Acid-treated carbon nanotubes and their effects on mortar strength

Abstract

In the present study, multi-walled carbon nanotubes (MWCNTs) were treated in an acidic mixture solution, made with nitric and sulfuric acids in a ratio of 3:1 by volume. The durations of the treatment were 100 and 180 min. The defects of these treated MWCNTs were examined using Raman spectroscopy. The attachment of hydroxyl functional groups to the walls of the MWCNTs were verified using FTIR spectroscopy. The dispersion of CNTs with acid treatment is assessed using UV-Vis spectroscopy and Scanning Electron Microscopy (SEM). The results indicate that the duration of the acid treatment has significant effect on both the degree of defects and functionality of the MWCNT. The compressive strength of mortar increased with the addition of the acid-treated MWCNTs; however, no appreciable difference was noted for the two treatment durations under this study.

Keywords

carbon nanotubes / concrete / composites / nanomaterials / cement

Introduction

Carbon nanotubes (CNTs) are artificially made hollow cylinders of ordered carbon atoms which could either form a single wall (single-walled) or multiple walls (multi-walled). CNTs have attracted a lot of attention due to their superior mechanical, electrical and thermal properties. There is an ever increasing use of CNTs in construction applications to enhance the mechanical strength of concrete. However, CNTs naturally tend to agglomerate in clusters, because of the p-electron clouds around their surface which creates van der Waals attractive forces between the various nanotubes [ 1], thus making their dispersion in the concrete mix quite a difficult task.

The dispersion of CNTs in concrete mixtures is vital to the properties of the CNT-modified concrete composite. Different techniques have been used to ensure proper dispersion of CNTs, and they can be mainly classified into physical and chemical methods. The commonly used physical method is ultrasonication, which has limited effectiveness by itself and is usually used together with a chemical method. Other mechanical processing methods for dispersing agglomerated particles in liquids include agitator discs, colloid mills, high-pressure homogenizers, triple roller mills, plenary and vertical vibratory ball mills, and beads mills [ 2]. Chemical methods include non-covalent adsorption of mediating molecules onto the CNT surfaces, and covalent bonding of functional groups. In the first approach, the attachment between the molecules and the nanotube walls is a weak non-covalent bond. The use of surfactants is an example of this approach. The second approach involves the formation of a covalent bond. In this case, functional groups are attached by a strong covalent bond to the walls of the nanotubes. An example of this approach would be acid treatment of CNTs [ 3].

Surfactants naturally tend to adsorb onto the surface of the CNTs and to forma coating. The repulsion between the same charged surfactant molecules on the surface of the CNTs will prevent the particles from agglomerating.

It shall be noted that the surfactants used for nanoparticle dispersion often facilitate air entrainment. Coating on cement particles and entraining air, surfactants used at a high concentration would lead to substantial delayed cement hydration and reduced strength of concrete. As a result, a defoaming agent is sometimes used along with the surfactant [ 1].

Numerous research efforts have investigated the issue of dispersion in cementitious materials. In a study by Konsta-Gdoutos et al. [ 4], the rheological properties of cement paste and its nanostructure was dependent on the effectiveness of dispersion. Successful dispersion was achieved by the use of ultrasonication and a surfactant-to-CNTs ratio close to 4. The study showed that the properties of cement paste could be improved even with low well-dispersed CNTs concentration of 0.08% by weight of cement. The work done by Metaxa et al. [ 5] proposed a new methodology to produce high concentrations of CNTs suspensions in water. The proposed process involves ultrasonication, followed by ultracentrifugation, decantation and finally ultrasonication of the resulting highly concentrated suspension. Another study performed by Sobolkina et al. [ 6] investigated the effect of using two different surfactants; polyoxyethylene laurylether (Brij 35) and sodium dodecyl sulfate (SDS). The two surfactants behaved similarly and a sonication time of 120 min was required for proper dispersion. No improvement in strength was noted, however, which was attributed to the negative effect of ultrasonication which result in fracturing, twisting and curling of the CNTs. Saez de Ibarra et al. [ 7] used gum arabic to disperse CNTs in water. CNTs used without dispersion led to a reduction in the strength of cement paste whereas a slight increase in strength was noted when gum arabic was used denoting better dispersion. Polycarboxylate superplasticizer and methylcellulose have also proven successful in dispersing CNTs as reported by Wansom et al. [ 8] where fiber-reinforced cement composites were analyzed by AC impedance spectroscopy to study fiber percolation.

CNTs are frequently functionalized by treating them with a solution of sulfuric and nitric acid [ 9]. Acid treatment of CNTs has shown to be very effective in enhancing its dispersion ability in water. The acids modify the CNTs surface by oxidation and addition of functional groups like hydroxyl and carboxyl onto it. The presence of these functional groups makes the CNTs more compatible with water, thus improving dispersion.

Yazdanbakhsh [ 10] showed that the mechanical strength of concrete is improved by using acid-treated CNTs. It was argued that the addition of the carboxylic group (COOH) to the surface of CNTs through acid-treatment helps the calcium silicate hydrate (C-S-H), which is the main component of cement, to deposit on the surface of the CNTs due to the covalent bond between C-S-H and COOH. This strong interaction between the cement and the functionalized CNTs increases the efficiency of load transfer and bond strength between the cement matrix and the CNTs.

However, in another study [ 11], a comparison was made between the use of untreated and acid-treated CNTs. The prepared concrete samples were tested under flexure to determine their bending capacity under load. The bending test was performed using a 3-point bending setup where a load was applied at the center of the beam which is supported at its ends. It was interesting to note that the flexural strength, as indicated by the modulus of rupture, was highest in case of untreated CNTs. Using acid-treated CNTs actually led to a substantial reduction in the strength of the concrete beams by a factor of 2.5. The concrete samples were also tested for compression. The compressive strength was seen to improve with the use of untreated CNTs whereas the addition of the acid-treated CNTs led to considerable drop in the strength.

Previous work on acid-modified CNTs has failed to provide conclusive evidence regarding their efficiency in concrete. It was argued that a possible explanation in the drop of concrete compressive strength when treated CNTs were used can be given in terms of the amount of functional groups attached to the CNTs walls [ 11]. Excessive functionalization would introduce a larger amount of functional groups which are hydrophilic in nature and will tend to adsorb water molecules on their surface thus leaving less water for the hydration of cement. A thermo gravimetric analysis (TGA) was done and revealed that cement was not completely hydrated in case of concrete using acid-treated CNTs [ 11]. Another explanation for the reduced strength by the acid-modified CNTs could be related to the introduction of defects on the walls of the CNTs as a result of excessive treatment.

The acid-treatment process involves a number of parameters which are all critical to the success of the process. The type of acid used, treatment duration and temperature are all essential parameters which need to be considered. The acidic mixture of sulfuric and nitric acid has been used extensively. The use of nitric acid only is also common but would yield mild acidic treatment. Nitric acid attacks the already existing active sites on the walls of the CNTs whereas the mixture of sulfuric and nitric acid creates new active sites and thus promotes further functionalization [ 12].

There are two different procedures to undergo acid-modification:ultrasonication and reflux. The ultrasonication procedure involves immersing the mixture in an ultrasonicator bath for a few hours at room temperature. This process however usually leads to a considerable damage in the CNTs structure due to continuous sonication [ 13]. The reflux procedure involves using a reflux setup to accelerate the process and reduce the need to ultrasonicate the mixture. For this reason, a reflux procedure has been adopted in the present study.

Earlier research attempts were undergone to incorporate CNTs in cement-based materials at different concentrations up to 2% by weight of cement [ 4, 11, 14, 15]. Several of these studies concluded that a small addition of CNTs can be more effective at improving the properties of the cementitious composite, if properly dispersed. Konsta-Gdoutos et al. [ 16] showed that the performance of CNTs depend not only on its concentration but also on its aspect ratio by testing two different types of CNTs denoted as short and long fibers added to cement at varying percentages from 0.025% to 0.08%. Higher concentrations of CNTs were needed for short fibers to be able to bridge nano-cracks. CNTs used in the percentage of 0.25% by cement weight led to an increase in compressive strength of up to 40% [ 6]. CNT-modified cement composites prepared using CNTs at 0.2% by weight of cement, showed an increase in flexural strength of about 35%.

In the present study, multi-walled carbon nanotubes (MWCNTs), at 0.2% by weight of cement, were treated in an acidic mixture solution, made with sulfuric and nitric acids (3:1 by volume), for 100 min and 180 min. The effects of the treatment on functionality of the MWCNTs and strength of the mortar containing acid modified MWCNTs were investigated.

Materials and experimental program

Materials

Ordinary Type I Portland cement (OPC) that meet the requirement of ASTM C150, was used as cementitous material in this study. The specific gravity of the cement is 3.15 g/cm³ and the Blaine fineness is 310 m² /kg, having a nominal strength of 32.5 MPa. Table 1 provides the chemical composition of the cement.

Fine sand graded according to ASTM C109 test standard was used with a specific gravity of 2.63, absorption of 0.3%, and fineness modulus of 2.36.

The MWCNTs had a diameter between 20 and 50 nm. Figure 1 below shows an SEM image for the as-received CNTs. Deionized water was used for mixing to assure no interference in the chemistry of the CNTs is caused by any ions in the mixing water.

Acid-treatment of CNTs

A reflux setup was used for the acid-treatment process as shown in Fig. 2. Using a reflux system was meant to prevent loss of acid by evaporation. The reflux setup uses a condenser with cooling water running continuously. As the acid evaporates, the condenser cools down the acid and returns it back to the reaction flask. CNTs in the amount of 1.2 g were added to 40 mLof a mixture solution of sulfuric and nitric acid (3:1 by volume). The mixture was ultrasonicated for 10 min prior to acid-treatment to disentangle the CNTs. An ultrasonic water bath from VWR International LLC, model no. 150HT was used. The average sonic power supplied by this ultrasonicator was 135W and is equipped with a temperature control thermostat to prevent excessive temperature rise of the water inside the bath. Excessive temperature rise is unfavorable as it would thermally degrade the CNTs. The temperature of the water bath was maintained at room temperature. The purpose of the ultrasonication step was to separate the CNTs prior to acid treatment in order to render the treatment more effective. Longer ultrasonication durations, up to 2 h, is usually required when CNTs are dispersed using surfactants to give sufficient time for the surfactants to completely adsorb to the external walls of CNTs. In this experiment, however, such excessive ultrasonication duration was unnecessary, as the ultrasonication step was only meant to de-agglomerate the CNTs before undergoing acid treatment. It has also been reported that excessive ultrasonication durations would lead to damage of CNTs as a result of the high ultrasonic energy, which is capable of disrupting the bond structure of the pristine CNTs. Excessive ultrasonication could result in structural defects, such as shortening, curling and fracturing of the CNTs [ 9, 17]. A study done by Rossell et al. [ 18] investigated the impact of ultrasonication on the structural integrity of the CNTs, using techniques such as transmission electron microscopy and electron energy loss spectroscopy, and concluded that the sp² structure of CNTs is significantly altered by introducing defect sites.

Following ultrasonication, the solution of CNT in water was heated up in a water bath up to a temperature of 100°C. A thermometer was inserted in the water bath to monitor the temperature. The mixture was magnetically stirred during the treatment to facilitate the reaction. The reaction was allowed to proceed for 100 min and 180 min. The top of the reflux tube was connected by a rubber hose to a bottle containing sodium hydroxide, as shown in Fig. 2, which served to neutralize any escaping acids. When the reaction was completed, the flask containing the mixture was cooled down slowly and then diluted carefully with deionized water. The diluted mixture was then filtered using a vacuum filter and 0.2 mm polytetrafluorethylene (PTFE) filters, as shown in Fig. 3. The filters were wetted with ethanol prior to using them. The filtration process usually takes a long time because CNTs tend to block the filters forming a CNT cake on top. To ensure continuous and speedy filtration, the CNT cake should be carefully removed with a spatula. The filtered CNTs are washed repeatedly until a neutral pH is obtained. Finally, the CNTs are dried in vacuum at a temperature of 40°C overnight.

In the present study, the untreated CNTs will be referred to as CN while the treated CNTs will be referred to as CN100 and CN180 to designate treatment durations of 100 and 180 min, respectively.

Mortar sample preparation

A mortar mix with a water/cement/sand ratio of 0.5:1:2.5 was prepared. The mix proportions were selected to obtain a workable mix as CNTs tend to reduce workability. A mix with good workability was desired to ease the CNTs dispersion. CNTs were added to the mix in the amount of 0.2% by weight of cement.

The acid-treated CNTs were first dispersed in water and ultrasonicated for 2 min to obtain a well dispersed mix. Alternatively, a shear mixer could have been used for a similar period of time for the sole purpose of ensuring the homogeneity of the aqueous CNTs suspension before mixing with cement. The mixing process followed ASTM C305 using a standard Hobart mixer.

Three mortar mixes were prepared, namely, plain mortar, mortar containing CNTs treated for 100 min and mortar containing CNTs treated for 180 min. For each mix, a set of three 50-mm cubes were casted to be tested for compressive strength. The samples were poured in oiled molds and covered with wet burlap and the molds of the samples were removed at 1 day after casting. Curing of samples was done by placing them in a moist room.

In the present study, PC, PCN100 and PCN180 will be used to denote mortar samples without CNTs, with acid-treated CNTs for 100 min, and with acid-treated CNTs for 180 min, respectively.

Test methods

The mortar specimens were tested for compressive strength at 28 days. The compression tests were performed according to ASTM C109 using a 400,000 lbs capacity compression testing machine.

FTIR spectroscopy (Thermo Scientific NICOLET 380 FTIR) was used to qualitatively identify the hydroxyl and carboxyl groups attached to the surface of MWCNTs. The specimens were prepared using KBr method, where a small sample of CNTs was mixed with KBr and grinded well and then pressed using a hydraulic press.

UV-Vis analysis was conducted using a UV-vis-NIR spectrophotometer (UV-Vis-NIR, Cary 5000-UV BROP, Agilent Technologies, Australia), in 1 cm quartz cuvettes over a wavelength range of 200 to 800 nm.

A Leo Supra 55 (ZEISS) field emission SEM device was used to study the dispersion of the treated CNTs in mortar.

Raman spectroscopy study was done using a laser excitation at a wavelength of 532 nm.

Results and discussion

Defects in CNTs examined using Raman Spectroscopy

The acid-treated CNTs were characterized using Raman Spectroscopy to determine the degree of defects resulting from the surface modification. As seen in Fig. 4, Raman spectrum of the CNTs studied exhibited three distinct bands, namely, D and G, and G’ bands. The D band, at 1350 cm^-1 is known as the disorder or defect mode, originates from edge configurations in graphene where the planar sheet configuration is disrupted. It gives an indication of the degree of defects in the nanotubes, such as the amount of amorphous carbon [ 19]. The G band, at 1500–1600 cm^-1, is the primary mode in graphene, and it indicates the tangential stretching of the C-C bonds. The G’ band appears at 2690 cm^-1 and indicates a second-order over tone in plane vibration of the graphitic structure. Another commonly seen band is the R band, at 100-400 cm^-1, not seen in Fig. 4 below, is also known as radial breathing band, and it is most sensitive to the diameter of the nanotubes [ 20].

In the present study, the ratio of D-band to G-band is calculated, as shown in Table 2, and used to assess the degree of disorder in the structure of the nanotubes.

As seen from the table, the intensity of the D-band, relative to the G-band, increased with the increase of the acid modification treatment duration denoting more defects being introduced in the process. An appreciable increase in defects was noted at the treatment duration of 100 min. No noticeable increase in defects, however, was noted as the treatment duration increased from 100 to 180 min. This result is possibly attributed to the low reaction temperature which was limited to 100°C due to the use of a water bath. Given a higher temperature, the effect of duration on the degree of defects could have been more considerable.

An initial increase in the ratio of the D- and G- bands followed by a period of insignificant variation in the ratio, as is the case here, was also reported in other studies Ref.[ 12], with the use of nitric acid only. The ratio of the D- and G- bands reflects the amount of amorphous carbon where the higher the ratio the more amorphous carbon. With acid treatment, this ratio initially increases, indicating more defects. Following this initial stage, the amount of amorphous carbon being generated as a result of the acid attack on the CNTs is continuously being consumed in the reaction, hence no significant change in the amount of amorphous carbon and accordingly the ratio of the D- and G- bands does not show any significant variation. It should be clear however that, regardless the ratio of the D- and G- bands, the CNTs are continuously being attacked by the acid and converted into amorphous carbon form, as the treatment duration proceeds. This can be verified by comparing the CNT mass before and after treatment. The work done by [ 12] showed that further treatment for prolonged period of time will lead to further decrease in this ratio indicating that the rate of generation of amorphous carbon is higher than the rate of its oxidation.

Degree of functionalization of CNTs using FTIR

The FTIR spectra for the treated and untreated CNTs are shown in Fig. 5. The untreated CNTs show a peak at about 3450 cm^-1 which corresponds to the O-H stretch mode of vibration. The O-H groups present in the untreated CNTs are possibly attributed to the air moisture attached to the nanotubes during sample preparation. This peak showed a slight increase in intensity with acid-modification however not very significant. No sign of carboxylic groups was found in the pristine sample. For acid-modified CNTs, a peak at 2361 cm^-1 was noted. This peak increased slightly in intensity as the treatment duration increased from 100 min to 180 min. The peak is believed to correspond to the O-H stretch bond in the –COOH group [ 21]. A small peak at 1721 cm^-1 correspond to C= O bond in carboxylic group. These peaks point to the presence of carboxylic groups in the treated carbon nanotubes.

Study of dispersion using UV-Vis

The dispersion of CNTs was further investigated using UV-Vis spectrophotometry. Only individual CNTs are active in the UV-Vis region. CNTs agglomerates do not show any absorbance possibly due to carrier tunneling between the entangled nanotubes [ 22]. The peak absorption intensity for individual MWCNTs has been reported by several researchers to be between 200 and 300 nm [ 23]. The absorbance is linearly related to the CNT concentration, hence the degree of dispersion, as given by Lambert-Beer law.

To evaluate the dispersion efficiency of the acid-treatment process, an amount of 0.08 g of acid-treated CNTs was added to 20 ml of water resulting in a concentration of 0.4g/100mL which is the same as the concentration of CNT in the tested mortar since CNT was used at 0.2% by weight of cement and the ratio of water to cement was 0.5. The solution was ultrasonicated for 2 min as a means of mixing prior to scanning with the UV-Vis spectrophotometer. The solution was then diluted with deionized water at a ratio of 1:100. The reason for dilution was to provide low concentration of CNTs since Lambert-Beer law is only well-obeyed at lower concentrations [ 24]. The resulting diluted solution had a CNT concentration of 0.04 mg/mL.

The absorption spectra for both CN100 (CNTs treated for 100 min) and CN180 (CNTs treated for 180 min) are shown in Fig. 6. The increasing absorbance intensity indicates increasing concentration of dispersed CNTs in the tested samples. In the figure, the presence of the absorption peak at around 240 nm was clearly shown. The intensity of the absorbance peak increased with the increase in treatment duration from 100 to 180 min, indicating better dispersion.

An attempt to calculate the dispersion efficiency was made by using Beer-lambert’s law which states that A= eCl, where A is absorbance, e is the extinction coefficient, C is the CNTs concentration in suspension, and l is the cuvette length. A study done by Rance et al. [ 24] investigated the extinction coefficients for different types of CNTs. According to this study, the extinction coefficient ranged from 44.8 to 54.5 mL·mg^-1·cm^-1. The concentration of CNTs in suspension was calculated, using the peak absorbance from Fig. 6 and the extinction coefficients reported by Rance et al. [ 24] with a cuvette length of 1 cm. The calculated concentrations for both CN100 and CN180 were 0.026–0.032 mg/mL and 0.029–0.036 mg/mL respectively. The degree of dispersion given as the ratio between the calculated concentrations from above and the initial concentration of 0.04 mg/mL for both CN100 and CN180 was about 65%–79% and 72%–88% respectively. These values clearly indicate that the acid-treatment process was successful to a great extent in dispersing the CNTs.

Compressive strength of CNT-mortar composite

The results of the compressive strength at 28 days for the tested specimens are shown in Fig. 7 below. It is clear that the compressive strength has increased upon the addition of the acid-treated CNTs. However, no significant change can be detected between the compressive strength for PCN100 and PCN180. The increase in treatment duration from 100 to 180 min did not lead to any change in the overall efficiency of the CNTs.

As the treatment process proceeds, two competing factors work against each other. With increasing treatment duration, better functionalization of the nanotubes is obtained hence better dispersion which ultimately results in a homogenous mix with improved mechanical properties. On the other hand, as the treatment duration is increased, more defects are introduced in the nanotubes as verified by Raman spectrum in Fig. 4. These defects affect the properties of the nanotubes and lower their effectiveness in increasing the mechanical properties of the mix. This suggests that a treatment duration of 180 min resulted in better dispersion however at the expense of the integrity of the nanotube structure. The two factors counterbalanced each other thus no change was noticed between the different durations studied.

SEM

Broken pieces of the tested samples were imaged using SEM to verify proper dispersion of the CNTs inside the cement matrix. The broken pieces were spattered with gold for image clarity. The images showed similar dispersion for both treatment durations. Figure 8 below shows the mortar matrix with C-S-H gel and some CNTs. No agglomeration of CNTs was noted.

Summary and conclusion

The treatment duration is a critical parameter in the acid-modification process of CNTs. This study investigated the effect of changing the treatment duration on the overall performance of the CNTs inside mortar. No appreciable difference was noted in the compressive strength of the specimens tested using CNTs treated at 100 and 180 min. However, it was verified that longer treatment durations result in better functionalization, resulting inimproved dispersion characteristics. On the other hand, a longer treatment duration introduced more defects into the CNTs structure leading to a reduction in their properties. The net effect of the functionalization and the extent of the defects determine the efficiency of the CNTs.

The treatment process followed in this study was not very severe as the temperature was set to only 100°C due to use of a water bath, which could explain why the two treatment durations selected for this study did not show a considerable variation in the results. Further study is necessary to investigate the effects of acid treated CNTs on cement hydration, microstructure development and flexural strength of mortar/concrete.

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