Microstructure of cement mortar with nano-particles

Hui Li*, Hui-gang Xiao, Jie Yuan, Jinping Ou

Harbin Institute of Technology, School of Civil Engineering, Harbin, 150090 People’s Republic of China

Received 10 August 2002; revised 20 March 2003

Abstract

The mechanical properties of nano-Fe2O3 and nano-SiO2 cement mortars were experimentally studied. The experimental results showed

that the compressive and flexural strengths measured at the 7th day and 28th day of the cement mortars mixed with the nano-particles were

higher than that of a plain cement mortar. Therefore, it is feasible to add nano-particles to improve the mechanical properties of concrete. The

SEM study of the microstructures between the cement mortar mixed with the nano-particles and the plain cement mortar showed that the

nano-Fe2O3 and nano-SiO2 filled up the pores and reduced CaOH2 compound among the hydrates. These mechanisms explained the supreme

mechanical performance of the cement mortars with nano-particles.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Nano-particles; Cement mortar; Microstructure; Mechanical properties

1. Introduction

Due to an ultrafine size, nano-particles show unique

physical and chemical properties different from those of

the conventional materials. Because of their unique

properties, nano-particles have been gaining increasing

attention and been applied in many fields to fabricate new

materials with novelty functions. Among all nano-

materials, the carbon nanotube composites are the most

abstracting [1]. Besides carbon nanotube composites, Ag/

Si3N4 nanostructured composites that were fabricated by

dry pressing showed unusual electric properties [2]. SiC

nanosized particles into Si3N4 matrix could lead to a

considerable improvement on its mechanical properties

[3]. If nano-particles are integrated with traditional

building materials, the new materials might possess

outstanding or smart properties for the construction of

super high-rise, long-span or intelligent civil infrastructure

systems. However, the present applications are limited to

produce antiaging, antiseptic, purified air composite paint

or other ecological building materials using nano-TiO2,

nano-SiO2 or nano-Fe2O3. There are few reports on

incorporating nano-particles in cement-based building

materials.

In view of the above-mentioned, cement mortars mixed

with nano-Fe2O3 or nano-SiO2 have been studied by the

authors to explore their super mechanical and smart

(temperature or strain sensing) potentials. This paper only

reports the mechanical properties and the SEM observation

of the mortars mixed with nano-Fe2O3 or nano-SiO2. It was

found that the compressive and flexural strengths of the

cement mortars with nano-particles were higher than those

of a plain cement mortar. The microstructure of the mortar

mixed with nano-particles was improved that was in

agreement with the strength enhancement. Therefore, it is

feasible to add nano-particles to make high performance and

smart concrete.

2. Materials and methods

2.1. Materials

A cement paste is composed of small grains of hydrated

calcium silicate gels, nanosized individual pores, capillary

pores (structural defects), and large crystals of hydrated

products. There should be rooms for nanophase materials to

improve the properties of pure cement paste [4]. However,

as nano-particles are easy to aggregate due to their great

surface energy, large quantity of these particles cannot be

uniformly dispersed. In this pilot study, the nano-particle

1359-8368/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S1359-8368(03)00052-0

Composites: Part B 35 (2004) 185–189

www.elsevier.com/locate/compositesb

* Corresponding author. Tel./fax: þ86-451-6282013.

E-mail addresses: lihui@hit.edu.cn, lihui6848@sina.com (H. Li).

contents in the mortar specimens were 3, 5 and 10% by

weight of cement.

Although there are various nanophase materials supplied

by some companies in China, nano-SiO2 and nano-Fe2O3

only were used in this study. The nano-SiO2 was purchased

from Zhoushan Mingri Nanophase Material Company,

Zhejiang province, China, and its main properties were

given in Table 1. The nano-Fe2O3 with a mean grain

diameter of 30 nm anda phase was purchased from

Fangyuan Nanophase Material Institute, China. The cement

used was Portland cement (P.O32.5). UNF-water reducing

agent (UNF) was added to disperse the nano-particles, and

defoamer was also used to decrease the amount of air

bubbles.

Eleven mixtures were cast with different mix proportions

(see Table 2). The water/binder (w/b) ratio for all mixtures

was 0.5, where the binder weight is the total weight of

cement, nanophase materials and silica fume.

2.2. Methods

A rotary mixer with a flat beater was used for mixing.

Defoamer and dispersant agent (if applicable) were

dissolved in water and then the nano-particles were added

and stirred at high speed for about 2 min. Then the cement

and silica fume (if applicable) were added to the mixer and

stirred for another 1 min. Afterwards, sand was added into

the mixture and auto-stirred for about 1.5 min. The well-

mixed mortar was poured into molds to form the cubes of

size 4 £ 4 £ 4 cm for all mixing proportion for compressive

testing and prisms of size 4 £ 4 £ 16 cm for mixtures A, B1,

B2, C1, C2 and D only for flexural testing. An external

vibrator was used to facilitate compaction and decrease the

amount of air bubbles. The samples were demolded after

24 h and then cured in air at room temperature for 7 and 28

days, respectively.

Six cubic specimens were made from each mixture.

Three cubes were tested at the 7th day and the other three

were tested at the 28th day to observe the influence of

different age strengths of mortars with nano-particles.

However, three prism specimens for mixtures B1, C1, and

D were cast for the flexural strength test at the 28th day.

Compressive testing for the strength at the 7th and 28th

day was performed using a hydraulic mechanical testing

system (MTS) under load control. Bending testing for

flexural strength at the 7th day and at the 28th day was

carried out on the long surface of prism specimens using a

bend tester under load control. In addition, the compressive

testing for strength of the specimens after the bending test

for mixtures A, C1, C2 and D was also conducted at the 28th

day, and the results so obtained were compared with those

of the cube specimens. After the mechanical tests, the

crushed specimens were selected for scanning electronic

microscope (SEM) tests.

3. Test results and discussions

3.1. Strength

Table 3 shows the compressive strength of eleven mortar

mixtures. It can be seen that the compressive strengths of

specimens with mixtures B1, B2 and B3 at the 7th day and

28th day were all higher than that of plain cement mortar

with the same w/b, as for the strength at the 28th day. The

effectiveness of the nano-Fe2O3 in increasing strength

Table 1

The properties of nano-SiO2

Item Diameter

(nm)

Surface-to-volume

ratio (m2/g)

Density

(g/cm3)

Purity

(%)

Target 15 ^ 5 15 ^ 5 0.15 99.9

Table 2

Mix proportion of the specimens

Mixture no W/b Mix proportion of the specimens (per 768 cm3)

Water

(ml)

Cement

(g)

Sand

(g)

Nano-SiO2

(g)

Nano-Fe2O3

(g)

UNF

(g)

Silica fume

(g)

A 0.5 225 450.0 1350 – – – –

B1 0.5 225 436.5 1350 – 13.5 3.4 –

B2 0.5 225 427.5 1350 – 22.5 6.5 –

B3 0.5 225 405.0 1350 – 45.0 11.2 –

C1 0.5 225 436.5 1350 13.5 – 6.8 –

C2 0.5 225 427.5 1350 22.5 – 11.2 –

C3 0.5 225 405.0 1350 45.0 – 22.5 –

D 0.5 225 427.5 1350 9.0 13.5 7.9 –

E1 0.5 225 427.5 1350 – 9.0 3.0 13.5

E2 0.5 225 405.0 1350 – 18.0 6.0 27.0

F 0.5 225 382.5 1350 – – 3.4 67.5

H. Li et al. / Composites: Part B 35 (2004) 185–189186

increased in the order: B1 . B2 . B3 (with the decrease on

nano-Fe2O3 volume fraction). Furthermore, that the strength

enhancement for B3 at the 7th day is evidently higher than

that at the 28th day. These results indicated that the optimal

content of nano-Fe2O3 for reinforcing concrete purposes

should be less than 10% (by weight of cement) under the

present dispersion condition.

The compressive strengths of the specimens with

mixtures C1, C2 and C3 at the 7th day and 28th day were

higher than that of the plain cement mortar with the same

w/b too. The effectiveness of the nano-SiO2 in increasing

strength increased in the order: C3 . C2 . C1 (with the

increase on nano-SiO2 volume fraction), which is opposite

to those for the nano-Fe2O3 test series.

Comparison of the strength of the cement mortars with

and without silica fume are listed in Table 3, it indicates that

the nano-particles are more valuable in providing strength-

ening than silica fume.

Table 4 shows the flexural strength at the 7th day and

28th day. It is increased by the addition of nano-SiO2 or

nano-Fe2O3. The effectiveness of the nano-particles in

increasing the flexural strength increased in the order: B1 .

B2 and C1 . C2, respectively.

The strength of the mortar mixed with nano-SiO2 and

nano-Fe2O3 together was lower than that of the mortar

mixed with only nano-SiO2 or nano-Fe2O3

3.2. Microstructure and discussion

Figs 1–5 show the microstructure of cement pastes with

and without nano-SiO2 and nano-Fe2O3. It was found that in

Fig 1 (the microstructure photograph of the plain cement

paste) that C–S–H gel existed in the form of ‘stand-alone’

clusters, lapped and jointed together by many needle

hydrates. At the same time, deposit CaOH2 crystals were

distributed among the cement paste. The Figs 2–4 show the

microstructures of mixtures B1, C1 and D, which are of

higher strength. They were different from that of the plain

cement paste, i.e., the texture of hydrate products was more

dense and compact. Big crystals such as CaOH2 were

absent. Although the cement paste pattern of these three

mixtures showed some differences, their microstructures

were uniform and compact.

The microstructure of the mixture B3, shown in Fig 5,

was different from those of the mixtures B1, C1 and

D. However, similar with that of the plain cement paste, i.e.

various hydrate products co-existed in different forms. Such

microstructure was consistent with the corresponding just

3.7% strength enhancement (listed in Table 3).

The mechanism that the nano-particles could improve

the microstructure and strength of cement paste can be

illustrated as follows. When a small quantity of

Table 3

Compressive strength of mixtures

Mixture no Compressive strength

at the 7th day

Compressive strength

at the 28th day

Target

(MPa)

Enhanced

extent (%)

Target

(MPa)

Enhanced

extent (%)

A 17.6 0 28.9 (29.7) 0

B1 21.4 22.7 36.4 26.0

B2 20.6 16.7 33.1 14.5

B3 21.1 20.0 30.0 3.7

C1 18.6 5.7 32.9 (33.4) 13.8 (12.6)

C2 21.3 20.1 33.8 (34.1) 17.0 (14.7)

C3 21.3 20.1 36.4 26.0

D 22.4 27.0 35.4 (34.8) 22.0 (17.1)

E1 19.4 10.0 29.8 3.0

E2 23.2 32.0 34.3 18.6

F 18.9 7.4 31.8 10.0

Note that the data in bracket were obtained from the corresponding

mixtures of 4 £ 4 £ 8 cm (by-specimens after bending test of specimens

with dimension 4 £ 4 £ 16 cm) at the 28th day.

Table 4

Flexural strength of mixtures at the 7th day and 28th day, respectively

Mixture

no

Flexural strength

at the 7th day

Flexural strength

at the 28th day

Target

(Mpa)

Enhanced

extent (%)

Target

(MPa)

Enhanced

extent (%)

A 3.28 0 4.9 0

B1 – – 5.8 17.8

B2 4.3 30 6.0 23.0

C1 – – 5.8 19.2

C2 4.2 28 6.2 27.0

D – – 6.0 21.8Fig. 1. SEM photograph of mixture A.

H. Li et al. / Composites: Part B 35 (2004) 185–189 187

the nano-particles were uniformly dispersed in the cement

paste, the hydrate products of cement will deposit on the

nano-particles due to their great surface energy during

hydration and grow to form conglomeration containing the

nano-particles as ‘nucleus’. The nano-particles located in

the cement paste as nucleus will further promote and

accelerate cement hydration due to their high activity. In the

consideration of the nano-particles uniformly disperse

situation, a good microstructure could be formed with the

uniformly distributed conglomeration. At the same time,

according to Wu’s ‘centroplasm’ hypothesis, the aggre-

gates, sands and other particles are considered as centro-

plasm that acts as skeleton, and gel as transmitter substance.

The binding force between centroplasm and transmitter

substance has an important effect on the strength of concrete

[5]. Innumerable nano-particles distributing in cement paste

as ‘sub-centroplasm’ can tightly bond with the hydrated

Fig. 2. SEM photograph of mixture B1.

Fig. 3. SEM photograph of mixture C1.

Fig. 4. SEM photograph of mixture D.

Fig. 5. SEM photograph of mixture B3.

H. Li et al. / Composites: Part B 35 (2004) 185–189188

products around the transition zone between the nano-

particle and hydrate products. On the other hand, the nano-

particles among the hydrate products will prevent the crystal

from growing, such as CaOH2 and AFm, and such fine

crystals are favorable for the strength of cement paste [6–8].

Also, the nano-particles will fill pores to increase the

strength as silica fume does. However, when the nano-

particles cannot be well dispersed, as the case of extensive

nano-particles content, the aggregating nano-particles will

create weak zone, in form of voids. Consequently, the

homogeneous hydrate microstructure could not be formed,

and low strength will be expected. On the other hand, as the

nano-SiO2 can participate in the hydration process to

generate C–S–H through reacting with CaOH2, the small

quantity of aggregating nano-SiO2 will not be a weak zone,

so the strength increases with the content of nano-SiO2

increases even when small quantity of nano-SiO2 is not very

well dispersed.

The strength of the cement mortars with nano-particles

has a preferably improvement, as demonstrated in this

study. Furthermore, it can be predicted that the strengthen-

ing effect of nano-particles would be further enhanced in

concrete because the nano-particles improve not only the

cement paste, but also the interface between paste and

aggregates.

4. Conclusions

The compressive and flexural strength of the cement

mortars with nano-SiO2 and with nano-Fe2O3 were both

higher than that of the plain cement mortar with the same

w/b. The SEM observations also revealed that the

nano-particles were not only acting as a filler, but also as

an activator to promote hydration proves and to improve the

microstructure of the cement paste if the nano-particles

were uniformly dispersed. The optimum mixing volume of

different nano-particles was not the same due to different

functions. Further study in this direction is recommended.

Acknowledgements

This study was financially supported by NSFC grant No.

50238040, and Ministry of Science and Technology grant

No. 2002AA335010 together.

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