Introduction

Nanotechnology and nanomaterials have been gradually changing architectural concepts [1]. Inspired by molecular nanotechnology, the visionary architect J. M. Johansen put forward the novel term “nanoarchitecture” to revolutionize the building environment [2]. He shared the evolving characteristics of nanotechnology and designed theoretical projects like Froth of Bubbles, Web, Mag-lev Theater, Space Labyrinth, etc. Dr. Wijdan brings the novel concept of nano-biomimicry into building design, employing new materials, computer modeling, and robotic strategies to construct new architectural forms [3]. Nanotechnologies improve the quality and efficiency of building materials and reduce consumption of raw materials and energy [4].

Nanomaterials have outstanding physical, chemical, and electrical properties and will serve as sustainable construction materials. Zhang et al. presents a frequency matrix of various properties of nanomaterials and points out that a certain proportion of nano-additives improves the mechanical properties of alkali-activated composites, including compressive, flexural, tensile and impact strengths, producing denser and more homogenous structures [5]. Nanomaterials could improve the efficiency of building materials and solve energy-related issues [6]. Verma and Yadav discuss the innovation of nanomaterials and the future application of nanomaterials in architecture [7].

Carbon nanotubes (CNTs) are composed of cylindrical tubes bonded by carbon atoms with a diameter of around 1 nm [8], which can be grouped into CNT forests and made into materials many meters long [9]. Since the early 1990s, CNTs have received increasing attention because of expanding research [10, 11], and their synthesis and functionalization awaits more discussion [12]. CNTs can be fabricated into novel forms and structures. For example, pillared carbon nanostructures have been designed for hydrogen storage [13]. Proposed by Dimitrakakis et al., the hybrid network structure is made of graphene sheets interconnected with single-walled carbon nanotubes (SWCNTs) [13], which exhibit better hydrogen storage capacity and special electronic properties. As an extraordinary performer in nanotechnology, CNTs have exceptional mechanical, thermal, electrical, optical, and electrochemical properties, with high strength, high flexibility, and light weight as main characteristics [14-16]. They could not only improve the quality and stability of buildings, but also reduce pollution [17]. In addition, CNTs are often used as an additive in other materials, such as glass, cement, metal, or wood, to enhance their characteristics [18]. CNTs are flexible and lightweight and could be used to reinforce the structural components in a building [18]. Moreover, with electrical and thermal properties, CNTs could provide energy collection, energy transmission, energy storage, heating, etc., displacing existing heating and cooling methods [19, 20]. The features of CNTs give wide use in the construction industry, ranging from large space-based structures to urban infrastructures [21]. The market has witnessed expansion of CNT products due to their superior mechanical properties, increasing demand for lightweight polymer composites, and large-scale production of CNT products [22].

There exist some review papers that report the research progress in CNT composite materials and their engineering application. For example, Garg A et al. summarizes the computational methods on properties prediction of SWCNTs and lists the applications of CNTs as reinforcement composite materials in beams, plates, and shells [23]. Khaniki and Ghayesh present a detailed literature review on the mechanics of CNT strengthened deformable structures and investigate the applications and advantages of these form types [24]. Soni et al. comprehensively reviews the functionally graded CNT reinforced composites and discusses the vibrational, mechanical, and thermal properties of the resulting composites [25]. Despite the reviews on the engineering application of CNTs, few literature reviews summarize the applications of CNTs in the building industry. Thus, this manuscript aims to address this gap by reporting the latest achievements and findings related to the application of CNTs in construction.

Publications on the application of carbon nanotubes in architecture were retrieved from ScienceDirect and ResearchGate, with a preference for the most recent papers. The selection criteria included a time frame from 2012 to 2022, correlation with central keywords (CNT + construction industry), and English language (Table 1). In the initial search, 992 papers were identified on Web of Science and the network of keywords concerning “carbon nanotubes” and “construction industry” was visualized on VosViewer (Fig. 1). Among the search results, the number of publications on specific CNT properties and forms was obtained (Fig. 2). There are 290 papers on thermal properties, 120 papers on mechanical properties and 69 papers on structural properties. Fibers, sheets, and panels are the top three CNT forms discussed among the literature.

Table 1 Case selection criteria1
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Fig. 1
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Network visualization of keywords on VosViewer

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Fig. 2
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Number of papers on CNT properties and forms on Web of Science

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A second selection process was conducted to preserve the papers with topics that are more centered around the building industry. Papers in other non-related subjects like chemistry, bioengineering or biochemistry were excluded. Finally, 63 papers met these criteria and were selected for final review. The framework of this paper includes five sections: (1) synthesis and fabrication methods of CNTs; (2) CNT properties and their advances for the construction industry; (3) different forms of CNT composites; (4) CNT applications in the construction industry; (5) challenges and prospects for using CNTs as construction material.

Synthesis and fabrication of carbon nanotubes

Synthesis

CNTs can be produced as SWCNTs or multi-walled nanotubes (MWCNTs) [26], with SWCNTs easily twisted and more pliable, while MWCNTs have more complex structures [27]. There are three main methods to produce CNTs [14, 21]. The first method is chemical vapor deposition [28], or methane pyrolysis, which heats methane gas in the absence of oxygen to 800 to 1,200 degrees Celsius and turns methane into hydrogen and CNTs. This method is especially for MWCNTs [14, 21]. Amama reports the influence of catalysts during CNT carpet growth via water-assisted chemical vapor deposition [29].

The second synthesis method is electric arc-discharge, and the third method is laser ablation, both of which produce SWCNTs [30]. It is potentially feasible to produce CNTs by mixing CO2 with methane, which offers a more sustainable production solution and contributes to the CO2 fixation goal [12]. Challiwala et al. uses CO2 utilization technology to convert CO2 and methane into MWCNTs. This novel method uses carbon (dioxide) capture and the sequestration technique in both the operation and generation cycles [12].

Fabrication

New technical tools have emerged to help researchers characterize and fabricate CNT-reinforced materials. For example, nanotechnological robots can help the nano-scale construction of carbon nanotubes [31]. Natarajan inclusively investigates the technical steps in direct forming (DF) value chain from CNT synthesis to post-processing and shows the positive value of this methodology for enhancing structural properties of CNT materials [32]. Natarajan et al. proposes a polymer infiltration approach into CNT porous structures and studies the primary parameters of infiltration for better performance of CNT composites [33]. With novel fabrication technologies, functional cementitious composites are modified by small addition of CNTs, and effects of CNT geometries, dispersion techniques, and organosilanes for the performance of cement materials are evaluated [20, 21, 34]. Kim et al. reviews various dispersion techniques of CNT particles in cementitious composites and evaluates their dispersion rates [35]. Goh et al. reviews the comparative approaches of fabricating polymer matrix composites with horizontally and vertically aligned CNTs and points out their advantages and limitations [36]. 3D printing technology assists in fabricating micro-structured sorbent membranes for heavy metal removal after enriching the polymer composite links of CNTs [37]. 3D-printed CNTs/Cu composites show higher compressive strength and structural integrity and can be used to develop complex structural applications [38].

Properties of carbon nanotubes

Regarding CNT properties, Fig. 3 shows the data of papers for each of the electrical, electronic, thermal, structural, mechanical, chemical, and acoustic properties and Table 2 presents the list of references on specific property. Within 43 papers in this part, 30 papers discuss mechanical properties, which far exceeds papers on other properties. There are 12 papers on thermal properties and 9 papers on electrical properties. A lack of research in acoustic properties is evident here.

Fig. 3
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Pie chart for the number of papers in each CNT property

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Table 2 43 papers on CNT properties
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CNT-based nanocomposites exhibit good mechanical and electrical properties as construction materials [3, 39, 70], and could also contribute to carbon dioxide absorption [68]. The reinforcement effects of MWCNTs coupled with PVA fibers are studied to optimize the mechanical properties of lightweight engineered geopolymer composites [53]. Structural lightweight concrete containing CNT additions also shows better compressive strength [54], indirect tensile strength [47], and light weight [52] than that with coarse aggregates in it [55]. Because of the filling, nucleation, and bridging effects of MWCNTs as a reinforcing nanomaterial, the compressive, split-tensile, and flexural strength of concrete can be elevated, and the compactness of the microstructure can be enhanced [34]. CNT-OH/PVA latex can improve the mechanical and fracture performance of cement mortars, and further research into high quality cementitious materials is suggested [56]. The chemical bonds between functionalized CNTs and resin influence the mechanical properties of concrete [57].

The thermal stability of CNT materials can help improve the performance of CNT concrete at high temperatures [58]. According to Yao and Hao, the compressive strength and failure mechanism of CNT concrete were examined under high temperatures and the effects of temperature on the performance of CNT concrete were explained. By altering the SiO2 shell thickness of CNTs, the electromagnetic absorption and shielding functions of the nanocomposites as a building material are enhanced and thus provide insight into other building materials [45]. Machine learning can aid the understanding and evaluation of the thermal, mechanical, electrical, and electronic properties of CNT at lower cost [40, 48, 59] and, for example, can predict the compressive strength of normal concrete with the increase of CNTs [54].

Different forms of carbon nanotubes

Researchers and material scientists have produced multiple forms and shapes of CNTs, such as CNT fibers, matrices, films, yarn, and sheet, and even novel forms like nano-chimneys [49], honeycomb-like networks [43], and tensegrity structures [61] testify to their mechanical, electrical, and thermal properties for engineering applications. Table 3 categorizes the references into each form type and Fig. 4 shows the number of papers in different CNT forms. In an experiment conducted by Newcomb et al. [62], CNT composite fibers are produced, and the stress transfer is analyzed. CNT fibers demonstrate high flexibility, high electrical conductivity, and good thermal conductivity in practical applications [41]. With CNTs of high aspect ratio and high purity, Carbon Hub from Rice University has produced carbon nanotube fibers (CNTF) with excellent electrical conductivity and tensile strength, and its higher production and wider application are expected [13, 71]. A structural design shows that well-aligned SWCNTs films display excellent mechanical and electrical properties in a ‘layer-by-layer’ mode [3]. Tensile experiments were performed on SWCNT films, and it was proved that the increase of SWCNTs fraction could improve the mechanical properties of laminated nanocomposites [70].

Table 3 34 papers on CNT forms
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Fig. 4
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Pie chart for the number of papers in each CNT form

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Directional alignment of CNT assembly in polymer matrix MWCNTs were distributed in a matrix nanocomposite sample for electrical and tensile tests [39]. PCM plate encapsulations with MWCNTs additions were made to explore the effects of CNT particles on the thermal conductivity improvement [46]. The dimension of the plates is 15 × 15 × 2 cm. Cubic, cylindrical and prismatic specimens of structural lightweight concrete with CNT additions showed improved mechanical properties [47].

Application in construction industry

Nanotechnology and carbon nanotube materials in construction

This part of the literature illustrates the influence of nanotechnology and CNT materials on architecture with findings claimed in selected studies. Table 4 shows the research question and nanomaterial in each study, and presents the ecological, social, and economic effects of CNT or the related technologies. Kuda and Yadav argue that nanotechnology can enable architects to design structures with thinner surfaces, superb strength, and more flexibility, and to contribute to the reduction of carbon dioxide emission, energy conservation, fire protection, and building repair [15]. The advantages for the construction industry can be seen in concrete improvements [71, 74], self-cleaning or air-purifying surfaces [74], cooling and heating equipment [4], efficient thermal regulation [75] and structural health monitoring [74].

Table 4 Use of nanotechnology and CNT materials in construction
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Applications of carbon nanotube-reinforced materials in architecture

The promising effects of using CNTs in building envelopes and structural elements have surfaced and architects have realized the potential of CNTs to be applied in composites manufacturing and architectural design [8]. CNT additives could reinforce the structural materials which are built into walls, roofs [42], beams [60], shells [63], and other components in a building. To show the application potential in the building industry, Table 5 presents the eight studies which discuss CNT properties and their potential advantages in architectural design, five of which demonstrate how CNT addition in the walls, roofs and floors of buildings helps create a better thermal environment. Cheng et al. explores the fabrication and thermal transfer effects of CNT-doped phase change materials in the shell of buildings [53], and similarly Niu et al. proves that CNT-modified phase change materials (PCM) have thermal stability impacts in indoor environments [50]. Sarrafha et al. finds that MWCNTs’ addition in roof and wall panels could increase the PCMs’ thermal conductivity [42], and thus produce increased heat flow and higher thermal comfort in the building in winter and autumn days [7]. Kazemi et al. experiments on the thermal effects of MWCNTs nanoparticles dispersed in air-PCM heat exchangers when used in buildings [46]. Zając proposes the use of CNT fibers as the main cables of tensegrity and illustrates the mechanical strength, flexibility, and light weight, etc. of the functional structures when applied in roofing, shelter, and walls [61]. Ong et al. suggests the reinforcement of CNTs in double beam systems for better structural stability [78].

Table 5 CNT-reinforced material applications and advantages for the building industry
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Applications of carbon nanotubes in other construction projects

With exceptional characteristics, CNT addition could reinforce the performance of functional materials in aerospace, automobile, marine [25], and water management [26]. CNT fibers have the potential to be applied in five industrial areas: artificial muscles, energy storage device, electronic device, sensors, and thermal management [41]. The CNT-reinforced double beam system has large application value in tracks of railway, vibration absorbers and cranes [78]. De Volder discusses CNTs’ use in rechargeable batteries, automotive parts, sporting goods, boat hulls, etc., and points out that wider commercial use would reduce energy consumption, waste production, and cost [11]. Cui et al. argues that CNT-reinforced cement-based materials can be used in national defense, military, earthquake prevention, and disaster reduction [80].

Challenges and prospects

Challenges of using carbon nanotubes in construction

While emphasizing the environmental advantages brought by CNTs, designers and architects are awaiting better performance of CNTs as the functional materials [48] and it is important to note their shortcomings or limitations. I selected exemplary application cases to demonstrate the challenges related to CNT applications in construction (Table 6).

Table 6 Challenges of using CNTs in construction
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Researchers have not yet tested its common use in construction, so its application deserves further research and experiment, including safety, comfort, cost, and efficiency issues of buildings. Up till now, CNTs’ use in construction has not been as mature as it is in energy storage [76, 82], water purification, concrete additives, aerospace, etc., so more research in theory and application is necessary for further development of CNT functional materials. There exists limited research on CNT reinforced cement-based composite as functional building materials [80] and the practical application requires extensive lifetime testing [79]. The production rate should be increased, and the cost should be decreased for wider adoption of CNTs [13]. From safety aspects, usage of nanoparticles incurs risks of toxicity and deserves our concern, so further recycling and separation operations are required, and stricter safety standards should be established [71]. Therefore, there should be stricter control over the life cycle of nano-technological products [74].

Futuristic projects using carbon nanotubes in architecture

Based on their advances in properties, CNT materials possess broad application prospects in cement composites [80], supporting structures [83], cables [84], and bundles [85]. The works listed in Table 7 are conceptual works or theoretical calculations. They pave the way for the visionary construction mode of CNT use in future architectural projects. Shimizu Mega-City Pyramid is a 2004-m-high futuristic city design plan proposed by Japanese architects. It is composed of 55 smaller pyramids in eight layers. The mega-structure requires exceptionally strong and lightweight materials like carbon nanotubes [81], and the traditional materials are too heavy for this huge construction. With their exceptional Young's modulus and tensile strength, CNTs have an advantage in producing ultra-strong cables of long-span bridges [86]. The two new cable-stayed bridges beside the existing Longfellow Bridge were designed by ASN and they have back spans of 128 m and a main span of 256 m. Tokyo's Obayashi Corp plans to build Space Elevation 2050, with a space station 36,000 km above Earth. Its 96,000 km-long cable will be built of CNTs, and an elevator will be built for sending tourists up [84].

Table 7 Futurist /conceptual projects using CNTs as a paradigm
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Conclusions

With great mechanical properties, CNTs could revolutionize the construction industry and bring more possibilities for architecture in the future, but up till now only small- scale applications have been done. This review summarizes the significance of works, the experimental findings, morphology, and choice of CNT material forms in selected literature. The use of CNTs for the building industry has great environmental potential. Based on the references reviewed, the approaches of CNTs as a functional construction material are analyzed in terms of synthesis and characterization methods, uses in construction, and visionary projects. Further research needs to be conducted to utilize CNTs as a building material and meet the practical requirements in architectural design. In the future, CNTs will offer a new material mechanism for construction as they can be widely used in buildings and infrastructure with sustainable impacts. It is hoped that this review can serve as a reference for the fabrication and processing of CNT composites in architectural design.