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Report on Research Project
Use of Solid Waste to Enhance Properties of Problematic Soils of Karnataka
to
CiSTUP
Indian Institute of Science
Bangalore 560 012
Investigator:
Prof. P V SIVAPULLAIAH
Department of Civil Engineering Indian Institute of Science
Bangalore 560012
April 2013
2
INTRODUCTION
The research project on the “Use of solid Waste to improve the properties of problematic
soils with solid waste” was sanctioned in April 2011.
The scope of the project for a duration of two years was
1. To improve geotechnical properties of soils amended with solid waste materials.
2. To promote safe uses of solid waste materials such as fly ash, red mud, blast furnace
slag, waste gypsum, lime sludge and rock flour etc in infrastructure development
projects such as roads, embankments, etc.
3. Study the leaching tests on stabilised materials
PROBLEMATIC SOILS
Two important soils that pose serious problems for geotechnical engineers in Karnataka are:
1. Dispersive soil
2. Expansive soils
SOLID WASTE MATERIALS
There has been a great deal of concern about land pollution since the onset of industrializa-
tion. The attention is mainly because of incidents of contamination, the scarcity of usable
land and increased general concern about the effect of industrial activity on the environment.
Till recently land disposal has been the only option available for the solid residues, which
may be concentrated with toxic contaminants. Use of the waste materials is big option in
finding solution to this problem. On the other hand these solid wastes may have potential for
reuse. Bulk utilization of industrial solid waste is very important. One such potential applica-
tion is for infrastructure development works in civil engineering. For example stabilisation of
problematic soils using waste can achieve great deal of economy and environmental safety.
Among all the wastes, the wastes which are produced in large amounts in India are the Fly
ash and the GGBS and are associated with the disposal and environmental problem. Thus the
two major solid wastes considered are:
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1. Fly Ash and
2. Granulated Blast Furnace Slag
APPROACH
1. Collection of waste materials from various industries and utilize them to modify ma-
jor types of soils which are not favourable for infrastructural development commonly
occurring in Karnataka.
2. To study the geotechnical properties and carry tests such as CBR tests to assess the
suitability of amended soil for bulk utilization for infrastructure projects such as
roads and embankments.
3. Sustainability of the improvement in soil properties.
DISPERSIVE SOILS
Problems associated with dispersive soils are reported. Soils that are dislodged easily and
rapidly in flowing water of low salt concentration are called dispersive soils.
A number of failures of earth dams, hydraulic structure, and road way embankments have
occurred due to erosion problems of these soils. Dispersive piping in dams has occurred ei-
ther on the first reservoir filling or, less frequently, after raising the reservoir to highest level.
Tunnelling failures commence at the upstream face when the reservoir is filled for the first
time, the settlement may accompany saturation of the soil, particularly if the soil was placed
dry of optimum and not well compacted. Settlement below the phreatic surface and arching
above can result in crack formation. Water moving through the cracks picks up dispersive
clay particles, with the rate of removal increasing as the seepage velocity increases.
Failures also have occurred in embankments, dams and slopes composed of clays with
low-to-medium plasticity (CL and CL-CH) that contain montmorillonite. Also, most studies
reported in the literature have shown that failures of structures built of dispersive clay soils
occurred on first wetting. All failures were associated with the presence of water and crack-
ing by shrinkage, differential settlement, or construction deficiencies. These failures empha-
size the importance of early recognition and identification of dispersive clay soils; if the dis-
persive soils are not identified beforehand, the problems they cause can result in sudden, ir-
reversible, and catastrophic failures.
4
Generally it was believed that clays soils were non-erosive by nature. However, it was found
that highly erosive clays do also exist in nature. Dispersive phenomenon is of a chemical-
physical state to condensed state; the repulsion force nature usually influenced by the type of
soil’s minerals between the particles and then dispersive potential of and chemical properties
of pore water.The tendency of clays to disperse or deflocculate depends on clay type and soil
chemistry. Such clays are eroded rapidly by slow-moving water, even when compared to co-
hesionless fine sands and silts. When the clay fraction in dispersive soil comes in contact with
water, it behaves like a single grained particle with less electrochemical attraction and does
not adhere with other soil particle. The electrical surface force (inter particle repulsive force)
exceeds the Van der walls attraction and the detached clay particles are carried away causing
piping in earth dams. The dispersiveness of soils is mainly due to the presence of sodium ions
in the soil structure and not due to the presence of sodium in the pore water.
Dispersive Soil Collection
Suddha Soil
Many earth dams, hydraulic structures and other structures like road way embankments in
Southern part of Karnataka have suffered serious erosion problems and have failed due to
the presence of dispersive soils. This has been attributed to occurrence of a native soil called
‘Suddha soil’. It is wide spread below a depth of 1.5 m from the ground level. It is known to
possess good strength in dry condition and upon increase in moisture content looses
strength. Failures of canal slopes, road bases, and foundation failures have occurred at
stretches where Suddha soil is present is a common observation. This soil has been selected
for detailed studies.
A soil, locally called Suddha soil, found in Southern parts of Karnataka was used. The soil
was collected by open excavation from a depth of 1 meter from the natural ground. Source,
collection and properties of these are described in the following sections. The physic chemi-
cal and geotechnical properties of soil which are carried out by standard methods listed in
Tables 1 and 2.
Table 1 Physico-Chemical Properties of Suddha Soil
Cations Value
5
Na+ 0.56 (meq/100 g
Ca++ 36.93 (meq/100 g )
Mg++ 13.96 (meq/100 g)
K+ 0.47 (meq/100 g)
Total 51.91 meq/100 g
Sodium Adsorption Ratio 7.7 Exchangeable Sodium Per-
centage 1.1 %
pH 7.2
Table 2 Geotechnical Properties of Suddha Soil
STABILIZATION OF SUDHA SOIL USING WASTE MATERIALS
Sl. No. Properties Suddha soil
1
Particle size analysis
Gravel (%) 4
Sand (%) 13
Silt (%) 55
Clay (%) 28
2 Liquid limit (%) 65
3 Plastic limit (%) 28
4 Plasticity index 35
5 Specific gravity 2.3
6
Compaction Characteristics
Optimum Moisture Content, (%) 27
Maximum dry density (kN/m3) 14.5
6
With Fly ash
The disposal of fly ash is becoming more expensive each year due to large land needs for its
disposal. The best way to solve the disposal problem of fly ash is to decrease the quantity for
disposal by bulk utilization for infrastructure development. Fly ash has been increasingly util-
ized in construction application, such as roads, embankments, fills, concrete, pavements,
wastewater treatment, landfill barrier material, grouts and others.
The addition of fly ash results in a high strength, swell-resistant monolithic solid, attain-
ing levels of strength similar to those of concrete products.
With respect to heavy metal release, the addition of fly ash was directly responsible for
the effective immobilization of both lead and hexavalent chromium, whereas it further en-
hances trivalent chromium immobilization. Extensive work has been carried out by investiga-
tor on the ability of fly ash to retain different metal ions.
Fly ash has been used as an admixture for stabilization of soils. Studies conducted on the
effect of fly ash on compaction and strength behavior of Suddha soil has been brought out
here. Fly ash is generated in vast quantities as a by-product of burning of coal at electric
power plants. About 80 million tonnes of fly ash are being produced in India annually. Less
than 5 percent fly ash produced is used in the manufacture of bricks, pozzolanic cement and
other products. Fly ash belongs to group of aluminosilicates or calciumaluminosilicates, a
pozzolanic material that by itself does not possess any strength property on its own, but in
infinitely divided state and in presence of water form cementitious products impairing a
characteristic hardness. Nearly 70 to 80 percent of the coal ash produced is fly ash and the
remaining 20 to 30 percent is bottom ash.
Fly ash has been used in large quantities in many geotechnical applications. Another
emerging area is to use fly ash to stabilize the problematic soils. The important properties in
stabilization of soils using fly ash are compaction and strength properties. Fly ashes often
contain both reactive silica and lime and produce cementitious products with the presence of
water. There are 3 different types of fly ashes from the consideration of pozzolanic nature-
self pozzolanic, pozzolanic and non pozzolanic fly ashes. Fly ashes with good reactive silica
and content and sufficient lime content are called self pozzolanic fly ashes. Fly ashes contain-
ing reactive silica but inadequate lime content are called pozzolanic fly ashes. They develop
good strength with the addition of lime. Fly ashes without reactive silica which cannot de-
velop strength even with the addition of lime are called non pozzolanic fly ashes.
7
The pozzolanic materials in the ash react with water and CaO, both from the lime and
from the cement. It forms similar products as in the cement reactions. Since the pozzolanic
reactions are very slow, the strength increase caused by fly ash is also very slow. The size
and the rate of the strength increase are mainly determined by the chemical composition of
the fly ash. It can vary significantly from ash to ash since fuel combustion and combustion
technology are the main factors deciding the quality of ash.
Mechanism of Stabilization of Soil Using Fly Ash
The efficiency of stabilization of soil using fly ash depends on the type of fly ash used. The
chemical and physical properties and hence the pozzolanic reactivity of fly ash depend on the
type and source from where it has been obtained. The class F fly ash produced from bitumi-
nous and sub bituminous coal requires lime to develop pozzolanic properties. In geotechnical
engineering fly ash has been extensively used for stabilization of different types of problem-
atic soils. Self pozzolanic fly ashes are directly used for stabilization of soils and other fly
ashes are used along with admixtures such as lime gypsum etc. The presence of free lime in
self pozzolanic fly ash aids in the chemical reactions with reactive silica in presence of water
to form cementitious gel. The soil particles are bound by the gel and get hardened with cur-
ing
Class F fly ash contains small amount of lime (CaO). This fly ash contains siliceous
and aluminous material that possesses little cementitious properties. Silica, alumina and sec-
ondary oxides of calcium, magnesium, iron and sulphur are the most common minerals found
in fly ash. Calcium oxide is the important chemical component of fly ash for soil modifica-
tion. Calcium oxides in presence of water dissociates into calcium cations and hydroxide
anions. The amount of calcium cations available from calcium oxide depends on the degree
of crystallization of the calcium oxide. Free or active calcium is present in large quantity in
poorly crystallized calcium oxide than in highly crystallized calcium oxide. On dissociation
calcium cations pozzolanically react with chemical compounds found in soil to form cementi-
tious materials. In pozzolanic reactions siliceous material reacts in presence of moisture and
calcium to form compounds having cementitious properties. The more the calcium cations is
available the greater the degree of soil modification.
The lime present in the fly ash hydrolyses and reacts with the soil as explained in chapter
5. Further, the fly ash also contains more reactive silica when compared with soil, which will
8
react with lime and produces more cementitious compounds. The products from both the
reaction- between lime and soil and between lime and fly ash will bind the soil particles
more effectively.
Three forms of CSH are known to be formed at different times after initial mixing of fly
ash and clay. The first CSH reaction product is a weak, very poorly crystallized, high-
calcium mineral called tobermorite gel. It is followed by mineralization of CSH (I), a low-
calcium well crystallized form of CSH. Since it is well crystallized, CSH (I) forms much
stronger cementing bonds than tobermorite gel. The third type of calcium silicate hydrate,
CSH (II) is low calcium, well crystallized, high silica compound that forms strong cementing
bonds. CSH (II) is formed only at extremely high temperatures and is normally not encoun-
tered in soil-fly ash mixtures cured at room temperatures (Marks and Halliburton 1972).
The type of Fly ash used in this study is Class-F Fly Ash. The properties of the Fly ash
used are presented in Table 3. The Suddha soil was treated with the following percentages of
fly ash 3, 5, 8 and 10 by weight for the respective tests.
Table 3 Properties of Fly Ash
Properties Value
Specific Gravity 2.01
Liquid Limit (%) 31.3
Plastic Limit (%) NP
Plasticity Index (%) NP
Shrinkage limit (%) -
Optimum Moisture Content (%) 22.0
Maximum Dry Density (kN/m3) 12.83
Effects of Fly Ash on Atterberg’s Limits of Suddha Soil
Liquid limit and plastic limit of fly ash treated Suddha soil
Table 4 shows the effect on Atterberg’s limits due to addition of fly ash. The liquid limit test
was carried out on Suddha soil with 3, 5, 8 and 10 percent fly ash by weight. Fly ash was dry
mixed with the soil and mixed with water and the samples were cured for one day after wet
mixing before conducting the test. It is observed that liquid limit continuously increases with
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increase in fly ash content. The effect of flocculation and cation exchange capacity leads to
the increase in liquid limit.
When fly ash is added to the soil, the reactions between the reactive silica in soil and lime
present in fly ash produce calcium silicate hydrates (C-S-H) of varying composition. At low
CaO/ SiO2 ratio, C-S-H I is formed, which is predominant and has a distorted structure. At a
higher ratio of CaO/ SiO2, C-S-H II is formed and this is of semi crystalline nature. Thus, ad-
dition of lower amounts of fly ash leads to the formation of C-S-H I, with relatively high wa-
ter holding capacity and with lower capacity to bind the soil particles. This in turn increases
the liquid limit. However the increase in liquid limit is very marginal indicating that the bind
action is sufficient to control the increase in liquid limit. It is observed that the increase is not
in proportion to the amount of fly ash added, though fly ash is almost non plastic. This might
be due to slight flocculation due to the presence of lime present in fly ash. The absence of
flocculation if any also adds to the increase in plastic limit.
Table 4 Effect on Atterberg’s Limits of Fly Ash treated Suddha Soil
Type
Liquid
Limit
(%)
Plastic
Limit
(%)
Plasticity
Index
(%)
Shrinkage
Limit
(%)
Suddha Soil 65 28 35 23
Suddha Soil + 3% Fly Ash 66 29 36.5 24
Suddha Soil + 5% Fly Ash 66.5 30 36.5 25
Suddha Soil + 8% Fly Ash 67 29.5 37.5 27
Suddha Soil + 10% Fly Ash 68 29 39 29
Shrinkage Limit of fly ash treated Suddha Soil
Shrinkage limit test was conducted on fly ash treated Suddha soil to study the shrinkage be-
haviour of soil on addition of fly ash. Table 4 shows the test results of the shrinkage limit of
fly ash treated Suddha soil. A marginal increase is seen in the shrinkage limit of fly ash
treated Suddha soil. This is due to binding of soil particles, making the particles slightly
coarser. Hence the predominant effect binding of soil particles over the effect of hydration of
the gel and water holding capacity (which can decrease the shrinkage limit) is seen from
shrinkage limit test results. Even with a small increase in liquid limit, the shrinkage limit in-
10
creases to a greater extent because of particle aggregation by bonding and also due to floc-
culation of particles.
Compaction Behavior of Suddha Soil Fly Ash Mixtures
The compaction test was carried out on Suddha soil with 3, 5, 8 and 10 percent fly ash by
weight to study the compaction behaviour. Figure 1 shows the compaction curves for soil fly
ash mixtures. It can be seen from the graph that for all the four percentages of fly ash the
value of optimum moisture content is slightly more than the optimum moisture content of
Suddha soil without the addition of fly ash and maximum dry density is less than the maxi-
mum dry density of Suddha soil without the addition of fly ash. The optimum moisture con-
tent has increased and the maximum dry density has decreased continuously with addition of
fly ash. Addition of 3% fly ash to Suddha soil exhibited the highest maximum dry density
when compared to other percentages. The Suddha soil-fly ash mixture exhibits highest den-
sity at 3% due to the favorable grain size distribution of fly ash.
Fig 1 Compaction Behaviour of Fly ash treated Suddha Soil
Effect of Fly Ash on Unconfined Compressive Strength of Suddha Soil
11
The unconfined compressive strength was conducted on Suddha soil treated with 3, 5, 8 and
10 percent by weight. The fly ash treated Suddha soil samples were compacted at the respec-
tive optimum moisture content and were subjected to the compression test. The curing pe-
riod of the samples was varied from one day to twenty eight days. Figure 2 shows the effect
of fly ash on unconfined compressive strength of Suddha soil. It can be seen that the strength
of the Suddha soil increases with increasing content of fly ash content. However, beyond a
threshold value of 8%, the opposite happens.
Fig 2 Unconfined Compressive Strength of Suddha soil
Although the density of Suddha soil- fly ash- mixture decreased with increase in fly ash con-
tent, the unconfined compressive strength of the Suddha soil-fly ash mixture exhibited a re-
verse trend upto 8% fly ash content. The strength increase is due to the soil particles being
bound by the reaction products formed in the presence of water on addition of fly ash to the
soil. However, the gain in unconfiened strength due to addition of fly ash is quite less. This
is due to Addition of non plastic fly ash reduces the cohesion by restricting the development
of soil skelton. The extent if strength developed due to addition of of fly ash depends on its
reactivity. The reactivity of fly ash is mainly dependent on the reactive silica and also the free
lime present in fly ash. Hence, when there is availability of sufficient free lime and reactive
silica, there can be effective formation of cementitious compounds. However, high order of
12
strength strenmgthgain is not seen as there is increasing losss of cohesion due to increasing
additions of fly ash. Thus there is an optimum fly ash content of 8% for the soil.
Effect of curing period on fly ash treated Suddha Soil
Figure 3 shows the effect of curing period on addition on fly ash to Suddha Soil. It can be
seen that irrespective of fly ash content added to the soil, the strength of the soil increases
with time. The highest strength for all percentages of fly ash is seen at a curing period of 28
days. Although, the gain in strength is not very appreciable, this could be due to the low re-
active silica and free lime present in the fly ash. A steep gain in strength can be seen on addi-
tion of 3% fly ash. The marginal increase in strength on further increase in fly ash content
can be attributed to the unfavorable grain size distribution, negligible binding effect and the
decrease in density not being able to compensate the unfavorable grain size distribution.
Comparing various percentages of fly ash added to the soil, 8% has shown the maximum
strength gain.
Fig. 3 Effect of curing period on fly ash treated Suddha soil
Optimum Fly ash content
The influence of fly ash on various properties of Suddha soil was elaborated. The tests were
conducted in order to observe the effect of fly ash and to infer the optimum fly ash content.
13
Through the pH test and unconfined compression test, an optimum fly ash content of 8%
could be arrived at, although the compaction test did not show the same result.
Effect of Fly ash on the dispersivity of Suddha soil
Addition of dispersing agent increased the dispersivity of the soil.
From unconfined compression Test
The unconfined compression test was conducted on the fly ash treated Suddha soil, with and
without dispersing agent to know the dispersivity of the soil and effect of addition of fly ash
in reducing the dispersivity. The gain in strength of the soil on addition of fly ash is less.
Similar is the trend in reduction of dispersivity when fly ash is added to Suddha soil treated
with dispersing agent. Table 5 shows the percentage dispersivity of fly ash treated Suddha
soil with and without dispersing agent. The dispersivity of the soil reduces to an extent when
3% fly ash is added to the soil. The soil has least dispersivity when 8% fly sh is added to it.
On further increase in fly ash content to 10%, there is not much effect in reduction in disper-
sivity of the soil.
Table 5 Unconfined compressive strength of fly ash treated Suddha soil with and without dispersing agent
Sl.
No
.
Fly Ash (%)
Unconfined Compres-
sive Strength with
Dispersing Agent
(kPa)
Unconfined Compres-
sive Strength without
Dispersing Agent
(kPa)
Dispersivity
(%)
1 0 120 256 53.1
2 3 173 336 48.5
3 5 212 362 41.4
4 8 240 390 38.5
5 10 238 381 37.5
Like Portland cement, blast-furnace slag also reacts with water (i.e. is hydrated) to form
specific hydrated calcium silicates known as tobermorite gels. However, unlike basic Port-
land cement, it forms this critical cementing agent (tobermorite gel) by consuming the slaked
lime, Ca(OH)2 provided by the hydration of the Portland cement. Removal of some of the
14
slaked lime is advantageous, since less of it in the waste form will lead to less dissolution of
the lime over time, and thus consequently less long-term waste from degradation. In addi-
tion, there will be less slaked lime available to potentially react with salts, and thus produce
undesired expansive and destructive minerals in the future.
Expansive Soils
The term expansive soil is associated with soils that are sensitive to changes in the moisture
regime. Expansive soils swell and shrink with change in moisture content and pose serious
problems to structures founded on them. If uncontrolled rapidly disrupt road surfaces, tilt
poles, crack buildings, break underground service pipes and generally cause great economic
loss. These soils are widely prevalent in many parts of northern Karnataka, particularly Bel-
gaum, Bijapur, Bagalkot and Gadag Districts of Karnataka. Also the mineralogy of soils var-
ies from place to place, though montmorillonite is the most common clay mineral.
Black cotton soil was obtained from Davangere, Karnaataka State which is an expansive soil
is used for the study. The soil was collected by open excavation from a depth of 1 m below
the natural ground level. The soil was air dried and passed through 420 micron sieve. The
major clay mineral in this soil was montmorillionite.
Stabilisation with Fly Ash
Considerable work has been done on stabilisation of expansive soil using fly ashes. Hence an
attempt has been made to stabilise expansive soil using GGBS.
Stabilisation with Blast Furnace Slag
Blast-furnace slag is produced as a by-product of the iron and steel production industries. Its
earthy constituents come from iron ore processing, and it consists of the same oxides as
Portland cement, but in different proportions. Immediately after its production, slag is usu-
ally quenched for rapid cooling in a process known as granulation. The granulation results
in a reactive amorphous glass and avoids any crystallization.
Soil stabilization has been used from historical times using lime (Brook Bradley, 1952).
In some cases, excessive swelling was observed in lime stabilized sulfate bearing clay soils.
Following this, research was carried out with ground granulated blast furnace slag (GGBS)
15
and it was found that GGBS can reduce the expansive tendencies of lime stabilized sulfate
bearing clay soils (Wild et.al, 1996; 1998). Using industrial by-products like fly-ash and
GGBS for soil stabilization is gaining momentum (Indraratna, 1996; Hughes and Glendin-
ning 2004; Phanikumar an Sharma 2004; Higgins et.al 1998; Higgins 2005; Kolawale 2006;
Tasong et.al1999; Wilkinson et.al 2010). Hence an attempt has been made to improve the
strength and swell behaviour of expansive black cotton soil using GGBS in this work.
EXPERIMENTAL INVESTIGATION
Black Cotton Soil:
Black Cotton Soil (BCS) was procured from a farm field near Bagalkot in Karnataka.
Ground granulated blast furnace slag:
Ground granulated blast furnace slag (GGBS) was obtained from Ready Mix Concrete Plant
of Ultra Tech cement Ltd. In Bangalore which had sourced it from Bhadravathi Iron & Steel
works Ltd.
Lime:
Commercially available pure hydrated lime Ca(OH)2 with 99% purity was used in this work.
Specimen preparation
Cylindrical specimens with height/diameter ratio of 2 were prepared at OMC using static
compaction. The specimens were moist cured for various time periods of 0, 3,7,14 & 28
days
The physical & engineering properties of the soil and GGBS are listed in Table 6.
METHODOLOGY
Compaction studies
The unit weight of GGBS-soil mixture is an important parameter because it controls the
strength, compressibility, and permeability. Densification improves engineering properties .
Mini Compaction tests designed by Sridharan and Sivapullaiah (2005) were performed on
the GGBS-BC soil mixtures at different GGBS-soil ratios. A premeasured amount of GGBS,
16
measured as percent of dry soil by weight, was mixed thoroughly to produce a homogenous
GGBS-soil mixture. Water was added slowly during mixing. The samples were then com-
pacted in 38.1 mm diameter moulds. The compaction tests were done on BC soil alone,
GGBS alone and on the soil-GGBS mixtures in weight proportions of 4:1, 3:2 and 2:3.
Table 6. Properties of BCS & GGBS
Sl.
N
o.
Property /Parameter For
BCS
For
GGB
S
1 Specific Gravity 2.65 2.82
2 Grain size analysis
i) % of Sand particles 25.2 76.3
ii) % of Silt size parti-
cles
34.8 23.0
iii) % of clay size particles 40 0.7
3 Atterberg’ Limits
Liquid limit, %
54.8
31.5
Plastic limit, % 33.64 NP
Shrikage limit, % 17.45 34
4 Plasticity index, % 16.19 NP
5 Free Swell, % 60 zero
6 Compaction charac-
teristics
OMC ( % ) 25.8 22.0
Max. dry density (
kN / m3 )
15.64 16.3
7 Unconfined Compres-
sion strength, kPa
231
zero
Unconfined Compressive Strength (UCS)
17
Compressive strength is one of the most important geotechnical properties that a material
like GGBS must possess when being considered for the stabilization of soils. The unconfined
compressive strength (UCS) varies with the GGBS-soil mixing ratio and water content.
Hence the UCS test was done on the soil-GGBS mixtures in different proportions. The sam-
ples for UCS test of height 7.6 cm and diameter 3.8 cm were prepared by statically compact-
ing the mixtures in the mould to their respective maximum dry density at corresponding op-
timum water content. The samples were then cured for different time periods in desiccators.
UCS test conducted on Fly ash alone showed that the strength achieved is very less even for
the 28 days curing period. So an attempt to enhance its strength characteristics UCS testing
was done for the different Fly-GGBS mixtures. The unconfined compressive strength test as
per the standard method [1] was then done on the cured samples at the end of the required
curing period. A constant strain rate of 0.061 cm/min was maintained for all the samples.
RESULTS AND DISCUSSIONS
Compaction Characteristics
The results of the dry unit weight as a function of GGBS-soil mixtures and moisture con-
tents are shown in Fig. 4. It is interesting to note that both OMC and MDD decrease with
increase in the GGBS content. Generally addition of silt or sand to fine grained soil de-
creases OMC and increases MDD. Similarly Fly ash addition has been reported to decrease
the optimum moisture content and increase maximum dry density. The decrease in OMC is
obviously due to the addition of GGBS which is relatively coarser relative to BC soil. Addi-
tion of coarser particles reduces the water holding capacity due to the reduction of the clay
content. The decrease in MDD, in spite of increase in OMC, is due to the predominant ef-
fect of high frictional resistance offered by relatively coarser GGBS due to size and surface
texture resisting the compactive effort effectively.
18
Fig. 4 Moisture density relationship of GGBS-soil mixtures
Unconfined Compressive Strength
The variation of the unconfined compressive strength test with GGBS content for different
curing periods has been shown in the Fig 5. From the figure it can be seen that the uncon-
fined compressive strength (UCS) of BC soil increases with the addition of small amount of
about GGBS which remains constant up about 40% addition of GGBS. With further addi-
tion of GGBS the UCS decreases continuously and reaches lowest value with the addition of
90% of GGBS.
The variations in strength can be explained by the following factors:
1. Reduction in cohesion of the soil due to addition of coarser materials
2. Increase in strength of soil due to cementation by pozzolanic compounds produced
3. The effect of compaction parameters as the soil GGBS mixtures are compacted to
their respective optimum conditions.
4. Occupation of GGBS particles by finer soil particles.
19
Fig. 5 Variation of UCS of BC soil with GGBS content
The reduction in cohesion of soil is least with the addition of 10% of GGBS because of soil
particle cohesion is disturbance is minimum which however increases with increasing GGBS
content. With increase in GGBS content the available pozzolanic material i.e. GGBS in-
creases but the available water for pozzolanic reactions becomes less due to decrease water
content. Further the moulding densities are also lower with increasing GGBS content. With
GGBS content higher than 40% all the effect of decreased moulding water content and den-
sity dominate and the strength decrease. Thus the effect of pozzolanic reactions is nullified
by lower densities and water contents. The 28 day curing period shows higher strength
which means that the UCS increases with higher curing periods.
Effect of lime
Following tests were conducted to see the effect of lime in the BC soil-GGBS mixtures.
Shear Tests.
Two identical specimens were tested in unconfined compression testing machine after 0
days, 3,7,14 & 28 days. The average values of undrained shear strength were calculated for
each time period. The various trial proportions of BCS, GGBS & Lime listed below were
used. Thus the combinations tested were:
20
i) 100 % BCS
ii) 80 % BCS + 20 % GGBS with 0, 2 & 4 % lime
iii) 60 % BCS + 40 % GGBS with 0, 2 & 4 % lime
The specimens prepared with the above said proportions were also used for free swell tests
& shrinkage limit tests.
RESULTS
For the various proportions of soil, slag & lime, the undrained shear strength as obtained are
tabulated in the Table 7, the variation of axial stress vs percentage strain for BC soil only is
shown in the Fig.6, for different proportions of GGBS, BC soil & lime are shown in Fig 7 &
8 and variation of UCC strength with time is shown in Fig 9 & 10.
Table 7. Variation of Su with proportions
Soil + Additives Average
Su in kPa
Black cotton soil only 231
80 % BCS + 20 % Slag + 0 %
Lime
1551
80 % BCS + 20 % Slag + 2 %
Lime
2134
80 % BCS + 20 % Slag + 4 %
Lime
3757
60 % BCS + 40 % Slag + 0 %
Lime
780
60 % BCS + 40 % Slag + 2 %
Lime
3853
60 % BCS + 40 % Slag + 4 %
Lime
4065
21
Fig.6 UCC Curve for 100% BC Soil
Fig. 7 UCC curves of 20% GGBS + 80% BCS
22
Fig 8. UCC curves of 40% GGBS + 60% BCS
Fig. 9. UCC curves of 20%GGBS + 80% BCS
23
Fig. 10. UCC curves for 40%GGBS + 60% BCS
It can see that the black cotton soil alone has an undrained shear strength Su of 231 kPa
only, whereas it increased with the addition of 20% GGBS without lime to about 600 kPa
after curing for 7days and to 1551 kPa after curing for 28 days. But when 40% GGBS
without lime is added the Su value was only 370 kPa after 28 days of curing and remained
the same even after curing for 28 days.If a small amount of lime 2%) is added, the Su value
started increasing at a faster rate up to 7 days & thereafter at a slower rate up to 28 days as
seen from Figures 9 & 10.Further addition of lime ( 4%), shows great improvement in the
strength in case of 20% GGBS + 80% soil as seen from the Fig.4, whereas in the case of
40% GGBS + 60% Soil, there is not much improvement in the strength as seen from the
Fig.5.
It is also seen from the Table 8 that the free swell of the soil has been reduced from 60%
to about 35% by adding 20% of GGBS without lime and to about 12% by adding 40%
GGBS without lime.
Addition of 2% lime along with 20% GGBS reduced the free swell to 20% & along with
40% GGBS reduced the free swell to about 12%. But further addition of lime has very little
effect or no effect. It can also be seen that, with increase in additives shrinkage limit has in-
creased from about 17% to nearly 42%. This also confirms the increased volume stability
with the addition of additives (40% GGBS + 4% lime).
24
Table 8. Effect of GGBS & lime on shrinkage properties of Black cotton soil
Summary of studies on Stabilisation of soil with GGBS
1. It is found that 20 % slag & 4 % lime ( 5:1 slag to lime ratio) to soil Su increased
phenominally almost 16 times and with 40% slag & 4 % lime ( 10:1 slag to lime ratio) 18
times.
2. It is also observed that the free swell of Black cotton soil which was 60 % has reduced to
11.75 % when we used 40% slag & 2 % lime along with BCS. Thus we could achieve
volume stability also.
3. But using 40% slag & 4% lime along with BCS reduced the free swell to 11.45% only,
that is it does not show further reduction in the free swell with higher slag content.
Materials used % Free
Swell
Shrinkage
Limit (%)
100% Black cotton Soil
80%soil + 20%GGBS +
0%lime
60
34.71
17.43
19.44
80%soil + 20%GGBS +
2%lime 20 38.12
80%soil + 20%GGBS +
4%lime 20 39.93
60%soil + 40%GGBS +
0%lime 14.28 20.89
60%soil + 40%GGBS +
2%lime 11.75 38.21
60%soil + 40%GGBS +
4%lime 11.45 41.93
25
4. Hence, to enhance the utiliation of slag and optimal improvement in properties of soil, it
is recommended to use 40% slag & 4 % lime along with Black Cotton Soil.
STUDIES ON LIME STABILIZED FLY ASH-GROUND GRANULATED BLAST
FURNACE SLAG (GGBS) MIXTURES FOR GEOTECHNICAL APPLICATIONS
GGBS contains higher amount of lime (CaO) content but lower amount of silica and alumina
where as Fly ash usually contains very high silica and alumina but very low amount of CaO
which accounts for 1-5% only. The idea behind mixing of both these wastes is that expected
that the combination of both can result in a product with much improved strength which
alone Fly ash and GGBS cannot produce. When both these wastes are mixed together, the
constituent which is excess in one waste material can be utilized by the other thereby en-
couraging the pozzolanic reaction which is not possible with wither of these wastes alone.
Lime addition to the mixture is also considered as sometimes there is a need of the activators
to speed up the hydration process.
The basic idea of this research is to find potential binders as alternatives to cement and
lime. This present study is an attempt to investigate the geotechnical properties of the Fly
ash-GGBS mixtures. Addition of different percentages of lime and its influence on the Fly
ash – GGBS mixtures is also considered. Compaction and Unconfined Compressive strength
(UCS) tests were conducted to determine the engineering properties of the lime stabilized
Fly ash-GGBS mixtures and also the effect of curing is discussed.
Experimental Methods
The testing program includes the characterization of testing materials and evaluation of the
engineering properties like Compaction and Unconfined compressive strength of the Fly ash-
GGBS mixtures at different percentages of lime.
Specific Gravity
The specific gravity of fly ash and GGBS were determined as per IS: 2720 (III) method.
26
Particle Size Analysis
The determination of the distribution of particle sizes in Fly ash and GGBS were done using
IS: 2720 (IV)-1985 method. From the particle size distribution curves it is found that almost
all of the fly ash particles contains fine sand whereas most of the GGBS particles are of fine
sand to silt size. The shape of the particle size distribution curve shows that both Fly ash and
GGBS are poorly graded material. (Fig.11)
Compaction
For Compaction studies, Mini Compaction test apparatus developed by Sridharan and
Sivapullaiah (2005) has been used. This testing apparatus not only require less quantity of
materials but also saves considerable time and effort.
Compressive Strength test
Unconfined Compressive strength (UCS) testing has been carried out as per the standard
method (ASTM 1989 (Designation D2166-85)). The samples for UCC testing have been
prepared by statically moulding the mixtures to their respective OMC and MDD. The dimen-
sions of the prepared specimens were 3.8 cm in diameter and 7.6 cm in length. During curing
the samples were kept in desiccators at 100% humidity. For each combination of the mix-
ture, three identical specimens were tested and the average UCS value was determined. The
samples were then tested after 7 and 14 days of curing period. The strain rate during UCS
testing was maintained as 1.2 mm per minute.
Testing Materials Properties
The Physical properties of the materials used in the study are given in Table 9.
Generally Fly ash particles are solid spheres or hollow cenospheres [Goodarzi and Sanei,
2009]. GGBS particles have shape with clear edges and angles.
Testing Procedure:
The details of the different Fly ash GGBS mixtures with lime on which the testing has been
done are given in the Table 10.
27
Table 9. Physical properties of the Fly ash and GGBS
Properties Materials
Fly ash GGBS
Colour Ash color Off-white
Specific gravity 2.01 2.84
Liquid limit (%) 32 40
Plastic limit (%) Non-Plastic Non-Plastic
Plasticity index (%) Non-Plastic Non-Plastic
OMC (%) 22% 22%
MDD (kN/m3) 12.46 15.80
Fig. 11. Particle Size distribution curve of Fly ash and GGBS
28
Table 10: Proportions of Fly ash and GGBS
S. No Fly Ash (%) GGBS (%) Lime (%)
1 90 10 0
2 90 10 2
3 90 10 4
4 90 10 8
5 80 20 0
6 80 20 2
7 80 20 4
8 80 20 8
9 70 30 0
10 70 30 2
11 70 30 4
12 70 30 8
13 60 40 0
14 60 40 2
15 60 40 4
16 60 40 8
Summary
1. Compaction Behavior:
Fig 12 below shows the compaction curves of the Fly ash-GGBS mixtures at different pro-
portions on volume basis. Since dry density is a dependent on both the specific gravity and
degree of packing, expressing the compaction curves of volume basis will count both the
effects and will give a clear picture about the dry density. Prashanth et al.1999 has explained
a procedure for converting dry density and water content into volume parameters. They have
mentioned that Solid Volume occupation (SVO) is a measure of degree of packing. Fig 12
shows the plot of solid volume occupation versus water volume content. It is observed that
solid volume occupation decreases with the increase in the GGBS content. Fly ashes and
GGBS are silt sized and non plastic materials. Shearing resistance at particle level is the main
29
controlling factor for its engineering properties (Prasanth et al. 1999). Shearing resistance is
more in the fly ash at lower water content because of the development of negative pore wa-
ter pressure. The fly ash and GGBS both have lower OMC value and hence negative pore
water pressure acts resulting in the increase in the shearing resistance. This increase in the
shear resistance forbids the fly ash and GGBS particles to come close by resisting to com-
pactive effort. The water volume content increases with the decrease in the solid volume oc-
cupation. This may be due to the fact that as the solid volume occupation increases, more
water is taken up in the voids and hence as a result the water volume content increases.
Fig 12. Solid volume occupation- water volume content relationship for the Fly ash-GGBS
mixtures
2. Unconfined compressive strength (UCS) :
The results of the unconfined compressive strength (UCS) of Fly ash-GGBS mixtures at dif-
ferent percentages of lime for different curing periods are given in Figures 13, 14, 15.
30
Fig 13. Variation of UCS with GGBS content for 7 day curing
Fig 14. Variation of UCS with GGBS content for 14 day curing
31
Fig 15. Variation of UCS with GGBS content for 28 day curing
Fig 13 shows the variation of UCS with GGBS content for 7 day curing period. It is ob-
served that the UCS of samples without lime increases nominally with the GGBS content.
GGBS reacts very slowly in the presence of the water. Its reactivity is dependent on the
breaking down of the glassy structure which is attained at higher pH environment [Newman
and Choo, 2003]. Hence addition of small amount of lime is encouraged which increases the
pH of the system. At higher pH conditions, GGBS particles starts to react very rapidly in the
presence of water and starts to form cementitious gel. This is what can be expected when a
small amount of lime i.e. 2% is added to the Fly ash-GGBS mixtures. With the addition of
small amount of lime there is tremendous increase in the UCS value. In the presence of lime
the formation of C-S-H gel, which is known for binding the particles, is enhanced there by
providing strength to the mix. However the addition of further addition of lime does not im-
prove in strength at lower GGBS content. Effect of higher amounts of lime (i.e. 4% and 8%)
in the strength development is only seen at higher GGBS content. This is because 2% of the
lime content is enough to increase the pH of the system and further addition is not required.
Also excess lime has not been utilized for the formation of cementitious compounds which
depends on the free lime to reactive silica ratio [Sivapullaiah et al.2000]. Similar trends oc-
cur for the 14 day and 28 day curing period (Fig. 14 & Fig. 15). The variation of UCC at
different lime contents for 7, 14 and 28 day curing period respectively are shown in Figures
16, 17 and 18. We see that the for any fixed GGBS content the strength increases rapidly
32
with just addition of 2% of lime but at higher percentages of lime the strength improvement
is very nominal. However at higher GGBS content above 30% the gradual increase in
strength can be seen with increase in lime content. It is also worth noting that Fly ash alone
does not achieve significant strength with lime as does the Fly ash- GGBS mixtures.
Fig 16. Variation of UCS with lime content for 7 day curing
Fig 17. Variation of UCS with lime content for 14 day curing
33
Although only GGBS gives the highest strength with increase in lime content, yet the differ-
ence between strength of the Fly-ash GGBS mix with that that only GGBS can be neglected
keeping in view of the utilization of Fly ash. The disposal problem by Fly ash is more than
that of GGBS and only utilization of GGBS cannot solve this problem. Hence the effort to
utilize maximum amount of Fly ash along with GGBS without compromising the strength
seems to be the best way to deal with the present day problem of disposal. Hence it can be
concluded that Fly ash-GGBS mixtures satisfies the strength requirement to be utilized in the
various geotechnical applications.
Fig 18. Variation of UCS with lime content for 28 day curing
34
3. Effect of Curing period
Fig.19 Variation of UCS with curing period without lime
Fig.20 Variation of UCS with curing period at 2% lime
35
Fig.21 Variation of UCS with curing period at 4% lime
Fig.22 Variation of UCS with curing period at 8% lime
Pozzolanic reaction is a long term process and hence formation of C-S-H gel formation de-
pends on the curing period (Sivapullaiah et.al 2000). Moreover, strength characteristics of
the materials which are to be utilized in the applications like subgrade and embankments are
associated with long term performance and expected to increase with curing time. It has
been reported that when Fly ash is used in concrete reaction starts only after one or more
weeks [Fraay et.al 1989]. Therefore, the UCS tests were conducted for periods of 7, 14 and
36
28 days on all the samples to observe how curing time affects the strength. The effect of cur-
ing period on the Fly ash GGBS samples without lime has been shown in Figure 19. It can be
observed that the strength of Fly ash-GGBS mix samples did not increase significantly be-
tween 7 and 14 days. Considerable increase in strength is found between 14 and 28 days cur-
ing. However, only GGBS follows a linear relationship in the UCS-Curing period plot. It is
interesting to see that Fly ash when combined with 30% and 40% of GGBS shows higher
strength than GGBS alone for a curing period of 28 days. Effects of curing period at various
percentages of GGBS at different lime content are shown in Figures 20-22. Worth noting is
the fact that addition of lime helps in gaining enormous strength within 7 day curing period.
Substantial increment in the strength is found beyond 7 day curing period.
Summary
1. UCS of the Fly ash-GGBS mixture increases nominally with the increase in the GGBS
content.
2. Addition of small amount of lime increases the strength tremendously at very early stage.
3. Fly ash with 40% GGBS mixture shows higher strength compared with either of Fly ash
or GGBS alone but only at higher curing period.
4. From the results obtained it can be concluded that Fly Ash-GGBS mixtures along with
very small percentage of lime have the potential to be used in various geotechnical applica-
tions such as highway subgrade, embankments etc.
CONCLUSIONS
1. Dispersive soils can be stabilised very effectively with fly ash. Curing with fly ash in-
creases the strength and reduces the dispersivity.
2. Black Cotton soil which is known to be effectively stabilised with fly ash can be stabi-
lised better with Ground Granulated Blast Furnace Slag (GGBS).
3. Mixtures of Lime, Fly ash and Ground Granulated Blast Furnace Slag (GGBS) can be
optimised for Geotechnical Applications.
4. The studies conducted in this project can help to improve the bulk utilisation of prob-
lematic soils reducing the environmental problems.
37
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