Graphene nanoplatelet-fly ash based geopolymer composites

Graphene nanoplatelet-fly ash based geopolymer composites

Cement and Concrete Research 76 (2015) 222–231 Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.e...

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Cement and Concrete Research 76 (2015) 222–231

Contents lists available at ScienceDirect

Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp

Graphene nanoplatelet-fly ash based geopolymer composites Navid Ranjbar a,⁎, Mehdi Mehrali b,⁎, Mohammad Mehrali b, U. Johnson Alengaram a, Mohd Zamin Jumaat a,⁎ a b

Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Mechanical Engineering and Center of Advanced Material, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e

i n f o

Article history: Received 9 October 2014 Accepted 2 June 2015 Available online xxxx Keywords: Graphene Alkali activated cement (D) Bending strength (C) Compressive strength (C) Microstructure (B)

a b s t r a c t Geopolymers show high quasi-brittle behavior because of their ceramic-like characteristics. Recent findings have indicated that graphene can be used as an additive to improve the mechanical properties of composites. In this study, we report the effect of the addition of graphene nanoplatelets (GNPs) on the microstructure and mechanical properties of a fly ash based geopolymer. The GNPs are relatively homogeneously distributed in the matrix of all composites. However, overlapping and agglomerate formation of GNPs was detected by FESEM. The results showed that the compressive and flexural strength of the geopolymer improved by 1.44 and 2.16 times, respectively, when adding 1% GNPs. The introduction of a GNP filler, even at low filler weight fractions, increased the toughness, stress and strain at the first crack and rigidity. Moreover, the wettability decreased with an increase in GNP content. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Geopolymers are currently being considered as a replacement of ordinary Portland cement and have drawn considerable attention for their cost efficiency, chemical stability, corrosion resistance, rapid strength gain, low rate of shrinkage and freeze and thaw resistance [1–3]. However, because of their ceramic-like characteristics, geopolymers show a quasi-brittle behavior similar to OPC; therefore, low flexural strengths and catastrophic failure during service usually affects their extensive application when safety-based structural design is considered [4,5]. Thus, improving the fracture properties of geopolymers is deemed necessary. Fractural strength improvement of geopolymer composites mainly depends on reinforcing materials, such as steel bars [6], steel fibers [7], carbon fibers [8,9], polymer fibers [10,11] and natural fibers [14]. Whereas fibers improve the fracture behavior of the entire matrix, the ones that improve the microstructural characters of the geopolymers are able to overcome the loss of fracture toughness because of the high brittleness of geopolymers. Generally, the utilization of fibers is proposed for controlling cracking and increasing the fracture toughness of the brittle matrix. Debonding, sliding and pulling-out of the fibers are local mechanisms that control the bridging action during both the micro- and macrocracking of the matrix. At the beginning of macro-cracking, the bridging action of fibers prevents and controls the opening and growth of the cracks. This mechanism increases the demand of energy for the crack ⁎ Corresponding authors. Tel.: +60 107014250. E-mail addresses: [email protected] (N. Ranjbar), [email protected] (M. Mehrali), [email protected] (M.Z. Jumaat).

http://dx.doi.org/10.1016/j.cemconres.2015.06.003 0008-8846/© 2015 Elsevier Ltd. All rights reserved.

to propagate. The linear elastic behavior of the matrix is not significantly affected for low volumetric fiber fractions. However, post-cracking behaviors can be substantially modified with the increasing strength, toughness and durability of the material [12]. The performance of reinforcements in geopolymers is highly dependent on the inherent properties of the fibers, fiber content, geopolymer precursors, curing conditions and age of the composites [13,14]. Graphene, a flat monolayer of carbon atoms in a two-dimensional (2D) honeycomb lattice with a high aspect ratio layer geometry and a high specific surface area, has attracted tremendous attention in recent years because of its exceptional thermal, mechanical, and electrical properties. In recent years, the incorporation of graphene in matrixes, such as polymers and ceramics, has shown dramatic improvements in the properties of the matrix, such as the elastic modulus, tensile strength, electrical conductivity, and thermal stability [15–18]. Compared to monolayer graphene, graphene nanoplatelets (GNPs) are formed by several layers of graphene and are less prone to agglomeration and entanglement because of the increased thickness of up to approximately 100 nm [19]. By contrast to graphene oxide (GO), graphene nanoplatelets (GNPs) have emerged as a competitive, alternate material for graphene because thermal annealing or chemical treatment can eliminate functional groups on GO to produce GNPs [19,20]. Furthermore, in comparison with carbon nanotubes (CNTs) for which only one side of the atomic lattice contacts the matrix and the other side of the lattice faces into the center of the tube, both faces of graphene contact the matrix. Consequently, graphene provides a stronger contact with binders [21]. Because of the increasing contact area and large size of particles providing a lengthy deflection path, the contribution of graphene in toughness enhancement can be greater than that in fiber reinforced composites [18]. Because of the

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aforementioned properties, GNPs have been used as a reinforcement in composite materials [22]. Because of the high specific area and aspect ratio, a small amount of graphene is sufficient to improve the properties of matrices. Our recent studies have shown that the mechanical properties of calcium silicate and hydroxyapatite ceramics can be significantly improved with relatively low graphene nanofiller loading [23,24]. All of the graphenereinforced ceramic matrix composites were found to exhibit a decreased tendency to fracture, mainly because of crack bridging, crack deflection, crack tip shielding, and crack branching. Moreover, few studies have investigated the effect of graphene on the properties of cementitious materials [25–28]. Lv et al. reported that the addition of 0.3% of graphene oxide in cement composites showed a remarkable increase of 78.6%, 60.7% and 38.9% in tensile strength, flexural strength and compressive strength, respectively [25,28]. Additionally, the thermal and electrical properties of the composites were enhanced compared to the material without graphene. Recently, the incorporation of rGO in geopolymer composites showed that rGO particles enhance the flexural strength, Young's modulus and toughness of the geopolymeric composites because they display a 2-dimensional structure and strong chemical bonding with the matrix. Further, the addition of low amounts of rGO sheets into geopolymers improved the mechanical properties and reduced the overall porosity of the composite specimens [29,30]. To achieve this potential material enhancement, the development of an efficient and capable method for the mass production of graphene for industrial applications has attracted increased attention. Among the proposed methods for graphene preparation, such as mechanical exfoliation, chemical vapor deposition, electrolysis, hydrothermal method, and chemical processing, the chemical reduction method is considered to be a promising route for cost-effective large scale production [31–33]. In this study, the variation of the mechanical properties of GNP/fly ash geopolymer composites with respect to the amount of GNPs in the matrix has been systemically investigated. The microstructures were studied using Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). The results showed that relative compressive and flexural strength of the composites were enhanced by approximately 1.44 and 2.16 fold, respectively, by incorporating 1% wt. GNP into the geopolymers. This increase results from various toughening enhancer mechanisms by the graphene nanoplates and stress uniformity through the composites. 2. Materials, synthesizing and analysis methods 2.1. Material characterization Low calcium FA (class F) was used in this research with a median particle size of 16.23 μm. The particle size distribution of the FA is shown in Fig. 1. The results of the XRF analysis and loss of ignition (LOI) of the fly ash, as determined by X-ray florescence on a PANalytical

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Axios mAX instrument, are shown in Table 1. The physical properties of the FA particles are given in Table 2. Alkaline activators in the investigation consisted of sodium silicate and sodium hydroxide solutions. Sodium hydroxide (NaOH) was prepared in pellet form at 99% purity and was ordered from Merck (Germany). The sodium silicate was used in liquid form (from PC Laboratory Reagent) with a modulus ratio of 2.5 (SiO2/Na2O, SiO2 = 30% and Na2O = 12%). 2.2. GNP preparation Graphene oxide was synthesized from graphite using a simplified version of Hummers' method according to our previously published study [23]. GNPs were obtained through the thermal reduction of graphene oxide. In this method, graphene oxide was placed in a quartz tube that was closed at one end. The other end of the quartz tube was covered using a rubber stopper. A high purity argon gas flow was inserted directly into the quartz tube for 15 min, and then, the tube was quickly inserted into a tube furnace (Carbolite) preheated to 1100 °C and held in the furnace for 30 s. To measure the size of the GNP, atomic force microscopy (AFM, Veeco Dimension AFM) in tapping mode was used and is shown in Fig. 2. The GNPs used in the AFM imaging were prepared by drop-casting a diluted suspension (0.05 mg/ml) onto a clean silicon substrate and dried at 50 °C for 24 h. The particle size distribution of the GNPs, as determined by an AFM analysis, is shown in Fig. 3. As observed, the GNP size varies between approximately 1 μm to 7 μm, and the median size was approximately 4 μm. Table 2 shows the physical properties of the fly ash and GNP. 2.3. GNP/fly ash geopolymer composites The alkali activator solution was prepared by mixing 16 M NaOH with the Na2SiO3 solution at a NaOH ratio of 2.5. The activator to binder ratio of 0.5 and additional water to fly ash ratio of 0.1 were maintained for all mixes, which was identical to our previous research [23]. GNP fly ash based geopolymer composites were produced in two separate stages of preparation of the geopolymer matrix and sonicated GNPs, which were ultimately mixed together. In the first stage, the GNPs were sonicated in the mixing water with a geopolymer using a Branson B3210 Ultrasonic for 5 min to separate the bundles into individual flakes, producing a uniform suspension. Fig. 4 shows the GNPs before and after sonication in the mixing water. Separately, alkali activators were gradually added to the fly ash and mixed for 7 min. The graphene suspension was added to the fresh geopolymer and mixed for 3 min to produce a homogenous and workable mixture. The material was poured into molds and vibrated for 30 s and maintained outside for 1 h. Afterward, the samples were maintained in a 65 °C oven for 24 h. Specimens were demolded and maintained in ambient conditions until testing. The concentrations of the GNPs in the composites were 0.1%, 0.5% and 1% of the geopolymer weight. Normal geopolymer specimens without GNP content were Table 1 XRF analysis of the fly ash.

Fig. 1. Particle size distribution of the fly ash.

Composition

Fly ash (%)

SiO2 Al2O3 CaO Fe2O3 TiO2 P2O5 MgO SO3 K2O Na2O MnO CuO LOI

54.72 27.28 5.31 5.15 1.82 1.12 1.10 1.01 1.00 0.43 0.10 0.01 6.80

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Table 2 Physical properties of the fly ash and GNP.

FA GNP

Median particle size (μm)

Specific gravity (g/cm3)

BET (m2/g)

16.23 ~4

2.18 2.1

2.96 80

prepared to serve as a comparison. Notably, 1 w% GNP was the maximum amount that could be sonicated uniformly without agglomeration in the geopolymer mixture when using a constant amount of water (as specified in the mix design) to achieve similar Si/Al/Na/H2O ratios in all of the specimens. Above this value, the sonication effect was nullified, and once the specimens were removed from the sonication bath, GNPs sedimented, separated and transferred into a gel form. This form is not suitable for the mixing method used in this research. However, with an altered mix design, additional water could be added. However, this additional water would lead to an increase in the porosity with a subsequent reduction in the mechanical properties of the geopolymer matrix [34]. Fig. 3. Particle size distribution of GNPs.

2.4. Analysis method The oxide composition of the materials was determined by X-ray fluorescence (XRF) using a PANalytical Axios mAX instrument. The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were evaluated on the basis of nitrogen adsorption isotherms measured at 77 K using a BELSORP-max nitrogen adsorption apparatus. Transmission electron microscopy (TEM) measurements were conducted on a CARL ZEISS-LIBRA120 microscope. The samples were prepared for TEM characterization by dispersing the powder in ethanol, placing the samples into a micro grid, and allowing the solvent to evaporate. Field emission scan electron microscopy (FESEM CARL ZEISS-AURIGA 60 and Quanta FEG 450) was used to observe the microstructures of the GNP/fly ash geopolymer composite and the distribution of GNPs. The X-ray diffraction (XRD) patterns were measured on an Empyrean PANALYTICAL diffractometer with monochromated Cu Kα radiation (λ = 1.54056 Å), operating at 45 kV and 40 mA with a step size of 0.026 deg and a scanning rate of 0.1 deg s- 1 in the 2θ range of 20 to 70 deg. Fourier transform infrared spectroscopy (FTIR) analysis was performed using a Perkin Elmer System series 2000 spectrophotometer in a frequency range of 4000 to 400 cm−1 to identify the functional group of the composites. The static water-contact angle of the GNP/fly ash geopolymer composites was measured by using a video-based optical contact angle measuring instrument (OCA 15EC; DataPhysics Instruments 8 GmbH; Germany). The specimen surface preparation is an important factor for the water contact angle test, and a difference in the surface roughness results in different measurements. In this study, after

14 days of curing, the top surfaces of the specimens not in contact with the mold were grinded by a cylindrical grinder machine to level the surface. Subsequently, the composites were polished with silicon-carbide emery paper and diamond paste (diameter: 1 μm). A single droplet of distilled water (2 μL) was applied to the surface of the sample, and the contact angle was measured after 30 s. The measurements were repeated three times at different locations for each sample, and the average value was calculated to show the variations of the hydrophobicity of the composites. The compressive strength was determined at 14 days on 25-mm cubes with a displacement control rate of 0.5 mm/min, and 25 × 25 × 100 mm prisms were prepared for three point bending with a rate of 0.1 mm/min and a span of 75 mm according to ASTM C293-10. The edges of the samples were lightly chamfered by a grinding machine to avoid a pseudo drop in the strength for the initial measurements of the specimens. An INSTRON-3369 machine was used to determine all of the mechanical properties. The compressive and flexural strength and Young's modulus were calculated as follows: σc ¼

σf ¼

F A

ð1Þ

3: F:l 2

2:b:d

Fig. 2. AFM images of GNP sheets.

ð2Þ

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Fig. 4. GNPs before (left) and after (right) sonication.



  3 F l : δ 4:b:d3

ð3Þ

3. Results 3.1. XRD

where F and δ are the applied vertical load and corresponding deflection at the midspan, respectively; A is the cross-section of the compressive specimens; l is attributed to the span or distance between supports; and b and d are the width and depth of flexural specimens, respectively. Flexural toughness was calculated based on the area under the corresponding load-deflection curves (as per the concepts of ASTM C1018-97) and is presented as relative values to the normal geopolymer specimen without GNP incorporation. The toughening mechanisms of the GNP-reinforced fly-ash based geopolymer samples were investigated by monitoring the generation of cracks via a Mitutoyo hardness tester (model AVK-C2, Mitutoyo, Kawasaki, Japan). First, the GNP/fly ash composite compacts were ground using progressively finer silicon carbide papers (up to 2000 grit size), and the samples were then polished to a mirror finish using diamond powders of various grades from 15 to 0.25 μm in an auto polisher (laboforce-3, Struers). A 5-kg Vickers load was applied to the polished samples for a loading time of 10 s.

Fig. 5 shows the XRD patterns of the fly ash, the fly ash based geopolymer and the fly ash based geopolymer reinforced by a 1 wt.% GNP geopolymer composite. The patterns show that the FA based geopolymer matrix consisted mainly of crystalline phases of quartz and mullite (originating from FA) and trace levels of albite. The XRD analysis confirmed that GNPs did not induce the formation of any other phase. In the GNP geopolymer composite, detecting GNPs by XRD is difficult because of their low content. 3.2. FTIR Fig. 6 shows the FTIR spectra of the pure geopolymer and 1 wt.% GNP/fly ash geopolymer composite. As observed, the major bands are similar for the pure geopolymer and 1 wt.% GNP/fly ash geopolymer composite. The distinct intensities band at 445 cm− 1 and 990 cm−1 are associated with the Si-O-Si bending vibration and Si-O-Si and SiO-Al asymmetric stretching vibration, respectively [35]. The small band of approximately 670 cm−1 represented the functional group Si-

Fig. 5. XRD patterns of the fly ash based geopolymer composites.

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Fig. 6. FTIR spectra of the pure geopolymer and GNP/fly ash geopolymer composite.

O-Al [36]. Moreover, the band at approximately 780 cm−1 is characterized as the crystalline phase of quartz. The band at region 1650 cm−1 is attributed to O-H bending [37]. The band at approximately 2300 cm−1 is assigned to the stretching vibration of OH and H-O-H [36]. From the inset of Fig. 6, the GNP/fly ash geopolymer composite exhibited clear absorption bands resulting from the asymmetric stretching of methylene groups (CH2) at 2920 cm− 1 and the symmetric stretching of CH2 at 2840 cm−1, which are inherent to GNP [24].

3.3. Water contact angle The water contact angle is a parameter that determines the hydrophilicity/hydrophobicity of the material. This angle is related to the value of the contact angle between the water droplet and the solid. In general, geopolymers are cementitious materials with porous structures and display a wide range of pore variation, from nanometers to millimeters, including entrapped air voids, entrained air voids, capillary pores and nanoscale gel pores. When the surface of a geopolymer is dry and subjected to aggressive liquids, the geopolymer absorbs the majority of the liquid because of the capillary force. This absorption leads to a durability problem and reduces the service lifetime of the product.

Therefore, the geopolymer must be protected from water penetration. The water contact angle is a significant factor because hydrophobicity will slow the water penetration through the material, improving the durability. Fig. 7 shows the contact angle images of water on the solid surface of a normal geopolymer and a GNP/fly ash geopolymer composite. As shown, the contact angle of distilled water (DW) on the surface of the geopolymer mixture is 22.1°, increasing to a maximum of 31.3° in the 1 wt.% GNP/fly ash geopolymer composite. Therefore, with an increasing contact angle, less wetting of the composite will occur. The resulting variations result from the presence of graphene in a composite that has already been recognized as a hydrophobic material. Moreover, the reduction of porosity is an additional parameter that might affect the water contact angle.

3.4. FESEM and TEM Fig. 8 shows the FESEM and TEM of the GNP geopolymer composites. As observed in Fig. 8.a and b, the GNP sheets are thin with a high aspect ratio and contain wrinkles and a crumpled texture on their surface. The rough surface increases the contact area between the geopolymer matrix and the GNPs, enhancing the mechanical properties by producing

Fig. 7. Water contact angle of GNP/fly ash geopolymer composites.

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Fig. 8. FESEM and TEM images of a GNP geopolymer composite.

Fig. 9. Flexural and compressive strength of the GNP geopolymer composites after 14 days.

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Fig. 10. Compressive to flexural strength ratio of the GNP/fly ash based geopolymer composites.

from 0.1 to 1 wt.%. Additionally, the compressive and flexural strength displayed maximum improvements of approximately 144% and 216% with the 1 wt.% GNP geopolymer composite, respectively. As indicated in Fig. 10, the compressive to flexural strength ratio of the specimens decreased by increasing the GNP content. Therefore, the incorporation of GNP will display a higher effect on the enhancement of the flexural properties than the compressive. Fig. 11 shows the nominal flexural toughness and Young's modulus of the materials. As observed, the nominal flexural toughness increased by ~300% in comparison to that of a normal geopolymer specimen without GNP content. Additionally, a significant increase in the stiffness was obtained in the GNP incorporated specimens, contributing to the high elastic modulus of GNP particles and the toughening actions. The strength of the composite is measured primarily in the tensile mode using the flexural or bending strength. As observed in Fig. 12, the flexural cracking deflection of the GNP geopolymer composites was postponed by approximately 30% by increasing the amount of GNPs to 1 wt.%. The specimens gained rigidity, and the flexural load capacity was increased by increasing the GNP content of the composites. However, for all of the specimens, sudden failures were observed without any post crack enhancement. 4. Discussion

a higher pull out and debonding energy. Moreover, the TEM image shows that single layers of GNP are attached to each other and formed into nanosheets. The GNP fly ash based geopolymer composite is shown in Fig. 8.c. This composite consisted of a geopolymer matrix, GNP sheets and unreacted fly ash particles. Randomly oriented GNPs are indicated in Fig. 8.d–h. Fig. 8.d shows a good distribution of GNPs in the geopolymer matrix and a strong contact among them. However, weak contact is observed in Fig. 8.e; the overlapped GNP sheets are accumulated parallel to each other, resulting in no contact with the geopolymer matrix at select surfaces and leading to an increased risk of sliding once exposed to a load. The agglomerated and uniform distribution of graphene particles in hardened geopolymer pastes are shown in Fig. 8.f–h. These distributions cause stress concentrations in that particular area. Generally, two major factors of GNP dispersion and its contact with geopolymers govern the mechanical enhancement mechanism of the composites.

3.5. Mechanical properties Fig. 9 shows the flexural and compressive strength of the GNP geopolymer composites. As observed, the compressive and flexural strength were improved progressively by increasing the GNP content

Geopolymer pastes are a ceramic-like material containing different cracks of different sizes under loading or environmental effects, such as shrinkage [38]. Depending on the orientation of the cracks and loading, the cracks can be loaded on all modes (I, II and III). Under uniaxial compression tests, visible cracks propagate parallel to the direction of the loading at a right angle of the local tensile stress. Fracture of the unreinforced geopolymer is associated with the development of visible vertical cracks and the separation of crack surfaces by breaking the interatomic bonds through tensile stress. Thus, the failure of the geopolymer under uniaxial compression is attributed to either a mode I fracture (once cracks are visible) or a combination with one or both of the other modes resulting from lateral tension because the sample is confined at both ends causing the stress distribution to be neither uniform nor uniaxial. The improvement of the compressive strength of the GNP geopolymer composite originated from the enhancement of two mechanisms. In a compression section in which the vertical stresses in different orientations transfer to the nanosheets, the high elastic modulus of the GNP alleviated the stress concentration in the matrix and transferred the stress uniformly to the other portions of the matrix. This mechanism will enhance the compression capacity of the composite by involving more area in carrying the stresses and to overcome the local failures. This action can be attributed to the ultrahigh elastic modulus of GNP sheets leading to a high rigidity and ability to not deform

Fig. 11. Relative flexural toughness and Young's modulus of the GNP geopolymer composites.

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Fig. 12. Flexural strength–deflection curves of the GNP geopolymer composites.

under such stresses at geopolymer failure. However, GNP particles decelerate the initiation and extension of visible cracks of mode I fractures and potential shear stress by bridging effects. Once a crack faces toward the surface of the GNP, for further propagation, the crack must pass the sheet. Passing the sheets demands a higher energy of fracture to pullout the nanoparticles. This increases the proportional limit and consequent ultimate strength of the composites. However, all of the composites failed in the brittle mode under a compression load. This failure is because of the high Young's modulus of the GNP particles and the sudden release of accumulated energy during the pull out of the nanosheets. The scheme of the load distribution in the compression section of a GNP geopolymer composite is depicted in Fig. 13. In ceramic-materials, a mode I fracture is smaller than II or III fractures once it is exposed to a bending load. Therefore, ceramics usually failed in this mode. This failure mode explains why the geopolymers are more sensitive to this load than to a compressive load. A nearly 300% improvement in a 1% GNP fly ash based geopolymer was observed when compared to the specimen, excluding the GNP. This high improvement in the flexural strength of GNP geopolymers is attributed to various toughening enhancer mechanisms resulting from the presence of GNP, including crack bridging, crack deflection, pull out and crack branching [23,24]. To assist with providing a detailed understanding of the improved fracture toughness of the GNP based composite, the FESEM images in Fig. 14 show the presence of GNP particles at the micro crack rejoins. These particles provide a higher resistance to indentation-

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induced crack propagation in comparison to that of the pristine geopolymer specimens. Fig. 14.a shows an indent point conducted on the surface of the composite samples to form the micro-cracks. Fig. 14.b shows a crack growing and meeting a GNP. Because the crack was harsh, the GNP particles were broken and pulled out (as they were resistant to crack propagation), resulting in crack branching. However, as observed in Fig. 14.c and .d, when the micro-crack reaches a GNP, the particles bridge the crack and resist its widening because of the high energy demand required for opening the crack, resulting in toughening. Further, the intrinsic GNP branching and deflection mechanisms increase the effective length of the crack and absorb more energy to release stress. Subsequently, Fig. 14.e and .f shows the GNP debonding once the micro cracks pass through the proximity of particles. Generally for a three-point bending test, cracks form at the middle of the beam at which the bending stress is the highest. The first crack starts once the bending stress has overcome the bending capacity of the geopolymer matrix at the extreme bottom fiber of the specimens. When the crack reaches the GNP sheet, the crack propagates depending on the contact of the GNP and geopolymer matrix. When the geopolymer and GNP maintain a strong contact, the accumulated stress in the particles might overcome the strength of the particles and penetrate through it, causing brittle cracks in the next GNP sheet. However, when the contact between the GNP and geopolymer is weak, as a crack propagates and meets with the GNP, the crack is obstructed and deflected in-plane or the crack might be branched depending on the interaction of the propagating crack and GNP of different sizes. Either the crack deflection or branching mechanism would increase the path of the crack to release the stress, which results in improvements in flexural toughness. Moreover, the energy absorption of the composite is also improved because of the additional resistance from the drag by graphene sheets once they are under crack bridging or pull out. However, the weak contact between the GNP with the geopolymer and overlapped particles, as shown in Fig. 8.e, nullifies the high pull out resistance of the composites. In addition, the pull out energy of a sheet is expected to be much greater than the pull out of a nanofiber because of the high toughness and large surface area of GNP [18]. Fig. 15 shows the typical mechanism of toughening enhancers in a graphene nanoplated geopolymer composite. In general, the increased strength of the GNP and its contact with geopolymers have a critical influence on the behavior of the composites. Therefore, the optimized mixing condition led to

Fig. 13. Scheme of a GNP/fly ash geopolymer composite under compression load.

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Fig. 14. Characteristic toughening mechanisms of GNP reinforced geopolymer based composites.

an improvement of the GNP distribution in the composite and stronger contact with geopolymers, thus improving the mechanical properties.

5. Conclusions This study evaluated fly-ash based geopolymer reinforced by GNP from the aspect of microstructures and mechanical properties. Based on our experiments, the following conclusions were drawn: Because of the high specific surface area and aspect ratio of the GNP, a small amount of the material is sufficient to enhance the mechanical properties. No chemical changes were detected in the GNP fly ash based geopolymer by XRD and FTIR. Moreover, regarding the watercontact angle, the products offer good behavior and improve the permeability class, as expected. The dispersion and contact of the GNP and geopolymer matrix are the major factors influencing the mechanical enhancement mechanism.

The existence of overlapped GNPs or their accumulation in particular regions are defective and result in nullifying the additional resistance from the drag by GNP sheets and stress concentration, respectively. The highest relative compressive and flexural strength were improved by the 1 wt.% GNP geopolymer composite by 1.44 and 2.16 times, respectively. The compressive and flexural strength, toughness and stiffness of the composites are increased because of toughening enhancer mechanisms and uniform stress distribution by increasing the GNP content.

Acknowledgments This research work was funded by the University of Malaya under a High Impact Research Grant (HIRG) No. UM.C/HIR/MOHE/ENG/36/ D0000036-16001 (Strengthening structural elements for load and fatigue). We would also like to acknowledge the contribution of Professor

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Fig. 15. Scheme of the GNP/fly ash geopolymer composite under a flexural load.

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