International Journal of Engineering Trends and Technology (IJETT) – Volume 9 Number 14 - Mar 2014
Soil Stabilisation Using Phosphogypsum and Flyash
Divya Krishnan.K1# , V.Janani2, P.T.Ravichandran3 , R.Annadurai3, Manisha Gunturi 1
1
2
PG Student, Department of Civil Engineering, SRM University, Kattankulathur, India
Asst. Professor, Department of Civil Engineering, SRM University, , Kattankulathur, India
3
Professor, Department of Civil Engineering, SRM University, , Kattankulathur, India
Abstract — Since ancient times, a number of stabilization
methods are being used to improve soil properties. Various
studies have been carried out on expansive soil after stabilization
with additives such as cement, lime, cement kiln dust, rice husk
ash etc. which shown promising results. Now-a-days easily
available industrial by-products are used for the enrichment of
soil properties. In present study, effects of stabilizing agents like
Phosphogypsum (PG) and Flyash (FA) have been studied for
strength improvement in varying percentages. This paper
describes a study carried out to check the improvements in the
properties of soil with phosphogypsum in various percentages
(i.e., 2, 4 and 6%) with fixed quantity of flyash (5%). The
unconfined compression test (UCC) and microstructure analysis
of the soil with different percentage of additives were determined
separately after curing the specimens for 3 days and 7 days.
Strength of stabilised soil is increase with increased amount of
stabiliser and curing periods.
Keywords— UCS Value, Flyash, Phosphogypsum, SEM.
I. INTRODUCTION
Expansive soils were been a tough task for Civil Engineers
in the design and construction of Infrastructure projects. The
major problems with clays, including low strength and high
compressibility, can cause severe damage to civil engineering
structures and can lead to very serious economic loss and
environmental hazards. Therefore, these soils must be treated
before commencing the construction operation to achieve
desired properties. This has led to the development of soil
stabilization techniques. Since the nature and properties of
natural soil vary widely, a suitable stabilization technique has
to be adopted for a particular situation after considering the
soil properties. The chemical technique is a common soil
stabilization approach, since it produces a better quality soil
with higher strength and durability than mechanical and
physical techniques.
In many countries of the world, stabilization of soils is
especially done if locally available natural/industrial resources
are available. The use of Phosphogypsum (PG) and Flyash
(FA) in soil stabilization can lead to low-cost construction and
can provide an environmentally friendly means of their
disposal and also enhance the engineering properties of soil.
Phosphogypsum is a by-product in the wet process for
manufacture of phosphoric acid (ammonium phosphate
fertilizer) by the action of sulphuric acid on the rock
phosphate. Flyash is one of the residues generated during the
combustion of pulverized coal and is transported from the
ISSN: 2231-5381
combustion chamber by exhaust gases. Flyash is mainly used
as stabilizer considering its construction potential as a
pozzolanic material.
In this paper, the study is aimed to investigate the strength
and the microstructure of treated clay using unconfined
compressive strength test and scanning electron microscope
(SEM) analysis.
II. LITERATURE REVIEW
Soils with significant plasticity may shrink and swell
substantially with changes in moisture conditions. These
changes in volume can cause a reduction in the density and
strength of the soil, leading to increase in potential of severity.
There is a substantial history of use of soil stabilisation
admixtures to improve the soil performance by controlling
volume change and increasing strength. A number of
innovative techniques have been established for construction
on this type of soil. FA and lime are effective in stabilizing
expansive soil for construction of road base, subbase and
embankments [1]. A combination of flyash and rice husk ash
(RHA) can stabilize black cotton soil[2]. Based on the CBR
and UCS tests, the optimum amount of FA and RHA were
found to be 12% and 9% respectively. Studies on subgrade
soil treated with Rice Husk and lime showed an increase in
strength and CBR value with increase in curing time[3].
Investigations were made on the microstructure and strength
of lime and cement stabilized clays which shows that the
strength development relates to the microstructural changes,
increase in curing time and admixture contents[4]. The
microstructural changes in cement-stabilized clay can improve
strength which can be explained with respect to the influential
factors, i.e., cement content, clay water content, flyash content
and curing time[5]. The unconfined compressive strength was
used as a practical indicator to investigate the strength
development.
III. MATERIAL PROPERTIES AND METHODS
The soil samples used in this investigation was collected at
a depth of 60cm below the ground level in an open excavation
after removing the top soil. The various laboratory tests
carried out on the virgin soils for obtaining geotechnical
characteristics in accordance with Bureau of Indian Standards
(BIS) include: Sieve analysis, Atterberg limits, Specific
gravity, Proctor compaction, Free swell index and Unconfined
compressive strength test. The results obtained are shown in
Table 1. Phosphogypsum(PG) and Flyash(FA) were used as
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International Journal of Engineering Trends and Technology (IJETT) – Volume 9 Number 14 - Mar 2014
TABLE - 1
GEOTECHNICAL PROPERTIES OF VIRGIN SOIL SAMPLES
Property
Value
Soil - S1
Soil - S2
2.23
2.65
2
2
28
32
70
66
82.5
75
40
39
42.5
36
CH
CH
7.71
5.6
120
109
1.5
1.58
25.73
18.52
75
112
Specific Gravity
Grain size
Sand,%
distribution
Clay, %
Silt, %
Liquid Limit, (%)
Plastic Limit, (%)
Plasticity Index, (%)
Soil classification
Shrinkage Limit, (%)
Free Swell Index, %
Maximum Dry Density, g/cc
Optimum moisture content, %
Unconfined compressive strength, kPa
specimens were tested at an axial strain rate of 1.2mm per
minute as per IS:2720-(Part X).
V. DISCUSSION OF TEST RESULTS
To study the effect of flyash and various percentages of
phosphogypsum on the soils S1 and S2, UCS tests were
performed on the soil samples prepared at their MDD
achieved in compaction test, after the curing period of 3 and 7
days. The stress – strain behaviour of the untreated and treated
soil samples S1 and S2 for different curing periods are
presented in Fig.1 and 2 respectively.
300
S1
2% PG, 3 Days
250
4% PG, 3 Days
Stress (kPa)
stabilizers to improve the properties of soil. Based on the
liquid limit, plasticity index and shrinkage limit both the soils
are classified as expansive soils (IS:1498-1970). The chemical
composition of soil samples, phosphogypsum and flyash are
shown in Table 2. For determining the effect of stabilizers on
soil samples, various percentages of PG (2, 4 and 6%) and
fixed quantity of FA (5%) were mixed with the soils.
6% PG, 3 Days
200
2% PG, 7 Days
4% PG, 7 Days
150
6% PG, 7 Days
100
50
0
0
TABLE - 2.
0.01
0.02
CHEMICAL COMPOSITION OF SOIL SAMPLES,
PHOSPHOGYPSUM AND FLYASH
Symbol
% by weight
Soil PG
S2
64.30
4.92
39.30
Insoluble residue
IR
Soil S1
84.92
Loss on ignition
LOI
13
14.25
16.5
2.74
Sulphur trioxide
SO3
0.12
0.23
44.56
4.21
Silica
SiO2
53.36
56.84
3.80
35.20
Calcium oxide
CaO
3.54
1.11
32.27
19.20
FA
0.05
0.06
0.07
Fig. 1. Comparison of stress-strain behaviour of treated and untreated soil S1
with different percentages of stabilizer (FlyAsh : 5%+PG%).
350
Magnesium oxide
MgO
0.73
0.6
---
1.73
Alumina
Al2O3
14.79
14.87
---
27.40
Ferric oxide
Fe2O3
7.76
6.95
---
6.83
IV. SAMPLE PREPARATION AND TESTS METHODS
For Unconfined Compressive strength test, samples of 38
mm diameter and 76 mm height were prepared by compacting
the samples at their Optimum Moisture Contents and
maximum dry density to maintain same dry density and water
content using split mould. For virgin soil the test was
conducted immediately after the sample preparation. For soil
treated with phosphogypsum and flyash, samples prepared
were placed in polythene covers which were cured by
covering them with wetted gunny bags. Samples prepared for
UCS tests were cured for 3 and 7 days and at the end of each
curing period, the specimens were tested until failure. All the
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0.04
Strain, %
S2
Stress (kPa)
Name of the
chemical
0.03
300
2% PG, 3 Days
250
4% PG, 3 Days
6% PG, 3 Days
2% PG, 7 Days
4% PG, 7 Days
6% PG, 7 Days
200
150
100
50
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Strain, %
Fig. 2. Comparison of stress-strain behaviour of treated and untreated soil S2
with different percentages of stabilizer (FlyAsh : 5%+PG%)
From the test results it can be observed that there is an
increase in stress with the addition of flyash and various
percentages of phosphogypsum for both the soils S1 and S2.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 9 Number 14 - Mar 2014
This trend was observed for both the curing period studied.
From the Fig. 1, it can be observed that the stress increased
rapidly and reached the peak at a particular strain for treated
soil. For the untreated soil, the peak stress is reached at
0.044% strain, whereas for the treated soil sample the peak
stress was observed at lesser strain of 0.031% and 0.025 % for
the curing periods of 3 and 7 days. The same behaviour was
observed for soil S2 which can be observed in the Fig.2. The
increase in UCS value for treated soil is compared with
untreated soil S1 and S2 and the increase in UCS values for
various conditions studied are given in Table-3.
VI. MICROSTRUCTURAL ANALYSIS
The scanning electron microscope technique was
employed using a FEI Quanta 200 FEG microscope to
qualitatively identify the microstructural developments in the
matrix of the soil, flyash, phosphogypsum and treated soil
specimens. Fig. 4 and 5 shows microstructural characteristics
of the stabilizers (ie., phosphogypsum and flyash) and soil
(ie.,virgin soil and treated soil).
TABLE – 3
COMPARISON BETWEEN PEAK UCS OF TREATED SOIL AND
UNTREATED SOIL
UCS Value (kPa)
Soil + % PG+ %FA
Sample S1
3 days
Soil + 0% PG+ 0%FA
7days
Sample S2
3 days
75.41
7 days
(a) Phosphogypsum
112
Soil + 2% PG+ 5%FA
204.9
224.9
240.6
268.1
Soil + 4% PG+ 5%FA
216.3
239.0
259.5
285.4
Soil + 6% PG+ 5%FA
223.5
266.5
266.5
292.1
The minimum increase in UCS value was 1.72 times as
that of the untreated soil for the stabilizer (FA:5%+PG2%)
addition = 2%, curing period = 3days in Soil S1 but for soil 2
the increase was 2.25 times that of the untreated soil. The
UCS value increased linearly with increase in stabilizer
content and was independent of the soil type, S1 and S2 (Fig.
3.)
350
(a) SEM photos of virgin soil S2
(b) SEM photo of treated sample S2
(6% PG & 5% FA)
Fig. 5. SEM photos of treated and untreated soil sample S2
It can be observed from the Fig. 4 and 5 that the samples
studied through SEM have quite different microstructures.
When flyash and phosphogypsum come in contact with water,
pozzolanic reaction between flyash and gypsum begins. Due
to these pozzolanic reactions occurring during the curing
period, the strength of soil treated with FA and PG is greatly
improved.
300
UCC value, kPa
(b) Flyash
Fig. 4. SEM photos of stabilizers
250
200
VII. CONCLUSION
150
Based on the experimental investigation conducted and the
analysis of test results, the following conclusions are drawn.
3 days S1
100
7 days S1
Strength of stabilised soils increased with increase in
amount of phosphogypsum in addition to flyash content of 5%.
3 days S2
50
7 days S2
0
0
2
4
6
8
Stabilizer, %
Fig. 3. Variation of UCS value with the addition of stabilizer on soil S1
and S2 (FA:5%)
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The curing period of the mix is a governing parameter as
the chemical reaction of stabilizers is depended on it. So it can
be concluded that the strength will increase with increase in
the curing period.
Unconfined compressive strengths of treated soils were
higher than that of untreated soils. The minimum increase is
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International Journal of Engineering Trends and Technology (IJETT) – Volume 9 Number 14 - Mar 2014
1.72 and 2.25 times for the Soil S1 and S2 on addition of 5%
flyash and 2% phosphogypsum at a curing period of 3 days.
Addition of combination of PG with FA stabilizer makes
the soil mixes durable, low-cost and effective for soil
stabilization.
ACKNOWLEDGMENT
The authors wish to thank Nano Technology Research
Centre, SRM University, Kattankulathur for the help in SEM
analysis.
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