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Interfacial bonding characteristics of multi-walled carbon nanotube/ultralight foamed concrete

  • Jing Zhang EMAIL logo and Xiaolei Zhang
Published/Copyright: August 19, 2024
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Science and Engineering of Composite Materials
From the journal Science and Engineering of Composite Materials Volume 31 Issue 1

Abstract

In the development of carbon nanotube (CNT)-reinforced cement-based matrices, one of the fundamental issues that investigators are confronting is CNT/cement-based matrix interfacial bonding, which determines the load transfer capability from the matrix to the CNT. In the present work, the stress transfer properties of multi-walled carbon nanotubes (MWCNTs) and ultralight foamed concrete matrices were studied using microscopic Raman spectrometry analysis. Two types of CNTs, such as MWCNT and MWCNT-COOH, were considered, wherein MWCNT-COOH was covered with fundamental COOH groups. The results show that the compressive and flexural strengths were 75 and 236% better for ultralight foamed concrete with a dry density of 200 kg/m3 with 0.4 wt% MWCNT-COOH addition, respectively. This indicates that the fundamental COOH groups of the MWCNT play an important role in determining the interfacial bonding characteristics between the MWCNT and the ultralight foamed concrete matrix. Therefore, the attachment of COOH groups with a reasonable concentration to the MWCNT surface may be an effective way to significantly improve the load transfer between the MWCNT and the ultralight foamed concrete matrix, leading to increased compressive and flexural strength values of composites.

Keywords: ultralight foamed concrete; multi-walled carbon nanotube; interfacial bonding characteristics; compressive strength; flexural strength

1 Introduction

Ultralight foamed concrete can improve the energy efficiency of buildings and plays an important role in non-structural partition walls and roof applications owing to its special porous structure and excellent low-density performance. Generally, the lower the density, the lower the thermal conductivity for the same type of material. About 40–50% of the energy consumed in buildings is spent on space heating and cooling, and lower thermal conductivity can effectively improve the energy efficiency of buildings, meaning that lower density can reduce heat wasting. However, its excellent low-density performance results in lower compressive and flexural strengths, leading to its restrictions on application in the construction field. Therefore, it is essential to improve the compressive and flexural strengths of ultralight foamed concrete.

To improve the compressive and flexural strengths, an alternative is to introduce macroscale fibers such as polypropylene and polyethylene fibers into the cementitious matrix. Nevertheless, it is more difficult to achieve the targeted properties because the bonding between the polymer fibers and cementitious matrix, which is determined by the structure and surface performance of the polymer fiber, is poor. Moreover, it is more difficult to achieve long-term structural performance and durability of cementitious matrices because polymer fibers easily fail over time.

The incorporation of silane-modified nano-CaCO3 into epoxy resin composites enhances interfacial adhesion through chemical bonding, resulting in improved toughness, impact resistance, and strength. This is particularly evident in the CF/EP system, where the tensile strength and flexural modulus were effectively increased. Conversely, multi-walled carbon nanotube carboxylic acid (MWCNT-COOH) benefits from the strong interaction between the –COOH group and epoxy resin and cement matrix, which facilitates stress transfer through chemical bonding. This results in an increase in the compressive and flexural strengths of ultralight foam concrete by 75 and 236%, respectively, which is suitable for applications requiring high strength and high interfacial bonding. The two mechanisms are distinct, and their selection should be based on the specific application, cost, and performance objectives [1].

There are satisfactory solutions to enhance interactions with cement hydrates using microscale fibers such as graphite nanomaterials, carbon nanomaterials, and carbon nanotubes (CNTs) [2–11]. CNTs have been extensively used as reinforcements in polymer composites because they offer high strength, modulus, aspect ratio, and specific surface area [12–14]. The strength of CNTs can be lowered by defects present on their surfaces. Although lower defect concentrations are desirable for achieving perfect mechanical properties, defects can be evaluated to improve the bonding properties with the cementitious matrix.

The application of MWCNT-COOH in carbon fiber composite repair demonstrates unique advantages over TEGO and MWCNT. The −COOH group on the surface of MWCNT-COOH forms a stronger chemical bond with the cement matrix, optimizing the stress transfer and significantly enhancing the compressive (+75%) and flexural (+236%) strengths of ultralight foam concrete. The enhancement mechanism is based on chemical bonding and interfacial stabilization, which promote uniform stress distribution rather than physical blocking. This approach is particularly effective in specific scenarios. Thus, MWCNT-COOH provides a new nano-strengthening strategy for repair systems that seek interfacial optimization [15]. Although CNTs as reinforcements in cementitious matrices are technically promising, the high cost of CNTs, especially single-wall carbon nanotubes (SWCNTs), is a major drawback in this application [16,17]. With the emergence of low-cost CNTs, such as multi-walled carbon nanotubes (MWCNTs), which are now produced at an industrial scale, MWCNT/cementitious composites could be developed at a viable cost for commercial applications.

The mechanical properties of cementitious nanotube composites depend on the CNT dispersibility in the cementitious matrix [18–20]. Thorough dispersion of MWCNT in the mixing water is the key step toward obtaining a uniform dispersion of MWCNT in the ultralight foamed concrete matrix. Different dispersion techniques for CNTs have been investigated, including surface modification and surfactants [20–22]. The surfactant method could not be adopted because the surfactant is likely to participate in the foaming process of foamed concrete, and part of it may even prevent the foaming process. The goal of achieving thorough dispersion of MWCNT in water can be accomplished through proper surface modification of MWCNT, such as tethering of carboxyl functional groups, which can form bonds with cement hydrates [23–28]. Hence, introducing MWCNTs covered with carboxyl functional groups on their surfaces could represent a new way to improve the mechanical properties of ultralight foamed concrete matrices.

It should be pointed out that the interfacial bonding between MWCNT-COOH and the ultralight foamed concrete matrix is the most important characteristic that determines the load transfer capability from the matrix to the MWCNT-COOH. Interfacial bonding and stress transfer between the MWCNT-COOH reinforcement and matrix are difficult to prove, not only because of the wide differences in measurements (millimeters and nanometers) but also because of the special porous structure of the matrix, which is especially difficult for ultralight foamed concrete. Recent investigations have demonstrated the influence of CNTs on the mechanical properties of cementitious matrices but less on the interfacial bonding characteristics between CNT reinforcement and the cementitious matrix. Therefore, it remains a major challenge to obtain the values of CNTs stretched through interfacial bonding and stress transfer under loading because of different measurements between millimeters and nanometers, which makes it difficult to investigate the interfacial bonding. Nevertheless, obtaining the interfacial bonding characteristics between CNT reinforcement and cementitious matrix has been a subject of intensive research because it takes advantage of the superior properties of CNTs, thus enhancing the mechanical properties of cementitious materials.

In view of the abovementioned reasons, in this work, two types of CNTs, MWCNT and MWCNT-COOH, were employed to reinforce ultralight foamed concrete in order to improve its compressive and flexural strength. It should be noted that the interfacial bonding characteristics between MWCNT-COOH and the ultralight foamed concrete matrix need to be investigated because of the contribution of COOH functional groups to the interfacial bonding. Because the interfacial bonding characteristics between MWCNT-COOH and the ultralight foamed concrete are difficult to achieve, two strategies were devised to accomplish this target. One approach is to employ a microscopic Raman spectrometer to test the interfacial stress because the movement and broadening of the characteristic Raman peak would be caused by the deformation of fiber in the fibrous composite materials [29,30]. A series of investigations have also demonstrated that [31–35] a blue shift is formed in the case of compressive stress because the final vibrational state is lower in energy than the original one, and the micro-Raman characteristic peak will shift from a low wave number to a high. On the contrary, a red shift is formed in the case of tensile stress, exhibiting micro-Raman characteristic peak shifts from a high wave number to a low. In addition, it was also demonstrated that [36,37] broadening of the characteristic Raman peak is caused by the deformation of fibers due to the load transfer between the fiber reinforcement and matrix.

Another approach is to obtain the interfacial bonding characteristics by observing the interfacial bonding between MWCNT-COOH and the matrix on the fracture surface using scanning electron microscopy (SEM). The challenge is to achieve the position of MWCNT-COOH on the fracture surface and separate the hydration covered on the outer surface of MWCNT-COOH because MWCNT-COOH and ultralight foamed concrete are mainly composed of carbon. In addition, defects in CNTs provide potential sites for functionalization, where carboxyl fundamental groups COOH can form bonds to cement hydrates [38,39]. Thus, the defects present on the surface of MWCNT-COOH were investigated to determine the impact of COOH groups on the interfacial bonding characteristics between MWCNT-COOH and ultralight foamed concrete composites. It has been demonstrated that increasing intensities of the D and/or G peaks of the microscopic Raman spectrometer indicate that the defects of the fiber surface and disorder degree increase [40], and the degree of defects and disorder on the surface of MWCNT and MWCNT can be confirmed.

In addition to the COOH groups, the relatively high specific surface area of MWCNT-COOH could benefit their bonding to the cementitious matrix. Based on the shear lag mold proposed by Cox [41] in 1952, the critical interface shear stress leading to interfacial debonding was obtained [42]. The interface shear stress is affected by the aspect ratio of the fiber, and the tensile stress is transmitted from the matrix to the fiber through the interface shear stress [43–45]. The stress along most of the length of MWCNT-COOH was very low because the aspect ratio of MWCNT-COOH was very large. In this work, the interfacial bonding characteristics between MWCNT/MWCNT-COOH and an ultralight foamed concrete matrix were confirmed through the compressive and flexural strengths of the composites.

MWCNT-COOH nanomaterials have been employed in an innovative manner to significantly enhance the mechanical properties of ultralight foam concrete. This was achieved through the strong chemical bonding of carboxylic acid groups on the surface of the nanomaterials with the matrix, resulting in a substantial increase in compressive and flexural strengths. In comparison to traditional reinforcing materials or methods, the modification strategy of MWCNT-COOH is more focused on optimizing interfacial interactions and chemical bonding. This approach opens new avenues for reinforcing composites, particularly in applications requiring high strength and durability.

2 Experimental details

2.1 Materials and chemicals

In this work, ultralight foamed concrete consisted of 42.5-grade ordinary Portland cement (Hohhot, China), dry ash (Hohhot City, China), industrial-grade naphthalene water reducer (Shanghai, China), industrial-grade FeCl3 (Hangzhou, China), 27.5 wt% H2O2 (Beijing, China), analytically pure calcium stearate (Beijing, China), and tap water (Hohhot, China). The chemical compositions of 42.5-grade ordinary Portland cement and dry ash are shown in Tables 1 and 2, respectively. The role of dry ash is as a nucleation site for the hydration of ordinary Portland cement, which can replace part of the cement. The water demand for complete hydration of ordinary Portland cement depends on the dosage of dry ash and cement, which is a maximum of about 30 wt%, while it is typical to add 20–35 wt% of dry ash to cement clinker.

Table 1

Chemical composition of 42.5-grade ordinary Portland cement (wt%)

CaO SiO2 Al2O3 Fe2O3 MgO TiO2 Others
63.8% 23.4% 5.6% 4.1% 1.2% 0.94% 0.96%
Table 2

Chemical composition of the fly ash (wt%)

Al2O3 SiO2 CaO Fe2O3 Na2O + K2O TiO2 Others
44.8% 46.4% 3.2% 2.3% 0.86% 1.4% 1.04%

Two types of CNTs, MWCNT and MWCNT-COOH, were used in this work. The MWCNT surface tethered to the carboxylic functional groups of COOH was characterized by FTIR spectroscopy. Figure 1 shows the FTIR spectra of MWCNT and MWCNT-COOH. In Figure 1(a) and (b), the peaks at 1,580 and 1,560 cm−1 are assigned to the C═C stretching mode associated with MWCNT surface defects. In Figure 1(b), the peak at 1,730 cm−1 is assigned to the C═O stretching mode of MWCNT-COOH, generating COOH groups on the MWCNT surface. The physical and mechanical properties of MWCNT and MWCNT-COOH are presented in Table 3. There are almost the same characteristics of MWCNT and MWCNT-COOH, except for some different values of outer diameter, which have scarcely influenced the matrix.

Figure 1 FTIR spectra of carbon nanofibers: (a) MWCNT and (b) MWCNT-COOH.
Figure 1

FTIR spectra of carbon nanofibers: (a) MWCNT and (b) MWCNT-COOH.

Table 3

Structural characteristics, tensile strength, and elasticity modulus of MWCNT and MWCNT-COOH

Samples Inner diameter (nm) External diameter (nm) Length (μm) Tensile strength (GPa) Price Elasticity modulus (TPa)
MWCNT 8–12 25–30 5–15 10–60 ¥20/g 1
MWCNT-COOH 8–12 25–40 5–15 10–60 ¥65/g 1

2.2 Preparation of MWCNT/MWCNT-COOH dispersion

To improve the dispersion of CNT in an ultralight foamed concrete matrix, the preparation of CNT dispersion in water is the first step. Water and alcohol are typically used for CNT dispersions. Alcohol could not be adopted because it is prone to participate in the fabrication process of foamed concrete. In order to disperse MWCNT/MWCNT-COOH in water thoroughly, after stirring for 10 min, MWCNT/MWCNT-COOH was dispersed in water using an ultrasonic cleaner at a temperature of 35°C for 30 min. The resulting dispersion of MWCNT/MWCNT-COOH in water was used as the mixing water for the preparation of ultralight foamed concrete following normal mixing procedures. The dispersibility of the MWCNT and MWCNT-COOH was determined by scanning electron microscopy. In Figure 2, MWCNT presented a serious agglomeration state, while the MWCNT-COOH was prone to dispersion by introducing COOH groups on its surface, as shown in Figure 3.

Figure 2 Dispersity of MWCNT.
Figure 2

Dispersity of MWCNT.

Figure 3 Dispersity of MWCNT-COOH.
Figure 3

Dispersity of MWCNT-COOH.

2.3 Sample preparation

All dry powder, including ordinary Portland cement, fly ash, and calcium stearate, was mixed at a speed of 14 rad/min using self-made foamed concrete mixing equipment for 3 min. To obtain wet mixture, the MWCNT/MWCNT-COOH dispersion solution was mixed with dry powder and stirred for 2 min. Then, FeCl3 and H2O2 were added to the wet mixture for 30 s at a speed of 30 rad/min to produce CNTs/ultralight foamed concrete composites. For each composite, three cubic samples were prepared for the dry density, compressive strength, and flexural strength tests. In the same batch of wet mixtures, two 100 mm × 100 mm × 100 mm cubes were prepared for dry density and compressive strength tests, and one 100 mm × 100 mm × 400 mm prism was prepared for flexural strength test. All the cubic and prismatic samples were further cured at a standard curing room temperature for 7 and 28 days, respectively. For comparative analysis, MWCNT/MWCNT-COOH dosages are 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, and 0.5 wt%, respectively. The objective of this study was to evaluate the optimization effect of MWCNTs and their surface-modified MWCNT-COOH on the physical and mechanical properties of ultralight foam concrete with different contents. To achieve this, a dose gradient design was employed, which allowed for a systematic evaluation of the effects of these materials on the properties of concrete [46,47]. A density of 200 kg/m3 of CNTs/ultralight foamed concrete composite material with different contents of MWCNT/MWCNT-COOH was prepared.

2.4 Testing method

2.4.1 Mechanical properties

The compressive and flexural strengths were determined according to the stipulations specified in the PRC Industrial Standard JG/T266-2011 foamed concrete formulated by the Ministry of Housing and Urban–Rural Development of the People’s Republic of China. Before the compression and flexural tests, all 100 mm × 100 mm × 100 mm and 100 mm × 100 mm × 400 mm samples were air-dried at a standard curing room temperature for 7 and 28 days, respectively. The reported compressive and flexural strengths were the averages of three samples from the same batch of mix and identical curing conditions, respectively (Figure 4).

Figure 4 Schematic of compressive and flexural tests of foam concrete samples. (a) Compressive test, (b) flexural test, and (c) interfacial stress transfer measurement methods.
Figure 4

Schematic of compressive and flexural tests of foam concrete samples. (a) Compressive test, (b) flexural test, and (c) interfacial stress transfer measurement methods.

2.4.2 Interfacial stress transfer measurements

Interfacial stress transfer measurements on the CNTs/ultralight foamed concrete samples were performed using a Renishaw inVia microscope Raman spectrometer (Britain), as shown in Figure 4(c). The wavelength was 785 nm, and 600 scans were added to ensure an adequate signal-to-noise ratio. MWCNT and MWCNT-COOH peaks were acquired prior to the processing of CNTs/ultralight foamed concrete. These spectra were compared with those of the MWCNT/MWCNT-COOH composite. This procedure allowed the local vibrational mode of the strained CNTs in the composite to be observed. The region between 100 and 3,200 cm−1 was also monitored for a peak assignable to the stretching vibration of CNTs. The change in this peak indicates that the interfacial stress transfer behavior between the CNTs and matrix occurred under the external force of the composites.

3 Results

3.1 Interfacial bonding characteristics

Figure 5 shows the micro-Raman spectrum curves of the MWCNT and MWCNT-COOH. Both MWCNT (Figure 10(a)) and MWCNT-COOH (Figure 10(b)) exhibit the D, G, and Gʹ peaks at the locations of 1,300, 1,600, and 2,600, respectively, similar to the three strongest peak locations of the Raman spectrum of CNT. These results indicate that the COOH groups adhered to the surface of MWCNT and did not affect the formation of the characteristic Raman peak.

Figure 5 Raman spectra of MWCNT and MWCNT-COOH: (a) MWCNT and (b) MWCNT-COOH.
Figure 5

Raman spectra of MWCNT and MWCNT-COOH: (a) MWCNT and (b) MWCNT-COOH.

Removing the Raman characteristic peak of the ultralight foamed concrete matrix caused by the fluorescence effect is the key first step to ensure the accuracy of the micro Raman data of MWCNT and MWCNT-COOH. Figure 6 depicts the Raman characteristic peak curves of ultralight foamed concrete. Ultralight foamed concrete exhibits a micro-Raman characteristic peak at 1,394 cm−1, similar to the 1,300 cm−1 peak of MWCNT and MWCNT-COOH. The characteristic Raman peak of ultralight foamed concrete is caused by the fluorescence effect on the matrix. It can be clearly observed that the strength of the characteristic peak exhibited by ultralight foamed concrete is 60,000, far higher than the MWCNT peak (2,400) and the MWCNT-COOH peak (2,800); thus, it is more likely to remove the characteristic peak of the matrix caused by the fluorescence effect due to the strength value difference between CNTs and ultralight foamed concrete matrix. In addition, it can also be observed from Figure 6 that the characteristic Raman peak did not appear at 2,600 cm−1, indicating that the G′ peaks of MWCNT and MWCNT-COOH are not affected by the ultralight foamed concrete matrix.

Figure 6 Raman spectrum of ultralight foamed concrete.
Figure 6

Raman spectrum of ultralight foamed concrete.

The interfacial stress transfer properties between the CNTs and ultralight foamed concrete matrix are indicated by the shifting of the Gʹ Raman peak. Figure 7 depicts the micro Raman spectrum curves of two types of CNTs, MWCNT and MWCNT-COOH, pulled out from the ultralight foamed concrete matrix. The pull-out load, which is mainly contributed by the interfacial frictional resistance, decays rapidly with an increase in pull-out distance. In Figure 7(a), the Gʹ peak of MWCNT shifted from 2,600 to a low wave number of 2,549, decreasing by 51. It can also be observed from Figure 7(a) that the width of the Gʹ peak increased from 204 to 1,000 and increased by 796. In Figure 7(b), the Gʹ peak of MWCNT-COOH shifted from 2,600 to a low wave number of 2,510, decreasing by 90, far greater than that of MWCNT, as Figure 7(c) shown. It can also be seen that the width of the Gʹ peak increased from 204 to 1,615, increasing by 1,411. These results indicate that both MWCNT and MWCNT-COOH were stretched by an external load. Although interfacial stress was generated by the applied loading stress, the additional stress could be effectively transferred to MWCNT/MWCNT-COOH depending on the interface with a higher bonding strength, which existed in the composite. The reason for the MWCNT being stretched is that the yielding behavior and sliding were generated as the applied loading stress increased. The yielding behavior and sliding were applied to the whole length of the MWCNT as the material strain increased, and then the stretching stress increased owing to the increase in interfacial shear stress.

Figure 7 Raman spectra of MWCNT, and MWCNT, MWCNT-COOH being stretched.
Figure 7

Raman spectra of MWCNT, and MWCNT, MWCNT-COOH being stretched.

This also indicates that the interfacial stress transfer properties between MWCNT-COOH and the matrix are superior to MWCNT. This is because carboxylic functional groups adhered to the MWCNT surface, leading to the increasing defects on the MWCNT-COOH surface, as shown in Figure 8, and the interfacial stress transfer between MWCNT-COOH and ultralight foamed concrete improved.

Figure 8 TEM images of surface morphology of (a) MWCNT and (b) MWCNT-COOH.
Figure 8

TEM images of surface morphology of (a) MWCNT and (b) MWCNT-COOH.

The effectiveness of carbon nanotubes in enhancing the compressive strength of ultralight foamed concrete was evaluated. The influence of MWCNT and MWCNT-COOH on the compressive strength of the composites at 28 days is presented in Figure 9, wherein the MWCNT and MWCNT-COOH addition is 0.05–0.5 wt%. These results suggest that the compressive strength of the ultralight foamed concrete matrix was effectively improved by adding MWCNT and MWCNT-COOH. It is worth noting that the compressive strength of the composite increased by 75% with the addition of MWCNT-COOH, whereas the compressive strength increased by 55% with the addition of MWCNT. This indicates that the interfacial bonding properties between MWCNT-COOH and the ultralight foamed concrete matrix are superior to the properties of the MWCNT and the matrix.

Figure 9 Effect of MWCNT/MWCNT-COOH on the compressive strength of the composite.
Figure 9

Effect of MWCNT/MWCNT-COOH on the compressive strength of the composite.

The effects of two types of CNTs, MWCNT and MWCNT-COOH, on the flexural strength of the composite are shown in Figure 10. These results indicate that the highest gain in flexural strength (increased by 236%) is obtained by adding 0.4 wt% MWCNT-COOH, while the flexural strength is increased by 181% by adding 0.4 wt%. An important finding of these comparative studies is that COOH functional groups on the surface of MWCNT enhance the contribution of MWCNT to the compressive and flexural strength of the ultralight foamed concrete matrix, suggesting that COOH functional groups enhance the tribological properties of the interfacial stress transfer between the MWCNT and the matrix.

Figure 10 Effect of MWCNT/MWCNT-COOH on the flexural strength of composite.
Figure 10

Effect of MWCNT/MWCNT-COOH on the flexural strength of composite.

3.2 Influence of COOH functional groups on the interfacial bonding

The COOH functional groups present on the surface of the MWCNT probably formed the strongest coordinate bonds with Ca2+ ions in the cement hydrates. The COOH functional groups can also form strong ionic interactions with Ca2+ ions in cement hydrates. The Ca(OH)2 constituents present in cement hydrates can undergo acid-based reactions with COOH, and ionic interactions can evolve into covalent bonds during steam curing. Some of these interactions were verified by the hydrate present on the surface of MWCNT-COOH, as shown in Figure 10. As shown in Figure 11(a), MWCNT-COOH was pulled out from the ultralight foamed concrete matrix, and its surface was covered by cementitious hydrates. The chemical component of the hydrate covering the surface of MWCNT-COOH was determined by EDS mapping, as shown in Figure 11(b). The oxidation interaction was further confirmed by FTIR spectroscopy, as shown in Figure 12. As shown in Figure 11, the following functional groups were identified: O–H stretching vibration (3,436 cm−1) and C═O stretching vibration (1,634 cm−1). According to these results, the hydrate covering the surface of MWCNT-COOH is composed of Ca(OH)2 and C–S–H, indicating that the interface between MWCNT-COOH and ultralight foamed concrete matrix exhibits good bonding strength; thus, the compressive and flexural strengths are enhanced remarkably. This can be attributed to the coordination bonds between the COOH groups present on the surface of the MWCNT and Ca2+ ions in the cement hydrates. In Figure 13, MWCNT-COOH is tightly surrounded by the ultralight foamed concrete matrix, in which remarkable nanometer gaps could not be observed, which provides a good foundation for interface stress transfer between MWCNT-COOH and ultralight foamed concrete matrix.

Figure 11 SEM image of MWCNT-COOH pulled out from the ultralight foamed concrete matrix: (a) SEM image of MWCNT-COOH pulled out from the ultralight foamed concrete matrix and (b) corresponding EDS mapping of MWCNT.
Figure 11

SEM image of MWCNT-COOH pulled out from the ultralight foamed concrete matrix: (a) SEM image of MWCNT-COOH pulled out from the ultralight foamed concrete matrix and (b) corresponding EDS mapping of MWCNT.

Figure 12 FTIR spectra of MWCNT-COOH pull-out from the matrix.
Figure 12

FTIR spectra of MWCNT-COOH pull-out from the matrix.

Figure 13 Longitudinal section image of the MWCNT/ultralight foamed concrete composites.
Figure 13

Longitudinal section image of the MWCNT/ultralight foamed concrete composites.

The defects caused by COOH functional groups can effectively enhance the interface bonding between MWCNT-COOH and the matrix. The defects on the surface of MWCNT and MWCNT-COOH were confirmed using a microscopic Raman spectrometer. Figure 8 shows the D and G peak intensities of MWCNT and MWCNT-COOH. As shown in Figure 14, the D peak intensity (ID) of MWCNT-COOH is 9,000, which is far greater than that of MWCNT (5,000). The G peak intensity (IG) of MWCNT-COOH is 9,000, which is far greater than that of MWCNT (4,000). The ID/IG values of MWCNT and MWCNT-COOH are listed in Table 4. The ID/IG value of MWCNT-COOH was 1.222, which was greater than that of MWCNT (1.044). These results indicate that the defect and disordering degree on the surface of MWCNT were increased by introducing COOH functional groups, probably leading to the MWCNT-COOH morphology of non-uniformity, bending, and/or twisting in the ultralight foamed concrete matrix, as shown in Figure 15. It is also indicated that there are no remarkable defects, including severe damage and/or C═C fracture of MWCNT-COOH/ultralight foamed concrete.

Figure 14 The G and D peaks of MWCNT and MWCNT-COOH were pulled out from the matrix.
Figure 14

The G and D peaks of MWCNT and MWCNT-COOH were pulled out from the matrix.

Table 4

I D/I G values of MWCNT and MWCNT-COOH

Samples MWCNT MWCNT-COOH
I D/I G 1.222 1.044
Figure 15 Morphological image of the fracture surface.
Figure 15

Morphological image of the fracture surface.

These results also revealed that the contribution of MWCNT-COOH to the compressive and flexural strengths was better than that of MWCNT, which can be attributed to the COOH functional groups. The high activity of the COOH groups, as well as the defect and disordering degree of MWCNT-COOH result in their bonding to the ultralight foamed concrete matrix, in addition to the excellent mechanical properties of CNTs, and thus their contributions to the mechanical properties of ultralight foamed concrete [48,49].

4 Discussion

The economic feasibility and scalability of MWCNT-COOH-reinforced ultra-lightweight foam concrete for large-scale construction must be evaluated through a comprehensive examination of production costs, integration into current manufacturing practices, and cost–benefit analysis. The high synthesis costs present a challenge that necessitates the optimization of production processes, exploration of cost-effective raw materials, and development of efficient dispersion methods for uniform incorporation into concrete. Despite the initial investment required, the long-term benefits of MWCNT-COOH, including improved structural performance and energy efficiency, outweigh the costs involved, as evidenced by comparative life cycle assessments. Consequently, overcoming these challenges is of paramount importance if the technology is to be fully realized and sustainable practices in the building materials sector are to be advanced.

When exploring the potential of MWCNT-COOH-reinforced ultra-lightweight foam concrete for a wide range of applications, it is important to consider its economic viability and scalability in construction. While MWCNT-COOH can significantly enhance the material properties, its cost and integration with existing manufacturing processes are issues that need to be addressed before commercialization. A cost–benefit analysis must encompass production costs, integration challenges, and the long-term benefits of the technology in question. This is necessary to ensure that the conversion of the technology is economically sound. Furthermore, it is of paramount importance to assess the environmental footprint of the material in question, including the full cycle of impacts from production, use, and disposal, in comparison with traditional materials. This is particularly critical for advancing sustainable building practices. Despite the outstanding performance of MWCNT-COOH in terms of material strength and energy efficiency, its environmental impacts, in particular the challenges of energy-intensive production and waste disposal, must be mitigated through the implementation of innovative manufacturing technologies and circular economy solutions in order to ensure the green sustainability of material applications.

5 Conclusions

In conclusion, this study successfully demonstrated the superiority of MWCNT-COOH as a reinforcing agent in ultralight foamed concrete, offering a marked improvement over unmodified MWCNT and highlighting its unique contribution to composite material science. By introducing –COOH functional groups onto the nanotube surface, the interface bonding characteristics between MWCNT-COOH and the ultralight foamed concrete matrix were significantly enhanced, leading to a 75% increase in compressive strength and a remarkable 236% improvement in flexural strength at a 0.4 wt% MWCNT-COOH addition rate.

Raman spectroscopy analysis (Figure 14 and Table 4) provided quantitative evidence for the higher defect and disordering degree on the MWCNT-COOH surface, with an I D/I G ratio of 1.222, compared to the lower 1.044 for unmodified MWCNT. This indicates a more effective stress transfer and load-bearing capacity due to the non-uniform, bent, or twisted morphologies (Figure 14) of MWCNT-COOH within the matrix without causing severe damage or C═C fractures.

Scanning electron microscopy (SEM) images (Figure 11) and Fourier transform infrared spectroscopy (FTIR, Figure 12) further confirmed the tight integration, showing MWCNT-COOH encapsulated by the matrix with minimal nanoscale gaps, facilitating efficient stress transfer. The absence of notable defects and the high specific surface area of MWCNT-COOH facilitated its exceptional bonding with the cementitious matrix, according to Cox’s shear lag model.

This study conclusively establishes that the strategic modification of MWCNTs with −COOH groups is a highly effective strategy to improve the mechanical properties of ultralight foamed concrete, overcoming the common issue of low strength associated with lightweight construction materials. This paves the way for wider applications of ultralight foamed concrete in sustainable construction, where energy efficiency and weight reduction are crucial while maintaining adequate strength and durability. Future research could delve deeper into optimizing the concentration of MWCNT-COOH to balance cost and performance and explore its recyclability to address environmental sustainability concerns in construction materials.

Acknowledgements

The authors would like to express their sincere thanks for the techniques that have contributed to this research.

  1. Funding information: This research was supported by Basic Research Projects of universities directly under the Inner Mongolia Autonomous Region (preparation and properties of carbon nanotube-foamed cement new environmental protection materials) (No.: NZJK202303).

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript, consented to its submission to the journal, reviewed all the results, and approved the final version of the manuscript. JZ designed the experiments, developed the model code, and performed the simulations and XZ prepared the manuscript.

  3. Conflict of interest: Authors state no conflict of interest.

  4. Data availability statement: The figures and tables that support the findings of this study are included in the article.

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Received: 2024-04-08
Revised: 2024-06-04
Accepted: 2024-06-05
Published Online: 2024-08-19

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/secm-2024-0028
Keywords for this article
ultralight foamed concrete; multi-walled carbon nanotube; interfacial bonding characteristics; compressive strength; flexural strength
Creative Commons
BY 4.0

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