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

: Graphene oxide (GO) is a two-dimensional derivative of graphene material, with carboxy, hydroxy group functional groups at the middle of the sheets, and oxygen-containing functional groups at sheet edges. It has multiple advantages, such as high strength, hydrophilicity, and strong reactivity. With the development of construction materials, GO has been widely used as a nano-reinforced material in cement-based composites (CBCs). Based on a large amount of relevant literature, the preparation and dispersion behavior of GO-reinforced CBC are summarized. Besides, the impact of GO on the workability, volume stability, mechanical performance, and durability of CBC are discussed. Moreover, the in ﬂ uencing mechanism of GO on the hydration of CBC is expounded. From the ﬁ ndings of this review, the following conclusions can be drawn: the ﬂ uidity of CBC will be decreased when GO is evenly dispersed in the cement slurry, which results in a loss of workability of CBC. Meanwhile, the addition of GO improves the volume stability of CBC, while the tensile, compressive, and ﬂ exural strengths are all improved to varying degrees. The improvement of GO on the durability of CBC is mainly re ﬂ ected in the corrosion resistance and permeability resistance. In addition, problems existing in the current research are summarized and future perspectives are put forward. The review work in this article could o ﬀ er important guidance for further research and implementation of GO-doped CBC in practical engineering.


Introduction
The role of cement-based composites (CBCs) is crucial in the construction sector, and global cement production has exceeded 3.6 billion tons, especially in rapidly developing China, which accounts for about 60% of global cement production [1,2]. Cement has low flexural strength and poor cracking and deformation resistance; therefore, as society has progressed, it has become more difficult to fulfill the criteria of the working performance of infrastructure systems [3][4][5][6], and the deformability, mechanical properties, ductility, toughness and bonding ability of conventional CBC need to be improved [7]. In actual construction, more additives such as steel fiber and carbon fiber are introduced to improve the performance of CBC but the performance improvement is all at the macro level and the problem of cracks and holes at the micro level still exists.
Concrete is a kind of common CBC, consisting mainly of a cementitious substance and the aggregate particles or fragments cemented in it. Concrete has not only high compressive strength but also low tensile and anti-cracking properties. How to improve the various performance of CBC is the main problem of current research. The key to solving these problems is to improve mechanical performance and durability, resulting in increased strength, toughness, and service life [8].
With the development of nanoscience and technology, on the one hand, people gradually discovered certain special effects of nanomaterials, such as small size effect, quantum effect, and nucleation effect [9]; on the other hand, C-S-H gels, the main hydration productions of CBC, are nanoscale in size. The application of nanomaterials provides new ways to enhance the mechanical performance and durability performance in CBC [10,11]. Moreover, the nano-admixture can be used to accelerate the increase in strength in CBC [12,13]. A thorough examination of the characteristics of fly ash-based geopolymers with nano-SiO 2 addition was carried out by Han et al. [14]. Zhang et al. [15][16][17] investigated the compressive strength and the fluidity of geopolymer mortars incorporating polyvinyl alcohol fibers and nanoparticles, respectively.
The mechanical performance and microstructure in recycled aggregate concrete (RAC) reinforced by nano-SiO 2 and basalt fibers were investigated by Zheng et al. [18]. The addition of carbon nanotubes (CNTs) could be used to promote early-age creep as well as enhance the cracking resistance in concrete [19]. All types of CNTs incorporated further reduce the pore content and size of the reactive powder concrete, thus improving their matrix compactness [20]. For example, reactive powder concrete's mechanical performance may be greatly improved by adding just 0.075 vol% of nickel-plated CNTs (0.03 vol% of CNTs) [21]. The incorporation of nano-SiO 2 can increase the engineered CBC after exposure to high temperatures [22]. Compared to the controlling group under a naturally occurring environment, the mechanical performance of geopolymer concrete reinforced with nano-SiO 2 was decreased under the coupling role of humid heat and chloride salt environment [23]. Nano-solution strengthening can fill the microcracks and pores in RAC and effectively improve the mechanical properties, particularly the early strength [24].
Based on morphological characteristics, nanomaterials could potentially be categorized into zero-dimensional (0D) particles, such as atomic clusters, one-dimensional (1D) fibers, such as CNTs, and two-dimensional (2D) sheets, such as graphene. Graphene is composed of a tightly packed honeycomb lattice, which is only one carbon atom thick and has an extremely large specific surface area [25], so it also has a very large contact surface with cement particles and can be effectively cemented with cement [26]. However, the potential of graphene to improve concrete properties is hindered by its high cost, insolubility in water, and tendency to agglomerate in water [9]. Among graphene-based nanomaterials investigated so far, graphene oxide (GO) accounts for the largest proportion [27]. GO is a kind of graphene derivative and is an intermediate product of the graphene oxidation process [28]. The representations of graphene and its derived substances are shown in Figure 1. GO has a honeycomb structure and is more active than graphene. After oxidation, it still maintains the monolayer flake structure of graphite. The structure of GO contains hydroxyl and carboxyl groups, etc., so it can exhibit better hydrophilicity. At the same time, GO has ultra-high strength and flexibility, and high specific surface area. GO has excellent application value in electricity, mechanics, thermology, and other aspects because of its unique structure [29].
GO incorporation into CBC could enhance the mechanical performance of the matrix, durability performance, etc. Many researchers in domestic and foreign countries have conducted experimental studies on this. However, at present, a review of the properties of GO-doped CBC has not been found. Therefore, for further research and application of GO-doped CBC in practical engineering, this review article provides a review of research results on GO in CBC by domestic and foreign researchers. In this article, the dispersion behavior and mechanism of GO action in CBC are described, and the implications of GO on the workability, bulk stability, durability performance, and mechanical performance of CBC are discussed. In addition, problems existing in the current research are summarized and future perspectives are put forward.

Preparation methods of GO
From the mid-nineteenth to the mid-twentieth century, researchers established three main methods for the preparation of GO by liquid-phase chemical oxidation, including the Brodie approach [30], the Staudenmaier approach [31], and the Hummers approach [32].
The Brodie approach uses a concentrated nitric acid system and it adopts potassium chlorate as the oxidizing agent to obtain GO with a molecular composition of C 8 O 2.13 H 1.60 by three consecutive oxidation treatments of the washed and dried yellow solid. The Staudenmaier approach uses a concentrated sulfuric acid system, which uses perchlorate and fuming nitric acid as oxidizing agents, and the temperature of the reaction system needs to be always maintained at 0°C. Perchlorate was added several times, and the final degree of oxidation of graphite could be controlled by controlling the reaction time. The Hummers approach uses concentrated sulfuric acid and nitrate system that takes permanganate as an oxidant, and the reaction process can be divided into three stages, including low-temperature (below 4°C), medium-temperature (around 35°C), and high-temperature (below 98°C) reaction. The preparation of GO by the Brodie and Staudenmaier approaches will produce toxic and hazardous substances such as nitrogen oxides and ClO 2 , which can pollute the environment. The Hummers approach is one of the most popular approaches because of its simple reaction process, short reaction time, high safety, and low environmental pollution.
The difference between the above three approaches is essentially the difference in the oxidant. The oxidant used in the three approaches has high oxidizing properties and also brings about high pollution and explosive problems. To solve these drawbacks, various modified Hummers approaches have emerged one after another. Marcano et al. [33] used weakly corrosive phosphoric acid instead of nitric acid for the pre-oxidation of flake graphite and then continued the oxidation with the strong oxidant KMnO 4 . In the whole process, NaNO 3 was not added, and using this approach GO could be prepared that contained substantial hydrophilic groups and a high oxidation degree. Shen et al. [34] were the first to use dibenzoyl per oxide as an oxidant to prepare GO, which shortened the reaction time. Chen et al. [35] directly removed sodium nitrate and prepared GO with the same structure and chemical properties as the Hummers approach. The current improvements to the Hummers approach are generally the use of an oxidizer to pre-oxidize the graphite to increase the degree of oxidation, or the use of an improved initiator instead of potassium nitrate to produce a high-quality GO while reducing environmental pollution.

Dispersion behavior of GO in CBC
GO possesses particularly a great specific surface area with high surface energy, which makes it easy for particles to coalesce during the preparation and use, and loses the function of nanoparticles. The dispersibility of GO in simulating porous solutions and cement pastes was investigated by Li et al. [39]; they found that it severely limits the widespread use of GO in CBC. An essential condition for enhancing the intensity and toughness of CBC is excellent dispersibility [36,37]. Therefore, in order to obtain high-performance CBC, GO needs to be evenly dispersed into water and then added to CBC. Commonly used dispersion methods include the ultrasonic treatment method, the mechanical dispersion method, etc. [38].
The dispersibility effect of GO varies in different liquids. It was discovered that using a considerable quantity of silica fume (SF) greatly increased GO dispersibility in the cement paste, and the SF can physically split the GO sheets and avoid aggregation. However, an overload of SF can negatively impact the compressive strength of the matrix. Wei et al. [40] combined reduced graphene oxide (rGO) and cement by a liquid-phase mixture technique, which caused the cement particles to stick to the rGO sheets and efficiently decrease the mutual interagglomeration level between rGO and strengthened the dispersal effectiveness of rGO in CBC. Moreover, applying GO on SF to enable the surface interface between SF and cement to be nanosized could optimize the dispersive quality of SF and the interaction between SF and cement [41].
By using the chemical modification method, GO dispersion solution can be prepared with fewer layers, smaller sizes, and uniform dispersion. Zhu [42] modified GO to obtain sulfhydryl-modified GO (SGO), carboxymethylated GO (CGO), polyacrylic acid-modified GO (AGO), branched-modified GO (VGO), and γ-propyl trimethoxy silane-modified GO (KGO). Figure 2 shows the absorbance of GO, SGO, CGO, AGO, VGO, and KGO dispersible liquid samples at 226 nm. The analysis of the binding phase shows that, in water, the dispersibility of the six samples is VGO > CGO > SGO > AGO > KGO > GO. Different functional groups are introduced into the GO lamellae by chemical modification so that they have different dispersibility in the aqueous phase. The dispersibility of the GO lamellae is significantly improved by the introduction of the Si-O bond and C-C double bond. The structure and properties of composites were related to the dispersibility of GO. The GO size range, GO oxygen content, and GO dispersion have a great influence on the CBC performance. Zhang [43] has conducted some relevant studies on this. The outcomes indicated that the regularity and compactness of the microstructure of CBC increased sequentially with the increase of GO size and GO oxygen content in the experimental range, which improved the pore structure and decreased the porosity, as well as conducive to enhancing the durability, toughness, and strength of CBC. The incorporation of fewer lamellar layers of small-size GO dispersion also resulted in the high strength, toughness, and durability of CBC.

Effect of GO on the workability of CBC
Workability, also known as compatibility, is the main parameter for evaluating the homogeneity, ease of transport, ease of placement, and compaction of fresh concrete mixes [44]. The addition of GO-like materials to CBC leads to a loss of compatibility, reduces the rheological properties of the material, and increases its viscosity [36,45]. The rheology of CBC is an exceedingly crucial matter in forecasting its intensity, durability, and application in the architectural industry [46]. GO influences the flowability of CBC; a significant number of oxygen-containing groups makes GO hydrophilic and binds to H atoms in water, consuming more water as the incorporation of GO increases, thus reducing the flowability of the specimen. In addition, the nanoscopic micro-sized function of GO impacts the adsorption of the two-electron layer formed by the cement granules and the water reducer, leading to a reduction in the rejection power between the cement granules, resulting in a decrease in flowability [47].
To investigate the effect of GO doping on cementitious composites, Wang et al. [48] conducted a systematic experiment. Figures 3 and 4 show the impact of GO content in terms of cement slurry rheological behavior and cement slurry setting time, respectively. From Figures 3 and 4, it can be observed that as the GO content increases, the fluidity of the slurry generally decreases; with the rapid increase of viscosity, the setting time was decreased. When the GO content reached 0.05%, compared with the   reference sample, the fluidity of the net cement slurry decreased by 70%, the viscosity increased by 1,850%, the initial setting time was decreased by 23.5%, and the final set time was decreased by 9%. This illustrated that the addition of GO could make the cement slurry even thicker and accelerate the hydration reaction of the cement. Lu and Ouyang [49] added different contents of graphene oxide nanosheets (GONSs) to concrete specimens. The results of the small slump flow experiments are presented in Figure 5. They stated that the incorporation of GO decreased both the slump and slump flow of the concrete.
Aiming to solve the problems of non-uniform dispersion and decreased fluidity when GO was incorporated into CBC, Gao and Ma [50] reacted GO with monomers of polycarboxylate (PC) water reducer by free radical copolymerization. The results indicated that GONSs were evenly dispersed into the polycarboxylate water reducer. The flowability of the cement slurry tended to decrease as the GO content increases. To keep the flowability of the cement mortar greater than 200 mm at 2 h, 0.02% PCs should be increased with the increase of 0.01% GO in the cement slurry [45]. Cui [51] mixed GO and fly ash in CBC and found that the incorporation of fly ash could considerably amend the negative impact on the flowability of the cement slurry by GO. As the fly ash content expanded, the yield stress and elastic viscosity decreased, while the flowability of the cement slurry increased. This was because fly ash had the characteristics of a "ball bearing" effect and low water demand.
To clarify the various impacts of GO on the rheological characteristics of cement mortar, a potential mechanism has been put forth. Shang et al. [52] investigated the effects of SF, SF, and GO co-mixing (SF + GO), and GO-encased SF (GOSF) on the rheological performance of net cement slurry. Research results indicated that the addition of nanomaterials could reduce the liquidity in the net cement slurry, and the effects of a significant degree of GOSF > SF > GO + SF. The highest fluidity of the GOSF-doped net cement paste is evidenced by its low yield shear stress and plastic viscosity, which is because of the synergistic interaction of the superficial activation of GO and the morphological influence of SF in GOSF. The investigation results of Lv et al. [53] revealed that the intercalated nanocomplex NS/ GO formed by naphthalene water reducer and GO could make up for the influence of GO on the fluidity of CBC.
In a word, the functional groups on the GO surface can promote hydration reactions and diminish free hydration in the cement paste. However, GO tends to agglomerate calcium ions and form new microstructures which have an impact on the macroscopic behavior and diminish the flowability of CBC [54].

Effect of GO on the volume stability of CBC
The volumetric stability of concrete refers to the ability of concrete to maintain its initial geometric scale under nonlinear loads during hardening [55]. In this process, various deformations will occur, including deformation under shortterm loads, deformation under long-term loads, and deformation under nonlinear loads. Concrete produces elastic deformation under shortterm load. Zhou [56] investigated the impact of different contents of GO on the mechanical characteristics of highperformance concrete (HPC) at low dosages. From the modulus of elasticity with different curing times and GO content ( Figure 6), it is found that the modulus of elasticity of each age of concrete increases with the increase of GO content. Moreover, from the increased rate of elastic modulus with different curing times and GO content, as shown in Figure 7, it can be concluded that GO mainly had a great impact on the development of the early modulus of elasticity of concrete, which might be mainly influenced by the rapid growth of the early strength of GO concrete. The increased rate of the elastic modulus at different curing times was roughly proportional to the GO content. GO can improve the 3 days modulus of elasticity of concrete by 6.07-27.45% and 28 days elastic modulus by 3.93-10.96%. Radman and Joorabchi [57] added ordinary Portland Figure 5: Influence of GONS content on the flowability of cement slurry [49]. cement (OPC) to 0.025 and 0.05% GO in order to compare the elastic modulus of concrete samples with plain concrete. It was observed that compared to the concrete without GO addition, the modulus of elasticity of the samples with GO incorporation marginally improved.
Under the action of continuous load, the deformation of concrete that increases with time is called creep. Zhou [56] found the influence of GO on the creep of HPC by testing the creep condition of HPC. By comparing the creep coefficient curves of each group of concrete specimens, a significant reduction in the creep coefficient of concrete with the inclusion of GO was found. During the same period, the creep coefficient of the component with a large content decreased more than that of the component with small content. Furthermore, the compressive strength of the material was increased by adding GO to concrete, and the compressive strength was positively correlated with the GO content, which is the main reason why GO can reduce the creep strain of HPC.
Non-load deformation includes drying shrinkage, selfshrinkage, and temperature deformation. Zheng [58] investigated the impact of adding GO through heat treatment tests on the high-temperature performance of concrete. The results showed that GO could form a tight and continuous cross-linking structure in the material system, which effectively improved the high-temperature mechanical properties of concrete and effectively reduced the degradation effect caused by heat treatment. Zhou [56] conducted dry shrinkage experiments for ordinary concrete mixed with GO, conducted shrinkage creep experiments for HPC mixed with GO and investigated how different levels of GO affected the mechanical performance of low-dose HPC. They found that GO increased the shrinkage strain of ordinary concrete and HPC, and increased further with the increase of the content; GO could reduce the creep strain and creep coefficient of HPC, and the magnitude of reduction increased with the increasing content.
The self-shrinkage phenomenon of CBC is closely related to capillary stress. Wang et al. [59] tested and characterized the self-shrinkage of fresh cement paste with different GO contents. Figure 8 shows the self-shrinkage characteristics of multilayer GO-doped concrete. The results indicated that GO incorporation would raise the free water in the gel pores, speed up the hydration rate of cement, and enhance the self-shrinkage. Besides, self-shrinkage would be more obvious with the increase of the admixture amount. GO helped to refine the internal pores and to transfer the large capillaries inside the cement paste to the small capillaries, leading to an increase in the capillary pressure, which in turn increased the self-shrinkage of CBC. It could be concluded that the accession of GO can dramatically improve the elastic modulus and the creep coefficient of CBC at each age, reduce creep strain, and increase shrinkage strain, and thus improve the volume stability of CBC.

Effect of GO on the mechanical performance of CBC
The extensive research outcomes demonstrate that GO could greatly improve the mechanical strength of cement mortar [15] and play a role in strengthening and toughening.  However, the increase in the strength rate varies greatly, which is probably associated with the oxygen concentration, size, content, and dispersed mode of the GO used [60]. The effect of GO can be divided into a single effect and a synergistic effect.

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  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-SiO 2 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 freezethaw 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].   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.

Synergistic effect of GO with dispersants
Strong interactions between GO and sand particles were observed [85]. In order to make GO uniformly and stably  [84] 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 acidbased 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 PCmodified 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/ 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.

Effects of GO on the durability of CBC
Structural designers, while considering the mechanical properties and initial cost of building materials, also evaluated the durability of building materials. Concrete with good durability retains its original shape, quality, and serviceable properties unchanged in its intended service environment. The wide range of factors affecting the durability of CBC, such as freeze-thaw damage, sulfate erosion, carbonation, steel corrosion, alkali-aggregate interaction, etc., has led to the complexity of durability mechanism investigation [94].

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.

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 saltfrozen resistance. 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.

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.

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 Figure 13: Porosimetry results of different specimens [96]. [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 SO 2− 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 SO 2− 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.

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 CO 2 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, CO 2 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, SiO 2 , and TiO 2 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.

Physical phase analysis of GO-doped CBC
GO is the product of graphene oxidation treatment, which maintains the layer structure of graphene but introduces numerous oxygen-containing functional groups into its monoliths. The addition of these functional groups makes the single GO structure very complex and requires some microscopic testing methods to characterize it.

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 polyhedrallike hydration crystals by intertwining and penetrating each other [114]. 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.

Energy dispersion spectroscopy (EDS)
EDS features easy operation, fast analysis, and intuitive results. Chintalapudi and Pannem [116,117] detected a natural mineral called wollastonite (CaSiO 3 ) in all OPC samples doped with GO. This implies that SiO 2 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 flowershaped 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.

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 CaCO 3 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.

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].

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].  Figure 19: XRD spectra of graphite and GO [121].
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 CaCO 3 . 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].

Effect of GO on the hydration reaction
The hydration reaction of cement is a complex process and so the hydration products of cement will have different changes in morphology and structure, resulting in irregularities in the morphology and the aggregation state of the hydration products formed after curing of cement mortar. This results in a large number of defects in the microstructure, which are responsible for poor ductility, resistance to deformation, and corrosion resistance of CBC [132]. For CBC, the structure of cement-hardened slurry is again mainly determined by the hydration products, including the morphology and their aggregation state (the spatial structure constructed by hydration products). The cement hydration products AFt, AFm, and CH are haphazardly distributed, mostly as needle-like, rod-like, and lamellar crystals; therefore, it is difficult to form regular arrangement and regular microstructure. In contrast, C-S-H generally forms amorphous bodies, and under certain conditions also forms short fibers and aggregates into clusters; therefore, it is difficult to form regular crystals and microstructures [133].
Several research studies show that GO can strengthen CBC but there is no unified understanding of the mechanism of strengthening and toughening [134]. According to the current research results, the main mechanism of action can be summarized into three aspects, as discussed in the following sections.

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 flowerlike 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].
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 Ca 2+ 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.

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, oxygencontaining 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.

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.
10 Conclusions and future perspectives

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 autoshrinkage 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.

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.