Accepted Manuscript

Mechanical properties and reinforcing mechanisms of cementitious composites

with different types of multiwalled carbon nanotubes

Xia Cui, Baoguo Han, Qiaofeng Zheng, Xun Yu, Sufen Dong, Liqing Zhang,
Jinping Ou

PII: S1359-835X(17)30359-7
DOI: https://doi.org/10.1016/j.compositesa.2017.10.001
Reference: JCOMA 4790

To appear in: Composites: Part A

Received Date: 20 July 2017

Revised Date: 14 September 2017
Accepted Date: 2 October 2017

Please cite this article as: Cui, X., Han, B., Zheng, Q., Yu, X., Dong, S., Zhang, L., Ou, J., Mechanical properties
and reinforcing mechanisms of cementitious composites with different types of multiwalled carbon nanotubes,
Composites: Part A (2017), doi: https://doi.org/10.1016/j.compositesa.2017.10.001

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Mechanical properties and reinforcing mechanisms of
cementitious composites with different types of multiwalled
carbon nanotubes
Xia Cui1, Baoguo Han 1, *, Qiaofeng Zheng1, Xun Yu2,3, Sufen Dong1, Liqing Zhang1, Jinping Ou 1, 4
1
School of Civil Engineering, Dalian University of Technology, Dalian 116024, China
2
Department of Mechanical Engineering, New York Institute of Technology, New York, NY 11568, USA
3
School of Mechanical Engineering, Wuhan University of Science and Technology, Wuhan, 430081, China
4
School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China

* Corresponding author: hithanbaoguo@163.com, hanbaoguo@dlut.edu.cn

Abstract

In this study, reinforcement effect of 12 types of multiwalled carbon nanotubes

(MWCNTs) on mechanical properties of cementitious composites was investigated

Research results showed that among pristine MWCNTs with different diameters and

lengths, the short MWCNTs with large diameter present the best reinforcing effect on

strength of composites. Functionalization of MWCNTs is beneficial for enhancing

strength of composites. Moreover, hydroxyl-functionalized MWCNTs feature a better

reinforcement effect compared to carboxyl-functionalized MWCNTs. The best

relative/absolute enhancements of 79%/74MPa and 64.4%/5.6MPa in compressive

and flexural strength of composites are achieved by incorporating 0.5% of

nickel-coated MWCNTs. XRD analyses revealed that the incorporation of MWCNTs

decreases the orientation of CH in matrix, which is consistent with SEM observations.

TG analyses showed that MWCNTs inhibit hydration of composites due to their

absorption effect. However, extensive MWCNT networks improve microstructure of

matrix and hinder the crack development under loading through fiber bridging and

pull-out.

Key words: A. Carbon nanotubes and nanofibers; B. Strength; B. Mechanical properties; D.

Mechanical testing

1. Introduction
 Cementitious composite is one of the most widely used construction materials in

the world because its abundant resources, strong adaptability and mature production

process. However, it is brittle and susceptible to cracking [1]. A traditional and

effective approach is to incorporate rebar and micorscale fibers into the cementitious

materials for improving the ductility of cementitious composites [2-5]. Recently, the

development of nanotechnology has brought a new dawn. Compared to rebar and

micorscale fibers, nanofillers can improve properties of cementitious composites at

nanoscale [6-8]. As a kind of nano materials, CNT was first discovered by Iijima in

1991 [9]. It is regarded as one of the most promising nano reinforcement fillers. CNT

has the distinct tube-like structure with a very high aspect ratio beyond 1000. The

tensile strength of CNT reaches 50-200GPa which is 100 times as that of common

steel, although its specific gravity is only one sixth that of the latter. Moreover, carbon

nanotube (CNT) has high elastic modulus and excellent deformability. Due to these

exceptional properties, a considerable variety of researches have been carried out on

CNT reinforced cementitious composites [10-15].

Makar et al. [16] were the first researchers studying CNT reinforced cementitious

composites in 2005. They proved that hydration process of cement can be accelerated

at early age by adding single-walled CNT (SWCNT) into cement paste. Since then,

several researchers have also observed the improved mechanical performances of

cementitious composites through the addition of CNTs. Yakovlev et al. [17] found that

the use of CNTs (0.05% by mass) as the reinforcement for production of foam

non-autoclave concrete, allows increasing its compressive strength up to 70%. De

Ibarra et al. [18] studied the impact of the addition of 0.05%/0.1% SWNTs and

0.1%/0.2% MWCNTs to cement paste, and reported modest gains in the Young’s

modulus and hardness. Branner et al. [19] fabricated CNTs filled concrete at different
CNTs’ content levels. They found that the use of 0.1% CNTs could increase the tensile

strength of cementitious composites, but reduce compressive strength. When at least

about 0.2% CNTs were added, both compressive and tensile strengths of cementitious

composites were slightly increased. The underlying cause for these different reported

results is that CNTs’ enhancement capability to cementitious composites depends on

many factors, among which the quality of CNTs’ dispersion, CNTs’ content level,

CNTs’ intrinsic structure and properties, composition of matrix, and the interfacial

bonding condition between CNTs and matrix will all make huge differences [20].

Shah et al. [21] and Konsta-Gdoutos et al. [22] studied the effect of MWCNTs’ length

(short versus long) on the fracture properties of the nanocomposites with a constant

weight ratio of surfactant to MWCNTs. The fracture test results indicated that small

content of MWCNT (0.048% and 0.08%) produced a 25% increase in flexural

strength of cement paste. It was also observed that higher concentrations of short

MWCNTs (10-30μm) were required to achieve the same mechanical performance

with respect to the long MWCNTs (10-100μm) due to the different effect of

dispersion. Abu Al-Rub et al. [23] also studied the effect of different concentrations

of long MWCNTs (high length/diameter aspect ratios of 1250–3750) and short

MWCNTs (aspect ratio of about 157) on cementitious composites. Results also

showed that composites with low concentration of long MWCNTs gave comparable

mechanical performance to the composites with higher concentration of short

MWCNTs. With the exception of length of CNTs, the diameter of CNTs as another

important factor also greatly affects mechanical properties of composites. Manzur and

Yazdani [24] investigated size effect (outside diameters) of CNTs on compressive

strength of cement composites. The compressive strength enhancement was ranged

between 15 % and 25 %. It has been found that the maximum compressive strength
was obtained when the composites was reinforced with the smallest MWCNT having

out diameter smaller than 8nm in majority of cases. In addition, surface

functionalization of CNTs will enhance reinforcement efficiency on mechanical

properties of composites because the surface functional groups can originate strong

chemical bonds between CNTs and matrix. Li et al. [25] employed

carboxyl-functionalized MWCNTs and obtained modest improvements of 19% and 25%

in compressive and flexural strengths, respectively. A higher increase (50%) in the

compressive strength of cement paste was observed by Cwirzen et al. [26] through the

addition of functionalized MWCNTs with carboxyl groups at an amount of

0.045%-0.15%. Luo [27] suggested that dispersion and cohesion with cementitious

composites could be improved by acid oxidation treatment of MWCNTs, where the

flexural strength and compressive strength of 0.5% MWCNTs reinforced cementitious

composites was increased by 34.6% and 21.7% compared with plain sample.

Moreover, Manzur and Yazdani [28] carried out the strength comparison between

CNTs-COOH and untreated MWCNTs reinforced cementitious composites. It was

found that in all cases, the addition of treated MWCNTs would result in higher

compressive strength than that of untreated ones. Musso et al. [29] obtained a

different result. Flexural and compressive tests of the composites containing

functionalized CNTs showed a significant reduction of the performances compared to

pristine cement. They also employed CNTs as-grown and annealed MWNTs in plain

cement paste and found that both pristine and annealed MWNTs induced an

improvement in the flexural and compressive properties of cementitious composites.

Recently, some special types of CNTs have been used to improve the performance of

cementitious composites. For example, Martínez-Alanis and López-Urías [30]

investigated cementitious composites containing nitrogen-doped multiwalled carbon

nanotubes (MWCNT-Nx) and oxygen-functionalized multiwalled carbon nanotubes

(MWCNT-Ox). Test results of mortars simultaneously mixed with MWCNT-Nx and

MWCNT-Ox exhibited an increase of approximately 30% in compressive strength.

Chen et al. [31] used a new type of CNTs synthesized by plasma process (p-CNTs) to

modify the mechanical properties of ultra high performance cementitious composites

(UHPC) at low dosages. It was found that the p-CNTs showed much improved

dispersion and stability as compared to the normal CNTs synthesized by chemical

vapor deposition. Moreover, a 69.6% increase in flexural strength of UHPC from 12.5

to 21.2MPa was observed though addition of well-dispersed p-CNTs at a dosage of

only 0.067%.

Generally, several factors play the important roles in reinforcement effect on

strength of CNTs filled cementitious composites: (1) Extensive distributing

enhancement meshwork of CNTs in the cementitious composites [25]. Extremely

small diameters and large aspect ratios of CNTs (about the same size as the distance

between layers in hydrated cement) mean that they can be distributed on a much finer

scale, which is beneficial for enhancing the integrity of materials. (2) Decrease of

primary cracks in cementitious composites. The addition of CNTs can enhance the

thermal conductivity of cementitious composites and easily transfer the hydration heat

of cementitious composites. Thus, the amount of autogenous cracking caused by

temperature stress is decreased [32]. (3) Improvement to the microstructure of

composites. CNTs will fill the micro-pores between cement hydration products such

as calcium silicate hydrate (C-S-H) and ettringite (AFt). Therefore, the porosity of

composites is reduced and the pore sizes are refined, thus leading to a denser

microstructure than that of control sample [25, 33]. (4) The strong bonding between

CNTs and cement hydration products. Due to great surface energy of CNTs, the
hydration production will deposit on the CNTs which are extensively distributed in

cementitious materials [20]. (5) Crack bridging, fiber pull-out, pinning effect, crack

deflection, fiber debonding and fiber breaking. When the cracks in the matrix

encounter well-distributed CNTs, the pinning effect and the efficient crack bridging

can inhibit the crack growth at the very preliminary stage of crack propagation within

composites [16]. The bridge coupling effect of CNTs guarantees the load-transfer

across voids and cracks [25]. Furthermore, other reinforcing behaviors such as fiber

pull-out, crack deflection, fiber debonding and fiber breaking also contribute to

mechanical enhancement of composites [20].

Based on the quantities of the tests results and theoretical analyses, it is persuasive

that CNTs are highly effective nanofillers to the cementitious composites. However,

the effect of CNTs depends on many different factors and tests results vary from time

to time, even leading to controversy between researchers. Therefore, it is necessary to

do further research on the mechanical properties and reinforcing mechanisms of CNTs

reinforced cementitious composites. By now, it is verified that type of CNTs involving

their structure and properties is another important factor, besides CNTs’ dispersion

and dosage, affecting performances of cementitious composites. However, previous

studies on reinforcement effect of CNTs’ types on cementitious composites are less

comprehensive and systematic. For example, only one index of CNTs’ size containing

diameter and length are taken in consideration. For functionalized treatment,

hydroxyl-functional CNTs are rarely investigated and have no comparison with

carboxyl-functional CNTs. Furthermore, no studies about nickel-coated, graphitized,

helical and large-inner thin-walled MWCNTs reinforced cementitious composites

have been reported.

This study is conducted to explore the effect of MWCNTs’ type and dosage level on
the performances and microstructures of cementitious composites. 12 different types

of commercially available MWCNTs are used as reinforcement fillers. Firstly,

comparison between strengths of cementitious composites having different sizes of

untreated MWCNTs are made for assessing the effect of CNTs’ size. Then

functionalized MWCNTs with hydroxyl and carboxyl groups are utilized as

reinforcement and are compared with the untreated MWCNT-reinforced cementitious

composites. Furthermore, MWCNTs with graphitization and nickel plating on surface

are used to modify cementitious composites and are compared with untreated

MWCNTs reinforced cementitious composites. In addition, special structures of

MWCNTs are given proper consideration as a new type that influences properties of

MWCNT-reinforced cementitious composites. Lastly, MWCNTs having the greatest

reinforcement effect and the optimum dosage level of MWCNTs are suggested

through comprehensive analysis.

2. Experiment

2.1 Material

The raw materials for fabricating cementitious composites with and without

MWCNTs are listed as follows. P.O.42.5R used as binder material is produced by

Dalian Onoda Cement Company. MWCNTs are provided by Chengdu Organic

Chemicals Co. Ltd., Chinese Academy of Sciences. A total of 12 types of MWCNTs

are employed in this study, including four different sizes (i.e. different length and

diameter) of untreated MWCNTs (SM1, SM5, M1, M5), functionalized MWCNTs

with hydroxyl (SMH1, MH1) or carboxyl groups (SMC1, MC1), graphitized

MWCNT (GM5) through heat treatment in inert gases at 2800℃ for 20h, nickel coated

MWCNT though electrode plating (NiM5), large-inner thin-walled MWCNT (LIM)

and helical MWCNT (HIM) through catalytic cracking. Their properties are shown in
Table 1. CNTs are synthesized by catalytic decomposition of catalyzer. Short CNTs

(SM1, SM5) are obtained by mechanical cutting of long CNTs (M1, M5) and then

dispersed. SMH1, MH1 or SMC1, MC1 are synthesized in H2SO4 solutions at

different temperatures and concentrations by oxidation of KMnO 4. The nickel content

in NiM5 exceeds 60 wt.%. GM5 is synthesized by heat treatment of the high-purity

CNTs in an inert gas at 2800℃ for 20h. HIM includes about 60 wt.% helical

MWCNTs. Fig.1 shows the microscopic morphology of some kinds of MWCNTs by

scanning electron microscope (SEM) or transmission electron microscope (TEM).

The RHEOPLUS 411 polycarboxylate superplasticizer used to disperse MWCNTs

and cement is provided by BASF’s Chemical Building Materials (China) Co. Ltd.

Table 1 Properties of MWCNTs

-OH -COOH Pure

Type of OD ID Length/ SSA/
content/ content/ density/
MWCNT /nm /nm µm (m2/g)
(wt.%) (wt.%) (g/cm3)
SM1 <8 2-5 0.5-2 - - >350 2.1
SM5 20-30 5-10 0.5-2 - - >120 2.1
M1 <8 2-5 10-30 - - >350 2.1
M5 20-30 5-10 10-30 - - >110 2.1
SMH1 <8 2-5 0.5-2 5.58 - >380 2.1
SMC1 <8 2-5 0.5-2 - 3.86 >270 2.1
MH1 <8 2-5 10-30 5.58 - >400 2.1
MC1 <8 2-5 10-30 - 3.86 >400 2.1
GM5 20-30 5-10 10-30 - - >90 2.1
NiM5 20-30 5-10 10-30 - - 70 -
LIM 30-60 20-50 1-10 - - >200 -
HIM 100-200 - 1-10 - - >30 -
OD=Out Diameter, ID= Inner Diameter, SSA=Specific Surface Area

2 0 0 n m 2 0 n m 2 0 0 n m

(a) (b)
2 0 0 n m 2 0 n m 2 0 0 n m

(c) (d) (e)

1μm 1μm 1μm

(f) (g) (h)
Fig.1 Microscopic morphology of (a) M5, (b) M1, (c) SM5, (d)SM1, (e) NiM5, (f)
GM5, (g) LIM and (h) HIM

2.2 Preparation

Previous studies showed that effective dispersion of CNTs in water can be achieved

by applying ultrasonic treatment and the addition of some commercially available

surfactants [22, 27, 34-37]. In this paper, combination of ultrasonic and surfactant is

used to disperse CNTs. As well known, cementitious composites are porous

materials, in which there exist large numbers of capillary pores and micro voids.

Moreover, water-cement ratio has significant influence on the number and structure

of pores and voids. Low water-cement ratio brings about more small-size pores, so

that CNTs can work more effectively at nanoscale. Considering the dispersing effect

of superplasticizer, the water-cement ratio of 0.2 is chosen. To avoid the influence of

superplasticizer, a unified amount of water reducing agent is employed. The mass

ratio of cement: water: superplasticizer was 1: 0.2: 0.0075 for guaranteeing good

shaping and no bleeding of specimens. The MWCNTs was added at the amount of 0,

0.1%, 0.5% and 0.8% by weight of cement, respectively.

The process of fabricating specimen is as following. Firstly, weigh the all the raw
materials according to the designed mixing proportions. Secondly, mix together water,

polycarboxylate superplasticizer and MWCNTs by ultrasonic for 5min to prepare

uniform and well-dispersed suspension with SCIENTZ-1200E ultrasonicator

(50-1200W, 20-25kHz) provided by Ningbo Scientz Biotechnology Co., Ltd.. Thirdly,

add the cement into the suspension and at the same time stir with a MXD-E1100

multifunctional mixer (Shanghai Mu Xuan Industrial Co., Ltd.) at the speed of

1000±100 r/min until disperse uniformly and then at speed of 2000±10 r/min for 60s.

Fourthly, put the cement paste into the corresponding oiled molds and put onto the

vibrating table until shaken to appear oozing slurry on the surface of specimens. There

were two sizes of molds used in the experiment. Three specimens were fabricated for

each size of the mold, of which ones with dimensions of 20mm×20mm×80mm were

used for flexural test and ones with size of 20mm×20mm×40mm were used for

compressive test. After molding, all specimens were put in standard curing box and

demolded after 24h and then cured in water at 20ºC for 28d. The specimens were

cured at room temperature for 180 days to guarantee full hydration of cement before

testing.

The samples for TG analysis and XRD were prepared by grinding cement powder

with diameter no more than 80μm. In addition, samples were dried at 40 ºC for 24h

before testing.

2.3 Testing

The tests of specimens include compressive strength, flexural strength and

microanalysis. The compressive strength was tested by a universal electronic testing

machine (WDW-200E, Jinan Times Shi Jin Test Machine Co., Ltd). The specimens

(20mm×20mm×40mm) were loaded to failure at constant loading rate of 1.2 mm/min.

The flexural strength was measured by a universal electronic testing machine

(WDW-2E, Changchun KeXin Test Machine Co., Ltd.). The specimens

(20mm×20mm×80mm) were loaded to failure at constant loading rate of 0.2 mm/min.

The average value of compressive and flexural strengths of 3 specimens in each group

was regarded as the final results if the difference between average and the maximum

and minimum strength was less 10%. TG analysis was performed using a METTLER

TOLEDO STARe system to get the amount of calcium hydroxide (CH) and other

hydration products. The TG analysis was under condition of nitrogen atmosphere at a

heating rate of 10ºC/min up to 1000ºC. XRD (Bruker D8 Advance, Bruker German)

was applied to study the change tendency of CH crystal which caused by MWCNT.

Field emission scanning electron microscope (Nova Nano SEM 450, American FEI

Ltd.) was used to carry out MWCNTs’ microscopic morphology and fracture surface

observation. Transmission electron microscope (Tecnai F30, American FEI Ltd.) was

used to observe microscopic morphology of MWCNTs.

3. Effect of size of untreated MWCNTs on cementitious composites

3.1 Compressive strength

Fig.2 shows the compressive strength of four different sizes of MWCNTs (based on

diameter and length) reinforced cementitious composites. Relative and absolute

increases in compressive strengths of cementitious composites compared with control

sample are listed in Tables 2 and 3, respectively.

 150 0 0.1%
Compressive strength/MPa 0.5% 0.8%

120

90

60

30

0
SM5 M1
M5 SM1
Types of MWCNT
Fig.2 Compressive strength of cementitious composites with untreated MWCNTs

Table 2 Relative increase of compressive strength of cementitious composites with

untreated MWCNTs/%

Contents of MWCNTs SM1 M1 SM5 M5

0.10% 7.8 23.7 37.6 32.7
0.50% 24.5 14.6 45.3 18.1
0.80% -21 20.5 47.1 30

Table 3 Absolute increase of compressive strength of cementitious composites with

untreated MWCNTs/ MPa

Contents of MWCNTs SM1 M1 SM5 M5

0.10% 7.3 22.3 35.3 30.7
0.50% 23 13.7 42.5 17
0.80% -19.8 19.3 44.2 28.2

As seen from Fig.2 and Tables 2-3, compressive strength of cementitious

composites reinforced with short and large-diameter MWCNTs (SM5) at

concentration of 0.8% is the largest, which increases by 47.1%/44.2MPa compared

with the control sample without MWCNTs. Besides, compressive strength of M1 and

M5 reinforced cementitious composites reach the maximum at concentration of 0.1%,

both increasing greatly as compared to the control sample. It is obvious that M5

produces a slight higher compressive strength than that of M1. Compressive strength
of SM1 reinforced cementitious composites reaches the maximum at 0.5% dosage

which increases greatly as compared to the control sample. However, compressive

strength of cementitious composites with SM1 decreases by 21%/19.8MPa at loading

of 0.8% compared with the control sample. This is principally because a large amount

of SM1 is not easily dispersed, the local agglomeration and winding in the hydration

products of cement is formed. Thus, void and pore in the cement matrix increase and

interface compatibility of CNTs is poor. Certainly, mechanical properties are

compromised. However, too few MWCNTs will have no obvious influence in the

mechanical properties of CNT filled cementitious composites. In order to improve the

mechanical properties of CNTs reinforced cementitious composites, the volume ratio

of fibers must be greater than the critical value. Shen et al. [38] proposed the critical

volume fraction based on composite model of fiber reinforced cementitious

composites. It can be expressed as:

(1)

where is coefficient of length of CNT, determined by the ratio of actual length of

carbon fiber and critical pull-out length of carbon fiber, is orientating coefficient

of CNT and is equal to 0.20 when fibers are 3D-distributed in matrix, is the

tensile strength of CNTs and is about 30GPa [39], is the elastic modulus of CNTs

and is about 600GPa [39], is ultimate tensile strain and is about 0.00015, is

the tensile strength of cementitious composites. There is a relationship between

compressive strength and tensile strength of fibers reinforced cementitious composites,

as shown in formula (2).

(2)

where is the compressive strength of cement paste got by test and shown in Fig.

2. Therefore, critical volume fraction of CNT is calculated according to formulas

(1)-(2). The minimum value of is about 0.08% when reaches the maximum,

i.e. 1. In this study, the volume fractions (Vf) of CNTs are listed in Table 4 and are all

larger than critical volume fraction, which guarantees effective enhancement for

strength of CNTs reinforced cementitious composites.

Table 4 Volume fraction of MWCNTs

Weight fraction Volume of the Mass of Density of Volume fraction of

of CNTs composites (V) CNTs (M) CNTs (ρ) CNTs (Vf=m/ρ/V)
0.1 wt.% 144cm3 0.32 g 2.1 g/ cm3 0.104 vol.%
0.5 wt.% 144 cm3 1.6 g 2.1 g/ cm3 0.528 vol.%
0.8 wt.% 144 cm3 2.56 g 2.1 g/ cm3 0.847 vol.%

Through the above analysis, conclusion can be drawn that compressive strengths of

cementitious composites with large-diameter MWCNTs is higher than that of

small-diameter MWCNTs reinforced cementitious composites.

3.2 Flexural strength

Fig.3 displays the flexural strength of four different sizes of MWCNTs (based on

diameter and length) reinforced cementitious composites. Relative and absolute

increases in flexural strength of cementitious composites compared with the control

sample are listed in Tables 4 and 5, respectively.

16 0 0.1%
0.5% 0.8%
Flexural strength/MPa

12

0
SM5 M5M1 SM1
Types of MWCNT
Fig.3 Flexural strength of cementitious composites with untreated MWCNTs

Table 5 Relative increase of flexural strengths of cementitious composites with

untreated MWCNTs/%

Contents of MWCNTs SM1 M1 SM5 M5

0.1% 4.6 48.3 55.2 44.8
0.5% 21.8 36.8 27.5 37.9
0.8% 3.4 42.5 36.8 44.8

Table 6 Absolute increase of flexural strengths of cementitious composites with

untreated MWCNTs/ MPa

Contents of MWCNTs SM1 M1 SM5 M5

0.1% 0.4 4.2 4.8 3.9
0.5% 1.9 3.2 2.4 3.3
0.8% 0.3 3.7 3.2 3.9

As seen from Fig.3 and Tables 5-6, flexural strength of cementitious composites

reinforced with short and large-diameter MWCNT (SM5) reaches maximum, which

presents a 55.2%/4.8MPa increase at concentration of 0.1% compared with the control

sample. However, flexural strength of cementitious composites reinforced with short

and small-diameter MWCNT (SM1) is smallest in all cases. Flexural strength of SM1

reinforced cementitious composites reaches the maximum at 0.5% dosage, which

increases by 21.8%/1.9MPa compared with the control sample. Like compressive

strength, variation in flexural strength between M1 and M5 reinforced cementitious

composites is similar to the changing contents of MWCNTs. However, the gain in

flexural strength of M1 and M5 reinforced cementitious composites is higher than that

in compressive strength. Judging from the test results, the long MWCNTs are better

for improving flexural strength in most cases against short MWCNTs.

In terms of compressive and flexural strengths, effect of MWCNTs’ size on

cementitious composites is summarized as follows. It can be concluded that the

large-diameter MWCNTs filled cementitious composites present relatively higher

compressive strengths since compressive strengths of SM5 and M5 filled cementitious

composites is higher than ones of SM1 and M1 filled cementitious composites.

However, Manzur and Yazdani [24] had found that MWNTs with OD 20 nm or less

obtained similar compressive strengths, with the highest compressive strength being

achieved by the smallest size of MWNT having OD smaller than 8nm in majority of

cases. They thought that smaller MWCNTs can be distributed at much finer scale, and

consequently filling the nanopore space within the cement matrix more efficiently.

The statement is partially right, since the reinforcement effect of CNTs is based on

their good and uniform dispersion inside the cement matrix. Theoretically, surface

energy of small-diameter MWCNTs is bigger than that of large-diameter MWCNTs.

Thus, compared with large-diameter MWCNTs, small-diameter MWCNTs is easily

agglomerate. Therefore, the large-diameter MWCNTs should achieve relatively better

dispersion. Moreover, the orientation index of CH indicates that large-diameter

MWCNTs are conducive to obtain better mechanical property compare with

small-large MWCNTs. Compared with short MWCNTs, reinforcement effects of long

MWCNTs (M1 and M5) on flexural strength of cementitious composites are enhanced.

It is, therefore, concluded that long MWCNTs have a better reinforcement effect on

flexural strength compared with short MWCNTs. The same result was observed in

studies by Al-Rub et al. [23] and Konsta-Gdoutos et al. [22]. Considering two factors

of diameter and length, MWCNTs with short length (0.5-2μm) and large diameter

(20-30nm) at concentration of 0.1% are suggested as reinforcement of cementitious

composites.

3.3 XRD analysis

Fig.4 shows XRD patterns of cementitious composites reinforced with four

different sizes of MWCNTs. In order to avoid the interference of concentration,

samples containing 5% of MWCNTs are selected in microscopic analysis. As seen

from Fig.4, hydration products such as CH, AFt, unreacted C3S and CaCO3 from

carbonization of CH all can be detected by XRD. Moreover, no special diffraction

peak in MWCNTs reinforced cementitious composites different from control sample

occurs, which indicates addition of MWCNTs don’t produce new materials. Crystal

face peak intensity of CH can be obtained and then the CH orientation can be

calculated according to the calculation method given in Ref. [40]. The calculation

results are listed in Table 7. It can be seen from Table 7 that the CH orientation degree

is reduced as MWCNTs are added into cementitious composites. This is due to the

fact that MWCNTs can accelerate C-S-H gel formation by means of increasing CH

amount at the early age, prevent the CH and AFt crystal from forming large size, and

modify the orientation index of CH crystal [32]. It is also found that size of MWCNTs

have influence on the CH orientation. By comparative analyses, MWCNTs with large

diameter are helpful in reducing the CH orientation of sample as compared to

small-diameter MWCNTs. The large-diameter MWCNTs have larger tube thickness

compared with the small-diameter MWCNTs. As a result, they are more effective to

restrain the directional growth of CH. The small CH orientation is beneficial for

increase of strength of cementitious composites. This is also identical with the results

of strength of cementitious composites. Meanwhile, the length of MWCNTs is not the

obvious factor affecting the CH orientation compared with the diameter of MWCNTs

because total length of MWCNTs is the same at the same concentration level of

MWCNTs.
 AAFt BCa(OH)2 CC3S DCaCO3
B(001) D
A C B(101)
A B C B BC M5

M1

SM5

SM1

Control

8 16 24 32 40 48 56 64
2/°
Fig.4 XRD patterns of cementitious composites with untreated MWCNTs

Table 7 Diffraction intensity and orientation of CH in cementitious composites with

untreated MWCNTs

Sample Control SM1 SM5 M5 M1

(001)CH 1794 1462 1458 1319 1442
(101)CH 898 868 952 881 853
CH Orientation 2.70 2.27 2.07 2.02 2.28
“Control” represents cementitious composites without MWCNTs.

3.4 TG analysis

TG and DTG (derivative of TG) diagrams of cemetitious composites are shown in

Fig.5. The mass loss between 50-300ºC is due to the dehydration of C-S-H gel,

ettringite (Aft) and physically-bonded water [41]. The mass loss between 400-550ºC

is resulting from the decomposition of CH. The mass loss between 600-800 ºC mainly

gives the credit to the decomposition of CaCO3. In addition, cement hydration degree

can be obtained based on the TG results by using the calculation method according to

Ref. [42]. Furthermore, the water required for full hydration is 0.23g /1g cement [43].
As shown in Table 8, cement hydration degree decreases when MWCNTs are added

into cementitious composites. This indicates that MWCNTs have certain inhibiting

effect on the cement hydration. This behavior is justified taking into account

adsorption effect of CNTs and small water-cement ratio in this study. As Musso et al.

pointed out, surface treated CNTs are so hydrophilic as to absorb most of the water

contained in cement paste, hence hampering the proper hydration of cement paste [29].

Due to large specific surface area and great surface energy, MWCNTs can serve as

adsorption nucleus of hydration products, water and ions [32]. Furthermore, low

water-cement ratio results in an incomplete hydration. Therefore, hydration of

MWCNTs reinforced cementitious composites is retrained. However, it is beneficial

to reduce hydration heat and primary cracks. As a result, strengths of CNTs reinforced

cementitious composites increase even if CNTs inhabit cement hydration. On the

other hand, it is found that cement hydration degree shows a negligible difference

when MWCNTs with different size are added into cementitious composites. It is

indicated that the size of MWCNT has unobvious influence on cement hydration

degree.

100 Control 0.0000

1st derivative of mass (%)

SM1
-0.0002
Mass Loss (%)

SM5
95
M1
M5 -0.0004
90
M1
-0.0006 M5
85
SM1
-0.0008 SM5
80 Control
-0.0010
200 400 600 800 1000 0 200 400 600 800 1000
Temperature(°C) Temperature (C)
(a) (b)
Fig.5 TG diagram (a) and DTG diagram (b) for cementitious composites with
untreated MWCNTs

Table 8 Hydration degree of cementitious composites with untreated MWCNTs

Sample Control M5 SM5 M1 SM1
Hydration degree (%） 69.5 62.7 62.6 61.9 62.8

3.5 Microstructure observation

Fig.6 shows the SEM micrographs of SM5 and M5 distributed in cement matrix. As

can be seen, SM5 form the extensive spatial distribution inside cementitious

composites and M5 can distribute within cement hydration and lap each other in space.

Assuming MWCNTs are in ideal geometrical shape and exist in single particle, the

number of MWCNTs per cm3 in the composites can be calculated. As shown in Table

9, the number of MWCNTs per cm3 can reach in the range from 5×1010 to 2.4×1014 as

the MWCNTs content is only 0.1 wt.%. This means that numerous of MWCNTs exist

in cementitious composites even at low filler concentration levels. The extensive

distribution of MWCNTs in cementitious composites is closely related to the number

of MWCNTs per unit volume, but also average center distance between two

MWCNTs in cementitious composites. Romualdi [44] built up the fiber spacing

theory in 1964. He stated that the average center-to-center spacing of fibers and the

cracking resistance have an inverse relationship and found that when the fibers are

disorderly 3D-distributed in the matrix, the average center-to-center spacing can be

expressed as:

(3)
Formula (3) indicates that the average center-to-center spacing of MWCNTs in

cementitious composites is only related with the diameter and contents of MWCNTs.

The average center distance between adjacent MWCNTs in matrix is shown in Table

10. It indicated that as the MWCNT content increases, average center distance

between adjacent MWCNTs decreases gradually and MWCNTs per unit volume can

carry high loading.

Table 9 Numbers of untreated MWCNTs per 1 cm3 in the composites

Types Out Inner Volume of single Number of

Length
of diameter diameter MWCNT MWCNTs per 1
(L)/μm
CNTs (2R)/nm (2r)/nm (Vc=π(R2-r2)L)/nm3 cm3
SM1 6-8 2-5 0.5-2 4.3×103-9.5×104 1.1×1013-2.4×1014
M1 6-8 2-5 10-30 8.8×104-1.4×106 7.4×1011-1.2×1013
SM5 20-30 5-10 0.5-2 1.2×105-1.4×106 7.4×1011-8.7×1012
M5 20-30 5-10 10-30 3×105-2.1×107 5×1010-3.5×1012

Table 10 Average center distance ( /μm) between adjacent MWCNTs in the

composites

Volume fraction M5 SM5 M1 SM1

0.1 vol.% 8.56-12.84 8.56-12.84 2.57-3.42 2.57-3.42
0.5 vol.% 3.8-5.7 3.8-5.7 1.14-1.52 1.14-1.52
0.8 vol.% 3-4.5 3-4.5 0.9-1.2 0.9-1.2

2μm 5μm
(a) (b)
Fig.6 SEM micrographs of (a) 0.8% of SM5 and (b) 0.8% of M5 distributed in cement
matrix

4. Effect of surface functionalization of MWCNTs on cementitious

composites

4.1 Compressive strength

The relationships between compressive strength of cementitious composites and

content of two sizes of functionalized MWCNTs are presented in Figs.7a and 7b,

respectively. For comparison, compressive strength of cementitious composites

reinforced with untreated and same size MWCNT is also shown in Fig.7. The

corresponding relative and absolute increases in compressive strength compared with

the control sample are listed in Tables 11 and 12, respectively.

0 0.1%
150
Compressive strength/MPa

0.5% 0.8%

120

90

60

30

0
M1 MH1 MC1
Types of MWCNT
(a)
0 0.1%
150
Compressive strength/MPa

0.5% 0.8%

120

90

60

30

0
SM1 SMH1 SMC1
Types of MWCNT
(b)
Fig.7 Compressive strength of cementitious composites with functionalized (a) M1
and (b) SM1

Table 11 Relative increase of compressive strengths of cementitious composites with

functionalized MWCNTs/%

Contents of MWCNTs M1 MC1 MH1 SM1 SMC1 SMH1

0.1% 23.7 37.5 30.5 7.8 0.6 -3
0.5% 14.6 52 64.6 24.5 43.5 61.9
0.8% 20.5 50.6 28.2 -21.1 -11.7 -0.7

Table 12 Absolute increase of compressive strengths of cementitious composites with

functionalized MWCNTs/MPa

Contents of MWCNTs M1 MC1 MH1 SM1 SMC1 SMH1

0.1% 22.3 35.2 28.7 7.3 0.6 -3.1
0.5% 13.7 48.9 60.5 23 40.9 58.2
0.8% 19.3 47.6 26.5 -19.8 -11 -0.7

As shown in Fig.7 and Tables 11-12, compressive strengths of samples having

carboxyl-functionalized and hydroxyl-functionalized MWCNTs enhance greatly as

compared to untreated and same size MWCNTs. Compressive strength of

cementitious composites with carboxyl-functionalized M1 (i.e. MC1) at

concentrations of 0.5% increases by 52%/49MPa compared to the control sample and

produces an improvement of approximately 38% compared with untreated and same

size M1. Samples containing hydroxyl-functionalized M1 (i.e. MH1) at loading of 0.5%

present a 64.6%/60.5MPa higher compressive strength compared with the control

sample, which increases by 50% as compared to M1. In another group having short

size of MWCNTs, compressive strength of cementitious composites filled with

hydroxyl-functionalized SM1 (i.e. SMH1) and carboxyl-functionalized SM1 (i.e.

SMC1) at concentration of 0.5% increases by 62%/58.2MPa and 43.5%/41MPa as

compared to control sample, respectively. Compared with untreated and same size

SM1, a 0.5% addition of SMH1 and SMC1 leads to 34.7% and 19% of increase in

compressive strength, respectively. According to the above analytical results, 0.5% of

MWCNTs with hydroxyl groups produce higher compressive strength of cementitious

composites than ones with carboxyl groups.

4.2 Flexural strength

The relationships between flexural strength of cementitious composites and

contents of two sizes of functionalized MWCNTs are shown in Figs.8a and 8b,

respectively. For comparison, flexural strength of cementitious composites reinforced

with untreated and same size MWCNTs is also shown in Fig.8. The corresponding
relative and absolute increases in flexural strength compared with control sample are

listed in Tables 13 and 14, respectively.

0 0.1%
0.5% 0.8%
Flexural strength/MPa

12

0
M1 MH1 MC1
Types of MWCNT
(a)
16 0 0.1%
0.5% 0.8%
Flexural strength/MPa

12

0
SM1 SMH1 SMC1
Types of MWCNT
(b)
Fig.8 Flexural strength of cementitious composites with functionalized (a) M1 and (b)
SM1

Table 13 Relative increase of flexural strength of cementitious composites with

functionalized MWCNTs/%

Contents of MWCNTs M1 MC1 MH1 SM1 SMC1 SMH1

0.1% 48.3 49.4 4.6 4.6 18.4 26.4
0.5% 36.8 34.5 24.1 21.8 54 75.9
0.8% 42.5 54 26.4 3.4 11.5 -10.3

Table 14 Absolute increase of flexural strength of cementitious composites reinforced

with functionalized MWCNTs/MPa

Contents of MWCNTs M1 MC1 MH1 SM1 SMC1 SMH1

0.1% 4.2 4.3 0.4 0.4 1.6 2.3
0.5% 3.2 3 2.1 1.9 4.7 6.6
0.8% 3.7 4.7 2.3 0.3 1 -0.9

As shown in Fig.8 and Tables 13-14, flexural strengths of samples having

carboxyl-functionalized and hydroxyl-functionalized MWCNTs enhance greatly as

compared to untreated and same size MWCNTs in most cases. MH1 is an exception

which produces negative effect on flexural strength of cementitious composites as

compared to M1. Compared with control sample and M1 reinforced cementitious

composites, flexural strength of cementitious composites reinforced with MC1 at

concentration of 0.8% increases by 54%/4.7MPa and 11.5%, respectively. In another

group, flexural strength variation of SMH1 and SMC1 reinforced cementitious

composites are both same as that of SM1, achieving the maximum at 0.5%

concentration. In addition, reinforcement effect of SMH1 on strength of cementitious

composites is more remarkable than that of SMC1 at less than 0.8% dosage level.

Flexural strength of cementitious composites reaches the maximum (15.3MPa) at 0.5%

content of SMH1, which increases by 76%/6.6MPa compared with control sample

and presents a 54%/4.7MPa increase than SM1.

The addition of functionalized MWCNTs results in higher strength of cementitious

composites than that of untreated MWCNTs in most cases. This is consistent with the

results in references [26-27]. It is known that the functionalized MWCNTs own

hydrophilic groups like hydroxyl or carboxyl groups on their surface which promote

the dispersion of MWCNTs in water and enhance the bond effect by their interfacial

interaction with hydrations (such as C–S–H or CH) of cement. The interaction leads

to a stronger covalent force on the interface between MWCNTs and matrix in the

composites, and therefore improves the load-transfer efficiency from matrix to

MWCNTs with respect to untreated MWCNTs [25-26]. Moreover, it is found that the
highest strength is obtained by hydroxyl-functionalized MWCNTs reinforced

cementitious composites in majority of case. Compared with carboxyl groups,

hydroxyl groups can more effectively enhance wettability of CNTs, thus leading to an

improvement of hydrophility of CNTs [45, 46]. As a result, hydroxyl-functionalized

MWCNTs can achieve better dispersion in matrix against carboxyl-functionalized

MWCNTs. This may be the reason that hydroxyl-functionalized MWCNTs produce

better reinforcing effect to matrix than carboxyl-functionalized MWCNTs. In

summary, functionalized MWCNTs with OH or COOH groups, especially OH group,

are recommended to modify cementitious composites.

4.3 XRD analysis

XRD patterns of two sizes of functionalized MWCNTs reinforced cementitious

composites are presented in Figs.9a and 9b, respectively. For comparison, XRD

patterns of cementitious composites reinforced with untreated and same size

MWCNTs are also shown in Fig.9. CH orientation of two sets of samples is listed in

Table 15. It is obvious that the CH orientation is reduced as functionalized MWCNTs

are added into cementitious composites as compared to control sample. Compared

with same size MWCNTs, MWCNTs with hydroxyl groups have an increase effect on

CH orientation, while ones with carboxyl groups decrease CH orientation. The

funtionalized MWCNTs, especial carboxyl-functionalized MWCNTs, will react with

CH in composites and consume CH, thus restraining grow of CH crystal. CH

orientation in MH1 reinforced cementitious composites is higher than that in M1

reinforced cementitious composites, which is one of reason that MH1 degrades

flexural strength compared with M1.

 AAFt BCa(OH)2 CC3S DCaCO3
B(001) D
C B(101)
A B C B M1
A BC

MC1

MH1

Control

8 16 24 32 40 48 56 64
2/°
(a)
AAFt BCa(OH)2 CC3S DCaCO3
B(001) D
C
B(101)
A B
A C B SMC1
BC

SMH1

SM1

Control

8 16 24 32 40 48 56 64
2/°
(b)
Fig.9 XRD patterns of cementitious composites with functionalized (a) M1 and (b)
SM1

Table 15 Diffraction intensity and orientation of CH in cementitious composites with

functionalized MWCNTs

Sample Control M1 MH1 MC1 SM1 SMH1 SMC1

(001)CH 1794 1442 1388 1211 1462 1518 1312
(101)CH 898 853 794 951 868 970 889
CH Orientation 2.70 2.28 2.36 1.72 2.27 2.11 1.99
4.4 TG analysis

TG and DTG diagrams of two sizes of functionalized MWCNTs reinforced

cementitious composites are shown in Figs.10 and 11, respectively. Cement hydration

degrees of two groups of samples are shown in Table 16, respectively. As seen from

Table 16, cement hydration degree of functionalized MWCNTs reinforced

cementitious composites is lower than that of control sample, but higher than that of

untreated MWCNTs reinforced cementitious composites. It is indicated that

MWCNTs with hydroxyl or carboxyl groups have a positive effect on cement

hydration. MH1 obtained highest hydration degree of cement among functionalized

MWCNTs, which is possibly a minus point for mechanical property of cementitious

composites due to the increase of primary cracks caused by temperature stress.

100 0.0000
1st derivative of mass (%)

M1
Control
-0.0002
Mass Loss (%)

95 MH1
MC1
-0.0004
90
-0.0006 M1
85 Control
MC1
-0.0008 MH1
80
-0.0010
200 400 600 800 1000 0 200 400 600 800 1000
Temperature(°C) Temperature (C)
(a) (b)
Fig.10 TG diagram (a) and DTG diagram (b) for cementitious composites with
functionalized MWCNTs

100 SM1 0.0000

1st derivative of mass (%)

SMH1
Mass Loss (%)

95 SMC1 -0.0002
Control
-0.0004
90
SM1
-0.0006 Control
85
SMH1
-0.0008 SMC1
80
-0.0010
200 400 600 800 1000 0 200 400 600 800 1000
Temperature(°C) Temperature (C)

(a) (b)
Fig.11 TG diagram (a) and DTG diagram (b) for cementitious composites with
functionalized and short MWCNTs

Table 16 Hydration degree of cementitious composites with functionalized MWCNTs

Sample Control M1 MH1 MC1 SM1 SMH1 SMC1

Hydration degree (%) 69.5 61.9 68 64.1 62.8 63.9 65.5

4.5 Microstructure observation

As shown in Fig.12, crack bridging and fiber pull-out effect are observed in

functionalized MWCNTs reinforced cementitious composites. When the cracks in the

matrix encounter well-distributed MWCNTs, the efficient crack bridging can inhibit

the crack growth at the very preliminary stage of crack propagation within composites

[16]. The enhancement mechanisms are different when the length or content of

MWCNTs are changing. The applied load will be transferred to the MWCNTs and

MWCNTs are pulled out or snapped. According to the references [38], if the MWCNT

length (Lf, μm) is larger than critical length ( , μm), the damage state of MWCNTs

will be mainly dominated by tensile failure. On the contrary, the damage state of

MWCNTs will be based on pull-out. The critical length of MWCNTs can be

calculated as follows.

Lf≤ (4)

Lf> (5)

where is the length of MWCNTs and is shown in Table 1, , and have

been known, is tensile strength of the composites and its relationship with

flexural strength is shown in formula (6) [47].

(6)

where is flexural strength of composites and is shown in Fig.8. Thus, the critical
length of MWCNTs can be calculated with formula (4)-(6). As listed in Table 17, the

critical length of MWCNTs increases as the MWCNT content increases. When

MWCNT content is at 0.1%, the actual length is all larger than the critical length, and

the MWCNTs will be snapped when the MWCNTs filled cementitious composites are

damaged. On the contrary, with the increase of MWCNT content, the actual length is

all shorter than the critical length, and the MWCNTs will be pulled out when the

MWCNTs filled cementitious composites are damaged. During the pulling-out or

snapping process, MWCNTs owning excellent stiffness can absorb energy in order to

overcome the friction force of the interface between MWCNTs and matrix, which can

increase the demand of failure energy of MWCNTs filled cementitious composites.

The calculation method of the pull-out energy of MWCNTs filled cementitious

composites is shown as formulas (7)-(8), based on Pigott’ theory [48]. The average

pull-out energy of one single MWCNT can be expressed as:

(7)

The pull-out energy of CNTs in the composites can be given by formula (8).

(8)

where N is number of MWCNTs per 1 cm2 in the composites. Table 18 shows the

pull-out energy of the composites when damage state of MWCNTs is based on

pull-out. Compared with untreated MWCNTs, functionalized MWCNTs consume

more pull-out energy.

Table 17 Critical length (μm) of MWCNTs at different dosage levels

Contents of MWCNTs 0.1wt.% 0.5 wt.% 0.8 wt.%

M1 6.07-18.2 15.48-46.44 19.21-57.62
MC1 6.03-18.1 15.61-46.83 18.48-55.43
MH1 7.63-22.9 16.25-48.74 20.39-61.17
SM1 0.38-1.53 0.82-3.28 1.13-4.51
SMC1 0.35-1.41 0.73-2.92 1.09-4.34
SMH1 0.34-1.35 0.68-2.73 1.21-4.84

Table 18 Fracture energy increments of MWCNTs filled cementitious composites

Types of ×10-12/J N×1012/cm2 Wp/(J/cm2)

MWCNTs Vf=0.5% Vf=0.8% Vf=0.5% Vf=0.8% Vf=0.5% Vf=0.8%
M1 0.45-2.43 0.37-1.96 21-6 34-9 9.58-14.6 12.5-17.7
MH1 0.43-2.3 0.35-1.85 21-6 34-9 9.13-13.9 11.8-16.6
MC1 0.45-2.4 0.38-2.0 21-6 34-9 9.5-14.5 13.0-18.4
SM1 0.02-0.15 0.02-0.11 419-86 672-138 9.02-13.2 10.5-15.4
SMH1 0.02-0.17 0.02-0.12 419-86 672-138 10.2-14.8 10.9-16.0
SMC1 0.03-0.18 0.01-0.1 419-86 672-138 10.8-15.8 9.8-14.3

1μm 2μm

(a) (b)
Fig.12 SEM micrographs (a) pull-out of 0.8% of SMH1 (b) crack bridge of 0.8% of
MC1 in matrix

5. Effect of special structure and surface modification of MWCNTs

on cementitious composites

5.1 Compressive strength

Fig.13 shows compressive strength of cementitious composites reinforced with four

special types of MWCNTs. For comparison, compressive strength of cementitious

composites reinforced with untreated M5 is plotted in Fig.13. The corresponding

relative and absolute increases in compressive strength compared with control sample

are listed in Tables 19 and 20, respectively.

 180
0
Compressive strength/MPa 0.1%
150 0.5%
0.8%
120

90

60

30

0
GM5 LIM HIM
NiM5 M5
Types of MWCNT
Fig.13 Compressive strength of cementitious composites with special types of
MWCNTs

Table 19 Relative increase in compressive strength of cementitious composites with

special types of MWCNTs/%

Contents of MWCNTs M5 GM5 NiM5 HIM LIM

0.1% 32.6 0.2 64.7 5.7 4.3
0.5% 18.1 44.8 78.8 65.3 20.4
0.8% 30 56 54 50 46

Table 20 Absolute increase in compressive strength of cementitious composites with

special types of MWCNTs/MPa

Contents of MWCNTs M5 GM5 NiM5 HIM LIM

0.1% 30.7 0.2 60.7 5.3 4
0.5% 17 42.1 74 61.3 19.2
0.8% 28.2 52.6 50.7 47.1 43.2

As shown in Fig.13 and Tables 19-20, compressive strengths of samples having

special types of MWCNTs enhance greatly as compared to control sample. Addition

of 0.5% nickel-coated MWCNT (NiM5) produces the maximum compressive strength

(168MPa) in this study, which is about 79%/74MPa and 61%/57MPa higher than that

of control sample and M5 reinforced cementitious composites, respectively. For

surface-modified MWCNTs, compressive strength of cementitious composites with

NiM5 and GM5 enhance greatly as compared to untreated and same size M5.

However, reinforcement effect of nickel-coated treatment on compressive strength is

better than that of graphitized treatment. As for MWCNTs with special structure, the

highest compressive strength is achieved by helical MWCNT (HIM) reinforced

cementitious composites at content of 0.5%, which is about 65%/61MPa higher than

control sample. Compared with HIM, compressive strength of large-inner thin-walled

MWCNT (LIM) reinforced cementitious composites is relatively small.

5.2 Flexural strength

Fig.14 shows flexural strength of cementitious composites reinforced with four

special types of MWCNTs. For comparison, flexural strength of cementitious

composites reinforced with untreated M5 is also plotted in Fig.14. The corresponding

relative and absolute increases in flexural strength compared with control sample are

listed in Tables 21 and 22, respectively.

16
0 0.1%
0.5% 0.8%
Flexural strength/MPa

12

0
GM5 LIM HIM NiM5 M5
Types of MWCNT
Fig.14 Flexural strength of cementitious composites with special types of MWCNTs

Table 21 Relative increase in flexural strength of cementitious composites with

special types of MWCNTs/%

Contents of MWCNTs M5 GM5 NiM5 HIM LIM

0.1% 44.8 5.6 57.1 21.8 4.6
0.5% 37.9 28.7 64.4 28.7 43.7
0.8% 44.8 39.1 34.5 56.3 17.2

Table 22 Absolute increase in flexural strengths of cementitious composites with

special types of MWCNTs/MPa

Contents of MWCNTs M5 GM5 NiM5 HIM LIM

0.1% 3.9 0.5 4.8 1.9 0.4
0.5% 3.3 2.5 5.6 2.5 3.8
0.8% 3.9 3.4 3 4.9 1.5

It can be seen from Fig.14 that the addition of these special MWCNTs improves

flexural strength of cementitious composites. The maximum flexural strength

(14.3MPa) is obtained from 0.5% NiM5 reinforced cementitious composites, which

increases by 64.4%/5.6MPa compared with control sample. Furthermore, compared

with M5, flexural strength of NiM5 reinforced cementitious composites increase,

while ones of GM5 reinforced cementitious composites reduce. As for MWCNTs with

special structure, the highest flexural strength is achieved by HIM reinforced

cementitious composites at content of 0.8%, which is about 56%/5MPa higher than

control sample. Flexural strength of LIM reinforced cementitious composites at large

concentration enhances greatly as compared to control sample. It should be noted that

reinforcement effect of MWCNTs with the special structure on flexural strength is

relatively smaller than that on compressive strength.

Through comprehensive analysis, it is observed that these four types of special

MWCNTs have good enhancement compared with control sample in most cases.

Moreover, strength enhancement in cementitious composites reinforced with special

kinds of MWCNTs at large amount is sometimes higher than that of pristine

MWCNTs. The maximum enhancement in compressive (78.8%/74MPa) and flexural

strength (64.6%/5.6MPa) are both achieved by incorporating nickel-coated MWCNTs

at content of 0.5% into cementitious composites. Moreover, comparing the result of

this study with the results obtained by present studies [19,22-23], it is observed that,

despite the lower CNT concentration used, strength increase for NiM5 reinforced

cementitious composites is higher. As we know, there are few studies on the

mechanical properties of these special kinds of MWCNTs reinforced cementitious

composites. Therefore, this paper only makes some assumptions and provides

direction for further research. The thermal conductivity of nickel is better than that of

carbon materials, which can greatly reduce primary cracks. Moreover, nickel plating

on surface can improve hardness of CNTs. In addition, graphitized treatment is a

purification method of CNTs, which is helpful to decrease structural defects of CNTs

and enhance degree of crystallization. The distinctive structure of helical MWCNTs is

just like the ribbed bars, which increases bonding strength between CNTs and matrix.

Moreover, the average bonding strength can be expressed as:

(9)

It can be calculated through combining with formula (4)-(6). Table 23 lists the

average bonding strength of helical and graphitized MWCNTs at 0.5 wt. % level. It is

obvious that bonding strength of HIM is higher than that of NiM5. The tube wall of

LIM is very thin and about 5nm. As listed in Table 24, LIM’s number per cm3 can

reach in the range from 1.5×1014 to 3.1×1015 as the MWCNTs content is only 0.1

vol.%. The numerous LIM can form an extensive distribution network and guarantee

to fully achieve enhancement effect.

Table 23 Average bonding strength of MWCNTs at 0.5wt.% level

Type of Dimater Length Critical length Average bonding strength

CNTs /nm /μm /μm /MPa
NiM5 20-30 10-30 16.44-49.32 18.25-9.12
HIM 100-200 1-10 1.86-18.58 807.47-161.49
Table 24 Number of LIM per 1cm3 in the composites

Diameter (d)/ Length Thickness of Volume Numbers of LIM per

3
nm (l)/μm wall(h)/nm (V=πdlh) /nm 1cm3 in the composites
30-60 1-10 5 3.2×10 -6.4×10 1.5×1014-3.1×1015
4 5

5.3 XRD analysis

Fig.15 shows XRD patterns of four special types of MWCNTs reinforced

cementitious composites. For comparison, XRD patterns of cementitious composites

reinforced with untreated M5 is also shown in Fig.15. Diffraction intensity and

orientation of CH are listed in Table 25. It can be observed that CH orientation of

samples containing HIM and LIM is larger than that of untreated MWCNT, even

more than that of control sample. Moreover, the main diffraction intensity of CH

(I(001)=2039 and I(101)=1007) in cementitious composites reinforced with LIM at

content of 0.8% are also higher than that of control sample. Because the helical

MWCNTs have large out diameter and are curly, their effect on the CH orientation is

weak. LIM also have no obvious effect on restraining the directional growth of CH

because of their small tube thickness. However, CH orientation of samples containing

GM5 and NiM5 has relatively small changes compared with untreated M5. It is

concluded that structure variety of MWCNTs has significant influence on the CH

orientation. By contrast, effect of nickel plating and graphitization of MWCNTs on

CH orientation is ignorable.
 AAFt BCa(OH)2 CC3S DCaCO3
B(001)
D
C B(101)
A B C B LIM
A BC

HIM

GM5

NiM5

Control

8 16 24 32 40 48 56 64
2/°
Fig.15 XRD patterns of cementitious composites with special types of MWCNTs

Table 25 Diffraction intensity and orientation of CH in cementitious composites with

special types of MWCNTs

Sample Control M5 GM5 NiM5 HIM LIM

(001)CH 1794 1319 1542 1512 2147 2046
(101)CH 898 881 964 980 1102 963
CH Orientation 2.70 2.02 2.16 2.08 2.63 2.87

5.4 TG analysis

TG and DTG diagrams for cementitious composites reinforced with four special

types of MWCNTs are shown in Fig.16. For comparison, TG and DTG diagrams for

cementitious composites reinforced with untreated M5 are also shown in Fig.16.

Cement hydration degree of cementitious composites reinforced with special kinds of

MWCNTs is shown in Table 26. As shown in Table 26, cement hydration degree

decreases as the MWCNTs are added into cementitious composites. Compared with

untreated MWCNTs, cement hydration degree for sample having GM5, LIM and HIM
increase slightly. It is indicated that surface modification and special structure of

MWCNTs have little influence on cement hydration degree.

100 GM5 0.0000

1st derivative of mass (%)

NiM5
-0.0002
Mass Loss (%)

95 LIM
HIM -0.0004
90 Control Control
-0.0006 LIM
85 HIM
-0.0008 NiM5
GM5
80 -0.0010
200 400 600 800 1000 0 200 400 600 800 1000
Temperature(°C) Temperature (C)
(a) (b)
Fig.16 TG diagram (a) and DTG diagram (b) for cementitious composites with special
types of MWCNTs

Table 26 Hydration degree of cementitious composites with special types of

MWCNTs

Sample Control M5 GM5 NiM5 LIM HIM

Hydration degree (%) 69.5 62.7 63.2 61.5 63.2 64.8

5.5 Microstructure observation

The size and surface morphology of CH crystals in cementitious composites

reinforced without and with MWCNTs are shown in Fig.17. It can be observed that

the size of CH in cementitious composites decreases due to the addition of MWCNTs.

XRD analyses also manifest the NiM5 at content of 0.5% decreases the orientation

index of CH crystals in matrix. Thus, it can be concluded that MWCNTs enhances

strength of cementitious composites by means of lowering orientation index of CH.

Fig.18 shows SEM micrographs of HIM and GM5 at content of 0.8% in cementitious

composites. It can be observed that helical MWCNTs and graphitized MWCNTs

bundles are anchored well inside the hydration products. The MWCNTs and matrix

tightly bond with each other.

 20μm 10μm

(a) (b)
Fig.17 SEM micrographs of CH in cementitious composites (a) without and (b) with
0.5% NiM5

4μm 1μm

(a)

2μm 500nm

(b)
Fig.18 SEM micrographs of (a) 0.8% of HIM and (b) GM5 cementitious composites

6. Conclusions
 Mechanical properties of different types of MWCNTs filled cementitious

composites were studied at levels of 0, 0.1%, 0.5% and 0.8%. The results obtained

from strength, XRD, TG, SEM and theory calculation had been summarized and

analyzed to identify the reinforcing effect of different types of MWCNTs on behavior

of MWCNTs reinforced cementitious composites in this article. It is verified that

incorporation of well-dispersed MWCNTs has significant enhancement for

mechanical properties of cementitious composites, while reinforcement effect of

different types of MWCNTs on cementitious composites is different from each other.

(1) Size of MWCNTs has a certain influence on strength of cementitious

composites. Compressive strength of cementitious composites with large-diameter

MWCNTs is higher than that of composites with small-diameter MWCNTs. Long

MWCNTs are better for improving flexural strength than short MWCNTs. However,

the largest increase in compressive (47%) and flexural strength (55%) compared with

control sample were both obtained by incorporating short and large-diameter

MWCNTs into cementitious composites. It is therefore concluded that MWCNTs with

short length (0.5-2μm) and large diameter (20-30nm) are suggested as reinforcement

of cementitious composites, and their optimum dosage is about 0.1%.

(2) Functionalized MWCNTs results in better strength of cementitious composites

than untreated MWCNTs. Moreover, hydroxyl-functionalized MWCNTs have a better

reinforcement effect on strength of cementitious composites than

carboxyl-functionalized MWCNTs. The highest increases in compressive (64.6%) and

flexural (76%) strength are obtained from hydroxyl-functionalized M1 and SM1

reinforced cementitious composites, respectively. Therefore, functionalized MWCNTs

with OH or COOH groups, especially OH group, are recommended to modify

cementitious composites, and their optimum dosage is about 0.5%.

 (3) Nickel-coated MWCNTs have better effect on strength than graphitized

MWCNTs. On the other hand, MWCNTs with helical structure have stronger effect

on strength than large-inner thin-walled MWCNTs. The maximum enhancement in

compressive (78.8%/74MPa) and flexural strength (64.6%/5.6MPa) are both achieved

by cementitious composites with 0.5% NiM5. It is, therefore, concluded that nickel

coated and helical MWCNTs are more suitable for modifying the mechanical property

of cementitious composites, and their optimum dosages are about 0.5% and 0.8%,

respectively.

(4) The reinforcement effect can be attributed to extensive distributing meshwork in

cement matrix, crack bridging, fiber pull-out effect, lowering orientation index of CH

crystal in hydration products and decreasing cement hydration degree. XRD analyses

indicate that orientation index of CH crystal in cementitious composites is decreased

due to the addition of MWCNTs. TG analyses suggested that MWCNTs have certain

inhibiting effect on the cement hydration due to adsorption effect of MWCNTs.

Decrease of hydration degree is helpful to lower hydration heat, thus reducing

primary cracks. In addition, phenomena of crack bridging and fiber pull out were

observed in SEM micrographs.

Acknowledgments

The authors thank the funding supported from the National Science Foundation of

China (51578110 and 51428801).

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