The dispersion behavior of graphene oxide in cement matrix is one important factor in enhancing cement performance. In this work, we investigated the dispersion of graphene oxide in cement by simulating alkaline environment with a solution of calcium hydroxide and studied the corresponding strategy of improving dispersion. The obtained results showed that graphene oxide would flocculate even if calcium hydroxide concentration was very low, which might be the main reason of the unstable properties of the graphene oxide-doped cement. In addition, we discovered that, compared to -OH group, the -COOH group and the long chain of polycarboxylate-based superplasticizer were more effective in delaying the flocculation of graphene oxide. Finally, we proposed a dispersion mechanism of polycarboxylate-based superplasticizer. The study provides inspiration for the design of graphene oxide-doped cement materials.
In recent years, several attempts have been made to enhance the mechanical, electrical and transport properties of cement by introducing graphene oxide (GO). Gong et al. (
The addition of GO enhances the performance of cement. However, GO easily aggregates in the alkaline environment of cement, and the poor dispersion could weaken the benefits associated with the original properties of GO. The major challenge for GO-added cement is how to evenly disperse GO in the cement matrix. Thus, experimental research on solving the dispersion problem of GO is of great importance.
Considerable research efforts have been devoted to improving the dispersion of GO in cement. However, the well dispersion of GO in the cement matrix has not achieved. To address this issue, a few studies have proposed several methods to achieve uniform distribution. Gong et al. (
This study evaluated the flocculation of GO in lime solution by carrying out the UV-Vis spectra and zeta potential investigation. Moreover, this research employed PC to disperse GO and investigated the influence of different PC functional groups on the dispersion of GO. The research could give guidance on the dispersion of GO in the alkaline environment of cement.
Graphite powder (8000 meshes, 99.95%), concentrated sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 36%), sodium nitrate (NaNO3), hydrogen peroxide (H2O2, 30%), potassium permanganate (KMnO4), calcium hydroxide (CH), glycerin, acrylic acid and polyacrylic acid were all analytical grade. Polycarboxylate-based superplasticizer (PC, 30 wt. %) was supplied by Sobute New Materials Co. Ltd (China).
GO was prepared through a modified Hummers’ method (
The specimens were dried in a vacuum environment at 45 °C for 24 h. The specimens were characterized by FT-IR, SEM, XRD and X-ray photoelectron spectroscopy (XPS). Fourier transform infrared (Nicolet 380 infrared spectrometer with a resolution of 0.5 cm-1 for 8 scans) was utilized to determine the functional groups of GO. KBr pellets were used in the analysis. Scanning electron microscopy (Zeiss EVO LS 15) was utilized to observe the microstructure of GO. Prior to the observation, the specimens were sputter-coated with gold-palladium. All images were obtained in the secondary electron mode with an electron beam of 20 kV. The XRD patterns were acquired using an X-ray diffractometer (Bruker D8 Advance from Germany, Cu k α, 40 kV voltage, 40 mA current). The scanning speed was 6°/min with a step size of 0.02° in the scattering 2° range of 5° to 60°. GO samples were manually ground into fine powder, and then fixed in a metal plate. The XPS pattern was acquired using X-ray photoelectron spectroscopy (Kratos Analytical Ltd, UK) and the measurement was carried out with 4 scans per eV.
A certain concentration of GO suspension was employed in this study. Based on the previous research, 0.05 mg/mL was chosen as adequate concentration to study GO dispersion. Different reagents (PC, glycerin, acrylic acid and polyacrylic acid) were added to GO suspension. Meanwhile, CH concentrations varied from 0 to 240 mg/L. The UV–Vis spectra (Shimadzu UV 4800) and zeta potential (Zeta PALS, Brookhaven) were performed to explore the dispersion of GO in lime solution (CH solution) with different reagents. In addition, to understand the interactions between PC and GO, the FT-IR analysis was carried out.
The results of the FT-IR analysis of graphite and GO were shown in
FT-IR spectra of original graphite and GO.
XPS spectrum of GO.
XRD patterns of original graphite and GO.
SEM images for: a) original graphite; b) GO.
Generally, in the cement hydration process, tricalcium silicate (Ca3SiO5) and dicalcium silicate (Ca2SiO4) formed calcium-silicate-hydrates (C-S-H) and calcium hydroxide (CH). Tricalcium aluminate (Ca3Al2O6) and tetracalcium aluminoferrite (Ca4AlnFe2-nO7) formed ettringite (AFt), monosulphates (AFm), 3CaO•Al2O3•6H2O (C3AH6) and 4CaO•Al2O3•13H2O (C4AH13). Thus, cement paste was alkaline and special attention should be paid towards dispersing GO in alkaline environments (
The dispersion of GO was investigated in CH solutions at different concentrations. In this study, the concentration of GO was set at 0.05 mg/ml. Firstly, the dispersion of GO in CH solution was visually shown in
Digital images of the dispersion of GO in different concentrations of CH solution.
The GO-CH flocculation and plain GO suspension were also characterized using the UV-Vis spectrum. The UV-Vis values of GO and GO-CH flocculation suspensions were received after subtraction the UV-Vis values of water and corresponding to CH solution, respectively. The zeta potential had been applied to measure the stability of colloid. As known, the electrostatic repulsion mechanisms could enable the formation of well-dispersed GO colloids in water. Hence, the zeta potential was used to analyze GO-CH flocculation suspensions.
The UV-Vis curve of GO suspension was shown in
UV-Vis absorption spectrum of GO.
The UV-Vis absorbance and zeta potential results of GO in different concentrations of CH solution.
UV-Vis values showed a sharp decrease when the concentration of CH increased from 20 to 30 mg/L, and depicted relatively stable trend later. The results were consistent with the visual observation (
The addition of CH exhibited a negative effect on the dispersion of GO. Flocculation of GO occurred when CH solution was introduced with an apparently low concentration. This could be explained by the cross-linking reactions between neighboring GO sheets induced by calcium ions from CH (
First, GO and PC were together dispersed in deionized water with continuous stirring. The content of PC was equal to the concentration of GO by weight. Then, CH was added into the PC/GO pre-mixture. The suspensions were subjected to the following tests.
The UV-Vis absorbance and zeta potential results of GO-PC suspensions with different concentrations of CH (The concentrations of GO and PC are both 0.05 mg/mL).
As the concentrations of CH moved from 60 to 240 mg/L, there was a slight jump down in absorbance but it remained much higher than zero. Besides, the zeta potential value was approximately -12 mV when the concentration of CH reached 240 mg/L, which was more stable than GO in CH solution. The relatively effective dispersion of GO was achieved with the use of PC.
In the consideration of the dispersion study of GO by PC presented above, the amount of PC was also chosen as equal to the concentration of GO by weight.
FT-IR spectra of PC and PC-GO.
FT-IR analysis was further carried out to verify the interaction between PC and GO in the reaction process. As shown in
In fact, PC had notable functional groups, such as -OH and -COOH. The possible cause of a better dispersion of GO by PC was that the functional groups played a pivotal role in the process. Therefore, this study paid attention to the functional groups of PC and selected the reagent containing functional groups to carry out the research. In a typical study, glycerin and acrylic acid were used to analyze the function of -OH and -COOH groups, respectively. The primary reason for the choice was that their structures contained only a single functional group.
The effect of functional groups on the dispersion of GO in CH solution was investigated.
The UV-Vis absorbance and zeta potential results of GO-glycerin suspensions with different concentrations of CH: (a) glycerin to GO weight ratios of 1; (b) glycerin to GO weight ratios of 3.
The results of the GO dispersion in CH solution with different amounts of acrylic acid were shown in
The UV-Vis absorbance and zeta potential results of GO-acrylic acid suspensions with different concentrations of CH: (a) acrylic acid to GO weight ratios of 1; (b) acrylic acid to GO weight ratios of 3.
It was mainly because the presence of -COOH group improving the dispersion efficiency of PC. The primary reason for the improvement was that the -COOH group on the edges of the PC could react with Ca2+ and reduced the reaction chance of GO and Ca2+. However, the effect of acrylic acid on the dispersion of GO was relatively poor in comparison to PC. This difference possibly could be related to the special long chain molecular structure of PC. Thus, polyacrylic acid was chosen to study the effect of long chain.
The UV-Vis absorbance and zeta potential results of GO-polyacrylic acid suspensions with different concentrations of CH: (a) polyacrylic acid to GO weight ratios of 1; (b) polyacrylic acid to GO weight ratios of 3.
The overall results suggested that -COOH group and the special long chain of PC were properly beneficial to prevent the flocculation of GO in CH solution.
In this study, a reasonable model of explaining the dispersion mechanism of GO by PC in CH solution was proposed in
Scheme showing the dispersion of GO by PC in CH solution.
In this paper, the flocculation of GO in CH solution and the corresponding improvement were studied experimentally. Based upon the test results, the following conclusions could be drawn:
An aqueous dispersion of GO was prepared with graphite oxidation and ultrasonic treatment. The FT-IR, XPS and XRD results confirmed the presence of -OH, -COOH and C-O groups in the GO structure. The SEM image showed that the surface of GO was rough and wrinkled.
The UV-Vis spectra and zeta potential results indicated that GO became unstable with the increase of CH content. At the CH concentration of 30 mg/L, GO suspension depicted a marked flocculation process. These results showed that incorporation of GO directly into cement was improper due to the appearance of CH which was produced by cement hydration.
The PC had an anticipated impact on the distribution of GO in CH solution. GO achieved a better dispersion with the incorporation of PC. Moreover, there might be weak intermolecular bonds between -COOH groups in PC and similar groups in GO.
The -COOH group was a better choice to be used as sacrificial agent than -OH group in terms of improving GO dispersion. In addition, the special long chain of PC also played a pivotal role in the process. These findings may be beneficial for enhancing the GO distribution in the cement paste. However, even with the improved dispersion, GO could also react with Ca2+ due to the presence of -COOH group on the edge. Therefore, it was necessary to modify GO in the aspects of the functional groups.
This research is financially supported by the National Nature Science Foundation of China (Grants 51272092 and 51772129), National Key Technologies R&D Program (Grants 2016YFB0303505) and the 111 Project of International Corporation on Advanced Cement-based Materials (No. D17001).