مرکزی صفحہ Construction and Building Materials Stabilization of residual soil using SiO2 nanoparticles and cement

Stabilization of residual soil using SiO2 nanoparticles and cement

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جلد:
64
زبان:
english
رسالہ:
Construction and Building Materials
DOI:
10.1016/j.conbuildmat.2014.04.086
Date:
August, 2014
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Construction and Building Materials 64 (2014) 350–359

Contents lists available at ScienceDirect

Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat

Stabilization of residual soil using SiO2 nanoparticles and cement
Sayed Hessam Bahmani a,⇑, Bujang B.K. Huat a, Afshin Asadi b, Nima Farzadnia b
a
b

Department of Civil Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor Darul Ehsan, Malaysia
Housing Research Centre, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor Darul Ehsan, Malaysia

h i g h l i g h t s
 Compaction characteristics and consistency of the soil were improved by nanosilica.
 Compressive strength of the soil increased under effect of nanosilica.
 Hydraulic conductivity decreased when nanosilica was added.
 pH of cement treated soil decreased when nanosilica was added.

a r t i c l e

i n f o

Article history:
Received 5 July 2013
Received in revised form 4 April 2014
Accepted 8 April 2014

Keywords:
Nanosilica
Cement treated residual soil
Mechanical properties
Chemical
Microstructural properties

a b s t r a c t
An experimental study was performed to determine the effect of SiO2 nanoparticles on consistency, compaction, hydraulic conductivity, and compressive strength of cement-treated residual soil. Also, SEM, XRD
and FTIR tests were carried out to identify the underlying mechanisms. The addition of nanoparticles was
found to advantageously affect the compactability, hydraulic conductivity. Besides, addition of 0.4%
nanosilica to the cement treated soil improved the compressive strength by up to 80%. XRD, FTIR and
SEM test results showed that silica nanoparticles promoted the pozzolanic reaction by transforming
Portlandite into calcium silicate hydrate (C–S–H) gel.
Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction
The use of stabilization techniques has increased significantly in
recent decades owing to new construction sites, increasingly being
located in areas of poor quality gr; ound. It is suggested that ground
improvement will be critically important in future geotechnical
practices to adopt cost-effective solutions, to achieve reductions
in quantities of material used and etc. [1–3]. One of the extensively
used techniques for the improvement of problematic soils in
relatively tropical countries is soil treatment with customary
cementitous additives such as cement, lime and fly ash.
Cement is often used as an additive to improve strength and
stiffness of residual soils in tropical areas. To achieve the maximum
possible strength for base construction, addition of 6–10% cement
in residual soils with plasticity indexes in the range of 10–20 has
been recommended [4–6]. Furthermore, benefits of cement treated
soil are not only limited to its enhanced strength but also the
compressibility of the cement-treated soil has much higher
⇑ Corresponding author. Tel.: +60 173180150; fax: +60 3 8946 4232.
E-mail address: h.bahmani.eng@gmail.com (S.H. Bahmani).
http://dx.doi.org/10.1016/j.conbuildmat.2014.04.086
0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.

pre-consolidation pressure than that of the untreated soil. High
pre-consolidation pressure leads to a sharp decrease in the void
ratio and permeability of the soil [5,7]. In regions where problems
of groundwater intrusion exist, alteration of the permeability is
often an important factor in the use of cement stabilization to construct cut-off walls [8,9]. So far, the effect of cement on some influential factors such as water content, curing time, and compaction
energy and its role on the microstructure and engineering characteristics of cement-treated soils have also been extensively studied
[6,10–12]. Improvement of the properties of cement-treated soil
has been mainly attributed to a soil–cement reaction [10,13],
which produces primary and secondary cementitious materials in
the soil–cement matrix [5,7,14]. The primary cementitious materials are formed by hydration reaction and are comprised of
hydrated calcium silicates (C2SHx, C3S2Hx), calcium aluminates
(C3AHx, C4AHx), and hydrated lime Ca(OH)2 [15–17]. A secondary
pozzolanic reaction between hydrated lime, silica and alumina
from the clay minerals leads to the formation of additional calcium
silicate hydrates and calcium aluminate hydrates. This soil–cement
reaction provides a clear basis by which to explain the improvement in strength of stabilized soil.

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S.H. Bahmani et al. / Construction and Building Materials 64 (2014) 350–359

In recent years, nanoparticles have attracted considerable scientific interest for many civil engineering applications. The types of
nanoparticles that are most commonly used in cementitous composites are SiO2, TiO2, Al2O3, and carbon nanotubes [18–21]. Of
all the introduced nanoparticles, nano-SiO2 plays the most significant role. Nanoparticles of SiO2 exhibit high pozzolanic activity due
to high amount of pure amorphous SiO2 [21–23]. According to
Sobolev et al. [20], the changes observed in mixtures modified with
nano-SiO2 particles are the result of a chemical reaction between
SiO2 and Ca(OH)2 during cement hydration. Furthermore, nanosilica accelerates hydration of cement due to its high surface energy
[20,21,24]. Also, nanosilica causes physical alterations such as
improvement in the packing density which corresponds to filling
effect of its particles [18,25–27]. Another known physical mechanism is nucleation effect by which hydration products envelop
the particles and hence a denser matrix with better distributed
hydration products is formed [24,28,29]. Experimental results have
shown that a joining effect of physical and chemical properties of
nanosilica results in up to 20% strength augmentation of cementitous composites [24].
This study addresses the development of cement-treated residual soil strengthened with nanosilica as a supplementary material.
Inclusion of nanosilica may reduce the cement consumption in the
soil and accelerate the stabilization process. This study tends to
investigate changes in consistency, compaction and hydraulic conductivity as well as unconfined compressive strength of cement
treated residual soil loaded with SiO2 nanoparticles. To further
elaborate the results, induced microstructural changes were also
traced. Apart from clarifying the underlying mechanisms that lead
to changes in engineering behaviour of residual soils due to the
inclusion of SiO2, the results may also be representative of the
engineering behaviour of other low plasticity soils after stabilization with cement and nano-SiO2.

Table 1
Properties of the residual soil.

2. Materials and methods

2.1.3. Portland cement
An ordinary Portland cement (OPC Type I) in compliance with ASTM C150,
obtained from the cement manufacturing company (Phoenix) in Malaysia, was used
in this study. The physical and chemical properties of the cement are given in
Table 3. The particle size distribution of the Portland cement particles, as determined by the BET method, is illustrated in Fig. 3. The specific gravity of the cement
is 1.7 g/cm3.

2.1. Materials
2.1.1. Soil
A typical residual soil, Malaysian granite soil, was used in this study. This soil
was tested to determine its physical properties—its specific gravity, liquid limit
(LL), plastic limit (PL), shrinkage limit and grain size distribution—using standard
procedures specified in BS 1377-2 (1990) [30]. The particle size distribution curve
for the soil is shown in Fig. 1. Table 1 shows the classification properties of the soil,
which is an inorganic clay with high plasticity (CH). The consistency limits of the
soil are a LL of 51.4% and a PL of 30%. The maximum dry density (MDD) and optimum moisture content (OMC) are 15.1 kN/m3 and 20%, respectively. A characteristic X-ray diffraction (XRD) plot of the soil, shown in Fig. 2, indicates that the soil is
predominantly a kaolinite clay mineral with a strong diffraction line at 3.6 A°,
which disappears when the clay is heated to 550 °C.
2.1.2. Nanosilica
To investigate the effects of different sizes of SiO2 nanoparticles on the properties of cement treated soil, particles with two different sizes of 15 nm and 80 nm in
powder form were purchased from Nanostructure & Amorphous Materials, Inc.,
(USA). Table 2 shows the chemical and physical properties of nanosilica particles.

Properties

Value

Physical properties
Natural water content (%)
Liquid limit (%)
Plastic limit (%)
Plasticity index (%)
Linear shrinkage (%)

21
51.48
30
20.48
12.12

Compaction properties
Maximum dry unit weight (kN/m3)
Optimum water content (%)
pH
Specific gravity
Unified soil classification system (USCS)

15.1
20
4.01
2.63
CL

Chemical properties
Silica (SiO2) (%)
Alumina (AL2O3) (%)
Iron oxide (Fe2O3) (%)
Potash (K2O) (%)
Magnesia (MgO) (%)
Loss in ignition (%)

71.3
15.55
6
1.5
0.17
1

Fig. 2. X-ray diffraction of the residual soil.

2.2. Laboratory tests
2.2.1. Atterberg limits
The Atterberg limits of the soil were determined in accordance with BS 1377-2
[30]. The residual soil was graded using a sieve with a diameter of 425 mm. The particles retained on the sieve were rejected. The particles smaller than 425 mm were
then oven dried for at least 2 h prior to testing. Atterberg limit tests were carried
out on the soils with different proportions of cement and nanoparticles.
2.2.2. Sample preparation
A modified Proctor compaction tests were carried out using a mini compaction
apparatus devised by Sridharan and Sivapullaiah [31]. The apparatus consisted of a
mould with an internal diameter of 48 mm and a height of 98 mm with a falling
hammer weighing 1.0 kg. Forty blows per layer were applied to three layers of soil
[31]. This apparatus is simple and quick to use, requires comparatively little effort,
and saves on soil. Samples for strength tests can be obtained quickly and with minimal disturbance. The compaction tests were carried out on the residual soil,
cement treated soil with 4%, 6%, and 8% cement with 0%, 0.2%, 0.4%, 0.8%, and 1%
nanosilica to evaluate the compaction properties of untreated and treated soils.
All the proportions are measured as percentage by weight of dry soil.

Table 2
The physical properties of SiO2 nanoparticles (adapted from Nanostructured &
Amorphous Materials, Inc., USA).

Fig. 1. Particle size distribution of the residual soil.

Diameter (nm)

Specific surface area (m2/g)

Density (g/cm3)

Purity (%)

15 ± 3
80 ± 9

640 ± 12
440 ± 32

<0.14
<0.14

>99.9
>99.9

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Table 3
Properties of the cement.
Properties

Cement

Physical properties
Specific gravity (g/cm3)
Fineness
Chemical composition
Silica (SiO2) (%)
Alumina (Al2O3) (%)
Iron oxide (Fe2O3) (%)
Calcium oxide (CaO) (%)
Potash (K2O) (%)
Magnesia (MgO) (%)
Sulphur trioxide (S03) (%)
Sodium oxide (Na2O) (%)
Loss on ignition (%)

1.7
3.12
21.89
5.3
3.34
53.27
0.98
6.45
3.67
0.18
3.21

Fig. 4. Sketch for the hydraulic conductivity test, falling-head method.

and allowed to stand for at least 8hr. The values of pH were then measured using
a calibrated pH probe. The pH outputs were determined as the average of three
measurements of the same samples.
Fig. 3. Particle size distribution of the cement.

The nanoparticles were mixed with distilled water using a magnetic stirrer at
120 rpm [32]. The mixture was then sprayed on the different samples to exchange
moisture among the particles, forming a homogeneous blend and preventing
agglomeration of the nanoparticles [33–36]. The mixtures were kept in sealed plastic bags for 24 h. The specimens were then prepared at 95% maximum dry density
(MDD) and on the wet side of optimum moisture content (OMC) with the mini compaction apparatus. The remoulded specimens were then cured in a plastic bag to
avoid evaporation at a temperature of approximately 23 °C and 90% humidity for
seven days as in BS 1377-5 (1990) [30].

2.2.3. Hydraulic conductivity
Hydraulic conductivity is a measure of the rate at which water can flow through
a soil or aggregate. The hydraulic conductivity of the residual soil with varying
amounts of different nanoparticles size of 15 and 80 nm was measured by falling
head test [37]. The hydraulic meter stand consisted a metal frame with a water
tank. This allowed monitoring the extent of hydraulic gradient, ‘‘i’’, applied on top
of the tested specimen. The value of ‘‘i ‘‘was calculated as the ratio of total head
of water under motion to the length of tested specimen. After opening the inlet
water valve on top of the cell, outflow was observed to ensure a continuous flow
regime where water constantly trickles out from the outflow valve (Fig. 4). After
ensuring continuous flow, the value of k was determined as follows:



Kðm=sÞ ¼

a
L
h1

 ln
A Dt
h2


ð1Þ

where a (cm2) is the cross-sectional area of the inlet water valve, A (cm2) the crosssectional area of specimen, L (cm) the height of specimen, and Dt (s) the time needed
for the total head to drop from clearly marked graduations h1 to h2 (Fig. 4).

2.2.4. pH value
The pH value of the soil specimens with different proportions of nanoparticles
were determined in accordance with BS 1377-2 (1990) [30]. This test describes the
procedure for determining the pH value, by the electrometric method, which gives a
direct reading of the pH value of a soil suspension in water. 30 g of soil specimens
was placed in a 100 ml beaker, then 75 ml of distilled water was added to the
beaker and the suspension was stirred for a few minutes. Then, it was covered

2.2.5. Unconfined compressive strength tests
Unconfined compressive strength tests were performed on the cylindrical specimens at 7 days, in accordance with BS 1377-7 [30]. A compression testing machine
with 0.2 N sensitivity and rating load of 1.5%/min was used in the test. Smooth
metal sheets were placed at the bottom and top of each specimens during the
unconfined compression test to minimise end effects [4–6,10,38,39]. The tests were
repeated on at least three identical specimens to minimise possible errors caused
by variation in the material and testing conditions and the average value was used
in the reports.
2.2.6. Chemical and microstructural tests
To understand the underlying mechanisms of the effects of nano-SiO2 particles
on cement-treated residual soil, scanning electron microscope (SEM) analyses,
X-ray Diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) of the
treated and untreated specimens were carried out by a Hitachi 4100 Field Emission
Scanning Electron Microscope (FESEM), Shimadzu XD-D1, X-ray Diffractometer and
Shimadzu’s IRPrestige-21 Fourier Transform Infrared spectrometer, respectively.

3. Results and discussion
3.1. Mechanical properties
3.1.1. Effect of nano-SiO2 on consistency limits
The effects of inclusion of cement and nanoparticles on liquid
limit (LL), plastic limit (PL), and plasticity index (PI) of the soils
are shown in Fig. 5. The plasticity index is defined as the difference
between the LL and the PL and indicates the range of moisture contents over which the soil remains plastic. Fig. 5 shows that there
was a direct proportion between consistency limits and the loaded
cement. It can be seen that addition of 4%, 6%, and 8% cement by
weight of dry soil increased the LL by 1%, 3% and 5%, respectively.
However, the increasing trend in the PL was shown with a higher
slope comparing that of the LL. This may be corresponded to the
deposition of cementitious products onto the surfaces of the flocculated clay clusters, which would lower the surface activity of
these clusters. The high rate of increase in the PL at a higher dosage

S.H. Bahmani et al. / Construction and Building Materials 64 (2014) 350–359

Fig. 5. Variation in consistency limits of the cement-treated residual soil.

of cement resulted in a decrease in plastic index which is in agreement with previous works [10,13]. Improvement in workability
and higher compressive strength were reported in line with the
PI augmentation [5,40,41].
Fig. 6a–c shows the effects of different percentages of SiO2
nanoparticles on the LL and the PL of cement-treated residual soil.
There were no apparent changes in the LL of the nano-SiO2-treated
soil at any cement level. However, the PL of the specimens initially
increased at nano-SiO2 content of 0.2% but then decreased at
higher nano-SiO2 contents. Nonetheless, the inclusion of higher

(a)

353

loads of nanosilica particles of 80 nm increased the PL at cement
level of 4%. In general, the changing trend was more remarkable
for nanosilica particles with the median size of 15 nm. As cement
dosage increased to 8%, the PL showed a more constant value at
different nanoSiO2 loads. As can be seen from the figure, the PL
index of samples with 15 nm silica particles were lower than that
of samples with 80 nm silica particles at cement dosages of 4% and
6%. The lower PL index with the presence of 15 nm silica may refer
to the increased packing density [23,42] and higher surface energy
of nanosilica [24,29]. A very thin layer of water molecules may
envelop the nanoparticles and hence less water was needed to
plasticize the matrix [20,43]. Albeit, as the cement percentage
increased to 8%, the PL of samples with 80 nm silica particles
decreased to a level below that of 15 nm silica particles. The hydration accelerating effect of small sized nanosilica with higher
amounts of cement may well explain the higher water absorption
in samples with 15 nm nanosilica [19,26].
Fig. 7a–c illustrates the effect of different percentages of SiO2
particles on the PI of cemented soil at different cement levels. In
all cases, there was a direct proportion between the percentages
of SiO2 nanoparticles and the PI of the specimens. As can be seen
from the figure, lower loads of nanosilica of up to 0.2% resulted
in the lowest PI for all samples with the cement levels of 4% and
6%. However, the nanosilica dosage associated to the lowest PI
increased to 0.4% at 8% cement level.
3.1.2. Effect of nanoSiO2 on the compactability
The variations in optimum moisture content (OMC) and maximum drying density (MDD) of the residual soil with different
percentages of cement are shown in Fig. 8. As can be seen, the

(a)

(c)
(b)

(b)
(c)

Fig. 6. Variation of LL and PL of the SiO2 – cemented soil (a) 4% cement, (b) 6%
cement and (c) 8% cement.

Fig. 7. Variation of PI of the SiO2 – cemented soil (a) 4% cement, (b) 6% cement and
(c) 8% cement.

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S.H. Bahmani et al. / Construction and Building Materials 64 (2014) 350–359

Fig. 8. Variation in compaction characteristics of untreated soil and cement-treated
soil.

addition of cement resulted in an increase in the OMC by 2% and a
decrease in the MDD of the soil by 1%. It is in agreement with previous works [4–7,10,12,44,45], which may be explained by the
flocculation and cementation of soil particles. As shown in Fig. 8,
the addition of cement has an influence on increasing the optimum
water content and decreasing the maximum dry unit weight of the
untreated soil.
Fig. 9 shows the effect of addition of SiO2 nanoparticles on the
compactability of the cement-treated soil at different cement levels. As can be seen, the addition of nanoparticles resulted in an
increase in the OMC and a slight decrease in the MDD of the specimens. This may be regarded to an immediate formation of secondary C–S–H gel with the presence of nanosilica which may reduce

compactability and hence increase the density of the treated soil.
Secondly, the addition of SiO2 nanoparticles may also improve
the packing density of particles, decrease the space between them
(free water decrease) and increase the internal friction between
solid particles. However, a greater increase in the MDD was
observed with the addition of nanoparticles with an average diameter of 80 nm than that of 15 nm (Fig. 9) at all cement levels. It was
also observed that an increase in the nanomaterial content resulted
in a decrease in the MDD but an increase in the OMC. The increase
of SiO2 nanoparticles more than the optimum limit may possibly
result in agglomeration of nanomaterial particles which in turn
may cause an increase in the OMC and consequently a decrease
in MMD due to hindrance in dispersion. This mechanism may be
more dominant in 15 nm particles because of their higher surface
area. According to Ferkel and Hellmig [46], the agglomeration of
nanoscaled powders increases the amount of necks between particles and therefore decreases the density of associated framework.

3.1.3. Effect of nano-SiO2 on the compressive strength
The effect of cement addition on the unconfined compressive
strength of the residual soil is shown in Fig. 10. It can be observed
that cement led to an increase in unconfined compressive strength
of the soil which is reported widely in previous works [10,13,47].
Fig. 11 depicts the effect of nanosilica on stress–strain curves of
cemented soil obtained from the unconfined compressive strength
test at curing age of 7 days. Generally, the shapes of stress–strain

Fig. 9. Variation in compaction characteristics of cement-treated soil with SiO2 nanoparticles with average diameters of 15 and 80 nm.

S.H. Bahmani et al. / Construction and Building Materials 64 (2014) 350–359

355

Fig. 10. Effect of the addition of cement on unconfined compressive strength.

Fig. 11. Effect of the addition of nano-SiO2 on unconfined compressive strength.

curves differed considerably as amount of nano-SiO2 changed. It
can be observed that the nanoparticles were obviously a very effective additive for enhancing the strength of the specimens. As
shown in Fig. 11, lower loads of SiO2 nanoparticle resulted in
higher strengths. The maximum strength was 673 kPa, 1020 kPa,
1611 kPa for 4%, 6%, 8% cement, respectively when silica particles
of 15 nm were used. The compressive strength of the samples
without nanosilica was 424 kPa, 450 kPa, and 515 kPa for 4%, 6%,
8% cement, respectively. It can be seen that the compressive
strength of cement-treated soil with 0.4% nano-SiO2 and 8%
cement was 85% higher than soil stabilized with 8% cement only.
However, the addition of higher percentage of the nanoparticles
(i.e. greater than 0.4%) led to a lower strength gain. Furthermore,

according to the test results, nanoparticles measuring 15 nm were
more effective in terms of strengthening the soil than those measuring 80 nm. This may be related to the higher specific surface
area of SiO2 with average size of 15 nm than that of 80 nm particles. The results from compressive strength were consistent with
those of compactability and consistency limit tests (Figs. 7 and 9).
Fig. 12a and b shows the strength development rate when
nanosilica was added to the soil at different cement levels. It can
be seen that the compressive strengths of cement-treated soil were
increased when a limited amount of nanosilica was added to the
soil. The highest rate was recorded when 0.4% nanosilica was
added to the cement treated soil with 8% cement. However, application of 0.2% nanosilica decreased the cement usage by 50%. The

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S.H. Bahmani et al. / Construction and Building Materials 64 (2014) 350–359

Fig. 13. Effect of the nanoparticles on hydraulic conductivity of soil.

specimens. Agglomeration of high amounts of nanosilica might
have reduced the chemical and physical effect of nanoparticles
on solidification of the soil, too.
3.2. Chemical and microstructural properties

Fig. 12. Relative strength development with cement dosage and nanoparticles SiO2
15 nm b) 80 nm.

changes observed in the mixtures modified with nano-SiO2
particles may be the result of a chemical reaction between SiO2
and Ca(OH)2 during cement hydration and production of additional
C–S–H clusters in the soil. Furthermore, nanosilica may accelerate
hydration of cement due to its high surface energy [21,42,48,49].
The reduction in the water content of samples with nanosilica
may well explain higher hydration rate caused by incorporation
of nanosilica. It can also be stated that nanosilica may cause
physical alterations such as improvement in the packing factor
which corresponds to filling effect of its particles. However,
addition of more than 0.4% nanosilica had an adverse effect on
the compressive strength which may correspond to dispersion
problems. It was also observed that inclusion of nanoparticles with
smaller size led to a higher strength development rat at all cement
levels at early ages of soil stabilization. The increased early
strengths with the incorporation of smaller sized nanosilica may
be related to their high specific surface areas.
3.1.4. Effect of nano-SiO2 on hydraulic conductivity
The hydraulic conductivity is a key parameter for most soil liners and covers. The relationship between hydraulic conductivity,
nanoparticle content and compaction effort is shown in Fig. 13.
The hydraulic conductivity increased with an increase in content
of both sizes of nanoparticles (15 and 80 nm), however, it can be
seen that the least conductivity was observed at 0.4% nanosilica
which is consistent with the results from compressive strength.
It confirms that incorporation of nanosilica led to a decrease in
large pores and eliminated the smaller pores in the soil which
may be due to formation of secondary C–S–H clusters in the soil.
The effect of 15 nm nanosilica particles on decreasing the hydraulic
conductivity was greater compared with the effect of the 80 nm
nanoparticles. This is possibly because the particle packing density
of 15 nm nanoparticles is greater than that of 80 nm nanoparticles.
However, when the percentage of nanoparticles exceeded 0.8%,
additional water was absorbed and held by the nanoparticles and
soil, which resulted in higher hydraulic conductivity of the

3.2.1. XRD
The most important peaks which may refer to the effect of
cement and nanosilica were the ones related to calcium hydroxide
at 2 theta of 18° and 34° [50] as shown in Fig. 14. The major hydration product which is C–S–H cannot be traced using XRD due to
amorphous nature of C–S–H clusters although consumption of
CH may implicitly represent the formation of C–S–H networks.
As can be seen from the figure, the addition of cement to the soil
caused the CH related peaks to appear at the aforementioned 2 thetas. However, inclusion of the nanosilica to the matrix of soil,
reduced the intensity of the peak which is attributable to the formation of secondary C–S–H through pozzolanic activity of nanosilica particles [20,24,29,43]. The results from XRD may well explain
the increased compressive strength of specimens with nanosilica.
Decreased hydraulic conductivity may also be corresponded to
the increased rate of gel formation throughout the soil matrix.
3.2.2. FTIR
Fig. 15 shows the FTIR spectra of Si–O–Si band in soil, cement
treated soil and cement treated soil with nanosilica at 7 day curing
time. The FTIR spectrum of the treated soil shows a broad group of
Si–O–Si band in the region of 600–1500 cm1. This band may be
related to the complex spectra of C–S–H. The vibration bands
appearing in the FTIR spectra were consistent with the characteristic signals of C–S–H gels previously described in the literature
[25,29,51]. In addition, the spectra centred around 1450 cm1
may be associated with calcium carbonate (CaCO3) as a result of
carbonation. Given these observations, the differences in the transmittance percentages and positions of the peaks in the untreated
soil, the cement-treated soil and the soil–cement mixture with
nanosilica may reveal that the nature and amount of the C–S–H
phase has changed and may confirm the additional formation of
C–S–H gel. From the aforementioned results, it can be concluded
that SiO2 nanoparticles readily reacted with water and calcium
hydroxide, a by-product of cement hydration, to produce additional C–S–H gel. The additional C–S–H may increase the compressive strength of nano-cement specimens. In addition to this effect
of C–S–H on the strength of the soil, the additional C–S–H may
reduce the porosity of the soil by filling the capillary pores and
thus improving the microstructure of the soil, which may also contribute to the increased compressive strength.
3.2.3. SEM
Three specimens (an untreated specimen, a specimen treated
with 8% cement and a specimen treated with 8% cement and
0.4% SiO2 nanoparticles) were subjected to SEM analysis (Figs. 16

S.H. Bahmani et al. / Construction and Building Materials 64 (2014) 350–359

357

Fig. 14. X-ray diffractograph of patterns of untreated, cement-treated and nanoparticles-treated soil. The reflection are labelled p (Portlandite), q (Quartz).

Fig. 15. FTIR patterns of untreated, cement-treated, and cement- and SiO2nanoparticle-treated specimens.

and 17). It was revealed that the untreated soil consisted of some
particle packs (Fig. 16a). This may be because with the presence
of water, clay particles adhere to each other to form large particle
packs, resulted in many micropores in the untreated soil. As can be
seen from Fig. 16b, some pores between the particles were filled
with cementitious gel, which resulted in particle packs with smaller pores contributing to a denser soil matrix.
Fig. 17 illustrates the micrograph of cement treaded soil with
the presence of nanosilica. As can be seen, a very dense matrix
was formed and pores were filled to a great extent. It may reflect
the formation of secondary C–S–H gel in reactions between the
cement products and the SiO2 nanoparticles. These reaction products envelope the soil particles and strengthen the soil. Also, nanofilling effect of SiO2 particles may increase the packing density of
the soil [7,42,49]. At the same time, nucleation effect of particles
may help to a better distribution of the C–S–H in the matrix
[21,24,29,52]. The SEM analysis is consistent with a study by Ltifi
et al. [27]. It was shown that more stable C–S–H gel was formed
when nanosilica was added to cement mortars which further densified the matrix.
3.2.4. pH value
The changes in pH level of the soil due to addition of SiO2 nanoparticles are shown in Fig. 18. The pH of the untreated sample was
4.0, indicating a strongly acidic soil. The pH increased to 10, 10.9,
and 11.9 when 4%, 6%, and 8% cement was added to the soil,
respectively. The formation of hydroxyl ions from CH due to
cement hydration may be the main reason for this phenomenon.
As can be seen from the figure, incorporation of nanosilica
decreased the pH of the samples up to 7 with presence of 1%

Fig. 16. Scanning electron micrographs of soil specimens: (a) untreated soil and (b)
cement-treated soil.

nanosilica. This may be mostly regarded to consumption of CH
by nanoparticles through pozzolanic reactions. The results from
pH are in agreement with XRD and FTIR findings. It was also
observed that addition of more than 0.8% nanosilica to soils with
higher cement levels caused an increase in pH level. It may be
regarded to insufficient amount of nanosilica to consume CH produced by cement hydration. It is safe to say that addition of nanosilica is beneficial when cement is used as a stabilization technique
to control the pH level of the soil.

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S.H. Bahmani et al. / Construction and Building Materials 64 (2014) 350–359

Fig. 17. Scanning electron micrograph of C–S–H gel formed in cement-treated
specimen with SiO2 nanoparticles.

Fig. 18. Effect of the nanoparticles on pH values.

4. Conclusions
The following conclusions can be drawn from this study:
 There were no apparent changes in the LL of the nano-SiO2treated soil at any cement level. However, the PL of the
specimens initially increased at nano-SiO2 content of 0.2%
but then decreased at higher nano-SiO2 contents. In general,
the PL index of samples with 15 nm silica particles were
lower than that of samples with 80 nm silica particles at
cement dosages of 4% and 6%. As the cement percentage
increased to 8%, the PL of samples with 80 nm silica particles decreased to a level below that of 15 nm silica particles.
Consequently, lower loads of nanosilica of up to 0.2%
resulted in the lowest PI for all samples with the cement
levels of 4 and 6%. The nanosilica dosage associated to the
lowest PI increased to 0.4% at 8% cement level.
 The addition of nanoparticles resulted in an increase in the
OMC and a slight decrease in the MDD of the specimens. A
greater increase in the MDD was observed with the addition
of nanoparticles with an average diameter of 80 nm than
that of 15 nm at all cement levels. It was also observed that
an increase in the nanomaterial content resulted in a
decrease in the MDD but an increase in the OMC.
 Addition of nanosilica increased the compressive strength of
samples dramatically. However, lower loads of SiO2 nanoparticle resulted in higher strengths. The maximum
strength was 673 kPa, 1020 kPa, 1611 kPa for 4%, 6%, 8%
cement, respectively when silica particles of 15 nm were

used. The compressive strength of the samples without
nanosilica was 424 kPa, 450 kPa, and 515 kPa for 4%, 6%,
8% cement, respectively. The addition of higher percentage
of the nanoparticles led to a lower strength gain. According
to the test results, nanoparticles measuring 15 nm were
more effective in terms of strengthening the soil than those
measuring 80 nm. It was also observed that inclusion of
nanoparticles with smaller size led to a higher strength
development rate at all cement levels at early ages of soil
stabilization. It may be stated that application of nanosilica
may accelerate the soil stabilization to certain levels.
 The hydraulic conductivity increased with an increase in
content of both sizes of nanoparticles (15 and 80 nm), however, it was seen that the least conductivity was observed
with addition of 0.4% nanosilica. The effect of 15 nm nanosilica particles on decreasing the hydraulic conductivity
was greater compared with the effect of the 80 nm
nanoparticles.
 Inclusion of the nanosilica to the soil, reduced the intensity
of the peaks related to the calcium hydroxide. Moreover, the
FTIR spectrum of the treated soil showed a broad group of
Si–O–Si band in the region of 600–1500 cm1. The differences in the transmittance percentages and positions of
the peaks in the untreated soil, the cement-treated soil
and the soil–cement mixture with nanosilica may reveal
that the nature and amount of the C–S–H phase has changed
and may confirm the additional formation of C–S–H gel.
SEM images also showed formation of a very dense matrix
in which pores were filled to a great extent.
 The pH increased to 10, 10.9, and 11.9 when 4%, 6%, and 8%
cement was added to the soil, respectively. Albeit, incorporation of nanosilica decreased the pH of the samples up to 7
with presence of 1% nanosilica. It is safe to say that nanosilica is beneficial when cement is used as a stabilization technique to control the pH level of the soil.
Acknowledgements
Financial assistance from the Research Management Centre
(RMC) of the Universiti Putra Malaysia for conducting this experiment is gratefully acknowledged.
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