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Strength Characteristics of Class F Fly Ash Modified with
Lime and Gypsum
Ambarish Ghosh1
and Chillara Subbarao2
Abstract: This paper presents the shear strength characteristics of a low lime class F fly ash modified with lime alone or in combination
with gypsum. Unconfined compression tests were conducted for both unsoaked and soaked specimens cured up to 90 days. Addition of
a small percentage of gypsum 共0.5 and 1.0%兲 along with lime 共4–10%兲 enhanced the shear strength of modified fly ash within short curing
periods 共7 and 28 days兲. The gain in unsoaked unconfined compressive strength 共qu兲 of the fly ash was 2,853 and 3,567% at 28 and 90
days curing, respectively, for addition of 10% lime along with 1% gypsum to the fly ash. The effect of 24 h soaking showed reduction of
qu varying from 30 to 2% depending on mix proportions and curing period. Unconsolidated undrained triaxial tests with pore-pressure
measurements were conducted for 7 and 28 days cured specimens. The cohesion of the Class F fly ash increased up to 3,150% with
addition of 10% lime along with 1% gypsum to the fly ash and cured for 28 days. The modified fly ash shows the values of Skempton’s
pore-pressure parameter, Af similar to that of over consolidated soils. The effects of lime content, gypsum content, and curing period on
the shear strength parameters of the fly ash are highlighted herein. Empirical relationships are proposed to estimate the design parameters
like deviatoric stress at failure, and cohesion of the modified fly ash. Thus, this modified fly ash with considerable shear strength may find
potential use in civil engineering construction fields.
DOI: 10.1061/共ASCE兲1090-0241共2007兲133:7共757兲
CE Database subject headings: Fly ash; Gypsum; Lime; Shear strength; Soil stabilization.
Introduction
Solid waste disposal has become an acute problem for many
countries due to rapid industrialization and urbanization. The de-
mand of power is increasing day by day. Major part of the power
is supplied by thermal power plants where coal is used as fuel and
a large quantity of fly ash emerges in the process. Fly ash creates
different environmental problems like leaching and dusting and
takes huge disposal area. Transforming this waste material into a
suitable construction material may minimize the cost of its dis-
posal and in alleviating environmental problems. Fly ash has
become an attractive construction material because of its self
hardening character which depends on the availability of free lime
in it. The variation of its properties depends on nature of coal,
fineness of pulverization, type of furnace, and firing temperature
共Raymond 1958; Gray and Lin 1972兲. According to ASTM clas-
sification ASTM C 618-03 共2003a兲 fly ashes fall in two types;
Class C and Class F. Class C fly ash high in calcium content
undergoes high reactivity with water even without addition of
lime 共Parsa
et al. 1996兲. Class F fly ash contains lower percentages of lime. It
lacks adequate shear strength for use in geotechnical applications
and requires stabilization with lime or cement and some admix-
tures to accelerate shear strength gain in short period. The fly ash
studied in the present investigation belongs to Class F.
Numerous studies on application of fly ash as bulk fill material
are available 共Raymond 1958; DiGioia and Nuzzo 1972; Gray
and Lin 1972; Joshi et al. 1975兲 which demonstrated the possibil-
ity of utilizing huge amount of fly ash in construction of embank-
ments, dykes, and road subgrade. A wide range of soils can be
stabilized using fly ash 共Chu et al. 1955; Goecker et al. 1956;
Viskochil et al. 1957; Ghosh et al. 1973; Vasquez and Alonso
1981; Lo and Wardani 2002兲. Other uses of fly ash are land rec-
lamation 共Kim and Chun 1994兲, and injection grouting 共Joshi et
al. 1981兲. Ghosh et al. 共2005兲 demonstrated the use of fly ash as
foundation medium reinforced with jute-geotextiles.
Undrained shear strength parameters of fly ash was reported
by Raymond 共1961兲. Gray and Lin 共1972兲 conducted undrained
triaxial test and unconfined compression test for fly ash specimens
cured up to 3.4 years. They showed through unconfined compres-
sion test results that lime stabilization enhanced the strength of
stabilized fly ash at elevated temperature or with long curing pe-
riod. Indraratna et al. 共1991兲 reported the unconfined compressive
strength and undrained triaxial strength for only fly ash. Perme-
ability and undrained shear strength parameters of solid waste
incinerator fly ash stabilized with lime and cement were reported
by Poran and Ahtchi-Ali 共1989兲. The strength characteristics
along with pore-pressure response study of stabilized Class F fly
ash have not received much attention of the previous researchers.
Sutherland et al. 共1968兲 reported that although the strength of
cement stabilized ashes is more compared to the corresponding
1
Assistant Professor, Dept. of Civil Engineering, Bengal Engineering
and Science Univ., Shibpur, Howrah-711 103, India 共corresponding
author兲. E-mail: ambarish@civil.becs.ac.in
2
Formerly, Professor and Head, Dept. of Civil Engineering, I.I.T
Kharagpur, Advisor–Consultant, Geo-Environ, D/3 Garud Heritage,
Pune-411 007, India. E-mail: csubbarao2005@yahoo.com
Note. Discussion open until December 1, 2007. Separate discussions
must be submitted for individual papers. To extend the closing date by
one month, a written request must be filed with the ASCE Managing
Editor. The manuscript for this paper was submitted for review and pos-
sible publication on June 15, 2005; approved on December 26, 2006. This
paper is part of the Journal of Geotechnical and Geoenvironmental
Engineering, Vol. 133, No. 7, July 1, 2007. ©ASCE, ISSN 1090-0241/
2007/7-757–766/$25.00.
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / JULY 2007 / 757
J. Geotech. Geoenviron. Eng. 2007.133:757-766.
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strength of lime stabilized ashes at early stages, the difference is
eliminated in three months in most of the cases.
The study presented herein is a part of the research work car-
ried out to investigate the suitability of Class F fly ash, containing
CaO as low as 1.4%, modified with lime and gypsum as a con-
struction material in different civil engineering fields. The modi-
fied Class F fly ash should satisfy the requirements of important
aspects like environmental impact, strength, durability, micro-
structural development, and longevity before it is recommended
for field applications 共Ghosh 1996兲. Fly ash may contain different
toxic metals depending on the sources of coal. Leaching of the
toxic metals may pollute the ground water or surface water.
Huang and Lovell 共1990兲 studied the leaching behavior of bottom
ash and its effect on ground water quality. Gidley and Sack 共1984兲
reported different solidification techniques for waste disposal
among which stabilization with lime was one of the promising
methods. Stabilization of the Class F fly ash with lime and gyp-
sum showed reduction in the quantity of metals leaching out from
compacted stabilized matrix compared to that of unstabilized mix
共Ghosh and Subbarao 1998兲.
Tensile strength is also a vital parameter to judge the suitabil-
ity of a stabilized fly ash to serve as a material in road construc-
tion 共Sobhan and Mashnad 2002兲. The study on tensile strength
characteristics and durability aspect of the Class F fly ash modi-
fied with lime 共10%兲 and gypsum 共1%兲 showed Brazillian tensile
strength and flexural strength values of about 22 and 29% of
unconfined compressive strength 共qu兲, respectively, at 45 days
curing, with medium high to high slake durability 共Ghosh and
Subbarao 2006a兲. From microstructural analysis, it is revealed
that the improvement of strength and durability of the Class F fly
ash, may be due to the formation of new reaction products such as
CSH1, due to the fly ash–lime reaction as well as the active par-
ticipation of gypsum in the reaction 共Ghosh and Subbarao 2001兲.
The interaction between fly ash and lime is complex and the
pozzolanic reaction is slow 共Croft 1964兲.
Though lime is used extensively for soil stabilization, Eades
and Grim 共1960兲 raised the question regarding longevity of lime
stabilized material. The stabilized material may be subjected to
leaching and lime may also be leached out from the matrix. The
strength of the matrix would depend on the amount of lime avail-
able for pozzolanic reaction. Hence, the leaching of lime should
be minimized to optimize the objectives of fly ash modification
with lime. The effectiveness of gypsum to reduce the leaching of
lime from stabilized matrix has been highlighted elsewhere
共Ghosh and Subbarao 2006b兲.
This paper presents the shear strength characteristics of a Class
F fly ash containing CaO: 1.4%, stabilized with lime 共4–10%兲 and
gypsum 共0.5 and 1.0%兲 through unconfined compression tests and
unconsolidated undrained triaxial tests with pore-pressure mea-
surements. Specimens were cured up to 90 days to study the long
term effect of lime and gypsum stabilization. The effects of lime
content, gypsum content, and curing period on shear strength
characteristics of the stabilized Class F fly ash are discussed
herein.
Based on the experimental findings and analysis of the test
results the following aspects of the stabilized Class F fly ash are
highlighted in this paper:
1. Shear strength characteristics of the fly ash stabilized with
lime and gypsum;
2. Pore-pressure response of the stabilized fly ash; and
3. Development of empirical relationships to estimate devia-
toric stress at failure and cohesion as function of unconfined
compressive strength of the stabilized fly ash.
Materials and Mix Proportions
The fly ash used in this study was collected in dry state from
Kolaghat Thermal Power Station, India, through electrostatic
precipitator. Grain size analysis 共Fig. 1兲 reveals that the fly ash
predominantly consists of silt-sized particles 共80%兲 with some
sand-sized particles 共13%兲, and clay-sized particles 共7%兲. The
uniformity coefficient 共Cu兲 and coefficient of curvature 共Cc兲 of
the fly ash are 5.44 and 3.12, respectively. The specific gravity of
this fly ash is 2.12. The chemical composition 共% by dry weight兲
of the fly ash is as follows: SiO2 =53.30%, Al2O3 =31.73%,
Fe2O3 =5.27%, CaO=1.40%, MgO=0.10%, loss on ignition
⫽5.50%, and others⫽2.70%. In accordance with ASTM classifi-
cation, this fly ash belongs to Class F type 共ASTM 2003a,b兲. This
fly ash was stabilized with hydrated lime having purity 69.1%.
The lime contents were 0, 4, 6, and 10% of the dry weight of fly
ash. To accelerate the fixation process analytical quality anhy-
drous gypsum was used in this investigation. The gypsum con-
tents were 0.0, 0.5, and 1.0%. Gypsum was added only to lime
stabilized fly ash mixes. The addition of gypsum was limited to
1.0% because higher percentages of gypsum may reduce the du-
rability of the stabilized matrix by producing more ettringite
共Rollings et al. 1999兲. In this paper, the mixes are designated in
the tables and graphs with a common coding system consisting of
three terms. The first term, FA stands for fly ash; the second and
third terms show the percentages of lime, L, and gypsum, G,
respectively. For example, a mix of fly ash, lime, and gypsum
containing 6% lime and 1% gypsum is designated as FA+6L
+1G. Total of ten mixes are used in the present study: FA+0L
+0G, FA+4L+0G, FA+6L+0G, FA+10L+0G, FA+4L+0.5G,
FA+6L+0.5G, FA+10L+0.5G, FA+4L+1G, FA+6L+1G, and
FA+10L+1G.
Moisture Density Relationship of Stabilized Fly Ash
Standard Proctor compaction tests were conducted in accordance
with ASTM D 698-92 共1992兲. Moisture content dry density rela-
tionships obtained from standard Proctor tests for the fly ash
mixes containing 0, 4, 6, and 10% lime are presented in Fig. 2.
The optimum moisture content 共OMC兲 varied from 31.5 to
35.4%, whereas the maximum dry density ranged from 1.045 to
1.103 Mg/m3
. Such low dry density of compacted fly ash was
Fig. 1. Grain size distribution curve of fly ash
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reported by a number of investigators 共Raymond 1961; DiGioia
and Nuzzo 1972; Indraratna et al. 1991兲. The nature of the com-
paction curve for unstabilized fly ash is fairly flat. Earlier inves-
tigators DiGioia and Nuzzo 共1972兲, and Indraratna et al. 共1991兲
also reported such type of compaction curve. This nature of fly
ash is beneficial for its field applications, as minor variation of
field moisture content may not alter the field dry density of the
compacted layer appreciably.
Specimen Preparation and Preservation
for Unconfined Compression Tests
and Triaxial Tests
Depending on the mix proportions, required amounts of materials
were mixed thoroughly in dry state. After dry mixing of the ma-
terials, water corresponding to OMC was spread over the dry mix
and thoroughly mixed. All specimens were prepared at maximum
dry density and optimum moisture content 共OMC兲 of the respec-
tive mixes 共Fig. 2兲 as obtained from standard Proctor compaction
test 共ASTM 1992兲. The values of dry density and molding water
content used for specimen preparation for the mixes with lime
along with gypsum were the same values for mixes with corre-
sponding lime content obtained from standard Proctor compaction
test. The specimens were compacted in layers into a split mold of
size 38 mm diameter and 76 mm height to achieve dry unit
weight corresponding to maximum dry density obtained from
Proctor compaction test at corresponding OMC. Each specimen
was extracted from the split mold after compaction, by pushing
it in the upward direction. The weights of the specimens and
moisture contents of the mixes were checked immediately after
specimen preparation. Those specimens having dry density and
molding water content not within ±0.15 and ±0.25% of maximum
dry density and optimum moisture content, respectively were re-
jected. Immediately after preparation, the specimens were kept in
moist-proof covers and placed inside humidity control chamber at
30±1°C temperature and humidity 艌95%. These specimens
were used for both unconfined compression tests and unconsoli-
dated undrained triaxial tests with pore-pressure measurements.
Unconfined Compression Tests
It is a common practice to determine the strength of stabilized
materials from unconfined compression test. Unconfined com-
pression tests were conducted in accordance with ASTM
D2166-85 共1985兲. To study the effect of pozzolanic reaction on
shear strength, specimens were cured for 7, 28, 45, and 90 days.
The stabilized fly ash may be subjected to inundation in the field.
To assess the effect of soaking two series of tests were conducted
on unsoaked and soaked specimens compacted at OMC. For
soaking specimens were immersed in water for 24 hours after
curing as this procedure was adopted by earlier researchers 共e.g.,
Chu et al. 1955; Schnaid et al. 2001; Lo and Wardani 2002兲.
Some of the specimens disintegrated while soaking due to lack of
bond between the particles and hence for those mixes, soaked
shear strength results are not available.
Results and Discussion—Unconfined Compression
Tests
Figs. 3, 4共a and b兲 illustrate the variation of unsoaked unconfined
compressive strength 共qu, kPa兲 with curing period of the fly ash–
lime mixes modified with 0.0, 0.5, and 1.0% gypsum, respec-
tively. Figs. 5共a and b兲 present the variation of soaked qu for fly
ash–lime mixes modified with 0.5 and 1.0% gypsum, respec-
tively. The effects of lime content, gypsum content, curing period,
and soaking on unconfined compressive strength of stabilized fly
ash are explained in the following sections.
Effect of Lime Content
It is observed from Fig. 3 that addition of lime has increased the
shear strength of the stabilized mixes due to increase in availabil-
ity of lime for pozzolanic reaction. The rate of gain in shear
strength is high for higher lime content 共Fig. 3兲. Fly ash mix
stabilized with 10% lime attains 5,901 kPa unsoaked qu at 90
days curing whereas the values are 172, 1,200, and 3,130 kPa for
mixes stabilized with 0, 4, and 6% lime, respectively. Table 1
shows the percentage increase in unconfined compressive strength
due to addition of lime to fly ash for unsoaked specimens only.
The values for the specimens disintegrated during soaking are not
presented in Table 1. The contribution of lime over unstabilized
mix for curing period from 7 to 90 days are about 10, 20, and 30
times of their corresponding shear strengths for the mixes con-
Fig. 2. Standard compaction test results of fly ash with varying
percentages of lime
Fig. 3. Unconfined compressive strength of fly ash versus curing
period for unsoaked specimens with varying percentages of lime
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taining 4, 6, and 10% lime, respectively. At higher curing period
共90 days兲 loss in shear strength of lime–stabilized fly ash, due to
soaking is restricted to 28% 共Ghosh 1996兲.
Effect of Gypsum Content
Addition of a small percentage of gypsum 0.5 or 1.0% to the fly
ash–lime mix increased the shear strength of compacted speci-
mens at lower curing periods 共7 and 28 days兲 compared to only
lime stabilized mixes 共Fig. 4兲. Table 2 presents the percentage
increase in unconfined compressive strength of lime stabilized fly
ash mixes due to addition of gypsum. It is revealed that the con-
tribution of gypsum at early stages of curing is significant at
curing periods up to 45 days and increases with increase in gyp-
sum content from 0.5 to 1.0%. But at higher curing period 共90
days兲 the contribution of gypsum is comparatively less because at
higher curing period only lime stabilized mixes itself attains high
shear strength. Addition of gypsum to lime stabilized fly ash re-
stricted the loss in shear strength due to soaking within 25% 共Fig.
5兲, whereas only lime stabilized specimens disintegrated due to
soaking except for 90 days cured specimens. It implies that addi-
tion of gypsum increases the bond strength between the particles
by accelerating the formation of pozzolanic reaction products.
Effect of Curing Period
The rate of gain in shear strength with curing period for lime
stabilized mixes 共Fig. 3兲 is low at the beginning but it increases
with increase in curing period 共45 days onward兲. Similar type of
behavior was reported by Consoli et al. 共2001兲 for soil–fly ash–
carbide lime mixture, when rate of gain in strength increased
appreciably after 90 days of curing. The low shear strength gain
may be due to the low pH values of the pore fluid in the first few
days 共Fraay et al. 1990兲. The pozzolanic reaction accelerates at a
later stage of curing. The enhancement of strength for 10% lime
addition to fly ash is 122, 303, 496, and 3,331% compared to that
of unstabilized mix at 7, 28, 45, and 90 days curing period, re-
spectively 共Table 1兲. Higher curing period 共90 days兲 can enhance
the shear strength 共unsoaked兲 of only lime 共10%兲 stabilized mix
period for unsoaked specimens with varying percentages of lime and
共a兲 0.5%; 共b兲 1.0% gypsum
period for soaked specimens with varying percentages of lime and 共a兲
0.5%; 共b兲 1.0% gypsum
Table 1. Percentage Increase in Unsoaked Unconfined Compressive
Strength 共qu兲 due to Addition of Lime to Class F Fly Ash
Curing period 共days兲
Mix 7 28 45 90
FA+4L+0G 70 134 145 598
FA+6L+0G 94 225 274 1,720
FA+10L+0G 122 303 496 3,331
Note: FA⫽fly ash; L⫽% lime; and G⫽% gypsum.
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to 5,901 kPa and for mix with 10% lime and 1% gypsum the
corresponding shear strength is 6,308 kPa. Increase in curing pe-
riod also increases the soaked shear strength of compacted speci-
mens stabilized with gypsum along with lime.
Effect of Soaking
It is observed that there is reduction in shear strength of stabilized
specimens due to soaking irrespective of mix proportion and cur-
ing period 关Figs. 4共a兲 and 5共a兲, 4共b兲 and 5共b兲兴. The loss of qu due
to soaking varies from 30 to 2% depending on the mix proportion
and curing period. Lo and Wardani 共2002兲 reported that soaked
unconfined compressive strength of silt stabilized by cement and
fly ash mixture was only 30% of that of the respective unsoaked
specimen. For unsoaked specimen, there is a possibility of suction
development in the pore fluid which gives rise to high compres-
sive strength 共Indraratna et al. 1991兲. Soaking of the specimens
may fill the voids to certain extent and reduces the chances of
development of suction in the pore fluid. While soaking, softening
of the specimens may take place reducing the shear strength,
whereas during soaking, the specimens can get sufficient moisture
for pozzolanic reaction and hence the shear strength may increase
on formation of reaction products. In this investigation, it is ob-
served that the shear strength has reduced on soaking for 24 h
which implies that the former two mechanisms i.e., probability of
low suction development in soaked specimens and softening of
the specimens have dominated over the third mechanism of gain
in shear strength due to pozzolanic reaction in presence of suffi-
cient moisture. Test results of saturated specimens may be used in
practice to avoid the assessment of development of suction in
partially saturated specimens. The reduction in strength due to
soaking is also governed by the hydraulic conductivity of the
stabilized matrix. The hydraulic conductivity of the Class F fly
ash stabilized with lime 共0.0–10.0%兲 and gypsum 共0.5 and 1.0%兲
varies from 4.4⫻10−6
to 1.0⫻10−7
cm/s 共Ghosh and Subbarao
1998兲. This low hydraulic conductivity of the stabilized matrix is
also beneficial to minimize the loss of strength due to inundation
in the field.
Unconsolidated Undrained Triaxial Tests with
Pore-Pressure Measurements
Shear strength parameters of stabilized fly ash used for field ap-
plications need to be assessed from triaxial test in which field
conditions are simulated. Depending on the field conditions, the
total shear strength parameters or the effective shear strength pa-
rameters should be used. Drained triaxial test of compacted sta-
bilized fly ash takes long time because the permeability of the
material is very low around 10−7
cm/s 共Ghosh 1996; Ghosh and
Subbarao 1998兲. For this reason the shear strength parameters of
the fly ash stabilized with lime and gypsum were determined from
unconsolidated undrained triaxial tests with pore-pressure mea-
surements in accordance with ASTM D 2850-03 共2003b兲. The
specimens were tested in a triaxial test set up AIM 049 connected
to a triaxial shear indicator 共Model SPL, Syscon make兲, used for
recording the data.
Three confining pressures 100, 200, and 400 kPa were adopted
in this study. This pressure range is similar to that adopted by
Mitchell and Wong 共1982兲 for triaxial tests on cemented tailings
sands. The strain rate was 0.105%/min chosen on the basis of the
permeability of the compacted stabilized specimens and in the
range recommended by Bishop and Henkel 共1957兲. Strips of filter
papers were used all around the specimens for efficient drainage
as suggested by Bishop and Henkel 共1957兲, as the permeability of
the compacted stabilized specimens was very low. The specimens
were sealed in 0.24-mm-thick rubber membranes with four O
rings around the pedestal and loading cap. Specimens were satu-
rated by applying back pressure and by measuring B-value as
described by Chaney et al. 共1979兲. To saturate the specimens, cell
pressure and back pressure were applied in steps of 25 kPa at a
time and after each increment sufficient time was allowed to
equilibrate the applied pressure. During this process, the value of
applied cell pressure was always maintained higher than that of
the backpressure. The maximum value of backpressure applied
for saturation of specimens was dependent on the mix proportions
and curing period; for unstabilized specimens, 175–200 kPa and
for specimens with 10% lime and 1% gypsum, 300–350 kPa. To
reduce number of cycles of loading for B-value check, the back
pressure was increased to the above mentioned values depending
on mix proportions. After raising the back pressure to the value as
mentioned above the cell pressure and the backpressure were re-
duced to zero to attain zero pressure gradient 共Chaney et al.
1979兲, step wise and then the cell pressure was applied to mea-
sure the developed pore pressure and to calculate the B-value.
The procedure was repeated till the asymptotic values of B were
obtained.
Results and Discussion—Unconsolidated Undrained
Triaxial Tests with Pore-Pressure Measurements
Unconsolidated undrained triaxial tests with pore-pressure mea-
surements were conducted for all the ten mixes cured for 7 and 28
days. Figs. 6–9 illustrate typical stress-strain graphs along with
pore-pressure response for the fly ash/stabilized fly ash. Table 3
presents the deviatoric stresses at failure for 7 and 28 days cured
specimens. The values of cohesion and angle of internal friction
of the stabilized fly ash are summarized in Table 4. In this inves-
tigation, the effects of different factors such as lime content, gyp-
sum content, curing period, and confining pressure on the shear
strength characteristics, stress–strain relationship, and pore-
pressure response of fly ash stabilized with lime and gypsum are
studied and explained as follows.
Shear Strength of Stabilized Fly ash
From Table 3 it is revealed that with the addition of lime, the
deviatoric stresses at failure 共qf兲 have increased for all the lime
Table 2. Percentage Increase in Unconfined Compressive Strength 共qu兲
due to Addition of Gypsum to Lime Stabilized Class F Fly Ash
7 28 45 90
Mix US S US S US S US S
FA+4L+0.5G 284 — 636 * 613 * 139 182
FA+6L+0.5G 290 — 460 * 454 595 45 56
FA+10L+0.5G 244 — 359 * 276 363 7 2
FA+4L+1G 681 — 808 * 851 * 228 236
FA+6L+1G 583 — 590 * 714 874 52 68
FA+10L+1G 591 — 634 * 489 677 7 6
Note: US⫽unsoaked test; S⫽soaked test; —⫽specimens disintegrated
when soaking; *⫽comparison is not feasible as corresponding lime sta-
bilized specimens have been disintegrated when soaking; FA⫽fly ash;
L⫽% lime; and G⫽% gypsum.
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Fig. 6. Stress–strain and pore-pressure response of fly ash, 7 days
curing
Fig. 7. Stress–strain and pore-pressure response of fly ash with 10%
lime and 1% gypsum, 7 days curing
Fig. 8. Stress–strain and pore-pressure response of fly ash, 28 days
curing
Fig. 9. Stress–strain and pore-pressure response of fly ash with 10%
lime and 1% gypsum, 28 days curing
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contents 共4, 6, and 10%兲. When a small percentage of gypsum
共0.5 or 1.0%兲 was added along with lime deviatoric stresses at
failure have increased considerably, compared with deviatoric
stresses of only lime stabilized specimens for curing period up to
28 days. There is increase in deviatoric stress with increase in
curing period for unstabilized as well as lime and gypsum stabi-
lized specimens. Effect of confining pressure on qf is more promi-
nent for the specimens of low strength mixes than for specimens
of high strength. The increase in shear strength for gypsum
addition along with lime is due to the development of more ce-
mentatious products in the stabilized matrix as a result of fly
ash–lime–gypsum interaction 共Ghosh and Subbarao 2001兲. From
Table 4, it is clear that addition of lime or lime combined with
gypsum increased the cohesion and angle of internal friction of
the fly ash. The increase in cohesion part of the shear strength of
stabilized fly ash is more significant due to development of bond-
ing between the particles on stabilization. The cohesion of lime
and gypsum stabilized mixes is always greater than the cohesion
of only lime stabilized mixes 共Table 4兲, proving the effectiveness
of the addition of gypsum. The total and effective cohesion of fly
ash specimens stabilized with 10% lime, cured for 28 days were
90 and 120 kPa, respectively. The total cohesion of lime-gypsum
stabilized fly ash varied from 315 to 700 kPa at 28 days curing.
The effective angle of internal friction of the lime-gypsum stabi-
lized fly ash mixes 共28 days curing兲 varied from 37.5 to 42.5°,
whereas the effective cohesion ranged from 360 to 720 kPa.
Stress–Strain Relationship and Failure Pattern
From Figs. 6–9 it is clear that the stress-strain response of stabi-
lized fly ash is affected due to modification of Class F fly ash with
lime and gypsum. Mixes containing lime only and cured for 7
days have shown stress–strain response similar to that of unstabi-
lized fly ash. For unstabilized and lime stabilized specimens,
bulging of the specimens without development of distinct failure
plane was observed 共Ghosh 1996兲. This type of failure may be
due to the low pozzolanic reaction of the Class F fly ash
共CaO:1.4%兲 with lime and due to a little change in microstruc-
ture of the matrix 共Ghosh 1996; Ghosh and Subbarao 2001兲.
Specimens stabilized with gypsum along with lime showed sharp
peak in the stress–strain curve and immediately after attaining
peak deviatoric stress there was rapid reduction in deviatoric
stress with increase in strain for both the curing periods 7 and 28
days. In this type of specimens, distinct failure planes developed
and with increase in lime and gypsum content the inclination of
the failure planes with vertical axis of the specimens decreased
共Ghosh 1996兲. At higher curing period 共28 days兲, specimens with
high lime and gypsum contents were observed to split nearly
along vertical plane.
Pore-Pressure Response of Stabilized Fly ash
From pore-pressure response curves 共Figs. 6–9兲 it is revealed that
the pore pressure has increased initially and then decreased with
strain. The values of the developed pore pressures, show decreas-
ing trend, with increase in shear strength of the specimens. It is
observed that the pore pressure attains its peak value before a
specimen has failed, possibly due to the development of minute
cracks in the specimen before deviatoric stress reaches its maxi-
mum value. The minute cracks may increase the void space or the
Table 3. Deviatoric Stress at Failure, qf 共kPa兲
7 28
Mix
␴3=100
共kPa兲
␴3=200
共kPa兲
␴3=400
共kPa兲
␴3=100
共kPa兲
␴3=200
共kPa兲
␴3=400
共kPa兲
FA+0L+0G 144 287 596 162 358 737
FA+4L+0G 257 443 813 344 637 1,272
FA+6L+0G 361 781 1,166 464 869 1,435
FA+10L+0G 447 803 1,333 630 993 1,537
FA+4L+0.5G 1,347 1,837 2,478 1,680 1,952 2,619
FA+6L+0.5G 1,477 1,916 2,606 2,350 2,670 3,382
FA+10L+0.5G 1,649 2,029 2,844 2,385 2,704 3,433
FA+4L+1G 2,384 2,900 3,688 2,898 3,133 3,865
FA+6L+1G 2,533 2,946 3,692 3,020 3,311 3,900
FA+10L+1G 2,875 3,297 3,928 3,324 4,192 4,668
Note: ␴3⫽confining pressure 共kPa兲; FA⫽fly ash; L⫽% lime; and G⫽%
gypsum.
Table 4. Total and Effective Shear Strength Parameters
Total shear strength parameters Effective shear strength parameters
Curing period 共days兲 Curing period 共days兲
7 28 7 28
Mix
c
共kPa兲
␾
共deg兲
c
共kPa兲
␾
共deg兲
c⬘
共kPa兲
␾⬘
共deg兲
c⬘
共kPa兲
␾⬘
共deg兲
FA+0L+0G 0.0 25.0 0.0 28.5 15.0 28.0 22.0 31.0
FA+4L+0G 19.0 30.0 16.0 37.0 38.0 32.5 50.0 38.5
FA+6L+0G 30.0 35.0 32.0 38.5 73.0 36.5 75.0 40.5
FA+10L+0G 50.0 36.0 90.0 37.0 76.0 39.0 120.0 39.0
FA+4L+0.5G 220.0 41.0 315.0 38.0 260.0 41.2 360.0 39.0
FA+6L+0.5G 240.0 41.5 400.0 41.5 295.0 41.3 475.0 41.0
FA+10L+0.5G 300.0 41.0 475.0 39.5 320.0 41.3 495.0 41.0
FA+4L+1G 420.0 43.0 600.0 39.0 360.0 41.0 655.0 38.5
FA+6L+1G 480.0 41.0 680.0 36.5 500.0 42.0 708.0 37.5
FA+10L+1G 560.0 41.0 700.0 41.5 615.0 41.0 720.0 42.5
Note: c⫽total cohesion 共kPa兲; c⬘⫽effective cohesion 共kPa兲; FA⫽fly ash; L⫽% lime; G⫽% gypsum; ␾⫽total angle of internal friction 共deg兲; and
␾⬘⫽effective angle of internal friction 共deg兲.
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interconnectivity of the pore channels, which causes the pore
pressure to decrease or to be at stationary stage. Skempton’s pore-
pressure parameter B varied from 0.68 to 0.13 for the whole range
of specimens tested herein. The values of B decreased with in-
crease of lime content or lime combined with gypsum, increase in
curing period, and increase in confining pressure; that is with
increase in stiffness of the modified fly ash matrix due to the high
strength of the skeleton of the stabilized fly ash specimens and for
the range of the confining pressures 共up to 400 kPa兲 used in this
investigation. Chaney et al. 共1979兲 reported the B-value as 0.20
for very stiff soils even at 99.5% saturation level. Wissa 共1969兲
also reported such response in case of stiff soils. Raymond 共1961兲
reported that the pore-pressure parameter B from undrained shear
test of ash ranged from 0.01 to 0.15 and attributed this low value
partly due to the high strength of the matrix. In the present study,
the values of Skempton’s pore-pressure parameter, Af, were in the
range of −0.065 to +0.057, indicating that the behavior of stabi-
lized fly ash is similar to that of stiff soils.
Correlation Study
In the present study, an attempt has been made to develop empiri-
cal relationships to estimate the parameters obtained from triaxial
test such as deviatoric stress at failure 共qf兲 and cohesion 共c兲 as
function of unsoaked qu. The shear strength characteristics of the
stabilized fly ash depend on a number of governing factors such
as type of fly ash, lime content, gypsum content, curing period,
and dry density. However, unconfined compressive strength 共qu兲
may be taken as the reference variable in estimating the above
mentioned parameters, as values of qu may represent the com-
bined effects of the important governing factors on the shear
strength characteristics.
Fig. 10 shows that the deviatoric stress at failure, qf obtained
from undrained triaxial tests with pore-pressure measurements,
can be expressed as a linear function of unsoaked qu of Class F fly
ash modified with lime alone or in combination with gypsum. The
empirical relationships along with the values of coefficient of
determination 共R2
兲 are presented in the respective figures for cor-
responding confining pressures and curing periods. It is revealed
from the Fig. 10 that the value of qf changes with curing period as
well as with the confining pressure 共␴3兲. The effect of curing
period may be taken care of by the values of unconfined compres-
sive strength. With this basis, a general empirical relationship for
qf 共kPa兲 is developed as function of unsoaked qu 共kPa兲 and ␴3
共kPa兲. Using multiple regression analysis 共Draper and Smith
1998兲 of the test results of all the ten mixes, two curing periods 共7
and 28 days兲 and three confining pressures 共100, 200, and
400 kPa兲, the empirical relationship for qf obtained may be ex-
pressed as follows:
qf = 0.9qu + 3.0␴3, R2
= 0.852 共1兲
Such a relationship for cemented sand was proposed by Schnaid
et al. 共2001兲 to estimate deviatoric stress at failure.
The linear relationship between total cohesion and unsoaked
qu of the stabilized Class F fly ash is presented in Fig. 11. The
values of the cohesion for both 7 and 28 days cured specimens
共Table 4兲 are considered in developing the model. The empirical
relationship for total cohesion c 共kPa兲 in simple form as function
of unsoaked unconfined compressive strength, qu 共kPa兲, along
with coefficient of determination 共R2
兲, obtained by applying the
least-squares regression technique 共Draper and Smith 1998兲, is
presented as follows:
Fig. 10. Relationship between deviatoric stress at failure and uncon-
fined compressive strength of stabilized fly ash for 共a兲 7; 共b兲 28 days
curing
Fig. 11. Relationship between cohesion and unconfined compressive
strength of stabilized fly ash
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c = 0.20qu, R2
= 0.950 共2兲
Similar relationship for effective cohesion is also obtained and the
relationship 关Eq. 共2兲兴 may be used to estimate both total and ef-
fective cohesion from unconfined compression test results.
The empirical relationships presented above in simple form
consider unconfined compressive strength 共unsoaked兲 as the inde-
pendent variable capable of representing the combined effects of
the factors that can influence the shear strength characteristics of
the stabilized fly ash. The models were selected based on the
physical significance. Due to paucity of relevant data in literature
for fly ash, the proposed relationships could not be checked for
other type of Class F fly ash. However, these relationships may be
refined when additional experimental results become available for
wide application in construction field.
Conclusions
The shear strength characteristics of a Class F fly ash were stud-
ied through unconfined compression tests and unconsolidated
undrained triaxial tests with pore-pressure measurements 共Table
5兲. The fly ash was stabilized with 4–10% lime alone or in com-
bination with gypsum 共0.5 and 1.0%兲. The specimens were cured
up to 90 days. Both soaked and unsoaked unconfined compres-
sion tests were conducted. Empirical relationships are developed
to estimate deviatoric stress at failure and cohesion as
functions of unsoaked unconfined compressive strength. The fol-
lowing conclusions may be drawn from the test results and the
discussions presented herein.
• Stabilization of a low lime Class F fly ash with lime 共up to
10%兲 is effective to improve the shear strength characteristics;
• Addition of a small percentage of gypsum 共0.5 and 1.0%兲
along with lime to fly ash enhances the gain in shear strength
at early curing periods 共7 and 28 days兲;
• Gypsum along with lime is effective to control the loss of
shear strength due to soaking for specimens cured for 28 days
or more. The loss of shear strength due to soaking of such
specimens is limited to 25%. Specimen stabilized with only
lime showed soaked qu about 72% of unsoaked qu at 90 days
curing;
• Fly ash stabilized with only lime requires longer curing period,
45 days and more, to gain considerable shear strength;
• Fly ash stabilized with 10% lime and 1% gypsum has achieved
unconfined compressive strength 共qu兲 of 6308 kPa at 90 days
curing;
• The pore-pressure response of the stabilized fly ash is similar
to that of stiff soils. The peak pore pressure develops before
the deviatoric stress reaches its maximum value irrespective of
the mix proportions. Skempton’s pore-pressure parameters Af
and B vary from −0.065 to +0.057 and 0.68 to 0.13, respec-
tively, for the stabilized fly ash specimens tested in this inves-
tigation; and
• Simple empirical relationships are recommended to estimate
deviatoric stress at failure and cohesion from unsoaked uncon-
fined compressive strength.
Thus, fly ash containing CaO as low as 1.4%, stabilized with
lime and a small percentage of gypsum may find potential appli-
cation in road and embankment constructions for its strength
characteristics, durability, longevity, and environmental safety.
The stabilized fly ash having low hydraulic conductivity and al-
kaline environment of pore fluid may find use in construction of
waste containment liners, cut off walls, and vertical barriers.
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