AU J.T. 14(2): 139-146 (Oct. 2010)
Pozzolanic Influence of Fly Ash in Mobilizing
the Compressive Strength of Lateritic Soil
Agapitus Ahamefule Amadi
Department of Civil Engineering, Federal University of Technology
Minna, Nigeria
E-mail: <agapitusahamefule4@yahoo.com>
Abstract
Soil mixtures with varying fly ash contents (0, 5, 10, 15 and 20%) prepared at
moulding water contents ranging from dry to wet of optimum moisture content and
compacted with British Standard Light (BSL), West African Standard (WAS) and British
Standard Heavy (BSH) compactive efforts were tested to determine shear strength using
unconfined compressive strength. Peak compressive strength values of soil mixtures
cured for 28 days were about 2 folds higher than the compressive strength of the
natural soil regardless of the compactive effort used. While BSL compactive effort
attained peak strength on application of 10% fly ash, BSH and WAS efforts achieved
peak strength value at 5% fly ash content. Expectedly, reductions in strength values
were observed as the moulding water content increased from dry to wet of optimum
even as the maximum strength realized in all compacted soil mixtures occurred at
moisture content 2% below optimum moisture content (OMC). Furthermore, it was
revealed that curing improved the strength of soil mixtures. Strength gain after a period
of 28 days curing was relatively low which could be attributed to insufficient calcium
oxide in the fly ash to sustain the formation of significant cementitious products.
Consequently, further tests with calcium based admixtures such as lime are suggested.
Keywords: Moulding, stabilization, compactive efforts, cementitious products.
seasons and continually leached by rainwater
causing a tendency for deterioration of its
strength characteristics. The structural elements
in the soils are often a less stable coarsegrained aggregation of variable strength which
may break down in performance. Furthermore,
their varying silt and clay content often render
them moisture sensitive. (Maigien 1966;
BRRI/Lyon Associates 1971; Ola 1978;
Charman 1988; Nicholson and Kashyap 1993).
The aforementioned properties give indication
of their limitations in engineering applications
or at best are restricted to minor engineering
projects. Common practice is to excavate and
cart away such materials and/or to import large
quantities of select fill material, both costly
remedies. A more economic solution for
improving the performance characteristics of
such problematic lateritic soils for development
and construction purposes is by appropriate
stabilization, which among other things
enhances cementation of aggregates, increases
Introduction
Originally, utilization of lateritic soil was
only discussed in connection with mining of
solid minerals such as Iron, Aluminum,
Bauxite, and Manganese (Maignien 1966).
Later, the civil engineering aspect mostly in
connection with road pavements, dams,
embankments, buildings became the subject of
numerous studies. Considering the vast areas
and abundance of lateritic soil in the tropics
and the present use of the soil, it is the apparent
that the diverse engineering potential of this
soil is utilized only to a very small extent.
Among the numerous questions that must
be answered in order to utilize this soil to the
full extent of its potentials are mainly directly
related to the strength properties.
Lateritic soils are weathered under
conditions of high temperatures and humidity
with well-defined alternating wet and dry
Technical Report
139
AU J.T. 14(2): 139-146 (Oct. 2010)
its strength, reduces the sensitivity of the soil to
moisture changes and subsequent loss of
strength.
Lateritic soils have been traditionally
stabilized by cement and/or lime but the current
drive towards sustainable development requires
that fewer wastes be generated and whenever
possible the wastes that are generated be reused in some other applications. This has given
an incentive to evaluate the beneficial reuse of
fly ash, an industrial by product of power
plants as a soil stabilizer by several researchers.
The improved engineering properties of
compacted lateritic soil resulting from the
addition of fly ash may be very appropriate in
this direction as it brings environmental and
economic benefits. The purpose of fly ash
addition is to ensure that there will be enough
pozzolans in the soil mixtures to combine with
the silica and alumina components of the
lateritic soil in both short and long term
chemical reaction to form insoluble
cementitious compounds made up of hydrated
calcium silicate gel (CSH) and calcium
aluminate gel (CAH). Such compounds as
shown in Eqns. 1 and 2 are capable of
producing strong permanent matrices and the
soil mixtures are transformed into a new
material that exhibits significant permanent
strength. It is also expected that other
engineering problems of low grade lateritic
soils will be substantially alleviated by the use
of this additive:

Ca    2OH   SiO 2  CSH
(1)
,
fly ash on strength properties of lateritic soil
with a view to exploiting the great possibilities
that the soil offer in engineering construction.
Materials and Methods
Study Soil
The study soil was obtained by disturbed
sampling from a borrow pit at depths of
between 1.0 and 2.0 m in Shika, Zaria (Latitude
11o15’ N and Longitude 7o45’ E) Nigeria. The
soil is reddish brown in colour and the clay
mineral identified in soil sample is primarily
kaolinite.
The
properties
and
oxide
composition of the soil are shown in Tables 1
and 2.
Table 1. Properties of soil mixtures.
Properties
Natural
Moisture
content
(%)
Specific
gravity
Liquid
Limit (%)
Plasticity
Index (%)
Linear
shrinkage
(%)
USCS
classificat
ion
pH
Color
gel
Ca    2OH   Al 2 O3  CAH
 .

(2)
gel
Much data is available on fly ash, soil
stabilization with fly ash, the mechanism of
strength development and are extensively
covered in various papers and studies (Usmen
and Bowders 1990; Edil et al. 1992; Chu and
Kao 1993; Ferguson 1993; Keshawarz and
Dutta 1993; Maher et al. 1993; Nicholson and
Kashyap 1993; Smith 1993; Consoli et al.
2001). But because of variability in soil
behaviour, it will be technically inappropriate
to extrapolate results obtained from testing one
type of soil to another. The present study
therefore was undertaken to provide research
data demonstrating the influence of pozzolanic
Technical Report
Dominant
Clay
Mineral
0
5.80
Fly Ash content (%)
5
10
15
-
20
-
2.76
2.66
2.55
2.49
2.42
42.20
36.50
34.00
31.24
29.53
22.22
15.87
12.06
7.78
3.54
9.50
7.82
6.70
4.56
2.88
CL
CL
CL
CL
ML
6.67
Reddish
brown
Kaolinite
8.14
8.60
8.80
9.20
Fly Ash
The fly ash used in this study was
obtained from an old stockpile at Oji River
thermal station, Enugu State, Nigeria. It is
black in color and has a specific gravity of
2.06. Only fractions passing BS sieve No. 200
(75 µm) was used throughout the test without
additional treatment. Mineralogical analysis by
140
AU J.T. 14(2): 139-146 (Oct. 2010)
Atomic Absorption Spectrometer showed that
the fly ash is composed of the following
principal constituents presented in Table 2.
(Nigerian General Specification 1997; Osinubi
1998).
Unconfined Compression Test
The unconfined compression test was
carried out in accordance with the procedures
outlined in BS 1377 (1990) and Head (1994b).
The test was conducted on specimens mixed
with tap water equivalent to 2% dry of
optimum, optimum and 2% wet of optimum
moisture content as determined from the three
compactive efforts namely, British Standard
light (BSL), West African Standard (WAS) and
British Standard heavy (BSH) used in this
study. Specimens were then extruded from the
mould, wrapped in cellophane bags and kept in
humid environment for the required curing
periods (7, 14, 21 and 28 days). The test was
performed on cylindrical specimens having a
diameter and length of 38mm and 76mm
respectively, which were trimmed from the
larger compacted cylinders. The samples were
tested in triaxial compression test machine
without applying cell pressure.
Table 2. Oxide composition of lateritic soil and
fly ash.
Oxide
CaO
SiO2
Al2O3
Fe2O3
MgO
SO3
MnO2
Na2O
K2O
Loss on ignition
Concentration (%)
Lateritic Soil*
Fly Ash
0.28
1.78
35.60
46.02
27.40
24.16
24.0
13.68
0.22
1.91
0.85
ND
2.0
0.56
ND
5.31
ND
5.58
1.46
1.3
* Adapted from Osinubi (1998);
ND - Not determined.
Compaction Test
Compaction tests were conducted in
accordance with BS 1377 (1990), Head (1994a)
as well as Nigerian General Specification
(1997) in order to establish the compaction
characteristics of laterite soil – fly ash
mixtures. Specimens were prepared to include
a wide range of compaction conditions and
lateritic soil – fly ash mixtures. For specimens
that contain fly ash, appropriate soil samples
were mixed with varying concentrations of fly
ash (0, 5, 10, 15 and 20%) and wetted with tap
water and blended to a uniform consistency.
Specimens were prepared with three
compactive efforts namely British Standard
Light (BSL), West African Standard (WAS) or
“intermediate effort” and British Standard
Heavy (BSH) compaction to represent the
range of compactive efforts expected in the
field without compaction delay. The WAS
compaction is the conventional energy level
commonly used in the region and consists of
the energy derived from a 4.5 kg rammer
falling 450mm onto five layers in a British
Standard mould, each receiving ten (10) blows
Technical Report
Results and Discussion
Index Properties
The index properties and classification of
the natural soil and soil – fly ash mixtures are
presented in Table 1. The particle size data for
the lateritic soil together with that of fly ash
used in this study are presented in particle size
distribution curves shown in Fig. 1. Although
the soil and the fly ash particle size distribution
curves are different, the addition of fly ash
improved the gradation characteristics of the
lateritic soil by reducing the amount of clay
size particles. The aggregation or clusters of
clay-minerals and clay-size mineral fragments
due to ion exchange at the surface of the soil
particles resulted in more stable silt-sand like
structures. With increasing percentage of fly
ash, the soil becomes more granular in nature.
141
100
90
80
70
60
50
40
30
20
10
0
0.001
Lateritic soil
Fly Ash
0.01
0.1
1
10
Maximum Dry Unit Weight
3
(kN/m )
The addition of fly ash to lateritic soil
obviously
affected
the
compaction
characteristics primarily due to alteration of
gradation of soil mixtures. Typical of such
mixtures, the maximum dry unit weight
decreased with higher fly ash content since the
ash is a low specific gravity material (Osinubi
et al. 2006). The optimum moisture content on
the other hand was raised by the addition of fly
ash. This increase could be as a result of the
increasing demand for water by the soil
mixtures for hydration reaction. The variation
of maximum dry unit weight with fly ash
content for the various soil mixtures is reported
in Fig. 2 while the data in Fig. 3 show the
variation of optimum moisture content with fly
ash content of soil mixtures.
WAS
BSH
17
16
15
15
20
Fly Ash Content (%)
Fig. 2. Variation of maximum dry unit weight
with fly ash content.
Technical Report
16
13.5
11
5
10
15
20
Strength development in soil mixtures
prepared over a range of moulding water
contents (dry to wet of optimum moisture
content) after a curing period of 28 days was
assessed by plotting values of UCS against fly
ash contents as reported in Figs. 3, 4 and 5 for
BSL, WAS and BSH compactive efforts
respectively. It is readily observed in Figs. 3, 4
and 5 that UCS values increased with higher
fly ash content up to the peak values after
which there were drops in strength values.
Generally, strength values dropped in the
region of higher fly ash content after attaining
peak value. Excessive fly ash content was
probably responsible for these reductions in the
UCS.
The influence of fly ash in improving the
strength of mixtures was evaluated by
comparing the measured strength values
obtained for the natural soil and lateritic soil fly ash mixtures after 28 days curing period. It
is pertinent to note here that curing was not
necessary for specimens without fly ash
additive. Peak UCS values for specimens
prepared at 2% dry of optimum moisture
content, optimum and 2% wet of optimum
moisture content were about 2 folds higher than
that of the natural soil regardless of the
compactive effort. These peak strengths were
established when 10% fly ash was added in the
case of BSL compactive effort (Fig. 4). Unlike
18
10
18.5
Unconfined Compressive Strength (UCS)
19
5
BSH
Fig. 3. Variation of optimum moisture content
with fly ash content.
Compaction Characteristics
0
WAS
Fly Ash Content (%)
Fig. 1. Particle size distribution curves.
BSL
BSL
21
0
Particle Size (mm)
20
Optimum Moisture Content (%)
Percent Passing
AU J.T. 14(2): 139-146 (Oct. 2010)
142
AU J.T. 14(2): 139-146 (Oct. 2010)
the BSL compactive effort, which achieved
peak strength at 10% fly ash content, WAS and
BSH compactive efforts had peak UCS values
at 5% fly ash content for dry of optimum as
well as optimum compaction (Figs. 5 and 6).
2
UCS (kN/m )
2300
2% Dry of Opt.
Opt.
2% Wet of Opt.
1400
500
2
UCS (kN/m )
1700
800
1150
950
200
750
0
550
5
10
15
20
Fly Ash Content (%)
350
Fig. 6. Variation of UCS with fly ash content for
soil mixtures compacted with BSH effort at the
various compaction states.
150
0
10
20
Fly Ash Content (%)
Several mechanisms are responsible for
strength development in soils stabilized with
fly ash. The main generator of strength in the
soil mixtures is the pozzolanic cements formed
from the reaction of silica and alumina
components of clay fractions of the soil with
calcium ions and water (Eqns. 1 and 2).
Additional strength is mobilized by granular
packing in the matrix formed with fly ash
acting as filler (Consoli et al. 2001).
The data in Figs. 7, 8 and 9 present peak
strength values established at the various fly
ash contents and strength at the optimum
moisture content for the various soil mixtures
compacted with BSL, WAS and BSH
compactive efforts respectively. These data
indicate that peak strength values are generally
higher than strength values at optimum
moisture content (i.e. maximum dry unit
weight). The implication is that moisture
control should be based on moisture – strength
relationship rather than moisture – density
relationship
for
applications
requiring
maximum strength. The maximum strengths
realized in soil – fly ash mixtures generally
occurred at moisture content 2% dry of OMC.
Lower UCS values were achieved for soil
mixtures prepared on the wet side of optimum
moisture content.
Fig. 4. Variation of UCS with fly ash content for
soil mixtures compacted with BSL effort at the
various compaction states.
2% Dry of Opt.
Opt.
2% Wet of Opt.
1750
1500
2
2000
1100
1350
UCS (kN/m )
2% Dry of Opt.
Opt.
2% Wet of Opt.
1250
1000
750
500
250
0
10
20
Fly Ash Content (%)
Fig. 5. Variation of UCS with fly ash content for
soil mixtures compacted with WAS effort at the
various compaction states.
Technical Report
143
AU J.T. 14(2): 139-146 (Oct. 2010)
Effect of Curing Period
Peak Strength
Strength at OMC
UCS (kN/m2)
1400
The trend of strength mobilization with
curing time monitored over a period of 28 days
for soil mixtures prepared at optimum moisture
content is presented in Figs. 10, 11 and 12 for
BSL, WAS and BSH compactive efforts
respectively. Evaluation of data in Figs. 10, 11
and 12 revealed that the UCS of all soil
mixtures improved with time. The strengths
achieved after 28 days of curing were
considerably (2 - 3 times) greater than those
realized for soil mixtures at 7 days. However, it
is interesting to note that a significant
percentage of this increase was recorded in the
first 14 days of curing. This behaviour is
suggested to result from the coupled effects of
the two main generators of strength in the soil
mixtures namely, the structure imparted by
compaction and the formation of cementitious
matrix in the first 14 days. As curing
progressed beyond 14 days, the increase in
UCS was at a lower rate, approximately
constant in some cases. It is suggested from the
behaviour observed in Figs. 10, 11 and 12 that
significant pozzolanic reaction could not be
sustained after 14 days. This is probably
correlated to low content of calcium oxide in
the fly ash utilized for this study (Table 1).
1200
1000
800
600
400
200
0
0
5
10
15
20
Fly Ash Content (%)
Fig. 7. Peak strengths and strengths at OMC
for soil mixtures compacted at BSL compactive
effort.
Peak Strength
1750
Strength at OMC
UCS (kN/m2)
1500
1250
1000
750
500
250
0
0
5
10
15
20
Fly Ash Content (%)
Fig. 8. Peak strengths and strengths at OMC
for soil mixtures compacted with WAS
compactive effort.
900
2
UCS (kN/m )
Peak Strength
Strength at OMC
2500
2000
2
UCS (kN/m )
5% Fly Ash
10% Fly Ash
15% Fly Ash
20% Fly Ash
1100
1500
700
500
1000
300
7
500
14
21
28
Curing Time (Days)
0
0
5
10
15
Fly Ash Content (%)
20
Fig. 10. Effect of curing time on UCS for soil
mixtures compacted with BSL compactive
effort.
Fig. 9. Peak strengths and strengths at OMC
for soil mixtures compacted with BSH
compactive effort.
Technical Report
144
AU J.T. 14(2): 139-146 (Oct. 2010)
Fig. 11. Effect of curing time on UCS for soil
mixtures compacted with WAS compactive
effort.
Test results revealed that the unconfined
compressive strength of all the mixtures
increased with higher fly ash content up to a
peak value, beyond which reductions in
strength values occurred. The strength
reduction is attributed to excess fly ash in the
soil mxtures beyond the peak strength values.
Results also showed that curing improved
strength of fly ash treated lateritic soil. Strength
gain was however comparatively low probably
due to the low calcium content of the ash
utilized which could not maintain adequate
pozzolanic reactivity to fully mobilize the
compressive strength of the mixture.
Further tests with additives such as lime
to initiate extensive pozzolanic reaction are
suggested.
5% Fly(BSH)
Ash
Curing
10% Fly Ash
References
15% Fly Ash
20% Fly Ash
5% Fly Ash
10% Fly Ash
15% Fly Ash
20% Fly Ash
2
UCS (kN/m )
1450
1250
1050
850
650
7
14
21
28
2
UCS (kN/m )
Curing Time (Days)
BRRI/Lyon Associates. 1971. Laterites and
lateritic soils and other problem soils of
Africa. An engineering study for USAID.
AID/csd- 2164, Baltimore, MD, USA.
BS 1377. 1990. Methods of tests for soils for
civil
engineering
purposes.
British
Standards Institutions, London, UK.
Charman, J.H. 1988. Laterite in road
pavements. Construction Industry Research
and Information Association (CIRCA),
Special Publication 47, Westminster,
London, England, UK.
Chu, S.C.; and Kao, HS. 1993. A study of
engineering properties of a clay modified by
fly ash and slag. Fly ash for soil
Improvement ’93. Sharp, D.K. (ed.). GSP
No. 36, ASCE, New York, NY, USA.
Consoli, N.C.; Prietto, P.D.M.; Carraro, J.A.H.;
and Heineck, K.S. 2001. Behaviour of
compacted soil - fly ash - carbide lime
mixtures. J. Geotech. Geoenv. Engrg.,
ASCE 127: 774-82.
Edil, T.B.; Sandstrom, L.K.; and Berthoux,
P.M. 1992. Interaction of inorganic leachate
with compacted pozzolanic fly ash. J.
Geotech. Engrg., ASCE 118: 1,410-30.
Ferguson, G. 1993. Use of self cementing fly
ashes as a soil stabilization agent. Fly ash for
Soil Improvement ’93. Sharp, D.K. (ed.).
GSP No. 36, ASCE, New York, NY, USA,
pp. 1-14.
1800
1400
1000
7
14
21
28
Curing Time (Days)
Fig. 12. Effect of curing time on UCS for soil
mixtures compacted with BSH compactive effort.
Summary and Conclusions
The present study is an attempt to assess
strength improvements resulting from fly ash
stabilization of lateritic soil with a view to
adopting it in remedying deficient soils/sites.
Compacted soil mixtures with varied amounts
of fly ash (0, 5, 10, 15, 20%) prepared at
different compaction states equivalent to 2%
dry of optimum, optimum and 2% wet of
optimum moisture content as determined from
the three compactive efforts namely, British
Standard light (BSL), West African Standard
(WAS) and British Standard heavy (BSH) were
tested for unconfined compressive strength.
Technical Report
145
AU J.T. 14(2): 139-146 (Oct. 2010)
ash for Soil Improvement ’93. Sharp, D.K.
(ed.). GSP No. 36, ASCE, New York, NY,
USA.
Ola, S.A. 1978. Geotechnical properties and
behaviours of some stabilized Nigerian
lateritic soils. Tropical Soils of Nigeria in
Engineering Practice. Ola, S.A. (ed.). A.A
Balkema, Rotterdam, The Netherlands, pp.
61-84.
Osinubi, K.J. 1998. Influence of compactive
efforts and compaction delays on limetreated soil. J. Transport. Engin., ASCE
124(2): 149-55.
Osinubi, K.J.; Amadi, A.A.; and Eberemu,
A.O. 2006. Shrinkage characteristics of
compacted laterite soil - fly ash mixtures.
NSE Technical Transaction 41: 36-48.
Smith, R.L. 1993. Fly ash for use in the
stabilization of industrial wastes. Fly Ash
for Soil Improvement ’93. Sharp, D.K. (ed.).
GSP No. 36, ASCE, New York, NY, USA,
pp. 58-72.
Usmen, M.A.; and Bowders, J.J., Jr. 1990.
Stabilization characteristics of class F fly
ash. Transportation Research Record 1288.
Transportation
Research
Board,
Washington, DC, USA, pp. 59-60.
Head, K.H. 1994a. Manual for soil laboratory
testing. Vol. I. Soil classification and
compaction tests. Halsted Press, New York,
NY, USA.
Head, K.H. 1994b. Manual of soil laboratory
testing. Permeability, shear strength and
compressibility tests. Vol. 2, 2nd ed.,
Pentech Press, London, England, UK.
Nigerian General Specification. 1997. Nigerian
General Specification for roads and bridges,
Federal Ministry of Works and Housing,
Abuja, Nigeria.
Keshawarz, M.S.; and Dutta, U. 1993.
Stabilization of South Texas soils with fly
ash. Fly Ash for Soil Improvement ’93.
Sharp, D.K. (ed.). GSP No. 36, ASCE, New
York, NY, USA.
Maher, M.H.; Butziger, J.M.; Disalvo, D.L.;
and Oweis, I.S. 1993. Lime sludge amended
Fly ash for utilization as an engineering
material. Fly Ash for Soil Improvement ’93.
Sharp, D.K. (ed.). GSP No.36, ASCE, New
York, NY, USA.
Maigien, R. 1966. Review of research on
laterites. Natural Resources Research IV,
United Nations Educational Scientific and
Cultural Organisaton, Paris, France.
Nicholson, P.G.; and Kasyap, V. 1993. Fly ash
stabilization of tropical Hawaiian soils. Fly
Technical Report
146