Mechanism of cement paste reinforced by graphene oxide/carbon nanotubes composites with enhanced mechanical properties

Abstract

This study presents the enhanced mechanical properties of cement paste reinforced by graphene oxide (GO)/carbon nanotubes (CNTs) composites. The UV-vis spectroscopy and optical microscopy results show that the dispersion of CNTs in the GO solution is much better than in an aqueous solution due to the higher electrostatic repulsion, which allows a completely new approach of dispersing CNTs rather than by incorporating a dispersant. More importantly, the GO/CNTs composite plays an important role in improving the compressive and flexural strength of cement paste by 21.13% and 24.21%, which is much higher than cement paste reinforced by CNTs (6.40% and 10.14%) or GO (11.05% and 16.20%). The improved mechanical properties of cement paste are attributed to better dispersed CNTs and enhanced interactions among CNTs by the GO incorporation. Finally, the space interlocking mechanism of the GO/CNTs/cement paste composite with enhanced mechanical properties is proposed.

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1. Introduction

Brittleness and lack of flexural/tensile strength are the two major limitations of cementitious materials.¹ Traditional methods to improve the flexural/tensile strength of cementitious materials include introducing fibers such as steel fibers, carbon fibers and polyvinyl alcohol (PVA) fibers, which can significantly enhance the crack-resistance of cement and restrain crack propagation in the macro scale. Another effective way is to combine cementitious materials with nanomaterials such as nano-silica, carbon nanotubes (CNTs) and graphene oxide (GO). Because CNTs and GO possess a high elastic modulus and tensile strength, the use of these carbon nanomaterials to strengthen cementitious composites has attracted most concern recently.

CNTs, including single wall (SWCNTs) or multi walls (MWCNTs), are one-dimensional carbon nanomaterial that may be viewed as rolled up from a single planar sheet of graphene. The unique mechanical properties make it as an attractive candidate for reinforcement of composite materials. Many attempts have been made to incorporate CNTs as reinforcement in cementitious materials and to investigate the mechanical behavior. However, the prerequisite for CNTs reinforcement is the uniform dispersion of CNTs. Three major methods for dispersing CNTs are the addition of surfactants, mechanical ultrasonication and functionalization of CNTs. Shah et al.² investigated the effects of ultrasonic energy and surfactant concentration on the dispersion of MWCNTs at an amount of 0.08 wt% of cement, and found that the appropriate dispersion of CNTs could be achieved using sonication and a surfactant-to-CNTs weight ratio of 4. Konsta-Gdoutos et al.³ demonstrated that the flexural strength of the cement paste with the addition of MWCNTs at a concentration of 0.08% was improved by 35% with the help of surfactants and ultrasonication. Parveen et al.⁴ determined that 0.1 wt% of SWCNTs improved the flexural modulus, flexural strength and compressive strengths of mortar by 72%, 7% and 19%, through a short dispersion route using Pluronic F-127 as a novel dispersing agent. Duan et al.⁵ demonstrated that the flexural strength, Young's modulus and fracture toughness of the cement paste were significantly improved using 0.55 wt% CNTs with a pre-treatment of 50 J mL⁻¹ ultrasonication energy. Although the addition of surfactants contributes to better dispersion of CNTs in cementitious materials, it has a weak interface between CNTs and cement matrix. Moreover, ultrasonication dispersion of CNTs before mixing with the cement matrix makes it more expensive, complicated and time-consuming, and excessive ultrasonication has damaging effects on the properties of CNTs. Furthermore, defect free CNTs are incapable of forming good adhesion with the cement matrix. Even if better dispersion of the CNTs can be obtained with the help of surfactants, the sliding of the CNTs still readily occurs due to the weak bonding between the CNTs and matrix, which leads to the poor reinforcing effect on the mechanical behavior of cementitious materials. Therefore, chemical functionalization of CNTs has been widely investigated and developed because of the improved chemical bonding between CNTs and the cementitious matrix. Cwirzen et al.⁶ investigated the surface decoration of MWCNTs on the mechanical properties of cement paste and indicated that the compressive strength can be improved to nearly 50% with only a small addition (0.045–0.15 wt%) of MWCNTs. Li et al.⁷ showed that the use of chemically functionalized CNTs at a concentration of 0.5% by weight of cement led to an increase in the compressive and flexural strength of cement mortar of 19% and 25%, respectively. However, the reinforcing efficiency of chemical functionalization of CNTs on the mechanical properties of cementitious materials greatly depends on the following two points: (1) dispersion of the functionalized CNTs. Although functionalized CNTs shows better dispersion in an aqueous solution than pure CNTs, because of the hydrophilic functional groups, it still can be improved to increase the dispersion of CNTs to generate better mechanical properties in the cementitious materials. The increasing concentration of functionalized CNTs may lead to further mechanical improvement of cementitious materials, but the agglomeration more readily occurs with excessive CNTs content, so how to improve the dispersion of functionalized CNTs with a fixed concentration is important. (2) Damage of the functionalized CNTs. The mechanical properties of the functionalized CNTs are not as good as pure CNTs due to structure damage. There is a trade-off between the improved chemical interactions of functionalized CNTs/cement matrix and the decreased mechanical properties of the functionalized CNTs itself. Therefore, a question that has arisen is ‘whether there is a way that not only improves the dispersion of functionalized CNTs but also has a positive effect on the mechanical properties of cementitious materials that can compensate for the mechanical loss of the functionalized CNTs’.

Differing from CNTs, GO is an excellent hydrophilic material with oxygen-containing functional groups such as hydroxyl, carbonyl and carboxyl. Therefore, the dispersion of GO in an aqueous solution is excellent and therefore it is much easier to mix with cement compared with CNTs. Duan et al.⁸ demonstrated that introduction of 0.05 wt% GO can increase the compressive strength and flexural strength of GO/cement composite by 33% and 59%, respectively. Saafi et al.⁹ reported that 0.35 wt% GO can improve the flexural strength, Young's modulus and flexural toughness of geopolymeric cement by 134%, 376% and 56%, respectively. The improved mechanical properties of cementitious composites were mainly attributed to the high specific surface area and excellent mechanical properties of GO.¹⁰

Although CNTs and GO make great contributions to the mechanical enhancement of cementitious materials, the co-effects of GO/CNTs composites on the mechanical behavior of cementitious materials have not been investigated. In addition, what might happen if the negative charged CNTs and GO are combined, and the question as to whether the dispersion of CNTs in a GO solution can be improved due to the electrostatic repulsion still needs to be settled.¹¹ In the present study, the carboxylic functionalized CNTs were first dispersed in a GO and an aqueous solution and the dispersion efficiency of the functionalized CNTs in both solutions was characterized by UV-vis spectroscopy and optical microscopy. Then, the mechanical behavior and microstructure of cement paste reinforced with 0.05 wt% functionalized CNTs, 0.05 wt% GO and 0.025 wt% functionalized CNTs/0.025 wt% GO composite were investigated by mechanical testing and scanning electron microscopy (SEM) coupled with the energy dispersive X-ray (EDX) spectroscopy technique. The chemical interactions between functionalized CNTs and GO were investigated by Fourier transform infrared (FTIR) spectroscopy. Finally, the space interlocking mechanism of the cement paste reinforced with GO/functionalized CNTs composite with enhanced mechanical properties is proposed.

2. Experimental methods

2.1 Materials

Ordinary Portland cement (OPC) type 52.5 (Green island, HK) and Class F were used to fabricate the cement paste. Carboxylic functionalization of CNTs was used in this study because it can generate strong chemical bonding between the CNTs and cement matrix,¹² which were provided by the Shenzhen Nanotech Port Co. Ltd in China, and its properties are shown in Table 1. In the following study, the term of CNTs is short for the carboxylic functionalization of CNTs unless it is expressly stated.

Table 1 Properties of CNTs

Diameter (nm)	Length (μm)	Aspect ratio	Specific surface area (m^2 g^−1)	Electric conductivity (s cm^−1)
40–80	5–15	800	40–300	(15–30) × 10^−3

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2.2 Preparation of GO

GO was prepared from graphite powder (Alfa-Aesar, 200 mesh) according to the modified Hummer's method (Hummers and Offeman, 1985). Graphite powder (3 g) was added to a solution containing K₂S₂O₈ (2 g), P₂O₅ (2 g) and concentrated H₂SO₄ (40 mL, 98 wt%) for 6 h mixing at 80 °C. The resulting mixture was then diluted with distilled water, filtered and washed until the pH value of the rinse water became neutral. The dried graphite oxide was re-dispersed into concentrated H₂SO₄ (100 mL, 98 wt%) in an ice bath. KMnO₄ (15 g) was gradually added and stirred for 2 h. The mixture was then stirred and mixed at 35 °C for another 2 h, followed by addition of 230 mL of distilled water. The resultant bright yellow reaction solution was terminated by adding 700 mL of distilled water and 15 mL 30% H₂O₂ and subjected to centrifugation and careful washing by 37% HCl and distilled water. After immersing the as-prepared suspension in dialysis tubing cellulose membranes for seven days it was finally centrifuged and collected for preparing different concentrations of graphene oxide solution. In this study, the concentration of the GO solution was 1.2 mg mL⁻¹. Fig. 1 shows the X-ray diffraction (XRD) pattern of the graphite and GO used in this study. It clearly indicates that the diffraction peak of graphite and GO are at 26.72° and 10.14°, respectively. The interlayer spacing (d) can be calculated by Bragg's equation. Compared with graphite (d_graphite = 0.34 Å), the d_GO increases to 0.87 Å due to the oxygen group addition during the oxidation process, which can enlarge the distance among different layers of GO. In addition, the oxidation content of the GO was characterized by X-ray photoelectron spectroscopy (XPS). Fig. 2 clearly shows four types of carbon bonds in GO, including the C–C at 284.4 eV, C–O at 286.4 eV, C=O at 288.3 eV and –COOH at 289.0 eV. The elemental analysis of the XPS results indicate that the C/O ratio and oxygen content of the GO in this study are 3.0% and 30.7%, respectively.

Fig. 1 XRD patterns of graphite and GO.

Fig. 2 XPS C 1s spectra of GO.

2.3 Preparation and characterization of CNTs suspensions

0.02 g of CNTs was dispersed in a 50 mL aqueous solution and GO solution using a 275 W ultrasonicator for 15 min, and the two solutions were diluted 100 times before the UV-vis spectrometer test. The dispersion of CNTs was characterized using UV-vis spectroscopy (Lambda 950, Perkin Elmer) with a wavelength range of 190–1100 nm and a typical optical microscope (BX51, Olympus). The absorbance (ABS) was measured at 600 nm.

2.4 Preparation and characterization of the GO/CNTs/cement paste composite

0.025% CNTs (0.025 g) by weight of cement powder (100 g) was dispersed in 20 mL GO solution (1.2 mg mL⁻¹) by using 275 W ultrasonicator for 15 min before mixing with the cement paste. Therefore, the solid content of GO was 0.025% by weight of cement powder. Based on the water to cement ratio (w/c) of 0.4, extra 20 mL water was added to the CNTs/GO solution to fabricate the GO/CNTs/cement paste composite. The 40 mL CNTs/GO solution was then mixed with cement powder in high mixing speed for 10 min, and placed into steel molds followed by 20 s of vibration. The specimens were then covered under polyethylene sheets for 24 h in the laboratory environment before demolding. After demolding, the specimens were cured for 14 days at a temperature of 20 °C and humidity of 98%.

For the mechanical property tests, a three-point bending test was conducted following the procedure prescribed by ASTM C78/C78 M-10. Three specimens with dimensions of 150 mm × 30 mm × 10 mm were measured with a span of 90 mm and a stroke control at a loading rate of 0.1 mm min⁻¹. Two linear variable differential transformers (LVDTs) were set up on each side of the specimen to measure the mid-point deflection. The compressive strength test was conducted by testing three cubes of size 40 mm × 40 mm × 40 mm. The samples were placed in a materials testing system and loaded at the speed of 1 kN s⁻¹. FTIR testing was conducted to investigate the chemical interactions between the CNTs and GO. The microstructures of GO/cement paste composite and GO/CNTs/cement paste composite were evaluated by SEM with EDX.

3. Results and discussion

3.1 Dispersion efficiency of CNTs/aqueous and CNTs/GO solution

Fig. 3 shows the UV-vis spectroscopy results for the CNTs/aqueous and CNTs/GO solutions. As shown in Fig. 3, it is evident that the ABS for the CNTs/GO solution is three times higher than that for the CNTs/aqueous solution. On the basis of Beer's law,¹³ ABS is proportional to the dispersed CNTs because only dispersed CNTs can effectively absorb light in the UV-vis region. Therefore, the dispersion of CNTs in a GO solution is much better than that in an aqueous solution.¹⁴ The mechanism can be attributed to the larger electrostatic repulsion in the CNTs/GO solution,^11,15 as shown in Fig. 4. Unfunctionalized CNTs tend to agglomerate due to the high surface energy, as observed in Fig. 4a. However, functionalized CNTs show better dispersion in an aqueous solution due to the hydrophilic oxygen-containing groups (–COOH), as observed in Fig. 4b. The negatively charged CNTs repulse each other due to the weak electrostatic repulsion as a result of ionization of the carboxylic acid groups. More importantly, functionalized CNTs show the best dispersion in the GO solution due to the electronegativity of the GO solution itself, which results from the ionization of the phenolic hydroxyl and carboxylic acid groups. The larger electrostatic repulsion leads to the increased distance among the CNTs, and thus the best dispersion of functionalized CNTs can be achieved in the CNTs/GO solution, as observed in Fig. 4c.

Fig. 3 UV-vis spectroscopy results for (a) CNTs/aqueous and (b) CNTs/GO solutions.

Fig. 4 Scheme showing the dispersion of (a) unfunctionalized CNTs and (b) functionalized CNTs in the aqueous solution and (c) functionalized CNTs in the GO solution.

To better investigate the dispersion of CNTs in aqueous and GO solutions, a typical optical microscope observation for the CNTs/aqueous and CNTs/GO solutions was carried out and the results are presented in Fig. 5. Although CNTs are dispersed by a pre-ultrasonication for 15 min, some agglomerates of the CNTs in the aqueous solution are still observed due to the large surface tension and energy, as observed in Fig. 5a. In contrast, the bundled CNTs disappear and a better dispersion of CNTs can be achieved in the GO solution, as observed in Fig. 5b. The microscope results are consistent with the UV-vis spectroscopy results, indicating the dispersion of CNTs in GO solutions is much better than in aqueous solutions.

Fig. 5 Typical optical microscope images for (a) CNTs/aqueous and (b) CNTs/GO solutions.

3.2 Mechanical properties of the cement paste reinforced by GO and CNTs

The compressive and flexural behavior of cement paste with and without the CNTs and GO composite is shown in Fig. 6. Table 2 lists the mechanical behavior of cement paste with different contents of CNTs and GO. It can clearly be observed that incorporation of 0.05 wt% CNTs lead to a 6.40% increase in compressive strength and 10.14% in flexural strength of the cement paste. Moreover, 0.05% GO shows a similar but stronger reinforcement with cement paste, leading to a 11.05% increase in compressive strength and 16.20% in flexural strength, indicating that GO can remarkably enhance the mechanical properties of cement paste, which was also reported in other studies.^8,16–18 The improved mechanical behavior of the GO/cement paste composite is attributed to the excellent mechanical properties of the GO itself and the pore-filling effect of the GO on the cement matrix. In this study, the pore-filling effect of GO was for the first time investigated by the SEM technique. Fig. 7 shows the SEM/EDX results of the GO/cement paste. It can clearly be observed that there no GO exists in the highlighted part in Fig. 7a, and no carbon (C) elements were found based on the EDX results. The pores or voids are obvious in the highlighted part. However, C elements, resulting from the GO incorporation, were detected in the highlighted part in Fig. 7b, which shows a more densified matrix with less pores or voids compared with that in Fig. 7a. Therefore, GO is definitely capable of filling the pores or voids in the cement matrix and thus improves the mechanical behavior.

Fig. 6 (a) Compressive and (b) flexural strength of the specimens.

Table 2 Mechanical behavior of cement paste with different contents of CNTs and GO

Specimen	Compressive strength (MPa)	Flexural strength (MPa)	Young's modulus (GPa)
Cement paste	25.60	13.64	12.12
0.05 wt% CNTs/cement	27.24	15.06	12.63
0.05 wt% GO/cement	28.43	15.85	14.31
0.025 wt% GO/0.025 wt% CNTs/cement	31.01	16.93	15.42

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Fig. 7 SEM images and EDX results of GO/cement paste: (a) highlighted part is without GO; (b) highlighted part is with GO.

More importantly, the GO/CNTs/cement paste composite shows the highest compressive strength (31.01 MPa), flexural strength (16.93 MPa) and Young's modulus (15.42 GPa), which is improved by 21.13%, 24.21% and 27.23%, respectively, compared with cement paste, as shown in Fig. 6 and Table 2. The reinforcement by the GO/CNTs composite is much higher that by GO or CNTs individually. Therefore, the GO/CNTs composite plays a more important role in reinforcing the mechanical strength of cement paste, which is attributed to the better dispersion of CNTs in the GO solution, as discussed and shown in Fig. 3 and 5. Better dispersion of CNTs, in turn, contributes more to the mechanical enhancement of the cement paste.

To verify the improved dispersion of CNTs with the help of GO in the cement paste matrix, the microstructures of the CNTs/cement paste composite and the GO/CNTs/cement paste composite were compared and investigated, as shown in Fig. 8. Some agglomeration of the CNTs in the cement paste can be observed in Fig. 8a, otherwise the mechanical improvement should be higher than the present results. However, the agglomeration of CNTs significantly disappears in the GO/CNTs/cement paste composite, as shown in Fig. 8b. Most of the CNTs tend to be uniformly distributed in the pores or voids of the matrix, rather than intertwining with each other. Therefore, it is reasonable to deduce that the better dispersion of CNTs, resulting from the GO incorporation, is the basis for the stronger reinforcement of CNTs on the cement paste. More importantly, as observed in Fig. 8b and c, some GO sheets exist in the middle of the CNTs, which is like a bridge linking the dispersed CNTs together by chemical bonding. It is considered that the space interlocking of CNTs by GO incorporation also contributes to the mechanical improvement of cement paste.

Fig. 8 SEM images of (a) CNTs/cement paste composite and (b) GO/CNTs/cement paste composite with low magnification and (c) high magnification.

FTIR analysis was conducted to investigate the chemical interaction between CNTs and GO sheets, as shown in Fig. 9. The characteristic peaks of GO at 1723, 1621, 1403, 1222 and 1058 cm⁻¹ indicate carboxyl or carbonyl C=O stretching, H–O–H bending band of the absorbed H₂O molecules, carboxyl O–H stretching, phenolic C–OH stretching and alkoxy C–O stretching.¹⁹ In addition, the characteristic peaks of CNTs at 2361, 1716, 1565, and 1182 cm⁻¹ indicate the O–H stretch from strongly hydrogen-bonded –COOH, C=O (carboxylic acid moieties), carboxylate anion stretching and C–O stretching, which shows the CNTs are decorated with carboxyl groups. However, the higher intensity and large width of these bands have the stronger interactions in the GO/CNTs composite. Particularly, the higher absorption at 1720 cm⁻¹, corresponding to the stretching vibration of the C=O ester groups formed between the carboxylic acid groups of the CNTs and the alcohol groups of GO indicates that the CNTs have indeed been covalently attached to the GO.

Fig. 9 FTIR spectra of the GO, CNTs and the GO/CNTs composite.

A number of studies have demonstrated the significant reinforcement mechanism of the cement matrix by CNTs or GO. Li et al.⁷ reported that chemical reactions occurred between the carboxylic acid of CNTs and the calcium silicate hydrate (C–S–H) or Ca(OH)₂ of the cement matrix. The strong covalent force on the interface between the CNTs and matrix can improve the load-transfer efficiency from the cement matrix to the CNTs. Duan et al.⁸ pointed out that GO sheets containing carboxylic acid groups can also form strong interfacial adhesion between the GO and the cement matrix, which has a similar reinforcement mechanism to CNTs. As a result, due to the excellent mechanical behavior of the functionalized CNTs and GO with better interaction with the cement matrix, the mechanical properties of the cement composite reinforced by GO or CNTs are clearly improved. In this study, it is the first time that the space interlocking mechanism of the GO/CNTs/cement paste composite with enhanced mechanical properties has been proposed, as shown in Fig. 10. The significant mechanical improvement of the GO/CNTs/cement paste composite is mainly attributed to two effects. First, the better dispersion of CNTs ensures that more CNTs contribute to the mechanical enhancement of cement paste. Second, the two separate phases, (C–S–H) and Ca(OH)₂, in the cement hydration product are likely linked by the GO and CNTs together. GO sheets can not only interlock the cement matrix together, but also bridge the CNTs by chemical bonding. More load can thus be transferred and shared by the GO and CNTs simultaneously. Finally, the improved chemical bonding among the CNTs by GO incorporation can result in a space interlocking structure, the [CNTs–GO–CNTs] structure, which helps to improve the load-transfer efficiency from the cement matrix to the GO/CNTs composites. As a result, the mechanical properties of the GO/CNTs/cement paste composite are significantly improved.

Fig. 10 Mechanism of GO/CNTs/cement paste composite with enhanced mechanical properties.

4. Conclusions

This study presents the co-effects of GO/CNTs composites on the mechanical properties of the cement paste. The UV-vis spectroscopy and optical microscopy results show that the dispersion of CNTs in GO solution is much better than in an aqueous solution due to the higher electrostatic repulsion. In addition, the flexural and compressive strength of the cement paste are greatly increased by 21.13% and 24.21% with incorporation of 0.025 wt% CNTs/0.025 wt% GO composite, which is much higher than that reinforced by 0.05 wt% CNTs (6.40% and 10.14%) or 0.05 wt% GO (11.05% and 16.20%). More CNTs contribute to the mechanical enhancement of cement paste due to the better dispersion by GO incorporation, and the improved interaction among the CNTs by GO addition helps to transfer more load from cement matrix to the CNTs or GO, which results in the mechanical behavior of cement paste improving significantly. Finally, the space interlocking mechanism of the GO/CNTs/cement paste composite with enhanced mechanical property was proposed.

Acknowledgements

The authors would like to acknowledge the financial support from the Hong Kong Research Grant Council under the Grant 615810, the China Ministry of Science and Technology under the Grant 2015CB655100, Information Technology of Guangzhou under the Grant 2013J4500069 and the Natural Science Foundation of China under the Grant 51302104.

References

1. Z. Lu, J. Zhang, G. Sun, B. Xu, Z. Li and C. Gong, Effects of the form-stable expanded perlite/paraffin composite on cement manufactured by extrusion technique, Energy, 2015, 43–53.
2. M. S. Konsta-Gdoutos, Z. S. Metaxa and S. P. Shah, Highly dispersed carbon nanotube reinforced cement based materials, Cem. Concr. Res., 2010, 40, 1052–1059.
3. M. S. Konsta-Gdoutos, Z. S. Metaxa and S. P. Shah, Multi-scale mechanical and fracture characteristics and early-age strain capacity of high performance carbon nanotube/cement nanocomposites, Cem. Concr. Compos., 2010, 32, 110–115.
4. S. Parveen, S. Rana, R. Fangueiro and M. C. Paiva, Microstructure and mechanical properties of carbon nanotube reinforced cementitious composites developed using a novel dispersion technique, Cem. Concr. Res., 2015, 73, 215–227.
5. B. Zou, S. J. Chen, A. H. Korayem, F. Collins, C. Wang and W. H. Duan, Effect of ultrasonication energy on engineering properties of carbon nanotube reinforced cement pastes, Carbon, 2015, 85, 212–220.
6. A. Cwirzen, K. Habermehl-Cwirzen and V. Penttala, Surface decoration of carbon nanotubes and mechanical properties of cement/carbon nanotube composites, Adv. Cem. Res., 2008, 20, 65–73.
7. G. Y. Li, P. M. Wang and X. Zhao, Mechanical behavior and microstructure of cement composites incorporating surface-treated multi-walled carbon nanotubes, Carbon, 2005, 43, 1239–1245.
8. Z. Pan, L. He, L. Qiu, A. H. Korayem, G. Li, J. W. Zhu, F. Collins, D. Li, W. H. Duan and M. C. Wang, Mechanical properties and microstructure of a graphene oxide–cement composite, Cem. Concr. Compos., 2015, 58, 140–147.
9. M. Saafi, L. Tang, J. Fung, M. Rahman and J. Liggat, Enhanced properties of graphene/fly ash geopolymeric composite cement, Cem. Concr. Res., 2015, 67, 292–299.
10. J. Du and H. M. Cheng, The fabrication, properties, and uses of graphene/polymer composites, Macromol. Chem. Phys., 2012, 213, 1060–1077.
11. D. Li, M. B. Müller, S. Gilje, R. B. Kaner and G. G. Wallace, Processable aqueous dispersions of graphene nanosheets, Nat. Nanotechnol., 2008, 3, 101–105.
12. S. Musso, J.-M. Tulliani, G. Ferro and A. Tagliaferro, Influence of carbon nanotubes structure on the mechanical behavior of cement composites, Compos. Sci. Technol., 2009, 69, 1985–1990.
13. J. Yu, N. Grossiord, C. E. Koning and J. Loos, Controlling the dispersion of multi-wall carbon nanotubes in aqueous surfactant solution, Carbon, 2007, 45, 618–623.
14. L. Qiu, X. Yang, X. Gou, W. Yang, Z. F. Ma, G. G. Wallace and D. Li, Dispersing carbon nanotubes with graphene oxide in water and synergistic effects between graphene derivatives, Chem.–Eur. J., 2010, 16, 10653–10658.
15. X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang and F. Zhang, Deoxygenation of exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation, Adv. Mater., 2008, 20, 4490–4493.
16. S. Chuah, Z. Pan, J. G. Sanjayan, C. M. Wang and W. H. Duan, Nano reinforced cement and concrete composites and new perspective from graphene oxide, Construct. Build. Mater., 2014, 73, 113–124.
17. S. Lv, Y. Ma, C. Qiu, T. Sun, J. Liu and Q. Zhou, Effect of graphene oxide nanosheets of microstructure and mechanical properties of cement composites, Construct. Build. Mater., 2013, 49, 121–127.
18. E. Horszczaruk, E. Mijowska, R. J. Kalenczuk, M. Aleksandrzak and S. Mijowska, Nanocomposite of cement/graphene oxide – impact on hydration kinetics and Young's modulus, Construct. Build. Mater., 2015, 78, 234–242.
19. M. Cano, U. Khan, T. Sainsbury, A. O'Neill, Z. Wang, I. T. McGovern, W. K. Maser, A. M. Benito and J. N. Coleman, Improving the mechanical properties of graphene oxide based materials by covalent attachment of polymer chains, Carbon, 2013, 52, 363–371.

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