The most recent studies on the applications of nanostructured carbon materials, including carbon nanotubes, carbon nanofibers, and graphene oxides, in constructions are presented. First, the preparation of nanostructured carbon/infrastructure material composites is summarized. This part is mainly focused on how the nanostructured carbon materials were mixed with cementitious or asphalt matrix to realize a good dispersion condition. Several methods, including high speed melting mixing, surface treatment, and aqueous solution with surfactants and sonication, were introduced. Second, the applications of the carbon nanostructured materials in constructions such as mechanical reinforcement, self-sensing detectors, self-heating element for deicing, and electromagnetic shielding component were systematically reviewed. This paper not only helps the readers understand the preparation process of the carbon nanostructured materials/infrastructure material composites but also sheds some light on the state-of-the-art applications of carbon nanostructured materials in constructions.
Nanostructured carbon materials, including carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene (GR), graphene oxide (GO), and fullerene, are promising elements that can be used in many practical areas [
The infrastructure materials are the most commonly used materials in the modern civilization. Some studies demonstrated that, with the addition of nanostructured carbon materials, the overall performances of the infrastructure materials can be modified from various perspectives. It was believed that the nanostructured carbon materials will change the pore structures and hydration process of the cementitious materials and thus change the mechanical properties or functionalize the infrastructure materials.
Currently, the research on nanostructured carbon in infrastructure materials is burgeoning. In the former studies, rapid progress and improvements of advanced nanocarbon materials have led to numerous studies for construction materials. Nanotechnology has demonstrated its promising merits in empowering the development of infrastructures with mechanical reinforcement and many other functionalities. In this paper, the most recent studies on the preparation and applications of nanostructured carbon materials, including carbon nanotubes, carbon nanofibers, and graphene oxides, in constructions are presented.
The final properties of the nanostructured carbon composites are determined by their fabrication process. Several methods were used to prepare the composites in the past decade. Thanks to its low cost, simplicity, and availability, high speed melt mixing process is the most widely used approach to prepare the composites. In this method, the nanostructured carbon material will be dispersed in a matrix material with a high shear mixing condition. The merit of the high speed melting mixing is that it can guarantee a good dispersion of the nanostructured carbon material in a matrix material; however, this process will damage the structure of the CNT, CNF, GR, or GO, which is another important factor governing the final properties of the composites. As a result, how to use low shear mixing speed to protect the structure of the carbon nanomaterials without sacrificing the dispersion condition is still a challenge to fabricate the composites.
Apart from the high speed melt mixing processing, the solution approach with help of sonication is another method to prepare the composites. In this process, the nanostructured carbon material will be dispersed in a liquid form solution by sonication before being mixed with the matrix. In addition, external cooling device has to be applied to avoid the temperature increase during the sonication process.
Unlike the applications in other areas, the applications of the nanostructured carbon materials in constructions have to satisfy some basic requirements before it can be widely accepted in the construction field. First, because the usage of the carbon nanomaterials will be extremely larger than other areas, the high speed melting mixing process in the field will not be realized as easy as in the lab. Second, for the construction applications, the cost of the composite fabrication has to be low before they can be widely applied. Accordingly, the quality and cost control are the top challenges for the applications of the nanostructured carbon materials in constructions.
Because the high speed melting mixing and solution methods are both not able to be easily realized in the construction fields, the surface treatment of the nanostructured carbon materials is becoming a promising approach to realize their good dispersion in a matrix material. In this process, various functional groups will be grafted on the surfaces of the carbon nanomaterials, and the compatibility between the matrix and the functional group will play a key role which decides the final properties of the composites. In some cases, the surface treatment was realized by oxidizing the surfaces of carbon nanomaterials by soaking them into acids at various temperatures followed by acylation. After that, the functional groups will be grafted on the surfaces of the carbon nanomaterials by the reaction between the carbon and the functional groups [
In the most recent years, the research of the carbon nanostructured composites in constructions has been focused on investigating effective dispersion methods of the carbon nanomaterials in the construction materials. Yu and Kwon [
The SEM image of the CNT/cement composites [
Another acids surface treatment method was applied to prepare well dispersed CNT/cement and CNF/cement composites [
The preparation of well dispersed CNT/cement composites was also investigated by another study with surfactant surface treatment method [
Effect of surfactant on the dispersion effect of the CNT in the cement matrix with CNT dosages of (a) 0%, (b) 1.5 wt.%, (c) 4.0 wt.%, and (d) 6.25 wt.% [
A few investigators have found the addition of the CNT/CNF in cement will largely affect the workability of the paste. As a result, how to maintain the workability of the CNT/cement composites became a top challenge. Collins et al. [
Meanwhile, Sobolkina divided the surfactant into anionic and nonionic types and investigated the dispersion effect via UV-vis spectroscopy with sonication time and surfactant concentration as variables [
Other than the sonication time, the sonication energy was also used as parameter to evaluate the dispersion effect of CNF/cement composites [
Comparing with the dispersion process of CNT or CNF in water, the dispersion of GO in water is relatively easier and more stable. In general, the GO nanosheet will be prepared via modified Hummer’s method [
Cement matrix with addition of 1.5 wt.% GO [
As another important infrastructure material, asphalt has been widely used as binder material for pavement construction, water proof layer at the building roofs, or crack sealer for pavement rehabilitations. Unlike the carbon/cementitious composite, which has to be cured in a water environment, the carbon/asphalt composite, on the contrary, has to be prepared without water, because the water damage is one of the most important factors that reduce the durability of the asphalt material. Therefore, the dispersion of carbon nanostructured materials in asphalt is more difficult than in cementitious materials. To the best of our knowledge, a few effective ways have been developed to realize the good dispersion of carbon nanostructured materials in asphalt. Although the dispersion problem is still a bottle neck to prepare the nanocarbon/asphalt composite, the investigators are still working on this project with their full enthusiasm.
Because asphalt is a viscoelastic material, it is much easier to prepare the nanocarbon/asphalt composite via high speed melting mixing method. Recently, this method was used to prepare the CNT/bitumen composites [
Combined with sonication and high shear mixing, Khattak developed a dispersion method and successfully prepared the well dispersed CNF/asphalt composite. In these studies, the CNFs were firstly thoroughly mixed with kerosene and followed by mixing with asphalt at 60°C. Slowly raise the oil bath temperature to 150°C and keep mixing for 175 min. During this process, the kerosene will be completely evaporated and the CNFs will be homogenously left in the asphalt matrix [
In addition to surfactants and sonication, a new study demonstrates that the addition of appropriate quantity of silica fume has positive effect on the CNF dispersion in cement paste [
Mechanical properties are always the first priority that needs to be considered before the construction materials can be used in the fields. The application of carbon nanostructured materials as reinforcement has been widely studied in the past decade and accepted as an effective way to enhance the mechanical properties of the infrastructure materials [
The compressive and splitting tensile strength of the CNT reinforced cement paste were studied by Kumar et al. [
By combining sonication and surfactants, Hu et al. considerably reduced the dosage of the CNT from 0.5 wt.% of cement to 0.1 wt.% [
The rheological performance of the CNT reinforced cement slurries was investigated recently [
Apart from the dosage, the effect of the aspect ratio of CNT on the mechanical properties was investigated as well [
The reinforcement on compressive and flexural strength of CNF/cement composites was investigated most recently [
There are some different voices arguing that the addition of the CNT or CNF has negative effects on the mechanical properties of cement paste [
In addition to cementitious materials, the mechanical performances of the asphalt modified by CNT/CNF were also investigated during the past years [
Apart from CNT/CNF modified cement composites, the GO addition is also another effective way to enhance the mechanical properties of cementitious materials. Although the results of this area are not quite fruitful, there still are some studies that show their mechanical reinforcement of the composite. A recent study demonstrated that the tensile and flexural strength were both increased with the dosage of GO increased from 0.01 wt.% to 0.03 wt.% and then decreased with the GO content being increased to 0.05 wt.%. Comparing with the control samples, the tensile and flexural strengths of the samples with addition of 0.03 wt.% GO increased 78.6% and 60.7%, respectively. Meanwhile, the highest compressive strength was found in the samples with addition of 0.05 wt.% GO, which increased 47.0% by comparing with the samples without GO addition [
Although some studies have demonstrated the overall performances of asphalt can be modified by adding CNT or CNF, the study of the asphalt material modified by GO is still very limited. In addition, there still are some bottle neck problems yet to be solved in the asphalt/nanostructured carbon material composites, such as how to effectively disperse the CNT or CNF in the asphalt, how to use GR or GO to modify the asphalt, and how the durability of the modified asphalt is. These questions will be the future study trends for the asphalt/nanocarbon composites.
Currently, the requirements for self-sensing have become an important characteristic to realize the smart constructions. The nanostructured carbon/cement composites, as a promising self-sensing infrastructure material, have been widely investigated in the past years [
Technically, the realization of the self-sensing nanostructured carbon/cement composites is originated from evaluating the bulk electrical conductivity/resistivity variation that resulted from the external condition changes, including stress/strain, humidity, temperature, or gas environment, because the electrical properties of the nanostructured carbon/cement composites can be evidently changed with the change of external conditions. It can accurately reflect not only the external conditions of the constructions but also the inside conditions of the composites.
Han et al.’s group did a great contribution in this area during the past decade. They systematically investigated the preparation, properties, and applications of the nanostructure carbon/cementitious composites and discussed their self-sensing performances from both academic and practical perspectives [
Via testing the variation of its piezoresistive property, which can reflect the stresses status of the materials, the CNT/cement composite was prepared as self-sensing pavement to test the traffic flow [
The electrical resistance as a function of the external loading of a self-sensing CNT/cement composite. (a) Corresponding to compressive loading of 6 MPa. (b) Corresponding to impulsive loading [
Via piezoresistivity measurement, another study demonstrated the pressure sensitivity was different with different direction of loadings, namely, compressive and tensile forces [
Other than the stress sensing, the temperature sensing property of the CNT modified cement composites was briefly investigated as well [
Heating and cooling plot of the electrical resistivity as a function of temperature with 0.4 wt.% CF and 2.0 wt.% CNT in a cement matrix [
Most recently, the GO/cementitious composites were prepared and used as the self-sensing elements to monitor the infrastructures [
Due to the huge negative impact of the deicing chemicals on the environment and the infrastructure materials [
Thanks to the decreasing fabrication cost of the nanostructured carbon materials, especially carbon nanofibers, they have been investigated as the heating elements to fabricate the self-heating deicing pavement. Due to its high chemical stability, magnificent electrical performances, and outstanding heating efficiency, it has been considered as an effective heating element to prepare the self-heating deicing pavement.
The deicing effects of the carbon nanofibers paper (CNFP) have been studied from numerical and experimental perspectives in recent years and demonstrated its high deicing efficiency. By using air temperature, wind speed, and thickness of the pavement or insulating layer as parameters, a finite element model was developed to evaluate the deicing effect of the CNFP [
In addition to numerical studies, the experimental research was also carried out to investigate the heating efficiency of the self-deicing pavement [
Schematic demonstration of a self-heating deicing pavement [
It has been widely accepted that the conductive concrete has the capability of electromagnetic (EM) wave absorption and can be used to build electromagnetic shielding infrastructures. Comparing with normal CF/cement composites, the CNT (CNF)/cement composites were considered with a higher EM absorption efficiency. Singh et al. [
Another study demonstrated the EM absorption efficiency of the CNT/cement composite in a relatively low frequency with low CNT dosages. In addition, the effect of the thickness on the EM efficiency of the CNT/cement composite was studied as well [
The EM wave absorption capability in a wider frequency range was evaluated [
Although the investigations of the nanostructured carbon/cement as energy harvesting materials are very limited, a few studies were still carried out to test the piezoelectric and thermoelectric performances of the CNT/cement composites. Unlike the piezoresistivity, which is used to evaluate the sensing capability, the piezoelectric performance is used for the energy harvesting from converting the mechanical energy to the electrical energy of the CNT/cement composites. The piezoresistivity of the CNT/cement composites was realized by the backbones or tunneling channels changes of the CNT networks corresponding to external force field change which can result in an electrical conductivity change and reflect the external or internal conditions of the concrete infrastructures; however, the piezoelectricity was realized by changing the polarization status of the CNT/cement composites under external force field and generating an induced electrical field to realize the energy harvesting. Gong et al. [
Other than piezoelectric performances, the thermoelectric performances of CNT/cement composites were also investigated recently. The Seeback coefficient of the CNT/cement composites with 0.5 wt.% CNT addition reached the highest thermoelectric power of 23.5
In this paper, the preparation methods of the nanostructured carbon/infrastructure materials composites, especially the dispersion methods of nanostructured carbon materials in the infrastructure materials matrix, were systematically reviewed. The high speed melt mixing, surface treatment, aqueous solution with surfactants, and sonication methods were presented to introduce the current preparation approaches. The surface treatment, aqueous solution with surfactants, and sonication methods were applied on the nanostructured carbon/cementitious materials composites, while the high speed melt mixing method was widely used in nanostructured carbon/asphalt composites. It was found that using surfactants or acid surface treatment to the nanostructured carbon materials is positive for their dispersion in cementitious materials, but different surfactants have different dispersion efficiency. Unfortunately, it is too early to draw conclusions of which surfactant is better than others because there are many influence factors determining various requirements of the composites. Another important factor that governs the dispersion of the carbon nanostructured materials in cementitious materials is the sonication energy. However, the investigation on this part is still very limited and should be further studied in the future studies. In addition to CNT and CNF, the investigation of the GO dispersion in a cementitious material matrix is still in an infancy stage. Many problems have not been solved so far to demonstrate an effective way of GO or GR dispersion in a cementitious material matrix.
The nanostructured carbon/cementitious composites can be used as self-sensing composites due to their capability to reflect external force field change via their specific piezoresistivity performance. As a self-sensing composite, the sensitivity and stability of the composites are a top challenge in the future studies. How to obtain a composite which has high piezoelectric sensitivity and stable performances in repeated loading cycles still needs to be systematically investigated.
For the thermoelectric converting investigation, the temperature gradient between the two ends and the
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the National Natural Science Foundation of China (51402074), the Lianyungang Scientific Plan-Industrial Program (CG1204), and the Six Talent Peaks Program of Jiangsu Province (ZBZZ-032).