aStrength Evaluation of CO 2 -Cured Cellulose Date Palm Fiber Reinforced Cementitious Boards

In recent years there has been an increasing demand to recycle wastes produced by the agricultural and industrial processing. The aim of this paper is to investigate the suitability of date palm (Phoenix dactylifera) as lignocellulosic materials for the production of wood-cement composite boards, in addition to enhance their compatibility with cement using physical pretreatment processes and accelerated carbonation curing. Experiments were performed to assess the physical properties (as density, flexural strength, toughness and E-modulus), and micro structural properties (as determined by scanning electron microscopy) of the produced cement boards. The results show an improvement in the physical and microstructural properties of cellulosic fiber-cement composites by using accelerated CO 2 curing method. In addition, excessive carbonation rate associated with pure gas carbonation does not necessarily lead to high strength and even detrimental strength development was found, which was shown by cement paste.


INTRODUCTION
he usage of waste fibers as reinforcement with the corporation of cementitious materials has great interest for civil engineering construction in terms of recycling. Besides, the accessibility of natural waste fibers encourages their promising usage to produce different building components through sustainable manners [1]. Iraq has relatively large quantities of lignocellulosic materials available in the form of agricultural and industrial residues since there are about 15 million trees of palmdate only [2].
Cellulose fiber, as a reinforcing material that is renewable and very high in performance to cost ratio, improves the ductility, flexural and tensile strength, fracture toughness, and machinability of the cement matrix, mainly through arresting and deflecting microcracks and through the pullout action of wood fiber at fractures [3]. Synthetic fibers are expensive, high-energy consumers and often manufactured from nonrenewable resources. While, natural fibers have renewable resources and are accessible worldwide. Therefore, the use of concrete reinforced with natural fibers could be a method to enhance both concrete strength and sustainable production [4]. CFCBs are as floor and wall sheathing, exterior siding, sound insulation, roof shakes and roof decking [5]. More uses that are specialized include permanent formwork, pre-fabricated house components, non-structural applications in both interior and exterior situations, sound barriers, and the construction of protective elements of fireproofing [6].
The accelerated hardening process with carbon dioxide revolutionized the manufacture of wood-cement composites [7]. The utilization of CO 2 gas as an accelerated curing process in fresh concrete has been submitted as a CO 2 sequestration method that contribute a value-added product and carbon dioxide cured process [8]. [Young,9] mentioned that, "the formation of C-S-H like gel as well as calcite (CaCO 3 ) that subsequently carbonated to silica gel and calcium carbonate in the first 3 minutes of carbonation of C 3 S and C 2 S".

Objectives
The ultimate goal of this study is to develop the industry manufacture of wood-cement composites in a way that the products can be made with higher productivity and better mechanical properties. The major objectives are: 1. Investigating the suitability of date palm (Phoenix dactylifera) as lignocellulosic materials for the production of wood-cement composite boards, in addition to enhance their compatibility with cement using physical pretreatment processes. 2. Studying the effect of accelerated hardening with carbon dioxide on the properties of woodcement composites made from recycled palm date fibers. 3. Studying the effect of manufacturing parameters, such as gas concentrations, chamber duration and chamber temperature, on the development of carbonation reaction, strength and microstructure of wood-cement composite boards.

Materials and Methods
Ordinary Portland cement, commercially known (MASS), was used to produce the fiberboards. Tables 1 and 2 show the chemical composition and physical properties of cement used throughout this work respectively. Results indicate that the cement is conformed to Iraqi specification No. 5/1984. Natural sand with 2.36 mm maximum size brought from Al-Ekhaider region was used as a fine aggregate in this research. Table 3 shows the grading of sand used in this work. The grading of sand is conformed to Iraqi specification No.45/1984 (zone 1). The sulfate content, the bulk density, specific gravity and the absorption of the sand were 0.07 %, 1500 kg/m 3 , 2.65, and 1.2 % respectively. Densified micro silica produced by BASF Company was used as pozzolanic admixture. The technical specifications and pozzolanic activity index are given in Table  4. The results show that silica fume used in this investigation conforms to the requirements of ASTM C-1240-05, ASTM C-618 specifications. The superplasticizer used in this work was based on a unique carboxylic with long lateral chains, which greatly improves cement dispersion. It is commercially known as GLENIUM 51. Table 5 shows the typical properties of it. This admixture is complying with type (F) according to ASTM C494-03.
Cellulosic fibers were derived from Iraqi date palm trees by dry mechanical processing method (Plate 1). A hummer mill used to make the fibers, and screened the resulting fibers to a final 4 mm mesh. The moisture content and density of the investigated fibers were measured according to the ASTM D-4442-03 and ASTM D 2395 -07a standards as shown in Table 6. Physical fiber pretreatment (hornification) aimed to remove extractives that inhibit cement curing. The hornification of the fibers was carried out according to procedures outlined by [Ali, 10].

Mix Proportions and Casting of Boards
Many trial mixes were carried out in the building materials laboratory to find a suitable mix that having the desirable properties in the fresh state to satisfy the ASTM C-208-01 and ASTM C-1186-08 requirements of cellulose fiber-cement boards in the hardened state. All samples with nominal thickness of 12 mm were obtained from the dry process. The semi-automatic dry process is the most employed in the fiber cement industry worldwide. The water and superplasticizer contents were chosen in order to achieve a reasonable fresh mix workability characteristics represented by a flow (ASTM C-1437) of 65-75 % at 1 min after mixing. All the specimens, which have dimensions of (305*152*12) mm, were kept inside their molds underneath wet burlap covered with a plastic sheet for 24 hrs as shown in Plate 2. They were then demolded, dried at 60 o C for 30 min. and cured for the specified time and temperature following that procedure outlined by [Alanbari,11]. The proportions of concrete mixes are summarized in Table 7.

Program of Work
The work consisted of two series of Cellulose Fiber Cement Boards (CFCBs) mixes, namely: control and CO 2 cured boards, to investigate the effect of several factors. In order to evaluate the strength of CFCBs, different factors were performed: CO 2 concentration, chamber duration and chamber temperature. One day after completion of processing, the fiberboards were carefully removed from the molds, dried at 60 o C for 30 min and then subjected to CO 2 curing. The drying process will ensure the availability of empty paths for CO 2 to take place. The CO 2 curing chamber process was performed at different ambient temperatures (25 o C, 50 o C and 75 o C) and CO 2 concentrations (0%-25%-50%-100%) for two duration times, 90 and 180 minutes. A comparison has to be made for the CO 2 cured specimens with control specimens.
The carbonation curing apparatus was used to subject samples to a low pressure of carbon dioxide gas and oxygen gas. Major components of the set-up included compressed gas tanks, pressure vessel, thermocouples, data acquisition, and vacuum with pressure and heat transducers. The samples in the curing chamber were treated with CO 2 gas at 6.9 MPa (1000 psi) at different temperatures for 90 and 180 minutes, respectively. In the specimen preparation of this section and the following ones, 20-minutes vacuum were applied before CO 2 injection, and the CO 2 flow rate was 10 L/min that mean about 25 min. is needed to reach 100 % concentration.

Mechanical Properties and Strength Tests Flexural Strength, Toughness and Modulus of Elasticity
Flexural tests are performed at 28-day according to ASTM C1185-12 and ASTM C1186-12. Board dimensions measured (305 mm) in length and (152 mm) in width with 12 mm thickness. The specimens were simply supported over a span of (255 mm) and loaded at a displacement rate of 0.5 mm/min (which were conducted in a displacement-controlled mode). A computer controlled data acquisition system is used to record the test data. The load-deflection curves are characterized by flexural strength, toughness (total area underneath the load deflection curve), and modulus of elasticity. Each value of the flexural strength was the average of the test results for three specimens.

Dry Density
Density was determined at 28-day in accordance to ASTM C 1185-08 (2012) with reference to ASTM C 20-07 using the water displacement method. Each specimen was weighed under water after being immersed for 48 hours. The saturated weight in air was then measured and the dry mass was obtained by drying each specimen to constant weight in an oven at 194 ± 4°F (90 ± 2°C).

Scanning Electron Microscopy (SEM) Observations
The textural study for the fractured surface of the samples was performed on a (SEM Model: TESCAN-VEGA/USA) with tungsten source and detector X-Flasb 5030, which operates at a voltage of 1-20 kV with a range of between 10 and 80,000 magnification, at a work distance from 1 to 12 mm. Four SEM micrographs were obtained for each composite treatment and just the typical images of the observed microstructure were used in this manuscript. Conducting this test requires to

Results and Discussion
Flexural Strength Fig. 1 shows a typical load-deflection curves that obtained with different CO 2 concentrations. It could be seen that, the behavior is elastic at the beginning of the loading; up to the first crack. Furthermore, this figure shows that the CFCB behaves as an elastic-degrading plastic material with significant loss of strength following the achievement of peak load.
After that, either the initiated crack had an unstable growth leading to separation of the body into parts for control mix, or to a macro-crack with a deflection around 0.55 mm, when the fibers could stop the crack growth for CO 2 cured boards.
Further, it is obvious that the flexural performance enhanced with the increasing of CO 2 concentrations. The toughness indices derived from results under bending test were (200-350) N.mm, and the results indicate that the CO 2 concentrations have a significant influence on the toughness of the fiberboards. A medium concentration (50 %) of CO 2 observed comparable to that obtained at 100 % CO 2 on flexural performance. An economic criterion encourages the use of lower CO 2 concentrations since the difference between them was negligible.  The CO 2 content in the solidified material increases with increasing temperature (up to 50 o C), possibly reflecting the leaching of Ca 2+ ions from the fibers. Above 50 o C, the CO 2 content decrease with further increase in reaction temperature, which may be due to the decreased solubility of CO 2 in water at elevated temperatures. This result suggests that the carbonation process may be controlled by the solubility of CO 2 and the amount of dissolved Ca 2+ ion in the water. Calcite (CaCO 3 ) products a layer coverage and associated loss of exposed board surface area and pore network and it was identified as the main factor limiting the rate, depth and extent of carbonation.

Toughness
Toughness is a measure of the absorbed energy per unit area of material and is usually defined as the area under the load-deflection curve. The toughness of wood-cement composites after CO 2 curing was illustrated in Figure 3. Similar to the flexural strength development, the toughness of the carbonated samples increased after the beginning of CO 2 exposure. The strength gain effect was significant for samples subjected to 50 % of CO 2 concentration, but a sharp increase was observed after the samples were cured by 25 % and 100 % of CO 2 concentrations. Moreover, an increase in the CO 2 concentration from (25% to 100%) increases the flexural toughness by (104%-152%), (36%-44%) and (44%-25%) for 25 O C, 50 O C and 75 O C respectively at 90 min. The rate of increase in flexural toughness were between (53%-81%), (1%-48%) and (47%-31%) for 25 O C, 50 O C and 75 O C respectively at 180 min. The flexural toughness kept increasing until 50 % of CO 2 curing. After that, there was no significant difference in toughness with the further increase of CO 2 concentration.
The examination of toughness properties represented by Figure 3, shows that for each term (90 or 180 min.) the flexural toughness, increases with an increase in the CO 2 concentration incorporated in the mix, with the maximum toughness, being obtained with the 50 % for both curing periods. In addition, there was no significant difference in toughness with the further increase of CO 2 concentration more than 50 %.
Further, as compared to control fiberboard, the palmdate fiberboard yielded higher flexural toughness by (152%-84%) at 50 % CO 2 concentration for 90 and 180 min respectively. It is also a fact that improving the bond between the fiber and the matrix (because of CO 2 curing) resulted in an improvement in the interfacial transition zone. The strain in the composite at a given stress depends on the length of debonded fibers and, hence, a greater bond leads to smaller failure strain and fibers are broken rather than pulled out. This behavior probably interprets the reduction in flexural toughness associated with increasing CO 2 curing [5]. .Journal, Vol.34,Part (A), No.6

E-Modulus
Initial modulus or initial stiffness is defined as the stiffness obtained through linear regression analysis of the load-deflection points for loads below 15% of maximum load [13]. The modulus of elasticity for all of the investigated cellulose fiberboards is shown in Figure 4. The test results in this Figure shows that the modulus of elasticity almost fluctuated as the CO 2 concentration is increased from 0 % to 100 %. Moreover, the stiffness of the wood cement fiberboards increases linearly with the increase of CO 2 concentrations to 50 % at 90 min. of chamber duration. However, increasing the CO 2 concentrations to (100 %) leads to a reduction in the static modulus by 25% and 36% for 90 min. and 180 min. respectively.
From the recorded results through the experimental work, it can be noticed that the 28-days modulus of elasticity for date palm fiberboards ranges between (

Dry Density
Apparently, the overall densities of control date palm fiberboards were lower than those for CO 2 cured fiberboards, as shown in Fig. 5; this could be attributed to the extra formation of CaCO 3 due to carbonation curing and the filling effect of these products in the fiber lumen. Moreover, an increase in the CO 2 concentration from (25 to 100) % increases the density of palmdate fiberboards by

Scanning Electron Microscopy (SEM)
SEM analysis of the carbonated material offered a qualitative view of the mineralogy that lies inside the board structure. Fig. 6 views the SEM micrographs of fractured surface of composite samples before and after accelerated carbonation. Further, it seems clear that the surface of palmdate fiber covered with Ca (OH) 2 (arrow 1) Fig. 6a. In general, it is obvious that the palm date fibers have a comparatively simple and uniform structure. It is appear that they are made up of a large number of cell types with the long pointed fibrous cells termed tracheids providing both the structural support and the conducting pathways in composite. These tracheids are empty (arrow 2), for non-carbonated samples, (0 % CO 2 ), and completely filled with CaCO 3 for carbonated boards.