POTENTIAL USE OF RICE STRAW AS FIBER IN ECO-COMPOSITE MATERIALS

Hosam El-din H. Seleem

HBRC, Cairo, Egypt; hosam_seleem@hotmail.com

ABSTRACT The current study is concerned with utilizing rice straw as fiber reinforcement to geopolymer mortar mixtures. The mixtures were cold pressed to form panel fiber boards with overall dimensions (100 * 25 * 2.5) cm. Five geopolymer mixtures were proposed based on the results of a preliminary investigation. All mixtures were designed such that the processed straw constitute 30% volume fraction and the geopolymer mortar 70% volume fraction of the total mixture volume. Two panel boards (two replicates) were cast from each mixture. Test specimens were cut from the cast panel boards and were tested for compressive strength at three ages; 7, 14 and 30 days, and modulus of rupture, bulk density, water absorption at 30 days age. The mixture which exhibited the least water absorption coefficient and in the same time good mechanical and physical attributes was selected as the best suitable for producing panel boards. This mixture was tested for the acoustical property sound absorption coefficient in an octave band covering frequency range 100 – 3200 Hz. Test results revealed that rice straw could be processed to yield geopolymer-based fiber boards with desirable mechanical and physical attributes and good acoustical properties. KEYWORDS: rice straw; geopolymer; fiber board; panel board. 1. INTRODUCTION Over the past ten years there has been a strong push to investigate the potential uses of ‘waste’ materials from the rice industry, including straw and hulls due to environmental concerns over air pollution and economic concerns because agricultural residues represent a renewable resource of raw materials that can be replaced annually. Among the various agricultural straws, rice straw could be very interesting material due to its good thermal stability compared to other agricultural waste [1]. The rice straw is mainly consisting of carbohydrate components such as hemicelluloses, cellulose, and lignin [2]. The

carbohydrates content is variable, as well as other components, such as ash and silica. The hydrophilic character of the rice straw is one of the reasons for relatively high moisture content, approximately 60% on a wet base, or 10-12% on a dry base. The ash content of up to 22% and low protein content result in a material decomposing not as readily as other straws [3]. The rice straw resistance to bacterial decomposition makes this material suitable as filler or fiber reinforcement in building composite materials. On the other hand, high content of silica (up to 20%) represents an additional potential benefit regarding the flame-retardant when used in building industry. From this point of view, rice straw has been studied as potential fiber in panel boards. Non-wood pulp production, which represents less than 10% of the total world’s pulp source, is based primarily on straw (46%), bagasse (14%), and bamboo (6%). Agricultural by-products account for 73% of the world’s non-wood capacity, while natural plants such as reed and bamboo account for 18%, and the remainder consists mainly of industrial crops [4]. The soda, kraft and neutral sulfite processes are employed for producing various types of pulps, ranging from high-yield semi-chemical to soft bleached grades [5-7]. Many techniques are readily available for processing cement-based fiber-boards and particle-boards. In USA the Slurry-dewatering is now the dominant means of processing fiber cement boards [8]. The experience of other countries, however, indicates that different fiber cement board production techniques can find their own niche in building construction markets [9]. Other technique focused on the use of wheat straw in production of cement-bonded particleboard, using a processing scheme which employs a combination of pressure, heat and carbonation reactions to ensure rapid curing and high performance of end products [10]. Recent efforts to reinforced fiber cement boards with agricultural residues have focused on the adverse effects of the water soluble constituents of such residues on hydration and strength development of cement. The inhibitory effects of cereal straws on hydration of cement are confirmed [11]. While hot water extraction proved effective in improving compatibility of inhibitory wood species with cement, this was not the case with cereal straws. The compatibility of rice straw and coconut coir dust with cement is investigated [12]. Results revealed that cold and hot water extractives of rice straw were detrimental to strength development of cement in conventional curing processes; coconut coir dust extractives, on the other hand, did not adversely affect strength development of cement. This observation highlights distinct attributes of various agricultural residues. Eusebio [13] recognized the retarding effects of a local fibrous agricultural residue, rattan shavings, and devised processing means of overcoming this deficiency. Rapid curing techniques involving carbonation reactions were employed to overcome this problem. The resulting boards exhibited satisfactory levels of strength and dimensional stability. Considerable research was conducted into alternative methods of producing wood pulp fibers, as well as extending the range of natural fibers suitable for reinforcing cement products [14, 15]. The search for a replacement of asbestos fibers resulted in many natural fibers (wood, bamboo, banana, flax, etc.) being examined in numerous laboratories around the globe. The range of research interests was wide taking in studies of ‘‘fiber curl’’[16], fiber compatibility to cement [17,18], relationships between fiber properties and composite performance [19], pressure effects during manufacture [20] and testing of the commercial product [21]. Binding materials other than cement was introduced as an attracting alternative to cement. Polypropylene filled rice straw composites have been examined [22]. Improved mechanical

properties, i.e. increased tensile modulus of the PP/rice straw composites with the increase of filler content are reported. Through this work, geopolymer mortar is suggested to bind rice straw fibers in manufacturing fiber boards. Geopolymer are a class of material developed by Davidovits in the 1970s [23] synthesized by the dissolution of alumino-silicate minerals in alkaline solutions, they have been reported to possess a number of excellent physical and chemical properties, such as high strength [24, 25], thermal stability [26, 27], environmental friendliness in that they do not produce carbon dioxide during processing, and corrosion resistance. 2. EXPERIMENTAL Utilization of rice straw in manufacturing panel fiber boards is the primary focus of this investigation. Processed rice straw was mixed with a geopolymer mortar and cold pressed to form panel board specimens with overall dimensions (100 * 25 * 2.5) cm. 2.1 Material Straw The long straws of rice were resized to produce a large surface area for fiber-to-matrix bonding. Rice straw were cut into 15 to 20 mm long strips, then were chopped using mechanical chopper to effectively reduce straw diameter, shown in Fig. 1. No attempt was made to pre-treating straw to account for the effects of water-and alkali-soluble products in straw as usually done for cement bonded products, as it was found thorough preliminary investigation that the mechanical and physical properties of the products are little affected by pretreatment. Pozzolans Two types of pozzolan were employed; low calcium class F fly ash has Blaine's specific surface of 350 m2/kg and metakaolin has Blaine's specific surface of 300 m2/kg. Kaolin was heated at about 850 oC for two hours then brought out the furnace at Lab temperature to cool down rather fast, thereby enables the formation of amorphous glassy phases with pozzolanic nature (metakaolin). The chemical compositions of the employed pozzolans are shown in Table 1. Filler additives Fine limestone (LS) powders passing sieve no.200 (75 micron) composed mainly of 99% CaCo3. Slaked lime (Ca (OH)2) was also used to take advantage of the pozzolanic reaction together with the geopolymeric reaction. Activator Laboratory grade sodium hydroxide in pellet form with 98 % purity and Sodium Silicate solution (Na2O= 8%, SiO2 =26.5% and 65.5% water) with silicate modulus ~ 3.3 and a bulk density of 1410 kg/m3. The alkaline activating solution was a mixture of Sodium hydroxide (NaOH) and Sodium silicate (nSiO2Na2O) solution and water as shown in Table 2. Resin and polymer A conventional water based phenol-formaldehyde resin was used as an additional binder in one of the mixtures (mix B1). This resin has, according to manufacturer's specifications, 55% solid content. In another mixture (mix B2), polymer Latex is used in a liquid form. The

employed Latex is styrene butadine rubber, has a solid part content (assigned experimentally) 40% of the total Latex weight.

FIG. 1- SIZE REFINEMENT OF RICE STRAW

Oxide Fly ash Metakaolin

Si O2 56 58.52

Al2 O3 29.8 35.54

Fe2 O3 3.58 1.15

Ca O 2.36 1.24

Mg O 0.3 0.19

Na2 O 0.6 0.25

K2 O 0.7 0.05

S O3 0.08 0.06

TiO2 1.75 0.83

L.O.I. 1.31 0.74

TABLE 1- CHEMICAL COMPOSITION OF THE POZZOLANIC MATERIALS

2.2 Composite Features and Preparation All mixtures were designed such that the processed straw constitute 30% volume fraction and the geopolymer mortar 70% volume fraction of the total mixture volume. Five geopolymer mixtures, shown in Table 2, were proposed based on the results of a preliminary investigation.

The pozzolans were first mixed with the activating solution in a pan mixer for about 2 minutes. Afterwards, the filler additives were added to the rotating mixer and mixing further continued for 3 minutes. The mix exhibited a thick sticky nature with good workability. Straw is dispersed manually to the rotating mixer in about 2 minutes and mixing further continued for 3 minutes. Then, panel board specimens with overall dimensions (100 * 25 * 2.5) cm were cast and cold pressed for 3 minutes under pressure of 20 kg/cm2. Two panel boards were cast for each mixture. The finished composite boards were conditioned in a laboratory at room temperature of 25 ± 1°C till the age of testing, and then cut into specimens of various sizes for testing.

water glass NaOH Water Fly ash Metakaolin Lime stone powder

Calcium hydoxide

A1 20 5 15 20 ___ 35 5 _____ ___

A2 20 5 15 ___ 20 35 5 _____ ___

A3 20 5 15 10 10 35 5 _____ ___

B1 14.5 5 10.5 10 10 35 5 10 ___

B2 16 5 9 10 10 35 5 _____ 10

Phenol formaldehyde Latex

Alkaline activating solution Pozzolan Filler additivesMix

TABLE 2- PROPORTIONS OF THE GEOPOLYMER MIXTURES (percentage by weight) 2.3 Performance testing Mechanical and physical properties of the cast panel boards were evaluated by cutting relevant specimens from the cast boards correspond to each test. Six test specimens from two replicated boards were tested for each performance criterion at each age. For each of the mixtures shown in Table 2, the following tests were conducted: • Compressive strength at three ages; namely 7, 14 and 30 days. For this purpose, cube

specimens with side length equals to the board thickness (25mm) were cut. Cutting was carefully done without exerting any shock to the specimens and the surfaces were grind if necessary to ensure parallel flat surfaces. All tests were done in a fully computerized displacement control universal testing machine having a 50 ton maximum capacity. The selected loading range was 10 ton and the rate of testing was 0.2 mm/minute.

• Modulus of rupture ((flexural strength) at 30 days age. Test specimens with overall dimensions 40 mm width, 150 mm length, and 25 mm thickness were prepared. The specimens were tested under three points loading over a span of 120 mm in the above described fully computerized displacement control universal testing machine. The selected loading range was 1.0 ton and the rate of testing was 0.5 mm/minute.

• Bulk density at 30 days age employing the specimens of the compressive strength test. The specimens overall dimensions were measured to the nearest 0.01 mm and then weighed using sensitive laboratory weighing balance (accurate to ± 0.001 g) based on the condition "air dry". A total of 6 replicate specimens were used for each mixture.

• Water absorption at 30 days age. The absorption potential was evaluated after 2 hours and 24 hours of soaking in water. Six cube specimens with 25 mm side length were tested for each soaking duration. The specimen's weights were recorded initially before water soaking (air dried) and after soaking for the prescribed duration. Before any measurement, the specimens were removed from water, and were brought to the saturated-surface dry

(SSD) condition. The absorption is expressed as the ratio of weight change to the initial weight of specimens.

These tests revealed that the mixture (A3) outperforms other mixtures regarding water absorption coefficient and in the same time exhibited good mechanical and physical properties. This mixture was further tested for acoustical property according to the following: • Acoustical property; the sound absorption coefficient was determined by the impedance

tube method, ASTM C 384. Fig. 2 shows schematic description of the used apparatus where measurements in an octave band covering frequency range 100 – 3200 Hz.

FIG. 2- SCHEMATIC DIAGRAM OF SOUND ABSORPTION APPARATUS

3. TEST RESULTS Figures 3, 4, and 5 show some of the cast panel boards. Test specimens relevant to each performance criterion were cut and tested. The cited results demonstrate the validity of panel boards production utilizing renewable agriculture residue employing unconventional binders other than cement, namely geopolymer mortar. It worth to emphasize that the proposed proportions of geopolymer mortar mixtures are based on the results of a preliminary investigation. 3.1 Compressive strength Fig. 6 shows compressive strength values of the investigated mixtures of fiber panel board at three ages; 7, 14, and 30 days. As could be seen, all the mixtures exhibited satisfactory level of strength suitable for walling application. The influence of geopolymer mortar constituents on compressive strength could be summarized as follows: • Fly ash seems more prone to strength enhancement at all ages compared to metakaolin

pozzolan. Apparently, the low water demand of fly ash represents a commonly observed advantage [28, 29] arises from the high-temperature treatment producing less porous and

more finer spherical particles with filling role in mixtures, which enables more free water and thus increases fluidity of mixtures.

FIG. 3- PANEL BOARDS FROM MIXTURES A1 AND A2

FIG. 4- PART OF BOARD FROM MIXTURE B2 (WITH LATEX)

FIG. 5- PART OF BOARD FROM MIXTURE B1 (WITH PHENOL FORMALDEHYDE)

• The inclusion of the two kinds of pozzolan is more efficient than each of them alone. Probably the two types with different fineness and different particle shapes enhance packing of the final product.

• The mixture (B1) with phenol formaldehyde resin exhibited the highest strength at all ages. The results assumed by this mixture were higher than those of the similar mixture but without phenol formaldehyde (A3), by about 17% at 7 and 14 days ages and by about 29% at 30 days age. This result implies that the phenol formaldehyde resin which usually employed in wood works could be utilized to further enhance the geopolymer-bonded-straw panel boards.

• The efficiency of phenol formaldehyde resin in increasing strength is more apparent with aging. Therefore, accelerated curing by steam is expected to significantly improve strength and even at early ages.

• Latex was added to one of the mixtures (B2) to enhance the product flexibility due to the rubbery nature of Latex. Unfortunately, Latex addition slow down the rate of strength gain as could be realized by comparing the results of this mixture with the similar one but without Latex (mixture A3). Compressive strengths of the Latex mixture are lower than the corresponding ones of mixture A3 by about 13.5%, 8%, and 5.7% at 7, 14, and 30 days respectively.

0

5

10

15

20

25

30

A1 A2 A3 B1 B2

Mixture

C7 C14 C30

FIG. 6- COMPRESSIVE STRENGTH VALUES OF THE INVESTIGATED MIXTURES

Com

pres

sive

stre

ngth

(MPa

)

3.2 Modulus of rupture Fig. 7 shows modulus of rupture (flexure strength) results of the investigated panel board mixtures at 30 days age together with the compressive strength values at the same age. As could be seen, the modulus of rupture results assumed trends different than those assumed by the compressive strength results. It is interesting to note that the ratios of modulus of rupture to compressive strength are relatively high ranged from 54% to about 80%. This observation could be attributed to the mutually exclusive effects of straw fibers on both kinds of strength, as compressive strength tends to decrease while tensile strength tends to increase. The figure shows that the moduli of rupture of all pozzolan mixtures are more or less the same regardless of the pozzolan kind. On the other hand, the latex pronouncedly increased the modulus of rupture strength by about 37% relative to the corresponding mixture without Latex (A3). The phenol formaldehyde resin was also efficient in increasing modulus of rupture by about 33% compared to the mixture A3. It was generally noted that the samples of

the Latex mixture experienced a relatively large deformations before failure, i.e. the ductility of this mixture is also improved due to inclusion of Latex. Anyhow, more research work is still needed to quantify the possible gains in ductility with Latex employment. 3.3 Bulk density Table 3 shows the bulk densities of the investigated panel board mixtures at 30 days age. The density values range from 1.58 gm/cm3 to 1.72 gm/cm3, i.e. lie within the range of medium density building materials. It is interesting to note that the density values are consistent with the compressive strength values but no simple relation could be found between them. The medium density of the produced panel boards is an advantage renders these boards as a valuable building alternative although more reduction in density is still needed for many reasons like optimization of building foundation, reduction in quake loads,..etc.

0

5

10

15

20

25

30

A1 A2 A3 B1 B2

Mixture

Stre

ngth

(MPa

)

Modulus of ruptureCompressive strength

FIG. 7- MODULUS OF RUPTURE OF THE INVESTIGATED MIXTURES

density

gm/cc 2 hours 24 hours

A1 1.65 2.4 7.3A2 1.58 3.85 7.8A3 1.71 3 5.2B1 1.72 4 23B2 1.6 2.6 6.25

Mixwater absorption %

TABLE 3- PHYSICAL PROPERTIES OF THE INVESTIGATED MIXTURES

3.4 Water absorption potential Table 3 shows water absorption of the investigated panel board mixtures at 30 days age. The absorption potential was evaluated after 2 hours and 24 hours of soaking in water. The 2 hours water absorption ranged from 2.4% to 4% with no signs of deterioration. After 24 hours soaking in water and except for the mixture (B1), the absorption values increased reasonably

to values ranged from 5.2% to 7.8% and the test samples are still sound with no signs of deterioration. On the other hand, the mixture (B1) with phenol formaldehyde resin experienced a relatively high absorption- about 23%- and the samples of this mixture were friable and broken during handling. The relatively high water absorption potential of straw -which is the main constituent of the produced fiber panel boards- render this test as a prime criterion for acceptance of these products. Based on this fact, the mixture (A3) which possessed the least absorption after 24 hours of soaking in water is regarded as the best mixture amongst the tested mixtures. 3.5 Sound absorption All materials can to a greater or lesser extent absorb sound. That is, they can convert the mechanical energy of molecular vibration into heat. The performance of a material is usually quantified in terms of its sound absorption coefficient α which is defined as

α = materialuponincidentenergysound

materialfromreflectednotenergysound

For a perfect absorber the value of α would be 1, while for a perfect reflector α would equal zero. Fig. 8 show the sound absorption coefficient of the mixture (A3) in the frequency range (100 – 3200) Hz. The results show that sound absorption increases with increasing frequencies till a value (α = 0.34) at a frequency of 2000 Hz, then start to decreases with further increase of frequencies. The cited sound absorption coefficients are generally higher than those of many of the building materials. This benefit together with other good mechanical and physical attributes qualifies the investigated panel boards as an appropriate to substitute conventional building materials and techniques.

00.05

0.10.15

0.20.25

0.30.35

0.4

100 200 400 500 800 1000 1250 1600 2000 2500 3150 3200

Frequency (Hz)

mixture A3

Soun

d ab

sorp

tion

coef

ficie

nt (α

)

FIG. 8- SOUND ABSORPTION COEFFICIENT OF THE MIXTURE (A3)

4. CONCLUSIONS This investigation leads to the following conclusions regarding utilizing rice straw as renewable resource of agricultural residue in manufacturing of panel fiber boards.

Versatile panel fiber boards capable of meeting the requirements in demanding applications are successfully produced by employing rice straw agricultural residues into quality value-added composite products using geopolymer mortar as a binder.

Abundantly available agricultural residues, including rice straw, can be valuable replacements for wood in fiber board building products.

Geopolymer mortar mixtures proved efficiency as a binder for agricultural rice straw residues, and even more dispense the need to pre-treating straw to account for the effects of water-and alkali-soluble products in straw as usually done for cement bonded products.

The inclusion of the two pozzolans; fly ash and metakaolin in a geopolymer mixture seems more efficient regarding mechanical and physical attributes than each of them alone.

The phenol formaldehyde resin which usually employed in wood works could be utilized to further enhance the mechanical properties of geopolymer-bonded-straw panel boards. Its efficiency is more apparent with aging and therefore, accelerated curing by steam is expected to significantly improve strength and even at early ages.

Latex addition to geopolymer mixtures slow down the rate of compressive strength gain. On the other hand, the latex pronouncedly increased the modulus of rupture strength by about 37% at 30 days age, and its test specimens possessed significantly high ductility.

The ratios of modulus of rupture to compressive strength are relatively high ranged from 54% to about 80% at 30 days age. This observation could be attributed to the mutually exclusive effects of straw fibers on both kinds of strength, as compressive strength tends to decrease while tensile strength tends to increase.

The relatively high water absorption potential of straw -which is the main constituent of the produced panel fiber boards- render this test as a prime criterion for acceptance of these products. Based on this fact, the phenol formaldehyde mixture does not prove efficiency in context with this requirement. Probably, a stabilizer or a special treatment is needed to overcome this deficiency.

The medium density of the produced panel fiber boards is an advantage, renders these boards as a valuable building alternative.

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