nanomaterials Article

Investigation of the Mechanical Properties and Microstructure of Graphene Nanoplatelet-Cement Composite Baomin Wang 1 , Ruishuang Jiang 1, * and Zhenlin Wu 2, * 1 2

*

Institute of Building Materials, School of Civil Engineering, Dalian University of Technology, Dalian 116024, China; [email protected] School of Physics and Opto-electronic Engineering, Dalian University of Technology, Dalian 116024, China Correspondence: [email protected] (R.J.); [email protected] (Z.W.); Tel.: +86-183-4087-9084 (R.J.); Fax: +86-411-8470-1400 (R.J.)

Academic Editor: Xiaoqiao He Received: 27 August 2016; Accepted: 28 October 2016; Published: 4 November 2016

Abstract: In this work, graphene nanoplatelets (GNPs) were dispersed uniformly in aqueous solution using methylcellulose (MC) as a dispersing agent via ultrasonic processing. Homogenous GNP suspensions were incorporated into the cement matrix to investigate the effect of GNPs on the mechanical behavior of cement paste. The optimum concentration ratio of GNPs to MC was confirmed as 1:7 by ultraviolet visible spectroscopy (UV-Vis), and the optical microscope and transmission electron microscopy (TEM) images displayed remarkable dispersing performance. The GNP–cement composite exhibited better mechanical properties with the help of surface-modified GNPs. The flexural strength of cement paste increased up to 15%–24% with 0.05 wt % GNPs (by weight of cement). Meanwhile, the compressive strength of the GNP–cement composite increased up to 3%–8%. The X-ray diffraction (XRD) and thermal analysis (TG/DTG) demonstrated that the GNPs could accelerate the degree of hydration and increase the amount of hydration products, especially at an early age. Meanwhile, the lower porosity and finer pore size distribution of GNP–cement composite were detected by mercury intrusion porosimetry (MIP). In addition, scanning electron microscope (SEM) analysis showed the introduction of GNPs could impede the development of cracks and preserve the completeness of the matrix through the plicate morphology and tortuous behavior of GNPs. Keywords: graphene nanoplatelets (GNPs); cement paste; mechanical properties; porosity; morphology

1. Introduction Ordinary Portland cement (OPC) is a crucial component of concrete, which is the most popular cementitious material for architectural structures. However, OPC paste is a typically brittle material due to low tensile strength, poor flexural strength and multiple initial cracks. Traditionally, various fibers or steel bars have been used to restrict the propagation of microcracks to improve the mechanical and electrical properties of plain cement materials [1–3]. For the last decade, nanomaterials including nanoparticles or nanofibres have been widely applied to cement-based materials as nanofillers because of advancements in nanotechnology [4,5]. These nanosized materials could control the formation and development of nanosized cracks. Moreover, many studies have been undertaken to create carbon nanomaterial-based cement composites, including carbon nanotubes (CNTs) and carbon nanofibers (CNFs) [6–9]. As an extensively attractive carbon nanomaterial, ideally, graphene is also able to remarkably reinforce cement-based materials due to its excellent mechanical properties. The average tensile

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strength and Young’s modulus of graphene are 125 GPa and 1.1 TPa [10,11]. In addition, the special surface area of graphene can theoretically reach up to 2630 m2 /g [12], which provides more potential sites for surface adsorption or other interactions between graphene and cement. In the last several years, graphene has been applied to polymers, ceramics or rubbers [13–15] as a reinforcing material. Meanwhile, the introduction of graphene into cementitious materials has attracted the comprehensive attention of many researchers and engineers. Gong et al. [16] found the incorporation of 0.03% graphene oxide (GO) increased the tensile and compressive strength of the cement paste by more than 40%, and the degree of hydration of GO–cement composites was promoted. Saafi et al. [17] observed that a 134% and 56% enhancement in flexural strength and flexural toughness of graphene–geopolymer composites were obtained for the addition of 0.35 wt % reduced graphene oxide (rGO). Other researchers have incorporated graphene or graphene oxide into cement-based composites as a filler material to enhance the flexural strength by 40%–60% and the compressive strength by more than 10% [18–20]. Graphene nanoplatelets (GNPs) consist of several graphene layers with a thickness in range of 3–100 nm [21]. Compared with single layer graphene, GNPs are not only a remarkable reinforcing material due to their morphological structure like monolayer graphene, but also low-cost, which further expands their application prospects. To date, GNPs have been extensively applied in polymeric or ceramic composites [22–25], whereas their use in cementitious materials has remained limited. Recently, Ranjbar et al. [26] reported that the compressive and flexural strength of a fly ash-based geopolymer were improved by 1.44 and 2.16 times with the help of GNPs. Peyvandi et al. [27] found that the reinforcement efficiency of different graphite nanomaterials in cementitious paste. The flexural strength of cement had various gains ranging from 27% to 73% with the addition of 0.13 wt % different GNP type and their oxide. Owing to the planer geometry and good chemical bonding with the matrix, GNPs have the ability to transfer the stress to the other positions and relieve the stress concentration in the matrix. Furthermore, GNPs may provide a larger thermal and electrical contact area due to its unique flat morphology, and thus Sedaghat et al. [28] introduced various quantities of graphene to measure the thermal diffusivity and electrical conductivity of graphene–cement composites. The electrical conductivity and thermal diffusivity of the composites were enhanced by the incorporation of graphene, and the content of needle or rod-shaped ettringite was reduced. Meanwhile, Du et al. [29] found that the GNPs significantly decreased the water penetration depth and aggressive ions ingress of cement mortar, because the layered structure of GNPs could increase tortuous paths. In a word, utilizing the outstanding properties of GNPs, the GNP-reinforced building materials can be endowed with high mechanical, electrical, thermal properties and excellent durability. However, it is a very formidable challenge to resolve the dispersion of GNPs in water and cement-based materials arising from their surface hydrophobicity and strong interlayer van der Waals forces [30]. As a traditional surfactant, methylcellulose (MC) is always applied for the dispersion of CNTs and CNFs because of its wettability, dispersibility and adhesion properties [31–33]. In this paper, methylcellulose (MC) was employed to dispersion GNPs in aqueous solution with the help of sonication. The ultraviolet visible spectroscopy (UV-Vis) was used to determine the optimum concentration ratio for dispersing. Moreover, the dispersing performance of GNP suspension was evaluated using optical microscope and transmission electron microscopy (TEM). Consequently, the homogenous GNP suspensions were incorporated into cement to investigate the mechanical properties of GNP–cement composites. In addition, the X-ray diffraction (XRD) and thermal analysis (TG/DTG) were used to explore the effect of GNPs on the degree of cement hydration. The pore size distribution and microstructure of GNP–cement composites were studied using mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM).

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2. Results 2.1. Dispersibility of GNPs in Aqueous Solution Nanomaterials Nanomaterials 2016, 2016, 6, 6, 200 200

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2.1.1. UV-Vis Absorbency Analysis of GNP Suspension

The The ability ability of MC to disperse GNPs in aqueous solution could be explored by UV-Vis absorbency The ability of of MC MC to todisperse disperseGNPs GNPsin inaqueous aqueoussolution solutioncould couldbe beexplored exploredby byUV-Vis UV-Visabsorbency absorbency spectra. According to the Lambert-Beer’s law, the higher absorbency implies that a better spectra. According to the Lambert-Beer’s law, the higher absorbency implies that a better dispersion spectra. According to the Lambert-Beer’s law, the higher absorbency implies that a better dispersion dispersion of GNPs in water. Figure 1 shows the absorption spectra of GNP suspensions at various of GNPs in water. Figure 1 shows the absorption spectra of GNP suspensions at various MC of GNPs in water. Figure 1 shows the absorption spectra of GNP suspensions at various MC MC concentrations. For all spectra, there exists an absorption peak of GNP suspension at the wavelength concentrations. For all spectra, there exists an absorption peak of GNP suspension at the wavelength concentrations. For all spectra, there exists an absorption peak of GNP suspension at the wavelength of of 260 260 nm nm (the (the red red line line in in Figure Figure 1), 1), which which is is specific specific absorption absorption peak peak of of individual individual GNPs. GNPs. When When the the of 260 nm (the red line in Figure 1), which is specific absorption peak of individual GNPs. When the MC concentration is 0.7 g/L, the absorption peak of GNP suspension reaches a maximum. At the MC concentration is 0.7 g/L, the absorption peak of GNP suspension reaches a maximum. At the MC concentration is 0.7 g/L, the absorption peak of GNP suspension reaches a maximum. At the lower or higher MC concentration, the of GNP order lower or orhigher higher MC concentration, the absorbency absorbency ofsuspension GNP suspension suspension is lower.toIn In order to to lower MC concentration, the absorbency of GNP is lower. is In lower. order understand understand the evolution of the absorbency of GNP suspension as a function of the MC concentration, understand the evolution of the absorbency of GNP suspension as a function of the MC concentration, the evolution of the absorbency of GNP suspension as a function of the MC concentration, the precise the of suspension was at maximum absorption the precise precise absorbency absorbency of GNP GNP was suspension wasatmeasured measured at 260 260 nm nm (the (the maximum absorption absorbency of GNP suspension measured 260 nm (the maximum absorption wavelength). wavelength). As in 2, in the of wavelength). As shown shown in Figure Figure 2, with with an an increasing in MC MC concentration, concentration, the absorbency absorbency of GNP GNP As shown in Figure 2, with an increasing inincreasing MC concentration, the absorbency of GNP suspension suspension increases gradually, and the value of maximum absorbency is achieved at 0.7 g/L MC, suspension increases gradually, and the value of maximum absorbency is achieved at 0.7 g/L MC, increases gradually, and the value of maximum absorbency is achieved at 0.7 g/L MC, which consists which consists with the result of Figure 1. The excessive MC has a negative effect on the dispersion which consists with the result of Figure 1. The excessive MC has a negative effect on the dispersion with the result of Figure 1. The excessive MC has a negative effect on the dispersion of GNPs. of The UV-Vis analysis indicates that optimum of to of GNPs. GNPs. The UV-Vis absorbency absorbency analysisthat indicates that the theconcentration optimum concentration concentration ratio of MC MC to The UV-Vis absorbency analysis indicates the optimum ratio of MC ratio to GNPs is 7:1. GNPs GNPs is is 7:1. 7:1. 0.60 0.60

0.2g/L 0.2g/L MC MC 0.3g/L 0.3g/L MC MC 0.4g/L 0.4g/L MC MC 0.5g/L 0.5g/L MC MC 0.6g/L 0.6g/L MC MC

Absorbency(a.u)

0.55 0.55 0.50 0.50

0.7g/L 0.7g/L MC MC 0.8g/L 0.8g/L MC MC 0.9g/L 0.9g/L MC MC 1.0g/L 1.0g/L MC MC

0.45 0.45 0.40 0.40 0.35 0.35 0.30 0.30

260 260 300 300

400 400

500 500

600 600

700 700

Wavelength/nm Wavelength/nm Figure nanoplatelet (GNP) 1. Ultraviolet Ultraviolet visible spectroscopy (UV-Vis) absorbance spectra spectraof of graphene graphene nanoplatelet nanoplatelet(GNP) (GNP) Figure 1. Ultravioletvisible visiblespectroscopy spectroscopy (UV-Vis) (UV-Vis) absorbance suspension with different different methylcellulose methylcellulose (MC) (MC) concentrations. concentrations. suspension with

Absorbency(a.u.)

0.6 0.6

=260 nm

0.5 0.5

0.4 0.4

0.3 0.3

0.2 0.2

0.4 0.4

0.6 0.6

0.8 0.8

1.0 1.0

CMC MC/(gL ) -1 -1

Figure 2. 2. The The absorbency absorbency of of GNP GNP suspension suspension as as aa function function of of MC concentration. Figure

2.1.2. 2.1.2. Optical Optical Microscope Microscope and and TEM TEM Analysis Analysis of of GNP GNP Suspension Suspension The The optimum optimum dispersing dispersing status status of of GNPs GNPs in in aqueous aqueous MC MC solution solution was was employed employed using using TEM. TEM. Figure Figure 3a 3a shows shows the the overlap overlap between between layer layer and and layer layer of of GNPs GNPs before before the the addition addition of of the the MC. MC. After After dispersed dispersed using using MC MC in in combination combination with with sonication, sonication, GNPs GNPs are are highly highly transparent transparent and and have have few few wrinkles wrinkles on on the the surface surface (Figure (Figure 3b). 3b). As As may may be be noted noted in in Figure Figure 3c, 3c, the the edges edges and and surface surface of of GNPs GNPs were were smooth smooth and and without without lager lager defects, defects, which which indicates indicates that that the the surface surface treatment treatment of of GNPs GNPs using using

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2.1.2. Optical Microscope and TEM Analysis of GNP Suspension The optimum dispersing status of GNPs in aqueous MC solution was employed using TEM. Figure 3a shows the overlap between layer and layer of GNPs before the addition of the MC. After dispersed using MC in combination with sonication, GNPs are highly transparent and have few wrinkles on the surface (Figure 3b). As may be noted in Figure 3c, the edges and surface of GNPs were smooth and without lager defects, which indicates that the surface treatment of GNPs using surfactant Nanomaterials 2016, 6, 200 4 of 15 can preserve the original structure of GNPs and does not introduce other defects.

Figure Transmission electron suspension: (a) (a) before before dispersion; dispersion; Figure 3. 3. Transmission electron microscopy microscopy (TEM) (TEM) images images of of GNP GNP suspension: (b,c) after (b) and (c)dispersion. after dispersion.

2.2. Workability By conducting slump flow test, the workability is evaluated and compared between the plain cement and and the theGNP–cement GNP–cementcomposite, composite,asas shown in Table 1. The mean spread diameter the shown in Table 1. The mean spread diameter of theofplain plain cement was probably 89 while mm, while the diameter of GNP–cement composite is merely 68 mm. cement was probably 89 mm, the diameter of GNP–cement composite is merely 68 mm. The The mean spread diameter of the cement paste is reduced by 30.9% with the addition of 0.05 wt % mean spread diameter of the cement paste is reduced by 30.9% with the addition of 0.05 wt % GNPs. GNPs. In previous literature, a distinct reduction workabilitywhen whenthe the nano nano additives In previous literature, there there was was a distinct reduction of of workability including GO or GNTs GNTs were were incorporated incorporated into into cement cement [16,19,34]. [16,19,34]. The result could be attributed to the large specific surface area area of of GNPs GNPs that that consumes consumes more more available available water water to towet wettheir theirsurfaces. surfaces. Table 1. Table 1. Workability Workability of of the the mixes. mixes.

Sample

Sample

Plain Cement

GNP/Cement

Mean Spread Diameter (mm)

89 ± 3

68 ± 2

Mean Spread Diameter (mm)

Plain cement 89 ± 3

GNP/cement 68 ± 2

2.3. Mechanical Properties of GNP–Cement Composite 2.3. Mechanical Properties of GNP–Cement Composite The results of flexural strength and compressive strength of plain cement and GNP–cement The results flexural compressive cement GNP–cement composite at 3, 7,of14 and 28strength days areand shown in Figure strength 4. From of theplain Figure 4, theand flexural strength composite at 3, 7, 14 and 28 days are shown in Figure 4. From the Figure 4, the flexural strength and compressive strength of both samples increase with reference to the ages, which is due toand the compressive strength both samples increase with reference the ages, which is dueof to the the samples ongoing ongoing hydration ofofcement. The results indicate that the to mechanical properties hydration of cement. results than indicate that the mechanical properties the samples reinforced reinforced with GNPsThe are higher the plain cement samples at variousofages. It is noted that the with GNPs are higher than the plain cement samples at various ages. It is noted that the flexural flexural strength of GNP–cement composite shows a more remarkable increment with respect to the strength of GNP–cement composite shows a more remarkable increment with respect to the compressive strength. The incorporation of GNPs enhanced the flexural strength by 15%–24% and compressive strength. The3%–8%, incorporation of GNPs flexural 15%–24% and compressive strength by respectively. At enhanced the age ofthe 7 days, the strength flexural by strength of plain compressive strength by 3%–8%, respectively. At the age of 7 days, the flexural strength of plain cement sample is increased by 23.5% to 10 MPa by the addition of 0.05 wt % GNPs by weight of cement sample is increased by 23.5% to 10 MPa by cement the addition ofis0.05 wt % GNPs of cement. Meanwhile, the compressive strength of plain sample increased by 7.5%bytoweight 63.3 MPa. cement. Meanwhile, the compressive strength of plain cement sample is increased by 7.5% to 63.3 However, when the age of the samples reaches 28 days, the growth rates of flexural strength and MPa. However, whenare the16.8% age of the samples reaches The 28 days, the growth ratesmay of flexural compressive strength and 1.3%, respectively. introduction of GNPs greatly strength enhance and compressive strength are 16.8% and 1.3%, respectively. The introduction of GNPs may greatly the strength of cement-based materials at early ages. enhance the strength of cement-based materials at early ages.

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Flexural strength(MPa)

14 Plain cement GNP/cement

12 10 8 6 4 2 0

3

7

14

28

Age(days) (a)

Compressive strength(MPa)

80 Plain cement GNP/cement

70 60 50 40 30 20 10 0 3

7

14

28

Age(days) (b) Figure 4. Mechanical properties of plain cement and GNP-cement composite at various ages. (a)

Figure 4. Mechanical properties of plain cement and GNP-cement composite at various ages. Flexural strength of plain cement and GNP-cement composite at various ages. (b) Compressive (a) Flexural strength of plain cement and GNP-cement composite at various ages. (b) Compressive strength of plain cement and GNP-cement composite at various ages. strength of plain cement and GNP-cement composite at various ages.

2.4. XRD Analysis

2.4. XRD Analysis

The effect of GNPs on the products of cement hydration can be estimated using X-ray diffraction

patterns. XRD patterns of plain cement and GNP–cement composite are shown Figures 5,6. The effectThe of GNPs on the products of cement hydration can be estimated usinginX-ray diffraction After the curing period of 7 days and 28 days, several representative hydration phases have been patterns. The XRD patterns of plain cement and GNP–cement composite are shown in Figures 5 and 6. in two samples, hydroxide 2), ettringite (AFt). Compared with Afterdepicted the curing period of 7 including days andcalcium 28 days, several (Ca(OH) representative hydration phases have been plain cement, there are no new phases of the GNP–cement composite, suggesting that the addition depicted in two samples, including calcium hydroxide (Ca(OH)2 ), ettringite (AFt). Compared with of GNPs cannot change the type and structure of final hydration products. plain cement, there are no new phases of the GNP–cement composite, suggesting that the addition of Furthermore, XRD analysis can also characterize the degree of cement hydration by monitoring GNPsthe cannot change the type and structure of consumption final hydration products. formation of hydration products and the of cement raw constituent qualitatively. Furthermore, analysis also characterize the degree of cement hydration by monitoring As can be seen XRD in Figure 5, the can intensities of peaks with respect to Ca(OH) 2 and AFt in GNP–cement the formation and the consumption cementthe raw constituent qualitatively. composite of arehydration both higherproducts than those in plain cement, whichof indicates higher crystalline degree of be Ca(OH) and AFt. Meanwhile, the intensity of unhydrated tricalcium silicate (C 3 S) in GNP–cement As can seen2 in Figure 5, the intensities of peaks with respect to Ca(OH) and AFt in GNP–cement 2 composite is much lower than that in in plain plain cement, cement, suggesting the higher hydration degree ofdegree the of composite are both higher than those which indicates the higher crystalline GNP–cement composite. The similar tendency also is observed at the age of 28 days, though the Ca(OH)2 and AFt. Meanwhile, the intensity of unhydrated tricalcium silicate (C3 S) in GNP–cement changeisismuch not very obvious Figure 6. Thecement, XRD analysis resultsthe demonstrate that the GNPs canof the composite lower thanin that in plain suggesting higher hydration degree accelerate the hydration of cement paste, especially at the early age. Additionally, the characteristic GNP–cement composite. The similar tendency also is observed at the age of 28 days, though the peaks of GNPs were not detected by XRD due to the low content of GNPs. change is not very obvious in Figure 6. The XRD analysis results demonstrate that the GNPs can accelerate the hydration of cement paste, especially at the early age. Additionally, the characteristic peaks of GNPs were not detected by XRD due to the low content of GNPs.

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a-Plain cement cement a-Plain b-GNP/cement composite composite b-GNP/cement



-Ca(OH) -Ca(OH)22 -AFt  -AFt

-CaCO -CaCO3  3

-C S -C33S



b-7 days days b-7

    

 

 

 



 

a-7 days days a-7

10 10

20 20

30 30

40 40

50 50

60 60

70 70

2theata(degree) 2theata(degree) Figure 5. 5. X-ray X-ray diffraction diffraction(XRD) (XRD)patterns patternsofof of(a) (a)plain plaincement cement paste and (b) GNP-cement composite Figure X-ray paste and (b)(b) GNP-cement composite at Figure 5. diffraction (XRD) patterns (a) plain cement paste and GNP-cement composite at the age of 7 days. the age of 7 days. at the age of 7 days.

a-Plain cement cement a-Plain b-GNP/cement composite b-GNP/cement composite

      

b-28 days days b-28 

-Ca(OH)2 -Ca(OH) 2 -AFt -AFt





-CaCO -CaCO3  3

-C S -C33S

 

 

a-28 days days a-28

10 10

20 20

30 30

40 40

50 50

60 60

70 70

2theta(degree) 2theta(degree) Figure 6. GNP-cement composite composite at at the the age age of of 28 28 days. days. Figure 6. XRD XRD patterns patterns of of (a) (a) plain plain cement cement paste paste and and (b) (b) GNP-cement GNP-cement composite at the age of 28 days. Figure

2.5. Thermal Thermal (TG/DTG) (TG/DTG) Analysis Analysis 2.5. Another common commonmethod method to investigate investigate the degree degree of hydration hydration is thermal thermal analysis. analysis. The Another to investigate the degree of hydration is thermal analysis. The TG/DTG common method to the of is The TG/DTG curves of plain cement and GNP–cement composite at the age of 7 days and 28 days have curves ofcurves plain cement GNP–cement composite at the age of age 7 days days been TG/DTG of plain and cement and GNP–cement composite at the of 7and days28and 28 have days have been presented presented in Figures Figures 7,8. The curves curves of of two two samples samples have have aa a similar similar trend trend at at various various ages, ages, presented in Figures 7 and7,8. 8. The The curves of two samples have similar trend at various ages, been in demonstratingthat thatthe theGNPs GNPs do not induce the formation of other any other other phase in accord accord with the demonstrating dodo notnot induce thethe formation of any phasephase in accord with the results demonstrating that the GNPs induce formation of any in with the results of XRD analysis. From two figures, there are four main mass losses in the DTG curve, of XRD of analysis. From twoFrom figures, are four main the DTG corresponding results XRD analysis. twothere figures, there aremass four losses main in mass lossescurve, in the DTG curve, ◦ C), ◦ C), corresponding to evaporable water (50–100 °C), portion nonevaporable water (105–200 °C), the the to evaporable water (50–100 portion nonevaporable water (105–200 the decomposition of corresponding to evaporable water (50–100 °C), portion nonevaporable water (105–200 °C), ◦ ◦ decomposition of calcium hydroxide (400–500 °C) and calcium carbonate (650–750 °C) respectively. calcium hydroxide (400–500hydroxide C) and calcium carbonate C) respectively. At the of 7 days, decomposition of calcium (400–500 °C) and(650–750 calcium carbonate (650–750 °C) age respectively. At the the age of ofof days, the contents contents ofand amorphous phases and Ca(OH) 2 in GNP–cement composite are the contents amorphous phases Ca(OH)phases in GNP–cement composite are higher than those At age 77 days, the of amorphous and Ca(OH) 2 in GNP–cement composite are 2 higher than those in plain cement in Figure 7. However, after curing 28 days, the contents of these in plain cement in Figure 7. However, after curing 28 days, the contents of these products is nearly higher than those in plain cement in Figure 7. However, after curing 28 days, the contents of these products is nearly nearly equal betweencomposite the GNP–cement GNP–cement composite andin the plain8. cement in Figure Figure 8. The The equal between the equal GNP–cement and thecomposite plain cement Figure The different trend at products is between the and the plain cement in 8. different trend at 7 days and 28 days matches the growth rates of mechanical properties of two 7 days and 28 days rates of the mechanical properties of two samples in Figure 4. different trend at 7 matches days andthe 28growth days matches growth rates of mechanical properties of two samples in Figure 4. The thermal analysis demonstrates further that the introduction of GNPs can The thermal analysis further that the introduction GNPs accelerate the hydration samples in Figure 4. demonstrates The thermal analysis demonstrates furtherofthat thecan introduction of GNPs can accelerate the hydration hydration process process of of cement. cement. process of the cement. accelerate

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100

0.0010 Plain cement-7days GNP/cement-7days

0.0005

80

0.0000

70

DTG DTG

TG(%) TG(%)

90

-0.0005

60 -0.0010 50

200

400

600

800

1000

Temperature(℃ ) Figure 7. Thermal analysis (TG/DTG)curves curves of of plain GNP-cement composite at theatage Figure 7. Thermal analysis (TG/DTG) plaincement cementand and GNP-cement composite the age of 7 days. of 7 days.

0.0010 Plain cement-28days GNP/cement-28days

90

TG(%) TG(%)

80

0.0005 0.0000

70

DTG DTG

100

-0.0005

60 -0.0010 50

200

400

600

800

1000

Temperature(℃ ) Figure 8. TG/DTG curves and GNP-cement GNP-cement composite at the 28 days. Figure 8. TG/DTG curvesofofplain plaincement cement and composite at the age age of 28ofdays.

2.6. Porosity Distribution 2.6. Porosity and and PorePore SizeSize Distribution The results of porosity test for cement paste with and without GNPs at 28 days are presented in

The results of porosity test for cement paste with and without GNPs at 28 days are presented in The results of porosity test for cement paste with and without GNPs at 28 days are presented Figures 9,10 and the total pore information is summarized in Table 2. Figure 9 shows the variation of in Figures 9 and 10 and the total pore information is summarized in Table 2. Figure 9 shows the cumulative pore area (m22/g) with pore size diameter (nm). The cumulative pore area of GNP–cement variation of cumulative pore area (m2 /g) with pore size diameter (nm). The cumulative pore area of composite displays a decrease at various pore sizes compared with plain cement paste. The trend is GNP–cement displays a decrease at various pore in sizes compared with plainof cement similar to composite the change of total intrusion volume and porosity Table 2. The total porosity cementpaste. The trend similar to the18.35% changetoof17.01% total intrusion volume 0.05 and wt porosity in Table 2. Theindicate total porosity of paste is decreases from by incorporating % GNPs. The results that cement decreases 18.35% the to 17.01% by incorporating wt % GNPs. The results indicate thepaste addition of GNPsfrom can increase compactness and decrease 0.05 the porosity of the matrix. As shown Figurecan 10, increase the relationship between theand Logdecrease differential pore that the addition ofinGNPs the compactness theintrusion porosity(mL/g) of theand matrix. size diameter (nm) is observed, representing the pore size distribution of the matrix. The Log and As shown in Figure 10, the relationship between the Log differential intrusion (mL/g) differential intrusion of plain cement paste reaches the maximum value of about 0.2088 mL/g at size pore size diameter (nm) is observed, representing the pore size distribution of the matrix. The Log diameter of 21 nm. Nevertheless, the Log differential intrusion of GNP–cement composite has a lower differential intrusion of plain cement paste reaches the maximum value of about 0.2088 mL/g at size value of 0.1862 mL/g at size diameter of 17.1 nm. Likewise, the average pore diameter of GNP–cement diameter of 21 nm. Nevertheless, the Log intrusion GNP–cement has ainlower composite is about 16 nm, whereas thedifferential value of plain cement of paste is about 18 composite nm. The GNPs valuecement of 0.1862 at decrease size diameter of porosity 17.1 nm.ofLikewise, pore of GNP–cement canmL/g not only the total the matrix,the butaverage also refine thediameter pore size distribution composite is about 16 nm, whereas the value of plain cement paste is about 18 nm. The GNPs in cement can not only decrease the total porosity of the matrix, but also refine the pore size distribution of cement based material. Consequently, the mechanical properties of cement are enhanced by GNPs due to the improved pore characterization.

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Nanomaterials 2016, 6, 200material. Consequently, the mechanical properties of cement are enhanced by GNPs8 of 15 of cement based

of cement based material. Consequently, the mechanical properties of cement are enhanced by GNPs due to the improved pore characterization.

Cumulative Pore Pore Area Area Cumulative

15 W/C=0.35 Plain cement GNP/cement

10

5

0

100000

10000

1000

100

10

Pore size Diameter(nm)

Log Differential Differential Intrusion(mL/g) Intrusion(mL/g) Log

Figure 9. Cumulative porearea areavs. vs. pore pore size plain cement and and GNP–cement composite at Figure 9. Cumulative pore sizediameter: diameter: plain cement GNP–cement composite 28 days. at 28 days.

0.25 W/C=0.35 Plain cement GNP/cement

0.20 0.15 0.10 0.05 0.00

100000

10000

1000

100

10

Pore size Diameter(nm) Figure 10. 10. Pore size distribution andGNP–cement GNP–cement composite 28 days. Figure Pore size distributionofofplain plain cement cement and composite at 28atdays. 2. Mercury intrusion porosimetry(MIP) (MIP) analysis analysis of and GNP–cement composite TableTable 2. Mercury intrusion porosimetry ofplain plaincement cement and GNP–cement composite at 28 days. at 28 days.

Total

Sample

Sample

Plain cement GNP/cement

Pore Median Median Pore Diameter Pore (Volume)/nm Diameter Volume/(mL/ Area/(m22/g) 0.0981 21.26 22 (Volume)/nm g)

Total Intrusion Pore Intrusion TotalTotal Volume/(mL/g) Area/(m2 /g)

0.0887

21

19

Median

Median Average Pore PorePore Average Pore Diameter (Area)/nm Diameter/nm

Diameter 13 (Area)/nm 12

Diameter/nm 18

16

Porosity Porosity/% /% 18.35

17.01

Plain 0.0981 21.26 22 13 18 18.35 cement 2.7. Microstructure GNP/ 0.0887 21 19 12 16 17.01 cement The comparison of microstructures between the plain cement paste and GNP–cement composite

is shown in Figure 11. The plain cement paste showed a distinct crack passing straight through the 2.7. Microstructure whole hydration product (Figure 11a). In contrast, no obvious cracks were found and the matrix Thedenser comparison of microstructures between the plain cement paste(Figure and GNP–cement composite showed the hydration products in the GNP–cement composite 11b). Some other studies is shown in Figure 11. The plain cement paste showed a distinct crack passing straight through the have proved that graphene can deflect or force the cracks due to its tortuous behavior [19,35]. Thus, whole hydration product (Figure 11a). In contrast, no obvious cracks were found and the matrix graphene can refine the large cracks and impede the development of cracks in cement based materials.

materials. In addition, GNPs performance in cement paste is found in Figure 11c,d. The GNPs can insert the hydration products of cement, combining well with the cement matrix (Figure 11c). As shown in Figure 11d, the GNPs have a plicate morphology and are wrapped by hydration products. This performance can prevent the crack propagation under the external load. The good interfacial Nanomaterials 2016, 6, 200 9 of 15 interaction between GNPs and cement matrix can transfer the stress effectively, resulting in enhanced flexural strength and compressive strength of the GNP–cement composite.

Figure 11. microscope (SEM) images of (a) cement paste paste and (b) the(b) GNP– Figure 11. Scanning Scanningelectron electron microscope (SEM) images of the (a) plain the plain cement and the cement composite at the age of 28 days. (c) The GNPs inserted into the hydration productions GNP–cement composite at the age of 28 days. (c) The GNPs inserted into the hydration productions of of cement. (d) in cement. cement. cement. (d) The The plicate plicate morphology morphology of of GNPs GNPs in

3. Discussion In addition, GNPs performance in cement paste is found in Figure 11c,d. The GNPs can insert the In recent years,ofsome studies have demonstrated thematrix introduction graphene can hydration products cement, combining well with the that cement (Figure of 11c). As shown remarkably enhance the mechanical properties of ceramicand andare cement composites [36–38]. It is a key in Figure 11d, the GNPs have a plicate morphology wrapped by hydration products. precondition for higher reinforcement GNP–cement composite to disperse GNPs into cement This performance can prevent the crackof propagation under the external load. The good interfacial matrix uniformly. InGNPs this work, the GNPs werecan dispersed aqueous solution using MC asindispersant interaction between and cement matrix transferinthe stress effectively, resulting enhanced with thestrength help of ultrasonic, and Figure 1 indicated that the optimum weight ratio of MC to GNPs was flexural and compressive strength of the GNP–cement composite. 7:1. Meanwhile, TEM test suggested a fine dispersibility of GNPs in cement matrix using the 3. Discussion dispersing method. The workability of the cement paste presented an apparent reduction with the addition of A lower flowability and weaker viscosity of fresh paste could affect the In recentGNPs. years, some studies have demonstrated that the introduction of graphene can remarkably compactibility and the mechanical GNP–cement composite. In previous reports, it was enhance the mechanical properties ofstrength ceramic of and cement composites [36–38]. It is a key precondition reasonable to incorporate superplasticizer to improve the workability of GNP–cement paste when a for higher reinforcement of GNP–cement composite to disperse GNPs into cement matrix uniformly. high content of GNPs is added [34]. However, the further study is needed to improve the workability In this work, the GNPs were dispersed in aqueous solution using MC as dispersant with the help of composite. of GNP–cement ultrasonic, and Figure 1 indicated that the optimum weight ratio of MC to GNPs was 7:1. Meanwhile, TEM test suggested a fine dispersibility of GNPs in cement matrix using the dispersing method. The workability of the cement paste presented an apparent reduction with the addition of GNPs. A lower flowability and weaker viscosity of fresh paste could affect the compactibility and the mechanical strength of GNP–cement composite. In previous reports, it was reasonable to incorporate superplasticizer to improve the workability of GNP–cement paste when a high content of GNPs is added [34]. However, the further study is needed to improve the workability of GNP–cement composite.

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In addition, the hydration process of cement paste can impact flexural and compressive strength In addition, the hydration process the of cement paste can nanomaterials impact flexuralon and compressive strength significantly. Previous research showed effect of carbon the hydration degree significantly. Previous research showed the effect of carbon nanomaterials on the hydration and products of cement paste [39–41]. In our work, the XRD and TG/DTG results indicatedegree that and products of paste [39–41].effect In our the XRD process and TG/DTG results that the the corporation of cement GNPs has a positive onwork, the hydration and does notindicate transform the corporation of GNPs has a positive effect on the porosity hydration process and size doesdistribution not transform the hydration products drastically. Furthermore, lower and finer pore were hydration products drastically. Furthermore, porosity and finer pore size distribution were observed in GNP–cement composite, enhancinglower the mechanical properties of the composite further. GNP–cement composite, enhancing mechanical properties composite further. Asobserved a porousinmaterial, the high porosity indicatesthe lower strength. Due to of thethe addition of GNPs, Ascement a porous material, the compacted. high porosity indicates lower strength. Due to the addition of GNPs, the the matrix becomes cement matrix becomes compacted. The interaction between the additives and the cement matrix also has a vital impact on the properties of cement composites. In the work, there is more obvious enhancement in the flexural strength than that of the compressive strength. Some studies found that the high improvement in the flexural strength of GNP composites was due to various toughening mechanisms, including stress dispersing, crack deflection, crack bridging, and cracking branching [26,42]. As shown in SEM images, the GNPs are wrapped by hydration products, and the large aspect ratio is propitious to enhance the interfacial bond strength between GNPs and cement matrix. The strong bondability can increase the load-transfer efficiency and eliminate the destruction of stress concentration. In addition, the GNP particles provide a higher resistance to crack propagation. For the three-point bending test, the first crack form at the middle of sample. When the crack reaches the surface of GNPs, the crack develops along the interface between the GNP and cement matrix. Thus, the presence of GNPs causes crack branching or deflection, as shown in Figure 11c,d. The crack branching or deflection mechanism could increase the path of crack development, which improves the mechanical strength of cement matrix. Moreover, the crack bridging mechanism of GNPs could absorb the more energy effectively, when the composite is under external load. The whole mechanical map is presented in Figure 12. In a word, due to the high tensile strength, high Young’s modulus, and unique two-dimension morphology, GNPs could reduce the stress concentration and prevent the development of the cracks, thus enhancing the mechanical strength of the cement matrix. These results are helpful in understanding the reinforcing effect of nanofillers used for cement-based materials.

Stress Crack deflection

Crack branching

Stress dispersing

Stress

Crack bridging

Figure 12. Scheme of GNP–cement composite under bending load. Figure 12. Scheme of GNP–cement composite under bending load.

4. Materials and Methods 4. Materials and Methods 4.1. Materials 4.1. Materials The cement was provided Dalian Onoda Cement Co., Ltd. (Dalian, China), which belongs The cement was provided byby Dalian Onoda Cement Co., Ltd. (Dalian, China), which belongs toto P.O 42.5R cement (Ordinary Portland Chinese Its Standards. Its chemical P.O 42.5R cement (Ordinary Portland Cement)Cement) accordingaccording to Chineseto Standards. chemical composition composition and physical parameters are shown in Tables 3,4. The GNPs (x-GnP-M25) were obtained and physical parameters are shown in Tables 3 and 4. The GNPs (x-GnP-M25) were obtained from from XG science, Inc. (Lansing, MI, USA). The physical properties of GNPs are presented in XG science, Inc. (Lansing, MI, USA). The physical properties of GNPs are presented in Table 5. Table 5. Methylcellulose (MC) as dispersant was purchased from SINOPHARM Chemical Reagent Co., Ltd. (Shenyang, China). The superplasticizer (SP) was provided by Dalian Mingyuanquan Group Co., Ltd. (Dalian, China). The tributyl phosphate (TBP) as defoamer was supplied by Tianjin Chemical Reagent Plant (Tianjin, China) in order to eliminate redundant bubbles introduced by dispersant.

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Table 3. Chemical composition of the cement. CaO

SiO2

Al2 O3

Fe2 O3

SO3

MgO

Na2 O

61.13

21.45

5.24

2.89

2.50

2.08

0.77

Table 4. Physical parameters of the cement. Loss on Ignition/%

Special Surface Area/m2 ·kg−1

Setting Time/min Initial Setting

Final Setting

175

235

3.50

Compressive Strength/MPa

346

Flexural Strength/MPa

3 days

28 days

3 days

28 days

6.0

8.5

30.0

53.5

Table 5. Physical properties of GNPs. Products

Particle Diameters/µm

Thickness/nm

Purity/%

x-GnP-M25

25

6–8

>99.5

Electrical Conductivity/(S/m) 107

Special Surface Area/m2 ·g−1 120–150

Methylcellulose (MC) as dispersant was purchased from SINOPHARM Chemical Reagent Co., Ltd. (Shenyang, China). The superplasticizer (SP) was provided by Dalian Mingyuanquan Group Co., Ltd. (Dalian, China). The tributyl phosphate (TBP) as defoamer was supplied by Tianjin Chemical Reagent Plant (Tianjin, China) in order to eliminate redundant bubbles introduced by dispersant. 4.2. Preparation of GNP Suspensions The 5 mg GNPs were added into 50 mL aqueous solution with different MC concentration (ranging from 0.2 g/L to 1.0 g/L), and the suspensions were mechanically stirred for 10 min and treated for 20 min using a sonicator (operating frequency 40 KHz, power 180 W, bottom area 450 cm2 ). The treated GNP suspensions were tested to determine the best dispersion condition with regards to MC concentration. 4.3. GNP–Cement Composite Processing The water to cement ratio of all samples was kept at 0.35. For GNP–cement composites, the amount of GNPs was fixed at 0.05 wt % by weight of cement. The uniform GNP suspension with weighted defoamer and dry cement were placed in an agitator kettle and the composites were mixed using a multispeed planetary mixer at low speed for 2 min and at high speed for 4 min. The mixture was then cast into 40 mm × 40 mm × 160 mm size steel molds and was vibrated for 1 min on an electric vibrator. After 24 h, all samples were demoulded and cured in 20 ◦ C water. These samples were tested for flexural strength and compressive strength at 3 days, 7 days, 14 days and 28 days. The mix proportion of all samples is shown in Table 6. Table 6. Proportion of the mixes. Sample Plain cement GNP/cement

Mix Proportion (wt %)

Water-Cement Ratio 0.35 0.35

GNPs

MC

TBP

SP

0 0.05

0 0.35

0.15 0.15

0.1 0.1

4.4. Test Methods 4.4.1. Characterizing the Dispersibility of GNP Suspensions UV-Vis absorption spectra of all suspensions were measured from 200 nm to 700 nm to understand the effect of MC concentration on dispersion of GNPs, using a T6-xinshiji spectrophotometer

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(PERSEE Co., Beijing, China). The blank was the aqueous MC solution under the same dispersing conditions as reference solution. TEM images of GNP suspensions were obtained by a Tecnai G2 Spirit (FEI Co., Hillsboro, OR, Nanomaterials 2016, 6, 200 12 of 15 USA) to represent the dispersing performance of GNP suspension. The GNP suspensions were pipetted onto a copper grid of carbon dried inwere air, obtained and then inSpirit TEM.(FEI Co., Hillsboro, OR, TEM images of GNPfilm, suspensions byobserved a Tecnai G2 USA) to represent the dispersing performance of GNP suspension. The GNP suspensions were pipetted ontoTest a copper grid of carbon film, dried in air, and then observed in TEM. 4.4.2. Slump Flow

According to Flow the Chinese standard “Methods for testing uniformity of concrete admixture”, 4.4.2. Slump Test the two composites were poured into a cone mould to perform the slump flow test. The dimensions According to the Chinese standard “Methods for testing uniformity of concrete admixture”, the of the cone arewere bottom diameter 60 mm, top mm, flow and test. height mm, as shown in two mould composites poured into a cone mould to diameter perform the36slump The 60 dimensions of Figure 13. The cone mould was placed on a glass pane and filled with cement paste. Then, the mould the cone mould are bottom diameter 60 mm, top diameter 36 mm, and height 60 mm, as shown in Figure 13. The cone was placedspread on a glass panepaste and filled cementThe paste. Then,spread the mould was raised vertically, andmould the horizontal of the waswith recorded. mean diameters was raised vertically, and the horizontal spread of the paste was recorded. The mean of two composites were presented in Table 1. The purpose of conducting the test is tospread estimate the diameters of two composites were presented in Table 1. The purpose of conducting the test is to effect of GNPs on the workability of the cement. estimate the effect of GNPs on the workability of the cement.

Figure 13. Geometry ofofcone usedforfor slump Figure 13. Geometry cone mould mould used slump flowflow test. test. 4.4.3. Mechanical Property Tests

4.4.3. Mechanical Property Tests

The flexural strength was carried out by three point bending fixture on a computer controlled

Theelectrohydraulic flexural strength carried out by threeatpoint fixtureAfter on athe computer controlled servowas universal tester (WDW-50) a speedbending of 0.5 mm/min. bending tests, electrohydraulic servostrength universal tester (WDW-50) at a was speed of 0.5 mm/min. After the bending the compressive of each fractured fragments conducted on a computer controlled servo universal (WHY-300) at a rate 0.01 mm/min. mechanical tests, theelectro-hydraulic compressive strength of eachtester fractured fragments wasofconducted on After a computer controlled property tests, theseuniversal fragments, including plain cementat paste and GNP–cement composite, were mechanical soak electro-hydraulic servo tester (WHY-300) a rate of 0.01 mm/min. After anhydrous ethanol to terminate hydration reaction. propertyintests, these fragments, including plain cement paste and GNP–cement composite, were soak The X-ray diffraction patterns of cement samples were recorded using an automatic in anhydrous ethanol to terminate hydration reaction. diffractometer (XRD, D8 ADVANCE, Bruker AXS Co., Karlsruhe, Germany) with Cu radiation. The Thescan X-ray diffraction patterns ofangles cement werea recorded automatic range was set for two-theta of samples 5° to 70° with rate of 0.5 using θ°/min.an The hardened diffractometer cement (XRD, D8 ADVANCE, AXSout Co., with Cu(TG/DSC, radiation. TheToledo scan range powder samples Bruker were carried onKarlsruhe, the thermal Germany) analysis instrument Mettler Stare, was set ◦ /min. The Mettlerangles ToledoofCo., Zurich, Switzerland). range was from 50 °Cpowder to 1000 °C at a were for two-theta 5◦ to 70◦ with a rate of The 0.5 θtemperature hardened cement samples heating rate of 10 °C/min. carried out on the thermal analysis instrument (TG/DSC, Mettler Toledo Stare, Mettler Toledo Co., The porosity and pore size distribution of composites were detected using mercury intrusion Zurich, Switzerland). The temperature range was from 50 ◦ C to 1000 ◦ C at a heating rate of 10 ◦ C/min. porosimeter (MIP, AUTOPORE IV 9500, Micromertics Instrument Corp., Norcross, GA, USA), with Theone porosity and pore sizetwo distribution composites were detected using mercury intrusion high-pressure station, low-pressureofstations and a maximum pressure of 33,000 psia for porosimeter (MIP, AUTOPORE 9500, Micromertics Instrument Corp., Norcross, measurements. In addition, anIV environmental scanning electron microscope (SEM, QUANTA GA, 450, USA), Co., Hillsboro,station, OR, USA) waslow-pressure performed to observe microstructure of pressure composites. SEMpsia for with oneFEI high-pressure two stationstheand a maximum ofThe 33,000 was used in energy dispersivescanning spectroscopy (EDS). microscope (SEM, QUANTA 450, measurements. Inconjunction addition, with an environmental electron FEI Co., Hillsboro, OR, USA) was performed to observe the microstructure of composites. The SEM 5. Conclusions was used in conjunction with energy dispersive spectroscopy (EDS). In this paper, GNPs were treated using MC as a dispersant to improve their dispersibility in

aqueous solution. The optimum MC to GNPs ratio of 7:1 by concentration was confirmed using UV5. Conclusions Vis absorbency, and the dispersing performance of GNPs was characterized by optical microscope

In this paper, GNPs were MC as a dispersant improve their dispersibility in and TEM. Consequently, thetreated uniformusing GNP suspensions were used toto reinforce the cement matrix as aqueous solution. The optimum MC to GNPs ratio of 7:1 by concentration was confirmed using UV-Vis absorbency, and the dispersing performance of GNPs was characterized by optical microscope and

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TEM. Consequently, the uniform GNP suspensions were used to reinforce the cement matrix as mixing water. The effect of GNPs on the mechanical properties and microstructure of cement based materials were investigated with the help of XRD patterns, thermal analysis, MIP, and SEM analysis. The incorporation of 0.05% GNPs by weight of cement can enhance the flexural strength by 15%–24% and the compressive strength by 3%–8%. The XRD pattern and thermal analysis demonstrate that the degree of cement hydration has been promoted by GNPs, especially at an early age. Moreover, a more compact microstructure and finer pore size were detected in the GNP–cement composite using MIP and SEM analysis. However, more research is needed to improve the dispersibility of GNPs in cement matrix and the interfacial interaction between GNPs and the hydration products of cement. This study can provide a suitable method to investigate the two-dimensional nanomaterial for cement composites. Acknowledgments: This work was sponsored by the National Natural Science Foundation of China (51578108, 51278086). Author Contributions: Baomin Wang provided suggestions on the experimental work, research methodology, and revised manuscript. Ruishuang Jiang carried out all the experimental work, data analysis and manuscript preparation. Zhenlin Wu participated in data analysis and revised the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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