Construction and Building Materials 35 (2012) 647–655
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
On the aspect ratio effect of multi-walled carbon nanotube reinforcements
on the mechanical properties of cementitious nanocomposites
Rashid K. Abu Al-Rub ⇑, Ahmad I. Ashour, Bryan M. Tyson
Zachary Department of Civil Engineering, Texas A&M University, College Station, TX, USA
h i g h l i g h t s
" Integration of carbon nanotubes (CNTs) into the cement paste fraction of concrete.
" CNT’s aspect ratio effect on mechanical properties of cement is investigated.
" Low concentrations of long CNTs are more effective than high volume of short CNTs.
" SEM and TEM images are obtained showing crack bridging and CNT pullout.
" CNTs’ effect on mechanical properties is investigated at 7, 14, and 28 days.
a r t i c l e
i n f o
Article history:
Received 29 January 2012
Received in revised form 28 April 2012
Accepted 29 April 2012
Keywords:
Carbon nanotubes
Cement
Aspect ratio
Strength
Surface area to volume ratio
a b s t r a c t
For their novel mechanical, thermal, chemical, and electrical properties, carbon nanotubes (CNTs) are widely
used in many fields of nanocomposite materials. In cementitious nanocomposites, CNTs can act as effective
bridges to minimize and limit the propagation of micro-cracks through the matrix, under the conditions of
well dispersion of the CNTs within the matrix and good bonding between the CNTs and the surrounding
hydrated cement matrix. This study focuses on the effect of different concentrations of long multi-walled
carbon nanotubes (MWCNTs) – high length/diameter aspect ratios of 1250–3750 – and short MWCNTs –
aspect ratio of about 157 – in cement paste. Flexural bending tests were performed to evaluate four major
mechanical properties of the cement/CNTs composites at ages of 7, 14, and 28 days. Results show that the
flexural strength of short 0.2 wt.% MWCNT and long 0.1 wt.% MWCNT increased by 269% and 65%, respectively, compared to the plain cement sample at 28 days. The highest increase in ductility at 28 days for the
short 0.1 wt.% MWCNT and the short 0.2 wt.% MWCNT was 86% and 81%, respectively. It is concluded that
nanocomposites with low concentration of long MWCNTs give comparable mechanical performance to the
nanocomposites with higher concentration of short MWCNTs. Clear evidence was obtained from scanning
electron microscope images for micro-crack bridging; many of the MWCNTs were stretching across the
micro-cracks showing CNTs breakage and pull-out.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Concrete is the most widely used construction material in the
world. However, the cement paste phase of concrete is a quasi-brittle material that has low tensile strength, low ductility, and early
development and propagation of micro-cracks due to shrinkage at
early ages. Therefore, there is a great desire to tailor the tensile
and flexural mechanical properties of the cement paste in order
to improve the damage and fracture resistance of concrete. Typical
reinforcement of cementitious materials is usually done at the
meso-scale (millimeter scale) and/or at the micro-scale using macrofibers and microfibers, respectively [1]. On the other hand, the
⇑ Corresponding author. Tel.: +1 979 862 6603; fax: +1 979 845 6554.
E-mail address: [email protected] (R.K. Abu Al-Rub).
0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.conbuildmat.2012.04.086
nano-scale and unique multifunction properties of carbon nanotubes (CNTs) make them promising reinforcements to many engineering materials [2–4]. CNTs occur as single-walled carbon
nanotubes (SWCNTs) and multi-walled carbon nanotubes
(MWCNTs). SWCNTs are composed of a single graphite sheet rolled
into a long hollow cylinder, whereas MWCNTs are nested arrays of
SWCNTs. The average diameter of an individual SWCNT is on the order of 1 nm whereas the average diameter of an individual MWCNT
is on the order of 10 nm. Typical properties of CNTs include an average Young’s modulus approaching 1000 GPa, exceptional tensile
strengths in the range of 20–100 GPa, and an ultimate strain of
12% [5,6]. Therefore, when compared to strongest steel, CNTs have
a modulus of elasticity of approximately five times higher, a tensile
strength 100 times stronger, and elastic strain capacities 50 times
greater, and yet a specific gravity one-sixth that of steel.
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By now, it is well-known that there are two major challenges
need to be solved in order to effectively obtain a successful
CNTs/cement composite; the well-dispersion of the CNTs within
the cement paste matrix, and the bonding or cohesive properties
between the surface of the CNTs and the cement paste around
them [7,8]. At the nano-scale the van der Waals forces between
CNTs are relatively high, and due to their large surface-area-to-volume (SA/V) ratio, CNTs tend to attract to each other and agglomerate, making it difficult to disperse and separate them. The use of
ultrasonic mixer with surfactants in aqueous solutions, with a specific amount of energy and time, a good dispersion could be
achieved. However, CNTs might dissolve in the solution or broke
down into smaller pieces if excessive amount of sonicating energy
was used. The compatibility of the surfactant used to disperse the
CNTs with cement is another important issue. The hydration and
the chemical reactions of cement could be badly affected; it could
delay or even stop the hydration and the hardening process of the
cement paste [9].
Despite the aforementioned challenges, there has been a growing interest in integrating CNTs in cement paste since 2004. A short
review of these developments is presented here. Makar and Beaudoin [10,11] are the first to integrate CNTs in cement paste and to
show successful micro-crack bridging by CNTs. Li et al. [12,13]
tested both functionalized (i.e. surface-treated) and non-functionalized MWCNTs within cement composites, and showed that the
compressive and flexural strengths for the functionalized CNTs/cement composites were slightly higher than the non-functionalized
MWCNTs/cement composites. Other researchers tried a novel approach by growing CNTs on the surface of the cement particles directly in order to enhance the dispersion and reduce the time and
efforts of mixing and dispersing the CNTs within the cement paste
[14,15]. In a series of studies, Shah and co-workers (e.g. [16–19])
have investigated the integration of various concentrations (as
low as 0.02 wt.% and as high as 0.1 wt.% by weight of cement) of
MWCNTs in the cement paste and consistently reported an
improvement in the mechanical properties of the nanocomposite
cement. They showed an increase in the flexural strength by 8–
40% and increase in the Young’s modulus by 15–55%. They also
showed that low concentrations of well-dispersed MWCNTs lead
to the largest enhancement. Through using nanoindentation of
the cement paste reinforced with MWCNTs, Konsta-Gdoutos
et al. [19] also reported a significant increase in the stiffness of
the calcium–silicate–hydrate (C–S–H) suggesting that, besides
the reinforcing action, CNTs produce significantly stronger and
tougher cementitious materials than traditional reinforcing materials through affecting the nano-scale processes that control the
C–S–H formation. Abu Al-Rub and co-workers (see e.g. [7–9,20])
have investigated the integration (dispersion and bond) of both
CNTs and carbon nanofibers (CNFs) of different concentrations
within the cement paste at different ages. Four mechanical properties have been investigated; strength, ductility, modulus of elasticity, and modulus of toughness, where modulus of toughness in this
paper refers to the total area under the stress–strain diagram. In
addition, the microstructure of the nanocomposites and dispersion
of CNTs and CNFs have been investigated using ultra-resolution
scanning electron microscope (SEM). The results showed that, generally, chemically-functionalized MWCNTs lead to degradation in
the mechanical properties over time due to formation of expansive
phases that weaken the formation of C–S–H, which is also shown
in [21]. However, most of the fabricated nanocomposites showed
improvements in the ductility compared to the plain cement
specimens.
Recently, Morsy et al. [22] have investigated the effect of different concentrations of MWCNTs with nano-clay particles on the
compressive strength of cement mortar, and concluded that large
concentrations of CNTs may agglomerate around the cement grains
leading to partial hydration of cement and the production of hydrated products with weaker bonds. Moreover, they have noticed
that the nano-clay particles improved the dispersion of CNTs within the mortar. Luo et al. [23] have tested MWCNTs with cement
paste with silica fume, at different relatively high concentrations
of MWCNTs (0.1, 0.5, and 1 wt.%), and reported 45% increase in
the 28 day’s flexural strength. Hunashyal et al. [24] have fabricated
hybrid composites of plain cement by integrating both carbon
micro-fibers (CMFs) and MWCNTs, and obtained 88% flexural
strength increase as compared to the plain cement specimens.
Generally, results to date have shown modest improvement in
the tensile/flexural strength of CNT/cement composites. Any
improvement depends on many factors (e.g. nanotube content
and aspect ratio, surfactants, sonication time and power, waterto-cement ratio, nanotube’s surface treatment). Since there has
been no special emphasis placed on investigating the effect of aspect ratio (length-to-diameter ratio) of CNTs on the mechanical
properties of cementitious nanocomposites, the objective of this
study is to investigate the integration of long and short MWCNTs
in the cement paste. The size and aspect ratio of CNTs mean that
they can be distributed on a much finer scale than commonly used
micro-reinforcing fibers. As a result, micro-cracks are interrupted
much more quickly and frequently during propagation in a nanoreinforced matrix; producing much lower crack widths at the point
of first contact between the moving crack front and the CNT. Fig. 1
shows graphically the relationships between the length-to-diameter aspect ratio and the surface-area-to-volume (SA/V) ratio for different lengths of SWCNTs, MWCNTs, carbon nano-fibers (CNFs),
and carbon micro-fibers (CMFs). It is noticed from this chart that
only CNTs can provide a very high SA/V ratio, which is one of the
most important and desired elements in fiber-reinforced composite systems in order to obtain the best and the most efficient composite materials. A SWCNT is the only material which has a SA/V
ratio that exceeds 2.0 nm-1, especially when considering the ultra-long SWCNTs that have aspect ratios that can reach several millions. A higher SA/V ratio means a larger contact area between the
fibers and the surrounding matrix, hence higher interaction with
the matrix and more efficient reinforcing. On the other hand,
Fig. 2 shows further physical relationships for SWCNTs and
MWCNTs for different parameters such as the aspect ratio, length,
and surface-area-to-volume ratio of the CNTs, in addition to cement paste/CNTs composite parameters such as mass fraction of
CNTs by weight of dry cement, and the approximate number of
CNTs obtained for that specific length and mass fraction. In fact,
Fig. 2 is very useful in showing the effect of several aspects of CNTs
(SA/V ratio, CNTs’ concentration, aspect ratio, length, and number
of CNTs). Therefore, based on the desired aspect ratio, one can find
the corresponding SA/V ratio and the needed concentration and the
type of CNTs. Fig. 2 demonstrates in a comprehensive manner the
related features of using CNTs as reinforcements. Figs. 1 and 2
show conceptually the importance of aspect ratio and SA/V ratio
of CNTs when used, particularly, in cementitious materials. Therefore, the focus of the current study is on investigating the effects of
these parameters on the mechanical properties of cementitious
materials reinforced with CNTs.
In this study, different low and high concentrations of short and
long MWCNTs are integrated in the cement paste with 0.4 waterto-cement ratio. Effective dispersion of CNTs within water was
achieved by applying ultrasonic energy and with the use of a commercially available surfactant that is commonly used in cementitious materials. Three-point flexural bending tests were
performed to evaluate four major mechanical properties (Young’s
modulus, flexural strength, ductility, and toughness) of the cement/CNTs composites at ages of 7, 14, and 28 days. The
microstructure at the fracture surfaces was investigated using a
high-resolution electron microscope images.
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Fig. 1. A chart showing the length-to-diameter (l/d) aspect ratio effect on the surface-area-to-volume (SA/V) ratio for different lengths of SWCNTs, MWCNTs, CNFs, and CMFs.
* Reading the mass fraction values shall be through the curves with “*” numbers.
Fig. 2. A chart showing the aspect ratio effect on the surface area/volume ratio for different lengths of SWCNTs and MWCNTs, and the relations between surface area/volume
ratio with mass fraction of CNTs by cement weight and their number per 1 g of cement.
2. Experimental work
2.1. Materials
The short MWCNTs are NC7000 multi-walled carbon nanotubes provided by
Nanocyl Inc. with an average aspect ratio (length/diameter ratio) of about 157,
whereas the long non-functionalized MWCNTs were provided by Cheap Tubes
Inc. with an average aspect ratio of 1250–3750. Both short and long MWCNTs are
produced by Catalytic Chemical Vapor Deposition (CCVD) process. The physical
properties for both the short and long CNTs as provided by the manufacturers are
shown in Table 1. The cement used in the mixtures is the commercial Type I/II Portland cement. A commercial water reducing admixture (polycarboxylate), provided
by Grace Corporation, named ‘‘ADVA cast 575’’ was used as a superplasticizer or as a
surfactant to aid in the dispersion of CNTs. All materials were used as received.
2.2. Preparation and dispersing of the MWCNTs
Three different batches for the short MWCNTs were made at three different
concentrations; 0.04%, 0.1%, and 0.2% of the weight of cement. For the long
MWCNTs, two batches were made at 0.04% and 0.1% by cement weight. All speci-
Table 1
Physical properties for the short and long MWCNTs.
Diameter
Length
Specific surface area
Purity
Short MWCNTs
Long MWCNTs
9.5 nm
1.5 lm
250–300 m2/g
P90%
<8 nm
10–30 lm
>500 m2/g
>95%
mens had a water/cement ratio of 0.4, and a superplasticizer of 0.4% by cement
weight. The reason why one more test of long MWCNTs at concentration of 0.2%
is skipped is based on the conclusions of [18] as mentioned before, where less
amount of long MWCNTs is needed to achieve the same level of composite characteristics. In addition to that, the dispersion process for a higher percentage of long
MWCNTs requires more energy than other specimens, and that may affect the consistency of the testing. On the other hand, the short MWCNTs have been used at all
three concentrations of 0.04%, 0.1%, and 0.2%.
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For each batch, in order to get representative measurements for the mechanical
characteristics, three to five replicates of each specimen were made and tested at
ages of 7, 14, and 28 days, then the average value and the standard error for the replicates were computed at each age.
CNTs were dispersed following the method described in [20] using an ultrasonic
wave mixer provided from Sonics & Materials (Vibra-Cell), the model is ‘‘VC-505’’,
with 20 kHz liquid processor with a ½ in. diameter titanium alloy probe was used
to disperse the nano-filaments within an aqueous solution of water with superplasticizer at room temperature. The sonication power used was 78.64 W of 70%
amplitude of the maximum (the power per solution volume was 0.4626 W/ml).
The mix design parameters and the sonication time are provided in Table 2. The dispersion quality was evaluated at the fracture surface of the hardened CNT/cement
paste composite using scanning electron microscope (SEM) imaging. Further information and discussions will be presented in the results subsection of this study.
2.3. Mixing and specimens preparation
After dispersion, the CNTs solution was added to the cement at a water/cement
ratio of 0.4, and then they were mixed together for 7 min using an electrical multispeed planetary blender. After mixing, the CNTs/cement mixture was placed in vacuuming chamber for 3 min to dispose any air entrained in the mixture in order to
reduce the air bubbles. The mixture was then poured into square acrylic molds
(of size 6.5 mm 6.5 mm 160 mm). During pouring process the acrylic molds
were placed on an electrical vibrator table to get rid of the air bubbles generated
during the casting process. All specimens were demolded at age of 7 days, and immersed in water saturated with lime for curing until testing.
Plain cement specimens were casted as a reference sample. The plain cement
mix was made with a water/cement ratio of 0.4, with addition of 0.1% of superplasticizer by the weight of cement. This amount of superplasticizer is equal to 25% of
what has been added to other CNTs/cement composite specimens. This is different
than the earlier work by the authors [20] where the same amount of superplasticizer has been used for both the reference and the nanocomposite samples. In this
study, the reason why less amount of superplasticizer has been used in the plain cement specimens is that larger amount of superplasticizer is needed in the nanocomposites in order to improve the dispersion of the nano-filaments (see [7,8]). Around
75% of the added superplasticizer to the composites with CNTs is consumed during
the dispersion process, and 25% of superplasticizer is left to enhance the workability
of the cement paste which is comparable to that consumed by the reference samples. Therefore, this will allow more objective comparisons between the results
from the reference and the nanocomposite samples. In fact, comparing the strength
results from the reference samples in the current study and that in [20] shows an
increase in the order of 100% due to the use of more superplasticizer in the reference samples in [20]. Therefore, the testing results from the nanocomposite specimens in [20] showed less gain in the mechanical properties due to the integration of
CNTs as compared to the results from the plain cement reference specimens. This
lack of objectivity when making the quantitative comparisons between samples
without and with CNTs is avoided in the current study. Also, in the current study,
the mixing time for the plain cement specimen has been reduced to 3 min after
the addition of the cement to the water. The air vacuuming, casting, demolding,
and curing of the plain cement specimen are the same as for all other composite
specimens.
2.4. Flexural testing
The testing apparatus used was a three-point bending testing frame of a small
scale of dimensions (170 mm 25 mm 114 mm), fixed with an actuator (NSA12
from Newport Corp.), load cell (from Strain Measurement Devices of 1 and 2.5 kg
capacity) and LVDT (from Macro Sensors) to measure the displacement, connected
a data acquisition board (from National Instruments), a controller (Newport
NSC200) with a LabView program was used to control the actuator and to measure
the load versus the displacement. See Table 3 for the specifications of the employed
actuator, which allowed us to get accurate load–displacement diagrams. The loads
and displacements were measured and recorded by the software and then converted to stresses and strains using Euler–Bernoulli elastic beam theory in order
to calculate some mechanical properties:
r¼
LC
F
4I
and
e¼
12C
L2
y
ð1Þ
Table 3
Newport NSA12 actuator specifications.
Feedback
Open loop, no encoder
Drive screw pitch (mm)
Travel range (mm)
Minimum incremental motion (lm)
Maximum speed (mm/s)
Axial load capacity (N)
0.3048
11
0.2
0.9
28
where r is the flexural tensile stress at the extreme tension fibers of the beam, e is
the elastic strain at the extreme tension fibers of the beam at each load step, L is the
span length of the beam (160 mm), C is the half-depth of the beam cross-section
(3.25 mm), I is the second moment of area of the square cross-section beam, F is
the applied force measured by the load cell at mid-span length, and y is the deflection measured by the LVDT at mid-span length.
3. Results and discussion
3.1. Flexural testing results
Four mechanical properties of the specimens were measured;
flexural strength, strain capacity (i.e. strain-to-failure which is referred to here as ductility), modulus of elasticity (Young’s modulus), and modulus of toughness (defined as the area under the
stress–strain diagram). Summary of the average values of the results compared to the plain cement reference specimens are shown
in Fig. 3 through Fig. 6. The column charts show the mean values
and the standard error of the mean for each specimen.
At age of 7 days, most of the MWCNT/cement composites show
an increase in the flexural strength, ductility, and modulus of
toughness when compared to the plain cement (reference) specimens. The highest improvement in the flexural strength was
shown in the short 0.04% MWCNT specimens with an increase of
66% compared to the reference specimens. The short 0.2% MWCNT
specimens show a decrease in their flexural strength by 38%, while
it showed the highest improvement in the ductility with an increase of 130% with respect to the reference specimens. The modulus of elasticity values for most of the specimens were close to the
reference samples. However, the short 0.1% and 0.2% MWCNT
show a slight increase compared to the reference samples. The
highest improvement in modulus of toughness was provided by
the short 0.04% MWCNT with a significant increase of 154% from
the plain cement samples.
On the other hand, considerable changes in the behavior of the
composites were observed at age of 14 days. Large drop in the flexural strength of most of the composites occurred, to be less than
the reference samples strength. The highest ductility obtained after
14 days was for the short 0.2% MWCNT with an increase of 72%
from the corresponding value of the reference samples. No significant changes in the general behavior of the composites regarding
the modulus of elasticity values with respect to the reference samples, but most of the specimens show decrease in the modulus of
elasticity in general compared to the values at 7 days. Many of
the nanocomposites showed degradation in modulus of toughness
values in comparing to the reference samples, large decrease is noticed in the long 0.1% MWCNT.
Table 2
Mix design of the test specimens.
Test specimens
Superplasticizer: % weight of cement
CNTs: % weight of cement
Ultrasonication time (min.)
Plain cement (reference)
Short CNTs 0.04
Short CNTs 0.1
Short CNTs 0.2
Long CNTs 0.04
Long CNTs 0.1
0.1
0.4
0.4
0.4
0.4
0.4
0.0
0.04
0.1
0.2
0.04
0.1
–
20
30
30
30
20
R.K. Abu Al-Rub et al. / Construction and Building Materials 35 (2012) 647–655
651
Fig. 3. Average flexural strength results for different MWCNTs composite specimens with the standard error of the mean.
Fig. 4. Average ultimate strain results for different MWCNTs composite specimens with the standard error of the mean.
After 28 days, all of the composites retrieved their strength values to become higher than the values at 14 days; however, the
short 0.2% MWCNT showed a significant large increase in the flexural strength, specifically 269% compared to the reference samples’
values. In addition, the long 0.1% MWCNT has also increased by
65%. Most of the composites show a reduction in ductility in general, where the highest composites in ductility were the short 0.1%
MWCNT and short 0.2% MWCNT; however, the short 0.04%
MWCNT showed almost a constant value for ductility from age of
7 days through age of 28 days. All specimens showed an increase
in their modulus of elasticity values compared to 14 days values,
and the short 0.2% MWCNT has the highest increase. Due to the
large increase in the flexural strength of the short 0.2% MWCNT,
it has recorded the highest modulus of toughness value at age of
28 days.
Therefore, from the reported results for the nanocomposites at
different curing periods, one can notice that in most cases, the
strength and Young’s modulus decreased from 7 day curing to
14 day curing and then increased (see Figs. 3 and 5). The opposite
is seen for the ductility (see Fig. 4). However, overall, this is almost
seen to be the case for the reference samples without the CNTs.
Therefore, the authors believe that this observed trend may not
be due to CNTs’ integration, but due to the curing process and
the resulting properties of the cement paste matrix, which is discussed later in this section.
It is interesting to note from Figs. 3–6 that cement nanocomposites with low concentration of long MWCNTs give comparable
mechanical performance to the nanocomposites with higher concentration of short MWCNTs. For example, the average modulus
of elasticity at 14 and 28 days for specimens with 0.04 wt.% of long
MWCNTs is almost the same as that for specimens with 0.1 wt.% of
short MWCNTs as shown in Fig. 5. This is in close agreement with
the results in [18], which can be attributed to: (a) comparable
effective interaction with the cement matrix of long CNTs with
low concentrations as compared to that of short CNTs with high
concentrations; (b) higher concentrations of short CNTs reduce
the CNT-free volume of the cement paste which helps in arresting
micro-cracks propagation; generally, one can see that the mechanical properties increase with the increase in the concentration
(0.04, 0.1, and 0.2 wt.%) of the short MWCNTs; and (c) high concentration of short CNTs are more effective in filling nano-sized voids,
which lead to enhancements in the packing density of the cement
paste.
The above conclusion of the effect of the CNT’s aspect ratio on
the Young’s modulus is emphasized in Fig. 7 which shows the variation of the normalized Young’s modulus (normalized with respect to the Young’s modulus of the reference cement paste
specimen) with the CNTs’ concentration for both short and long aspect ratios. The data points plotted in Fig. 7 for the normalized
Young’s moduli are obtained from the literature as well as the
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R.K. Abu Al-Rub et al. / Construction and Building Materials 35 (2012) 647–655
Fig. 5. Average modulus of elasticity results for different MWCNTs composite specimens with the standard error of the mean.
Fig. 6. Average modulus of toughness results for different MWCNTs composite specimens with the standard error of the mean.
current study and are summarized in Table 4. The data points are
fitted using a power-law relation. One can clearly see from Fig. 7
that the Young’s modulus increases as the long-CNTs’ concentration decreases, whereas the Young’s modulus increases as the
short-CNTs’ concentration increases. Also, one can notice from
Fig. 7 that low concentrations of long-CNTs can lead to a much
higher increase in the Young’s modulus as compared to higher
Fig. 7. Variation of the normalized Young’s modulus with the CNT’s concentration
for two different aspect ratios; long and short.
concentrations of short-CNTs. For example, from Fig. 7, 0.04 wt.%
of long-CNTs yields a comparable Young’s modulus to a 0.2 wt.%
of short-CNTs. Although this conclusion is of a very high importance from the cost point of view; the lower the concentration of
CNTs the lower the cost of the nanocomposite, achieving well dispersion of high concentrations of long CNTs in the material matrix
is highly desirable for other multifunctional properties (e.g. electrical and thermal conductivity). Note that the rate of increase in the
Young’s modulus with decreasing the concentration of long-CNTs
is higher than the rate of increase in the Young’s modulus with
increasing the concentration of short-CNTs. Therefore, it is more
desirable to achieve better dispersion of long-CNTs since it is expected to lead to a much higher increase in the mechanical properties due to the higher SA/V ratio as indicated in Fig. 1. However,
Fig. 7 shows that the Young’s modulus decreases at high concentrations of long-CNTs indicating the difficulty in dispersing high
concentrations of long-CNTs as compared to short-CNTs. These results show that the aspect ratio of CNTs plays an important role in
the reinforcement of cementitious materials.
However, it should be emphasized that although the above
observations are obvious for the Young’s modulus, but is not as
obvious for the other mechanical properties. Moreover, the results
from 0.1 wt.% long and 0.2 wt.% short MWCNTs are not similarly
related due to the poor dispersion of long MWCNTs with higher
concentration than 0.04 wt.%, and the ability of the short MWCNTs
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R.K. Abu Al-Rub et al. / Construction and Building Materials 35 (2012) 647–655
Table 4
Summary of obtained Young’s modulus values from the literature for CNT/cement composites.
CNTs: % weight of cement
Nanocomposite Young’s modulus (MPa)
Control Young’s modulus (MPa)
Water/cement ratio
Aspect ratio
References
Short CNTs
0.048
0.048
0.08
0.1
0.2
0.04
0.1
0.2
19.4
11.3
13
15.9
19.2
12.4
15.2
16.98
16.7
8.8
8.8
14.8
14.8
14.38
14.38
14.38
0.3
0.5
0.5
0.4
0.4
0.4
0.4
0.4
300
300
300
150
150
150
150
150
[16,18–19]
[16,18–19]
[16,18–19]
[20,29]
[20,29]
Current study
Current study
Current study
Long CNTs
0.025
0.048
0.025
0.048
0.2
0.04
0.1
22
19
13.8
12.7
17.5
15.25
12.99
16.7
16.7
8.8
8.8
23
14.38
14.38
0.3
0.3
0.5
0.5
0.4
0.4
0.4
1600
1600
1600
1600
2500
2500
2500
[16,18–19]
[16,18–19]
[16,18–19]
[16,18–19]
Current study
Current study
Current study
Fig. 8. An example of the stress–strain diagrams for some samples of the 0.04%
short MWCNT and 0.1% long MWCNT at ages of 14 and 28 days. The multi-peaks
behavior can be observed.
with higher concentration to reduce the CNT-free volume of the cement paste that help in arresting micro-cracks and filling nanosized voids. Therefore, the integration of 0.2 wt.% of short MWCNTs
is more effective than integrating 0.1 wt.% of long MWCNTs.
Based on the observation of the stress–strain diagrams of all
specimens, one can notice the multi-peaks behavior that is related
to the CNTs pull-out from the matrix. This is important for the fracture energy stored in the composite, which will affect the strain
capacity (ductility) and modulus of toughness of the composite.
An example of the stress–strain diagrams for some specimens is
shown in Fig. 8.
It has been noticed that although all samples of a batch are
identical and casted from the same mix, there is variability in the
results from one sample to another of the same composite. The
uniformity of the distribution of the MWCNTs within the aqueous
solution does not guarantee a uniform distribution of the nano-filaments within the composite, hence the stresses distribution in the
composite beams will be non-uniform, and will cause variability in
the flexural behavior from sample to sample.
Besides the non-uniform dispersion and weak bonding, the degradation in the flexural strength in many specimens could be related to formation of weak hydration products as C–S–H that is a
main product of the Portland cement hydration, which has an
important contribution to the strength of the cement hydrate in
different stages of hydration, [21] and [25], or due to existence of
Fig. 9. SEM image showing the micro-crack bridging and breakage of the MWCNTs
within the cement paste composite.
harmful components like excessive formation of expansive ettringite, more details were discussed in the work of [20].
Another reason that the authors have investigated was the curing methods used to cure all the beam specimens. Leaching of Calcium Hydroxide (CH) [Ca(OH)2] from the cement paste submerged
into the lime water, would increase the porosity (capillary voids) of
the matrix and hence degrade the mechanical properties of the cement paste. Carde et al. [26] have shown experimentally that the
compressive strength of plain cement paste of meso-cylinders
(diameters of 10–30 mm) had degraded significantly due to the
leaching process of CH. It was proposed that leaching of CH from
the external layers (leaching zone) of the plain cement paste specimens and the loss of Calcium [Ca2+] ions content due to the progressive decalcification of the C–S–H, have a major contribution
in the degradation of the mechanical properties of the cement
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R.K. Abu Al-Rub et al. / Construction and Building Materials 35 (2012) 647–655
Fig. 10. SEM image of MWCNTs agglomerations within a small area of cement
paste.
paste [27]. For this study, the authors believe that although the
leaching kinetics is usually slow, but due to the special case in this
study of testing a very small specimens’ size (6.5 mm 6.5 mm
cross-section) with large surface area to volume ratio – as in our
case – will expedite the leaching process due to the increase in
the leaching area to total area ratio. Crumbie et al. [28] have tested
concrete specimens with w/c ratio of 0.4 and 0.6. The specimens
were cured in saturated lime solution after 24 h of casting. The
SEM back scattered images and the methanol-exchange porosity
testing confirmed that the porosity and the permeability had increased near the surface. Also, they reported that the surface region
was depleted in CH content. However, leaching of CH occurred
although the specimens were immersed in saturated CH solution.
CNTs breakage implies a good bonding between the CNTs surfaces
and the surrounding cement paste. However, it is clear that the
existence of the CNTs will affect the chemical reaction of the hydrated cement. The SEM images also indicate the high elastic flexural properties of the long MWCNT allow them to sustain large
elongations without breakage.
Although individual CNTs can be identified on the fracture surface in Fig. 10, relatively large agglomerations of the long MWCNTs
can be seen even after the aforementioned dispersion and mixing
processes. It was noticed from the SEM scanning of the fracture surface that some regions have no CNTs, while in other regions large
number of agglomerated CNTs can be seen. This requires more
investigations for improving the current dispersion techniques in
cementitious materials. However, in order to effectively relate the
SEM images to the aspect ratio effect on the mechanical properties
of the nanocomposites, it should be noticed that the actual aspect
ratio retained in the composite is a function of the unbroken length
of the CNTs, i.e. the sonication process impose large amount of energy on the CNTs in order to well disperse them, but that also could
break them into shorter fragments, especially the long ones.
Obtaining transmission electron microscope (TEM) images for
CNTs within the hardened cement paste is a very challenging task
due to the brittleness of cement paste and in turn the difficulty in
processing the cement paste sample for TEM imaging. Fig. 11 shows
successfully obtained cryogenic-TEM images of the short and long
MWCNTs within a thin strip of hardened cement paste. One can
clearly see that the MWCNTs are nicely imbedded into the hydration
products of the C–S–H phase, but closely spaced and to certain extent
agglomerated. Also, one can notice that the long MWCNTs bridge the
neighboring hydration products allowing for better crack-bridging
and interaction with the cement matrix. This might explain the comparable increase in the mechanical properties of cement nanocomposites with low concentration of long MWCNTs as compared to
the nanocomposites with high concentration of short MWCNTs.
3.2. Electron microscopy images
4. Conclusions
In order to visually explain and justify the results obtained,
many SEM images were taken for the tested samples. An example
of the micro-crack bridging by the MWCNTs within the cement
paste is shown in Fig. 9 with a scale of 100 nm. The images indicate
clearly that many CNTs are bridging the micro-crack. Both pull-out
and breakage of CNTs can also be observed from images. In fact, the
The results obtained showed improvements in the flexural
strength and ductility for all of the nanocomposites at age of
28 days when compared to the plain cement samples. It is observed
that nanocomposites with low concentration of long MWCNTs give
comparable mechanical performance to the nanocomposites with
Fig. 11. Cryo-TEM images of short MWCNTs (left) and long MWCNTs (right) within the hardened cement paste.
R.K. Abu Al-Rub et al. / Construction and Building Materials 35 (2012) 647–655
higher concentration of short MWCNTs. Moreover, it was found
that the short MWCNT at 0.2% concentration by cement weight
has better results than other specimens at age of 28 days. Those
observations could be due to that short CNTs result in a relatively
better dispersion of filaments within the cement paste, reduction
in the filament-free volume of the cement paste, and effective filling
of nano-sized voids.
At early ages (age of 7 days) all specimens showed higher ductility than the plain cement (reference) samples. It was noticed that
at age of 28 days, for the same concentrations of 0.1% and 0.04%,
the long MWCNTs composites showed higher strength and less
ductility than the corresponding short MWCNTs composites which
can be due to breakage of long CNTs during the sonication process
that in turn led to a relatively better dispersion of the CNTs.
The nanocomposite specimens showed multi-peaks in the
stress–strain response, which indicates multiple pull-out actions
of the MWCNT from the hydrated cement matrix. However, this
multi pull-outs behavior, from the stress–strain diagrams, could
not improve significantly the strength or the ductility of the
CNTs/cement composites in all cases; this could be due to the
low bonding between the CNTs and the cement matrix or due to
the formation of low-stiffness hydration products in the cement
paste matrix due to the existence of the CNTs.
The SEM images clearly show successful crack-bridging by the
MWCNTs and pull-outs and breakage. The CNTs breakage implies
a good bonding between the CNTs surfaces and the surrounding
cement paste. The TEM images show clear embedment of CNTs
within the cement hydration products, and bridging of neighboring
hydration products by long MWCNTs indicating that MWCNTs
with higher aspect ratios are more effective reinforcements if
well-dispersed.
Extensive studies are needed on the effects of the existence of
the CNTs within cement paste on the chemical reactions and
hydration process of cement. Improvements on dispersing techniques are also needed in order to effectively utilize long MWCNTs
in cement composites. Also, novel multiscale computational modeling of CNT/cement composites that can be used carefully for
investigating the effects of aspect ratio and various reinforcing
mechanisms by CNTs is highly desirable.
Acknowledgements
The authors wish to acknowledge the financial support by Texas
Department of Transportation through the Southwest University
Transportation Center (SWUTC). Also, R.K. Abu Al-Rub would like
to acknowledge the financial support provided by Qatar National
Research Fund (QNRF) through the National Priority Research Program Project 4-1142-2-440. The QNRF funding supported the
microscopy images in Section 3.2.
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