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Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2
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1. Introduction
Nano-particles have been gaining increasing attention
and been applied in many fields to fabricate new materials
with novelty function due to their unique physical and
chemical properties. If nano-particles are integrated with
cement-based building materials, the new materials might
possess some outstanding properties. The pozzolanic
activity of nano-SiO2 is more obvious than that of silica
fume. Nano-SiO2 can react with calcium hydroxide
(Ca(OH)2) crystals, which are arrayed in the interfacial
transition zone (ITZ) between hardened cement paste and
aggregates, and produce C–S–H gel. Thus, the size and
amount of calcium hydroxide crystals are significantly
decreased, and the early age strength of the hardened
cement paste is increased [1–3]. Nano-SiO2 can behave as a
nucleus to tightly bond with cement hydrates. The stable gel
structures can be formed and the mechanical properties of
hardened cement paste can be improved when a smaller
amount of nano-SiO2 is added [4]. Nano-SiO2 can improve
the pressure-sensitive properties of cement mortar [5,6]. Fly
ash concrete with nano-SiO2 has the higher density and
strength [7]. Ref. [8] indicated that high-strength concrete
with nano-SiO2 has higher flexural strength.
Penetrability in cement mortars and concrete (such as
high-strength lightweight aggregate concrete, and concrete
containing high-reactivity metakaolin, et al.) were studied
extensively [9–14]. However, up to now, there is no
published report on the durability of concrete. A water
permeability test and an ESEM test were performed to
investigate the durability of concrete with nano-SiO2 in this
paper. The research results reveal that nano-SiO2 can
improve the microstructure of the interfacial transition zone
(ITZ) between aggregates and binding paste matrix and the
water permeability resistant capacity of concrete.
2. Experimental investigation
2.1. Material properties
All materials used in these experiments are produced in
China. The cement is the grade 42.5 Porland cement, and a
class 1 fly ash (Chinese Standard) is used, as shown in Table
1. The superplasticizer TW-7 is naphthalene-type with a
solid content of 40%. The coarse aggregates used are the
continuous grading crushed gravels, with the maximum
0008-8846/$ - see front matter D 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cemconres.2005.07.004
* Tel.: +86 591 83736529; fax: +86 591 83737442.
E-mail address: jt72[at]163.com.
Cement and Concrete Research 35 (2005) 1943 – 1947
particle size of 26.5 mm. The fine aggregates are river
sands, with a fineness modulus of 2.7. Nano-SiO2 is
produced by Zhoushan Mingri Nanophase Material Company,
Zhejian province, China. The properties of nano-SiO2
are shown in Table 2.
2.2. Mix proportions
‘‘NC’’ and ‘‘SC’’ are the mix proportions of normal
concrete and concrete with nano-SiO2 (called the nano-SiO2
concrete), respectively. In the nano-SiO2 concrete, part of
cement is replaced by nano-SiO2. Due to the vast difference
between the density of nano-SiO2 and that of cement, the
wet density of each concrete was measured to adjust the mix
proportions to give 1 m3 of concrete, as reported in Table 3.
The content of superplasticizer TW-7 in the nano-SiO2
concrete is determined according to a principle that the
slumps of the normal concrete and the nano-SiO2 concrete
are the same.
2.3. Concrete mixture test
2.3.1. Testing procedure
The superplasticizer TW-7 was dissolved in water, and
then the nano-SiO2 was added and stirred at a high speed for
2 min. Though nano-SiO2 can not be dissolved in water, a
smaller amount of nano-SiO2 can be dispersed evenly by the
superplasticizer TW-7.
The cement, fine aggregates, coarse aggregates and fly
ash were mixed in a rotary mixer for 30 s. The ready-mixed
liquid including water, TW-7 and nano-SiO2 was poured
into the rotary mixer slowly. The concrete mixture was
mixed for another 1.5 min. After mixing, the slump and the
slump flow of the concrete mixture were measured. The
well-mixed concrete mixture was poured into molds to form
the cubes of the size 151515 cm for the compressive
strength testing. The samples were demolded after 24 h and
then cured in a curing cabinet (relative humidity in excess of
95%, temperature 20T3) for 28 days.
2.3.2. Testing results
Table 4 shows the slump, the slump flow and the 28-day
strength of the normal concrete (NC) and the nano-SiO2
concrete (SC). It can be seen that the slumps of NC and SC
are the same, while the slump flow of NC is larger than that
of SC. Namely the ratio of slump to slump flow of NC is
larger than that of SC. It means that the nano-SiO2 concrete
is stickier than the normal concrete [15].
2.4. Water permeability test
2.4.1. Testing method
The HS-4 type concrete permeability device produced by
Sheng Fei Testing Mechanical Factory in Zhejiang province
of China was used. The testing procedure was in consistent
with GBJ 82-85 [16].
The tapered cylinders with a height of 150 mm and a
diameter of 175 mm at one end and 185 mm at the other end
were used to determine the water permeability of NC and
SC. Each of mixtures had six such tapered cylinders. The
tapered cylinders were demolded after 24 h and then cured
in a standard curing cabinet. The water permeability test was
performed at the curing age of 28 days.
The tapered cylinders were taken out from the cabinet at
the curing age of 27 days and dried in the air. The side of
each tapered cylinder was sealed with moisture-insensitive
Table 2
Properties of nano-SiO2
Item Diameter
(nm)
Surface– volume
ratio (m2/g)
Density
(g/cm3)
Purity
(%)
Target 15T5 160T20 <0.15 >99.9
Table 1
Characteristics of cement and fly ash
Characteristics Cement Fly ash
Density (g/cm3) 3.18 2.55
Special area (cm2/g) 4168 6910
Bulk density (g/cm3) 1.101 0.741
Ratio of water requirement (%) – 84.8
Special surface mean diameter (Am) 15.32 12.33
Table 3
Mix proportions
Mixture no. Nano-SiO2
(kg/m3)
Cement
(kg/m3)
Fly ash
(kg/m3)
Water
(kg/m3)
Sand
(kg/m3)
Gravel
(kg/m3)
TW-7
(kg/m3)
Density
(kg/m3)
NC 0 389 80 190 654 1100 5.2 2418
SC 13.9 370 79 188 647 1088 13.5 2400
Table 4
Test results
Mixture no. Slump
(mm)
Slump flow
(mm)
The ratio slump
to slump flow
Compressive strength
of 28 days (MPa)
Water pressure
H (MPa)
The average penetration
depth (mm)
The permeability
grade S
NC 175 420 0.417 47.5 0.5 146 S4
SC 175 405 0.432 44.0 >3.2 81 >S12
1944 T. Ji / Cement and Concrete Research 35 (2005) 1943– 1947
epoxy to prevent the water leakage. The tapered cylinders
were fixed into the permeability test rigs of the HS-4 type
concrete permeability device pre-heated in an oven. Then
the rigs were installed in the permeability device for testing.
At the beginning, the water pressure of 0.1 MPa was
applied at the bottom of the specimens. Then, at interval of 8
h, the additional water pressure of 0.1 MPa was added. If the
top surface of tapered cylinders is carefully observed by
eyes to be wet due to water penetration, it can be said that
the water penetration phenomenon rises. If three of the six
cylinders had the water penetration phenomenon, the test
was stopped, and the water pressure was recorded. At the
end of the test, each specimen was removed from the rig and
split into two halves lengthways for determining the water
penetration depth. Average depth was taken from five
equidistant spots along each face of the split specimens.
2.4.2. Testing results
When the water pressure H reached 0.5 MPa, three of the
six cylinders of NC had the water penetration phenomenon.
The average penetration depth was 14.6 cm as listed in
Table 4. While for SC, when water pressure H of 3.2 MPa
was applied, there were still only two cylinders having the
water penetration phenomenon. Then the test was stopped.
The average penetration depth of 8.1 cm was measured and
reported in Table 4.
The permeability grade S (GBJ 82-89) is defined as
S ¼ 10H 1 ð1Þ
where H is the water pressure when the water penetration
phenomenon of three of the six cylinders appears. The
permeability grades of NC and SC are also listed in Table 4.
It can be seen that nano-SiO2 can improve the water
permeability resistant capacity of concrete.