1 Introduction
Cement mortar is one of the most commonly used building materials due to its characteristics of high compressive strength and good durability. However, there are requirements for the higher compressive strength of cement mortar with the wide application of super high-rise structure and long-span structure. It is also noticed that cement mortar has problems of low flexural strength and easy crack formation. Therefore, the current research focuses on further improving the compressive and flexural strength of cement mortar.
Researchers tried to modify the cement mortar by adding nanomaterial with the progress of nanomaterial. As we all know, small particle size, large specific surface area, and high surface energy are the distinguishing features of nanomaterial [
1]. The compressive and flexural strength of cement mortar can enhance effectively by adding nanomaterial. Graphene oxide (GO) is a derivative of graphite by chemical oxidation stripping [
2]. Therefore, GO has abundant oxygen functional groups and a large specific surface area, providing more condensed cores and accelerating hydration reaction [
3–
5]. Furthermore, the functional groups of GO can enhance the covalent bond between GO and hydration products of cement, which can raise the load transfer efficiency between cement matrix and GO plate [
6]. The cement mortar prepared by cement, deionized water, and stably dispersed graphene oxide has excellent compressive and flexural properties [
7]. Researchers have studied the influences of GO on the compressive strength, flexural strength, and microstructure of cement mortar [
8,
9].
Lv et al. [
10] used poly acrylic acid diallyl dimethylammonium chloride (PAAD) as a dispersant to prepare a few layers of GO dispersion, which can be uniformly dispersed in cement paste. The researcher claimed that GO dispersed by PAAD had a template effect which can regularize hydration products to form an ordered structure. Therefore, GO cement mortar has the advantages of high compressive strength, high flexural strength, few cracks, and small average pore size. Peng et al. [
11] studied the effect of
w/
c ratio and GO content on the filling effect, nucleation effect, and hydration promoting effect of GO. The optimal compressive and flexural strength of cement mortar could be obtained with 0.35
w/
c ratio and 0.05 wt% GO. Wang et al. [
12] found GO improved the mechanical properties of cement mortar by reducing the pore volume and improving the compactness. With 0.05 wt% GO, the compressive strength of cement mortar increased by 43.2%, 33%, and 24.4% and the flexural strength of cement mortar increased by 69.4%, 106.4%, and 70.5%, compared with the control group at 3, 7, and 28 d. Kudžma et al. [
13] investigated the compressive and flexural strength of cement mortar modified by GO with a low C/O ratio which is approximately 4. The researcher found the compressive strength at 28 d gradually increased (10.5%–21.7%) with the increases of GO content (0.02%–0.04%) and the increase of compressive strength of cement mortar with a low C/O ratio was greater than the flexural strength. Chintalapudi and Pannem [
14] reported the compressive strength of cement mortar increased by 27.64%, 31.70%, and 46.34% at 28 d with 0.02 wt%, 0.03 wt%, and 0.04 wt% GO. The researcher reported that the oxygen-containing functional groups on the GO surface reacted with C
3S, C
2S, and C
3A to form flower-shaped hydration products, which filled the hydration cracks and holes. Abrishami and Zahabi [
15] claimed that NH
2 functionalization could to improve the adhesion between GO nanoflakes and cement composites. The researcher reported that the compressive strength of cement mortar with 0.10 wt% NH
2 functionalization GO increased from 39 to 54.23 MPa. Zhao et al. [
16] claimed that the improvement of cement mortar’s compressive and flexural strength with GO was attributed to the strong interface between GO and cement mortar. The researcher found the strong interface could delay the initiation of crack and effectively prevent crack propagation. The cement mortar compressive strength with 0.022 wt% GO was 34.10%, 26.90%, and 22.59% higher than that with 0 wt% GO at 3, 7, and 28 d, the flexural strength increased by 30.37%, 24.49%, and 24.56%, respectively. Gong et al. [
17] found the compressive strength and tensile strength of cement mortar with 0.03 wt% GO could increase by more than 40%, respectively, owing to pore volume decrease. Li et al. [
18] reported that the compressive strength of cement pastes with 0.02 wt%, 0.04 wt%, 0.06 wt%, and 0.08 wt% GO increased by 42.3%, 43.4%, 48.5%, and 56.3%, respectively, compared with the control group. The researcher found that the accelerated hydration effect of GO on cement and the strong interface adhesion between cement paste and GO were the reasons for the increased strength.
In previous studies, GO with the thickness of 0.7–3 nm (single layer or few layers GO) was widely used [
6,
11,
12,
19–
21]. At present, GO is usually prepared by Hummers method because of its short reaction time and no toxic gas ClO
2 emission. In recent years, many researchers have simplified the preparation process by improving the Hummers method, to reduce the cost of GO [
22–
24]. Generally, the dried GO powder is obtained by the freeze-drying method, low-pressure drying method, or the low-temperature drying method. The quality and cost of the final product are vastly different with different drying methods. Low pressure or freeze-drying is usually used to prepare single-layer or few layers GO, but the cost of production is high. Therefore, it is difficult to apply a single layer, or few layers GO in practical engineering. In contrast, the low-temperature drying method for preparing multilayer graphene oxide (MGO) has a lower cost and better economy. The price of a single-layer or few layers GO is about 100–200 times that of MGO. At present, MGO has been used in producing high-performance energy dissipation materials [
25], Laser-driven propulsion [
26], gas separation [
27] and water treatment [
28,
29]. In addition, a previous study [
30] has shown that MGO can reduce carbonation of concrete and sulfate attack on concrete. However, very few comprehensive investigations have been conducted on the influence of MGO on the mechanical properties and microstructure of cement mortar. For instance, Chen et al. [
31] investigated the mechanical properties of integrated MGO carbon-fiber composites, but the research on MGO cement mortar was not comprehensive.
This study investigated the effect of MGO on the mechanical properties and microstructure of cement mortar. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and Fourier transform infrared spectroscopy (FTIR) were used to characterize MGO. Several characterization techniques, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and nitrogen isotherm adsorption analysis, were utilized to discuss a possible mechanism of the cement mortar reinforcement with MGO.
2 Materials and methods
2.1 Materials
In this study, MGO powder provided by Suzhou carbon graphene Technology Co., Ltd was used. The specific surface area of MGO was 100–300 m
2/g. Type P.II.52.5 Portland cement provided by Nanjing Sany Building Materials Co., Ltd conforms to GB175-2007 [
32] standard, and its composition is shown in Table 1. ISO standard sand provided by Xiamen ASO Co., Ltd and Polycarboxylate-based superplasticizer (S.P.) conforming to JG/T 223–2007 [
33] provided by Shanghai Qinhe Chemical Co., Ltd were used. The particle size distribution of cement and standard sand measured by Laser Particle Size Analyzer (Malvern Mastersizer 2000) was shown in Fig. 1. The bulk density, pH, and water reduction of polycarboxylate-based superplasticizer (S.P.) were 650–850 g/L, 7.0–9.0, and greater than or equal to 23%, respectively.
2.2 Preparation and characterization of MGO
Dispersed MGO was prepared by ultrasonic dispersion in a sonication bath (Fangao YM-100S) for 90 min. The ultrasonic frequency was 40 kHz, and the power was 120 W.
The size and thickness of MGO were characterized by atomic force microscopes (AFM, Bruker Dimension Icon). Scanning electron microscopies (SEM, JEOL JSM-6510LA) were used to perform the microscopic morphology. Fourier transform infrared spectrometer (FTIR, Thermo Scientific Nicolet iS5) was used to measure the chemical bonding of MGO.
2.3 Preparation of MGO-cement mortar
The mix proportions of MGO-cement mortar are shown in Table 2. According to previous studies [
34,
35], the dosage of MGO varies from 0.02%, 0.04% to 0.06% by cement mass. The sample was mixed with a JJ-5 mixer. The mixing procedure of cement mortar was carried out according to Method of testing cements-Determination of strength (GB17671-1999) [
36], and the detailed procedure is shown in Fig. 2. The fresh cement mortar was poured into the 40 mm × 40 mm × 160 mm molds. Specimens were removed from the molds after curing for 24 h. Then all specimens were placed in the curing chamber at 20°C and 95% relative humidity until further test.
2.4 Testing methods
2.4.1 Workability
The fluidity of fresh mortar was tested by the flow table. The fresh mortar was added into the test mold in two layers, and the tamping rod was used to tamp 15 times and 10 times respectively from the edge to the center after each addition of mortar. After tamping, the test mold was lifted gently vertically upward. Finally, 25 beats were performed at a rate of one per second. The detailed test procedure was shown in the Test method for the fluidity of cement mortar (GB/T 2419–2005) [
37].
2.4.2 Mechanical strength
The loading rate was 0.04–0.06 and 2.2–2.6 kN/s during the flexural and compressive tests, respectively. Each mixture design’s compressive and flexural strength was the average of three specimens’ test results. The strength of cement mortar was tested according to the Method of testing cements––Determination of strength (GB 17671–1999) [
36].
2.4.3 X-ray diffraction
The phases of cement mortar were detected by the XRD technique. To prevent cement hydration, the broken samples were immersed in anhydrous ethanol. After drying at 40°C for 24 h, the sample was ground into powder. In the 2θ range of 5°–80° at a voltage of 40 kV, current of 20 mA, and a scan rate of 2°/min, an X-ray diffractometer (Rigaku Ultima IV) were used to conduct the XRD procedure.
2.4.4 Scanning electron microscopy (SEM)
A scanning electron microscope (JEOL JSM-6510LA) was used to observe the microscopic morphology of samples of 3 and 28 d. During the test, samples were taken out from anhydrous ethanol and sprayed with gold on the surface. The working voltage was set at 20 kV for microscopic analysis.
2.4.5 Nitrogen isotherm adsorption analysis
Compared with traditional cement mortar, MGO-cement mortar has a denser microstructure and a higher proportion of small pores. Therefore, the pore structure of samples with or without MGO was quantitatively evaluated by Nitrogen adsorption and desorption isotherms. Before testing, the samples were dried at 40°C for 24 h.
A nitrogen gas adsorption analyzer was used to carry out nitrogen adsorption and desorption under vacuum condition. The BJH model was used to calculate the pore size distribution and pore volume of N2.
3 Results and discussion
3.1 Dispersion and characterization of MGO
3.1.1 Dispersion of MGO
The dispersion of GO is essential to play the role of MGO in cement mortar. However, GO is easy to aggregate in cement mortar [
38], as it reacts with divalent cations and hydroxide ions in the cement mortar. In this study, MGO was evenly dispersed in cement mortar by adding S.P. To ensure the uniform dispersion of MGO in cement mortar, 3 g cement and 0.6 g S.P. were added to the MGO dispersion in Fig. 3(c) and 3 g cement was added to the MGO dispersion in Fig. 3(b). The method was to simulate the reaction of MGO in cement mortar and test the dispersion of MGO. Figure 3(a) is the diagram of 100 mL MGO dispersion. The MGO dispersion was black and opaque, and there was no precipitation and stratification in the dispersion, which indicates that MGO was well dispersed in an aqueous solution before it was added to cement mortar. The mixture in Fig. 3(b) is layered indicates that MGO agglomerates in a cement mortar environment. However, the mixture in Fig. 3(c) did not show stratification and its state was similar to that without cement, which indicates that MGO was well dispersed in this study.
3.1.2 Characterization of MGO
The atomic force microscopy (AFM) image of MGO sheets is presented in Figs. 4(a) and 4(b). It shows that the shape of MGO sheets was irregular, and the size was about 1μm. The thickness of each part of the MGO sheet was not consistent, and the surface of the MGO sheet was undulating. Its average thickness was about 6 nm, and it was a multilayer graphene oxide. This indicates that MGO sheets have good dispersion and exfoliation and that MGO reached nanoscale.
The MGO dispersion was observed by a SEM. The images obtained at 50000 times magnification are shown in Fig. 4(c). As shown in the image, there were folds on the flakes of MGO, which enlarged the specific surface area of MGO. In addition, the folding thickness of MGO was about 12 nm, so the tiling thickness of MGO was about 6 nm, which was consistent with the AFM results. The results of SEM show that the prepared MGO dispersion has a large specific surface area, so it has large surface energy and can be well combined with cement mortar.
The Fourier-transform infrared spectroscopy (FTIR) spectra of MGO is presented in Fig. 4(d). The wavenumber 3154.36 cm
−1 is the peak of hydroxyl (–OH), and the wavenumber 1713.16 and 1612.44 cm
−1 correspond to the peaks of Carboxyl (–COOH) and carbonyl (–C=O), respectively. The peaks at 1165.20 and 1035.07 cm
−1 refer to C–O–C/–C–O [
10,
11]. These characteristic peaks present in the result indicate that MGO has rich oxygen-containing functional groups. These oxygen-containing functional groups give MGO good hydrophilicity and make it easy to disperse in an aqueous solution.
3.2 Workability of the fresh MGO cement mortar
As shown in Fig. 5, the fluidity of fresh MGO-cement mortar with different content had no noticeable change, and the fluidity test results of each sample were about 180 mm. In conclusion, MGO does not reduce the fluidity of cement mortar, which is different from previous studies [
17,
20]. This may be due to the use of thicker MGO.
3.3 Mechanical properties of MGO-cement mortar
3.3.1 Compressive strength of cement mortar incorporating MGO
Figure 6 shows the effect of the content of MGO on the compressive strength of cement mortar with increasing curing time. According to Fig. 6, the compressive strength of cement mortar at the 3, 7, and 28 d increased to different degrees after the inclusion of MGO. Among them, the compressive strength of the G6 sample reached a maximum at 3 d, which was 13.42% higher than the G0 sample and reached 50.55 MPa. When the curing age was 7 d, the compressive strength of the G4 sample reached 51.88 MPa, which increased by 12.08% compared with the G0 sample. Compared with the compressive strength of MGO-cement mortar at 3 and 7 d, the compressive strength of MGO-cement mortar at 28 d had little improvement. The compressive strength of the G4 sample only increased by 5.85%. It can be concluded that the compressive strength of cement mortar increases after incorporation of MGO, but MGO has no significant effect on the compressive strength of mortar after 7 d. Similar results have been found in previous studies [
14,
39,
40], which used single or few layers GO. Therefore, the results of this study show that the effect of MGO on the compressive strength of cement mortar is similar to that of single or few layers GO.
3.3.2 Flexural strength of cement mortar incorporating MGO
The results of the effect of MGO on the flexural strength of cement mortar are shown in Fig. 7. It can be seen that the flexural strength of MGO-cement mortar at 3 and 7 d was improved compared with the G0 sample. Yet, it is worth noticing that the flexural strength of MGO-cement mortar was not significantly different with the G0 sample at 28 d. When the age was 3 d, the flexural strength of the G6 sample increased by 8.28% compared to the G0 sample, reaching 8.79 MPa. The flexural strength of the G4 sample reached the maximum at 7 d. The flexural strength of the G4 sample at 7 d increased by 13.43% compared with the G0 sample, reaching 9.46 MPa. The flexural strength of the G4 and G6 samples was slightly higher than the G0 sample. Based on the above results, it can come to a conclusion that adding a certain amount of MGO into cement mortar can improve the flexural strength of MGO-cement mortar at 3 and 7 d, but the contribution to the improvement of the flexural strength of MGO-cement mortar at 28 d is not significant. The influence of MGO on the flexural strength of MGO-cement mortar is similar to that of MGO on the compressive strength.
3.4 Microstructure characterization
3.4.1 X-ray diffraction analysis
The XRD patterns of cement mortar with different MGO content at 3 d are displayed in Figs. 8(a) and 8(b). The results show that the peak positions of samples with different MGO content were the same, which indicates that the types of hydration products of cement mortar were not changed by adding MGO. Because specimens contain standard sand, there were many diffraction peaks of SiO
2 crystals in the test results. According to the enlarged drawing of diffraction peaks of CH crystals in Fig. 8(b) and the Debye-Scherrer equation, the grain size of CH crystals in cement mortar with different MGO content had little difference, which indicates that MGO did not change the grain size of hydration crystals. The grain size will affect the mechanical properties of the cement mortar. The larger the grain size is, the more serious the atomic mismatch at the grain boundary is, and the more prone to form microcracks, which leads to the decrease of strength. A previous study [
39] found that single-layer GO can change the grain size of CH, thus affecting the mechanical properties of cement mortar. In the current research, MGO did not improve the compressive and flexural strength of MGO-cement mortar by changing the grain size. The different thickness GO may attribute to the conflicting results. The Debye-Scherrer Eq. (1) is as follows:
where D is the grains size in the direction perpendicular to the crystal plane, λ is the wavelength of X-ray, β is the half-width of the diffraction peak, θ is the diffraction angle, and K is a constant equal to 0.89.
3.4.2 Morphology analysis
The mechanical properties of cement mortar are affected by its microstructural characteristics. Figures 9(a)–9(h) show the microstructure images of MGO-cement mortar at 3 d (variant GO content of 0, 0.02 wt%, 0.04 wt%, and 0.06 wt%). The hydration products of the G0 sample were relatively low in density and loose in the microstructure. The needle-like and polyhedral hydration products were disorderly distributed with many holes and pores inside. In contrast, the microstructure of the MGO-cement mortar was denser than the G0 sample. The hydration products in the shape of needle, flake, and gel were interwoven in the G2 sample. The pores in the MGO-cement mortar were smaller than those of the G0 sample, but there were still a few small pores and cavities. The gelatinous and flaky hydration products of the G4 sample were uniformly distributed.
Moreover, the hydration products of the G4 sample bridged to form a relatively regular structure. At MGO dosages of 0.06 wt%, the hydration products of MGO-cement mortar were connected as a whole and formed a more compact structure. In conclusion, MGO can accelerate the hydration rate and regulate the hydration products to form a more compact structure, thus improving the compressive and flexural strength of MGO-cement mortar.
The influence of MGO on the microstructure of cement mortar at 3 d is mainly related to the following two points. 1) MGO can provide more condensed cores and promote hydration reaction. First of all, previous studies [
41] found that the oxygen-containing functional groups of GO can provide condensed cores to accelerate the hydration rate. The results of XRD show that MGO had rich oxygen-containing functional groups. Secondly, the larger specific surface area of the smaller particles can also provide the condensed cores to accelerate hydration rate [
42], and the results of AFM showed that the size of MGO reached the nanoscale. According to two aspects, the incorporation of MGO promoted the hydration reaction and produced more hydration products. 2) Due to the template effect of MGO, the hydration products of MGO-cement mortar can grow more regularly, and the structure of MGO-cement mortar can be more compact [
10].
Figures 10(a)–10(d) show the effect of without MGO and different content of MGO (0.02 wt%, 0.04 wt%, and 0.06 wt%) on the microstructure of cement mortar at 28 d. It can be seen from SEM and EDS images that the G0 sample had a loose structure, and hydration products were disorderly distributed. Compared with the G0 sample, more CH crystals were formed in the G2 sample. The lamellar CH crystals in the G2 sample overlapped each other to form a structure. Table 3 shows that the weight percentage of Al in G4 and G6 samples was larger than that in the G0 sample, indicating that G4 and G6 samples generated more AFt than G0. Furthermore, the microstructure of G4 and G6 samples was denser than that of the G0 sample, but fractures and pores still existed.
From the above results, one can conclude that the content of CH crystals in specimens increased after incorporating 0.02 wt% MGO. A previous study [
43] showed that the content of CH crystals was closely associated with the hydration degree of cement mortar. More CH crystals were generated in the G2 sample, indicating that MGO promoted the hydration reaction and improved the degree of cement hydration. This finding is consistent with that reported in Ref. [
44] that the surface and interlayer of hydrophilic GO can absorb extra water molecules. For the later stage of curing, the free water required for further hydration will be released by GO. Due to the additional hydration, the content of CH crystals increased at 28 d.
3.4.3 Pore structure analysis
Cement mortar is a heterogeneous material with many pores inside. The porosity is closely associated with the mechanical properties of cement mortar. Therefore, studying the pore structure of cement mortar will contribute to understanding the influence of MGO on the mechanical properties of cement mortar. According to pore size, there are four categories of pores in cement mortar (gel pores, transition pores, capillary pores, and big pores). The pores smaller than 10 nm are gel pores, and the pores inside the gel particles and the pores between the hydrated calcium silicate gel particles are all gel pores. Transition pores are a type of pore with the size of 10–100 nm, and transition pores include the pore between the external hydration products. The size of capillary pores is between 100 and 1000 nm. Capillary pores mainly refer to the void which is not filled by hydration products and the void left by water-filled space. Big pores refer to pores larger than 1000 nm in size [
45]. Among these pores, the size less than 20 nm is harmless, and the sizes 20–50 nm, 50–200 nm, and greater than 200 nm are less harmful, harmful and more harmful, respectively [
46].
Figures 11(a) and 11(b) illustrate the effect of MGO on the pore structure of cement mortar at 28 d. From the pore size distribution curve in Fig. 11(a), it can be seen that the most probable pore size of G2 and G6 samples shifted toward a finer pore size relative to the G0 sample, and its peak value was higher than that of the G0 sample. Figure 11(b) cumulative pore volume distribution shows that the cumulative pore volume of G2 and G6 samples was higher than that of G0 sample in the range of 5–20 nm. The above results illustrate that the pore volume of the gel pores of cement mortar increased with the incorporation of MGO. And similar results [
9,
17] have been obtained by using single layer GO before. The promotion of MGO on cement hydration generates more C-S-H gel, which may increase the gel pore volume of MGO-cement mortar [
47]. In addition, the harmful pore volume (>50 nm) of the G2 sample was higher than that of the G0 sample. According to the SEM results, a lot of CH crystals were generated in the G2 sample. Therefore, the increase in harmful pore volume may result from the existence of a spatial structure formed by the overlapping CH crystals in the G2 sample. The harmful pore volume of G4 and G6 samples was similar to that of the G0 sample. The results of pore structure analysis are consistent with the results of macro mechanical tests.
4 Conclusions
In this study, the mechanical properties and microstructure of cement mortar with MGO content of 0, 0.02 wt%, 0.04 wt%, and 0.06 wt% were investigated by mechanical properties tests, XRD, SEM, and BET.
1) The shape of MGO was irregular, and the thickness of each part was not consistent. Its average thickness was about 6 nm, which shows that MGO has reached the nanometer scale. MGO had folds on the surface, which increases its specific surface area. In addition, the MGO surface was rich in oxygen-containing functional groups, such as –COOH, –OH, –C= O, etc. These characteristics of MGO can promote the hydration of cement and improve the compressive and flexural strength of cement mortar.
2) After adding MGO, the compressive and flexural strength of cement mortar increased at 3 and 7 d. MGO made little contribution to the increment of compressive and flexural strength of cement mortar at 28 d. The optimum MGO content of cement mortar with 3 d was 0.06 wt%, and the optimum MGO content of cement mortar with 7 and 28 d was 0.04 wt%.
3) The results of XRD show that MGO did not change the type of hydration products and grain size of CH crystals in cement mortar. Therefore, the improvement of compressive strength and flexural strength of MGO-cement mortar was not due to the formation of new types of hydration products or the change of grain size.
4) The microstructure of cement mortar at 3 d shows that the microstructure of MGO-cement mortar was denser than that in the G0 sample after incorporating MGO and became denser with the increase of dosage. The reason is that rich oxygen-containing functional groups of MGO and large specific surface area provided more condensed cores to accelerate the hydration rate. Furthermore, MGO had the role of template, which contributes to the regular growth of cement hydration products and makes the structure more compact. When the age is 28 d, adding 0.02 wt% MGO into cement mortar can promote the formation of CH crystals, and the incorporation of 0.04 wt% and 0.06 wt% MGO can make the hydration products grow regularly and improve the microstructure of cement mortar to a certain extent.
5) Pore structure analysis indicates that the promotion of MGO on cement hydration generates more C-S-H gel, which may increase the gel pore volume of MGO-cement mortar. The harmful pore volume of the G2 sample is higher than that of the G0 sample, which may be due to the existence of a spatial structure formed by the overlapping CH crystals. The harmful pore volume of the G4 and G6 samples is similar to that of the G0 sample. The results of pore structure analysis are consistent with the results of macro mechanical tests.
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