Hindawi Publishing Corporation

Journal of Nanomaterials
Volume 2013, Article ID 710175, 19 pages
http://dx.doi.org/10.1155/2013/710175

Review Article
A Review on Nanomaterial Dispersion, Microstructure,
and Mechanical Properties of Carbon Nanotube and Nanofiber
Reinforced Cementitious Composites

Shama Parveen,1 Sohel Rana,1 and Raul Fangueiro1,2

1
Fibrous Materials Research Group (FMRG), School of Engineering, University of Minho, 4800-058 Guimaraes, Portugal
2
Department of Civil Engineering, University of Minho, 4800-058 Guimaraes, Portugal

Correspondence should be addressed to Sohel Rana; soheliitd2005@gmail.com

Received 11 March 2013; Accepted 28 May 2013

Academic Editor: Tianxi Liu

Copyright © 2013 Shama Parveen et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Excellent mechanical, thermal, and electrical properties of carbon nanotubes (CNTs) and nanofibers (CNFs) have motivated the
development of advanced nanocomposites with outstanding and multifunctional properties. After achieving a considerable success
in utilizing these unique materials in various polymeric matrices, recently tremendous interest is also being noticed on developing
CNT and CNF reinforced cement-based composites. However, the problems related to nanomaterial dispersion also exist in
case of cementitious composites, impairing successful transfer of nanomaterials’ properties into the composites. Performance of
cementitious composites also depends on their microstructure which is again strongly influenced by the presence of nanomaterials.
In this context, the present paper reports a critical review of recent literature on the various strategies for dispersing CNTs and CNFs
within cementitious matrices and the microstructure and mechanical properties of resulting nanocomposites.

1. Introduction concrete reinforcement in order to prevent the transforma-

tion of nanocracks into microcracks [2, 3]. Nanoparticle
Civil infrastructures are the building blocks of any country’s addition to cement paste was found to improve mechanical,
highway structures, bridges, pavements, runways for airport, chemical, and thermal properties of cementitious matrix.
and so forth, and concrete is the primary material for There are various types of nanoparticles, especially SiO2
their construction. Concrete generally consists of Ordinary and Fe2 O3 , which when incorporated into cement led to
Portland Cement (OPC, which is known as the principal considerable improvement in the compressive strength [4–
binding agent), coarse aggregates, and fillers such as sand, 9]. Nanosized TiO2 has been added to accelerate the rate
admixtures, and water. Cementitious materials are charac- of hydration and increase the degree of hydration [10].
terized by quasi-brittle behaviour and are susceptible to Moreover, the photocatalytic characteristic of TiO2 helped
cracking. The cracking process within concrete begins with to remove the organic pollutants from concrete surfaces,
isolated nanocracks, which then conjoin to form microcracks which were directly exposed to UV radiation [11]. Carbon
and in turn macrocracks. Reinforcement is required because nanomaterials present a large group of functional materi-
of this brittle nature of concrete, and as reinforcements, als with exceptional physical properties. Extensive research
polymeric fibers as well as glass and carbon fibers were used endeavors over the last few years demonstrated the appli-
during the 1970s, 80s, and 90s, respectively [1]. Recently, cation potential of various carbon nanomaterials, mainly
the use of microfiber reinforcements has led to significant carbon nanofiber (CNF) and carbon nanotube (CNT), in
improvement in the mechanical properties of cement-based polymeric matrices. This fact has motivated the scientists and
materials by delaying the transformation of microcracks into researchers worldwide to use these nanomaterials in concrete
macroforms, but they could not stop the crack growth. This as well, in order to utilize their extraordinary mechanical,
fact encouraged the use of nanosize fibers or particles for electrical, and thermal properties [12, 13]. In addition to
2 Journal of Nanomaterials

that, in nanometer length scale, CNFs and CNTs offer the

possibility to restrict the formation as well as growth of
nanocracks within concrete, thus creating a new generation of
crack-free materials. So, concrete reinforcement using carbon
nanomaterials is a rapidly growing research area in recent
times. However, there exists a large difference in the structure
and chemistry between a polymeric and a cementitious,
matrix, and, therefore, a great deal of research activities
is being directed towards understanding the interaction
between these nanomaterials and cementitious matrices for
their successful application.

2. Structure of Cement
Figure 1: The molecular model of C-S-H: the blue and white spheres
A dry portion of Portland cement is composed of 63%
are oxygen and hydrogen atoms of water molecules, respectively;
calcium oxide, 20% silica, 6% alumina, 3% iron (III) oxide, the green and gray spheres are inter- and intralayer calcium ions,
and small amount of other materials including some impu- respectively; the yellow and red sticks are silicon and oxygen atoms
rities. These materials when react with water cause an in silica tetrahedral [14].
exothermic reaction forming a mineral glue (known as
“C-S-H” gel), calcium hydroxide, ettringite, monosulfate,
unhydrated particles, and air voids. Molecular structure of C- as arc discharge, laser ablation, thermal and plasma enhanced
S-H gel was not fully understood till recent past, but some chemical vapor deposition (CVD), and many other recently
researchers in Massachusetts Institute of Technology (MIT, developed methods [21–34]. CNTs possess outstanding prop-
USA) [14] recently proposed a structure, and according to erties such as the highest Young’s modulus (1.4 TPa), tensile
that, cement hydrate consists of a long chain silica tetrahedral strength (above 100 GPa), current density (109 A/cm2 ), and
and calcium oxide in long range distances, where water thermal conductivity (above 3000 W/mK) among the known
causes an intralayer distortion in otherwise regular geometry materials. Additionally, CNTs are flexible and have high
(Figure 1). The distortion in the structure due to addition breaking elongation (20–30%).
of water makes the cement hydrate robust. The density of Vapour-grown carbon nanofibers (VCNFs) are another
C-S-H has been determined as 2.6 g/cc [15], and the elastic type of carbon nanomaterial which was first explored in
modulus of different cementitious phases were determined 1889 by Hughes and Chambers [35], and their hollow
as follows [16]: 35 MPa for the Ca(OH)2 phase, 26 and graphitic structure was first revealed in the early 1950s by
16 MPa for high and low stiffness C-S-H, respectively, and Radushkevich and Lukyanovich [36]. Because of their low
10 MPa for the porous phase. One of the major drawbacks production cost and higher availability as compared to CNTs
of cement structure is its proneness towards crack formation and excellent properties (although lower than CNTs), VCNFs
and degradation. The amorphous phase of cement, that are receiving tremendous research attention in recent times.
is, C-S-H gel, is itself a nanomaterial, and, therefore, the VCNFs can be synthesized by catalytic CVD of a hydrocar-
degradation mechanisms within concrete start at nanoscale, bon (such as natural gas, propane, acetylene, benzene, and
spreading then to micro- and macroscales. Degradation of ethylene) or carbon monoxide using metal (Fe, Ni, Co, Au)
concrete can be due to physical reasons such as abrasion or metal alloy (Ni-Cu, Fe-Ni) catalysts at a temperature of
and erosion, freeze thaw cycles, leaching and efflorescence, 500–1500∘ C [37–43]. The dimension and structure of CNF are
drying shrinkage, and so forth or chemical reasons such as highly dependent on the manufacturing and post-treatment
aggregate-paste reaction, sulfate and acid attack, carbonation, methods [44]. CNFs are hollow core nanofibers comprising
and so forth [17–22]. either a single layer [44] or double layer of graphite planes as
shown in Figure 3 [45]. The graphite planes can be stacked
parallel or at a certain angle from the fiber axis and nested
3. Carbon Nanomaterials with each other to form different structures such as bamboo-
After the discovery of buckyball (a ball-like molecule made like, parallel, and cup-stacked [46–49].
of pure carbon atoms) in 1985 by Kroto et al. [23], a tubular
form of carbon was reported by Iijima [24] in 1991 and 4. Dispersion of CNTs and CNFs
named carbon nanotubes (CNTs). These nanotubes (called
multiwalled carbon nanotubes or MWCNTs) consisted of Dispersion of CNTs and CNFs is one of the major factors that
up to several tens of graphitic shells with adjacent shell strongly influence the properties of nanocomposites. These
separation of ∼0.34 nm, diameters of a few nanometers, nanomaterials have strong tendency to agglomerate due to
and high length/diameter ratio. About two years later, he presence of attractive forces (Van der Waals), originated from
reported the observations of single-walled carbon nanotubes their polarizable extended 𝜋-electron systems. Infiltration of
(SWCNTs), which consist of a single graphite sheet seam- agglomerates with matrices is very difficult, and their pres-
lessly wrapped into a cylindrical tube [25], as shown in ence is therefore the source of potential defects in nanocom-
Figure 2. CNTs can be produced by various techniques such posites. The process of deagglomeration and subsequent
Journal of Nanomaterials 3

Roll-up

Graphene sheet SWNT

(a) (b)

Figure 2: Schematic representation of SWCNT (a) and MWCNT (b) [25].

20 nm 100 nm

(a) (b)

Figure 3: TEM micrograph of CNF showing a single layer (a) and double layer (b) [44, 45].

distribution of nanomaterials within matrices or solvents is strongly influences the properties of cement-based nanocom-
called dispersion. Dispersion can occur either due to abrupt posites. The approach of dispersing CNF/CNT directly within
splitting up of agglomerates into small fragments under high cement paste during mixing is not feasible, as the thickening
stress (rupture) or due to continuous detachment of small of cement paste begins within a short period after addition
fragments at a comparatively lower stress (erosion). The dis- of water [75]. The mixing process using a Hobart mixer,
persion behaviour of CNF and CNT depends on a few critical commonly used to prepare mortar paste, cannot ensure
factors such as length of nanomaterials, their entanglement proper dispersion of CNT within cementitious matrix [76],
density, volume fraction, matrix viscosity, and attractive resulting in large CNT clusters within the hydrated paste
forces. Different chemical methods have been tried till date (Figure 4). To avoid this situation, the strategy commonly
to achieve homogeneous dispersion of carbon nanomaterials employed for mixing CNTs/CNFs with cementitious matrices
in water and various polymers such as using solvents [50], is to disperse these nanomaterials first in water, followed by
surfactants [51–54], functionalization with acids [55], amines mixing of nanomaterial/water dispersion with cement using
[56], fluorines [57], plasma [58, 59], microwave [60] and a conventional mixer. However, the methods of dispersing
matrix moieties [61], noncovalent functionalization [62], nanomaterials in water should be carefully selected so that
using block polymers [63, 64], wrapping conjugated polymers they do not interfere with the hydration and processing of
[65], and other techniques [66, 67]. On the other hand, cement nanocomposites. Many surfactants that are success-
the basic physical technique used for carbon nanomaterial fully used to disperse carbon nanomaterials in polymeric
dispersion is the ultrasonication, which is often used in com- matrices have been reported to create problems in cement
bination with the other methods mentioned above [68–74]. hydration, entrap air in the cement paste or react with the
water-reducing admixtures [77].
Dispersion of CNF/CNT in cement is even more difficult
5. Dispersion of CNFs/CNTs in as compared to the polymeric matrices. One of the reasons for
Cementitious Matrices poor dispersion may be the size of cement grains. As CNFs
or CNTs are separated by the cement grains, the presence of
Similar to polymeric matrices, dispersion of carbon nanoma- larger grains than the average leads to absence of CNFs/CNTs
terials in cementitious matrices is also a critical issue which in some areas, whereas they can be present in higher quantity
4 Journal of Nanomaterials

5.2. Chemical Methods

5.2.1. Use of Surfactant. Surfactants can improve aqueous
dispersion of nanomaterials by reducing surface tension of
water and, moreover, lead to stable dispersion as a result of
electrostatic and/or steric repulsions between the surfactant
molecules adsorbed on the nanomaterials surface. However,
the dispersion capability of surfactants strongly depends on
their concentration, and an optimum surfactant to nano-
materials ratio should be used for preparing cementitious
composites. Among the various concentrations studied, it
was observed that surfactant/CNT ratios of 4.0 and 6.25 (by
UFSJ 2010/11/19 NL D3.5 ×100 1 mm
weight) were efficient in preparing homogeneous aqueous
Figure 4: SEM image of CNT/cement paste after hydration [76]. dispersion of 0.16 wt.% MWCNT using ultrasonication pro-
cess (operated at 500 W, amplitude of 50%, energy of 1900–
2100 J/min and at cycles of 20 s) [81]. Moreover, the dis-
persion homogeneity was preserved in the nanocomposites
in other areas where the cement grains are much smaller in
due to the better dispersion stability, and only individual
size [78]. Although reduction of cement particle size using
nanotubes were observed in the fracture surface (Figure 5).
ball milling can improve nanomaterial dispersion, small grain
Lower surfactant/CNT ratios (0 and 1.5), however, could
cement has many other disadvantages such as high water
not disperse CNTs well, leading to presence of large CNT
consumption, thermal cracking, more chemical and autoge-
clusters within the composites. Similarly, VCNFs (0.048 wt.
nous shrinkage, and so forth [79]. Recently performed 3D
of cement) could also be dispersed homogeneously (Figure 6)
simulation study suggested that a homogeneous distribution
using a surfactant/CNF ratio of 4.0 [82].
of nanomaterials within cement is possible only when the
cement particles are also distributed homogeneously with- Besides concentration, it has been observed that the type
out any agglomeration [80]. Therefore, to improve carbon and structure of surfactant also have significant influence
nanomaterial dispersion in cementitious matrices various on the dispersion of carbon nanomaterials in water and
approaches have been employed till date, carefully consider- subsequently within cementitious composites. Among the
ing the issues discussed above, and can be broadly categorized various surfactants such as Sodium dodecylbenzenesulfonate
into physical and chemical techniques, as discussed in the (SDBS), sodium deoxycholate (NaDC), Triton X-100
following sections. However, it should be noted that the var- (TX10), Gum Arabic (GA), and cetyl trimethyl ammonium
ious chemical routes (such as using surfactant, polymers, or bromide (CTAB), the anionic one (SDBS) provided the best
functionalization) cannot directly disperse nanomaterials in aqueous dispersion of MWCNTs (prepared using surfactant
water; instead, they help in the dispersion process by wetting concentration of 2 wt.% and magnetic stirring for 10 min at
the nanomaterials with water and improving the dispersion 300 rpm combined with ultrasonication using a tip sonicator
stability. Therefore, these chemical routes are always used in at 40 W for 90 rounds, each of 90 s and 10 s rest in between),
combination with the physical routes (such as ultrasonica- which was stable after 70 minutes of ultracentrifugation and
tion), which can directly disperse the nanomaterials. 60 days of sitting [83]. The result was even better when SDBS
was used in combination with Triton X-100 (nonionic) in the
weight ratio of 3 : 1. The better stabilization in case of SDBS
5.1. Physical Techniques
was attributed to the benzene ring in the hydrophobic chain,
5.1.1. Ultrasonication. In an ultrasonic processor, electrical smaller charged SO3 2− head group, and relatively longer alkyl
voltage is converted to mechanical vibrations, which are hydrophobic chain [84]. The dispersion ability of various
transferred to the liquid medium (water or solvent) and lead surfactants was found in the following order: SDBS and TX10
to formation and collapse of microscopic bubbles. During this > SDBS > NaDC and TX10 > NaDC > AG > TX10 > CTAB.
process (known as cavitation), millions of shock waves are The cationic surfactant CTAB showed the lowest dispersion
created and a high level of energy is released [75], leading to capability because of the absence of benzene ring on the
dispersion of nanomaterials in the liquid. A short duration long chain and the positive charge which might have
of ultrasonic treatment (15 minutes at 20 kHz frequency and neutralized the negative charge of MWCNTs in aqueous
amplitude setting of 50%) using a titanium probe was found solution. The fracture surface of cement nanocomposite
successful to prepare homogeneous aqueous dispersion of containing 0.2 wt.% MWCNTs dispersed using SDBS/TX10
VCNFs (1.14 wt.%) [78]. However, this technique could not combination showed a very uniform distribution of CNTs.
ensure homogeneous distribution of CNTs within cement, Sodium dodecyl sulfate (SDS) has also been reported as
meaning that a homogeneous nanomaterial dispersion in an effective anionic surfactant for fabricating CNT/cement
water does not guarantee a good dispersion in the nanocom- nanocomposites [85]. However, one drawback of using sur-
posites as well. This fact necessitates the use of various chem- factants as nanomaterial dispersant is the lack of connectivity
ical routes in combination with ultrasonication to improve of nanomaterials within cementitious matrix due to blocking
the dispersion stability, thus preserving the nanomaterial by surfactant molecules, and this fact affects the electrical
dispersion up to the composite stage. and piezoresistive properties of nanocomposites [85].
Journal of Nanomaterials 5

(a) (b)

1 𝜇m 1 𝜇m

(c) (d)

1 𝜇m 1 𝜇m

Figure 5: Dispersion of MWCNT within cementitious composites prepared using different surfactant to MWCNT weight ratio: (a) 0, (b) 1.5,
(c) 4.0, and (d) 6.25 [81].

(a) (b)

500 nm 500 nm

Figure 6: Fracture surface of CNF/cement nanocomposites, showing individually dispersed CNFs [82].

Surface decoration of carbon nanomaterials using poly- lignosulfonate, very stable dispersions stable up to 9 days
meric surfactants has been reported to introduce steric repul- were obtained with the air entrainer, polycarboxylate, and
sion between the nanomaterials, leading to their homoge- lignosulfonate in the sedimentation test. However, the use of
neous dispersion. The surface of MWCNTs could be covered high concentration of lignosulfonate required for good CNT
with acrylic acid polymer through ultrasonication in water, dispersion is not recommended to avoid delay in the setting
as can be seen from Figure 7 [86], and this led to very good time of Portland cement [91]. Also, despite of a good aqueous
aqueous dispersion of CNT (Figure 8). Methylcellulose is dispersion, the use of alkylbenzene sulfonic acid could not
another polymer which has been used to prepare highly stable lead to a homogeneous CNT dispersion in the hardened
aqueous dispersion of CNT for fabricating cementitious cement paste. On the contrary, the use of polycarboxylate
nanocomposites [87–89]. resulted in a very good dispersion of MWCNT in water as
well as in the hardened cement paste and, therefore, proved
5.2.2. Use of Cement Admixtures. Polycarboxylate, which is to be the best dispersant among the various admixtures used
commonly used as a superplasticizer within cement paste, in cement.
was also found to be an effective dispersant of CNT [90]. Silica fume, an amorphous polymorph of silicon dioxide,
Among the various cement admixtures such as alkylbenzene is also used as a pozzolanic material in concrete production
sulfonic acid (air entraining agent), styrene butadiene rubber [92–94]. Silica fume consists of spherical particles with
copolymer latex, aliphatic propylene glycol ether including average diameter of 150 nm and has been found to improve
ethoxylated alkyl phenol, polycarboxylate, calcium naphtha- microfiber dispersion within cement [95, 96]. The influence
lene sulfonate, naphthalene sulphonic acid derivatives, and of silica fume on carbon nanomaterial dispersion has also
6 Journal of Nanomaterials

0.20 𝜇m 3.00 nm
×13000 ×800000

(a) (b)

Figure 7: TEM image of MWCNTs showing presence of acrylic acid polymer on the surface at magnifications of 13000x (a) and 800000x (b)
[86].

2.00 𝜇m 1.00 𝜇m

(a) (b)

Figure 8: TEM image of MWCNT dispersion in water without any treatment (a) and with acrylic acid polymer and sonication (b) [86].

been studied [97]. It was observed that the cement nanocom- Frequently, carbon nanomaterials have been treated with
posites prepared through dry mixing of 2 wt.% CNFs with strong acids such as nitric acid or a mixture of sulfuric and
cement and silica fume (10 wt.%) using a conventional three- nitric acid (3 : 1) to oxidize the surface and create functional
speed mixer (followed by water addition) showed both CNF groups such as carboxylic. Covalent functionalization using
agglomerates as well as individually dispersed nanofibers. acid mixture has been found successful to disperse CNTs
However, in absence of silica fume only CNF agglomerates individually within cementitious matrix [99]. Moreover,
were observed, indicating positive influence of silica fume CNTs became tightly wrapped by the C-S-H phase of cement,
[98]. The better dispersion in the presence of silica fume was due to covalent bonding between COOH or C-OH groups
attributed to the smaller size (100 times smaller as compared of nanotubes and C-S-H. Similar observations were also
to anhydrous cement particles) of silica fume particles which made in case of surface-functionalized CNFs using 70% nitric
could disrupt the Van der Waals forces between individual acid [100]. However, although surface-treated CNTs could
CNFs, thereby mechanically separating some of them during be homogeneously dispersed within cementitious matrix,
the dry mixing process and reducing the CNF clumps. the dispersed CNTs could not form a well-connected three-
Additionally, the silica fume particles present within the CNF dimensional network (as evident from Figure 9) required for
clumps as well as individual CNFs could also act as the silicon good electrical conductivity or piezoresistive properties due
source for the formation of Ca-Si-rich phases and nucleation to fewer contact points and covering of surface by C-S-H
sites for the self-assembly of Ca-Si-rich phases. phases [101].
Functionalization of CNTs with strong acids forms
5.2.3. Covalent Functionalization. The most common ap- carboxylated carbonaceous fragments (CCFs), which are
proach to improve the dispersion ability of CNTs/CNFs in organic molecules consisting of condensed aromatic rings
water or polymeric matrices is the covalent functionalization. with several functional groups [102]. Although CCFs have
Journal of Nanomaterials 7

(a) (b)

(c) (d)

Figure 9: SEM image of cement nanocomposites with untreated CNTs ((a), (b)) and acid-treated CNTs ((c), (d)) [101].

functional groups which can react with cement, they do (sodium dodecylbenzene sulfonate) to homogeneously dis-
not contribute to the mechanical properties as they are perse carboxyl-functionalized MWCNTs within cementi-
only small fragments and do not have proper structure tious matrix was found to be very effective [103].
to carry mechanical loads. CCFs can be removed through
washing of functionalized CNTs using acetone. CNTs, either
5.3. Novel Routes of CNT Dispersion. In order to avoid
containing CCFs or free from CCFs, resulted in floccules
the problematic and time-consuming process of dispersing
formation when Ca(OH)2 was added to the dispersion,
CNTs within cementitious matrix, an innovative method of
indicating reaction between the surface functional groups
fabricating cementitious nanocomposites through growth of
of CNT and Ca2+ ions. The hydration of cement on the CNTs onto the cement particles has been recently reported
surface of functionalized CNTs was also observed, as shown [104]. CNTs were grown in a chemical vapour deposition
in Figure 10. (CVD) reactor at 400–700∘ C using acetylene as the main car-
bon source and carbon monoxide and dioxide as the additives
5.2.4. Combination of Various Chemical Methods. The com- to enhance the yield. Cement powder was feed in the reactor
bination of surface functionalization with polymers has been continuously at a speed of 30 g/h, and the oxides (Fe2 O3 )
found to provide more stable aqueous dispersion of CNT present in the cement acted as catalysts for CNT growth,
than using only polymers or functionalized CNTs [86]. The without the need for an additional catalyst support used in
dispersions of nonfunctionalized MWCNTs using acrylic the conventional CVD process. The concept of preparing
acid polymer or gum arabic were found stable only up to cement nanocomposites using this route has been illustrated
2 hours after which sedimentation was observed. Similarly, in Figure 11. The TEM images of CNT-grown cement particles
aqueous dispersion of functionalized nanotubes also showed showed complete coverage of cement particles by carbon
poor long-term stability. On the contrary, functionalized nanomaterials and formation of MWCNTs as well as CNFs,
MWCNT dispersion prepared using acrylic acid polymer as shown in Figure 12. More recently, CNTs were also grown
showed stability more than 2 months. The long polyacrylic on the silica fume particles, impregnated with iron salt, using
acid polymers were adsorbed on the surface of function- acetylene as the carbon source [105]. CNTs with 5–10 walls
alized nanotubes and increased the steric barrier towards and diameters of 10–15 nm were grown at 600∘ C and with 12–
their agglomeration. In a similar way, use of surfactant 20 nm diameters were produced at 750∘ C (Figure 13). Silica
8 Journal of Nanomaterials

10 nm

100 nm 20 nm

(a) (b)

Figure 10: TEM image of CCF-free FWCNTs after hydration for 1 hour (a) and 5.5 hour (b) [102].

Cement particles Cement hybrid material Composite material

Figure 11: Schematic diagram showing concept of incorporating CNTs/CNFs within cementitious composites by their direct growth on
cement particles [104].

(a) (b) (c)

5 𝜇m 200 nm 100 nm

Figure 12: TEM image showing complete coverage of cement particles by carbon nanomaterial (a), formation of MWCNT (b), and CNF
formation (c) [104].
Journal of Nanomaterials 9

10 nm

500 nm 500 nm 50 nm

(a) (b) (c)

Figure 13: SEM images of pristine silica particles (a), growth of CNTs on silica particles at 600∘ C (b), and TEM image of CNTs grown on
silica particles at 600∘ C (c) [105].

fume, which is used as an admixture, can therefore be utilized the mesopores (size less than 50 nm), between the hydration
to introduce CNTs within cementitious matrices. products, and, thereby, produced a denser microstructure
than the unreinforced cement. Moreover, this also resulted
5.4. Large-Scale Production of CNT Dispersion. A technique in very good interaction between the hydration products and
for producing highly concentrated MWCNT/water suspen- dispersed CNTs, which were seen densely inserted between
sions that can be used for developing cement nanocomposites the C-S-H and CH phases of cement (Figure 14). Similar
at large scale has been recently developed [106]. In this findings were also made in case of cement containing 0.5 wt.%
process, MWCNTs were homogeneously dispersed in water surface-treated MWCNTs [100], which resulted in 64% lower
using surfactant (MWCNT to surfactant weight ratio of porosity and 82% lower pores with size more than 50 nm.
4.0) using a tip sonicator. When this CNT dispersion was On the contrary, cement composites containing microscale
centrifuged at 28,000 rpm using a swing bucket rotor, the fibres such as carbon showed much higher porosity than the
dispersed MWCNTs started precipitating at the bottom of Portland cement samples. Nanoindentation tests also showed
the tube, and complete sedimentation was achieved after 11 lower probability of porous phase in a cement nanocomposite
hours. The supernatant solution was then decanted down to containing 0.08 wt.% MWCNT than Portland cement, indi-
keep only 20% of the initial volume of the solution, and CNTs cating lower porosity in case of nanocomposites [81].
were redispersed in this solution through ultrasonication
for 40 minutes. The concentration of MWCNTs increased 5
times using this process, as revealed by optical absorbance 7. Mechanical Properties of CNT/CNF
spectroscopy. The concentrated MWCNT solution, when Reinforced Cementitious Composites
diluted by adding the same amount of water previously
decanted, showed same concentration as the reference non- Early investigations showed that CNTs have strong influence
concentrated MWCNT suspension. Moreover, it was quite on the hydration process and hardness of cementitious com-
interesting to note that the cementitious nanocomposites posites [108]. In spite of inhomogeneous CNT dispersion in
prepared using the concentrated MWCNT suspension (after nanocomposites with cement/CNT ratio of 0.02 (by weight),
dilution) exhibited similar mechanical properties as those Vickers hardness improved up to 600% in case of 0.4 and 0.5
obtained using the reference non-concentrated MWCNT water/cement ratios in the early hydration stages, although
suspensions, indicating that the dispersion of MWCNTs was no improvement in hardness was observed after 14 days of
preserved even after the concentration process through cen- hydration. These early results reflected the potential of CNT
trifugation. Therefore, this process can be utilized to prepare for improving mechanical properties of cement. However, as
large-scale production of CNT admixtures for developing in case of polymer, the reinforcing efficiency of CNT/CNF in
cementitious nanocomposites. cementitious matrices and the resulting mechanical proper-
ties of nanocomposites also depend on several critical factors,
as discussed in the following sections.
6. Microstructure of Carbon
Nanomaterial/Cement Nanocomposites
7.1. Influence of Dispersion. Dispersion of nanomaterials has
It has been reported by several researchers that carbon been identified as one of the principal factors which influence
nanomaterials can significantly change the microstructure the mechanical properties most. Therefore, the parameters
of cement, and this is one of the principal reasons for which control the dispersion behaviour have strong influence
improvement in mechanical properties. Significant difference on the mechanical properties also. For example, the type
between the porosity of Portland cement and cement/CNT and structure of surfactant were found to be very important
nanocomposites was observed [107]. The total porosity and with respect to the mechanical properties. Among the various
surface area both decreased with CNT addition. This was surfactants such as SDBS, NaDC, TX10, AG, and CTAB, the
attributed to the fact that CNTs filled in the pores, mainly highest flexural and compressive properties were achieved
10 Journal of Nanomaterials

(a) (b)

10 𝜇m 1 𝜇m

Figure 14: SEM micrographs of 1 wt.% CNT/cement paste at 28 days of hydration at different magnifications [107].

with NaDC, whereas the lowest variation as well as second- Homogeneous dispersion of CNTs/CNFs achieved
best flexural and compressive strengths were obtained in through their growth onto cement particles was reported
case of 3 : 1 mixture of SDBS and TX10. The improvements to provide 2 times higher compressive strength than the
in case of NaDC were 35.45% and 29.5% as compared to pristine cement composites after 28 days of hydration
plain cement paste. The highest improvement in case of [104]. This dispersion process led to well distribution of
NaDC was due to good dispersion of MWCNTs as well as CNTs and CNFs embedded into the hydration products of
formation of strong interface between cement matrix and C-S-H phases and, therefore, bridged the adjacent cement
MWCNTs. Similarly, better mechanical properties in case of particles (Figure 15), resulting in strong improvements in
SDBS and TX10 mixture resulted from the best dispersion compressive strength. Although a homogeneous dispersion
ability of this combination and also good bonding between of carbon nanomaterials is extremely necessary for enhancing
MWCNTs and matrix. Microscopy study in case of this mechanical performance of cementitious composites, it has
surfactant combination suggested that MWCNTs were well been observed that even when they are poorly dispersed,
distributed within the cement matrix as a net-like structure they can prevent the formation of shrinkage cracks and
and acted as bridges between the microcracks, resulting in significantly improve the mechanical performance, especially
superior mechanical performance [83]. Similarly, among the when cuing is done in absence of moisture for the first 24
various cement admixtures, improved dispersion of CNT in hours [111].
water as well as within cement was observed only in case of
polycarboxylate, and, therefore, the cement paste containing
0.8% polycarboxylate and 0.5% CNT showed very good flow 7.2. Influence of Nanomaterial Surface Treatment and Inter-
behaviour even with low water ratio (0.35) and presented a face. The interface between nanomaterials and cementitious
compressive strength 25% higher than the control cement matrix controls the load transfer between them and, there-
samples [90]. The length and concentration of CNTs also fore, significantly influences the mechanical properties of
influence their dispersion behaviour and, therefore, are con- composites. Formation of covalent bonding between COOH
trolling factors for mechanical properties of nanocomposites or C-OH groups of functionalized CNTs and C-S-H phases
[81]. It was noticed that short MWCNTs (10–30 𝜇m) provided of cement matrix has been observed through FTIR studies
better dispersion and flexural properties even when used [99] and was also supported by microscopy studies which
at higher concentrations (0.08 wt.%), whereas long MWC- showed tight wrapping of functionalized CNTs by C-S-H
NTs (10–100 𝜇m) should be used at lower concentrations phases. Cement nanocomposites containing surface-treated
(0.048 wt.%) to maintain better dispersion and to achieve MWCNTs presented much better flexural and compres-
good flexural properties. It was also observed that short CNTs sive properties as compared to plain cement paste. Flex-
at higher concentrations were better in terms of mechanical ural and compressive strength improved up to 25% and
properties due to relatively better dispersion, reduced CNT- 19%, respectively, using 0.5 wt.% functionalized CNT. It has
free volume of cement paste, and better filling of nanosized been observed that ensuring a good dispersion through
voids [109]. However, reduction of CNF’s aspect ratio due acrylic acid polymer wrapping does not ensure improved
to either debulking process or ultrasonication was found mechanical properties of composites, due to improper load
detrimental to mechanical properties, and it was observed transfer at the interface [86], whereas 0.045% of functional-
that a higher ultrasonication energy than optimum led to ized MWCNTs showed nearly 50% increase in compressive
reduction in nanomaterials’ aspect ratio and deterioration of strength when dispersed using the same process, indicating
mechanical properties [110]. strong influence of the interface. Improvement of mechanical
Journal of Nanomaterials 11

(a) (b)

Figure 15: SEM image of hardened cement paste (28 days) after mechanical test at different magnifications [104].

(a) (b)

Figure 16: Post-compression testing structural integrity of plain cement paste (a) and cement paste containing 0.5 wt.% surface-treated CNFs
(b) [100].

properties using functionalized nanomaterials can be further composites resulted due to the restriction in crack prop-
enhanced through removal of CCFs (see Section 5.2.3) from agation by the entangled clumps of CNF inside cement
the nanomaterials surface [102]. It has been reported that the cavities, leading to bridging of cracks, and also due to
incorporation of functionalized CNTs (0.01 wt.%) containing individually dispersed CNFs within the cement matrix. It
CCFs resulted in only 13% improvement in compressive was also observed that, after decalcification using ammonium
strength, whereas after removal of CCFs using acetone nitrate solution for 95 days, the samples containing CNFs
resulted in very strong improvement in compressive strength, showed better ductile behaviour with slow load dissipation
up to 97% using only 0.03 wt.% CNT. This was attributed after failure, as presented in Figure 17. This indicates better
to the fact that functionalized CNTs became less accessible durability of CNF/cement nanocomposites as compared to
for the reaction with cement hydration products and their the plain cement paste.
nucleation, due to presence of these CCFs. Similarly the In spite of several benefits of using functionalized
presence of surfactant molecules on the nanomaterial surface nanomaterials, surface functionalization method should be
was also found detrimental to the mechanical properties, used carefully in case of cementitious matrices. There is a
due to blocking of direct contacts between surface functional possibility that functionalized CNTs can absorb water present
groups and cement hydration products, and a reduction of in the cement paste due to their hydrophilic nature and may
65% in compressive strength was observed using 4% SDS. adversely affect the cement hydration. It has been noticed that
Use of surface-treated CNTs/CNFs also improves the the cement nanocomposites containing 0.5 wt.% carboxyl-
posttesting mechanical integrity of cement nanocomposites functionalized MWCNTs led to formation of lower amount
[100]. Cement samples containing 0.5 wt.% surface-treated of tobermorite gel due to improper hydration process and
CNFs were found to maintain better structural integrity than significantly deteriorated the mechanical properties [112].
the control samples after compression testing, as shown in Besides surface functionalization, the interface in a car-
Figure 16. Better structural integrity in case of CNF/cement bon nanomaterial/cement composite also depends on the
12 Journal of Nanomaterials

60 of different scales such as micro and nano. Hybrid cement

PC pastes
nanocomposites containing both CNTs and nano metakaolin
50 Reference 0.5 wt.% CNF
(NMK) have been reported [121]. NMK is a silica-based
Compressive load (MPa)

material, which can react with Ca(OH)2 to produce C-S-

40 H gel at room temperature, and incorporation of NMK
into concrete was found to significantly improve the early
30 strength, increase resistance to alkali-silica reaction and
0.5 wt.% CNF-AN95d
sulfate, and can increase toughness and durability [122–125].
20 Additionally, homogeneous dispersion of exfoliated NMK
was found to significantly improve the compressive strength
10 of cement (18% using 6 wt.% NMK) due to reduction of
Reference-AN95d porosity and improvement in the solid volume and bond
0 strength of cement through pozzolanic reaction between
0 1 2 3 4 5 silicon and alumina elements present in NMK and the
Displ. (mm) elements of calcium oxide and hydroxide in cement. Also, the
presence of NMK could probably disrupt the attractive forces
Figure 17: Comparison of compressive behaviour of cement paste
between CNTs during dry-mixing process and could separate
and CNF/cement composites before and after decalcification [100].
them leading to their individual dispersion. Additionally, the
presence of NMK particles mixed with the dispersed CNTs
surface feature of nanomaterials. The CNF variety with could act as Si source for the formation of Ca-Si-rich phases,
rougher surface containing conically shaped graphitic planes and CNTs could further act as the nucleation sites for the self-
was found to be very effective in enhancing mechanical assembly of Ca-Si phases. Due to these reasons, addition of up
properties as compared to the CNFs having smoother surface to 0.02% CNT resulted in 11% higher compressive strength as
containing CVD layers [110]. compared to the mortar containing only NMK.
Hybrid cement nanocomposites containing polyvinyl
alcohol (PVA) micro-fibres and CNFs were also developed
7.3. Influence of Microstructure. The improvement of me- and reported to have higher Young’s modulus, flexural
chanical properties achieved in case of well-dispersed strength, and toughness than plain cement, cement contain-
MWCNT/cement nanocomposites has been found to be ing only PVA micro-fibres, or CNFs [82]. It was observed
much higher than that predicted using theoretical equations that cement containing CNFs presented much higher load
[81]. There may be several possible reasons for this fact. carrying capability at the same CMOD (crack mouth opening
The decrease in cement porosity and improvement of its displacement) during the early stages of loading (Figure 19).
microstructure is certainly one of them. The increase in the Using only 0.048% CNFs, Young’s modulus, flexural strength,
amount of high stiffness C-S-H phases in presence of CNT, and toughness improved up to 75%, 40%, and 35%, respec-
as revealed from the nanoindentation tests, is another rea- tively. On the contrary, use of PVA micro-fibres improved
son for such strong improvement in mechanical properties. the Young’s modulus and flexural strength only marginally,
Improvement of microstructure is also the primary cause for but the fracture toughness increased tremendously, retaining
mechanical property enhancements in case of non-autoclave the load for ten times higher CMOD than plain cement.
foam concrete. It was found that the use of CNTs (0.05% Therefore, in the hybrid composites, the pre-peak behaviour
by mass) as the reinforcement of foam concrete stabilized was mainly controlled by CNFs, whereas the post-peak
its structure by decreasing the pore wall percolation and behaviour was influenced by mainly PVA micro-fibres. The
ensuring better pore size uniformity (Figure 18) [113]. This fracture surface study suggested good bonding between
resulted in strong improvement in the compressive strength cement and both CNFs and PVA microfibers and bridging
(70%) associated with a decrease in the average density of micropores by PVA fibres and pores at nanolevel by
of concrete from 330 kg/m3 to 309 kg/m3 . Improvement of CNFs. The hybrid cementitious composites showed up to
microstructure and resulting enhancements in mechanical 50% improvement in flexural strength, 84% improvement
properties due to CNT addition has also been noticed in in Young’s modulus, and 33 times (3351%) improvement
case of fly ash cement [114–119]. Fly ash cement samples in fracture toughness over plain cement matrix. Similarly,
containing CNTs presented higher density than control fly hybrid cement nanocomposite bar containing 2.25% short
ash and PC samples, due to filling of cement pores by CNTs, carbon fibres and 0.5% MWCNT was found to have much
and a denser microstructure [120]. The compressive strength higher tensile modulus (60%), load carrying capacity (54%),
of fly ash cement composites containing 1 wt.% CNT reached and failure strain (44%) as compared to plain cement bars
that of PC at 28 days and 60 days, which is usually higher [126].
than the compressive strength of fly ash cement due to its slow A list of important literature on the dispersion techniques
hydration rate. as well as improvement of microstructure and mechanical
properties of cementitious composites using carbon nano-
7.4. Mechanical Properties of Hybrid Cement Nanocomposites. materials is provided in Table 1. However, besides mechan-
Hybrid cement nanocomposites are analogous to the mul- ical properties, degradation behaviour of cement is another
tiscale polymer nanocomposites containing reinforcements important characteristic that influences the durability of
Journal of Nanomaterials 13

140 𝜇m 140 𝜇m 50 𝜇m 50 𝜇m

(a) (b) (c) (d)

Figure 18: Structure of cement-foam concrete: (a) without nanotubes, (b) with 0.05% CNT (pore walls), (c) without CNT (perforated), and
(d) stabilized with addition of 0.05% CNT [113].

300 70 300 70
28 d CP w/c = 0.5 CP + CNFs 28 d CP w/c = 0.5
250 60 250 60
50 50
200 200
CP
Load (N)

Load (N)

Load (lb)
Load (lb)

40 CP + micro-PVA 40
150 150
30 30
100 100
20 20
CP
50 10 50 10
0 0 0 0
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
CMOD (mm) CMOD (mm)
(a) (b)
300 70 300 70
28 d CP w/c
/ = 0.5 28 d CP w/cc = 0.5
250 60 250 60
CP + CNFs + micro-PVA
50 50
200 200 CP + micro-PVA
Load (N)

Load (lb)
Load (lb)

Load (N)

40 40
150 150 CP + CNFs + micro-PVA
30 30
100 100
20 20
CP + micro-PVA
50 10 50 CP + CNFs 10

0 0 0 0
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
CMOD (mm) CMOD (mm)
(c) (d)
Figure 19: Load-CMOD curves for (a) plain cement paste and cement paste containing CNFs, (b) cement paste and cement paste containing
PVA microfibers, (c) cement paste containing PVA microfibers and hybrid cement paste, and (d) cement paste containing CNFs, cement
paste containing PVA microfibers, and hybrid cement paste for CMOD values less than 0.04 mm [82].

concrete structures. Although some initial studies demon- reviewed. Various dispersion techniques have been pre-
strated a higher corrosion rate of steel bars inside a CNF sented, and the major issues affecting the mechanical prop-
reinforced mortar, subjected to aggressive environments such erties were discussed. It can be concluded that the disper-
as carbonation and chloride attack [127], more research is sion of CNTs/CNFs is the main factor which controls the
necessary in this direction to understand the degradation microstructure as well as the mechanical performance of
behaviour and durability of nanoreinforced concrete and the cement nanocomposites. The conventional method of mixing
steel reinforcements present inside the concrete sections. nanomaterials within mortar paste using a standard mixer
cannot ensure homogeneous dispersion and, therefore, dete-
riorates the mechanical properties. Addition of admixtures
8. Summary and Conclusions such as silica fume during the mixing process can signif-
In this paper, current research activities on the carbon nano- icantly improve the nanomaterial dispersion. However, the
material reinforced cementitious composites have been best route to achieve homogeneous nanomaterial dispersion
14 Journal of Nanomaterials

Table 1: Summary of different techniques used for carbon nanomaterial dispersion in cementitious matrix and resulting improvement in
microstructure and mechanical properties.

Type and concentration of Improvement in micro-structure

Dispersion techniquea Researchers and reference
nanomaterial and mechanical properties
Total porosity and surface area
MWCNT, 1.0 wt.% Ultrasonication Nochaiya and Chaipanich [107]
decreased by 16% and 23%
Compressive strength improved
CNT, 1.0 wt.%, and fly ash cement Ultrasonication Chaipanich et al. [120]
by 10%
Compressive and flexural
Functionalization with strength improved by 19% and
MWCNT, 0.5 wt.% HNO3 /H2 SO4 mixture and direct 25%, total porosity reduced by Li et al. [101]
mixing with cement 64%, and pores with diameter
>50 nm reduced by 82%
Compressive strength improved
Solvent (acetone) and
MWCNT, 0.5 wt.% by 11% and 17% for pristine and Musso et al. [112]
ultrasonication
annealed CNTs
Ultrasonication and Flexural strength and ductility
Short MWCNT, 0.2 wt.% Al-Rub et al. [109]
superplasticizer improved by 269% and 81%
Young’s modulus and flexural
MWCNT, 0.08 wt.% Ultrasonication and surfactant strength improved by 45% and Konsta-Gdoutos et al. [81]
25%
Flexural strength and Young’s
Ultrasonication at optimum
CNF, 0.048 wt.% modulus improved by 50% and Metaxa et al. [110]
conditions and surfactant
75%
Magnetic stirring, Flexural and compressive
MWCNT, 0.2 wt.% ultrasonication, and surfactant strength improved by 35.4% and Luo et al. [83]
(NaDC) 29.5%
Magnetic stirring,
Compressive strength improved
MWCNT, 0.5 wt.% ultrasonication, and Collins et al. [90]
by 25%
polycarboxylate admixture
Carboxylic functionalization,
Compressive strength improved
MWCNT, 0.045 wt.% ultrasonication, and polyacrylic Cwirzen et al. [86]
by 50%
acid polymer
CNT growth onto cement Compressive strength improved
Hybrid CNT-cement particle Nasibulin et al. [104]
particles more than 2 times
Compressive strength improved
CNT (0.02 wt.%) and NMK
Dry mixing with cement by 11% than mortar containing Morsy et al. [121]
(6 wt.%)
NMK
Flexural strength, Young’s
CNF (0.048 wt.%) and PVA modulus, and toughness
Ultrasonication and surfactant Metaxa et al. [82]
micro-fibres (0.54 wt.%) improved by 50%, 84%, and 33
times
Load carrying capacity and
CNF (0.5 wt.%) and short carbon Acetone, ultrasonication, and
failure strain improved by 54% Hunashyal et al. [126]
fibres surfactant
and 44%
a
The techniques for preparing aqueous dispersion of carbon nanomaterials are only provided here; subsequent mixing with cement mortar is performed using
a standard mixer.

is to disperse the nanomaterials first in water, followed by well as strong enhancement in the mechanical properties
mixing of aqueous dispersion with mortar paste. Various of cementitious composites. Alternatively, carbon nanoma-
chemical techniques attempted to achieve uniform and stable terials can be grown directly onto cement or silica fume
CNT/CNF aqueous dispersion are using surfactants, poly- particles to fabricate cement nanocomposites with homo-
mers, cement admixtures, functionalization, and combina- geneous dispersion and significantly improved mechanical
tion of various techniques. Anionic surfactants such as SDBS, performance. Well-dispersed CNTs/CNFs lead to filling of
polymers such as acrylic acid, cement admixtures such as pores within cement, improve its microstructure, restrict the
polycarboxylates, acid functionalization, and combination propagation of nanocracks to form micro- and macrocracks,
of functionalized nanomaterials with acrylic acid polymers and thereby improve the fracture behaviour and mechanical
were found to provide very good aqueous dispersion as properties. Hybrid cement nanocomposites containing both
Journal of Nanomaterials 15

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