Status and Perspectives for Mechanical Performance of Cement/Concrete Hybrids with Inorganic Carbon Materials

Abstract

The rapid advancement of modern infrastructure and construction industries demands cementitious materials with superior mechanical performance, durability, and sustainability, surpassing the limitations of conventional concrete. To address these challenges, carbon-based nanomaterials—including carbon nanofibers (CNFs), carbon nanotubes (CNTs), and graphene—have gained significant attention as next-generation reinforcement agents due to their exceptional strength, high aspect ratio, and unique interfacial properties. This review presents a critical analysis of the latest technological developments in carbon-enhanced cement and concrete composites, focusing on their role in achieving high-performance construction materials, as there is a shortage of reviews of cement concretes based on carbon nanoadditives. We systematically explore the underlying mechanisms, processing techniques, and structure–property relationships governing carbon-modified cementitious systems. First, we discuss advanced synthesis methods and dispersion strategies for carbon nanomaterials to ensure uniform reinforcement within the cement matrix. Subsequently, we analyze the mechanical enhancement mechanisms, including crack bridging, nucleation seeding, and interfacial bonding, supported by experimental and computational studies. Despite notable progress, challenges such as long-term durability, cost-effectiveness, and large-scale processing remain key barriers to practical implementation. Finally, we outline emerging trends, including multifunctional smart composites and sustainable hybrid systems, to guide future research toward scalable and eco-friendly solutions. By integrating fundamental insights with technological advancements, this review not only advances the understanding of carbon-reinforced cement composites but also provides strategic recommendations for their optimization and industrial adoption in next-generation construction.

1. Introduction and Background

It was not until the 18th century that the first iron railway was built after the development of pig iron and wrought iron in the 17th century. Later, steel was developed, and the first 11-storey high-rise building was built in the late 19th century. With the advent of Portland cement in 1824, the subsequent reinforced concrete project flourished [1,2,3,4]. In modern civil engineering, in spite of the replacement of traditional soil and stone materials by new types of materials, cement concrete, reinforced concrete, and steel remain irreplaceable structural materials. New alloys, ceramics, glass, organic materials, and other synthetic materials have become increasingly important in civil engineering [5,6,7,8,9].

Compared with the past, the physical and mechanical properties of contemporary civil engineering materials have also been significantly improved. With the improvement of the performance of modern ceramics and glass, the application range has also changed significantly. For example, the strength, durability and other characteristics of cement and concrete have been improved. Although civil engineering materials have made great progress in terms of variety and performance, there is still a large gap between the general public’s expectations when it comes to their performance requirements. Generally speaking, the basic requirements for materials in civil engineering for their mechanical performance are that they must have sufficient strength and are secure enough to withstand design loads, and the material quality can be designed to be light for reducing the load on the substructure and foundation. For example, cement is a major adhesive material in the construction of concrete and has been widely used in buildings and architecture. As mentioned earlier, cement and concrete are very easy to crack because of the quasi-brittle characteristics, which cause low strain capacity and tensile strength [10,11,12,13,14,15,16].

Based on these demands, currently, new types of carbon materials, such as carbon nanotubes (CNT), carbon fiber, graphene (graphene oxide or reduced graphene oxide), graphite, etc., have been considered as new fillers for metal, polymer, and ceramic matrix composites because of their special mechanical, chemical, thermal, and electronic properties with the primary purpose of strengthening their mechanical performance, such as fracture strength and other functional features. These carbon materials can enhance the mechanical properties of cement and concrete-based materials; however, they cannot restrain the conformation of cracks in these materials. Among them, graphene is one of the most prevalent nanomaterials; it possesses unusual properties, such as excellent heat and electricity conductivity, is 200 times stronger than steel and other materials and has drawn much enthusiasm from many researchers [17,18,19].

In this review, types of carbon materials including carbon nanotubes, carbon fiber, graphene, and graphite in concrete and cement hybrids have been summarized. Typically, civil materials made up of these carbon materials have higher mechanical performance than their single-material counterparts. As a result, additive carbon materials can lead to stronger mechanical performance due to their outstanding pristine mechanical properties. In addition, the mechanisms associated with mechanical performance in these carbon hybrids are analyzed and summarized in this work. We have classified them as two main mechanisms: a physical compounding and chemical bonding mechanism, respectively. In particular, 2D carbon materials possess extraordinary chemical and physical performance due to their strong electron–electron interactions within one plane, which can lead to a confinement effect and some evident anisotropy presentation. Compared with traditional concrete and cement, carbon components contain various advantages and unique mechanisms, which can be presented in the following context: it is vital to summarize the recent progress in carbon-based hybrids and structures based on concrete and cement and to discuss the trends for future traditional civil materials [20,21,22].

In our review paper, we provide an overview of the state-of-the-art perspectives in the research field of carbon-based cement and concrete hybrids for reinforcing their mechanical properties. Firstly, various carbon-based hybrid structures are introduced, followed by some developments in the synthesis of these new designed materials in the civil engineering field. Secondly, some potential applications of black phosphorus and some meta-materials in civil materials are discussed. Finally, many perspectives on how to strengthen mechanical performance in traditional concrete and cement are presented for the near future.

2. The History and Current Application of Carbon Composites in Civil Materials

2.1. One-Dimensional Carbon Nanofiber

As is known, the advantages of carbon fiber are its specific high strength and modulus, and making it into a lightweight and high-strength structure is the main aim of future development. The comprehensive properties of carbon fiber are excellent, and it can solve the problems which other materials cannot solve in some situations as both structural and functional materials. The following is a brief history of the development of carbon fiber [23,24].

Fiber Reinforced Polymers were created in the 1940s and initially made in the USA. In the 1950s, an American air force base obtained CFRP by dragging rayon under a high temperature of 2000 °C. In 1959, the U. S. Union Carbide Company made cellulose-based CFRP from viscose fiber. In 1962, the Japan Carbon Corporation realized the industrial production of low-modulus polyacrylonitrile-based CFRP. In 1963, the British Institute of Aeronautical Materials developed high-modulus polyacrylonitrile-based CFRP, and in 1965, Gunma University of Japan tried to produce a general-purpose CFRP based on asphalt or lignin. In 1969, Japan’s Otani Sylvester produced a high strength and elastic fragrance material from a special copolymerized PAN. In 1972, Dupont produced Aramid CFRP with a density value of 1.2 t/m to 1.5 t/m and a strength of 3000 MPa. In 1980, the American Emery Company developed activated carbon fiber with phenolic fiber as the raw yarn and successfully placed it on the market. In 1996, the total global production of carbon fiber reached 17,000 tons, of which polyacrylonitrile-based fibers accounted for 85%, and the rest were pitch-based fibers. In 2002, the global polyacrylonitrile-based carbon fiber production capacity was about 31,000 tons, of which 75% were small-tow carbon fibers and 25% were large-tow carbon fibers. Carbon fiber materials are mainly produced in Japan, followed by the United States, while other countries produce only a small amount of the material [25,26,27,28].

In the application of carbon fiber reinforcing cement, a certain amount of high-performance carbon fiber and high-efficiency admixture are usually blended in cement and its products. The main focus of research is carbon fibers reinforcing mortar and concrete. How to effectively control the plastic shrinkage and early cracking of cement and its products, improve the compactness of cement and its products, and increase the strength of cement and the related products are the key targets of research. Carbon fiber for cement reinforcement is mainly chopped carbon fiber. According to the different uses of cement products, it can also be divided into two types: structural materials and functional materials. As a functional material, carbon fiber can improve anti-seepage, crack resistance and temperature difference, wear resistance, and it can be used for waterproof and anti-corrosion layers in harsh environments. As a structural material, carbon fiber can enhance the tensile, impact and bending strength of cement products, and can also improve toughness and earthquake resistance. In addition, carbon fiber can also introduce conductive and anti-magnetic functions into cement, and could be made into various cement fiber boards such as electromagnetic shielding boards and anti-static floors. The research into carbon fiber-strengthened cement for mechanical performance began in the early 1980s, and was applied in the late 1980s. Carbon fibers for reinforcing mechanical performance are not widely used due to the low price of fiberglass and synthetic fibers.

2.2. One-Dimensional Carbon Nanotubes

The invention of carbon nanotubes allowed for a much more effective method for enhancing cement-based materials in mechanical performance. Carbon nanotubes are flexible, lightweight, have unique and extraordinary high-yield stress strength, large surface curvature, a high surface area and high heat resistance, and also have excellent conductive properties, where the current transfer capability is 1000 times that of metal copper wire. Therefore, the novel nano-scale carbon nanotubes with high elastic modulus, high strength, and high durability have become urgently needed toughening and strengthening materials for overcoming the disadvantages of ordinary cement concrete which include low tensile strength, low flexural strength, high brittleness, poor crack resistance, and the fact it is a material that is overly relied on. Until now, there have been many reports about carbon nanotubes’ mechanism of improvement of the mechanical properties in cement or concrete materials. In addition, carbon nanotubes have a large aspect ratio, and there is a strong attraction between the tubes called van der Waals, which are generally agglomerated and bundled, and are nearly insoluble in organic solvents and water. As is known, for composite materials, the dispersion uniformity of the additive phase in the matrix is an essential indicator of influencing the properties of the materials. That is, the more uniform the dispersion, the more obvious the effectiveness, and the better the overall performance. Consequently, how to enhance the dispersion capability of carbon nanotubes and eliminate the agglomeration is a prerequisite for the synthesis of carbon nanotube-based composites [29,30]. According to the relevant references, the methods for the dispersion of CNTs are divided into physical dispersion and chemical dispersion, and the primary dispersion methods are nothing more than mechanical stirring (such as ball milling, ultrasound, high-speed shear, etc.) after chemical surface modification dispersion (surface chemical modification, mainly including hydroxylation, carboxylation, etc.), and surfactant dispersion after treatment (such as using Tween 100, SDS, CTVB, etc.)

H. Wagnert used the traditional Kelly–Tyson method to calculate the interfacial shear degree of single-walled carbon nanotube–polymer composites [31]. It is found that carbon nanotubes have a large aspect ratio and can afford large interface transfer stress; they can also make full use of the modulus and strength. The effective stress is transferred from the substrate to the carbon nanotubes through the interface. Likewise, Simone Musso et al. have compared the three cement slurry composites with different kinds of MWCNTs and analyzed the influence of carbon nanotubes on the mechanical performance of the composites. The results indicate that the specimen rupture modulus of the additive normal and directional CNT increases 34% and 9%, respectively, and the compressive strength improves by 10–20% [32].

2.3. Two-Dimensional Graphene and Graphene Oxide

Graphene, known as two-dimensional graphite sheet, is a perfect hexagonal symmetric crystal, ideally formed by a single layer of carbon atoms connected by covalent bonds. In 2004, physicists Andrew Gem and Konstantin Novoselov of the University of Manchester in the United Kingdom successfully separated graphene from graphite, confirming that it can exist on its own. It is another new low-dimensional carbon material introduced after fullerene (Cso) and carbon nanotubes (CNTs), with a thickness of only one hundred-thousandth of the diameter of a hair. It is about 0.335 nm thick and is the thinnest layered material found so far. Every carbon atom connects the other three carbon atoms by the strongest σ bond, and these strong C-C bonds make the graphene nanosheet possess outstanding mechanical performance and structural rigidity. Just due to its unique two-dimensional structure and excellent crystal quality, graphene possesses excellent mechanical, thermal, magnetic, and electrical properties, and it is applied in materials, gas sensors and energy storage fields, etc. Its thermal conductivity and mechanical strength are comparable to those of macroscopic graphite materials, and the breaking strength is comparable to that of CNTs. However, the insertion of graphene in cement-based materials has been certified to be comparatively hard and not easy to execute due to the difficulty of dissolving it in water and its expensive cost.

The exfoliation of graphite and chemical oxidation are the most ordinary methods for generating bulk kilograms of graphene materials, as the derivative of graphene. The primary two types of graphene are graphene oxide (GO) and reduced graphene oxide (rGO), which are graphene layers with some oxygen-containing functional groups on the surface and reduced functional groups, respectively. Because of these functional groups, GO is non-conductive, amorphous, has a high aspect ratio and large surface area, and also presents lower mechanical strength compared to graphene [33]. Also, due to its functional groups, GO is hydrophilic and highly soluble in water. Due to this distinctive water dispersion function, GO is more suitable for being incorporated into cement for obtaining a uniform hybrid, and consequently, most of the identified works expressed a preference to adopt GO to enhance the mechanical performance and the durability of cement or concrete-based composites [34,35,36,37,38,39]. In addition, there are limited reports about pure graphene and rGO reinforcing the mechanical performance of cement or concrete-based composites in spite of rGO being partially soluble in water and not an entirely defect-free GO.

Many papers have analyzed the effect of GO, rGO or graphene on mechanical properties, microstructure, workability, early age hydration and transport performance. And they have found that different characteristics such as functional groups, interlayer distance (d spacing), size, molecular structure, and mechanical strength all influenced the performance of the strengthened composites. Generally speaking, the improved mechanical performance of GO composites mainly originates from its high dispersibility in water as well as high number of functional groups which exist in chemical bonding with cement or concrete hydration products, which can largely bridge the composite matrix at a level of nano/micro size. In addition, in rGO composites, the improved mechanical performance may be due to its high physical strength and maintaining lower functional groups which can form a bond with the products through cement hydration.

3. Reinforcing Civil Materials by Carbon Materials

3.1. The Strengthening by Physical Compound Route

3.1.1. One-Dimensional CNT in Cement/Concrete Composites

CNTs can be widely used in the construction industry due to their high tensile strength and high elastic modulus compared to some traditional fibers. CNTs have a large aspect ratio of about 1000 to 10,000, and also have very high toughness, strength, and Young’s modulus due to its sp² bonding of carbon with carbon. In addition, its density is only one-sixth of that of steel, but the tensile strength is 100 times higher than that of steel; also, its Young’s modulus is around 1 TPa and the fracture strain is about 280%, which is more than nine times of that of high-strength steel. And ignoring the factors of weak shear interactions between the alongside tubes which can cause a reduction in the effective tensile strength, CNTs which have ideal microstructures can reach a tensile strength of 800 GPa. Therefore, CNTs can act as an optimal choice for fiber-reinforced materials [40,41,42,43].

Above all, research on the mechanical, electrical, chemical, and other properties of CNTs has achieved outstanding advances, including in the area of modifying concrete, and other traditional civil materials. Many researchers have adopted some zero-dimensional (0 D) nanomaterials for modifying cement, which can act as a nuclei site for cement hydration and could densify the hydration products because of their high reactivity. Even though they can enhance the final strength of the nanocomposites, they afford almost no resistance for micro-crack propagation. Below is a summary of some works about the CNTs reinforcing cement or concrete for mechanical performance. Meanwhile, the microstructures, dispersion, mechanisms, and workability in these works are also discussed and analyzed.

Matikas et al. have investigated the influence of carbon nanotubes on chloride penetration in mortars, and report on the mechanical performance of the material. The mortars were corroded artificially, and the flexural and compressive strength were measured as well, whereas the effect of varying carbon nanotube concentrations was appreciable. It was proved that its concentration affects the permeability of the mortars remarkably. The enhancement of flexural and compressive reaction compared with virgin specimens in a salt spray fog test originated from the decrease in the porosity in the materials because it is full of sodium chloride pores. The salt spray chamber and the sealed specimens inside the chamber are listed in Figure 1a,b [44]. In their work, air content and mortar machinability were maintained constant, and therefore not influencing the ion penetration in the modified specimens, and they obtained the rationalization of the findings by using various carbon nanotubes. The influence of CNT concentrations on the flexural and compressive strengths 100 days before and after exposure in the salt spray is shown in Figure 1c,d. With regard to CNT content, the raw samples displayed insignificant fluctuation for the compressive strength, and as flexural strength, it exhibited a slight rise when the loading CNT reached up to 0.4%, due to the lower porosity of the samples with 0.6% and 0.8% CNTs. It is proved that the samples exposed to sea fog resulted in both enhanced flexural and mechanical performances, no matter the CNT concentration. The largest increase compared to the unexposed specimens was 22% with 0.8 wt.% CNT content. In addition, the compressive strength seems to enhance after exposure to salt spray for all kinds of loadings of CNTs, and generally remains at about 10% [44].

Also, some programs were developed to appraise the influence of including multi-walled carbon nanotubes (abbreviated as MWCNTs; and for the sake of convenience, MWCNT will be abbreviated as CNT later on) on elevated temperature properties for normal weight concrete (NWC) and lightweight concrete (LWC). The mechanical properties including tensile strength (f’_t, T), compressive strength (f’_c, T), compressive toughness (T_c), mass loss (M_T), elastic modulus (E_T) and stress–strain response under the condition of unstressed and residual were in the varying range of 23 °C to 800 °C. They were tested by heating some cylindrical specimens to 200 °C, 400 °C, 600 °C, and 800 °C at a heating rate of 5 °C/min [45].

The addition of CNTs in cementitious matrices could improve fire endurance, and the relative maintenance of the weight and mechanical strength of concrete modified with CNTs was even higher, also, the stress–strain response of the samples was tougher. The cryo-fractured specimens indicated that the reinforcements in host matrix are homogenously dispersed, and further, researchers obtained mathematical relationships for illustrating the mechanical performance of modified mix samples as a function of temperature. They have found that the addition of CNTs could enhance the compressive strength before and after exposure to fire; further, the samples displayed better strength retention at elevated temperatures which originated from the reinforcing effect of nanotubes as well as some crack-bridging influence. The effect of nanotubes enhanced the tensile strength by 16% in the surrounding area. In addition, the presence of CNTs could be important in lightweight matrices at elevated temperatures because of the crack accumulation in the loading of the diametric, and also, the stress–strain response of the specimens with CNTs is ductile. For the elastic modulus curve, it can be shown that the samples with CNTs enhanced the elastic modulus retention at higher temperatures. The micro-structural images illustrated that the CNTs in concrete matrix are homogeneous and were shown to be bridging the cracks, which can maintain the effective bearing area, and it conversely enhanced the mechanical properties of the modified samples at higher temperatures. In addition, CNTs compensate shrinkage cracks while keeping the mix properties and enhance the packing density of cementitious matrices [46,47]. Therefore, the most common reported effect of CNTs is their refinement and densification of the micro-structure [48,49].

Also, Zhang Chunwei’s group have studied the influence of CNT loading on the physical properties both before and after the inclusion of tetraethyl orthosilicate (TEOS), such as density and compressive strength for foam concrete which plays a significant role in the roof, thermal pipe insulation layer, and non-load bearing walls due to its outstanding insulation performance and specific porous construction. They found that CNTs could enhance the pore structure and reduce the pore diameter of the foamed concrete. When the weight of CNTs varies between 0% and 0.1%, the dry density of the composites changes between 290 and 320 kg/m³, in addition, the modified maximal compressive strength could reach up to 0.302 MPa, and increased over 27% compared to the plain. In the paper, CNTs were dispersed in an aqueous employing surfactant sonication method first, then they were incorporated into cement slurry, and finally, composite foaming agents were added to synthesize the foam concrete with reinforced CNTs. As is known, CNTs act as the nucleating agents with different nano-sizes, and can reshape the sizes of micro-pores in foam concrete, and also improve the strength by 70% just with a 0.05% addition. Moreover, in the paper, polycarboxylate-based cement superplasticizer was adopted as the dispersant integrated with an adequate ultrasonic treatment for obtaining homogeneously dispersed CNTs in water, thus, resulting in an increased interfacial interaction between the foam concrete base and CNTs [50]. In conclusion, the hydration process of cement affected by CNTs originated from the attachment sites for the C-S-H gels, which can be considered as a filler leading to a higher strength and denser architecture of the matrix. The strength was found to be enhanced with the addition of CNTs, and it is affected by the content, length, diameter, and type of CNT. Also, the interaction between the cement hydration products and CNTs is mainly reflected in the crack bridging and de-bonding of CNTs for the reinforcement of matrix toughness [51]. Similarly, six different CNT concentrations have been fabricated in cement and formed CNT/cement composites by using the dispersion method of surfactant decoration and ultrasonic treatment. Not unexpectedly, with a presence of 2.0% carbon nanotubes, the flexural strength of the nanocomposite increases about 32% compared to the reference. It is assumed that the interfacial “stick–slip” capacity and micro-crack bridging of nanotubes between cement matrixes could contribute to the improvement of the flexural strength. As shown in Figure 1e, the flexural strength is smoothly improved by the weight fraction of MWCNT (wfM). It can be shown that the improvement degree of the flexural strength dropped with the increasing wfM. Specifically, the flexural strength of the composite with 0.5% wfM is 3.65 MPa, and that of the sample with 2.0% wfM is 3.87 MPa, respectively, as shown in Figure 2c. That means that the maximal growth is just 31.63%, compared with the reference of 2.94 MPa [52]. In addition, the net-like CNT fibers during bending within the matrix are presented in Figure 2a,b, and at high wfM, the fibers could strengthen the pulling-out effect and bridging in the process of flexural bending. Moreover, some local agglomerates emerged inevitably and were embodied by the macro property, which is also certified by the homologous microstructure. In any case, the enhancement of flexural strength could originate from the viscous multi-phase boundary friction with a large area and various forms of slippage between hydrated cement products and CNT whilst vibrating. And favorable transmitters between the bridges also favor flexural strength, whereas at high wfM, the aggregation formed by the fibers could jeopardize the enhancement to a certain extent [52].

To solve the issue of CNT aggregation and enhance the dispersion for activating the graphite surface and enhancing the interfacial interaction, apart from numerous methods of surface coating and functionalization, the adoption of a surfactant or other admixtures and optimum physical blending, Talachi et al. have used surface-functionalized CNTs and cement paste for overcoming the obstacles mentioned above [53]. They examined CNTs with polar impurities end groups OH and COOH, and also researched types of CNTs, including non-functionalized and dispersed in water solution, non-dispersed and non-functionalized, and a control group containing no CNTs. They measured the compressive and flexural strengths of different mixes, and it revealed that the functionalized CNT composite caused a much higher improvement in both flexural and compressive strengths, compared with other specimens. The interfacial interaction which derived from surface functionalized CNTs and hydration products such as calcium silicate hydrate (C-S-H) brings about strong bonding among the matrix composites. The result indicated that the addition of 0.15% CNT mixed in the cementitious material reinforced the compressive strength by 8%, and initially dispersed CNTs in water suspension improved the compressive strength by 8.5%. Among different kinds of CNTs, non-dispersed and non-functionalized CNTs brought about the lowest rise, and the CNTs dispersed in water solution initially led to the highest increase in compressive strength. It is worth noting that the researchers have adopted silica fume to improve the effective dispersion of CNTs which causes the bonding which effectively reinforces the mechanical performance of the composite.

Also, when using non-surface functionalized CNTs and surface functionalized CNT (CNT-OH and CNT-COOH), the flexural strength grew by 35% and 50%, respectively, compared with the no-CNT samples (control sample). The mechanism in the enhancement of CNT-COOH may be related to the modification of carboxylic acid groups which could generate chemical interactions between calcium silicate hydrate (C-S-H) or Ca(OH)₂ and carboxylic acid. SEM analysis also proved that micro cracks could be bridged by the interaction resulting from the strong covalent forces between the matrix and CNT reinforcement and revealed that CNTs could fill the voids in the mortar, by not dispersing evenly and forming clumps, and therefore improving the compressive strength. They have certified in previous work that surface functionalization and better dispersion bring about an enhancement for mechanical performance, and also the improvements are finite, particularly at a low concentration of CNTs. The compressive and flexural test set up are presented in Figure 1f,g, and also Figure 1h,i show the stress–strain plot versus no CNT sample and CNT-COOH sample; it was concluded that the mechanical properties with surface-functionalized CNTs are significantly reinforced [53].

In a word, CNTs could be used as a reinforcement modifier widely in concrete and cement composites, and we have analyzed the various mechanisms of CNTs when applied to composites, as well as provided some significant information for research on the improvement of CNT mechanical performance. The appropriate dispersion of CNTs is definitely beneficial for designing CNT mixed-function cementitious hybrids. In addition, the content of CNTs for obtaining the effect of enhancing mechanical performance ranges from 0.01 to 0.15% by weight of cement. The addition of CNTs into the composites contributes to conquering the limitations of the composites used as conventional conductive fillers. Overall, further studies should be executed to test the durability and intensity of the composites in view of understanding the cement chemistry.

Figure 1. (a) Salt spray chamber [44]; (b) sealed specimens inside the chamber [44]; (c) mechanical properties of the nano-modified mortars under different weight fractions of MWCNT [44]; (d) the exposure to the salt spray chamber [44]; (e) the flexural strength of MWCNT/cement composites [44]; (f) compressive test for a cubic specimen [53]; (g) flexural test for a beam specimen [53]; (h) stress–strain plot of No CNT-30 composite [53]; (i) CNT-COOH-30 composite [53].

Figure 1. (a) Salt spray chamber [44]; (b) sealed specimens inside the chamber [44]; (c) mechanical properties of the nano-modified mortars under different weight fractions of MWCNT [44]; (d) the exposure to the salt spray chamber [44]; (e) the flexural strength of MWCNT/cement composites [44]; (f) compressive test for a cubic specimen [53]; (g) flexural test for a beam specimen [53]; (h) stress–strain plot of No CNT-30 composite [53]; (i) CNT-COOH-30 composite [53].

3.1.2. Two-Dimensional Graphene and GO in Cement/Concrete Composites

In recent years, graphene and its derivatives such as 2D GO nanosheets have been widely studied. Because of its special mechanical, thermal, electrical, and optical properties, as well as high surface area-to-volume ratio, combining it with cement is expected to have many applications and thus a promising future. As is known, 2D GO nanosheets could afford an extra dimension to connect with the cement and concrete matrix. To synthesize reinforced and pre-stressed concrete architectures, Young’s modulus E is an essential factor which is required to be defined. And the raw GO is considered to be about 1 TPa; therefore, graphene will be able to enhance the toughness and strength of cementitious materials. In addition, the structure formed by porous calcium silicate hydrate (C-S-H) gel is significantly associated with the mechanical performance of the cement paste [54].

Mijowska et al. have assessed the effects of 3 wt.% of GO mixed with cement on the microstructure and physical-mechanical performance of cement hybrids. It is illustrated that the presence of 3 wt.% GO in cement could lead to a great improvement of elastic modulus. In addition, the experimental data demonstrate that the GO interacts with the hydration production from the cement and also has the possibility of being adopted in concrete. Figure 2a shows the synthesized program, and Figure 2b shows the topography of the prepared GO assessed by the AFM. It can be concluded that bigger changes exist in the thickness compared to the initial graphite. The raw starting graphite flakes’ thickness varies from 156 to 171 nm, which was certified in previous works, while the GO in their work revealed a thickness of approximately 0.86 nm [55]. Figure 2c presents the values of Young’s modulus of the reference sample and nanocomposite, and it is evident that the nanocomposite has a wider elastic modulus distribution range, of 5–20 GPa, than that of the reference materials, which have a range of 1–10 GPa. This outcome means that the addition of the GO into the cement may be a prospective method for the improvement of the mechanical properties in future civil materials. In conclusion, the presence of GO leads to a positive effect on the hydration process, which could be converted to mechanical properties directly [56].

Unsurprisingly, researchers have compared the influence of GO and rGO on flexural strength for cement-based materials. They have used a typical form of GO with the C/O ratio of 54:46 and rGO form with a C/O ratio of 82:18 for examining the properties in the cement hybrids. They found that different characteristics of GO and rGO including molecular structure, d spacing, size, and physical strength all affected properties such as mechanical properties. The result demonstrated that the highest flexural strength was obtained at 0.04 wt.% of GO and 0.06 wt.% rGO composite, respectively. Also, the improvement degree was 75.7% and 33.7%, respectively, compared to the raw mix. Despite both the GO and rGO composites obtaining higher mechanical properties than the control composites, interestingly, the GO composites have more flexural strength, while rGO composites have higher compressive strength. In addition, with the increasing concentrations of rGO, the mechanical performance shows the same trend. Meanwhile, there is an optimal proportion, which is 0.04%. The reasons for the improved mechanical properties for GO composites are mainly derived from its high dispersion in water and also with large amounts of functional groups which could participate in the hydration products formed by chemical bonding and may densify and bridge the composite at a nano-micro level. However, the improved mechanical performances obtained for the rGO composites are mainly due to its high physical strength as well as its fewer remaining functional groups which could constitute the bonding in cement hydration products. The mechanical strengths of different composites at 28 days are shown in Figure 2d,e [57].

Also, many researchers have adopted multiple modifiers including GO, for example, Liu’s group have used both GO and nano-silica for improving the mechanical properties of oil well cement. The compressive and flexural strength of cement with 0.03% GO and 1.5% nano-silica were grown by 43.2% and 42%, respectively, as shown in Figure 2f,g [58]. The values were much better than the cement with GO or nano-silica solely; this denotes that GO and nano-silica have a synergetic effect on reinforcing mechanical performance, including better dispersion, higher degree of pozzolanic reaction, and hydration process. In their work, they have designed a special hybrid with covalent joints between the active groups of GO and Si-OH which could make SiO₂ particles disperse on average in alkaline cement. They found that GO and nano-silica have hybrid effects of a higher degree of pozzolanic reaction and network structures from hydration products, all of which are favorable for enhancing the mechanical properties of the cement composites. They also have proved the conclusion of some previous works, such as that the presence of GO could facilitate cement hydration and decorate the microstructure of the cement hydration crystals when the concentration of GO is just 0.03% [59]. In another example, researchers have found that with the addition of GO, the pore sizes could largely be reduced which could result in an enhancement for the mechanical properties [60].

There is another example for GO as a modifier on the surface properties. The GO/CF composite fibers were synthesized by a designed depositing method which indicated that the hybrids had a rougher surface which could lead to enhancement of the friction when carbon fiber was drawn from the cement matrix. They have conducted a three-point bending test on the GO/CF hybrid, and it showed that the flexural strength was reinforced by 14.58%, and was able to be further reinforced by 10.53% when predispersion was conducted in the GO solution following mixing with cement powders. The better dispersion of GO/CF composite fibers in the GO solution derived from bigger steric stabilization and electrostatic repulsion may have an important role in further reinforcing the mechanical properties of cement paste. That means that better dispersion may strengthen the bonding between cement hydrates and GO, and consequently, it makes GO both a kind of surface modifier and a dispersant. The set up of the system they used and mechanical performance are shown in Figure 3a,b [61].

Figure 2. (a) Schematic formation of cement/GO nanocomposite [54]; (b) topography of the synthesized GO appraised by the AFM [54]; (c) Young’s modulus values of the reference sample and nanocomposite [54]; (d) the compressive strength [57]; (e) the flexural strength of different concentrations mixed for 28 days [57]; (f) compressive strength of the four different types of cement [58]; (g) flexural strength of four different types of cement [58]; (h) the illustration scheme of hybrid nanomaterials’ synergetic effect on the hydration process of the cement [58].

Figure 2. (a) Schematic formation of cement/GO nanocomposite [54]; (b) topography of the synthesized GO appraised by the AFM [54]; (c) Young’s modulus values of the reference sample and nanocomposite [54]; (d) the compressive strength [57]; (e) the flexural strength of different concentrations mixed for 28 days [57]; (f) compressive strength of the four different types of cement [58]; (g) flexural strength of four different types of cement [58]; (h) the illustration scheme of hybrid nanomaterials’ synergetic effect on the hydration process of the cement [58].

In short, there are more studies focused on the mechanical performance for GO strengthening cement composites; however, the durability problem is still to be studied. Through the analysis and summary of past works related to graphene and graphene oxide reinforcing cement/concrete composites, some suggestions about near future investigations could be provided. Further studies including the adaptation of the microstructures of GO in the cement composites and the formation mechanism of cement hydration products should be paid much attention when it comes to achieving the required mechanical properties of the composites. Another important issue, the durability of different types of GO (including nanotubes, nanosheets, fibers, etc.) and volume stability, remains unclear, and needs to be further researched. In the long term, researchers should focus on experimental works with graphene and GO from the perspectives of structural considerations and the material itself.

3.1.3. Three-Dimensional Graphite in Cement/Concrete Composites

Besides the 1D and 2D carbon materials for the modifiers, 3D graphite material could also be used in modifying cement. For example, Balachandra et al. have used low-cost graphite and polyvinyl alcohol fibers for obtaining the high mechanical properties of concrete. They have identified that the desired degrees of nano- and micro-scale strengthening systems and the synergistic effects derived from nano- and micro-scale in concrete may be due to the activities occurring at various scales. The reason for the balanced gains of different engineering performances was that graphite nanomaterials could control the rate and spread of the moisture into the concrete. The complementary effects were noticed in the concrete with good properties, and they could be owing to the alleviation of the formation and propagation of micro-cracks which could lead to a higher compressive strength and flexural strength compared to raw cement. To illustrate these results, the SEM images and the mechanical performance of the composites are shown in Figure 3c–f [62].

Figure 3. (a) Flexural and compressive strength of the specimen [61]; (b) EPD system of GO/CF fibers [61]; (c) SEM image of graphite nanoplatelets dispersed in high-performance concrete [62]; (d) high magnification image of the graphite nanoplatelet; for the mass production of GO/CF hybrid fibers [62]; (e) compressive strength, and (f) flexural strength of concrete materials with different volume fractions of graphite nanomaterials and PVA fiber [62].

Figure 3. (a) Flexural and compressive strength of the specimen [61]; (b) EPD system of GO/CF fibers [61]; (c) SEM image of graphite nanoplatelets dispersed in high-performance concrete [62]; (d) high magnification image of the graphite nanoplatelet; for the mass production of GO/CF hybrid fibers [62]; (e) compressive strength, and (f) flexural strength of concrete materials with different volume fractions of graphite nanomaterials and PVA fiber [62].

There are not many works about graphite enhancing-cement or concrete composites, but graphite with a 3D spatial net structure could cause a rise in the contact areas with the cement or concrete, which may be favorable for improving their mechanical performance. Therefore, the research into the application of graphite-based cement/concrete composites needs to be widely carried out due to both its excellent performance and its low cost.

The differences in mechanical performances between different carbon materials strengthening cement/concrete hybrids are listed in

Table 1. Cement and concrete composites with carbon modifiers and their related mechanical properties.

Raw Material	Modifier	Enhanced Compressive Strength (%)	Enhanced Flexural Strength (%)	Reference
mortars	CNT	10	22	[44]
foam concrete	CNT	70	70	[50]
cement	CNT	/	32	[52]
oil well cement	GO	43	42	[58]
cement	GO/CF	15	/	[61]
concrete	graphite	500	300	[62]

.

3.1.4. Carbon Material Applications in Other Civil Composites

Apart from traditional cement and concrete, many other construction materials, such as SiC, alumina ceramics, and slag-based materials with carbon modifiers are also listed in

Table 2. Other composites with carbon modifiers and their related mechanical properties compared with concrete and cement composites.

Raw Materials	Modifier	Mechanical Properties	References
15R-SiAlON polytype	MWCNT	HV& KIC achieved within 0.25–0.5 wt% MWCNT loading in 15R matrix.	[63]
ZrB2	CNT	The flexural strength and fracture toughness of the nanocomposites were 1184 ± 52 MPa and 10.8 ± 0.3 MPa·m1/2, which is two times of that pure ZrB2.	[64]
3 mol% yttria stabilized zirconia (3YSZ)	CNT	The fracture toughness increased from 5 MPa m1/2 (in 3YSZ) to 10.1 MPa m1/2 with CNT reinforcement (for 12 vol%)	[65]
silica matrix	Carbon fabric	Flexural strengths of the 3D CMC were doubled compared with 2D counterparts.	[66]
SiC	Graphene	Young’s modulus has enhanced 31.7% with 5% single layer graphene sheet.	[67]
Alumina ceramic	Graphene and MWCNT	The hardness and fracture toughness increased prominently with 0.2 wt.% and 0.2 wt.% graphene and hybrid-functionalized CNT.	[68]
Slag-based nanocomposite	Graphene	The compressive strength of graphene reinforced slag-based nanocomposite increased by 31.10% and the flexural strength added up to 96.2% at a curing age of 28 days.	[69]
Poly(propylene carbonate) (PPC)	SiC/GO	Both the tensile strength and the ductility of PPC were improved by adding only a small amount of the SiC/GO hybrid (0.1 wt%).	[70]

. These examples can be referred to as a reference and were inspired by researchers. For example, Bandyopadhyay et al. reinforced dense 15R-SiAlON polytype with CNT, and among the different samples, 0.5 wt.% CNT/15R-SiAlON hybrid provided 9.6–17% higher hardness and as well as 12–13% fracture toughness over the monolith. They found that the influencing factors which enhance the tribo-mechanical properties of the composites are mainly due to elongated matrix grains and reinforcement by survived CNTs as well solid lubrication and refined matrix grains by the pulled-out CNT and dispersed CNT, respectively [63]. Another example is when, over nano-ZrB₂ nanocomposites, CNTs were synthesized in situ by using Ni/Y catalysts. The flexural strength and fracture toughness of the CNT/ZrB₂ nanocomposites are approximately 1184 MPa and 10.8 MPa·m^1/2, respectively. Similarly, in situ dispersion of CNTs could enhance the performance, which renders CNT/ZrB₂ composites with the best mechanical performance compared with those monolithic ZrB₂ nano-ceramics. The strengthening mechanisms involved the CNT de-bonding, pull-out, and fracture effect, besides crack deflection and the bridging effect [64]. Also, Balani et al. modified 3 mol% yttria-stabilized zirconia (3YSZ) by the addition of CNTs, and the fracture toughness increased two times from 5 MPa m^1/2 to 10.1 MPa m^1/2 due to an energy dissipating mechanism such as CNT bridging, crack branching, and deflection, which is evidenced by the micrographs of 3YSZ-CNT composites [65].

Also, as a one-dimensional carbon material, carbon fiber (abbreviated as: C_f) has been adopted for reinforcing fused silica composites [66]. A new type of 3D orthogonal woven structure carbon fiber strengthened silica ceramic matrix composites were fabricated by using a facile slurry impregnation and hot-pressing method. The 3D composite of C_f/SiO₂ displayed a larger flexural strength in both weft (263.6%) and warp (201.6%) directions than the raw SiO₂. The experimental results demonstrated that the maximum impact energy absorption of this 3D composite was 96.9 kJ/m², which was nearly four times bigger than that of other pure carbon fiber-reinforced ceramic matrix composites.

Except for CNTs, graphene could also be widely used in other civil materials. For example, (SiC) has been reinforced by graphene; the single layer sheet and double layer sheets of graphene have been used as the strengthening modifiers at 3 at.% and 5 at.%, respectively [67]. When the volume fraction reaches 5%, it brings an increase of 31.7% for the Young modulus. In addition, they also found that with the increasing number of graphene layers, there is a reduced mechanical behavior, which may originate from the occurring interlayer sliding from the weak van der Waals interactions. The excellent strengthening result was due to the non-smooth surface of graphene sheets for improving the interlocking between the reinforcement phase and the matrix. Derived from different samples, it is analyzed that with the addition of graphene, there is an interlayer slippage in graphene which denotes the weak properties of the nano-architecture in strengthening ceramic materials.

Guo’s group have adopted both graphene and multi-walled carbon nanotube as the agents for toughening the alumina ceramic coating [68]. The results demonstrate that the composite is dense, and both the hybrid treated with CNT and graphene are evenly dispersed, with 0.2 wt.% graphene and 0.2 wt.% functionalized CNT bringing about an eminent rise in hardness and fracture toughness. Also, the composite has a much lower wear depth and grinding crack width compared with initiative alumina coating. Also, graphene as a modifier has been used for reinforcing slag-based nano composite (GRSN) by incorporating it into the alkali-activated slag-based cementitious-material (ABSC) [69]. The result of mechanical testing revealed that the flexural and compressive strengths of ABSC were remarkably improved with the presence of graphene. When incorporating 0.02 wt.% graphene into ABSC, the compressive and flexural strength increased by 31.1% and 96.2%, respectively. Apart from graphene, GO as a substitute material could also be used as an agent for enhancing the performance of ceramic silicon carbide (SiC) nanowires [70]. The scroll-like nanostructure of the nanocomposites improves the compatibility of SiC with poly(propylene carbonate) (PPC) and the adhesion, whilst avoiding aggregating the GO sheets in the PPC. They have revealed synergistic effects with superior mechanical properties compared to the ones used as individual nanocomposites. With the presence of only 0.1 wt% of the SiC/GO composite, the enhancement of the tensile strength and ductility performance of the PPC-based nanocomposites displays a notable improvement.

From this, it can be shown that a large number of publications keep on researching ceramic/carbon materials although it remains a new area. Evidently, carbon materials could enhance the toughness of ceramics, which could bring about a prospective effect of structural ceramics. However, the research into the mechanical properties of carbon material-modified ceramic composites has the potential for enormous development in the future.

3.2. The Strengthening by Chemical Bonding Route

To analyze the strengthening mechanisms in the cement materials more precisely, many researchers have focused on the influence of different contents of short CNTs (with an aspect ratio of about 157) and long CNTs (with a high aspect ratio of 1250–3750) in the cement matrix. Bryan M. Tyson et al. found that the flexural strength of 0.2 wt.% short CNT and 0.1 wt.% long CNT enhanced by 269% and 65%, respectively, besides the virgin cement specimen for 28 days, and concluded that the nanocomposite with long CNTs of a low concentration afford equivalent mechanical properties to that of short CNTs of higher concentration, as shown in Table 3. In addition, the short CNT with 0.2% concentration has the best result among all the specimens at an age of 28 days, which may contribute to a relatively better dispersion in short CNTs, and cause a reduction in filament-free volume, and following an effective nano-sized pore filling. It was observed that the long CNT hybrids showed higher strength and less ductility than the relevant short CNT composites, which may be because of significantly more breakage of long CNTs that reversely cause an especially better dispersion. Also, to prove the results, the SEM and TEM images shown in Figure 4 illustrate that triumphant crack-bridging exists through the CNT modifier and pull-outs and breakage behaviors. Also, the TEM images display an evident embedment of CNTs and bridging of closing hydration products by long CNTs, which demonstrate that CNTs of higher aspect ratios are more advantageous modifiers if they are well-dispersed [71]. The stress–strain plots of the samples for different kinds of CNTs are displayed in Figure 4d.

Some researchers [72] have adopted carbon fibers and yarns as modifiers by using a facile and novel method to enhance the interactions with the cementitious matrix to improve the bond and the following mechanical performance in cement-based composites. The bond between carbon fiber and the cement-based matrix can be enhanced by anodic oxidation in cement pore solutions, and during the process, oxygen-involving groups formed on the fiber surface, which could form calcium carbonate by reacting with calcium in the form of an inorganic dense coating among the fibers. The results are that the shear-bond strength of carbon fiber was enhanced by 37.9% with a low voltage of 3 V and duration of 15 min, while the decrease in shear bond in longer treatment may have occurred due to the surface disruptions in alkaline electrolyte, as presented in Table 3. They believed that anodic oxidation could be a good approach to strengthen the compatibility between the carbon fiber and cement matrix by selecting some appropriate parameters. The surface morphology of carbon fibers and mechanical performance are shown in Figure 4e–g [72].

Figure 4. (a) SEM image illustrating the micro-crack bridging and breakage of the MWCNT into the cement matrix [71]; (b) cryo-TEM image of short MWCNTs and (c) long MWCNTs within the hardened cement paste [71]; (d) the stress–strain diagrams for the samples for the 0.04% short MWCNT and 0.1% long MWCNT at periods of 14 and 28 days [72]; (e) the tensile strength of carbon fiber through the effect of treatment voltage and duration [72]; (f,g) the shear bond strength obtained from single fiber pullout tests for different modification parameters [72].

Figure 4. (a) SEM image illustrating the micro-crack bridging and breakage of the MWCNT into the cement matrix [71]; (b) cryo-TEM image of short MWCNTs and (c) long MWCNTs within the hardened cement paste [71]; (d) the stress–strain diagrams for the samples for the 0.04% short MWCNT and 0.1% long MWCNT at periods of 14 and 28 days [72]; (e) the tensile strength of carbon fiber through the effect of treatment voltage and duration [72]; (f,g) the shear bond strength obtained from single fiber pullout tests for different modification parameters [72].

Li Zongjin et al. [73] have investigated the roles of graphene and GO on cement paste when it comes to hydration, architecture, and mechanical performance in-depth. They found that graphene could decrease the hydration growth and mechanical properties because of the poor dispersion in alkaline conditions, also, by the introduction of 0.16 wt.%, GO could improve the flexural strength by 11.62% originating from the higher nano-filler effect, hydration extent, and cracking-bridging function. Further, they investigated two different mechanisms by using reactive force field molecular dynamics at the macro, micro, and atomic levels. They disclose that the functional hydroxyl groups from GO afford sites of non-bridging oxygen accepting hydrogen-bonds from interlayer water molecules between the calcium silicate hydrate (abbreviated as C-S-H). Protons transmit from the hydroxyl groups in GO to the sites of non-bridging oxygen, which is still favorable for the polarity of the surface of GO and as well strengthens the bonding between the adjacent species. Apart from the H-bond interconnections, the Al³⁺ and Ca²⁺ ions in the neighboring surface of C-S-H take on a moderate role in bridging oxygen atoms from the silicate chains and -OH groups in GO, which remedies the defective GO structure and also enhances the silicate chain length. From the kinetics, the functional -OH groups, calcium ions, and aluminate-silicate chains could create some “networks as cages”, and avoid the smooth diffusion of the interface water molecules intensely, which maintain the affiliation between GO and C-S-H. The strong H-bonds and calcium aluminate network may be responsible for the high cohesive force and improved plasticity for the GO-introduced cement hybrids. Inversely, the poor bonding and the unsteadiness of atoms on the surface may be accountable for the poor mechanical behavior of the graphene/calcium-silicate hydrate composite. According to analysis based on the macro results, the compressive and flexural strength are promoted by 3.21% and 11.62%, respectively, in GO modified cement composites. Otherwise, with the hydrophobic benzene-ring structures, graphene with an interior surface with less friction may accelerate the transmitting process of the water molecules, which could decrease the binding stability among the systems of graphene and C-S-H. That means the mechanisms between the graphene and GO modifier are mainly derived from the different cohesive forces and stability of the atoms in the interface region, which is consistent with the experimental results. Interestingly, Al atoms can reconnect the C-O or C-OH groups and slow down the propagation of the cracks and cause the following enhancing plasticity. The researchers investigated the chemical reactions of GO/C-S-H systems to analyze the reason for the improvement of cohesive strength and plasticity. As demonstrated in Figure 5a, the amount of various hydroxyl groups is annulled with the development of tensile strain. The number of other hydroxyl groups is unchanged at the early stage of the tensile progress, but as the strain rises from 0.1 to 0.3 Å/Å, the numbers of Si-OH groups and water molecules decrease, and those of C-OH and Ca-OH also decrease. The variation in the hydroxyl number means that water association and disassociation occur constantly within the tensile procedure. The hydrolytic reaction has been dissertated on C-S-H gel for tensile loading, that is, water molecules strike the Si-O-Ca bonds, impair the bond strength and are dissociated to H⁺ and OH⁻, eventually bringing the same amount of Si-OH and Ca-OH bonds. The response clarified that the water amount declines and both Si-OH and Ca-OH numbers enhance concurrently after 0.3 Å/Å, which is shown in Figure 5c,d. In addition, the mechanism of “hydrolytic reaction” with water isolating to Si-OH and Ca-OH, has been found for the sample of the GO-C-S-H composite underneath tensile loading. Furthermore, the compressive and flexural strength of graphene and GO-modified cement samples are also shown in Figure 5e,f [73]. Additionaly, mechanical property data comparing it with other carbon materials are also presented in Table 3.

Figure 5. Hydroxyl number evolution with tensile strain in (a) GO/C-S-H [73]; (b) GO/CASH; ionization pathway for (c) Si-OH production [73]; (d) water production. Among them, the red, gray, green, yellow, white, and purple balls denote oxygen, carbon, calcium, silicon, hydrogen, and aluminum atoms, respectively. Also, the gray stick is the C-C bond, the yellow-red stick is the silicate bond, and the white-red stick denotes the hydroxyl bond [73]; (e,f) compressive and flexural strength of graphene and GO-modified cement paste [73].

Figure 5. Hydroxyl number evolution with tensile strain in (a) GO/C-S-H [73]; (b) GO/CASH; ionization pathway for (c) Si-OH production [73]; (d) water production. Among them, the red, gray, green, yellow, white, and purple balls denote oxygen, carbon, calcium, silicon, hydrogen, and aluminum atoms, respectively. Also, the gray stick is the C-C bond, the yellow-red stick is the silicate bond, and the white-red stick denotes the hydroxyl bond [73]; (e,f) compressive and flexural strength of graphene and GO-modified cement paste [73].

In view of the various advantages of GO, many researchers have focused on its engineering application in civil materials. In another example, GO was adopted for enhancing the mechanical performance of Strain Hardening Cementitious Composites (SHCCs), including compression, flexure, and tension. Lu et al. [74] have found that with the presence of 0.08 wt.% GO could lead to about a 24.8% rise in compressive strength, a 37.7% rise in tensile strength, and an 80.6% rise in flexural strength, respectively, which denotes evident strengthening results. The reasons for the strengthening mechanism are primarily the stable chemical bonding between the cement paste and the polyvinyl alcohol by the addition of GO. Specifically, the wrinkled GO may interconnect various phases together and consequently raise the crack tip toughness. Although GO plays a big role in the strengthening process, too high of a concentration of GO will produce a fracture of polyvinyl alcohol (PVA fiber) before being drawn out from the matrix and therefore should be prevented in the process of material design. The enhanced behavior for the SHCCs due to the declined porosity which is called the pore-filling effect, the increased toughness at the level of nanoscale by the interconnecting mechanism, the related performances, and the mechanism explanation are all shown in Figure 6a–c [74]. Finally, they also found that moderate content of GO is advised due to de-bonded process and the following negative effect from too much GO [74,75,76,77,78,79,80].

Also, Duan’s group [81] modified the interface between mortar and polyvinyl alcohol (PVA) fibers with painting GO, and experimental results displayed that the GO@PVA fibers enhanced the tensile strength by 35.6% compared with that of raw PVA fiber reinforced cementitious composites (FRCCs). Similarly, a theoretical investigation demonstrated that the chemical bond energy at the fiber/matrix interface would be responsible for this major enhancement. The chemical bond energy rose more than 80 times, and changed from adhesive failure to cohesive failure, which is consistent with previous works about the insufficient bonding strength at the interface between the fiber and matrix. The tensile strength of the GO-modified PVA FRCCs enhanced 35.6% compared to untreated PVA FRCCs, which may be attributed to the increased chemical bond energy from 3.58 J/m² to 290.26 J/m², as summarized in Table 3 [81,82]. Compared with other reports about GO-modifying oil agent materials, this work suggested that the mechanical behaviors of FRCCs could be significantly strengthened with little content of GO, illustrating the high capability of GO, shown in Figure 6d–f [81].

Figure 6. (a) Compressive strength plots of the specimen [74]; (b) uniaxial tensile plots [74]; (c) mechanism of the enhanced mechanical performance of the GO-reinforced SHCCs [74]; (d) schematic sketch of the synthesis procedure of GO@PVA fibers [81]; (e,f) tensile strength and compressive strength of fiber reinforced cementitious composites (FRCCs) [81].

Figure 6. (a) Compressive strength plots of the specimen [74]; (b) uniaxial tensile plots [74]; (c) mechanism of the enhanced mechanical performance of the GO-reinforced SHCCs [74]; (d) schematic sketch of the synthesis procedure of GO@PVA fibers [81]; (e,f) tensile strength and compressive strength of fiber reinforced cementitious composites (FRCCs) [81].

In addition, Li et al. [83] have recently found that the GO aggregates’ shape originated from the chemical cross-linking of calcium cations in the cement matrix. They firstly characterized GO aggregates through the particle size measurement, and investigated its influence on the tensile strength, hydration, as well the sorptivity of cement paste. GO nanosheets were cross-linked chemically to constitute larger aggregates in a saturated solution of Ca(OH)₂, which have bigger aspect ratios and dimensions than those of the former GO nanosheets. The tensile strength was largely enhanced by 67% with the addition of just 0.04 wt.% GO compared with that of the original cement sample due to the substituting role of fibers which could strengthen cracking strength, toughness, and resistance, as seen in Figure 7a–f [83].

Similarly, GO modified by Portland cement (PC) has been used for achieving uniform dispersion in alkaline cement paste. With the presence of PC@GO, the fresh cement mortar has a beneficial fluidity for liable compactness. It is discovered that it can enhance the compressive strength and flexural strength by 27.64% and 26.74%, respectively, with the introduction of only 0.022% GO, also summarized in Table 3. The reason for the improvement is mainly due to the improved extent of cement hydration and uniformly distributed hydration crystals including processed cracks. The special features of high specific surface area and oxygen functional groups make GO an active addition which could afford favorable nucleation sites for the hydration process and facilitate the hydration of the cement. Also, GO could control the shape and congregation of the products from the hydration process as well as the initiation and propagation of the cracks at the nano scale level. In all, the GO strengthening mechanisms may be outlined as an advanced degree of hydration, superior transfer efficiency, condensed microstructure, and crack refinement. The SEM images in this work and the mechanical performance are illustrated in Figure 7g–q [84].

Figure 7. (a–d) Optical images of GO aggregates at different magnification factors [83]; (e) size distribution of GO aggregates by optical image analysis [83]; (f) tensile strength of different contents of GO composites [83]; (g) TEM images of GO nanosheets [84]; (h,i) TEM images of dispersion states of PC@GO in saturated CH solution [84]; (j–l) SEM images of crack pattern in plain-cement composites at 7 days [84]; (m–o) SEM images of various fine cracks in GO-cement composites at 7 days [84]; (p,q) standard deviations of compressive strength results and flexural strength results of both the plain-cement and GO-cement samples at various curing times [84].

Figure 7. (a–d) Optical images of GO aggregates at different magnification factors [83]; (e) size distribution of GO aggregates by optical image analysis [83]; (f) tensile strength of different contents of GO composites [83]; (g) TEM images of GO nanosheets [84]; (h,i) TEM images of dispersion states of PC@GO in saturated CH solution [84]; (j–l) SEM images of crack pattern in plain-cement composites at 7 days [84]; (m–o) SEM images of various fine cracks in GO-cement composites at 7 days [84]; (p,q) standard deviations of compressive strength results and flexural strength results of both the plain-cement and GO-cement samples at various curing times [84].

In addition, Duan’s group have also adopted GO aggregates for the modifier in cement paste. They have used silica fumes to enhance the GO dispersion and improve the characteristics successfully. With a better dispersion of GO, improving mechanical properties becomes more effective [85]. Apart from cement, graphene can also be used in concrete material by engineering technologies. As the most used construction material all over the world, its chemical and physio-mechanical performance should be significantly improved.

However, the GO strengthening of cement still faces major challenges. As is known, GO is hydrophilic and could absorb the water remaining in the cement mortar, and hamper the hydration, thus making the dispersion of GO become difficult. Many methods include making multiple additives linked to the oxidation and changing the molecular structure of GO sheets by introducing some defects, which may lead to uncertainties in the molecular interactions of GO–cement. Also, this method is costly, which turns production unfeasible for scale up. In view of another aspect, most previous investigations about the GO modifying the cement were conducted on small samples which cannot be directly suitable for concrete because of altering mechanical behaviors after the introduction of sand and aggregate.

To solve these problems, Craciun’s group [86] reported an unprecedented result of improved mechanical properties compared to standard concrete, that is an improvement of up to 146% and 79.5% for compressive strength and flexural strength, respectively. Furthermore, comparative mechanical data against other carbon materials is also presented in Table 3. The fresh concrete pastes were poured in standard 10 cm × 10 cm × 10 cm steel molds, and the evolution of compressive strength over time as one of the most important mechanical properties was measured compared to standard concrete. Besides outstanding mechanical performance, this novel graphene–concrete hybrid can have multiple applications, deriving from the peculiar properties of graphene. The investigated reinforcing of concrete through the incorporation of graphene could be due to the terms of the modification process in cement hydration. The cement powder undergoes physical transformations to fibrous crystals including calcium silicates, calcium hydroxide, and alumino-ferrites, and by forming 40 kinds of silicate crystals, they form the calcium silicate hydrate (C-S-H) gel, which is the main influencing factor responsible for mechanical performance in concrete. As the paper denoted, graphene could interact with multiple elements and form a vast group of C-S-H, and make the structure of the hydration crystals change. Specifically, C-S-H may bond to graphene and act as the nucleation site, and promote the growth of C-S-H gels along the flakes of graphene, which causes an improvement in the bonding strength. On the other hand, the degree of crystallinity is another vital physical parameter charge for mechanical properties such as strength and Young’s modulus. When analyzing XRD results, there were some microstructural changes in the early period, which are the higher strength charge of the concrete at later periods (7 and 28 d). In addition, linking C-S-H with the E_c of 23.8 GPa with graphene which has an E_c of 2 TPa should give rise to a significant increase for the E_c of the composite. Apart from these factors, the degree of porosity may determine the compressive strength simultaneously, which is derived from empty pores within the cement matrix because of the leaching of Ca(OH)₂ or unhydrated crystals. When concrete is faced with fresh water, Ca(OH)₂ crystals tend to leach out due to the high solubility and nano-scale level. Therefore, this procedure raises the porosity of concrete, and decreases the strength in the meantime. That means that graphene could have an effect on lessening the degree of porosity, which is consistent with previous works referred to above. With the presence of GO sheets, the microstructures of cement matrix have become denser and finer, resulting in an improvement for the strength and durability. The plots with ultrahigh compressive strength and flexural strength in this work are displayed in Figure 8 [86].

Table 3. Cement/mortar/concrete composites with carbon modifiers and their related mechanical properties.

Raw Material	Modifier	Mechanical Performance	Enhanced Strengths (%)	Reference
cement	long and short CNT	flexural strength	65; 269	[71]
cement-based matrix	carbon fiber	shear bond	38	[72]
cement	graphene and GO	compressive and flexural strength	7, 43	[73]
mortar	GO@PVA	tensile strength	36	[81]
Portland cement	GO	compressive and flexural strength	28, 27	[84]
concrete	graphene	compressive and flexural strength	146, 80	[86]

Table 3. Cement/mortar/concrete composites with carbon modifiers and their related mechanical properties.

Raw Material	Modifier	Mechanical Performance	Enhanced Strengths (%)	Reference
cement	long and short CNT	flexural strength	65; 269	[71]
cement-based matrix	carbon fiber	shear bond	38	[72]
cement	graphene and GO	compressive and flexural strength	7, 43	[73]
mortar	GO@PVA	tensile strength	36	[81]
Portland cement	GO	compressive and flexural strength	28, 27	[84]
concrete	graphene	compressive and flexural strength	146, 80	[86]

4. Conclusions

This comprehensive review demonstrates that carbon-based nanomaterials (CNTs, graphene, etc.) significantly enhance the mechanical properties of cementitious composites through multiple mechanisms, including crack bridging, pore refinement, and interfacial strengthening. Scientific results confirm that even low concentrations of these modifiers can improve compressive strength, flexural toughness, and durability by densifying microstructures and reducing water permeability [87,88,89,90]. Crucially, the effectiveness depends on uniform dispersion and interfacial bonding, with chemical functionalization (e.g., –COOH groups on CNTs) proving crucial for compatibility with cement hydrates [91,92].

For practical applications, carbon-modified concrete shows promise for high-performance infrastructure (e.g., seismic-resistant structures, thin pavements) and smart systems (e.g., self-sensing bridges) [93,94]. However, scalable production requires resolving cost barriers and standardizing dispersion techniques (ultrasonication, surfactants). Hybrid systems combining carbon materials with low-cost mineral admixtures (e.g., fly ash) may offer a balanced solution. Future work should prioritize industry collaborations to translate lab-scale successes into real-world implementations, guided by lifecycle assessment and standardized testing protocols [95].

5. Summary and Perspectives

In this review, we have introduced the mechanical properties and mechanisms of various civil materials with carbon modifiers. We also conclude that carbon materials have enormous potential in concrete and cement materials originating from their special mechanical performance characteristics and other related behaviors. The introduction of carbon nanomaterials in concrete or cement or some other related civil materials can enhance their compressive, flexural, and tensile strength [96,97]. Recent investigations have considered nanomaterials such as CNT, graphene, GO, rGO, nano-alumina, nano-silica, nano-titania, nano-ferric oxide, etc. The inclusion of these nanomaterials in concrete/cement can provide denser microstructures, thus decreasing water absorption, and leading to improved workability [98].

Therefore, undoubtedly, researchers should adopt and develop the advantages of carbon materials such as the king of the materials—graphene—and design particular hybrids in traditional civil materials to achieve comprehensive mechanical performances. In addition, researchers may focus on some emerging 2D crystalline materials which could substitute carbon materials due to their special characters, such as black phosphorus molybdenum disulphide, and MXenes, which may offer new probabilities for multifunctional and high-efficiency civil materials in the future [99]. Nevertheless, there are also a series of challenges remaining for the research topic. Taking representative carbon modifier graphene as an example—how to yield defect-free scale-up graphene with tunable surface chemistry, how to minimize structural damages during the creation process, and how to adjust the layer numbers accurately are all critical problems related to the development of graphene modifiers. Likewise, the same questions apply for the adoption of black phosphorous, which has considerable potential as a next-generation alternative for graphene, which must be addressed and resolved in the near future. Researchers should develop different mechanical cement and concrete hybrids with synergistic effects. Some graphene analogs, such as metal chalcogenide, silicene, stanene, MXenes [100], BP [101], and two-dimensional (2D) oxyhalides, may provide new possibilities as next-generation modifiers compared with carbon materials. Above all, similar and diverse modifier structures must be developed in the area of civil materials in the future [102,103]. Naturally, some graphene-like structures, such as BP, and some 2D topological insulators [104,105,106] and transition metal dichalcogenides may be used as graphene-alternative modifiers to achieve effective mechanical properties in cement/concrete composites. A schematic illustration of the process is shown in Figure 9.

In addition, 2D material-based hetero-structures will certainly find their applications in civil materials in the future. As we summarized here, 1D/2D and 2D/3D hybrid-architectures may be positively designed in the civil field and so far, there are few reports about the applications of the mechanical performance of these series of structures, such as CNT@graphene, graphene@graphite, etc. [107]. On the other hand, 2D/2D hybrid structures have also been rarely reported on, probably because of the intrinsic stresses for synthesizing 2D/2D hetero-structures, which require advanced technical methods.

Based on the available theory, there is weak Van der Waals coupling between the organic layers and inorganic layers. It is also possible to create hybrids of some organic layers with other 2D materials for designing optimum modifier materials [108]. Although considerable progress has been made, studies of multiple carbon materials in reinforcing civil materials are still in the initial stage and there is much more space for their development. The preparation and design of modifier materials and structures require further exploration. Specifically, the interplay of influence derived from the carbon modifiers in civil materials requires further theoretical investigation [109].

In conclusion, the application of nanotechnology in cement/concrete-based materials is still in an infant stage. The results of experimental tests conducted on carbon nanomaterial-strengthened cement/concrete specimens have shown that they can improve the mechanical strength and durability of concrete [110]. Furthermore, these carbon nanomaterials can result in a novel generation of cement-based composites with strain-sensing abilities for damage inspection and structural health monitoring [111]. Finally, if the mechanisms behind carbon modifiers are clear and mature, the limitations which hinder the transformation from laboratory research to industrial application are still challenging, as they are associated with the costs of initial investments and carbon nanomaterial. Indeed, the researchers could consider using other more standardized materials to replace expensive carbon materials. Many new materials (such as MXenes, black phosphorus, carbon dots and some other nanostructures, etc.) also have excellent properties and can be considered to be used for substitution [112,113]. Additionally, advanced computer modeling techniques, including molecular dynamics simulations and machine learning-assisted design, could accelerate the optimization of these materials by predicting interfacial interactions, dispersion efficacy, and long-term stability under operational conditions [114].