J. Cent. South Univ. (2016) 23: 919-925

DOI: 10.1007/s11771-016-3139-4

Effect of graphene on mechanical properties of cement mortars

CAO Ming-li(曹明莉), ZHANG Hui-xia(张会霞), ZHANG Cong(张聪)

School of Civil Engineering, Dalian University of Technology, Dalian 116024, China

 Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: Functionalized graphene nano-sheets (FGN) of 0.01%-0.05% (mass fraction) were added to produce FGN-cement composites in the form of mortars. Flow properties, mechanical properties and microstructure of the cementitious material were then investigated. The results indicate that the addition of FGN decreases the fluidity slightly and improves mechanical properties of cement-based composites significantly. The highest strength is obtained with FGN content of 0.02% where the flexural strength and compressive strength at 28 days are 12.917 MPa and 52.42 MPa, respectively. Besides, scanning electron micrographs show that FGN can regulate formation of massive compact cross-linking structures and thermo gravimetric analysis indicates that FGN can accelerate the hydration reaction to increase the function of the composite effectively.

Key words: functionalized graphene nano-sheets; cement mortars; mechanical strength; microstructure

1 Introduction

Cement composites are the most common and widely used construction materials in the world. Although cement gelatin materials possess relatively high compressive strength, the brittle natural and consequent tendency to cracking is still its main drawback. There are a number of different routes that may overcome these serious shortcomings and lead to the production of stronger and tougher concrete products, such as the addition of superplasticizer, active mineral admixtures and fibers [1-6]. However, the effectiveness of these approaches is not significant in the aspect of improving the brittleness characteristics of cement based materials.

The Feynman vision of a powerful and general nanotechnology, based on nano-machines that build with atom-by-atom control, promised great opportunities [7-8]. This viewpoint also opened a new prospect for utilization of nano-materials in various application fields. In recent years, a variety of nano-materials including carbon nanotubes (CNTs), nanosilica and carbon nanofiber (CNF) have been used as electrical conductors, reinforcements and adsorbents, etc [9-15]. In 2004, the discovery of graphene added a new dimension of nano-reinforcement. The unique mechanical, thermodynamic and chemical properties of graphene make it close to ideal reinforcing materials in the carbon family [16]. The thickness of monolayer graphene sheet is only 3.35 , while its theoretical elastic modulus and intrinsic strength are as high as 1 TPa and 130 GPa, respectively [17]. Additionally, the thermal stability and structural stability of this material is also outstanding. These attractive properties have led to considerable research on graphene-based composites, such as polymer, metal and ceramic matrix materials [18-20]. Nevertheless, there have been only limited investigations of graphene used in civil infrastructure applications. According to ALKHATEB et al [21], the addition of functionalized graphene nano-platelets of 0.5% leads to a 23.04% increase in elastic modulus of the cement paste matrix. LV et al [22-23] reported that the flexural strength of graphene oxide (GO)-cement composites was found to be higher than that of blank samples. Early results indicated that adding graphene to cementitious matrices can remarkably enhance the toughness of cement-based composites. However, the effect of graphene nano-sheets on the cement hydrates such as ettringite (AFt), calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH) is always neglected.

In this work, the nitric acid tread graphene nano- sheets was added to cement to produce cement-graphene composites in order to determine the influence of functionalized graphene nano-sheets (FGN) on the properties of the newly formed composites where fluidity, compressive strength and flexural strength of theses mixes were then investigated. Above all, scanning electron micrographs (SEM) was used to study the morphology and microstructure of cement hardened mortar, and X-ray diffraction (XRD) and Thermo gravimetric (TG) analyses were employed to characterize the phase composition of FGN-cement composites.

2 Experimental

2.1 Materials

Graphene was prepared by chemical method at Cheap Tubes Inc. USA. The physical parameters are given in Table 1. Ordinary Portland cement P·O42.5R was supplied by Dalian Onoda Cement Plant, and the chemical composition of cement is illustrated in Table 2. The standard sand was provided by Xiamen ISO Standard Sand Co., Ltd. (China). All reagents were of analytical reagent grade and used as received. Nitric acid (70% HNO₃, mass fraction) and ammonium hydroxide (25% NH₃·H₂O, mass fraction) were brought from Sinopharm Chemical Reagent Co., Ltd. (China). Water used in all experiments was distilled water.

Table 1 Physical parameters of grapheme

Table 2 Chemical composition of cement (mass fraction, %)

2.2 Graphene nano-sheets functionalization

The agglomerates of graphene due to strong van der Waals interactions result in poor dispersion and limit the efficiency of graphene in cement based materials. The experiment employed an oxidation/ultrasonic procedure to improve the dispersion of graphene in water. Briefly, predetermined mass of graphene (3 g) and 70% nitric acid (300 mL) were mixed in a 500 mL glass flask and ultrasonically stirred for 30 min. Next, the mixture was stirred for 24 h with a magnetic stirrer, filtered, rinsed with moderate alkaline (NH₃·H₂O) until the pH was adjusted to 7 or so, and then dried for 3 h in the oven at 60 °C [24]. Finally, the dried film-like material was ground into fine powder with a porcelain mortar, and the prepared sample was named FGN.

2.3 Specimen preparation and test

FGN was used as an additive material at 0% (PO), 0.01% (N1), 0.02% (N2), 0.03% (N3), 0.04% (N4), and 0.05% (N5) by mass fraction of cement. The compressive strength and flexural strength tests of mixes were conducted using water/cement ratio (w/c) of 0.5 and sand/cement ratio (s/c) of 3.0 after curing in standard curing room for 3, 7, 14 and 28 days.

For homogeneous structure, the FGN was firstly dispersed in some part of the mixed water and ultrasonicated for 10 min at room temperature, and then was added to cement. Then, the blends were mixed with remaining water and sand. The mixing procedure is revealed in Fig. 1.

After that, the mortars were poured into lubricated moulds (40 mm×40 mm×160 mm), compacted and surface-smoothened. All mortar samples were de-molded after 24 h and then cured in standard curing room until testing.

Three-point bending test was employed in the process of the flexural tests according to NF-EN-196-13 [25]. The compressive strengths of cement mortars broken in flexural test were measured with a microcomputer controlled automatic press (YAW-300) at a crosshead speed of 2.4 kN/s according to ISO 679.

2.4 Structural characterization of FGN and microstructure of cement mortars

The surface properties and composition of GN and FGN were investigated by Fourier transform infrared (FT-IR) spectroscopy using an EQUINOX 55 FT-IR spectrometer over the wavenumber from 4000 to 400 cm^-1. Microstructures of cement mortars were observed by means of scanning electron microscopy (SEM) in a JEOL JSM-5600LV microscope. Identification of the phases present in cement-based composites was performed with X-ray diffraction (XRD) and thermo gravimetric analysis (TGA).

3 Results

3.1 Structural characterization of FGN nano-sheets

The FT-IR spectra of FGN and GN are presented in Fig. 2. The FT-IR studies of FGN indicate that the method of acid treatment generates oxygenic groups on the surface of graphene: as shown in Fig. 2(a), the intensity of hydroxyl group (—COOH, 3400 cm^-1) and carboxyl group (—OH, 1626 cm^-1), and the peaks at 1400 cm^-1 corresponding to the asymmetric and symmetric —COO— is higher than that of GN. The new peaks at 1727 and 1069 cm^-1 can be ascribed to the vibration of C—O, C—N groups introduced on the surface of FGN by HNO₃ treatment, respectively. These polar groups on the modified surface can make FGN layers mutually excluded by electrostatic force and weaken the interaction between the layers, so as not to form agglomerates and improve its hydrophilicity and dispersibility further.

Fig. 1 Mixing procedure for fresh cement mortar mixtures

Fig. 2 FTIR spectra of FGN (a) and GN (b)

3.2 Effect of FGN on flow properties of cement composites

In the present work, the effects of FGN on fluidity of the specimens were investigated by the jump table method. The results are given in Table 3. From Table 3, it can be concluded that the fluidity of FGN-cement composites decreases with increasing FGN content and the smallest fluidity loss is obtained when 0.01% FGN is used. The reason for the degradation of mortar liquidity may lie in the fact that the large specific surface area of FGN and the oxygen-containing functional groups (hydroxyl, carboxyl, etc.) on its surface increase the interaction between the cement particles. FGN here plays the role of thickening tackifier, thereby reducing mortar mobility. In addition, the increase of FGN content leads to the increase of the quantity of slurry, which, by wrapping FGN, declines the continuous of slurry consequently.

Table 3 Fluidity (mm) of cement mortar with different FGN contents

3.3 Effect of FGN on mechanical properties and microstructure of cement mortar

Table 4 gives the test results of flexural and compressive strengths of the samples. Generally speaking, the addition of FGN can increase the mechanical strength of FGN-cement composites, especially the flexural strength. The strength at preset test time of Portland cement mortars reinforced with different FGN contents is found to be highest with 0.02% FGN addition where the flexural strengths at 3, 7, 14 and 28 days are 6.317, 7.567, 9.900 and 12.250 MPa, respectively. This is noticeably higher than that of cement composites without FGN. However, increasing the FGN content to 0.05%, the flexural strength of FGN-cement composites shows a slight increase by 6.81% at 28 days compared with blank samples. Here, it is assumed that further addition of FGN may cause graphene restacking together due to van der Waals force, weakening the efficiency of the mechanical improvement. Compared with the similar researches on other nano-materials reinforced cement composite, we find the improvement caused by little dosage of FGN is significant, as shown in Table 5. This result indicates that the right amount of FGN can improve the brittleness of mortars to a certain degree.

Similar trend in the results is also observed in the compressive strength test where the compressive strength of the specimens with FGN remains higher than that of the reference samples without FGN, and rises at first and goes down later with the increasing of FGN content. The highest increment in compressive strength of cement- based composites with the additional FGN is approximately 20% for Sample N2 at 28 days (the mixture compressive strength is about 52.42 MPa). Previous studies concluded that the addition of functionalized graphene nanoplatelets can improve the elastic modulus, shear modulus, tensile strength and compressive strength of cement-based composites [29-31]. The results obtained from mechanical testing for cement matrix composites are in a good agreement with values reported early.

Table 4 Flexural and compressive strength of Portland-FGN cement composites at different curing time

Table 5 Comparison of enhanced extent of flexural strength in mortars reinforced with different kinds of nano-materials

The mechanical properties of cement-based composites mainly depend on its microstructure (solid phase and pore types, quantity and distribution). The corresponding SEM images of the cement composites were investigated to define the relationship between mechanical strengths and microstructure. SEM micrographs of cement mortars mixed with different FGN contents after curing for 28 d are shown in Fig. 4. A distinct shape change is observed as the dosage increases. The structure of the hydrated cement without the addition of FGN shows the formation of many disorderedly stacked needle-like and bar-like crystals, which are cement hydration crystals of ettringite and calcium hydroxide (Fig. 4(a)). With FGN content of 0.01%, the hydrated sample is more compact, with fewer needle-shaped hydration crystals emerged in the fracture surface (Fig. 4(b)). For dosage of 0.02%, the shape of the hydration crystals resembles regular and complete polyhedral. The polyhedron-like crystal hydration products form a compacted structure, and there is a certain space to absorb movement simultaneously, so the mechanical strengths are enhanced (Fig. 4(c)). However, increasing FGN content from 0.03% to 0.05%, the order degree of cement hydration products decreases (Figs. 4(d)-(f)). The main origin of this phenomenon is that the hydrophilic groups on the surface of graphene absorb part of the water and prevent the hydration process of cement pastes.

These results confirm that FGN can regulate the shape of cement hydration crystals to form massive compact cross-linking structures according to FGN dosages, which increases the mechanical properties of cement composites. But, this phenomenon is applicable more at lower content compared to higher content. Within the ranges of this work, FGN has good effect on the reinforcement of mortar, and the optimum content of FGN is 0.02%.

3.4 Effect of curing time on mechanical properties and microstructure of cement mortar

Cement-based composite materials are widely used in bridges, dams and other infrastructures, and the durability of concrete structures is closely related to the early mechanical properties. It can be seen that the mechanical properties of prepared graphene-cement composites increase with the preservative age prolonging. Compared with the blank sample, the flexural strength increases by 13.21%, 26.51% and 29.75%, and the compressive strength increases by 9.72%, 17.51% and 18.59%, at 3, 7 and 14 d, respectively, when the content of graphene is 0.02% (Table 4). Adding a small amount of FGN can significantly improve the early strength and decrease the damage of cement mortars. Therefore, graphene can be used as early strength enhancer, and will provide a new research direction to improve the early age performance of cement-based materials.

Fig. 4 SEM images of specimens after curing for 28 d:

The SEM images of cement mortar with a fixed FGN content of 0.02% at different curing times are displayed in Fig. 5. Figure 5(a) is from the sample hydrated for 3 d and shows a loose construct. The microstructure of the paste is more uniform and dense at 7 and 14 d compared with that of 3 d. After 28 d, these hydration crystals show a tendency to form massive compact cross-linking structures through the regular polyhedra. The results indicate that a sprinkling of FGN can control the shape and arrangement of the cement hydration products.

3.5 XRD analysis of cement mortar

In order to further prove the influence of FGN on the microstructure of these samples, the XRD patterns of hydration crystals over time are also investigated (Fig. 6). The XRD patterns of the hardened mortar containing 0.02% FGN are similar to those of ordinary cement mortar. This demonstrates that the hydration products consist of AFt, CH, AFm and C—S—H, and the amount of these crystals increases with increasing hydration time. In addition, the crystal peak intensities of cement-FGNcomposites are higher than those of cement mortar without FGN, which confirms that FGN has the role of inducing, promoting the growth of cement hydration crystal products.

3.6 Thermal stability analysis of cement mortar

Figure 7 shows the results from TG/DTG test for the system of Portland cement and FGN-cement composites at 28 d. It can be seen that there are three typical endothermic peaks in the cement-based composite: C—S—H ( °C), Ca(OH)₂ ( °C) and CaCO₃ ( °C) [32]. C—S—H is the main source of cement strength, and Ca(OH)₂ is one of the main hydration products. The morphology of ettringite and hydration process is related with the content of C—S—H and Ca(OH)₂ in hardened cement. The content of C—S—H and Ca(OH)₂ can be calculated from the following equations, respectively:

                    (1)

                 (2)

where W_C—S—H and are the mass fractions of C—S—H and Ca(OH)₂, %, respectively; L_C—S—H and are the decomposition rates of C—S—H and Ca(OH)₂ at decomposition temperature, %; M_C—S—H,and are the molecular masses of C—S—H (182.32 g/mol), Ca(OH)₂ (18.02 g/mol,) and H₂O(74.10 g/mol), respectively.

Fig. 5 SEM images of cement hydrates with 0.02% FGN at different curing times:

Fig. 6 XRD patterns of graphene-cement composites at different hydration time

Fig. 7 TG/DTG curves taken at 28 d for Portland cement composites (a) and FGN-cement composites (b)

Table 6 C—S—H and Ca(OH)₂ contents of FGN-cement composites

The calculated results of C—S—H and Ca(OH)₂ content are listed in Table 6. Apparently, more C—S—H and Ca(OH)₂ are generated after the additional of 0.02% FGN. This phenomenon may be explained by the chemical interaction between cement matrix and FGN where hydration products give further responsibility for the strength of FGN-cement composites.

3.7 Regulation mechanism of FGN in cement hydration products

According to experimental results and discussion above, the possible mechanism that FGN can improve the strength of cement mortar can be illustrated as follows (Fig. 8).

1) The interfacial interaction between FGN and cement hydrates. Due to the fact that the modification of FGN involves surface functional groups (—OH,—COOH) on the graphene, acid-base reactions take place between —COOH groups and Ca(OH)₂ of cement hydrates [9]. The interaction results in a strong covalent force on the interface between FGN and cement matrix, which further increases the load-transfer efficiency from matrix in the composites to FGN. As a result, the compressive/flexural strengths of the cement composites are enhanced.

2) The template effect of FGN. As demonstrated by SEM analysis, the hydration products, consisting of various AFt, AFm, CH and C—S—H, exhibit in a ordered way and form regular polyhedral in pores, cracks or loose hardened paste. The uniform and compacted hydrated crystals have great contribution to improving mechanical properties of cement composites.

Fig. 8 Mechanism of cement hydration products by FGN

4 Conclusions

Incorporation of FGN in cement mortars shows modifications in microstructure and mechanical properties of the composites. The fluidity of FGN-cement composites decreases with increasing FGN content. The flexural and compressive strengths of the specimens with FGN are higher than those of the reference samples without FGN. The highest strength obtained for all graphene-cement mortars is found at 0.02% level of FGN addition where the flexural strength and compressive strength at 28 d are 12.917 and 52.42 MPa, respectively. SEM micrographs show that FGN can act as a regulate phase resulting in massive compact cross-linking structures. TG/DTG test suggests that there are chemical reactions between cement matrix and FGN, which form more hydration products to give further responsibility for the strength of FGN-cement composites.

References

[1] HOUST Y F, BOWEN P, PERCHE F, KAUPPI A, BORGET P, GALMICHEC L, MEINSC J F L,LAFUMAC F,FLATTD R J,SCHOBERD I,BANFILLE P F G,SWIFTE D S, MYRVOLDF B O, PETERSENF B G, REKNESF K. Design and function of novel superplasticizers for more durable high performance concrete (Superplast Project) [J]. Cement and Concrete Research, 2008, 38(10): 1197-1209.

[2] van TITTELBOOM K, de BELIE N, LEHMANN F, GROSSE C U. Acoustic emission analysis for the quantification of autonomous crack healing in concrete [J]. Construction and Building Materials, 2012, 28(1): 333-341.

[3] BENTE D P. Influence of silica fume on diffusivity in cement based materials II: Multi-scale modeling of concrete diffusivity [J]. Cement and Concrete Research, 2000, 30: 1121-1129.

[4] BISCHOFF P H. Tension stiffening and cracking of steel fiber-reinforced concrete [J]. Journal of Materials in Civil Engineering, 2003, 15(2): 174-182.

[5] LAWLER J S, ZAMPINI D, SHAH S P. Microfiber and macrofiber hybrid fiber reinforced concrete [J]. J Mater Civ Eng, 2005: 17: 595.

[6] LU Chun-hua, CUI Zhao-wei, LIU Rong-gui, LIU Qi-dong. Chloride diffusivity in flexural cracked Portland cement concrete and fly ash concrete beams [J]. Journal of Central South University, 2014, 21: 3682-3691.

[7] TOUMEY C P. Reading Feynman into nanotechnology [J]. Techné: Research in Philosophy and Technology, 2008, 12(3): 133-168.

[8] DREXLER K E. Nanotechnology: From Feynman to funding [J]. Bulletin of Science, Technology & Society, 2004, 24(1): 21-27.

[9] LI Geng-ying, WANG Pei-ming, ZHAO Xiao-hua. Mechanical behavior and microstructure of cement composites incorporating surface-treated multi-walled carbon nanotubes [J]. Carbon, 2005, 43(6): 1239-1245.

[10] LI Yan-hui, WANG Shu-guang, WEI Jin-quan, ZHANG Xian-feng, XU Cai-lu, LUAN Zhao-kun, WU De-hai, WEI Bing-qing. Lead adsorption on carbon nanotubes [J]. Chemical Physics Letters, 2002, 357(3): 263-266.

[11] MAHMOUD M E, ALBISHRI H M. Supported hydrophobic ionic liquid on nano-silica for adsorption of lead [J]. Chemical Engineering Journal, 2011, 166(1): 157-167.

[12] SHIH J, CHANG T, HSIAO T. Effect of nanosilica on characterization of Portland cement composite [J]. Materials Science and Engineering A, 2006, 424(1): 266-274.

[13] RAO Mu-min, SONG Xiang-yun, CAIRNS E J. Nano-carbon/sulfur composite cathode materials with carbon nanofiber as electrical conductor for advanced secondary lithium/sulfur cells [J]. Journal of Power Sources, 2012, 205: 474-478.

[14] METAXA Z S, KONSTA-GDOUTOS M S, SHAH S P. Carbon nanofiber-reinforced cement-based materials [J]. Transportation Research Record: Journal of the Transportation Research Board, 2010, 2142(1): 114-118.

[15] G, JANTUNEN H,LAJUNEN M, SOLDANO C,TALAPATRA S, VAJTAI R, AJAYAN P. Inkjet printing of electrically conductive patterns of carbon nanotubes [J]. Small, 2006, 2(8/9): 1021-1025.

[16] GEIM A K, NOVOSELOV K S. The rise of graphene [J]. Nature Materials, 2007, 6(3): 183-191.

[17] LEE C, WEI Xiao-ding, KYSAR J W, HONE J. Measurement of the elastic properties and intrinsic strength of monolayer graphene [J]. Science, 2008, 321(5887): 385-388.

[18] PANG Huan, CHEN Tao, ZHANG Gang-ming, ZENG Bao-qing, LI Zhong-ming. An electrically conducting polymer/graphene composite with a very low percolation threshold [J]. Materials Letters, 2010, 64(20): 2226-2229.

[19] BLANTER Y M, MARTIN I. Transport through normal- metal–graphene contacts [J]. Physical Review B, 2007, 76(15): 155433.

[20] WALKER L S, MAROTTO V R, RAFIEE M A, CORRAL E L. Toughening in graphene ceramic composites [J]. Acs Nano, 2011, 5(4): 3182-3190.

[21] ALKHATEB H, ALOSTSAZ A, CHENG A H, LI Xiao-bing. Materials genome for graphene-cement nanocomposites [J]. Journal of Nanomechanics and Micromechanics, 2013, 3(3): 67-77.

[22] LV Sheng-hua, MA Yu-juan, QIU Chao-chao, JU Hao-bo. Effects of graphene oxide on microstructure of hardened cement paste and its properties [J]. Concrete, 2013(8): 51-54. (in Chinese)

[23] LV Sheng-hua, SUN Ting, LIU Jing-jing, MA Yu-juan, QIU Chao-chao. Toughening effect and mechanism of graphene oxide nanosheets on cement matrix composites [J]. Acta Materiae Compositae Sinica, 2014(3): 644-652. (in Chinese)

[24] ROSCA I D, WATARI F, UO M, AKASAKA T. Oxidation of multiwalled carbon nanotubes by nitric acid [J]. Carbon, 2005, 43(15): 3124-3131.

[25] EN N F. Methods of test for cements-Part I: Determination of mechanical resistance [S]. French Standard, 2006. (in French)

[26] LI Hui, XIAO Hui-gang, YUAN Jie, OU Jin-ping. Microstructure of cement mortar with nano-particles [J]. Composites Part B: Engineering, 2004, 35(2): 185-189.

[27] NAZARI A, RIAHI S, RIAHI S, SHAMEKHIA F, KHADEMNO A. Mechanical properties of cement mortar with Al₂O₃ nanoparticles [J]. Journal of American Science, 2010, 6(4): 94-97.

[28] CWIRZEN A, HABERMEHLC K, PENTTALA V. Surface decoration of carbon nanotubes and mechanical properties of cement/carbon nanotube composites [J]. Advances in Cement Research, 2008, 20(2): 65-73.

[29] LV Sheng-hua, MA Yu-juan, QIU Chao-chao, ZHOU Qing-fang. Regulation of GO on cement hydration crystals and its toughening effect [J]. Magazine of Concrete Research, 2013, 65(19/20): 1246-1254.

[30] ALKHATEB H, ALOSTAZ A, CHENG A H, LI Xiao-bing. Materials genome for graphene-cement nanocomposites [J]. Journal of Nanomechanics and Micromechanics, 2013, 3(3): 67-77.

[31] LV Sheng-hua, MA Yu-juan, QIU Chao-chao, SUN Ting, LIU Jing-jing, ZHOU Qing-fang. Effect of graphene oxide nanosheets of microstructure and mechanical properties of cement composites [J]. Construction and Building Materials, 2013, 49: 121-127.

[32] DWECK J, FERREIRA D S, BCHLER P M, CARTLEDGE F. Study by thermogravimetry of the evolution of ettringite phase during type II Portland cement hydration [J]. Journal of Thermal Analysis and Calorimetry, 2002, 69(1): 179-186.

(Edited by YANG Bing)

Foundation item: Project(51102035) supported by the National Natural Science Foundation of China

Received date: 2015-01-12; Accepted date: 2015-06-11

Corresponding author: CAO Ming-li, Associate Professor, PhD; Tel: +86-15840902911; E-mail: caomingli3502@163.com