Research progress on properties of cement-based composites incorporating graphene oxide

6.1 Single effect of GO

GONS enhances the toughening effect on CBC, and in order to accurately investigate the mechanism of action, Lv et al. [61] incorporated GONSs into cement mortar and found that GONSs could significantly improve the strength of extension and the bending strength of plaster. Table 1 displays the mechanical performance of cement paste at various doses to GO. When the GO content was 0.03 wt%, the tensile strength, bending strength, and compressive strength of the cement mortar were increased by 65.5, 60.7, and 38.9%, respectively, in 28 days. Moreover, high-performance and ultra-high-performance CBC were prepared with varied GO contents, and the mechanical performance of CBC was significantly enhanced at varied water/cement ratios and GO contents [62].

Similar studies have been conducted by other scholars. Jiang [63] introduced two diffractions of graphene and found that both GONSs and GO were able to improve the bending and compressive properties of cement slurry; the strength enhancement effect of GO was better than those of graphene and nanosheets, especially for the early strength enhancement of cement mortar. Zeng et al. [64] conducted compressive and splitting tensile tests on concrete specimens with different GO dispersions and ages. Figure 9 shows the impact of age and GO content on the compressive strength of concrete, and it was found that compressive and splitting tensile strengths of the test samples are more significantly improved, which offers an investigational basis for preparing high strength, long-life concrete. Gui et al. [65] compared GO-modified recycled CBC with non-GO-doped recycled CBC, and found that 0.02% GO increased the bending and compressive strengths of GO/recycled CBC by 13.7 and 13.6%, respectively. Wang et al. [66] added GONSs to OPC to significantly improve the compressive strength and splitting tensile strength of CBC. The compressive strength and splitting tensile strength showed a tendency to increase first and then decrease with the increase of GONSs. The addition of 0. 02% (mass fraction) GONSs increased the compressive strength by 13%, and 0.03% (mass fraction) GONSs increased the splitting tensile strength by 41%.

Figure 9

Effect of age and GO content on the compressive strength of concrete [64].

The addition of steel fibers can improve the fracture properties of the material [67]. Just like steel fibers, GO may also enhance the crack resistance of the cement matrix. Li and Luo [68] tested the fracture performance of GO-reinforced cement slurry specimens by a three-point bending beam method, and the analysis of the double-K fracture model indicated that an appropriate GO content raised the fracture toughness of cement mortar specimens and had a good strengthening and toughening effect on the concrete mortar [69]. Figure 10 displays the impact of GO content to fracture energy of GO/cement slurry. Murthy et al. [70] and Murthy and Ganesh [71] also used this method to study the impact of steel fibers and nano-SiO₂ on the fracture performance in moderate- and high-strength concrete. When the GO content (mass ratio to CBC) was 0.03%, the crack initiation load was maximum, which represented a 30.3% increment in comparison to the control group; the crack initiation toughness and instability toughness were increased by 25.4 and 18.3%, respectively. He and Deng [72] used the ball milling method to prepare GONSs of different sizes and found that the bending and compressive strengths of CBC specimens with the addition of GO increased with the decrease in the GO size. However, regardless of the size, the optimal amount was 0.25% because as the amount of GO exceeded 0.25%, there was a corresponding decrease in the flexural strength of CBC. Wang [73] conducted 300 freeze–thaw cycles, and Figure 11 shows the compressive strength of cement slurry after freeze–thaw cycles. An appropriate amount of GO can protect against the expansion of cracks in cement mortar ranging from a scale of nanometers to microns, thus improving the mechanical performance; however, once the critical amount is exceeded, the GO lamellae in the cement matrix restack and produce micro-cracks, and the mechanical properties will be decreased [74].

Figure 10

Impact of GO content on the fracture energy of GO/cement slurry [68].

Figure 11

Compressive strength of cement mortar containing GO after freeze–thaw cycles [73].

The influence mechanism of GO on CBC differs under different curing systems. Lu et al. [75] studied the mechanical properties of GO-enhanced ultra-high performance concrete (UHPC) in both steam curing and standard curing. The results indicated that in both steam curing and standard curing conditions, compressive strength was enhanced by the incorporation of GO. The reason was that steam curing can accelerate the hydration rate and GO can promote hydration to some extent. Li et al. [76] used a uniaxial compressive acoustic emission test to study the influence law of GO with various doping amounts on mechanical properties and acoustic emission parameters (amplitude, ringing count, frequency) of concrete, and the results showed that the compressive strength of concrete at each age increased sequentially with the increasing amount of the GO admixture; the damage form of concrete had better integrity, the range of acoustic emission frequency of concrete widened gradually, and the dominant frequency of concrete became larger. Balasubramaniam et al. [77] evaluated the compressive and tensile strength enhancement with RAC incorporating 30% recycled coarse aggregate with varying amounts of rGO and found the optimum rGO percentage for RAC to be 0.6.

The type of cement was different, and GO had different impacts on its mechanical performance. The cement investigated was Portland cement, while the concrete was ordinary concrete [78]. Lu et al. [79] found the compressive and bending strength of magnesium potassium phosphate cement (MKPC) was improved, the hydration degree was, and the porosity was lower when an appropriate amount of GO was added. Du et al. [80,81] investigated the mechanical properties and microstructure of hybrid GO/CNTs on MKPC paste and analyzed the effect of different GO/CNT ratios. The results demonstrated that the reinforcement effect of hybrid GO/CNTs on mechanical properties was better than that of the same doping amount of single added GO and CNTs, respectively. Compared with MKPC, GO enhances the mechanical performance of Portland cement. Liu et al. [82] investigated GO concrete with different amounts of GO and found that with the increase of the GO amount, the fluidity, compressive strength, and bending strength of the composite materials tended to first increase and then decrease. Both slump and porosity show a trend of first decreasing and then increasing. Gong et al. [83] investigated the strengthening efficiency of CNTs and GO toward Portland cement. When 0.03% GO was added to Portland cement, its compressive strength and tensile strength increased by more than 40%. Liu et al. [84] incorporated 0.04 wt% of GO into sulfoaluminate cement (SAC) mortar. The results indicated that incorporation of trace amounts of GO resulted in banded AFt creation and development, and thereby improved the ultra-early strength of sulfoaluminate CBC. Table 2 provides the results of different researchers regarding the effect of GO on the improvement of the mechanical performance of the material.

6.2 Synergistic effect of GO with dispersants

Strong interactions between GO and sand particles were observed [85]. In order to make GO uniformly and stably distributed in CBC, dispersants are often added, which not only play a dispersive role but also have no negative impact on the enhancement effect of flexural, tensile, and compressive strength of GO-doped CBC.

With the increasingly superior performance of dispersant products on the market, polycarboxylate superplasticizers (PCs) with good compatibility with cement systems are selected by many investigators because of their high properties and simplicity of use [86]. Table 2 shows the compressive and bending strengths of GO-doped CBC with the addition of dispersants by different researchers. Babak et al. [87] used PC to improve GONSs’ dispersity in cement and studies the effects of GO and superplasticizers on the mechanical performance of cementitious nanocomposites. The tensile strength of cement slurry increases as the GO amount increases, and increases by 48% when the GO content reaches 1.5%. Both Gao and Ma [50] and Lv et al. [88] used the copolymerization reaction of polyacrylic acid-based water-reducing agents (PCs) and GO to form GO-PCs, which improved the dispersive effectiveness of GO in cement slurry and achieved enhancement of CBC using a small amount of GO. The corresponding data in Table 3 show that the dispersant in CBC can improve the dispersion effect of GO after incorporation of GO; an enhancement of cementitious materials was achieved by using a small amount of GO. Similarly, Li et al. [89,90] studied the mechanical performance and strengthening regime of PC-modified GO-reinforced CBC. PC@GO was dispersed uniformly in the alkaline cement matrix and increased the mechanical performance. The compressive strength of cement with 0.242% PC@GO (0.22% for PC and 0.022% for GO) reached 25.59% and flexural strength increased by 24.56%.

GO can also be introduced into CBC via a copolymerization reaction between GO and dispersions to produce a copolymer. Zhao et al. [91] prepared poly(AA-GO), a copolymer of acrylic acid (AA) and GO, by the copolymerization reaction of GO sheet dispersion with AA. The results of Table 2 show that the compressive and flexural strengths were dramatically increased, especially the flexural strength increased significantly. When 0.03% of GO was incorporated, the compressive and flexural strength of CBC improved up to 80.1 and 105%, respectively, in contrast to the controlled sample without GO addition at 28 days, indicating that the strengthening and toughening effect of GO could be improved by the poly(AA-GO) copolymer. Lv et al. [92] compared CBC with GO/P(AA-AM) and the aqueous dispersions directly incorporated with GO. Compared with the blank control sample, CBC could be uniformly distributed in the form of a few lamellae [43], and the compressive strength and bending strength of CBC significantly increased. Ding [93] prepared naphthalene water reducing agent/graphene oxide (NS/GO) composites by the solution blending method, and investigated the influence of various doping amounts of composites on the shape of cement hydration products, the net cement paste flow, the setting time, the cementite pore structure, and mechanical properties. The relevant results of the experiments are displayed in Table 2. When the NS/GO amount was 4.4%/0.06% (mass fraction of cement), the setting time of the cement paste was decreased, the harmless pore porosity of cementite increased, the compressive strength reached 65.4 MPa, the flexural strength reached 10.6 MPa, and compared to the control sample, there was an increase of 29.2 and 29.3%, respectively.

To sum up, GO could obviously ameliorate the mechanical performance of CBC, mainly including flexural, tensile, and compressive strengths, with some improvement in ductility and toughness [36,48]. However, a wide range of GO content has been used in various experiments, and there is no exact relationship between GO content and reinforcement efficiency, and more systematic and in-depth studies are needed.

7.1 Effect of GO on permeability resistance

The permeability resistance of CBC refers to their ability to resist the invasion of various media, including the penetration of gases, liquids under pressure, or ions in CBC under a chemical potential or electric field. The permeability of CBC at the macroscopic level depends on the pore structure at the microscopic level, and the migration of liquid within them depends largely on the liquid conductivity, also called the permeability coefficient [95].

There are three test methods mostly used to evaluate the permeability resistance of CBC including the permeability coefficient method, the ionic diffusion coefficient method, and the electrical parameter method. Since the incorporation of GO significantly changes the electrical conductivity of the specimen, Lin [96] used the permeability coefficient method to conclude that GO and CNTs can significantly reduce the permeability coefficient of the specimen. However, he found that the porosity of concrete increases with the increase of porosity, and the two are not simply linear; instead, the porosity and pore linkage and curvature are jointly determined. Zhang et al. [97,98] found that modified gelling materials covered with polymeric coatings had lower penetration heights and chloride diffusion coefficients than uncoated polymeric coatings, and the carbonation depth and weight loss after freeze–thaw cycles were reduced. Du [99] examined the impact of concrete permeability with various sizes of GO by determining the diffusion coefficient of chloride ions. The experiment results indicate that as the size of GO increases, the barrier effect on micro-cracks in cement stone also increases, which helps to improve the permeability resistance of concrete. Figure 12 displays the impact of GO on the chloride ion diffusion coefficient. In Figure 12, PC is the blank sample and the rest are 0.1% pure GO slurry specimens, in which the HGC sample is mixed with GO obtained by the oxidation of improved Hummers, the TGC sample is mixed with GO purchased from the Chengdu Organic Chemistry Company of Chinese Academy of Sciences, and the LGC sample is mixed with large GO prepared by the thermal expansion method.

Figure 12

Effect of GO on the diffusion coefficient of chloride ions [99].

7.2 Effects of GO on the freeze–thaw resistance of CBC

The severe cold climate will cause freeze–thaw damage to hydraulic buildings, and freeze–thaw resistance is an important element of durability. Lin [96] and Gong and Lin [100] conducted freeze–thaw cycle tests on 300 specimens and quantitatively analyzed the total pore volume of each group of specimens. With the increase of GO doping, the total pore volume roughly tends to decrease first and then increase slightly, reaching a minimum value of 0.06% GO doping. Figure 13 shows the porosimetry results of different specimens. Similar experiments were carried out by Xu and Fang [101]. The results showed that GO incorporation could enhance the compressive strength and strengthen the deformation resistance of salt-frozen concrete. At 0.03% GO, the concrete had the least salt-frozen damage and the best salt-frozen resistance.

Figure 13

Porosimetry results of different specimens [96].

The incorporation of GO improves the freeze–thaw resistance of cement paste, and Tong et al. [102] and Lv et al. [103] confirmed this conclusion. Li et al. [104] prepared a new type of CBC by mixing GO and slag powder (SP) and conducted a systematic study on its fluidity and freeze–thaw damage characteristics. The results demonstrated that the cooperative interaction of GO and SP may be effective in improving the freeze–thaw damage mechanical properties of CBC. From the test results of mechanical properties of fluidity and freeze–thaw damage, it was concluded that 0.05% GO and 50% SP have the most obvious synergistic effect on CBC.

7.3 Effect of GO on the transport properties of CBC

The transportation process of concrete is caused by different dynamics (such as pressure difference, concentration difference, etc.) under certain conditions. There are mainly two different views on the relationship between transportability and concrete strength. One point of view is that the compressive strength is part of the determining element that affects concrete permeability, and the permeability decreases with the increase of compressive strength but the specific relationship differs. Although the concrete durability damage mechanism varies and is very complex, the common denominator is the ability to transmit with the concrete.

Recent studies have shown that the key to obtaining high durability and long life of concrete is to improve the permeability resistance of concrete [105]. Mohammed et al. [106] have carried out extensive work on the impact of GO integration into the cement matrix and its transport properties by water adsorption, chloride ion penetration, and mercury pressure tests. GO was dispersed into cement mortar and GO cement composites were prepared at 0.01, 0.03, and 0.06% cement admixtures. Figure 14 displays the water absorbance of all compounds as a function of the square root of time, and Figure 15 displays the chlorine depth permeation plots of all compounds. It is evident that the very low level of GO (0.01%) prevented the entry of chloride ions, and the addition of 0.03% GO could remarkably enhance the adsorption properties. Therefore, the addition of GO into the cement matrix could effectively lead to the improvement of the transport properties of the cement matrix and thus enhance its durability.

Figure 14

Water absorbance of the cement mixture with various GO contents [106].

Figure 15

Chlorine permeation depth of the cement mixture with various GO contents [106].

7.4 Effect of GO on the erosion resistance of CBC

The erosion resistance of CBC generally refers to the ability of the material to resist the action of erosive media. Fang [107] dispersed GO and modified graphene oxide (FGO) into acrylate monomers and prepared GO/low surface energy composite acrylic resin coatings by in situ polymerization. The results showed that GO and FGO effectively enhanced the toughness and resistance of the coatings to seawater erosion; the dispersion performance of FGO in the resin was superior to GO. GO was compared with traditional grinding aids triethanolamine and propanetriol by Yang et al. [108]. They found that GO also had similar grinding aids. The improvement in the compressive strength and fluidity of the concrete was achieved, as well as a remarkable increase in the erosion resistance factor. This is attributed to the fact that GO contains polar anionic reactive groups OH⁻ and COOH⁻, which can neutralize the positive charge on the surface of slag cement granules and adsorb them on the surface of slag cement granules. In this way, it prevents the intrusion of OH⁻ and SO²⁻ ₄ in the solution and improves the erosion resistance of the concrete slurry. Li [109] investigated the effect of GO content on the mechanical performance, microstructure, and resistance to salt solution erosion of CBC, and the results indicated that the GO content in a definite range could reduce the porosity of CBC and prevent the erosion of Cl⁻ and SO²⁻ ₄ and reduce the mass loss and the erosion of CBC impregnated in the salt solution for a long time. The mass loss and decay of mechanical properties in a salt solution significantly improved the durability.

The rapid chloride migration method is often applied to assess the concrete strength against chloride ion penetration. Xue [110] used a chloride diffusion coefficient meter to determine the effect of GO on the chloride erosion resistance of CBC and expressed chloride ion attack-resistant properties of concrete in terms of non-stationary chloride ion migration coefficient. The findings revealed that compared with CBC without GO, the unsteady chlorine ion mobility coefficient of CBC incorporated GO gradually decreased as the GO amount increased and the chloride ion erosion depth gradually decreased and was evenly distributed. Du [99] performed intensive research on the influence of GO on general concrete performance. It was found that at a 0.4 water/cement ratio, the chloride diffusion coefficient of concrete gradually grows with the increase of GO. This indicated that GO could significantly improve the chloride ion erosion resistance of CBC.

To further characterize the collective effect of graphene nanosheets (GONs) and GONS, especially the effect of their content on concrete properties, Liu et al. [111] conducted a comprehensive investigation. Through a series of tests on slurry specimens with various water/cement ratios and various levels in GONs and GONS, the strength and resistivity were determined in a well-structured manner, and the process of piezoresistive reaction was investigated. Figure 16 shows the penetration depth of concrete with different GON contents along with the corroded days. It can be inferred from the figure that the concrete water/cement ratio and workability are significant to utilize the benefits of GONs and GONSs. Low proportions of GONs and GONS dramatically increase the concrete strength at an appropriate water/cement ratio. However, to obtain the required low resistance and precise piezoresistive response, the GON levels should be increased considerably. Additionally, rapid chloride corrosion experiments indicated that there was a decrease of chloride migration as GONs were added to the cement slurry.

Figure 16

Penetration depth along with corroded days [111].

7.5 Effects of GO on the carbonation resistance of CBC

Carbonation of concrete is a chemical corrosion of the concrete, also known as neutralization. The chemistry between CO₂ gases in the atmosphere and alkaline substances produces carbonate and water, making the concrete less alkaline in a process called concrete carbonation. The concrete carbonation process can be divided into three stages: cement hydration, CO₂ diffusion, and carbonation of carbonizable substances. These three stages proceed gradually from the surface of the concrete to the interior, with no clear boundaries. Concrete carbonation and its resulting corrosion of reinforcement are both important concrete durability problems. However, there is no report on the effect of carbonization on GO concrete on its performance, and the corrosion of reinforcement inside the GO concrete caused by carbonization is still not clear [112].

Compared with fly ash, slag, silica powder, SiO₂, and TiO₂ nanoparticles, there is very little domestic and international research literature on the carbonation resistance performance of GO-doped CBC. Only Lv et al. [103] prepared GO dispersions with good dispersion in CBC and mixed them into cement mortar. Table 4 shows the carbonation resistance testing results on GON/cement composites. The results showed that the carbonization depth of the GO cement mortar was 0.6 and 1.2 mm on 7 and 28 days, which was significantly below those of the common cement mortar (2.8 and 3.5 mm) during the same period. This is due to the ability of GO to regularize the formation of flower-shaped or multifaceted cement hydration products, which can form a regular and orderly microstructure after aggregation, and thus, has a significant ability to reduce pores and cracks and improve the carbonation resistance of cement mortar.

8.1 Scanning electron microscopy (SEM)

To illustrate GO nanosheet layers’ modulating effect on cement hydration products and their microstructures in CBC, Lv et al. [113] performed SEM analysis of CBC without GO and doped with 0.03% GO. Figure 17 displays the SEM morphology of CBC without GO nanospheres, and Figure 18 displays the SEM morphology of CBC doped with 0.03% GO. From the SEM images, it can be observed that the microstructure of CBC without GO nanosheet layer doping is irregular in shape overall, with large cracks. The hydration products are less in number, unevenly distributed, disorderly accumulated, and mainly needle-like, rod-like, and sheet-like. In contrast, for the small magnification SEM images of CBC doped with GONSs (Figure 18a), the overall microstructure was significantly improved, the morphology was regular and dense, and no clumped or isolated hydration products were found to exist, and no obvious cracks and pores existed. Figure 18b–f shows the microscopic morphology of different parts of Figure 8a after local enlargement. It can be observed that the microstructure with regular morphology was formed by clusters of polyhedral-like hydration crystals by intertwining and penetrating each other [114].

Figure 17

SEM images of control specimens without GO and after 28 days. Magnification: (a) 500×; (b) 5,000×; and (c) 200,000× [113].

Figure 18

SEM images of CBC doping with 0.03% GO at 28 days. (a) Large- scale regular microstructure; (b–d) flower-like motifs in different positions; (e) interwoven structure of polyhedron-like cluster crystals; and (f) aggregation structure of polyhedron-like crystals [113].

By using SEM analysis, Gao [115] analyzed the SEM images of cement stone hydrated with 0.05% GO for 7 days and cement net slurry hydrated with no GO for comparative observation. It can be observed that the microstructure of GO-doped cementite is denser and the pores are more uniform and smaller than those of the undoped cementite in cross-sectional morphology. Moreover, calcium hydroxide was significantly reduced in the hydration products of GO-doped cement net slurry, which caused substantial changes in the cement mechanical performance, indicating that the impact of GO on cement hydration was primarily manifested in its mechanical properties. In conclusion, the SEM images of the cement matrix doped with GONSs show that GONSs may modulate the cement hydration products into regular-shaped crystals, including polyhedra and rods, and aggregated into regular and ordered microscopic morphologies.

8.2 Energy dispersion spectroscopy (EDS)

EDS features easy operation, fast analysis, and intuitive results. Chintalapudi and Pannem [116,117] detected a natural mineral called wollastonite (CaSiO₃) in all OPC samples doped with GO. This implies that SiO₂ reacts with water to form calcium silicate hydrate (C–S–H), which confers strength to the hydrated products such as silicate cement. Peng et al. [118], in their study, analyzed the EDS energy spectrum on the marked points in the SEM images of different GO-enhanced cement mortar contents at a water/cement ratio of 0.35 in order to deeply study the reasons for the formation of microscopic morphology. The analysis data are shown in Table 5. After GO incorporation, the elemental composition of C–S–H at the flower-shaped morphological center appeared to be C, the proportion of C elements was larger and the proportion of O elements increased. This may be attributed to the fact that GO was more abundant in the center of the flower morphology after incorporation. With the large surface energy, GO adsorbed extensive water molecules and ions, which served as growth points for hydration products, enabling hydration crystals to be formed in a regular and refined manner [119]. Thereby, GO was almost integrated with the set-hardened hydrides and the aggregates formed a stable multiphase network structure, which greatly improved the cementitious sand structure. This indicated that the GO nanosheet layers were doped by the regulation of the cement hydration product shape and the macroscopic properties of CBC were improved.

8.3 X-ray diffraction (XRD)

XRD image analysis is usually used for assessing the hydration level of the concrete. The XRD analysis of graphite and GO was carried out by Peng et al. [120] and Pang and Wang [121]. From the XRD spectra of graphite and GO in Figure 19, it is evident that the GO layer distance increased from 0.33855 to 0.83859 nm, and this increase of the interlayer distance indicated that oxygen-containing groups were introduced into the carbon atom layer following the oxidation of graphite. Under the influence of oxidation, oxygen can interact with graphite through covalent bonding, which can cause the original stacking structure of graphene sheets to be altered, thus resulting in a significant movement of the GO diffraction peaks. Zeng et al. [47] performed the XRD structure analysis of cementitious sand with 0 and 0.1% GO doping at 3 and 28 days of aging, respectively. The analysis of experimental results showed that when the ages were the same, the cement mortar mixed with 0.1% GO had higher diffraction intensity than the hydration products AFt, AFm, CH, and C–S–H from blank specimens. This showed that GO mixed into the cement mortar helped the growth of hydrated crystals. With the increase of age, the contents of unhydrated tricalcium silicate, tricalcium aluminate, C–S–H gel, and the gypsum of cement were stronger than the diffraction intensity of GO blending. Chintalapudi and Pannem [122,123] also observed significant calcite deposition and CaCO₃ precipitation in the characteristic peaks confirmed by XRD. These results indicated that the reactive oxygen-containing groups incorporated into GO blending have a catalysis effect on cement hydration.

Figure 19

XRD spectra of graphite and GO [121].

8.4 Mercury intrusion porosimetry

The microstructure of CBC is heterogeneous, and there are a certain number of voids in them, including pores, capillary pores, and gel pores. These voids are intimately associated with the structure and macroscopic characteristics of the material. In the microstructure, the pore size, pore distribution, pore arrangement, and connection are random [124]. The pore structure is an important part of the microstructure of CBC. It mainly includes the pore area, median pore size, pore size distribution, average pore size, and porosity [43]. The permeability of CBC is strongly correlated with inner microstructural features like pore diameter allocation, porosity, and critical pore diameter [125,126]. The influence of GO size on CBC can be illustrated by the pore structure. Figure 20 shows the pore size distribution of CBC at 28 days, and the findings revealed that GO in the size range of 5–410 nm had a strong ability to reduce the pores. Table 6 shows the results of the pore structure analysis of GO-doped CBC. It is evident from the table that the pore area, median pore size, pore size distribution, average pore size, and porosity of GO-doped CBC significantly decrease [43]. This conclusion was echoed by the trial findings of Liu et al. [127].

Figure 20

Pore size distribution of CBC at 28 days [43].

8.5 Fourier-transform infrared spectroscopy (FTIR)

A modern and essential analytical tool and method is FTIR analysis. This method is used for assessing hydration processes in composites, compositional analysis of samples, etc. [125]. Both Peng et al. [120] and Pang and Wang [121] conducted FTIR tests on GO. In these two experimental results, the number of vibrational peak waves for each group and the vibrational peaks are also very similar. Figure 21 displays the FTIR spectra of graphite and GO. The presence of a characteristic peak suggested that oxygen-containing groups, which were incorporated during GO fabrication, like carboxyl, epoxy, and carboxyl groups, indicate successful oxidation of graphite. The interfacial interaction between carboxyl groups and hydration products was revealed by the analysis of Zhao et al. [128]. They suggested that such interaction is the cause of the strengthened mechanical performance of GO-doped CBC. By FTIR analysis, Chintalapudi and Pannem [129,130] concluded that the O-containing functional groups in hydrated cement mortars exhibit GO reaction properties at an optimal dosage to regulate the precipitation of CaCO₃. However, the C–S–H structure is not changed in GO cement-based materials; this conclusion is derived from the study of Yang et al. [131].

Figure 21

FTIR spectra of graphite and GO [120].

9.1 Filling effect

One of the reasons for the high brittleness and low toughness of CBC is that the cement matrix contains numerous open and closed pores, which are mostly micron-sized. The commonly used methods of fiber doping are difficult to fill these harmful pores due to the large fiber size and may cause the pores to extend in the direction of fiber distribution to form cracks. Thanks to the layered morphology of GO and the fact that the GO dimension is several magnitudes smaller than that of the hazardous pores, GO can play a more adequate “filling role” in CBC [135].

To further explore the regulatory mechanism, Lv et al. [103] regulated cement hydration products by adjusting the size of GONSs. The results revealed that when the dimensions of GONSs were 5–140, 5–260, and 5–410 nm, the cement hydration crystal products with nanometer needle-like, petal-like, and polyhedral shapes could be obtained, respectively. GO interaction with cement in the hydration process accelerates the aggregation effect which significantly increases toughness by dense structure formation of the hydration product [116]. This conclusion was confirmed by the results of Du, for GO at a small radius, the influence on flower-shaped hydrated crystal creation is significant at low ratios, whereas the influence on regular polyhedral crystal creation is significant at high ratios [136]. GONSs can facilitate flower-shaped and polyhedral structure formation through rod-shaped crystals, which eventually form a dense structure. The cement slurry pore structure plays a significant role in hydration crystal development and morphology. It is easy to produce flower-like crystals in pores and fractures of cement slurry, creating multi-point networks in pores and fractures, thus reducing porosity and the pore radius. In the end, a dense and interconnected structure is inclined to result for all crystal types [137]. The forming mechanism of the petal-like organized microstructure of CBC hydration products is shown in Figure 22 [103].

Figure 22

Schematic diagram of the formation mechanism of order microstructure with flower-like patterns: (a) hydration reaction; (b) assembling influence of GONs; (c) flower-like crystals; (d and e) assembling influence of GONs; and (f) flower-like patterns [103].

It is worth mentioning that Wang et al. [138] submitted an imaginative 3D mechanism model of GO-regulated structure, as shown in Figure 23, which suggests that the surface of the GO lamella contains abundant oxygen-containing groups and –COOH at the edge of the lamellae can chemically react with the cement hydration product Ca(OH)₂. To form a 3D network structure, GO layers were connected in both vertical and horizontal directions. The COO–Ca–OOC structure was formed by Ca²⁺ of the hydration product Ca(OH)₂ and –COOH at the GO edge to link the 3D mesh architecture. Simultaneously, the hydration product directly pervaded the 3D architecture to densify the microstructure, thus CBC can be strengthened and toughened.

Figure 23

Model of GONS-modified cement [138].

9.2 Template role

When the cement comes in contact with water, the hydration reaction occurs and the reactive groups contained in the GO participate in the hydration reaction. The hydration products are preferentially fixed on the GO lamellae to form growth sites, and the hydration crystals CH, Aft, and AFm are formed on the GO lamellae template with initial flower-like morphology. Lv et al. [137] suggested that the incorporation of GO accelerates nucleation site formation for the hydration process and binds the hydration products together to form a dense microstructure. Lin et al. [139] showed that during cement hydration, oxygen-containing functional groups provided adsorption sites for cement, formed nucleation sites for cement water compounds, and water molecules on GO constituted reservoirs and water transport channels for further hydration of cement. Li et al. [140] observed the acceleration of the hydration reaction by adding GO to a Portland cement slurry. Characterized by XRD and FTIR, they concluded that GO offered a point where water molecules and cement components could be adsorbed; in addition, the cement hydration rate increased as a result of the abundant of oxygen-containing functional groups at the GO surface.

9.3 Ink bottle effect

The “ink bottle effect” of GO in CBC is a special phenomenon under the synergistic effect of the “filling effect” and “template effect.” GO is not directly involved in the cement hydration reaction. The strengthening and toughening effect of GO on CBC mainly relies on the filling of GO nanoscale particles and the regulation of the growth pattern and morphology of hydration products. Thus, the pore structure and pore distribution of CBC are optimized and the microstructure of CBC is more compact.

The study of the hydration process and hydration products of GO cement by Wang et al. [48] indicated that GO had an important influence on structural characteristics of the gel pores formed after cement paste hardening, and with increasing content of GO, it can make more free water and refine and close the pore structure to a certain extent. However, a different view was held by Jing [141], who investigated the effect of rGO and GO on the hydration process of cement. They revealed that hydration exothermic curves of cement specimens doped with both graphene materials had essentially the same shape compared to the control group, and the chemically bound water content was also approximately the same. Therefore, it was concluded that rGO and GO had little effect on cement hydration properties.

Similarly, Wang et al. [142] compared the hydration heat curves of Portland cement with different GO contents, as shown in Figure 24. They found that the heat of hydration curves of the cement paste with GO was almost the same as those of the control group. Therefore, they concluded that GO did not have an obvious influence on the cement hydration process; in other words, the mechanical and durability performance improvement of CBC had no obvious relationship with the influence of the hydration rate. Currently, much contradiction exists in the research regarding the impact of GO on the cement hydration process. More experiments and studies are needed to further analyze the mechanism.

Figure 24

Impact of GO on the hydration exotherm rate of cement pastes [142].

10.1 Conclusions

By combining the current research status of GO-doped CBC in terms of workability, mentioned stability, mechanical properties, durability performance, and microscopic action mechanism, the conclusions are as follows:

1. The incorporation of GO into CBC can consume more water, resulting in the loss of workability and a decrease in the fluidity of the material. However, the incorporation of appropriate water-reducing agents could compensate for the effect of GO on the fluidity of CBC.

2. The addition of GO increases the elastic modulus and improves the ability of CBC to resist elastic deformation. The creep coefficient and creep strain of concrete is significantly decreased. The shrinkage strain and auto-shrinkage are increased and the volume stability of CBC is improved.

3. The incorporation of GO could significantly enhance the tensile, compressive, and flexural strengths of CBC but the degree of improvement of flexural strength is greater than that of the compressive strength. Moreover, the enhancement effect was more significant when GO acted in concert with the dispersant.

4. The durability of CBC involves a wide range. The influence of GO on the durability of CBC is mainly reflected in the corrosion resistance, permeability resistance, freezing resistance, carbonization resistance, and transport performance, which have all been improved.

5. GO can promote flower-like crystals formation, fill the pores and cracks of the cement paste, and reduce porosity. Meanwhile, oxygen-containing functional groups at the GO surface provide adsorption sites for cement, accelerate the hydration rate, and make the microstructure of CBC more compact.

10.2 Future perspectives

Although many meaningful results have been obtained in the study of GO-doped CBC, some aspects need further research work in the following aspects:

1. Solving the dispersion problem: GO agglomeration has a serious effect on the properties of CBC. Although investigators had already suggested approaches like copolymer modification and water-reducing agent incorporation, yet viability for such approaches is not applied to large-scale engineering applications.

2. Improving the problem of decreased fluidity: Although some methods have been proposed to compensate for the decrease of the fluidity of CBC, such as adding a water-reducing agent, the GO addition, nevertheless, exacerbates fluidity loss and complicates construction over time.

3. Volume stability aspects: The volume stability of CBC is directly related to the cracking sensitivity of concrete structures, and various deformation mechanisms are different. At present, there are few reports on the volume stability of GO-doped CBC.

4. Long-term mechanical aspects: There are relatively more studies on the mechanical properties of GO-doped CBC but none of them involves the development trend of long-term mechanical properties. The mechanical properties of GO-doped CBC under high-temperature conditions also deserve in-depth study.

5. Durability aspects: The research on GO-doped CBC in terms of carbonation resistance, alkali–aggregate reaction, long-term performance, and other durability studies are basically in the initial stage. In addition, there is a lack of in-depth mechanism analysis and corresponding model construction, and the durability investigation needs to cover a longer period.

6. Modulation mechanism aspects: The mechanism of GO regulating the strengthening and toughening of hydration products is not yet perfect, and the strengthening mechanism of GO-doped CBC and the interface interaction need to be clarified by further studies between GO and hydration products.

7. Promotion and application: The current research objects are mostly the application of GO in cement paste, while the most used in practical engineering is concrete. Future research should focus on the application of GO in concrete, which would lay a solid foundation for the practical application of GO-doped CBC.