Influence of Nanosilica on Properties of Green ... - Springer Link

Arab J Sci Eng (2014) 39:2631–2640 DOI 10.1007/s13369-013-0935-0

RESEARCH ARTICLE - CIVIL ENGINEERING

Influence of Nanosilica on Properties of Green Cementitious Composites Filled with Waste Sulfite Pulp Fiber and Aminosilane Reza Hosseinpourpia · P. Hosseini · S. R. Mofidian · Rezvan Hosseinpourpia · A. Varshoee

Received: 15 June 2012 / Accepted: 15 December 2012 / Published online: 31 January 2014 © King Fahd University of Petroleum and Minerals 2014

Abstract Developments in the field of green cement-based products are characterized as an important approach to sustainable development and are being devoted much attention by the construction industry. Numerous types of materials are utilized; however, based on other published studies, the use of waste material as a filler normally deteriorates the performance of cementitious products. Appropriate additives thus need to be employed to improve the performances and properties of green products. As a consequence, the aim of this study has been to investigate the properties of a novel green cement-based composite—a hybrid system composed of cement, waste natural fiber, silica nano-particles, and aminosilane. Experiments were performed to assess the physical properties (density and flowability), mechanical properties (compressive strength and bending performance), and

microstructural properties (as determined by scanning electron microscopy) of the cement sheets. The results demonstrated an improvement in the mechanical and microstructural properties of green cement-based composites by using this hybrid system. Keywords Waste management · Sulfite pulp fiber · Cement-basedcomposite · Aminosilane · Nanosilica

Reza Hosseinpourpia (B) Young Researchers Club, Islamic Azad University, Chalous Branch, Chalous, Iran e-mail: [email protected] Reza Hosseinpourpia Department of Wood Biology and Wood Products, Georg-August University, 37077 Goettingen, Germany P. Hosseini Department of Civil Engineering, Sharif University of Technology, Tehran, Iran S. R. Mofidian Department of Computer Engineering, Mazandaran University of Science and Technology, Babol, Iran Rezvan Hosseinpourpia Department of Civil Engineering, Islamic Azad University, Jouybar Branch, Jouybar, Iran A. Varshoee Department of wood Science & Industry, Islamic Azad University, Chalous Branch, Chalous, Iran

1 Introduction The use of green cement-based materials has become prevalent, as the concrete industry aims towards a sustainable development. Accordingly, a number of attempts have been made to determine the optimal application of different types of panels and environmentally friendly cement sheets. These panels have been used for various construction applications,

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such as roofs and exterior walls. However, the production of these products should be consistent with the objective of reducing environmental impacts. Several natural waste materials, such as natural fibers, have been used in the production of construction materials. Among the many types of cellulose fibers, fully treated pulp fibers produced by sulfite processes are of extreme importance. These fibers are used commercially as sheets of varying thicknesses in the manufacture of such items as tissues, printing paper, and personal hygiene products [1]. Sulfite pulp fiber (SPF) has more cellulosic components than many other types of cellulose fibers. Due to their unique chemical structure, cellulose fibers possess numerous active sites in the amorphous region that are available for reactions [1]. However, using natural fibers may lead to a decrease in the strength of these green cement-based composites due to their weak physical structures [2,3]. For this reason it has become increasingly popular to reinforce cement-based composites with nanoscale particles. Among the available nano-particles, silica nano-particles (nanosilica) have been frequently used in the production of cement-based composites due to their high content of amorphous SiO2 (more than 99 % in most products) and large specific surface area [4]. Calcium hydroxide (Ca(OH)2 ) crystals in the interfacial transition zone (ITZ) are affected by silica nano-particles, which contribute to the formation of a dense calcium-silicatehydrate (CSH) gel. Consequently, as the size and number of Ca(OH)2 crystals decrease considerably, the strength of the hardened cement is amplified [5–8]. To improve the properties of the composite, a material capable of creating bonds between organic fibers and a mineral material should be used. The effectiveness of silane for this purpose depends on the reactivity of its molecular ends with –OH groups, which are present on the surface of both the silica and cement particles. Accordingly, the use of a ternary system of aminosilane-micro silica-cement leads to an increase in the compressive strength of the cementbased materials. Moreover, the hydrophilic nature of silane is expected to improve the workability of the mixture and enhance the dispersion of admixtures in the matrix [9]. For these reasons, aminosilane has been employed in cement composites reinforced with natural or synthetic fibers [10]. Aminosilane reacts strongly with cement and causes some changes in the macroscopic properties of fresh and hardened cement paste. These changes include an increased workability, less water being required, a significant delay in cement hydration, lower compressive and flexural strengths at early ages (due to a delay of the hardening process), and an improvement of mechanical properties at 28 days [11]. Mechanical and microstructural investigations of kraft pulp fiber modified by two types of coupling agents, i.e., methacryloxypropyltri-methoxysilane (MPTS) and aminopr-

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opyltri-ethoxysilane (APTS), revealed that the optimal amount of coupling agent is 6 % of the fiber weight [12,13]. These results indicate that using aminopropyltriethoxysilane (APTS) contributed to a greater number of fiber cavities in the mineralization state than with methacryloxypropyltri-methoxysilane (MPTS). The presence of APTES led to proper adhesion between organic material (i.e., the fiber) and mineral material (i.e., the cement) [12]. Iranian manufacturers of personal hygiene products use needle milling to obtain commercial SPF in the appropriate size. Due to a limited access to proper technology, most manufacturers remove fibers shorter than 3 mm from their production procedure and use them as waste material. In accordance with the above-mentioned paragraphs, the research described in this paper has dealt with the effects of nanosilica on the mechanical, physical, and microstructural properties of green cement-based composites filled with aminosilane.

2 Experimental Programs 2.1 Materials The cement used in the study was an ASTM type II Portland cement. Its chemical and physical properties are presented in Table 1. Tap water was used in the experiments. Conventional bleached softwood pulp, manufactured by Sappi group-Germany, was purchased. The fiber was obtained from commercial SPF and its properties are listed in Table 2. It should be noted that after passing through a needle mill, most fibers were within the favorable range (i.e., longer than 3 mm), and the remaining fibers were considered as waste.

Table 1 Chemical and physical properties of cement (ASTM type II) Item

Chemical composition (%) Cement

SiO2

20.4

CaO

63.0

Al2 O3

4.9

Fe2 O3

3.9

MgO

1.7

SO3

2.0

Na2 O+K2 O

0.9

Loss on ignition

1.5

Physical properties Specific gravity (g/cm3 )

3.12

Specific surface (m2 /kg) (Blaine)

295

Average particle size

26 µm

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2633 Table 3 Physical properties of nanosilica

Table 2 Basic properties of SPF Basic weight (g/cm2 )

Dry density (g/cm3 )

Wet (%)

Fine (%)

Brightness (%)

pH

Fiber length (mm)

0.7

0.8

9%

12 %

84 %

7

98 %

221.37

SPF pastes produced with nanosilica to achieve an acceptable workability for pouring the cement pastes into the molds. The blending process was split into three stages. In the first step, cement and fibers were combined as solids. In the next step, all liquid materials (water, nanosilica, coupling agent) were mixed at a moderate speed (200 rpm) for 2 min. Finally, both liquid and solid components were stirred in the mortar mixer at a high speed (600 rpm) for 4 min. The well-mixed cement pastes were subsequently poured into oiled molds to form 50-mm cubes for compressive strength testing. The samples were de-molded after 24 h and were cured in a water tank (at 20 ± 2 ◦ C) for 3, 7, and 28 days. Compressive strengths at 3, 7, and 28 days were determined according to ASTM C109 [14]. Bending tests were performed using the Instron-4489 servo-controlled universal testing machine. The three-point bending configuration was employed to evaluate the load-

Fig. 1 Micro-scale picture of SPF captured by SEM: a ×1,500 magnification; b ×6,000 magnification

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Table 5 Chemical properties of coupling agent

Reagent

Chemical formula

γ -Aminopropyltriethoxysilane

SPF

Cement

Water

Nanosilica (colloidal)

Coupling agent (liquid)

A Ac1

75 75

1,500 1,500

1,500 1,500

– –

– 4.5

Ac2

75

1,485

1,477.5

37.5

4.5

Ac3

75

1,455

1,432.5

112.5

4.5

B

150

1,500

1,500

–

–

Bc1

150

1,500

1,500

–

9

Bc2

150

1,485

1,477.5

37.5

9

Bc3

150

1,455

1,432.5

112.5

9

C

225

1,500

1,500

–

–

Cc1

225

1,500

1,500

–

13.5

Cc2

225

1,485

1,477.5

37.5

13.5

Cc3

225

1,455

1,432.5

112.5

13.5

Alkoxy

–NH2

OCH2CH3

The obtained results are presented in Table 7 and discussed in this section.

3.1 Compressive Strength

deflection curve, the modulus of rupture (MOR), and the modulus of elasticity (MOE) of the specimens (Eqs. 1 and 2) in accordance with DIN 68763 [15]. A 180-mm span and a loading rate of 2 mm/min were adopted in the bending tests to determine the MOR and MOE. Specimens with dimensions of 15 × 50 × 280 mm were prepared for this purpose and were tested after 28 days. MOE =

P1 .L 3 4B H 3 Y1

(1)

MOR =

3P.L 2B.H 2

(2)

Here, H is the thickness (mm), B is the width (mm), P is the maximum load (N), L is the major span (mm), P1 is the proportional load (N) and Y1 is the proportional elongation (mm). The bulk density of cement sheets was determined in accordance with ASTM C948 [16]. After compressive testing of the 28-day old specimens containing 10 % SPF (Bc1 and Bc3), the crushed specimens from fractured surfaces were selected and coated with gold for SEM investigations. The instrument used was a Philips XL 30 SEM, with a testing voltage of 25 kW.

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Functional group

3 Results and Discussion

Table 6 Mix proportions of different mixtures (g) Mix code

Chemical structure

The effects of aminosilane (APTES) on the matrix of cementbased materials were assessed from various viewpoints. The first effect of aminosilane was on the mechanical and strength properties of the matrix. For the sake of clarity, we first explain the performances of aminosilane in cementitious media. Aminosilane can bond to the surface of mineral and organic materials according to the following reactions: Since, during the mixing process, the liquids (water and aminosilane) are first mixed together, and then subsequently added to the mixture of cement and fibers, hydrolysis of this material in water occurs according to Eq. 3 as shown in Fig. 2. As a result of the reaction between water and aminosilane molecules, one alkoxy (–OR) from aminosilane reacts with one hydrogen atom H + from a water molecule, causing ROH to be released. This alkoxy is then replaced by OH− from the water molecule in the aminosilane molecule (Eq. 3). Finally, according to Eq. 4 shown in Fig. 2, the ≡Si–OH groups produced during the hydrolysis reaction react with ≡Si–OH groups present on the surface of mineral matter, which leads to the formation of a bond between two silicon centers. A water molecule is released during this reaction [17]. It should be mentioned that the amino group is responsible for the high reaction capability of aminosilane. This is the result of nitrogen atoms being present in the amino group which can easily hydrogen bond to hydrogen-donating groups, such as the hydroxyl group. This is believed to be the reason that aminosilane is soluble in water [18]. However, covalent bonds are more prevalent prior to hydrogen bonding. Therefore, in the event that the amino molecules bond with organic and mineral materials, hydrogen bonding might be disbanded. The following section briefly describes the bonding mechanism of aminosilane with SPF. According to Eq. 5 shown in Fig. 2, SPF has a high potential for reaction and bears many hydroxyl groups on

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Table 7 Experimental results of different tests Mix code

A Ac1

Compressive strength (MPa) ± SD 3 days

7 days

3.20 ± 0.75 –

5.65 ± 0.95 3.12 ± 0.64

MOR (MPa) ± SD (28 days)

MOE (MPa) ± SD (28 days)

Density (g/cm3 )

– –

– –

1.64 1.60

28 days 8.98 ± 1.06 7.92 ± 1.11

Ac2

1.94 ± 0.41

5.41 ± 0.92

10.57 ± 1.03

–

–

1.55

Ac3

2.54 ± 0.22

6.54 ± 1.08

11.37 ± 1.21

2.92 ± 0.68

942.24 ± 24

1.46

B

2.88 ± 0.35

5.08 ± 0.80

7.33 ± 0.98

–

–

1.55

Bc1

–

3.38 ± 0.55

9.56 ± 1.39

2.82 ± 0.75

1049.33 ± 45

1.52

Bc2

–

6.02 ± 1.14

13.81 ± 0.87

4.19 ± 0.83

1160.15 ± 26

1.47

Bc3

–

9.61 ± 1.21

16.34 ± 1.28

4.33 ± 0.72

888.78 ± 17

1.46

C

2.76 ± 0.33

4.93 ± 0.81

7.22 ± 1.04

–

–

1.51

Cc1

–

3.74 ± 0.73

10.37 ± 1.35

–

–

1.47

Cc2

–

5.74 ± 1.05

13.94 ± 1.05

–

–

1.43

Cc3

–

7.52 ± 1.03

15.37 ± 1.22

3.95 ± 0.74

832.29 ± 31

1.41

Fig. 2 Schematic view of aminosilane interaction

its surface due to it not having any extractive materials in its structure. Accordingly, the aminosilane molecules that were hydrolyzed in water lost their hydroxyl groups and bonded with oxygen atoms thus becoming attached to the fiber surface. The remaining hydroxyl groups in the aminosilane molecule could cause couplings between the cement and SPF through bonding with mineral material such as cement. The fundamental reaction controlling the coupling effect of aminosilane is Eq. 6, which was caused by the combination of chemical processes in Eqs. 3 and 4 (Fig. 2). According to these equations, the amino molecule formed a bond between its silicon center and the silicon centers located on the sur-

faces of mineral matter, which resulted in a coupling between mineral particles. According to Eq. 6, shown in Fig. 2, an alkoxy departed from an aminosilane and reacted with the H + of OH− present on the surface of mineral matter, consequently producing an alcohol molecule. Due to the type of aminosilane (APTES) and its alkoxy coupled with the silicon center of the aminosilane molecule (–O–C2 H5 ), the alcohol molecule that was released was ethanol (HO–C2 H5 ). On the other hand, by releasing H + from the surface of the mineral materials, the amino molecule, from its ≡Si– side, which had lost R–O− , participated in a condensation reaction with the ≡Si–O– side

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on the surface of the mineral matter to form an oxo bridge between two silicon centers. It is apparent from Table 7 that the retarding effect of aminosilane molecules on the strength development was more pronounced when a greater amount of cement was used. For lower cement contents, along with higher amounts of fiber (increased fiber content in a specific volume of the composite), the coupling effect of aminosilane molecules could overcome the retarding effect faster. According to other studies, the turning point for the strength (i.e., the point at which the retarding influence on the hardening caused by aminosilane was eliminated and beneficial effects, such as the coupling property, were increased, thereby increasing the strength compared to specimens without aminosilane), occurred within 14 to 24 days, depending on the type and amount of aminosilane [11]. However, this finding also held true for a matrix of cement paste without fibers. The results of this study indicated that low levels of fibers (5 %) combined with aminosilane postponed the turning point to more than 28 days, while higher levels of fibers (10 and 15 %) combined with aminosilane yielded results similar to those obtained for composites without fibers, i.e., with only aminosilane. It is interesting to note that due to the strong retarding effect of the amonisilane used (APTES), the compressive strength testing could not be performed at early ages. One reason for this could be the bonding of amino molecules to cement surfaces or mineral particles, which led to a reduction of material reactivity. On the other hand, the production of alcohol as a byproduct, according to Eq. 6 (Fig. 2), which is known as the principal and final reaction of aminosilane coupling to mineral matter in the matrix of cement composites, can be considered as one of the factors delaying the hardening of cement paste containing aminosilane [19]. Due to their small, highly specific area, and many unsaturated bonds, silica nano-particles have a favorable effect on the cement composite matrix. By using nanosilica, considerable cost savings may be achieved due to the lower price of silica nano-particles as compared to other types of nanoparticles. The fourfold effect of silica nano-particles on the matrix of cement-based composites is listed below [4]. However, it should be noted that the intensities of these effects vary and that not even the setting time of each can be precisely determined. 1. 2. 3. 4.

Control of the crystallization process Higher production of CSH gel Nucleation effect and development of hydration products Micro- and nanofilling

When nano-particles are added to a cement paste matrix, the particles have several unsaturated bonds, such as ≡Si– and ≡Si–O–, depending on their chemical properties [20]. There-

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fore, the chemical effects of these particles on the structure of cement paste are better compared to those of silica microparticles such as a conventional pozzolan. Nanosilica has numerous silicon centers that can react with cement hydration products. According to the pozzolanic reaction (i.e., the reaction of silica nano-particles with Ca(OH)2 crystals) expressed in Eq. 7 [20,21], nano-particles are helpful in developing a dense CSH gel. ≡ Si − OH + Ca2+ + 2OH− → CSH

(7)

The findings of this study indicate that the effects of the presence of silica nano-particles and aminosilane in the cement composite are complicated. It is clear that silica nanoparticles improve the strength of cement specimens by accelerating the setting time [20], while aminosilane molecules delay setting, particularly early ages [11]. Conversely, both of these admixtures improve the long-term strength of cement composites [11,20]. Based on Table 7, a hybrid system of silica nano-particles, aminosilane and 5 % fibers increased the strength at an early age (3 days) when compared to mixtures with aminosilane and without nanosilica. This result was attributed to a lower amount of aminosilane at small amounts of fiber (5 %) for the same quantity of nano-particles (since the ratio by mass of aminosilane to fibers was constant in all mixtures). This resulted in an increase of the ratio of nano-particles to aminosilane molecules at lower fiber levels. Therefore, the setting-accelerating effects of silica nano-particles overcome the setting-retarding effects of aminosilane. Moreover, in the presence of aminosilane, an increase in the quantity of silica nano-particles resulted in an increase of the strength of the specimens. This signifies that, as a result of increasing the quantity of silica nano-particles, aminosilane did not alter the trend of increasing the strength of the cement-based composites. One should note that ≡Si–O– groups of aminosilane can react directly with crystals of calcium hydroxide obtained from the hydration process and produce other products, as well as hydration products. Furthermore, ≡Si–O– groups of silica nano-particles can react directly with aminosilane and cement and cause a hydration reaction to develop, and also improve the coupling effect. In addition, aminosilane molecules can bond to silica nano-particles and, by forming a network of covalent coupling among silica nano-particles, enhance the strength properties of cement composites containing aminosilane and nanosilica. The effect of composite systems of nanosilica and aminosilane on a cement-based material matrix was considerable since nano-particles could increase the effect of aminosilane at ages by accelerating the setting of the cement. Therefore, the synergistic effects of these materials improved the mechanical properties of cement composites at early ages

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as well as over extended periods. On the other hand, the silanol and amino groups reacted strongly with particles and hydration products to develop new complexes in the structure of cement paste. This finding was more pronounced in the simultaneous effects of nanosilica and aminosilane molecules in a cement composite matrix. In other words, in addition to producing new products, the hydration reaction and its common products should be developed. 3.2 Bending Performance As shown in Fig. 3, increasing the quantity of nanosilica in the presence of 10 % fibers resulted in an improvement of the bending strength. Moreover, by adding 3 % nanosilica with a coupling agent, we obtained a maximum bending load for a fiber content of 10 %. According to Fig. 4, the presence of aminosilane and silica nano-particles with various fiber concentrations increased the bending strength of the specimens and introduced an opti-

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mum system in the Bc3 mixture. Based on all of the observations mentioned above, the presence of nanosilica was concluded to improve the bending strength of cement composites reinforced with natural fibers and a coupling agent. 3.3 Investigation of the Modulus of Rupture According to Table 7, it was possible to improve the strength properties of cement-based sheets using a hybrid system of waste natural fiber, nanosilica, and aminosilane. This finding can be attributed to an enhancement of the bonds between the fibers and the hydrated cement, which is the fundamental factor in the breaking resistance of cement sheets after the formation of principal cracks. Increasing the quantity of nanosilica in the presence of aminosilane improved the ultimate bending strength of cement sheets. This was due to enhanced microstructural properties of the cement composites thanks to the addition of silica nano-particles. However, the presence of aminosilane

Fig. 3 Bending load-deflection curve of cement composites with 10 % SPFs, aminosilane and different dosages of silica nano-particles at 28 days

Fig. 4 Bending load-deflection curve of cement composites with different levels of SPFs (5, 10, and 15 %), and aminosilane incorporating 3 % silica nano-particles at 28 days

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alone increased the strength properties of the cement sheets, a result due to the coupling effect of aminosilane between hydrated cement and fibers. On the other hand, an improvement of the strength of the cement sheets could be achieved by increasing the fiber content up to 10 % aminosilane and 3 % nanosilica. Also, the ultimate bending strength of cement composites with 10 % fibers, 3 % silica nano-particles and aminosilane (Bc3) was higher than that of other mixtures with the same amount of nanosilica, which implies that the maximum strength of the cement sheets had been achieved. 3.4 Investigation of the Modulus of Elasticity The variation in the modulus of elasticity is one of the more complicated behaviors of cement composites containing SPFs, nanosilica, and aminosilane. According to Table 7, the presence of aminosilane in the cement matrix could increase the modulus of elasticity. This occurred as a result of the improvement of the bonding properties between the cement paste and fibers as a consequence of the coupling effect of aminosilane, as well as an improvement of the microstructural properties of the cement paste in the presence of aminosilane molecules (except at early ages). As shown in Table 7, the modulus of elasticity increased in the presence of aminosilane and with the replacement of cement by 1 % silica nano-particles. This was attributed to the outstanding pozzolanic behavior of silica nanoparticles, which developed the microstructure and consequently enhanced the adhesion of fibers and cement hydration products. Additionally, it should be noted that the production of CSH gel (the principal factor behind the brittle nature of cement-based composites) also increased. Moreover, based on Table 7, it is clear that higher fiber contents caused decreases in the modulus of elasticity because of the low stiffness of the fiber induced by reduction of the stiffness of the cement paste matrix.

The presence of aminosilane can cause air entrainment in the cement matrix [9–11] and can consequently give rise to a decrease in the density of the hardened specimens. Simultaneously adding silica nano-particles and aminosilane resulted in increased air entrainment in the matrix and consequently induced a further decrease in density. 3.6 SEM Results As stated previously (cf. Sect. 3.1), aminosilane improved the bonding between cement paste and fibers in such a way that increased quantities of hydration products, particularly CSH gel, were observed on the fibers and in the lumen pores (Fig. 5). Figure 6 shows the proper coupling of fibers with hydration products, particularly CSH gel, in the mixture containing

Fig. 5 The adhesion between fibers and cement paste is improved in the presence of aminosilane (Bc1)

3.5 Bulk Density According to Table 7, at a constant volume of the cement composite, an increase in fiber content corresponded to a decrease in the amount of cement. Since the density of cement is greater than those of the fibers and silica nano-particles, an increase in fiber content for a constant volume of cement composite resulted in a decrease in the density of the mixture. Adding nanosilica alone can also lead to a decrease in the density. As a result of adding nano-particles, more air bubbles can be introduced into the structure of the cement composite [22,23]. Consistent with other published studies, the addition of nano-particles resulted in a lower density of the cement specimens.

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Fig. 6 Improvement in the microstructure of cement paste and transition zone of fiber-cement in the presence of aminosilane and silica nano-particles (Bc3)

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– The simultaneous addition of silica nano-particles and aminosilane resulted in an increased air entrainment in the matrix, which consequently induced a further decrease in density. – Due to the structure of the natural fibers, as seen in SEM images, aminosilane, silica nano-particles, and hydration reaction products (specially Ca(OH)2 crystals) were able to penetrate into the cavities of the surface of the fibers and improve the strength of the fibers by producing a dense CSH gel during the pozzolanic reaction and a proper coupling between the fiber and hydration products.

Fig. 7 Presence of CSH gel on the surface and inside of fiber cavities (mixture with hybrid system of aminosilane and silica nano-particles) (Bc3)

nano-particles and aminosilane (Bc3). The presence of silica nano-particles and aminosilane led to an improvement of the cement paste microstructure, as well as to proper coupling between the fiber and hydration products. Therefore, the bending strength of the mixture with silica nano-particles and aminosilane was higher than that of other pastes. Figure 7 illustrates the increased quantity of hydration products on the fiber surface in the microstructure of the mixture with a hybrid system of aminosilane and silica nanoparticles. Due to the presence of high-strength CSH gel in the lumen and on the fiber surfaces, the performance of natural fibers improved in the cement-based composites.

4 Conclusions The following conclusions can be drawn from the present study: – Aminosilane molecules were involved in the coupling between cement particles, fibers, and silica nano-particles. Incorporating aminosilane together with nanosilica helped overcome negative effects of aminosilane at early ages (3 and 7 days) and enhanced the performance of aminosilane at moderate age (28 days). – Increasing the content of nanosilica in the presence of aminosilane increased the ultimate bending strength of the cement sheets (at 28 days). For a constant quantity of SPFs (10 %) incorporated with aminosilane, a higher nanosilica content (from 0 to 3 %) resulted in an increase followed by a decrease of the modulus of elasticity. – The mixture containing a coupling agent, 10 % fibers, and 3 % silica nano-particles (Bc3), exhibited the best behavior among the mixtures studied, at both 7 and 28 days.

Acknowledgments This study is a part of research project with the title of Investigation on replacing of asbestos with natural fiber on cement composites, and the authors gratefully acknowledge the financial support of the Young Researchers Club, Islamic Azad University, Chalous branch, Chalous, Iran.

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