In recent years, great achievements have been made in the preparation, functionalization and application of graphene. So far, graphene has been widely used in supercapacitors, biosensors, fuel cells, thin film materials, liquid crystal materials, etc., but its application in cement-based materials has been rarely reported. At present, cement concrete is still the cheapest and most widely used building material in construction. However, as our country continues to build high-rise and super-high-rise buildings, as well as projects such as cross-sea and cross-river bridges and undersea tunnels, traditional concrete materials have exposed more and more problems. If the defects of concrete are not fully understood and properly solved, its application in various fields will be restricted. In the process of promoting the development of concrete towards high strength, high performance, green environmental protection, high durability and intelligence, the first problem to be solved is to improve the defects of concrete such as low tensile strength, poor toughness and poor permeability. Graphene's excellent mechanical, electrical, and thermal properties make it an important material for enhancing the tensile strength, toughness, and electrical properties of cement-based materials. With its volume effect, surface effect, and filling effect in the matrix, graphene shows good application prospects in improving the tensile strength, toughness, permeability, and durability of cement-based materials. 1 Study on the dispersion of graphene in aqueous solution The π-π stacking effect and strong van der Waals force between graphene sheets make it difficult to disperse in aqueous solution or other common solvents. The agglomeration of graphene in aqueous solutions greatly limits the application of graphene. Therefore, obtaining a highly dispersible graphene aqueous solution is the advance and foundation for its application in cement-based materials. At present, graphene dispersion methods include mechanical stirring, ultrasonic treatment, surface modification (covalent modification and non-covalent modification), etc., as well as the combined use of different methods. Lotya et al. used sodium cholate as the dispersant, combined with a low-frequency ultrasonic dispersion method, ultrasonic peeling of graphite with an initial concentration of 5 mg/mL and then centrifugation to obtain a high-concentration graphene suspension. By selecting appropriate process parameters, it was found that a dispersion with better dispersion effect can be obtained at a centrifugal speed of 500~2000r/min. The results show that the maximum concentration of graphene aqueous solution is 0.3 mg/mL, and as the ultrasonic time increases, the concentration of graphene suspension increases. Transmission electron microscopy shows that the number of graphene layers is less than 10, the single-layer content reaches 20% (mass fraction), and the average number of graphene layers is 4. In addition, Raman spectroscopy showed that graphene prepared by exfoliating graphite in a dispersant and then ultrasonic treatment formed new boundaries. In graphene with fewer layers, more defects were generated. Khan, U et al. used N-methylpyrrolidone as the dispersant, dissolved graphite in the dispersant aqueous solution, and peeled it off under medium-frequency ultrasonic conditions to prepare a graphene aqueous solution and explore the effects of different ultrasonic times on the concentration of the graphene aqueous solution. The results show that by extending the ultrasonic time, a graphene aqueous solution with the highest concentration of 1.2 mg/mL was prepared, and the proportion of graphene single layer reached 4% (mass fraction). The size of graphene will decrease as the ultrasonic time increases. Small. Raman spectroscopy showed that new defects were created at the boundaries of graphene, but no defects existed in the basal plane. This graphene aqueous solution can be used to prepare a thin film with excellent electrical conductivity and mechanical properties. Lotya et al. believe that the dispersion of graphene is similar to the dispersion behavior of carbon nanotubes in surfactants. They dispersed graphite in sodium dodecyl benzene sulfonate (SDBS), exfoliated it, and prepared a graphene aqueous solution. A UV/visible spectrophotometer was used to measure the absorbance of the dispersed graphene aqueous solution to characterize the concentration of the dispersion. TEM shows that the proportion of graphene with less than 5 layers is 40% purity (mass fraction), and the proportion of single-layer graphene is 3%. Atomic force microscopy observed that dispersed graphene has fewer defects. Research shows that the reason why a stably dispersed graphene aqueous solution can be obtained is because the addition of surfactant creates Coulomb repulsion between the sheets, ensuring that graphene does not agglomerate. Pu Nianwen from Taiwan Yuan Chi University used four different surfactants: cetyltrimethylammonium bromide (CTAB), polyoxyethylene nonylphenyl ether (CO890), and sodium dodecyl sulfate (SDS). ), polycarboxylic acid activator (H14N), non-covalent modification of graphene. The absorbance of different dispersions after centrifugation was measured, combined with standing and centrifugal sedimentation tests, to characterize the dispersing effect of different dispersants on graphene. The surface morphology of dispersed graphene was observed through TEM, and a graphene film was prepared and its conductivity was measured. performance. The results show that when the concentration of CO890 is 0.3 mg/mL, graphene has the best dispersion effect, followed by SDS. In addition, films prepared using CO890-dispersed graphene have the lowest resistivity and excellent electrical conductivity. Wei Wei uses sodium dodecyl sulfonate (SDS), sodium dodecyl benzene sulfonate (SDBS), polyvinyl alcohol (PVA), sodium lignosulfonate (SLS), cetyltrimethyl bromide Ammonium chloride (CTAB), deoxyribonucleic acid (DNA), polyvinylpyrrolidone (PVP), etc. are used as dispersants, combined with ultrasonic treatment, to disperse graphene into an aqueous solution. The dispersed graphene aqueous solution is centrifuged and its absorbance is tested. . The results show that PVP has the best dispersion effect. When the PVP concentration reaches 10 mg/mL, the concentration of graphene dispersion can reach 1.3 mg/mL. The thermogravimetric test also shows that the graphene dispersion concentration reaches 1.3 mg/mL. The reason why PVP can effectively disperse graphene is because the nitrogen and oxygen atoms in the long chain are not bonded, and the electrons have a strong interaction with the dangling bonds on the surface of graphene. They are easily adsorbed on both sides of graphene and interact through electrostatic repulsion and van der Waals forces. , prevent the agglomeration between graphene and maintain the stability of the dispersion. Most researchers use a combination of surfactants and ultrasound to achieve stable dispersion of graphene in the matrix. The addition of surfactant can produce electrostatic repulsion or steric hindrance on the surface of graphene, thereby achieving stable dispersion. This non-covalent surface modification method will not damage graphene and can maintain the original structure and excellent properties of graphene to the greatest extent. 2 Research progress on graphene cement-based composite materials At present, research on graphene at home and abroad is mainly focused on graphene polymer composite materials, and there is less research on graphene cement-based composite materials. In recent years, domestic and foreign researchers have successively carried out Research on the physical and mechanical properties and electrical conductivity of graphene cement-based composite materials has not formed a systematic research theory, and the relevant experimental support is not yet comprehensive. 2.1 Study on the mechanical properties of graphene cement-based composite materials Mohaned Saafi et al. prepared reduced graphene oxide/silica fume geopolymer cement-based composite materials. First, graphene oxide is reduced with alkali and treated with ultrasonic for 1 hour. Then reduced graphene oxide (rGO) and silica fume were mixed and ultrasonic treated for 3 hours, and then added to geopolymer cement-based materials. The mechanical properties and morphology of geopolymer composites were studied. The test results show that the incorporation of rGO and silica fume improves the physical properties of geopolymer cement-based composites, such as flexural strength and Young's modulus. When the rGO content is 0.35% (mass fraction), the enhancement effect is greatest. . This is because the two-dimensional structure of rGO enables it to have a good chemical interaction with the matrix; at the same time, rGO can also effectively fill into disordered pores and cracks, and its wrinkled morphology plays a positive role in promoting it; in addition, The rGO sheets can adhere to the surface of silica fume particles and form hybrid clusters. This is because of the cross-linking and functionalization between rGO and the surface of silica fume. Under the traction of silica fume, rGO fills the pores in the matrix, reducing the The total porosity of the matrix. Hunain Alkhateb et al. [29] added original graphene and functionalized graphene treated with nitric acid to cement-based materials, tested their mechanical properties, and explored the effect of the incorporation of graphene on hydrated calcium silicate. Atomic force microscope observation found that in cement-based materials mixed with graphene, graphene was present in the generated calcium silicate hydrate. At the same time, the content of high-density calcium silicate hydrate increased, and the overall mechanical properties of the composite material were enhanced. Molecular dynamics showed that the addition of functionalized graphene improved the interface strength and integrity of cement-based composites. In addition, phase analysis shows that the incorporation of graphene and functionalized graphene can affect the phase composition and surface toughness of the product, which is why the overall toughness and plasticity of graphene cement-based composites are improved. Navid Ranjbar et al. used graphene (GNPs) as a filling material to enhance the mechanical properties of geopolymer composites. The results show that when 1% (mass fraction) graphene is added, the compressive and flexural strengths of the geopolymer composites increase by 1.44 and 2.16 times respectively. Atomic force microscopy has proven that although graphene is evenly dispersed in the matrix, there are still overlapping and agglomeration phenomena. ZhuPan et al. used a chemical exfoliation method to prepare graphene oxide (GO) and added it to ordinary Portland cement to enhance the mechanical properties of cement-based composites. The flow expansion test shows that adding 0.05% (mass fraction) graphene oxide reduces the flow expansion diameter of the slurry by 41.7%. At the same time, the addition of 0.05% graphene oxide increased the flexural strength of cement paste by 41% to 59% and the compressive strength by 15% to 33%. Test results of the slurry surface area and pore structure show that graphene oxide with carboxylic acid groups can chemically react with hydration products in cement, promote the hydration of cement, and generate more hydrated silicic acid. Calcium gel, thus increasing the surface area of the slurry and more gel pores. Scanning electron microscopy (SEM) results show that when there is no graphene oxide, the cracks will continue to develop and directly penetrate the entire hydration product; when graphene oxide is added, the cracks will shift and twist, becoming discontinuous cracks, graphite oxide Alkene can refine cracks, block crack expansion, lead to branching of cracks, and discontinuous cracks. Lu Shenghua et al. used an improved Hummers method and ultrasonic dispersion method to prepare graphene oxide (NGO), and then incorporated it into cement slurry to test the fluidity, viscosity, compressive and flexural strength of cement slurry at different dosages. The results show that the addition of nano-graphene oxide reduces the fluidity of cement slurry and increases the viscosity of the cement slurry; when the dosage of nano-graphene oxide is 0.05% (mass fraction), the flexural strength of cement stone increases by 102.4%; Scanning electron microscopy (SEM) was used to observe the microscopic morphology, and it was found that nanographene oxide had an impact on the microstructure of the crystals in the cement stone. Graphene has a promotion and template effect on the formation of cement hydration crystal products, and can promote the cement hydration products to form neat and regular flower-shaped nano-scale microcrystals, thus achieving the effect of strengthening and toughening. Wang Qin et al. added graphene oxide suspension (GO) into cement-based materials and measured the effects of graphene oxide on the viscosity, setting time and hydration heat release of cement paste. They also measured the effects of graphene oxide on cement paste and mortar. Compressive and flexural strength. The addition of graphene oxide can increase the viscosity of cement slurry, shorten the setting time, and effectively reduce the cement hydration heat release. When the dosage is 0.05% (mass fraction), the pressure resistance and The flexural strength increased by 40.4% and 90.5% respectively. Graphene oxide may participate in the cement hydration reaction, accelerating the formation and growth of crystal nuclei. At the same time, the adsorption of graphene oxide is conducive to the separation of hydration products and unhydrated cement particles, thereby accelerating the hydration process of cement. 2.2 Study on the electrical properties of graphene cement-based composite materials Sedaghat, A et al. studied the thermal diffusion coefficient and electrical conductivity of graphene cement-based composite materials, cement stone density, particle size distribution of particles, microscopic morphology after hydration and cement stone mineral composition. Conductivity tests show that the presence of graphene increases the electrical conductivity of cement stone, and as the content increases, the electrical conductivity increases. When the graphene content is 10% (mass fraction), the electrical conductivity increases to 10-2S/m; Thermal diffusion coefficient test shows that 1% graphene has no effect on the thermal diffusion of cement stone. When the dosage is 5%, the thermal diffusion coefficient increases, and increases with the dosage of graphene. It increases with the increase; SEM shows that graphene can adhere to the surface of hydrated calcium silicate and calcium hydroxide, fill into the micron-sized capillary pores between the hydration products, improve the compactness of the matrix, and at the same time, graphene changes the calcium The morphology of vitriol reduces the generation of needle-rod ettringite. When the graphene content is 10%, almost no needle-rod ettringite is formed. The incorporation of graphene reduces temperature cracks in cement-based materials and improves the temperature integrity and structural durability of the matrix. Huang from Tsinghua University used DarexSuper20 as a dispersant, combined with ultrasonic technology, to disperse graphite nanoplatelets (GNPs) into mortar to measure the mechanical and electrical properties of graphite nanoplatelet mortar composites. When 1.5% (mass fraction) of graphite nanosheets is added, the compressive strength of the mortar increases by 20% and the flexural strength increases by 17%. This is due to the small size effect, surface effect, and filling effect of the graphite nanosheet particles. Crack blocking effect, thereby improving the transition zone of the mortar matrix and improving the overall mechanical properties of the composite materialable. At the same time, the incorporation of graphite nanosheets reduces the resistivity of the mortar and improves its conductivity, allowing the composite material to obtain a self-diagnostic function for cracks. 2.3 Study on the durability performance of graphene cement-based composite materials DuHongjian et al. used naphthalene sulfonic acid-based water reducing agent (SP) and ultrasonic energy to effectively disperse graphene (GNP) into water, and studied the permeability of graphene to cement mortar. The influence of chloride ion diffusion and chloride ion migration properties was studied, and the pore size distribution of cement mortar was analyzed by mercury porosimetry. In the cement mortar mixed with 2.5% GNP + 1.25% SP (mass fraction), the water penetration depth, chloride ion diffusion coefficient and chloride ion migration coefficient of the composite material were reduced by 64%, 70% and 31% respectively. The mercury porosimetry test results show that the addition of graphene significantly reduces the volume of macropores and refines the critical pore size. The cumulative porosity decreases as the amount of graphene increases, but the addition of graphene affects mesopores. Not much impact. Changes in the pore size structure of cement mortar show that the added graphene can provide nucleation sites for the formation of hydration products, promote the formation of hydration products, fill pores, refine the pore size structure of the mortar, and form a more dense microstructure. . ZhouFan selected different grades of graphene (GC, GM) and different grades of graphene oxide (GOC, GOM) to prepare graphene cement-based composite materials, and tested the compressive strength of cement slurry and the freeze-thaw resistance of cement mortar. and corrosion resistance. The incorporation of graphene improves the compressive strength of cement-based materials to varying degrees; the incorporation of C-grade graphene and M-grade graphene oxide improves the corrosion resistance of cement mortar, but cement containing M-grade graphene The corrosion resistance of the mortar test blocks has declined; after the freeze-thaw cycle of the cement mortar test blocks containing graphene, the length and mass have increased, but compared with the test blocks without adding graphene, they have shown a decreasing trend. This is It may be that the presence of graphene increases frost heave pressure, and the deeper reason remains to be further studied. Du Tao used polycarboxylate superplasticizer to disperse graphene oxide into cement-based materials, and studied its impact on the mechanical properties and durability of cement and concrete as well as related mechanisms. The test results show that adding 0.5‰ (mass fraction) graphene oxide can reduce the pores inside the 28-day-old cement stone, while making the gel in the cement more uniform and dense, and can also reduce the formation of needle-like ettringite, making The cement stone structure becomes denser; at the same time, larger graphene oxide can improve the mechanical properties and chloride ion permeability resistance of cement, reduce the calcium hydroxide content in cement, refine the crystallite size, and reduce Ca/Si ratio, increasing the degree of polymerization of hydrated calcium silicate gel; in addition, the incorporation of graphene oxide can also improve the pore size distribution of concrete and improve the resistance to chloride ion penetration of concrete. 3 Problems (1) Graphene cannot yet achieve industrial production. The preparation of graphene can be divided into physical methods and chemical methods. Physical methods include using micromachines to peel off graphite, or dissolving graphite in an organic solvent and directly peeling it off to obtain graphene. These methods have easy-to-obtain raw materials, simple operations, and high purity of graphene. However, they are time-consuming, labor-intensive, difficult to precisely control, and repetitive. It has poor performance and is not suitable for large-scale production. Chemical methods mainly include chemical vapor deposition (CVD), crystal epitaxial growth, redox methods, etc. The CVD method can prepare graphene on a large scale, but its high cost and complex process restrict the use of CVD to prepare graphene. Development; however, the crystal epitaxial growth method requires high temperature and high vacuum, and graphene is difficult to separate from the substrate, so it cannot be a method for preparing large quantities of graphene. Among them, the redox method has low cost and large output, and is considered the most likely method to achieve industrialized production of graphene. This method first oxidizes graphite and connects oxygen-containing functional groups such as hydroxyl, epoxy and carboxyl groups to the graphite base layer and its edges, thereby greatly reducing the van der Waals force between layers and making peeling easier. However, the electrical conductivity of graphene oxide is greatly reduced. After reduction treatment, although its structure has a high repair effect, reduced graphene oxide still has large structural defects, and the excellent physical and chemical properties of graphene cannot be fully expressed. . (2) Graphene is difficult to disperse in water. The small size, high specific surface area and high van der Waals force between the layers of graphene make graphene tend to agglomerate in aqueous solutions and are difficult to disperse. Before use, it has become a crucial step to carry out various treatments on graphene. In existing research, many scholars focus on dispersing graphite into a dispersant and then peeling it off to prepare a high-concentration graphene dispersion. However, there is no in-depth study on the dispersion of original graphene. Similar to the dispersion of carbon nanotubes and nanocarbon fibers, the dispersion method of dissolving graphene in a surfactant solution and combining ultrasonic treatment is simple and effective. This will be one of the research hotspots for obtaining high-concentration graphene dispersions. (3) The mechanism of graphene’s action in cement-based materials is still unclear. Researchers in recent years have shown that graphene not only plays a nanofilling effect and crack blocking effect in cement-based materials, but also has a certain impact on the formation of cement hydration products. However, there is no systematic and comprehensive test to support it. , the research on the strengthening mechanism is not perfect; especially for graphene to significantly improve the toughness of cement-based materials, more comprehensive and in-depth research is needed; in addition, there are also reports that the addition of graphene increases the toughness of cement-based materials. The thermal diffusion coefficient of the base material reduces micro-cracks in the base body due to temperature stress. The mechanism of graphene's action in cement-based materials still requires long-term and in-depth research. (4) The compatibility of graphene and cement matrix has not been studied yet. Cement-based materials are composite materials containing various mineral admixtures and admixtures. When considering the dispersion of graphene in cement-based materials, the compatibility of the treated graphene with different components in the matrix must also be considered. problem to ensure that each component can promote each other and achieve good performance of cement-based materials. (5) There are few studies on the durability of graphene cement-based composites. At present, there is almost no research on the durability of graphene added to cement-based materials. Durability is a key guarantee for the application of graphene-cement-based high-performance composite materials. In-depth research on durability issues is of vital significance. . Comprehensive and systematic research on the long-term durability properties of graphene cement-based composite materials such as freeze-thaw resistance, carbonization resistance, penetration resistance, and alkali-resistant aggregates can provide a strong theory for the application of graphene cement-based high-performance composite materials in engineering. support. 4 Research trends At present, some researchers have initially explored the impact of graphene on the macroscopic properties of cement-based materials. The addition of graphene can significantly improve the mechanical properties of cement-based materials, increase their electrical conductivity, and improve impermeability and resistance. Corrosive and other durable properties. The author believes that research should be conducted from four aspects: (1) For the stable dispersion of graphene in the matrix, it is necessary to focus on the selection of surfactants, and a large number of experimental studies are required to ensure low cost of graphene dispersion and the combination of surfactants and surfactants. On the premise of good compatibility of the matrix, the maximum dispersion of graphene in the matrix is achieved. (2) Utilize the excellent electrical and thermal conductivity properties of graphene cement-based composite materials to develop smart cement concrete materials to achieve real-time health monitoring of concrete structures. At the same time, use its good thermal conductivity properties to improve temperature cracks in large-volume concrete. (3) Enrich the research on the durability performance of graphene cement-based composite materials, and consider as comprehensively as possible the various factors that cause durability problems if laboratory conditions permit. (4) The mechanism of graphene's action in cement is not clear, and the effect of graphene's addition on the hydration products and microstructure of cement has not been systematically studied. In order to clarify the mechanism of graphene's action in cement-based materials, molecular dynamics theory, finite element analysis and other methods can be used to study it. 5 Conclusion The emergence of graphene not only enriches the family of carbon materials, but also broadens the application fields of traditional cement concrete materials. With the continuous development of various types of high-strength and high-performance concrete in our country, there is an urgent need for various high-tech materials to improve cement concrete's low flexural strength, low toughness, easy cracking and a series of inherent defects. Graphene has high strength, high flexibility, high specific Excellent properties such as surface area and high electrical conductivity have brought broad prospects for the development of high-performance, intelligent cement-based materials. At present, the large-scale production of graphene is still a thorny problem, which requires the cooperation of relevant researchers to find the simplest and most effective method for preparing graphene. Once the industrial preparation of graphene is realized, it will not only drive the development of a series of related high-tech fields, but also realize the widespread application of graphene in the cement concrete industry.

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