Self-heating and electrical performance of carbon nanotube-enhanced cement composites

Highlights

• CNTs can improve electrical and mechanical properties of cement/mortar.

• This study investugated effects of mixing method on properties of MWCNT-cement composites.

• MWCNTs are incorporated into composites using three different methods.

• Composites with MWCNT dispersion and MWCNT films show best performance.

• This is because CNTs are evenly distributed and form dense networks.

Abstract

This work hypothesizes that mixing carbon nanotubes with cement improves the thermal and electrical properties of bulk cement composites. To test this, two different methods of combining cement and dispersed multi-walled carbon nanotubes (MWCNTs) were considered. In the first method, cement composites were produced by adding a dispersion of MWCNTs to cement. In the second, MWCNT-based thin films were spray-coated and integrated with cement to produce cement composites. A third group of specimens was produced using both MWCNT dispersions and MWCNT thin films. The experimental parameters considered were the mixing method, MWCNT concentrations, number of curing days, and voltages applied. Furthermore, field emission scanning electron microscopy revealed that the MWCNTs were evenly dispersed within the composites and formed a percolated network. In addition, X-ray diffraction analysis confirmed that the products formed during hydration of the composites (i.e., C-H and C-S-H) were the same as those generated using ordinary mortar. Upon testing these mortar-based specimens, it was found that the cement composites formed using a combination of MWCNT dispersions and MWCNT-based films exhibited the highest heating performance and lowest electrical resistance. Finally, thermal imaging showed that increased MWCNT concentrations during specimen casting led to a corresponding increase in their surface temperature upon voltage application.

Introduction

Various technologies for designing and constructing large-scale concrete structures have been developed in recent years. Concrete materials are regularly used in various structures worldwide because of their excellent strength and durability. However, the use of concrete also leads to various problems related to the construction environment and global warming. Moreover, concrete structures can experience significant damage in the winter because of black ice caused by snow. Thus, multifunctional concrete with improved strength and durability that can melt snow and ice must be developed to address the aforementioned problems related to existing concrete structures.

Multifunctional concrete can potentially be synthesized by incorporating nanomaterials in existing construction materials [1], [2], [3]. Nanomaterials, which have particle sizes ranging from 0.1 to 100 nm, are attracting significant attention in various areas because of their high specific surface area per unit weight and excellent properties [4], [5], [6], [7]. Nanomaterials can be mixed with construction materials based on ordinary cement to improve the mechanical, electrical, and thermal performances of the materials. Carbon nanotubes (CNTs), whose thermal conductivity and electrical conductivity are 7.5 and 100 times higher, respectively, than those of copper, have been mixed successfully with cement-based materials [8], [9], [10], [11], [12]. Furthermore, there have been numerous studies on the mechanical and physical characteristics of cement composites formed using carbon nanomaterials to improve their strength, electrical conductivity, and heating performance [13], [14], [15], [16], [17], [18].

Li and Zhao [13] fabricated specimens of ordinary mortar and its composites with multi-walled CNTs (MWCNTs) and carbon nanofibers (CNFs) and studied their compressive strengths. The results showed that the compressive strength of the composite with MWCNTs at 28 days was 19% higher than that of the composite consisting of ordinary mortar and CNFs. Furthermore, the load transfer efficiency increased as more MWCNTs were added to the ordinary mortar, because this resulted in a greater number of chemical bonds between the cement hydrates (C-S-H and calcium hydroxide). Chaipanich et al. [14] analyzed the compressive strength of cement mortar in which CNTs and fly ash had been mixed. The CNTs were added in concentrations of 0.5 and 1.0 wt% (cement weight). The results indicated that, for the same fly ash content, the compressive strength of the mortar increased with an increase in the CNT concentration, because both the internal density and the extent of the CNT networks within the composite increased. Morsy et al. [15] fabricated mortar composites containing clay with nano-sized particles and MWCNTs and studied their compressive strengths. The clay content was fixed at 6% of the cement weight, while MWCNT concentration was varied, and composites with dimensions of 50 mm × 50 mm × 50 mm were fabricated. The composite with 0.02 wt% (by cement weight) MWCNTs exhibited a compressive strength 11% higher than that of ordinary mortar. On the other hand, the composite with 0.1 wt% MWCNTs showed a lower compressive strength. Hamzaoui et al. [16] studied the mechanical properties of mortar and concrete containing CNTs, measuring the compressive strength of the composites containing CNTs in different concentrations. The compressive strength was the highest when CNTs were added in amounts of 0.01 and 0.003% by cement weight. The compressive strength increased with the addition of the CNTs, because they served as bridges between the pores and cracks. Choi et al. [17] fabricated cement mortar by mixing dispersions of MWCNTs in distilled water and studied its compressive strength. It was found that the use of distilled water to form the MWCNT dispersions had little effect on the dispersibility of the CNTs. Kang et al. [18] studied the effects of the dispersibility of CNTs on the compressive and tensile strengths of cement composites. CNTs were dispersed using surfactants, high-performance plasticizers, and an ultrasonic treatment. The results showed that the use of both ultrasonic treatment and high-performance plasticizers improved the compressive and tensile strengths by 10% as compared to the other dispersion methods.

As for studies on the electrical and heating performance of composites formed using nanomaterials, Nan et al. [19] elucidated the relationship between CNT content and improvements in thermal conductivity for composites containing CNTs. Liu et al. [20] studied the thermal conductivity of nanofluids containing MWCNTs and found that the thermal conductivity of the nanofluids increased linearly with MWCNT concentration. Li et al. [21] used CNTs dispersed in sulfuric acid and nitric acid as well as unmixed CNTs to fabricate composite specimens with dimensions of 40 mm × 160 mm × 40 mm and measured their electrical resistances. Both the composite containing CNTs dispersed in sulfuric acid and nitric acid and those containing the unmixed CNTs exhibited significantly reduced electrical resistances, because CNTs were uniformly dispersed and formed networks within the composites. Zhang and Li [22] studied the road deicing performances of MWCNT-containing cement composites. The results showed that a composite with a thermal conductivity of 2.83 W/m$·$K was formed when the MWCNTs were mixed in a concentration of 3% by cement weight. Thus, it is possible to melt ice formed on roads using MWCNT-cement composites. Kim et al. [23] studied the mechanical and electrical properties of cement composites containing silica fume and CNTs. It was found that the cement composites with low silica fume and CNT contents exhibited improved mechanical and electrical properties, owing to the decomposition of the aggregated CNTs into small clusters. Konst and Aza [24] analyzed the electrical properties and piezoresistive sensitivity of cement composites containing CNTs and CNFs. The cement composites formed using both CNTs and CNFs in concentrations of 0.1% by cement weight exhibited the highest piezoresistive sensitivity. Lee et al. [25] fabricated cement composites using MWCNTs and studied their electrical and thermal properties. The effects of the mixing method and MWCNT concentration were elucidated. The mixing methods used were forming a CNT coating on the fine aggregates and mixing the CNTs using a dispersion. MWCNTs were added in concentrations of 0.125 and 0.25 wt% by cement weight. The results showed that the composite formed using the former method exhibited better heating performance. Lee et al. [26] studied the heating performance of CNT-based cement composites. Different types of CNTs were used in varying concentrations, with the applied voltage also being a parameter. Specifically, single-walled carbon nanotubes (SWCNTs) and MWCNTs were used in combined concentrations of 0.0625 and 0.125% (by cement weight), while the voltages used were 50 and 100 V. The composite containing MWCNTs and SWCNTs with a total concentration of 0.125% exhibited better heating performance than those with 0.0625 wt% CNTs. Thus, the heating performance increased with the total CNT concentration. Moreover, when a voltage of 100 V was supplied to the 0.125 wt% composite, its temperature rose by 70.6 °C.

Finally, considering studies on nanomaterial-coated films, Hone et al. [27] fabricated an SWCNT film by filtration and desorption through a magnetic field and studied its heating performance. The SWCNT film exhibited a thermal conductivity of 200 W/m·K, which was similar to those of graphite and highly crystalline diamond. Haung et al. [28] studied the thermal conductivity of CNT films exhibiting different arrangements. For the same CNT concentration, the thermal conductivity was higher when the CNTs were arranged in the same direction. Kim et al. [29] fabricated CNT films with dimensions of 10 mm × 10 mm by electrostatic spray deposition without using any binders and studied their electrical conductivity. The CNT films showed good electrical conductivities. In addition, the capacitance of the films was linearly proportional to their CNT concentration. Pham et al. [30] analyzed the electrical resistances of films of conductive CNTs and polymers under tensile strains. The results showed that the electrical resistance increased with an increase in the tensile strain, owing to a decrease in the density of the conductive CNT network and an increase in the intertube distance. Park et al. [31] studied the heat diffusion performance of films of CNTs and polymers. The thermal diffusivity of the films was analyzed visually based on thermal images. Jang and Park [32] fabricated 25 mm × 25 mm films of composites with dispersed CNTs. The CNT concentration and film thickness were varied and the electrical conductivities and heating performances of the films were analyzed. The electrical resistance of the composite nanofilms decreased, because the CNT networks became denser with an increase in CNT concentration. Furthermore, the electrical conductivity increased as film thickness was increased. For temperatures of −5 to 5 °C, the films with lower thicknesses exhibited greater temperature sensitivity.

In this study, the heating performance and electrical properties of cement composites containing MWCNTs were analyzed. A widely employed method of incorporating MWCNTs into cement is to disperse MWCNTs in a solution and then mix them with cement [33], [34], [35], [36], [37], [38]. In this study, however, both an MWCNT-containing solution and MWCNT-coated films were added to cement, and their effects on the composite's heating performance and electrical resistance were measured. In particular, the effects of the mixing methods used, MWCNT concentrations, number of curing days, and applied voltages were investigated. As stated above, the mixing methods used included adding a solution containing the dispersed MWCNTs with ordinary mortar, adding MWCNT-coated films to ordinary mortar, and adding both the MWCNT solution and the MWCNT-coated films. The MWCNT concentrations used were 0.125, 0.25, and 0.5 wt% (by cement weight), and the mortar samples were cured for 7 and 28 days. The surface temperatures and thermal distributions of the composite samples were analyzed using thermal images. Finally, the internal microstructures of the MWCNT-cement composites were analyzed using field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) analysis.