Mechanical properties and microstructure of a graphene oxide–cement composite
Introduction
Ordinary Portland cement (OPC) is extensively used worldwide for building and construction. However it has limited structural applications due to poor tensile strength and strain capacity. The weak tensile strength is associated with pre-existing flaws. By incorporating steel reinforcing bars and fibres, it is possible to delay the development of microcracks and therefore improve the resistance of the structural composites to tension [1], [2]. Recent advancements in nanotechnology have produced nanosized particles/fibres (e.g. nano-silca and carbon nanotubes) that could be used as reinforcements to hinder the formation and propagation of microcracks at the outset. Nano-reinforcements in cementitious materials are more effective than conventional steel bar/fibre reinforcements (at millimeter scale) because they can control nano-size cracks (at the initiation stage) before they develop into micro-size cracks [3]. For example, improved mechanical performance by introduction of nano-silica in cement matrix has been reported [4]. Nano-silica has a spherical shape (low-aspect-ratio) with diameters less than 30 nm and with a specific surface area of 300 m2/g [5]. The improved mechanical performance is generally attributed to two mechanisms [6]. Firstly, as nuclei for cement phase, nano-silica can promote cement hydration due to its high specific surface area. Secondly, as filler, nano-silica can highly densify the microstructure because the size of nano-silica is comparable to that of gel pore in cement matrix. However, due to its low-aspect-ratio, nano-silica lacks ability to arrest microcracks derived from nano-size cracks, thus affecting reinforcement efficiency of nano-silica.
Compared to nano-silica’s spherical shape, carbon nanotubes (CNTs) can be regarded as a one dimensional tube (high-aspect-ratio). Depending on whether single walled CNTs (SWCNTs) or multi-walled CNTs (MWCNTs), they generally have the diameter of 1–3 nm or 5–50 nm, respectively [7]. The length of CNTs can be up to centimeters, which gives an aspect ratio exceeding 1000. CNTs also exhibit extraordinary strength with moduli of elasticity on the order of TPa and tensile strength in the range of GPa [8]. With the concurrent benefits of high aspect ratio and excellent mechanical performance, CNTs have been found to improve cementitious materials’ elastic modulus [9] and strength [10], [11]. However, the incorporation of CNTs in cement composites has proven to be rather complex and sometimes yields contrasting results. Several researchers found that the addition of CNTs results in little change in strength or even a reduction in strength of the composite in some cases [12], [13]. The reasons for this are generally attributed to the poor dispersion of CNTs and weak bonding between the CNTs and the cement matrix. Owing to strong Van der Waal’s attractive forces between particles, CNTs tend to form agglomerates or bundles which may become defect sites in the composites. Sáez de Ibarra et al. [14] tested CNT-cement composite by nanoindentation measurements and found that the samples containing CNTs without dispersing agent had worse mechanical properties than the plain cement paste. They also observed a non-uniform distribution of CNT bundles within the matrix which provoke the decrease in the mechanical properties. In contrast, the mechanical properties increased with respect to the samples containing CNTs when dispersing agents were used. Besides poor dispersion, another problem that limits the efficiency of CNTs in the cement matrix is the difficulty in achieving adequate CNT-matrix bonding. A CNT can be thought of as a graphene sheet that has been rolled up into a tube structure. The tube-shaped CNTs reduces interfacial contact area since the outermost CNT shields the internal tubes from the matrix [15]. The lack of interfacial areas between CNTs and the cement matrix reduces CNT’s reinforcing efficiency, even though CNTs exhibit excellent mechanical properties. A study by Cwirzen et al. [13] indicated that MWCNTs introduced as a water suspension with added surfactant admixture did not increase the compressive and flexural strength, even though good dispersion was obtained. One of the possible reasons is that the bonding between the MWCNTs and the cement matrix to be rather weak. As a result the MWCNTs are easily pulled from the matrix when subjected to tensile stresses.
Similar to CNTs, graphene also comprises sp2-bonded carbon atoms [16], providing graphene with excellent mechanical properties. The intrinsic strength and Young’s modulus are estimated to vary between 60 and 130 GPa and 1 TPa, respectively [17]. In addition, graphene is a flat sheet of carbon atoms with only one atom thickness [18]. The planar structure of graphene sheets creates a lot more contact area with the host material because both the top and bottom surfaces of a graphene sheet are in close contact with the host material. Moreover, the aspect ratio (defined as the ratio between the lateral size and the thickness) of a single graphene sheet can reach more than 2000 and value of surface area of a single graphene sheet can theoretically reach 2600 m2/g, which are much higher than those of CNTs [19]. The greater exposed surface areas of graphene sheets generate more potential sites for advantageous chemical or physical interactions, which in turn improve bonding between graphene sheets and host material. An initial investigation on the performance of epoxy based composites shows that graphene outperform CNTs for developing stronger graphene/epoxy composites [15]. Note that only 0.1% weight (of the composite) graphene is needed to enhance the mechanical properties of the composite to the same degree as that by adding 1% CNTs. However, difficulties in dispersing graphene and high cost of production limits its widespread applications.
As a graphene derivative, graphene oxide (GO) is mono-layer of sp2-hybridized carbon atoms derivatized by a mixture of carboxyl, hydroxyl and epoxy functionalities [20]. The oxygen functional groups, attached on the basal planes and edges of GO sheets, significantly alter the van der Waals interactions between the GO sheets and therefore improve its dispersion in water [21]. The recent research [22] reported the homogeneous dispersion of GO in cement matrix. GO also exhibits high values of tensile strength, aspect ratio and large surface area [23]. In addition, GO can be easily acquired from natural graphite flakes (inexpensive source) by strong oxidation and subsequent exfoliation. The extraordinary mechanical properties combined with the highly dispersible in water and inexpensive source make GO a promising material for enhancing the mechanical properties of composites. A recent study [24] has shown that the addition of 1 wt% GO could simultaneously improve the strength and toughness of GO-chitosan composites. This improvement has been attributed to the enhanced nanofiller-matrix adhesion/interlocking arising from their wrinkled surface and two-dimensional geometry of graphene platelets. An improvement in elastic modulus of 33% has been reported for the addition of 0.01 wt% functionalized graphene sheets (FGS) in poly(methyl methacrylate) (PMMA) [25]. It was suggested that FGS contains pendant hydroxyl groups across the surfaces which may form hydrogen bonds with the carbonyl groups of PMMA. This surface chemistry leads to stronger interfacial interactions with PMMA and thus large influence on the properties of the host polymer. With regards to surface functionality of GO, the oxygen functional groups may also favor the utilization of these carbon nanostructures in cement composites because they can react with hydration products to form covalent bonds. For example, Li and co-workers [26] reported that carboxyl acid groups can react with calcium silicate hydrate (C–S–H) to form strong covalent bonds, and thus notably improve the mechanical properties of CNT-cement composite.
While GO has been extensively studied in polymeric composites [24], [25], [27], [28], its use in cement has, to date, remained limited. A recent study of GO–ferrofluid–cement system [22] found that the presence of GO and ferrofluid in the cement leads to strong polarizations and magnetic losses that consequently result in higher shielding effectiveness compared to pristine cement. However, effect of GO on mechanical properties of the cement matrix has not been investigated.
The aim of this paper is to investigate the compressive strength and flexural strength of OPC paste reinforced by GO sheets. The results obtained in GO–cement composite are compared with that of plain cement paste. Remarkably, only 0.05% weight of GO sheets is needed to increase the flexural strength of OPC paste by 41–59% and the compressive strength by 15–33%. The mechanisms for reinforcing effect are further discussed by the characterization of microstructure of GO–cement composite.
Section snippets
Materials
The cement is provided by Cement Australia, West Footscray, and conforms to the requirements of Type I – Normal Cement, as defined by the ASTM C 150. Graphite, with an average particle size of 44 μm was obtained from Zhongtian Co., Ltd. (Qingdao, China). According to Li et al. [29], the synthesis of GO dispersions is described in the following section.
Preparation of GO
Graphite oxide was synthesized from natural graphite using the modified Hummers method [30] that involves three steps, i.e. oxidation,
Characterization of GO
The thickness of a GO sheet is around 1 nm [34] while the size of GO sheets is characterized by SEM. Fig. 1(a) displays SEM image of GO sheets from 0.005 mg/ml aqueous GO solution. According to the SEM image, the sizes of GO sheets are widely distributed from less than 1 μm2 to over 200 μm2. Fig. 1(b) shows a TEM image of GO sheets from 2 mg/ml aqueous GO solution. It can be seen that the grid appears to be entirely filled with the GO sheets. This ‘visual’ effect is due to the enormous surface area
Conclusions
High performance GO–cement composites have been prepared. The incorporation of GO in cement paste significantly increases the volume of gel pores in the composite. As a result, the surface area of the samples has been increased from 27.3 m2/g to 64.9 m2/g by the addition of GO. The addition of 0.05% GO sheets improves the compressive strength and flexural strength by 15–33% and 41–58%, respectively. There are several possible reasons for this, which include improved mechanical interlocking
Acknowledgement
Authors thank the financial support from the Australia Research Council in conducting this study. Authors acknowledge use of the facilities within the Monash Centre for Electron Microscopy.
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