites at 28 days: (a) CSF 680 mL, (b) CSF 220 mL, (c) CSF 20 mL, and after 100 aging cycles: (d) CSF 220 mL.

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Hydration and carbonation products have been formed with the accelerated cycles and promoted the reduction of the matrix pores, similar as demonstrated by Mehta and Monteiro [25]. The volume of major pores attributed to the fibers (around 1,000 nm) was not much affected by the wet/dry cycles. Conclusion This experimental study was developed to evaluate the behavior of composites containing Brazilian sisal pulp subjected to different degrees of refinement. The studies showed that the refinement causes intense fibrillation of the plant fibers, favoring their plasticization and mechanical anchorage in the cement-based matrix. However, excessive refinement also caused higher damage to the filaments, with expressive shortening and increase of fines. At initial ages, sisal Kraft pulp with intermediate refinement degree (CSF 220 mL) conducted to significant improvement of the modulus of rupture of cement-based composites. Such a behavior was a consequence of the strong adhesion between the cement paste and the refined fiber. The pulp beating favored the composite packing and the interfacial bonding of the two phases. The excessive refinement (CSF 20 mL) caused some decay in the mechanical performance probably due to the shortening of fibers and fines generation. The better adhesion of the fibers reduced the incidence of fiber pull-out during the composite fracture with the consequent damage to the toughness of the material. Pulp beating also played an important role in composites subjected to accelerated aging tests. In this situation the predominant effect seems to be the improved densification of the composite. The better packing provided by the plasticized fibers and the continued hydration of the composites subjected to wet/dry cycling were combined to an additional enhancement in the modulus of rupture, limit of proportionality and modulus of elasticity. The water absorption also presented a significant reduction that can be partially explained by the gradual carbonation of cement matrix. These results seem very promising in view of the potential applications of such non-conventional composites for cost-effective housing and infrastructure. Additional studies are required for the analysis of composites behavior under natural weathering in real applications. Acknowledgments Financial support for this research project was provided by Financiadora de Estudos e Projetos (Finep), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes) and Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp), in Brazil. The authors were supported by grants offered by CNPq and Capes, Brazil. They also acknowledge the support offered by the National Science Foundation (NSF), in the USA. Sisal Kraft pulp was provided by Lwarcel Celulose e Papel Ltda. and cementitious raw-materials were kindly furnished by Infibra Ltda.

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