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Draft

Performance of carbon nanotubes/cement composites using

different surfactants

Journal: Canadian Journal of Civil Engineering

Manuscript ID cjce-2016-0570.R1

Manuscript Type: Article

Date Submitted by the Author: 21-Feb-2017

Complete List of Authors: ElKashef, Mohamed; The American University In Cairo, Department of Construction Engineering Abou-Zeid, Mohamed Nagib; The American University in Cairo, Department of Construction Engineering

Is the invited manuscript for consideration in a Special

Issue? :

N/A

Keyword: Carbon nanotubes, Concrete, UV-Vis, Dispersion, Raman

https://mc06.manuscriptcentral.com/cjce-pubs

Canadian Journal of Civil Engineering

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1

Performance of carbon nanotubes/cement composites using different

surfactants

M. Elkashefa*

, M.N. Abou-Zeida

a Construction Engineering Department, The American University in Cairo, Egypt.

*Corresponding author. Email: [email protected]

Abstract

The performance of Carbon Nanotubes (CNTs) in cement-based composites relies to a great

extent on its degree of dispersion. In this work, the performance of two commonly used

surfactants; Sodium dodecyl sulfate (SDS) and Triton X-100, is being compared. The effect of

surfactant-to-CNT ratio on dispersion efficiency is studied using UV-Vis spectrometry, to

determine the optimum surfactant dosage. For the optimum ultra-sonication energy, Raman

spectroscopy is used to assess the degree of imperfections on CNTs. CNTs-reinforced mortar

specimens prepared using Triton X-100 and SDS are tested for compressive and flexural

strength. Triton X-100 is shown to exhibit better dispersion efficiency than SDS, leading to more

improvement in flexural and compressive strength. An ultra-sonication time of 60 minutes (19.4

KJ/ml) is shown to be sufficient to achieve proper dispersion, however notable degradation of

CNTs was noted beyond 30 minutes (9.7 KJ/ml) of dispersion leading to a strength reduction.

Keywords: Carbon nanotubes, cement, concrete, dispersion, UV-Vis, Raman.

Introduction

Since its discovery in 1991, CNTs have been under extensive study by researchers (Iijima 1991).

CNTs are tubular in shape with a diameter that is typically in the nanoscale range. They are

entirely made of carbon atoms which are bonded together in a hexagonal pattern to form the

walls and a pentagonal ring structure to form the end caps. They can be made up of single walls

(Single-walled CNTs), double walls (Double-walled CNTs) or multiple-wall (Multi-walled

CNTs) and are characterized by a very high aspect ratio that can go up to 2,500,000 (Kumar et

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al. 2011) and possess superior mechanical and electrical properties. CNTs-reinforced cement

composites have shown enhanced mechanical properties (Konsta-Gdoutos et al. 2010b; Li et al.

2005), exceptional electromagnetic shielding (Singh et al. 2013) and self-sensing characteristics

(Han et al. 2009). To ensure utmost improvement in properties, CNTs need to be properly

dispersed in the cement-based matrix. Dispersion of CNTs is a difficult task since they tend to

agglomerate due the van der Waals attraction, caused by the π-electron clouds on their surface,

which is considerable owning to their large surface area.

The various methods to achieve CNTs dispersion generally fall under a mechanical and/or

chemical approach. The mechanical approach utilizes a physical technique such as ultrosnication

whereas the chemical approach involves altering the outer structure of the CNTs, by covalent or

non-covalent interaction with other chemical moieties. Ultrasonication is a frequently used

mechanical method which involves applying ultrasonic energy. The ultrasonic energy generates

cavitation bubbles which when collapsed results in high pressure and temperature. Such high

pressure, which could be in excess of 500 atm, is sufficient to debundle the CNTs agglomerates

(Hilding et al. 2003). Ultrasonication is used to disperse CNTs in low viscosity liquids such as

water. Ultrasonication can be done using a water bath sonicator or a tip sonicator, which is

limited to small specimens as the ultrasonic energy is provided through a tip with a narrow

diameter. The degree of dispersion of CNTs depend primarily on the sonication energy (Yu et al.

2007). The sonication energy depends on the sonicator power, sonication time and volume of

liquid being dispersed. The role of sonication energy using an anionic surfactant SDBS was

studied by Yang et al. (2013) and it was concluded that the optimum sonication energy is

independent of the surfactant-to-CNT ratio and is only dependent on the diameter of the CNTs

due to the interrelation between the van der Waals attraction forces and nanotubes diameter,

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where lower forces are associated with larger diameters. A number of other mechanical

dispersion techniques include agitator discs, colloid mills, high-pressure homogenizers, triple

roller mills, and bead mills (Inkyo et al. 2006). These techniques were used to disperse different

type of nanoparticles, specifically bead mills were used with titanium dioxide nanoparticles

(Inkyo et al. 2006).

Chemical methods include covalent or non-covalent attachment of molecules onto the CNTs

surface. Using surfactants or polymers is an example of non-covalent bonding which do not

disturb the chemical structure of the CNTs. Surfactants have both a hydrophilic part which is

mainly the head group and a hydrophobic part which consists of a chain of hydrocarbons.

Surfactants can be classified into cationic, anionic, and nonionic or zwitterionic based on the

charge on their head group (Vaisman et al. 2006). The mechanism of adsorption of surfactant

molecules onto the CNTs surfaces can be through coulomb attraction in case of ionic surfactants

or through π- π interaction in nonionic surfactants (Rastogi et al. 2008). The ionic and steric

repulsion between the surfactant molecules adsorbing onto the CNTs walls prevent them from

agglomerating.

The use of surfactants is usually accompanied by ultrasonication. Ultrasonication separates the

CNTs and creates gaps which are penetrated by the surfactant to wrap individual CNTs. CNTs

dispersed by ultrasonication alone tend to reagglomerate , hence the use of surfactants to provide

a stable CNTs suspension (Kumar et al. 2011). It is noted that surfactants increase air void

content which could potentially reduce concrete strength and delay cement hydration hence a

defoaming agent is sometimes used (Gopalakrishnan et al. 2011). Alternatively, the fresh paste

could be placed inside a vacuum chamber to remove large air voids (Al-Rub et al. 2011; Tyson et

al. 2011).

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Earlier research on surfactants showed that there exists an optimum surfactant-to-CNTs ratio at

which maximum dispersion is achieved (Rastogi et al. 2008). Higher surfactant concentrations

beyond this optimum ratio leads to CNTs flocculation and a reduction in dispersion efficiency.

This behavior was explained in terms of the theory of micelle formation, as with increasing

surfactant concentrations multilayers of the surfactant molecules build up around the CNTs

walls. In this arrangement, the outermost surfactant molecules have their hydrophilic tails in

contact with water, which is not favorable in terms of energy, hence interaction between the

surfactant molecules on neighboring CNTs take place to reduce the surface energy which

eventually leads to CNTs flocculation.

Another chemical approach involves covalent attachment of functional groups onto the walls of

CNTs which could be done by acid treatment (Tchoul et al. 2007). A solution of sulfuric and

nitric acid is usually used for this treatment (Hilding et al. 2003). The acids induce oxidation of

the CNTs leading to attachment of hydroxylic and carboxylic functional groups which renders

the nanotubes readily dispersible. It is also believed that theses functional groups bond with

hydration products, mainly calcium silicate hydrate, providing a strong interaction which

improves load transfer efficiency between the CNTs and the surrounding cement matrix

(Yazdanbakhsh et al. 2009). Excessive acid treatment may lead to a degradation of the CNTs

which could potentially result in a drop in the mortar strength (Elkashef et al. 2016)

Extensive research has been conducted to investigate dispersion in cement-based materials. The

rheological properties and nanostructure of cement paste was shown to rely heavily on the

dispersion efficiency of CNTs. It was shown that a considerable improvement in properties can

be achieved even at low CNTs concentration of 0.08% by weight of cement, when a surfactant-

to-CNT ratio of 4 accompanied by ultrasonication was used (Konsta-Gdoutos et al. 2010a). A

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recent study compared the performance of two different surfactants; polyoxyethylene laurylether

(Brij 35) and SDS and concluded that both behaved similarly and an ultrasonication time of 120

minutes was required to ensure proper dispersion of CNTs with either of the two surfactants

(Sobolkina et al. 2012). However, using the dispersed CNTs in mortar led to no improvement in

strength, which was attributed to a possible degradation of the CNTs as a result of long

ultrasonication times. In another study, gum arabic was used as a dispersing agent and a slight

increase in strength was noted (Saez de Ibarra et al. 2006). Methylcellulose and polycarboxylate

superplasticizer were also used successfully to disperse CNTs as verified by AC impedance

spectroscopy (Wansom et al. 2006).

CNTs have been added to cement-based materials at different loading ratios up to 2% by weight

of cement (Cwirzen et al. 2008; Konsta-Gdoutos et al. 2010a; Li et al. 2005; Makar et al. 2005).

It is currently established that low additions of CNTs can be very effective when properly

dispersed. CNT concentrations ranging from 0.025% to 0.08% were used to reinforce cement

paste (Konsta-Gdoutos et al. 2010a). CNTs with lower aspect ratios require higher

concentrations to be able to bridge nano-cracks. An increase in compressive strength of up to

40% was noted with CNTs added at 0.25% by weight of cement (Sobolkina et al. 2012). CNT-

modified cement composites prepared using CNTs at 0.2% by weight of cement, showed an

increase in flexural strength of about 35% (Al-Rub et al. 2011).

In this work, CNTs are added at 0.2% by weight of cement and dispersed using both an anionic,

SDS, and a nonionic, Triton X-100, surfactants and ultrasonicated for a duration of 30, 60 and 90

minutes. A quantitative analysis of the dispersion efficiency of CNTs, at different surfactant-to-

CNT ratios of 1, 2.5 and 5, is done using UV-Vis spectrometry. The damage caused by

ultrasonication is assessed using Raman spectrometry. The dispersed CNTs are used to prepare

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mortar mixes and tested for compressive and flexural strength. The percentage of improvement

in strength is correlated with the dispersion efficiency and degree of damage results from UV-

Vis and Raman.

Materials and methods

Materials

Ordinary Portland Cement (OPC) Type 1 that meets ASTM C150 requirements was used for this

study. The specific gravity of the cement is 3.15 and its Blaine fineness is 330 m2/kg. The

chemical composition of the cement is listed in Table 1.

Table 1: Chemical properties of OPC

Fine sand graded as per ASTM C109 was used having a specific gravity of 2.63, absorption of

0.3%, and a fineness modulus of 2.36. The CNTs were multi-walled with a diameter between 20-

50 nm as shown in Figure 1, which provides an SEM image of the as-received CNTs. Deionized

water was used for mixing to prevent any ion interference from the water with the CNTs

chemistry.

Figure 1: SEM image of pristine CNTs

CNTs dispersion

In order to select a surfactant-to-CNT ratio for this study, dispersion of CNTs were prepared

using different surfactant-to-CNT ratios. A solution of CNTs in deionized water was prepared

using 0.08g of CNTs in 20ml of water resulting in a concentration of 0.4g/100mL, which is the

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same concentration of CNTs in mortar. An amount of 0.08, 0.2 and 0.4g of either SDS or Triton

X-100 was added to the solution resulting in a surfactant-to-CNT ratio of 1, 2.5 and 5

respectively. The prepared suspensions were then utlrasonicated for 30, 60 and 90 minutes using

an ultrasonic water bath from VWR International LLC, model no. 150 HT with an average

power of 135W and a temperature control thermostat to maintain the temperature at 21oC. The

ultrasonicated suspension was then diluted with deionized water at a ratio of 1:40 before

analyzing them using UV-Vis. The reason for dilution was to provide low concentration of CNTs

since Lambert-Beer law is only well-obeyed at lower concentrations (Rance et al. 2010). The

resulting diluted solution had a CNT concentration of 0.1mg/mL. The dispersion efficiency of

the surfactant and the ultrasonication process was then assessed using UV-Vis spectrometry. The

UV-Vis spectra for the different surfactant-to-CNT ratios and ultrasonication times were then

analyzed to select the appropriate surfactant-to-CNT ratio and optimum ultrasonication time

required for casting the mortar specimens.

UV-Vis

The significance of using UV-Vis to study the dispersion of CNTs stem from the fact that only

individual CNTs are active in the UV-vis region. Entangled CNTs do not show absorbance in

this E/M region, between 200 to 1200 nm (Yu et al. 2007). Accordingly, the detected absorbance

is entirely related to the individual CNTs dispersed in the solution. The intensity of the

absorption spectrum is linearly proportional to the concentration of dispersed individual CNTs.

The peak absorption intensity for CNTs has been reported by several researchers to be between

200 and 300 nm (Grossiord et al. 2005; Jiang et al. 2003; Yu et al. 2007). This absorbance peak

is associated with Plasmon resonance of the π-electron cloud on the CNTs.

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The linear relation between absorbance and concentration, as given by the Lambert-Beer law, is

not applicable at higher concentrations. Accordingly, the CNT solution in water had to be diluted

to lower concentrations for the purpose of the UV-Vis analysis.

The analysis was done using a UV-vis-NIR spectrometer (UV-Vis-NIR, Cary 5000-UV BROP,

Agilent Technologies, Australia), in 1 cm quartz cuvettes over the range 200 to 600 nm. An

absorbance spectrum, covering this wavelength range, was obtained for each sonication time for

each of the two surfactants, at different surfactant-to-CNT ratios. For each tested sample, a blank

containing water and the same amount of surfactant as in the tested sample but no CNTs, was

used as a baseline and subtracted from the obtained absorbance spectrum.

Beer-Lambert’s law correlates the concentration of CNTs in suspension with the absorbance

measurement as given by the following equation:

A = ε C l (1)

where A is absorbance, ε is the extinction coefficient, C is the CNTs concentration, and l is the

cuvette dimension.

The extinction coefficient, ε, varies with wavelength and CNTs dimensions. An extensive study

done on both single-walled and multi-walled carbon nanotubes produced by different methods

including arc discharge and chemical vapor deposition, showed that the absorbance peak

appeared between 206.1 and 251.2 nm with a corresponding extinction coefficient in the narrow

range of 44.8 to 54.5 mL mg-1

cm-1

(Rance et al. 2010). Other studies reported different values of

extinction coefficients at other wavelengths, such as 30.3±-0.2 mL mg-1

cm-1

at a wavelength of

1035.3 nm (Jeong et al. 2007) for single-walled carbon nanotubes and 16.9 mL mg-1

cm-1

at a

wavelength of 530 nm (Ikeda et al. 2006; Marsh et al. 2007). The study done by Rance,

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however, covered different types of carbon nanotubes and reported the extinction coefficient

within the range at which the maximum absorbance peak would occur (Rance et al. 2010). For

this reason, a value of 54.5 mL mg-1

cm-1

was used in this study, as the extinction coefficient for

multi-walled CNTs, as reported by Rance (Rance et al. 2010).

Raman Spectroscopy

Previous studies have shown that ultrasonication cause shortening, curling and fracturing of the

CNTs (Hilding et al. 2003; Lu et al. 1996). Raman spectroscopy was used to characterize the

ultrasonicated CNTs in order to assess the degree of defects caused by ultrasonication. A Raman

device with laser excitation of wavelength of 532 nm was used. A comparison was made

between the as-received CNTs, CNTs ultrasonicated for 30 minutes and CNTs ultrasonicated for

60 minutes. The CNTs ultrasonicated for 90 minutes were not analyzed since the UV-Vis results

showed no considerable improvement in dispersion beyond 60 minutes.

Specimen Preparation and Strength Testing

Mortar cubes of size 50 mm and prisms measuring 40*40*160 mm were casted for compressive

and flexural strength testing respectively. The mix used had a water/cement/sand ratio of

0.5:1:2.5. The mix was designed to provide good workability to ease the CNTs dispersion. CNTs

were added at 0.2% by weight of cement. CNTs were dispersed with both Triton X-100 and SDS

using a surfactant-to-CNT ratio of 2.5, as determined from the UV-Vis analysis. The

ultrasonication time was limited to 30 and 60 minutes only as the UV-Vis results showed no

notable improvement in dispersion above 60 minutes. Mortar specimens without CNTs, and

containing the same amount of surfactant as the CNTs-reinforced specimens, were used as the

control. The mixing procedure was done according to ASTM C305 using a Hobart mixer with

flat beater. Following mixing, the fresh mortar was placed in a vacuum chamber for 5 minutes to

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eliminate the air voids caused by the surfactant. The air content of the fresh mortar was measured

before and after vacuum application as per ASTM C185.

The mortar was then filled in oiled molds and covered with wet burlap and de-molded after 1

day. The samples were cured in a moist curing room for 28 days before testing. Compressive

testing was performed according to ASTM C109 using a 400,000 lbs capacity compression

testing machine. Flexural testing was conducted according to ASTM C348 using an INSTRON

type universal testing machine with 22,000 lbs capacity testing machine. The compressive and

flexural strength results reported, at each test condition, are the average of three test specimens.

Results and discussion

Effect of different surfactant-to-CNT ratios

The intensity of the peak absorbance in the UV-vis spectra for the CNTs aqueous suspensions

prepared with different surfactant-to-CNT ratios and sonication durations was recorded. Figure 2

and 3, shows the variation of the peak intensity with varying surfactant-to-CNT ratios for SDS

and Triton X-100 respectively. It can be seen that the absorbance increases with an increasing

sonication time denoting better dispersion with time regardless of the type of surfactant used. For

SDS, the highest absorbance and hence maximum dispersion, was obtained at SDS-to-CNT ratio

of 1 indicating that the optimum concentration for SDS is achieved at SDS-to-CNT ratio of 1 or

less. As for Triton X-100, increasing the ratio from 1 to 5 led to an increase in dispersion. Note

that in Figure 3, the results of Triton X-100/CNT ratio of 5 is not shown because the absorbance

was higher than the limit of detection of the device. It was thus be concluded that the optimum

concentration of Triton X-100 is above the concentrations used in this study. Nevertheless, the

concentrations of Triton X-100 used in this study were very efficient at achieving dispersion as

indicated by the high absorbance values compared to SDS.

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Based on the above analysis, a surfactant-to-CNT ratio of 2.5 was selected for the two

surfactants. At this concentration, the efficiency of SDS was very much close to the maximum

efficiency noted at a ratio of 1. Also, the efficiency of Triton X-100 was reasonably high. Using

a higher Surfactant-to-CNT ratio, above 2.5, would have decreased the efficiency of SDS

notably. In the same way, using a lower Surfactant-to-CNT ratio, less than 2.5, would lower the

efficiency of Triton X-100.

Figure 2: UV-vis peak absorbance for CNTs aqueous suspension using SDS

Figure 3: UV-vis peak absorbance for CNTs aqueous suspension using Triton X-100

Analysis of dispersion efficiency

The dispersion efficiency of both SDS and Triton X-100 was quantitatively assessed using Beer-

lambert law and an extinction coefficient of 54.5 mL mg-1

cm-1

(Rance et al. 2010). The

calculations were done for the CNTs suspensions prepared using the selected surfactant-to-CNT

ratio of 2.5 only. The complete UV-vis spectra is shown in Figure 4.

Figure 4: UV-vis spectra for CNT in water dispersions using Triton X-100 and SDS for different

sonication times

The maximum peak absorbance occurred at a wavelength range between 210 to 226 nm.

Lambert-Beer’s law was used to compute the CNTs concentration. A dispersion factor was

calculated as the ratio between the computed concentration and the initial CNT concentration,

which was equal to 0.1mg/mL. This dispersion factor is representative of the dispersion

effectiveness of the surfactant used and the sonication process. Table 2 lists the dispersion

factors for each of the different combination of surfactant and sonication time. The factors given

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in Table 1 show that Triton X-100 exhibited significant dispersion power compared to SDS. The

dispersion factor did not increase considerably from a sonication time of 60 to 90 mins.

Table 2: Dispersion factors for different surfactants and sonication times.

SDS is an anionic surfactant with a sulfate hydrophilic head group and a long organic

hydrophobic tail group. The existence of long chains improve dispersion because they provide

larger steric hindrance which results in greater repulsion between the CNTs preventing them

from agglomerating (Islam et al. 2003). On the other side, Triton X-100 is a nonionic surfactant

with a hydrophilic group of polyethylene oxide and an alkyl-phenol hydrophobic group. The

benzene structure of the hydrophobic part of Triton X-100 provides better adhesion to the surface

of the CNTs due to the π-π interaction (Cyr et al. 1996). The superior dispersion power of Triton

X-100, as noted in this work, can thus be attributed to the presence of the benzene ring which

drastically increase the binding and surface coverage of the surfactant onto the CNTs.

Raman Spectroscopy

As seen in Figure 5, Raman spectrum of the CNTs showed two main peaks, namely D and G

bands. The G band, noted at 1500-1600 cm-1

, is a characteristic mode of graphene-like ordered

structures which is attributed to the tangential stretching of the C-C bonds. The D band, seen at

1350 cm-1

, is an indication of the defects and disorder in the structure. It is mainly linked to the

disruptions in the planar sheet graphene structure at the edges and amount of amorphous carbon

(Natsuki et al. 2013). The ratio of D-band to G-band is of particular significance as it relates

directly to the imperfections in the CNTs. In Figure 5, the peak at the D-band was normalized to

1 so that the normalized intensity of the peak at the G-band is used for comparison. The value of

the normalized intensity at the G-band, which equals to the ratio of D-band to G-band, was

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tabulated in Table 3 below. It can be concluded that the degree of defects, as indicated by this

ratio, increased with the increase of ultrasonication. The change of this ratio with 30-minutes

ultrasonication was insignificant, however a noticeable increase was noted as the ultrasonication

time increased to 60 minutes.

Figure 5: Raman Spectra for as-received and ultrasonicated CNTs

Table 3: Ratio of D- to G-bands

Air content

The results of the air content before and after vacuum application is shown in Table 4. The

determined air content was 7.2% and 6.6%, following vacuum application, compared to an air

content of 11.6% and 11.1% before vacuum application, for SDS and Triton X-100 respectively.

These results also indicate that the entrapped air was not completely removed by vacuum suction

however the use of vacuum resulted in a considerable elimination of the air voids. Earlier

research has showed that anionic surfactants like SDS generate more stable air voids (Dodson

1990), compared to nonionic surfactants such as Triton X-100, which agrees with the finding of

this study.

Table 4: Air content before and after vacuum for the mixes with SDS and Triton X-100

Compressive Strength

A summary of the compressive strength for all specimens is shown in Figure 6 and Table 5,

along with the percentage improvement in strength with CNTs. It was noted that the strength of

the control specimens made with SDS was lower than that made with Triton X-100 which could

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possibly be due to more air voids being trapped in the case of SDS as evidenced by the air

content measurements. The percentage improvement in strength, shown in Table 5, indicate that

using Triton X-100 resulted in a higher percentage increase in strength amounting to 38% as

compared to an increase of 20% in the case of SDS, at 30-minute ultranosication. These results

agree with the UV-Vis analysis earlier which revealed that Triton X-100 is more efficient than

SDS in dispersing the CNTs. With better dispersion, the strength improvement due to CNTs is

more pronounced. The strength obtained at an ultrasonication time of 60 minutes did not further

improve the strength. According to UV-Vis analysis, when ultrasonication time increased from

30 to 60 minutes, the dispersion efficiency factors slightly increased by about 10-15%. However,

this increase in dispersion efficiency did not cause further improvement in strength as evidenced

by the compressive strength results. This can be explained in terms of defects introduced by the

ultrasonication process which disrupts the integrity of the CNTs and reduce their mechanical

properties.

Figure 6: Compressive Strength of tested mortar specimens

Table 5: Compressive strength values and percentage improvement

Flexural Strength

The flexural strength results are shown in Figure 7 and Table 6, along with percentage

improvement with CNTs. The control specimens, without CNTs, showed similar strengths for

both SDS and Triton X-100 with the latter being slightly higher. Using CNTs ultrasonicated for

30 minutes, the improvement in strength was 46% and 29% for Triton X-100 and SDS

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respectively. The percentage improvement in flexural strength dropped considerably to 33% and

9% for Triton X-100 and SDS respectively, when CNTs ultrasonicated for 60 minutes were used.

Clearly the defects introduced by ultrasonication primarily affected the tensile strength of the

CNTs which determines its ability to bridge cracks and enhances the flexural strength of the

composite.

Figure 7: Flexural strength of tested mortar specimens

Table 6: Flexural strength values and percentage improvement

Summary and conclusions

In this study, two commonly used surfactant, namely SDS and Triton X-100 were used to

disperse CNTs in water. The performance of the two surfactants were investigated using UV-Vis

spectrometry and it was shown that Triton X-100 proved to be more efficient due to the presence

of the benzene ring in their structure which improves its ability to bond to the CNTs. This

finding agrees with previous research which also pointed out to the effectiveness of Triton X-100

as compared to SDS (Rastogi et al. 2008). The dispersion factor, which is an indication of the

dispersion efficiency, for Triton X-100 at a surfactant-to-CNT ratio of 2.5 and an ultrasonication

time of 30 minutes reached up to 0.69 compared to a dispersion factor of 0.24 for SDS. An

analysis of the ultrasonicated CNTs using Raman showed that the CNTs degraded notably

beyond an ultrasonication time of 30 minutes.

Mortar specimens containing 0.2% of CNTs by weight of cement showed an improvement in

both compressive and flexural strength. The improvement was higher in case of Triton X-100,

compared to SDS, due to their better dispersion capabilities. With Triton X-100, an increase of

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38% and 51% in compressive and flexural strength respectively was obtained at a surfactant-to-

CNT ratio of 2.5 and an ultrasonication time of 30 minutes. Mortar specimens using CNTs

ultrasonicated for 60 minutes showed no improvement in compressive strength and a significant

reduction in flexural strength, which is believed to be related to the damage in CNTs due to

ultrasonication. Obviously, the damage in CNTs has affected its ability to bridge cracks and

improve flexural strength. In this regard, additional work might be necessary to further

understand the difference in response between compressive and flexural specimens with

increasing sonication times. In conclusion, an ultrasonication time of 30 minutes, providing an

energy of 9.7 KJ/ml, was considered optimal, for both types of surfactants used in this study,

since it provided satisfactory level of dispersion without considerably degrading the CNTs as

evidenced by Raman spectroscopy, leading to higher improvement in strength.

Acknowledgements

This work was funded by the American University in Cairo (AUC), Egypt.

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Figure 1: SEM image of pristine CNTs

Figure 2: UV-vis peak absorbance for CNTs aqueous suspension using SDS

Figure 3: UV-vis peak absorbance for CNTs aqueous suspension using Triton X-100

Figure 4: UV-vis spectra for CNT in water dispersions using Triton X-100 and SDS for different

sonication times

Figure 5: Raman Spectra for as-received and ultrasonicated CNTs

Figure 6: Compressive Strength of tested mortar specimens

Figure 7: Flexural strength of tested mortar specimens

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Table 1: Chemical properties of OPC

SiO2 Al2O3 Fe2O3 CaO MgO SO3 LOI

20.89 4.68 3.43 63.33 2.18 2.95 2.20

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Table 2: Dispersion factors for different surfactants and sonication times.

Surfactant Sonication Time

30 minutes 60 minutes 90 minutes

Triton X-100 0.69 0.83 0.83

SDS 0.24 0.27 0.29

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Table 3: Ratio of D- to G-bands

As-Received CNTs CNTs 30mins CNTs 60mins

Ratio of D- to G-bands 1.03 1.05 1.12

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Table 4: Air content before and after vacuum for the mixes with SDS and Triton X-100

Before

Vacuum

After

Vacuum

SDS 11.6% 7.2%

Triton X-100 11.1% 6.6%

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Table 5: Compressive strength values and percentage improvement

Control

CNT 30

mins.

CNT 60

mins.

SDS 20 24.1 (20%) 24.4 (22%)

Triton X-100 24.5 33.8 (38%) 32.9 (34%)

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Table 6: Flexural strength values and percentage improvement

Control CNT 30 mins. CNT 60 mins.

SDS 4.7 6.06 (29%) 5.14 (9%)

Triton X-100 4.8 7.03 (46%) 6.39 (33%)

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 20 40 60 80 100

Peak Absorbance

Sonication time, minutes

SDS/CNT = 1

SDS/CNT= 2.5

SDS/CNT = 5

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0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 20 40 60 80 100

Peak Absorbance

Sonication time, minutes

Triton X-100/CNT = 1

Triton X-100/CNT = 2.5

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0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

200 300 400 500 600

Absorbance

Wavelength , nm

Triton X-100 90minsTriton X-100 60minsTriton X-100 30minsSDS 90minsSDS 60minsSDS 30mins

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0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1100 1300 1500 1700 1900

Norm

aliz

ed I

nte

nsi

ty

Wavelength, cm-1

As-Received CNTs

CNTs 30 mins

CNTs 60 mins

G

D

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