Enhancement of Cement-Based Materials: Mechanisms, Impacts, and Applications of Carbon Nanotubes in Microstructural Modification

Abstract

Carbon nanotubes (CNTs) exhibit high strength and high modulus, excellent electrical and thermal conductivity, good chemical stability, and unique electronic and optical properties. These characteristics make them a one-dimensional nanomaterial with extensive potential applications in fields such as composite materials, electronic devices, energy, aerospace, and medical technology. Cement-based materials are the most widely used and extensively applied construction materials. However, these materials have disadvantages such as low tensile strength, brittleness, porosity, shrinkage, and cracking. In order to compensate for these shortcomings, in recent years, relevant scholars have proposed to integrate CNTs into cement-based materials. Incorporating CNTs into cement-based materials not only enhances the microstructure of these materials but also improves their mechanical, electrical, and durability properties. The characteristics and fabrication process of CNTs are reviewed in this paper. The different effects of CNTs on the physical properties and hydration properties of cement-based materials due to the design parameters, dispersion methods, and temperature were analyzed. The results show that the compressive and flexural strength of CNT cement-based materials with 0.02% content increased by 9.33% and 10.18% from 3 d to 28 d. In terms of reducing the shrinkage and carbonization resistance of the cement base, there is an optimal amount of carbon nanotubes. The addition of dispersed carbon nanotubes reduces the resistivity, and the nucleation of carbon nanotubes promotes the hydration reaction. In general, under the optimal dosage, carbon nanotubes with uniform dispersion and short length–diameter ratio have a significant effect on the cement-based lifting effect. In the future, CNT cement-based materials will develop high strength, multifunctionality, and low cost, realizing intelligent self-sensing and self-repair and promoting green and low-carbon manufacturing. Breakthroughs in decentralized technology and large-scale applications are key, and they are expected to help sustainable civil engineering with intelligent infrastructure.

1. Introduction

The performance of conventional cement-based materials can no longer meet the demands of special structural engineering projects (such as large-scale, high-rise, and smart structures). The tensile strength of concrete is much lower than its compressive strength, which means that it is prone to cracking under tensile stress [1,2]. This usually requires materials such as rebar to increase its carrying capacity. Under harsh environmental conditions (such as high humidity, salt erosion, extreme temperatures), ordinary concrete may crack, flake, and other damage, reducing its service life. Ordinary concrete is relatively heavy, which may put higher requirements on structural design and increase the difficulty of transportation and construction [3,4,5]. Ultra-high performance has become a prevailing trend in the development of cement-based materials. Ultra-high-performance concrete [6] material shows good toughness and ultimate peak ductility under pressure load, its failure form is stable, no surface spalling, and no brittle failure of concrete collapse. The ultimate bending strength can reach 6~8 MPa, and the ultimate deflection can reach about 38 mm.

To enhance the properties of cement-based materials and improve their mechanical performance, the incorporation of reinforcing materials is a common practice. Among these, the effective utilization of nanomaterials in the construction industry has emerged as a prominent research topic. Nowadays, a wide range of nanomaterials are graphene, nanosilica, carbon nanotubes, etc. The main advantage of adding CNTs to cement-based materials compared with graphene and nano-SiO₂ is their unique comprehensive properties (

Table 1. Advantages of carbon nanotubes over graphene and nanosilica.

	CNTs Dominance	Specific Reason
Graphene	Dispersion	CNTs have a high length–diameter ratio
	mechanics	CNTs fibrous structure can effectively bridge cracks
	Electrical conductivity	CNTs conductive network is easier to form
	Cost and process	CNTs are mature in industrial production and relatively low in cost
Nanosilica	Versatility	CNTs have both mechanical tensile and cracking resistance and electrical and thermal conductivity
	Toughness	CNTs bridging effect of inhibition of crack propagation and significantly increased toughness
	Long-term stability	CNTs had little effect on the hydration process

) [7,8].

As a one-dimensional nanomaterial, CNTs possess a lightweight nature, high specific surface area, and an exceptionally high aspect ratio, which contribute to their excellent mechanical, electrical, and chemical properties [9]. When combined with other matrix materials to form composites, even a small quantity of CNTs can significantly enhance the strength, elasticity, and electrical properties of the composite, thereby greatly improving its overall performance [10,11]. CNT cement-based materials can significantly improve the strength, durability, and functionality of concrete, promote smart infrastructure (such as self-monitoring and self-healing), reduce cement carbon emissions, and meet the stringent needs of high-rise buildings, marine engineering, and other important engineering and environmental considerations. Based on the properties of CNTs, this paper will study the performance improvement of cement-based materials by the incorporation of CNTs and study the future direction and prospects of CNTs.

The latest research progress of carbon-nanotube-cement-based materials was systematically reviewed. The preparation technology, strengthening mechanism, dispersion technology, and multifunctional properties of carbon-nanotube-cement-based materials were described to provide theoretical guidance for the development of high-performance, intelligent, and sustainable cement-based materials and promote their innovative application in civil engineering, as well as the innovative development of nanomodified cement.

2. The Fundamental Properties and Fabrication Processes of CNTs

2.1. Composition of CNTs

CNTs are one-dimensional quantum materials with a distinctive structure, characterized by a radial dimension on the nanometer scale and an axial dimension on the micrometer scale, with closed ends. The radial dimensions of CNTs are relatively small, typically ranging from a few nanometers to several tens of nanometers, making them ideal one-dimensional functional materials. CNTs are tiny, hollow cylindrical structures formed by the rolling of a hexagonal lattice similar to graphite, exhibiting advantages such as a light weight and perfect hexagonal connectivity. The arrangement of carbon atoms along specific crystallographic directions determines the electronic structure and properties of CNTs.

Based on the number of graphene layers, CNTs are categorized into single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) (Figure 1) [12,13]. SWCNTs are composed of a single layer of carbon atoms rolled into a hollow tubular structure, whereas MWCNTs consist of multiple concentric layers of carbon atoms rolled into a concentric tubular structure. Compared with MWCNTs, SWCNTs have a narrower diameter distribution, fewer defects, and higher uniformity, but are more expensive. MWCNTs, on the other hand, have achieved large-scale production and are cost-effective, leading to their widespread application in cement-based materials. Due to their excellent mechanical, electrical, and chemical properties, CNTs find extensive applications in various industries, including construction, automotive, aerospace, and medical fields [14].

Figure 1. Types of CNTs [15].

Figure 1. Types of CNTs [15].

2.2. Process of CNTs

At present, MWCNTs can be produced on a large scale. However, the production of SWCNTs is largely still at the experimental stage. Some synthesis methods have not yet clarified the growth mechanisms of CNTs, and precise control over their structural characteristics, such as wall thickness, helicity, diameter, length, and metallicity, as well as yield and purity, remains elusive. Future advancements in carbon nanotube synthesis technologies will address these issues, providing robust support for the widespread application of CNTs across various fields (Table 2) (Figure 2).

Table 2. Preparation technology of CNTs and its advantages and disadvantages.

Method	Craft	Product Type	Advantages	References
Arc discharge	Control pressure, adjust make-up rate, catalyst	SWCNTs, MWCNTs	Easy to operate, features simple procedures, is cost-effective, and has a short production cycle	F Y Zhang [16]
	Coal-based composite carbon rod was used as anode material	SWCNTs, MWCNTs	Higher yields	J S Qiu [17]
	A method for mass production of SWCNT by temperature-controlled electric arc furnace	SWCNTs	High purity	Y N Liu [18]
CVD	Method for fabricating carbon nanotube arrays by wire-assisted CVD	CNTs arrays	High crystallinity, intact structure and low impurity content	N Q Zhao [19]
	Preparation of CNT by hydrogen-free CVD	SWCNTs, MWCNTs	High purity, good structural integrity, thin, long pipe diameter	M Y Liu [20]
	Bulk preparation of bamboo CNT by CVD	Bamboo CNTs	Continuous batch preparation	F Y Kang [21]
Electrolytic process	High-temperature electrolysis of CO2 to CNTs	CNTs	Increase speed and reduce production costs	H J Wu [22]
	Electrolysis of CO2 to prepare graphene or CNTs	CNTs	Recyclable, environmentally friendly, easy to operate	S Q Jiao [23]
	Preparation of CNT-coated metal materials by molten salt electrochemical method	CNT-coated metal material	Simplify processes, save costs	W Xiao [24]

Table 2. Preparation technology of CNTs and its advantages and disadvantages.

Method	Craft	Product Type	Advantages	References
Arc discharge	Control pressure, adjust make-up rate, catalyst	SWCNTs, MWCNTs	Easy to operate, features simple procedures, is cost-effective, and has a short production cycle	F Y Zhang [16]
	Coal-based composite carbon rod was used as anode material	SWCNTs, MWCNTs	Higher yields	J S Qiu [17]
	A method for mass production of SWCNT by temperature-controlled electric arc furnace	SWCNTs	High purity	Y N Liu [18]
CVD	Method for fabricating carbon nanotube arrays by wire-assisted CVD	CNTs arrays	High crystallinity, intact structure and low impurity content	N Q Zhao [19]
	Preparation of CNT by hydrogen-free CVD	SWCNTs, MWCNTs	High purity, good structural integrity, thin, long pipe diameter	M Y Liu [20]
	Bulk preparation of bamboo CNT by CVD	Bamboo CNTs	Continuous batch preparation	F Y Kang [21]
Electrolytic process	High-temperature electrolysis of CO2 to CNTs	CNTs	Increase speed and reduce production costs	H J Wu [22]
	Electrolysis of CO2 to prepare graphene or CNTs	CNTs	Recyclable, environmentally friendly, easy to operate	S Q Jiao [23]
	Preparation of CNT-coated metal materials by molten salt electrochemical method	CNT-coated metal material	Simplify processes, save costs	W Xiao [24]

Figure 2. Test preparation process [25]: (a) Structure of arc discharge experimental device. (b) CVD. (c) High-temperature electrolysis.

Figure 2. Test preparation process [25]: (a) Structure of arc discharge experimental device. (b) CVD. (c) High-temperature electrolysis.

2.3. Characteristics of CNTs

2.3.1. Physical Characteristics

CNTs exhibit high strength, high modulus, exceptional electrical and thermal conductivity, good chemical stability, and unique electronic and optical properties. As a one-dimensional nanomaterial, they hold significant promise for applications in composite materials, electronic devices, energy, aerospace, and medical fields [26,27].

The mechanical properties of CNTs, particularly their high strength, have garnered considerable attention in engineering and technological applications. Their strength surpasses that of most materials, with a specific strength far exceeding that of steel. The low density of CNTs, due to the lightweight nature of carbon atoms, provides advantages in applications requiring weight reduction, such as in aerospace.

In terms of electrical properties, CNTs possess unique conductivity due to their high melting point. This makes them highly promising for applications in electronics, energy storage, and conversion. With a structure similar to graphene layers, CNTs exhibit excellent electrical performance. Their atomic diameter is in the nanometer range, categorizing them as one-dimensional materials with constrained geometric dimensions. Their electronic conductivity outperforms traditional conductive materials, offering substantial potential for electronic device applications.

CNTs also demonstrate remarkable flexibility and can be stretched. In reinforced fiber materials, the mechanical performance is significantly influenced by the length–diameter ratio. CNTs possess a length–diameter ratio far exceeding the typical 20:1 of conventional materials, making them ideal as high-strength fiber materials. Their hardness is approximately 100 times that of steel for the same volume, while their mass is only one-sixth to one-seventh that of steel.

2.3.2. Microscopic Characteristics

The addition of CNTs can improve the pore structure of cement-based materials, mainly through physical filling effects and hydration regulation, thereby reducing porosity, refining pore size distribution, and improving durability. Physical filling effect refers to the reduction in nanoscale pores in cement-based materials due to the addition of CNTs. Hydration regulation refers to the addition of CNTs to promote hydration reaction. As a nucleation site, CNTs accelerate cement hydration and generate denser C-S-H gel. The decrease in pore volume and the change in size distribution can be measured by MIP and nitrogen adsorption experiments.

CNTs exhibit good dispersibility, referring to their uniform distribution within a solution. Due to their high specific surface area and unique structure, CNTs tend to aggregate or agglomerate in conventional solvents, which may impact their effectiveness and performance in practical applications. Therefore, to achieve a uniform distribution of CNTs in a solution or solid matrix and to minimize or prevent aggregation, physical and chemical methods are employed (Figure 3).

Physical methods include ultrasonication, mechanical stirring, and ball milling. Ultrasonication provides energy to overcome inter-particle interactions. Ball milling can fragment CNT aggregates caused by intermolecular forces but may reduce the aspect ratio of CNTs and their functionality in the matrix. Mechanical stirring, which encompasses both manual and magnetic stirring, is typically combined with ultrasonication. Chemical methods improve dispersibility by enhancing the wettability of CNT surfaces through the introduction of covalent or non-covalent bonds, as well as functional groups such as hydroxyl and carboxyl groups.

Nowadays, the dispersion effect of MWCNTs is characterized by SEM, Zeta potential, Fourier transform infrared spectroscopy, Raman spectroscopy, and other methods [29,30]. The surface morphology changes in MWCNTs could be observed by SEM. The centrifugal method was used to determine the surface morphology changes according to the duration of apparent delamination. The Zeta potential was determined according to the changes in Zeta potential on the surface of MWCNTs (Figure 4a), the changes in surface functional groups could be characterized by Fourier infrared spectroscopy (Figure 4b), and the surface defects could be characterized by Raman spectroscopy (Figure 4c).

Cement-based materials are widely utilized across various fields of construction and production due to their favorable mechanical properties and durability, making them one of the most commonly used building materials. However, their inherent brittleness, relatively low tensile strength, and susceptibility to cracking adversely affect the durability of structural applications in practical engineering.

Researchers, both domestically and internationally, have explored the modification of cement-based materials by incorporating various admixtures to enhance their performance. Recently, the application of nanomaterials such as CNTs and graphene in cement-based materials has become increasingly prevalent. Among these, CNTs, known for their exceptional properties, have a significant impact on both the microstructure and macroscopic performance of cement-based materials. As an advanced material, CNTs can substantially improve the mechanical properties, durability, and electrical conductivity of cement-based materials. Additionally, they offer advantages such as lightweight characteristics and improved processability, indicating their extensive potential for application in construction engineering, infrastructure, and high-tech fields.

3. Effect Mechanism of Carbon Nanotubes on Cement-Based Materials

Cement-based materials are the most widely used and in-demand construction materials in the building industry. Traditional concrete, being a brittle material, no longer meets the functional requirements for modern construction materials. As a result, cement-based materials have evolved towards multifunctionality and higher added value. To improve the performance of cement-based materials and enhance the properties of cement, the incorporation of reinforcement materials is a common approach. Among these, the effective utilization of nanomaterials in the construction industry has become a prominent research topic. Most of the modeling studies on composite structures use numerical model methods [31,32,33,34]. This paper summarizes the progress in research on the impact of adding CNTs on the physical and chemical properties of cement-based materials.

3.1. Physical Characteristics

The incorporation of CNTs into cement-based materials significantly enhances their physical properties. Uniformly dispersed CNTs reduce the electrical resistivity of cement-based materials, thereby improving their electrical conductivity. Additionally, the inclusion of CNTs reduces harmful porosity and inhibits crack formation at the microscopic level. The bonding and adsorption between CNTs and cement enhance the mechanical properties of the cement-based materials. Furthermore, porosity and pore structure are critical factors in determining the depth of carbonation. Under equivalent conditions, a higher porosity facilitates the diffusion of CO₂ within the material, leading to increased carbonation depth and diminished carbonation resistance. The bonding and adsorption effects also significantly mitigate early-age shrinkage deformation.

3.1.1. Effects of Mechanical Properties

CNTs possess exceptionally high strength and modulus of elasticity. When uniformly dispersed within cement-based materials, they can effectively bear loads, inhibit crack propagation, and enhance the overall strength of the material. The diameter of CNTs typically ranges from tens to hundreds of nanometers, which allows for significant improvements in the microstructure of cement-based materials at the nanoscale. Uniform dispersion of CNTs can bridge microcracks within the cement-based matrix, prevent crack propagation, and increase the material’s flexural strength. Additionally, CNTs form a network structure within the cement-based materials, which efficiently distributes and transfers external loading stresses, thereby improving the overall strength and toughness of the material. The incorporation of CNTs also enhances the durability of cement-based materials, such as resistance to cyclic loading and chemical corrosion, which indirectly improves their compressive and flexural strengths.

• Static Strength

Compressive strength and flexural strength are commonly used mechanical properties of cement-based materials and are also important indicators for evaluating their mechanical performance [35,36]. The enhancement of mechanical properties of cement-based materials by CNTs occurs in two main ways: firstly, uniformly dispersed CNTs act as reinforcing bridges at the microscopic level, inhibiting the formation and propagation of pores and cracks in the matrix, thereby increasing material strength. Secondly, the high hardness of CNTs compared with ordinary concrete materials can improve overall strength and increase compressive capacity. However, some scholars argue that the uneven dispersion of CNTs in cement-based materials, forming clustered aggregates, introduces new pores and cracks, increasing the porosity and degrading the density of the cement-based material. Additionally, localized stress concentrations may occur near these aggregates, leading to a reduction in the material’s mechanical properties (Figure 5).

Li et al. [36] investigated the effects of MWCNTs treated with sulfuric and nitric acids on the mechanical properties of cement-based composites. Their study demonstrated that, at 28 days of curing, the incorporation of CNTs significantly improved the compressive strength of the cement-based materials. Liu dispersed surfactant-treated and ultrasonicated carbon nanotube dispersions into reactive powder concrete (RPC), finding that RPC materials with 0.025 wt% CNTs exhibited increases in compressive strength and flexural cracking strength by 7.2% and 36%, respectively, compared with the control group, with tensile cracking strength and ultimate strength also improving by over 8%, as well as a substantial increase in RPC energy absorption index. Chaipanich et al. [37] examined the impact of incorporating 0.5% and 1% CNTs on the mechanical properties of fly ash–cement composites (Figure 6a). Over hydration periods of 7, 28, and 60 days, the compressive strength of the fly ash–cement matrix increased with CNT addition by up to 1%, beyond which it plateaued. Li [38] conducted microstructural analyses of MWCNTs in cement paste using XRD phase analysis, thermogravimetric analysis, and SEM, revealing that up to 0.5% MWCNTs significantly enhances the flexural and compressive strengths of cement paste (Figure 6b,c). Tian [39] explored the mechanisms by which MWCNTs influence the mechanical properties and microstructural features of concrete after high-temperature exposure, finding that MWCNTs optimize pore distribution during the heating-cooling cycle, leading to higher residual compressive strength (Figure 6d).

2.

Bending Strength

Cement-based materials are typically brittle materials with high compressive strength but relatively low flexural strength due to their poor ductility. Under flexural loading, they are highly prone to damage and failure. The incorporation of CNTs effectively enhances the toughness and fracture toughness of cement-based materials, thereby improving their flexural strength.

Konsta-Gdoutos et al. [40] investigated the impact of MWCNTs with identical diameters but varying lengths on the flexural strength of cement-based materials, focusing on 0.048% and 0.08% CNTs. The results demonstrated that MWCNTs significantly improved the flexural strength of cement pastes, with shorter CNTs at higher dosages showing a faster rate of increase in flexural strength at a 3-day hydration period. The flexural strength of cement pastes increased with the CNT dosage when short CNTs were added. Metaxa et al. [41]. explored CNTs prepared using different centrifugation techniques, finding that the incorporation of CNTs dispersed by suspension techniques led to significant increases in flexural strength. Wang [42] investigated the enhancement mechanism of different-aged CNTs on cement-based materials, noting that a 0.02% CNT dosage improved the strength of the cement-based composites at 3, 7, and 28 days compared with ordinary cement-based materials. The increases in flexural strength were 1.01%, 5.14%, and 9.33% respectively, and the compressive strength increased by 7.88%, 6.35%, and 10.18% at the same ages.

3.

Dynamic Strength

The incorporation of CNTs enhances the performance of cement-based materials under dynamic loading conditions, including improved strength, hardness, toughness, impact resistance, overall durability, and resistance to aging. These attributes render CNTs highly promising for applications in construction engineering, infrastructure development, and other industrial domains.

Li et al. [43] and others have primarily investigated the optimal length–diameter ratio of cement-based composites in dynamic testing. Their research indicates that for a large-scale Hopkinson bar device with a diameter of 75 mm, the test specimens with lengths ranging from 30 to 75 mm (equivalent to a length–diameter ratio L/D ≈ 0.4–1.0) yield more accurate and reliable results in Hopkinson bar tests. Wang et al. [44] utilized a split-Hopkinson pressure bar (SHPB) device to study the dynamic mechanical properties of cement paste samples with different dosages of CNTs. The results show that, with a constant impact load, the dynamic compressive strength, elastic modulus, and peak toughness of the samples initially increased and then decreased with the rising dosage of CNTs. The maximum values for these properties were observed at a carbon nanotube dosage of 0.1 wt.%, which improved by 34.1%, 70.0%, and 15.4% compared with the control group, respectively. At this dosage, the degree of damage in the samples also significantly decreased. Wu et al. [45] employed dynamic mechanical analyzers to study damping under different strains, conducted compressive and flexural tests to assess mechanical property changes, and used scanning electron microscopy and mercury intrusion porosimetry to explore the reasons for mechanical property variations and the microscopic mechanisms of enhanced damping. They found that, at 28 days of age, the flexural strength of cement-based composites with 0.1% CNTs increased by 43% compared with the control group, while compressive strength decreased by 15%, the loss factor increased by 55.9%, and the loss modulus increased by 8.5%. Ruan et al. [46] performed dynamic compression tests on RPC modified with different types of CNTs at strain rates of 200 s⁻¹, 500 s⁻¹, and 800 s⁻¹. The results indicate that appropriate types and dosages of CNTs could increase the compressive strength growth factors of RPC by 73.7%, 82.0%, and 23.3%, respectively. Wang et al. [42]. dispersed hydroxylated and carboxylated modified CNTs into ultra-high-performance concrete (UHPC) and found that at strain rates of 200–800 s-1 and carbon nanotube dosages of 0.25–0.50%, all specimens exhibited varying degrees of improved impact performance. The bridging effect of the fibers effectively prevented crack propagation under impact loading, thereby reducing the material’s strain rate sensitivity.

The study of the static and dynamic mechanical properties of cement-based materials modified with CNTs reveals that the optimal dosage of CNTs and their performance enhancement effects vary due to differences in factors such as cement matrix, water–cement ratio, aggregate types, fiber types, and treatment methods.

From the above, we can conclude that CNTs can significantly improve the mechanical properties (10–50% increase in compressive and flexion strength), toughness (3–5 times increase in fracture energy), and durability of cement-based materials, as well as enhance bending strength and dynamic properties by bridging cracks and optimizing pore structure. However, it is necessary to control the dosage (0.1–0.5 wt%) and dispersity, because excessive will lead to agglomeration and reduce the density.

3.1.2. Effect of Contractility

Early shrinkage deformation in ordinary concrete can lead to significant cracking issues, greatly compromising the safety and durability of structures [47,48]. The incorporation of CNTs into the concrete can significantly mitigate early shrinkage deformation and enhance its early cracking resistance. Due to external factors and experimental conditions, the shrinkage behavior of cement-based materials varies. The impact of different types, concentrations, and aspect ratios of CNTs on shrinkage is analyzed.

• Different types

The differences in the types of CNTs, including their intrinsic physicochemical properties, surface characteristics, dimensions and shapes, interaction mechanisms with cement-based materials, compatibility, and stability, as well as their dispersion and stability within the materials, also lead to varying effects on the shrinkage of cement-based materials.

Feneuil et al. [49] investigated the impact of pristine and carboxylated CNTs on the autogenous shrinkage of sulfate-resistant Portland cement paste with a water–cement ratio of 0.3. Their results demonstrate that CNTs significantly reduce the shrinkage of sulfate-resistant Portland cement paste, with carboxylated CNTs exhibiting superior performance. Bogas [50] compared the effects of pristine, carboxylated, and hydroxylated CNTs on the total shrinkage of concrete with a water–cement ratio of 0.55, finding that the shrinkage values for the control group, pristine CNTs group, carboxylated CNTs group, and hydroxylated CNTs group were 43, 27, 20, and 27 μm/m at 2 days, respectively. The test results indicated that carboxylated CNTs were the most effective in reducing shrinkage. Hawreen [51] examined the shrinkage properties of cement mortar with pristine and hydroxylated CNTs, revealing that the incorporation of carboxylated CNTs and pristine CNTs mitigated the shrinkage of cement-based materials, with carboxylated CNTs demonstrating a more pronounced effect (Figure 7).

Different types of CNTs exhibit varying degrees of effectiveness in reducing shrinkage in cement-based materials, with carboxylated CNTs demonstrating the most optimal performance in suppressing early-age shrinkage.

2.

Different dosage

The concentration of CNTs significantly affects the shrinkage of cement-based materials, due to factors such as their dispersion and uniformity within the material, the network structure they form, and their influence on the cement hydration process. CNTs, as nanomaterials, are incorporated into cement-based materials in concentrations ranging from 0.01% to 1%, with varying concentrations leading to different impacts on shrinkage. An optimal concentration and good dispersion of CNTs can effectively improve the shrinkage characteristics of cement-based materials, enhancing their overall performance and durability [52].

Konsta-Gdoutos [41] investigated the effect of different dosages of CNTs (0.025% and 0.048% by cement mass) on the autogenous shrinkage of cement paste within 96 h. At 24 h, a significant difference in shrinkage values was observed, with the order being control > 0.025% CNT > 0.048% CNTs, and this trend persisted up to 96 h. The results indicate that the dosage of CNTs has a significant impact on shrinkage. Shi Tao et al. [53] studied the effect of incorporating CNTs on the shrinkage of hardened cement mortar and found that the incorporation of CNTs has a suppressive effect on shrinkage, which is further enhanced with an increase in the water–cement ratio (

Table 3. The dosage and water–cement ratio shrink the cement mortar.

Water Cement Ratio	The Dosage Shrinks the Cement Mortar/(μm/m)
	0	0.05%	0.1%	0.15%
0.30	−1397	−1178	−1119	−1318
0.35	−1178	−884	−780	−990
0.40	−1132	−728	−638	−981

). Xiao [54] discovered that, under the same water–cement ratio, the self-shrinkage of carbon nanotube–cement composites initially decreases and then increases with increasing carbon nanotube content. With an increasing water–cement ratio, the self-shrinkage of the cement-based material significantly decreases. At a water–cement ratio of 0.35 and a test age of 7 days, the optimal dosage of MWCNTs was found to be 0.05 wt%, which resulted in a 46.74% reduction in self-shrinkage compared with the control sample.

measured the shrinkage of cement paste (water–cement ratio of 0.25) with CNT dosages of 0.3% and 0.6% over 47 days, finding that the incorporation of CNTs increased the shrinkage of the cement paste. Under the same age conditions, an increase in CNT dosage led to greater shrinkage. At 47 days, the shrinkage of the 0.6% CNTs group increased by 18.4% compared with the control group (

Table 4. The shrinkage value decreases by percentage with the content of carbon nanotubes.

CNT content	0.05%	0.1%	0.15%	0.3%	0.6%
Shrinkage value decreased by percentage	15%	20%	5%	−5%	−18%

).

The incorporation of CNTs exerts a certain degree of suppression on the shrinkage of cement mortar. An increase in the water–cement ratio enhances the shrinkage-reducing effect of CNTs. At a constant water–cement ratio, cement shrinkage decreases with increasing carbon nanotube content; however, beyond a certain threshold, further increases in dosage do not continue to reduce shrinkage and may even increase it. This indicates the presence of an optimal dosage of CNTs for minimizing shrinkage in cement-based materials.

3.

Different Length–Diameter Ratio

The effect of the aspect ratio of CNTs on the shrinkage behavior of cement-based materials primarily manifests in their dispersion, enhancement effectiveness, bridging ability, and control over the cement hydration process. Selecting CNTs with an appropriate aspect ratio can more effectively improve the shrinkage characteristics of cement-based materials, thereby enhancing their overall mechanical properties and durability.

Hawreen [50] analyzed the impact of CNTs with aspect ratios of 300 and 667 on the shrinkage of mortar. At 2 days of curing, the shrinkage of both sets of samples was identical. At 7 and 90 days, there were no regular changes in shrinkage values between the two sets. At 180 and 365 days, the samples with an aspect ratio of 300 exhibited lower shrinkage. Overall, the influence of different aspect ratios of CNTs on the shrinkage of various cement-based materials showed inconsistent patterns. Diego (Figure 8) [55] investigated the effect of aspect ratios of 400 and 1500 on mortar shrinkage. The shrinkage values of CNT-incorporated samples were consistently higher than those of the control samples. At 7 and 14 days, the shrinkage of samples with an aspect ratio of 1500 was higher, while at 21 and 28 days, their shrinkage values were lower than those of samples with an aspect ratio of 400.

Due to the influence of materials and testing methods, the length–fineness ratio of CNTs has different effects on the shrinkage of cement-based materials.

According to the above content, it can be concluded that CNTs can effectively inhibit the early shrinkage of concrete, and the optimal effect should meet three conditions: choose carboxylated carbon nanotubes (shrinkage reduction of more than 50%); control the dosage of 0.05–0.1 wt% (the optimal dosage can reduce the shrinkage by 46.7%); and adopt medium length–diameter ratios (300–500). However, excessive addition (>0.5 wt%) or improper length–diameter ratios will increase and shrink. The inhibition effect is best when the water–cement ratio is 0.40.

3.1.3. Effect of Crack Resistance

The incorporation of CNTs into the matrix can mitigate the formation of fine cracks in the bonding structure on a microscopic level, preventing further cracking and secondary fissures [56]. The addition of CNTs facilitates the formation of a bridging network structure that acts as a fiber anchoring mechanism, linking cracks and enhancing densification. Furthermore, the early cracking resistance of cement-based materials is significantly improved with the inclusion of CNTs [57,58]. When the incorporation level is 0.10 wt%, the onset of cracking is delayed to the latest point, whereas at 0.05 wt%, the cracking time is extended to the longest duration.

Li [59] Employed a ring constraint method under fully sealed conditions, revealing that mortar specimens with 0.05 wt% CNT content exhibited the longest cracking time and optimal early cracking resistance. When the CNT content is 0.05 wt%, a higher water–cement ratio enhances the early cracking performance of the specimens. According to Zhuang, scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM) analyses of MWCNTs concrete specimens indicate that the addition of CNTs inhibits the further propagation of cracks in the cement matrix, shortens crack lengths, and enhances densification. FESEM images reveal a bridging network structure of CNTs within the cement matrix that prevents further crack development and demonstrates excellent tensile properties. Zheng et al. [60] analyzed the fracture performance of concrete enhanced with 0.15% CNTs by mass of cement using dual-K fracture parameters, finding that the crack initiation toughness, instability toughness, and fracture energy of the concrete increased by 31.0%, 32.4%, and 24.6%, respectively. Wang et al. [61] investigated the mechanical properties of cement-based composites reinforced with CNTs and polyvinyl alcohol (PVA) fibers. When the CNTs and PVA contents were 0.5% and 1.0%, respectively, CNTs/PVA were uniformly dispersed in the concrete, forming a three-dimensional network that refined pores, bridged cracks, and improved the microstructure of the concrete. Fakhim et al. [62] used SEM to observe that MWCNTs in cement-based materials act as bridges in microcracks, with proper dispersion of MWCNTs being crucial for the mechanical properties of cement-based materials. However, excessive MWCNTs can lead to internal microstructural damage due to agglomeration. Shi et al. [53]. incorporated CNTs into cement-based materials (

Table 5. Relationship between mass fraction of CNTs and cracking time and cracking strain.

CNTs Mass Fraction	Closed Experiment
	Cracking Time/h	Cracking Strain/10−6
0	212.2	−142
0.05	223.2	−133
0.10	310.2	−138
0.15	248.5	−121

), finding that the addition of CNTs improved the cracking resistance of the cement matrix. At 0.10 wt% CNT content, the cracking time was delayed to the latest point, but overall cracking time was not significantly influenced by the presence or absence of CNTs.

According to the above content, the conclusions are summarized as follows: Carbon nanotubes can significantly improve the cracking resistance of cement-based materials: 0.05–0.1 wt% of the content forms a three-dimensional bridge network, inhibits crack propagation, extends the cracking time by 46% (310 h), and increases the fracture energy by 24.6%. However, excess (>0.15 wt%) will reduce the effect due to agglomeration. The crack resistance is the best when the optimal dosage is 0.05 wt%, and the effect can be further enhanced with an increase in water–cement ratio.

3.1.4. Effect of Electrical Conductivity

The piezoresistive behavior of CNTs cement-based composites is characterized by their electrical conductivity [63]. The inclusion of uniformly dispersed CNTs reduces the resistivity of the composite. When two CNTs are in close proximity, electron tunneling can occur, thereby enhancing electrical conductivity. External factors such as experimental conditions can also affect conductivity.

Ying et al. [48] developed CNT-cement-based materials by treating CNTs with H₂SO₄ and HNO₃ followed by ultrasonic dispersion. This treatment effectively increased the electrical conductivity of the cement-based materials. Wansom et al. [64] observed a reduction in resistivity with the incorporation of MWCNTs into cement-based materials, indicating the presence of a CNTs percolation network. Han et al. [65] found that the electrical percolation threshold of composites is significantly influenced by the geometry of conductive fillers. Materials with high aspect ratios, such as CNTs, can achieve the percolation threshold at lower concentrations. Lu et al. [66] demonstrated that quantum tunneling plays a major role in the conductivity of CNT-cement composites; when CNTs are close together, electrons can tunnel from one CNT to another. Li et al. [67] showed that when MWCNTs are evenly dispersed in the cement matrix, the resistivity of the sample decreases with increasing MWCNT content. However, when MWCNTs are unevenly dispersed, the conductivity of the sample does not have a clear correlation with MWCNT content (Figure 9). The resistivity of MWCNT-cement paste decreases with increasing test temperature, and when the temperature exceeds 80 °C, it stabilizes. Additionally, when MWCNTs are well dispersed, the effect of temperature on the resistivity of the cement paste decreases with higher MWCNT content. A better dispersion of MWCNTs in the matrix results in an increased rate and amplitude of temperature rise caused by electrical current, as MWCNT content increases.

According to the above content, the conclusions are summarized as follows: Carbon nanotubes can significantly improve the electrical conductivity of cement-based materials: when uniformly dispersed, a conductive network can be formed, and the resistivity decreases with an increase in the dosage and becomes stable after 80 °C. Electron tunneling is the main conduction mechanism, but uneven dispersion results in no clear correlation between conductivity and dosage.

3.2. Chemical Characteristic

The impact of CNTs on the hydration rate of cement-based materials involves several mechanisms. CNTs can act as nucleation sites for C-S-H gel formation, allowing the cement paste to bypass the “formation of C-S-H nuclei” step during hydration, thereby accelerating the hydration reaction rate and improving the morphology of the hydration products. Conversely, CNTs can also have adverse effects by adsorbing cement particles onto their surfaces, which increases the contact area between cement and water. The adsorption of calcium ions on CNT surfaces raises the local calcium ion concentration. CNTs possess hydrophilic properties; their walls and interiors can adsorb a certain amount of moisture, which is released and participates in the surrounding hydration reactions during the process.

Cui et al. [68] tested the heat of hydration for a 1.0 wt% MWCNTs cement paste, finding that the 12 h hydration heat rate was significantly higher than that of other cement pastes, indicating that CNTs facilitate the hydration reaction process through nucleation effects. Zhao et al. [69] showed that increasing CNT content shortens the setting time of cement paste. XRD analysis of CNT-incorporated cement paste revealed higher hydration levels, increased amounts of calcium silicate hydrate gel, and reduced crystallinity of calcium hydroxide. Makar et al. [70] investigated the role of SWCNTs in accelerating the hydration of tricalcium silicate in cement, observing changes in the morphology of the initial hydration products of alite and belite, which suggests that SWCNTs act as nucleating agents for tricalcium silicate. The adsorption of calcium silicate hydrate gel on CNT surfaces generates stress. Li [71] performed XRD phase analysis, thermogravimetric analysis, and SEM analysis to microscopically examine MWCNTs cement paste, concluding that well-dispersed CNTs act as “nucleation sites” during the growth of hydration products (Figure 10), thereby promoting the formation of calcium hydroxide.

Based on the above analysis, the nucleation effect of CNTs promotes the hydration reaction, facilitating the formation of hydration products such as C-S-H gel and Ca(OH)₂ and accelerating the hydration process of C₃S.

3.3. Environmental Adaptation Characteristics

3.3.1. Effects of Resists Carbonization

Cement-based materials are inherently brittle and porous, with relatively poor carbonation resistance. The incorporation of CNTs can address these deficiencies by reducing the porosity of cement-based materials. Porosity and pore structure are critical factors determining the depth of carbonation. Under identical conditions, greater porosity facilitates easier diffusion of CO₂ within the material, leading to increased carbonation penetration and reduced carbonation resistance. This study explores and analyzes the effects of carbon nanotube dosage, dispersion method, and aspect ratio on porosity.

• Different Dosage

The incorporation of an appropriate amount of CNTs can reduce the porosity of cement-based materials, inhibit CO₂ diffusion and absorption within the matrix, decrease carbonation depth, and thus enhance carbonation resistance. However, excessive incorporation of CNTs can cause aggregation due to van der Waals forces, making pore size distribution more challenging and increasing porosity, which deteriorates the pore structure.

The addition of CNTs improves the carbonation resistance of cement mortar. Xu et al. [56]. found that for cement-based specimens containing 0.025%, 1%, and 2% MWCNTs, the total pore volume decreased by 2.33%, 4.83%, and 5.96%, respectively, compared with the control group, with corresponding reductions in porosity of 1.54%, 3.9%, and 4.5%. During the experiments, porosity decreased with increasing CNT dosage, and pore size distribution became more uniform. Liu [72] studied the evolution of microscopic pores in mortar with 10% silica fume and different CNT dosages. Results show that as the CNT dosage increased from 0.0% to 0.5%, the matrix porosity decreased. At a CNT dosage of 0.5%, the total porosity was higher compared with a 0.2% dosage but still lower than the control group. Xie investigated the carbonation depth of cement mortar with CNT dosages of 0.00%, 0.05%, 0.10%, and 0.15% over different carbonation ages (Figure 11b,c). Carbonation depth was positively correlated with carbonation age. At lower CNT dosages, carbonation depth was higher, indicating poor improvement in carbonation resistance; at a dosage of 0.10%, carbonation depth was minimized, showing excellent carbonation resistance. Liu examined the carbonation resistance of four groups of specimens with different CNT dosages (Figure 11a) and found that CNT incorporation enhanced the carbonation resistance of cement mortar, with increased dosages leading to reduced carbonation depth and improved resistance.

The incorporation of CNTs adversely affects the microscopic pore structure of cement-based materials [73]. Zhuang et al. [74] compared the pore structures of cement-based materials modified with CNTs, finding that with carbon nanotube contents of 0.05%, 0.08%, and 0.15%, the porosities decreased by 24.53%, 28.69%, and 17.59%, respectively. However, further increases in carbon nanotube content led to clustering within the cement matrix, resulting in localized defects. Camacho et al. [75] revealed that the porosity of cement mortar modified with various amounts of CNTs increased by 7% to 13% compared with the control group, indicating that CNTs somewhat deteriorate the matrix pore structure. Zhang et al. [76] conducted mercury intrusion tests on cement-based composites with different carbon nanotube contents and found that both the pore diameter and porosity of the matrix increased with higher carbon nanotube content. Excessive incorporation of CNTs causes aggregation due to van der Waals forces, making pore sizes less dispersed and increasing porosity. Additionally, during the mixing and vibration of cement-based materials, the use of dispersants for CNTs introduces a certain amount of gas, leading to an increased total porosity.

The incorporation of CNTs should be optimal; an excessive amount of CNTs can lead to strong van der Waals forces, causing aggregation. This aggregation results in the formation of numerous voids and cracks around the nanotubes, increasing the porosity of the material. Consequently, the enhanced porosity accelerates the infiltration of gaseous media such as CO₂, thereby diminishing the material’s resistance to carburization.

2.

Different Dispersion Methods

Uneven dispersion of CNTs can inhibit the cement hydration process, create concentrated stresses due to aggregation, affect the effectiveness of filling and bridging actions, and consequently lead to localized cracking and deterioration of the pore structure. Improving the dispersion of CNTs is crucial for ensuring their effective application.

Dispersion methods for CNTs are categorized into physical and chemical approaches, with various methods being investigated for their effectiveness in optimizing the matrix porosity and pore structure. Li et al. [77] utilized mechanical stirring to prepare carbon nanotube–cement mortar, and their dispersion treatment significantly improved the material’s pore size distribution and porosity. Compared with the control cement mortar, the total porosity decreased by 4.7%, and the harmful pore porosity (d ≥ 50 nm) reduced by 0.25%. Han et al. [78] combined two dispersion methods and found that compared with pure carbon nanotube cement paste, the permeability and gas permeability of the treated composite paste decreased by 43–65% and 15–20%, respectively, indicating a refinement of the pore structure. Wang et al. [79] discovered that cement specimens with MWCNTs treated with active dispersants and ultrasonic dispersion showed a 27.52% reduction in porosity after 28 days compared with the control group.

When dispersing CNTs, selecting an appropriate dispersion method is essential for improving the pore structure and optimizing the porosity of the composite material, thereby enhancing its density.

3.

Different Length–Diameter Ratio

The agglomeration of CNTs is attributed to their high aspect ratio, which causes them to coil under the influence of van der Waals forces. Research on the aspect ratio aims to improve the porosity structure of CNTs from their intrinsic properties [80].

Carrio et al. [81] investigated concrete made with CNTs of two different aspect ratios and found that concrete with CNTs of lower aspect ratio exhibited the best carbonation resistance. This was confirmed through SEM and thermogravimetric analysis, which revealed the presence of crack bridging and nucleation effects. AI-Rub et al. [82] studied the microstructural characteristics of cement-based composites containing long MWCNTs with aspect ratios ranging from 1250 to 3570 and short MWCNTs with an aspect ratio of approximately 157. They found that a high concentration of short CNTs could reduce the free volume of CNTs within the matrix, effectively filling nanovoids and decreasing porosity. Manzur et al. [83] noted that MWCNTs with outer diameters of 20 nm or smaller significantly enhance the matrix. TEM scans revealed that nanotubes with smaller outer diameters are tightly embedded into the C-S-H phase hydration products, thereby better achieving crack bridging and optimizing the porosity structure.

Research on the influence of CNTs on the carbonation resistance of cement-based materials suggests that using short CNTs with a small aspect ratio is preferable. When considering the dosage of CNTs, a combination of high concentrations of short CNTs and low concentrations of long CNTs is recommended to mitigate the impact of the nanotubes’ aspect ratio on carbonation.

According to the above content, it is concluded that carbon nanotubes can significantly improve the carbonization resistance of cement-based materials, and the key points include the optimal content is 0.05–0.1 wt%, the porosity is reduced by 30%, and the carbonization depth is minimum. The dispersion process (ultrasonic/chemical) needs to be optimized to avoid agglomeration resulting in increased porosity. The effect of length–diameter ratio is significant, and the short tube makes it easier to fill the nanopores. Excessive addition (>0.5 wt%) will reduce the carbonization resistance due to agglomeration and reverse porosity increase by 15%.

3.3.2. Effect of Durability

The durability of concrete refers to the ability of concrete to maintain its form and performance under various environments, such as seasonal changes, ionic corrosion, etc. [84]. In recent years, some scholars have carried out systematic experimental research on the durability of carbon-nanotube-modified cement-based composites, in which freeze–thaw resistance and chloride ion penetration resistance as two core evaluation dimensions of durability have received special attention.

• Freeze–Thaw Resistance

Yu et al. [85] prepared carbon-nanotube-cement-based composite materials by adding carbon nanotubes into cement base at 0%, 0.1%, 0.2%, 0.5%, and 1.0% of cement mass, respectively, and analyzed its freezing resistance, finding that the added carbon nanotubes improved the freezing resistance. Qi [86] conducted a rapid freeze–thaw cycle test with samples of different diameters and found that the addition of carbon nanotubes increased the relative elastic modulus and reduced the loss of bending and compressive strength. When the cycle times exceeded 50 times, the improvement of the anti-freezing performance of carbon nanotubes was fully reflected, and the improvement effect was inversely proportional to the dosage and diameter.

2.

Resistance to Chloride Ion Penetration

Liu et al. [87] showed that the addition of CNTs to concrete at the dosage of 0.03%, 0.05%, and 0.10% could reduce the chloride ion diffusion coefficient by 22.8%, 24.0%, and 8.8% because the addition of CNTs refined the internal pore diameter of concrete. Wang et al. [88] prepared cement-based samples with different carbon nanotube content and studied them by chloride ion penetration method 28 days later. It is found that adding carbon nanotubes can improve the chloride ion penetration resistance of cement-based materials. It was found that when the mass fraction of carbon nanotubes was 0.1% of the mass of cement, the durability of the composite was the best. Sun [89] prepared cement-based samples with different carbon nanotube content and found that when the carbon nanotube content was 0.5%, its chloride ion resistance was 33.3% higher than that of standard samples. Chen [90] changed the dosage of CNTs, Na₂SO₄, and NaCl and concluded that the incorporation of CNTs improved the resistance to chloride ion erosion, which was attributed to the excellent filling and bridging effect of CNTs.

The research content of this chapter is summarized in the following

Table 6. CNTs lift cement-based materials.

	CNTs Are Lifted by Adding Cement Base
Mechanical	Static strength	Within 60 days of water age, the content was less than 1% of CNTs, and the compressive strength increased with the increase in CNTs
	Bending strength	The bending strength increased with the increase in CNT content and increased with the extension of age
	Dynamic mechanics	When the dosage is 0.1%, it reaches the highest, and the dynamic compressive strength is increased by about 30%
Contractility	Type	The effect of carboxyl carbon nanotubes is the most significant
	Dosage	The optimal dosage is 0.05–0.1%, and the increase in water–cement ratio will shorten the shrinkage effect
	Slenderness ratio	Medium aspect ratio (300–500)
Crack resistance		The optimal dosage is 0.05%, and >0.15% will reduce the effect due to caking
Electrical conductivity		The optimal dosage is 0.1% and tends to be stable at 80 °C with the increase in temperature
Hydrability		CNTs are hydrophilic and promote hydration
Carbonization resistance	Dosage	The optimal content is 0.05–0.1%, the pore ratio is reduced by 30%, and the carbonization depth is minimum
	Slenderness ratio	The length–diameter ratio of short pipe is easier to fill
	Dispersion method	The reasonable dispersion method can reduce the porosity by about 30%
Durability	Freeze–thaw property	When the number of cycles increases and the content is 0.1%, the loss of bending and compressive strength is minimal
	Chloride ion resistance	The optimal dosage is 0.1%, which is attributed to the bridging effect

.

4. Prospects for the Application of CNTs in Cement-Based Materials

CNT cement composites have widespread applications in the field of sensors due to their electrical conductivity and mechanical strength. When fabricated into sensors, these composites can be utilized for measuring various physical quantities such as pressure and strain. Furthermore, carbon nanotube cement composites can be employed in 3D printing of concrete [91,92]. By incorporating an appropriate amount of CNTs into the concrete, it is possible to enhance the material’s electrical conductivity and mechanical properties, enabling customized printing of concrete. This results in the production of concrete components with excellent conductivity and strength, suitable for applications in building structures, electromagnetic shielding structures, and other domains.

• Traditional architecture

CNTs cement-based materials exhibit high strength and durability, enhancing the compressive and flexural strength of tunnel linings, thus providing greater stability under dynamic loads and ensuring safe tunnel operation. Additionally, carbon-nanotube-cement-based materials possess excellent resistance to carbonation, mitigating CO₂ erosion and extending the tunnel’s service life. The incorporation of CNTs can fill micropores within the cement matrix, improving pore size distribution and pore connectivity, which reduces internal water leakage issues in tunnels.

When applied to high-rise buildings, carbon-nanotube-cement-based materials can enhance the building’s wind and seismic resistance, improving the stability and safety of concrete structures, foundations, and flooring. They offer excellent thermal and corrosion resistance, prolonging the building’s lifespan. Moreover, their use can reduce energy consumption during construction, lower operational costs, and minimize environmental impact. The application of carbon-nanotube-cement-based materials in building construction holds significant development potential and market prospects.

2.

Cement-Based CNT Sensor

The sensor itself is cement-based, and CNTs, due to their small size, high strength, and uniform dispersion within the cement matrix, endow the carbon-nanotube-cement-based sensors with high strength and long service life. When embedded within structural health monitoring systems, carbon-nanotube-cement-based sensors require infrequent replacement.

The piezoresistive effect of CNTs extends the conductive network of CNTs via electrodes, which are connected through wires to measurement devices such as multimeters. When a structure embedded with these sensors is subjected to force, the force transmitted to the sensor causes a change in its resistance. Experimental results demonstrate that this resistance change is highly consistent with measurements from other sensors, suggesting that these sensors possess comparable or even superior sensitivity to traditional sensors, with broad application prospects (Figure 12).

3.

Three-Dimensional Printed Concrete

Three-dimensional printing concrete technology offers an efficient construction method for the building industry. Referred to as additive manufacturing, 3D printing involves using computer-generated 3D data models to print components in various shapes. This process involves successive layers of printing to construct the intended product. The technology demands high performance from 3D printing materials, which must exhibit good processability and excellent properties. Carbon-nanotube-cement-based materials, with their superior strength, hold promise as potential 3D printing materials. The use of carbon-nanotube-cement-based materials in 3D printing can effectively reduce curing times, enhance production efficiency, improve print quality, and fill voids in printed products, thereby increasing their toughness and stability after extrusion.

Compared with conventional concrete, carbon-nanotube-cement-based materials offer superior mechanical properties, high sensor sensitivity, excellent durability, and good compatibility with the matrix, presenting significant potential for broad applications.

4.

Future Research Directions

Relative to the carbon nanotube material itself. Current production processes for CNTs face challenges such as low yield and inconsistent quality. New production techniques should be investigated to enhance yield and ensure stable quality. Optimization of nanotube performance can be achieved by controlling structural parameters such as diameter and length. Although various effective dispersion techniques for CNTs have been proposed, the dispersion and stability of high-concentration nanotube dispersions still require further enhancement.

Compared with carbon-nanotube-cement-based materials. It is crucial to determine the optimal CNT dosage, water–cement ratio, aggregate type, fiber type, and treatment methods to maximize the mechanical properties and durability of cement-based materials. Due to the high specific surface area and van der Waals forces between molecules, the dispersion of CNTs in cement-based materials can easily lead to aggregation or re-aggregation, which affects their performance and may even damage the internal microstructure of the material, impacting its macro properties. Therefore, a deeper exploration of the dispersion mechanisms of CNTs in cement-based materials is needed. Experimental testing of the impact of CNTs on cement-based materials is costly, and the number of samples and test points is limited; thus, further research is needed to improve the reliability of test results.

5. Conclusions

The latest progress of CNTs cement-based materials was systematically reviewed, and its preparation process, strengthening mechanism, dispersion technology, and multifunctional characteristics were expounded. This paper provided theoretical guidance for the development of high-performance, intelligent, and sustainable cement-based materials and promoted its innovative application in civil engineering and the innovative development of nanomodified cement.

5.1. Effects After the Addition of CNTs

The properties of cement-based materials were significantly improved after the addition of CNTs:

• Mechanical properties: Compressive and flexographic strength increased by 10–50%, toughness increased (fracture energy increased by 3–5 times), and CNTs bridge cracks and disperse stress.
• Contractility: Carboxylated CNTs inhibited early contraction by 46.7%, and excessive dosage (>0.5 wt%) may reverse the contraction.
• Carbonization resistance: Porosity is reduced by 20–30%, CO₂ diffusion is blocked, and the carbonization depth is minimum when 0.1–0.5 wt% is added.
• Electrical conductivity: A seepage network is formed, the resistivity decreases, and the electrical conductivity increases with the increase in the dosage when the dispersion is uniform.

5.2. Current Technological Challenges

• Challenges with CNTs

The production of CNTs often suffers from low yield and inconsistent quality. Current synthesis methods, such as CVD or arc discharge, may produce nanotubes with varying diameters and lengths, leading to difficulties in achieving uniformity and reproducibility. CNTs tend to agglomerate due to strong van der Waals forces, which complicates their dispersion in various media. Achieving a stable and uniform dispersion, particularly at high concentrations, remains a significant challenge. The high cost of production and purification processes limits the widespread use of CNTs in practical applications.

2.

Challenges with CNT Cement-Based Materials

Determining the ideal amount of CNTs to add to cement-based materials is challenging. Excessive or insufficient quantities can adversely affect the material’s properties, including workability and mechanical performance. Concerning compatibility with the cement matrix, CNTs must interact favorably with the cement paste to enhance properties such as strength and durability. Ensuring that the nanotubes do not adversely affect the setting time, hydration process, or long-term stability of the cement-based materials is crucial. The integration of CNTs into cement-based materials can be costly, both in terms of the nanotubes themselves and the associated processing techniques. Developing scalable and economically viable methods for incorporating nanotubes into construction materials is essential for practical applications.