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Claramunt, J., Ardanuy, M., and Fernandez-Carrasco, L. J. (2015). "Wet/dry cycling durability of cement mortar composites reinforced with micro- and nanoscale cellulose pulps," BioRes. 10(2), 3045-3055.

Abstract

A combination of reinforcements at different levels can have a synergetic effect on the final properties of a composite. The aim of this work was to produce, evaluate, and compare the wet/dry cycling durability of the exposure of cement composites reinforced with conventional pulps at the micro-scale level, with nanofibrillated cellulose fibers at the nano-scale level, and with combinations of both reinforcements (hybrid composites). To evaluate the durability of their mechanical properties, the composites were tested under flexural loading after 28 days of humidity chamber curing and after 20 wet/dry accelerating aging cycles. Composites reinforced with the nanofibrillated cellulose exhibited significantly higher flexural strength and flexural modulus, but they had lower fracture energy values than those reinforced with conventional sisal fibers. Moreover, the hybrid composites with a high content of nanofibrillated cellulose maintained or even improved their properties after aging.



Full Article

Wet/Dry Cycling Durability of Cement Mortar Composites Reinforced with Micro- and Nanoscale Cellulose Pulps

Josep Claramunt,a Mònica Ardanuy,b,* and Lucia J. Fernandez-Carrasco c

A combination of reinforcements at different levels can have a synergetic effect on the final properties of a composite. The aim of this work was to produce, evaluate, and compare the wet/dry cycling durability of the exposure of cement composites reinforced with conventional pulps at the micro-scale level, with nanofibrillated cellulose fibers at the nano-scale level, and with combinations of both reinforcements (hybrid composites). To evaluate the durability of their mechanical properties, the composites were tested under flexural loading after 28 days of humidity chamber curing and after 20 wet/dry accelerating aging cycles. Composites reinforced with the nanofibrillated cellulose exhibited significantly higher flexural strength and flexural modulus, but they had lower fracture energy values than those reinforced with conventional sisal fibers. Moreover, the hybrid composites with a high content of nanofibrillated cellulose maintained or even improved their properties after aging.

Keywords: Nanofibrillated cellulose; Cement mortar composites; Mechanical performance; Durability

Contact information: a: Departament d’Enginyeria Agroalimentària i Biotecnologia, Universitat Politècnica de Catalunya, E-08860, Spain; b: Departament d’Enginyeria Tèxtil i Paperera, Universitat Politècnica de Catalunya, E-08222, Spain; c: Departament de Construccions Arquitectòniques I, Universitat Politècnica de Catalunya, E-08028, Spain;

* Corresponding author: monica.ardanuy@upc.edu

INTRODUCTION

The use of cellulosic fibers as reinforcements for cement composites represents an interesting option for the building industry (Tonoli et al. 2009, 2010; Silva et al. 2010; Claramunt et al.2011). These fibers provide adequate stiffness, strength, and bonding capacity to cement-based matrices for the substantial enhancement of their flexural strength, toughness, and impact resistance. However, two main drawbacks restrict the performance of the material: (1) the maximum weight content of cellulosic fibers that can be incorporated into the composites in the form of short fibers (about 8 to 10 wt%), and (2) the long-term durability of the composite.

Concerning the first drawback, the problem is the agglomeration of the fibers during the mixing with the cement. Even using the Hatschek methodology, which allows a good dispersion of the fibers, the maximum content described in references is around 10 wt.%. On the other hand, for higher contents of 6 to 8 wt.% of pulp fibers, although there is an increase of the toughness, the strength and modulus are not improved (Savastano et al. 2001) (Claramunt et al. 2013). One possible alternative for increasing the reinforcement capacity of the fibers without increasing their percentage content above 6 to 8 wt.% is to use the fibers at the nanoscale level. It is well-known that the reinforcing capability of fibers can be increased by reducing their size to the nanometer scale. Nanofibrillated fibers can be obtained from vegetable fibers by subjecting them to mechanical, chemical, or enzymatic treatments. These fibers, which consist of alternating crystalline and amorphous domains, have a high specific area and an extensive hydrogen-bonding ability (Eyholzer et al. 2009; Lavoine et al. 2012).

Research related to nanofibrillated cellulose in polymer composites is widespread (Reddy et al. 2013). However, to our knowledge, research related to the use of these fibers as reinforcements in cement mortar matrices is scarce (Ardanuy et al. 2012).

The durability problem of cellulose cement composites is associated with an increase in fiber fracture and a decrease in fiber pull-out, which is caused by a combination of weakening of the fibers by alkali attack, fiber mineralization caused by the migration of hydration products to lumens and porous spaces, and volume variation caused by their high water absorption. This causes a reduction in the post-cracking strength and toughness of the material (Savastano and Agopyan 1999; Roma et al. 2008). The problem of the presence of calcium hydroxide can be overcome by modifying the composition of the matrix in order to reduce or remove this alkaline compound (Toledo Filho and England 2003; Mohr et al. 2007). The problems of the dimensional changes induced by moisture changes are a consequence of the composition of the vegetable fibers, which have a cellulose structure and other hydrophilic components with an affinity for water, thus favoring its penetration into the amorphous regions of the fibers. Various treatments have been used to minimize this problem, such as subjecting the fibers to previous water heating and drying treatments (Claramunt et al. 2011; Ferreira et al. 2014) or modifying the surface of the fibers to make them more hydrophobic (Tonoli et al. 2009). One of the advantages of using nanofibrillated cellulose is that higher dimensional stability is expected since the cellulose microfibrils have high tendency to bond with each other reducing the swelling and cellulose accessibility (Pönni et al. 2012). Moreover, as the nanofibrillated fibers will be mixed with the cement matrix dispersed in water, it is expected a better dispersion than in non-polar matrices, where the main problem is the agglomeration of the pulp (Eyholzer et al. 2009; Abdul Khalil et al. 2014).

The aim of this work was to produce, evaluate, and compare the durability to wet/dry cycling exposure of cement composites reinforced with conventional pulps at the microscale level, nanofibrillated cellulose fibers at the nanoscale level, and combinations of both reinforcements. For this purpose, composites reinforced with nanofibrillated cellulose were assessed and compared with those reinforced with conventional cellulose fibers and combinations of both reinforcements. The mechanical performance of these composites was tested under flexural loading after 28 days of humidity chamber curing and after 20 wet/dry accelerating aging cycles.

EXPERIMENTAL

Materials

UNE-EN 197-1:2000 Type I cement supplied by CIMENCAT (Spain) was used as a cement matrix. Based on previous studies, silica fume was used to replace 10 wt% of the cement (Fernández-Carrasco et al. 2014). The sand used, “quartz flour,” Hispania U-S500, has a similar particle size distribution to the cement and was supplied by Sibelco (Spain). Sika Viscocrete-3425 fluidizer, obtained from Sika, Spain, was used at a maximum dosage rate of 40 g/1000 g of cement to aid workability.

Sisal (Agave sisalana) pulp from a soda-anthraquinone cooking process was kindly supplied by CELESA (Spain).

Methods

Nanofibrillated cellulose was prepared by the application of a high-intensity refining process in a Valley Beater. Following the ISO 5264/1-1979 (E), 360 g of oven-dried sisal pulp were added to deionized water in such a way as to give a final volume of 23 L, corresponding to a consistency of 1.57% (m/m). The mixture was placed in the Valley Beater device, where the cutting and fibrillation of the sisal fibers took place as a result of the mechanical action. A refining time of 6 h was used (Ardanuy et al. 2012). The microstructure and morphology of the pulps were analyzed by scanning electron microscopy (SEM) in a Jeol JSM 6400 (USA).

In order to study and compare the reinforcing effect provided by the incorporation of the sisal microfibers and nanofibers, as well as the combination of both reinforcements, five series of composites were prepared following the same procedure described previously (Ardanuy et al. 2012). Cement/silica fume/sand proportions (by weight) for all the composites were 0.9:0.1:1. Table 1 shows in detail the composition of the samples prepared. To prepare the components, firstly a dispersion of water, fluidizer, and nanofibrillated pulp is prepared by mixing mechanically. The water content used corresponds to the final mixture of water/cement ratio equal to 1. Then cement and sand are mixed with the dispersion of the fibers. Finally, the mixture is divided in 3 equal portions which are pressed in the molds at 4.5 MPa for 24 h. During the first minutes of the compression process, samples lose the excess water. The final w/c ratio depends on the characteristics of the mixture, being higher for the composites with higher content of nanofibrillated pulp.

Table 1. Reference and Composition of Prepared Cement Mortar Composites

Rectangular solid specimens were prepared for the flexural tests. The mold used was UNE-EN 196-1:2005 with internal dimensions of 40 × 40 × 160 mm3, modified to allow the compression of the specimens to a 10-mm thickness. After demolding, the specimens were cured for 28 d at 20 ±1 °C and 95% relative humidity. Three-point bending tests were performed using an Incotecnic press equipped with a maximum load cell of 3 kN and controlled by the cross-head displacement at a rate of 2 mm/min. The modulus of elasticity (MOE) and the flexural strength were calculated following the Standard UNE-EN-12467 and the fracture energy following the TFR4 test of RILEM “The determination of energy absorption in flexure of thin fiber reinforced cement sections”. The composite durability was estimated with accelerated aging tests, a methodology widely used to study the degradation of vegetable fibers in cement matrices (Toledo Filho and England 2003; Mohr et al. 2005). For this purpose, half of the specimens prepared for each series were subjected to 20 wet/dry cycles after curing. The wet/dry cycle used was 96 h of soaking in water at room temperature followed by 72 h of drying in an oven provided with open air circulation at 60 °C (Claramunt et al. 2011). To characterize the fiber-matrix interface, the fracture surface of the composites was observed using a JEOL JSM-S610 microscope at an accelerating voltage of 10 kV. A focused ion beam (FIB) was also used to resolve the nanofibrillated pulps.

RESULTS AND DISCUSSION

Characterization of Nanofibrillated Cellulose

Figure 1 shows the microstructure of the initial sisal pulp and of the pulps obtained after 6 h of refining.

As Fig. 1 shows, initially the sisal fibers had a diameter ranging from 10 to 20 m in width. Six hours of refinement yielded highly-branched fibers on the nanometer scale, between 25 and 250 nm. The increase in the aspect-ratio could enhance the reinforcing capabilities of these pulps because their high specific surface area favors interaction with the matrix, giving way to a better stress transfer (Tonoli et al. 2007; Ardanuy et al. 2012).

Fig. 1. SEM micrographs of the initial sisal pulp (2,000x) and sisal pulp after 6 h refinement (10,000x)