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2.3.4 Soil Stabilization with Traditional and Non-Traditional Stabilizers
The artificial traditional admixtures in order of their usage are:
Portland Cement (and Cement-Fly Ash)
Lime (and Lime-Fly Ash)
Fly Ash
Fly Ash with Cement or Lime
Bitumen and Tar
Cement Kiln Dust (CKD)
In recent years an increasing number of non-traditional additives have been
developed for soil stabilization purposes. These stabilizers are becoming popular due
to their relatively low cost, ease of application, and short curing time. Since the
chemical formulas of the products are modified often based on market tendency, it is
rather difficult to evaluate the performance of a single product. Non-traditional
stabilizers are:
Polymers Based Products
Copolymer Based Products
Fiber Reinforcement
Calcium Chloride
Sodium Chloride
2.3.4.1 Cement Stabilization
Portland cement as an additive modifies and improves the quality of soil for
the purpose of increasing strength and durability. Cement also has been used to
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control the erosion of inorganic soils (Oswell, and Joshi, 1986). Oswell, and Joshi
(1986) found a good correlation between unconfined compressive strength and
erosion resistance. As the compressive strength increases the erosion rate decreases.
Cement can be applied to stabilize any type of soil, except soils with organic
content greater than 2% or having pH lower than 5.3 (ACI 230.1R-90, 1990). Kezdi
(1979) reports that cement treatment slightly increases the maximum dry density of
sand and highly plastic clays but it decreases the maximum dry density of silt. In
contraststudies by Tabatabi (1997) shows that cement increases the optimum water
content but decreases the maximum dry density of sandy soils. Cement increases
plastic limit and reduces liquid limit, which mainly reduces plasticity index (Kezdi,
1979). The other significant effects of cement-soil stabilization is reduction in
shrinkage and swell potential, increase in strength, elastic modulus, and resistance
against the effect of moisture, freeze, and thaw. Cement treated soils show a brittle
behavior compare of non-treated soils. Addition of cement can affect strength and
durability of the treated soils as follows.
2.3.4.1.1 Strength
The effect of cement content and curing time on unconfined compressive
strength are shown in Figures 4 and 5. Figure 4 shows that unconfined compressive
strength for both fine-grained and coarse-grained soils increases with increasing
cement content. The 28-day unconfined compressive strength is proportional to the
cement content; it varies from 40 percent of cement content for fine-grained soils to
150 percent cement content for coarse-grained soils (Mitchell, 1976).
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Figure 4: Relationship between unconfined compressive strength and cement content
( Mitchell, 1976)
The unconfined compressive strength increases by increasing curing time
(Figure 5).Improvement in unconfined compressive strength due to curing time for
coarse-grained soils is more significant compared to fine-grained soils.Equation 1
shows the empirical relationship between unconfined compressive strength and
curing time for a given soil and cement content (Mitchell, 1976):
+=
0
log)()(0 d
dKUCSUCS dd (1)
Where:
:)( dUCS Unconfined compressive strength at age of ddays (psi)
0)( dUCS : Unconfined compressive strength at age of 0d days (psi)
K=70C for coarse-grained soils and K=10C for fine-grained soils,
(C: cement content percent by weight)
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Figure 5: Effect of curing time on unconfined compressive strength of
cement (Mitchell, 1976)
Lo and Wardani (2002) report that addition of stabilization agent increases the
cohesion significantly. Figure 6 shows the effect of cement content on cohesion for
coarse-grained and fine-grained soil. Equation 2 shows that cohesion is a function of
unconfined compressive strength (Mitchell, 1976).
)(225.00.7 UCSc += (2)
Where UCSis unconfined compressive strength (psi) and cis cohesion.
Cement increases both cohesion and internal friction angle of the soil (Uddin
et al., 1997; Bragdo eral, 1996) though for some cement treated soils internal friction
angle remains constant (Balmer, 1958).
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Figure 6: Effect of cement content on cohesion for several coarse-grained and fine-
grained soils (Mitchell, 1976)
Unconfined compressive strength increases with increasing relative
compaction as well (White and Gnanendran, 2005). Figure 7 shows the relationship
between the uniaxial compressive strength and relative compaction.Delay in mixing
and compaction decreases the unconfined compressive strength.
Figure 7: Unconfined compressive strength versus relative compaction
for cement treated material (White and Gnanendran, 2005)
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2.3.4.1.2 Durability
In most of soil stabilization projects achieving a maximum durability is
desirable. Cement treated soils have a good reputation for having a good resistance
against freeze-thaw and wet-dry cycling tests. Figure 8 shows the relationship
between unconfined compressive strength and durability of cement treated soils. It is
evident that resistance against freeze-thaw and wet-dry cycling increase with
increasing unconfined compressive strength.
Figure 8: Relationship between unconfined compressive strength and durability of
cement treated soils (ACI 230, IR- 90, 1990)
2.3.4.2 Lime Stabilization
Lime is one of the additives, which is widely used in stabilization of fine-
grained soils. Various forms of lime such as hydrated high-calcium lime ( 2)(OHCa ),
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monohydrated dolomitic lime ( MgOOHCa 2
)( ), and dolomitic quicklime (
)MgOCaO have been successfully used as stabilizing agent for many years. Quick
lime (calcium oxide) is delivered in the form of coarse-grained powder. It reacts
quickly with water producing hydrated or slaked lime, generating heat and volume
change (Equation 3):
molkJOHCaOHCaO /3.65)( 22 ++ (3)
Quick lime must be handled with care; it can burn the skin in the presence of
moisture it also can cause corrosion of equipment (Kezdi, 1979). The main
contribution of lime to the strength of soil is from its ability to create cementation
between soil particles. The higher the surface area of the soil, the more effective this
process of lime cementation is.
2.3.4.2.1 Chemical Reactions in Lime treated Soils
Several reactions occur when lime is added to clay in the presence of water.
The reactions are cation exchange, flocculation-agglomeration, carbonation, and
pozzolanic reaction (Mallela et al., 2004). Cation exchange and flocculation-
agglomeration reaction occur immediately after mixing and these reactions cause
immediate changes in strength, plasticity index, and workability of the soils (sections
2.3.5.2.2 and 2.3.5.2.3). Carbonation is reaction of carbon dioxide in the open air or
voids in the ground with lime, which forms a relatively weak cementing agent.
Cementation caused by carbonation on the clay surface results a rapid initial increase
in strength (Hausmann, 1990). Pozzolanic reaction occurs between lime and silica
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and alumina of the clay mineral and produces cementing material including calcium-
silicate-hydrates and calcium alumina hydrates. The long term result of pozzolanic
reactions (Equation 4 and 5) is solidification of the soil (Hausmann, 1990). Rate of
the pozzolanic reactions depends on time and temperature.
OHSiOCaOSiOOHCa 2222)( + (4)
OHOAlCaOOAlOHCa 232322)( + (5)
2.3.4.2.2 Stress Strength Behavior
Lime treatment leads to significant increase in strength. The immediate
increase in strength results from flocculation-agglomeration reaction and leads to
better workability, whereas long-term strength gain is due to pozzolanic reactions
(Thompson, 1966). Figure 9 shows that, as lime content increases unconfined
compressive strength increases (Giffen et al., 1978).
Figure 9: Relationship between unconfined compressive strength and lime content of
the treated soils with lime (after Giffen et al., 1978)
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2.3.4.2.3 Atterberg Limits
Lime changes the Atterberg limits of the soils. An increase in lime content
decreases the liquid limit, increases plastic limit and that leads to a significant decease
in plasticity index (Figure10).
Figure 10: Relationship between Atterberg limits with lime content
(after Giffen et al., 1978)
2.3.4.2.4 Compaction Characteristics
Several changes occur when lime is added to the soil. Addition of lime
increases optimum water content but decreases maximum dry density (Figure 11).
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Figure 11: Change in compaction curve of a lime treated soil
(after Giffen et al., 1978)
2.3.4.2.5 Swell Potential
As lime content increases, swell potential decreases significantly (Figure 12).
It is evident that reduction in plasticity index leads to a significant decrease in swell
potential (Giffen et al., 1978).
Figure 12: The effect of lime on shrinkage and swelling properties of soils
(after Giffen, 1978)
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2.3.4.2.6 Fatigue and Durability
Fatigue strength is the number of load cycles that a metrical can carry at a
given stress level. Studies show that the immediate strength of lime is an important
factor in resisting to higher number of freeze-thaw cycles (Mallela et al., 2004).
2.3.4.2.7 Optimum Lime Content
The required amount of lime to be added to the soil depends on the
application. For modification purposes 2% to 3% lime by dry weight of soil is
sufficient (Maher et al. 2005). For stabilization purposes, normally 5% to 10% lime
by dry weight of the soil is suitable. To determine the optimum lime content for soil
stabilization several methods have been suggested. Hilt and Davidson (1960) suggest
the following equation for the optimum lime content:
25.135
%+=
weightbyclayofWieghtbyContentLimeOptimum (6)
2.3.4.3 Fly Ash Stabilization
Fly ash is a by-product of coal combustion in power plants. Fly ash contains
silica, alumina, and different oxides and alkalis in its composition (Das, 1990). Its
general appearance is light to dark gray powder and the size is the same as silt. The
specific gravity of fly ash ranges from 1.9 to 2.5. There are two types of fly ash: type
C and type F. Type C fly ash has significant amount of free lime. This type of
fly ash causes pozzolanic and cementitious reactions. Addition of fly ash to lime and
cement can improve the engineering properties of soil like lime or cement. However,
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fly ash properties are highly variable and depend on chemical composition of coal
and combustion technology.
2.3.4.4 Soil Modification with Fly Ash and Cement or Lime
Addition of mixtures of lime (L) or cement (C) and fly ash (F) to aggregates
(A) results in LFA, CFA or LCFA. For cohesionless soils with low plasticity fly ash
treatment with cement will be more effective than lime, and for plastic soils fly ash
treatment either with cement or lime is more effective (Hausmann, 1990). Less
permeable layer is created by stabilization of a sandy road base with fly ash-cement
mixture rather than cement alone. It is also convenient that cement-flyash-sand or
cement-flyash-gravel mixtures shrink less than soil-cement mixtures (Natt and Joshi,
1984). Lime and fly ash reduce the maximum dry density of clay; the corresponding
optimum water content tends to increase (Hausmann, 1990). Results of the research
by While and Genendran (2005) indicate that one hour delay between mixing and
compaction lead to significant increase in unconfined compressive strength of lime-
fly ash treated soils. Construction of runway 9-27 at Houston International Airport is
an example of the applications of Lime-Cement-Fly ash stabilization (Little et al.,
2000). The engineering properties of mixture of fly ash with cement or lime are
summarized as follow:
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2.3.4.4.1 Compaction and Strength Characteristics of Fly Ash with Cement or
Lime
Figures 13, 14, and 15 show the compaction and strength characteristics of
compacted fly ash with addition of cement or lime. Figure 13 shows that fly ash tends
to improve the dry density of soil better when combined with cement compared with
lime (Hausmann, 1990). Figure 14 illustrates the relationship between maximum dry
density and fly ash content for different percentage of cement (Hausmann, 1990).
Figure 13: Compaction curves for stabilized soils with 5% fly ash (class F)
(after Hausmann, 1990)
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Figure 14: Correlation between maximum dry density of sand-fly-ash-cement mixes
with fly ash content (after Giffen et al., 1978)
Figure 15 shows that fly ash improves the soil strength better when combined
with cement compared with lime (Hausmann, 1990). Figure 16 illustrates the
relationship between unconfined compressive strength of fly ash treated soils and
water content with mixture of cement or lime (Giffen et al., 1978). Figure 17 shows
by increasing the fly ash content, the uniaxial compressive strength of sand-fly
ash-cement increases (Giffen et al., 1978).
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Figure 15: Unconfined compressive strength of fly ash (class F) as a function of the
additive content (after Hausmann, 1990)
Figure16: Unconfined compressive strength of fly ash (class F) as a function of the
water content of compaction (after Hausmann, 1990)
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Figure17: Relationship between the 7-day compressive strength of a medium sand
fly-cement with fly ash content (after Giffen et al., 1978)
2.3.4.5 Bitumen and Tar
Bitumen is a by-product that remains after distillation or evaporation of crude
petroleum. Tar is the result of destructive distillation of coal and other carbonaceous
material. Asphalt consists of mineral particles impregnated or cemented by bitumen.
Most suitable bitumen admixtures are used in sandy gravel, sands, silty sands, fine
crashed rocks, and highly plastic clays. Bitumen is not as common as other stabilizers
like lime and cement, mainly because of its relatively high cost. The effectiveness of
bitumen on cohesion and waterproofing depends on the nature of the soil. The amount
of fine soil particles is important in workability of bitumen. Too much fines could
present problem with mixing, stability, and uniformity. Lack of fine could result an
unstable mixture, causing loss of adhesion (Kezdi, 1979). Bitumen and Tar have
several affects on soil engineering properties of soil as follows:
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2.3.4.5.1 Compaction Characteristics
Kezdi (1979) and Ingles (1973) reported that maximum dry density with
constant compactive effort decreases with increasing bitumen content. However
results of studies by Giffen et al., (1978) show a different behavior in soils that are
stabilized by tar (Figure18). Figure 19 shows that optimum amount of water
necessary to reach the maximum dry density decreases with increasing tar content
(Giffen et al., 1978).
Figure 18: Relationship between the maximum dry density and tar content
(after Giffen et al., 1978)
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Figure19: Relationship between the optimum water content and tar content
(after Giffen et al., 1978)
2.3.4.5.2 Strength
The strength of compacted stabilized soil with bitumen is measured in terms
of unconfined compressive strength. Figure 20 shows that initially there is an increase
in strength with quantity of binder added until maximum strength is reached and after
the peak there is a slow drop in unconfined compressive strength of the soil (Giffen et
al., 1978).
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SiO2(%) Al2O3(%) Fe2O3(%) CaO(%) MgO(%) SO3(%) Na2O(%) K2O(%) Total Alkali(%)
12.47 2.89 1.58 41.84 0.59 7.25 0.9 1.21 1.69
15.05 4.43 2.23 43.99 1.64 6.02 0.69 4 3.32
Figure 20: Unconfined Compressive strength of tar-stabilized clayey sand
(after Giffen et al., 1978)
2.3.4.6 Cement Kiln Dust (CKD)
CKD is a by-product of Portland cement manufacturing process. CKD is a
fine material that is carried by hot gasses in a cement kiln and collected by a filter
system during the production of cement. CKD contains mostly dried raw materials
like limestone, sand, shale, and iron ore. Table 1 shows the percentage of chemical
compositions of CKD. Values in the first row are provided by Lafarge North America
(2007) taken form their cement plants. The second row are the mean values of 63
different CKD types calculated from published data by Sreekrishnavilasam et al.
(2006).
Table1: Cement kiln dust (CKD) chemical compositions
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CKD has variety applications in agriculture, construction, and waste
stabilization. In agriculture, the high concentration of soluble potassium in CKD is
found to be a good source of potassium for growing plants in treated soils (Lafond
and Simard, 1999). CKD can also be used for soil stabilization in road construction.
For example it has been used as a stabilizer in road base in Oklahoma and also as a
filler in asphalt pavements (Miller and Azad, 2000). CKD and asphalt binder can
create low ductile asphalt that has been successfully used in Europe for bridge
waterproofing and protection (Bghdadi, Fatani, and Sabban, 1995). Another
important application of CKD is in soil stabilization and modification of waste
materials. CKD has been used to stabilize the coal mine waste effluents (Haynes and
Kramer, 1982). Nearly 4 million tons of CKD is disposed every year in the Unites
States (Miller and Zaman, 2000). The cement industry loses money with CKD
disposal because of the raw material and energy that was wasted to produce CKD.
Therefore using CKD would be much more cost effective than just throwing it away
as a waste (Kessler, 1995). The effects of CKD on geotechnical properties of soils are
discussed in sections 2.3.4.6.1 through 2.3.4.6.4.
2.3.4.6.1 Strength
Addition of CKD to the soil increases the unconfined compressive strength
(Miller and Azad, 2000). Also by increasing the curing time uniaxial compressive
strength of CKD treated soils increases (Miller and Azad, 2000). Other studies such
as those conducted by Baghdadi (1995) show that after 28-days, unconfined
compressive strength of kaolinite samples mixed with 16% (by weight) CKD
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increased from 210 kPa to 1115 kPa. Based on the experimental studies given by
Miller and Azad (2000) the stiffness of CKD treated soils increases and the failure
occurs at a smaller axial strain compared to untreated soils (Figure 21).
Figure 21: Stress strain results from unconfined compressive strength tests on CKD
treated soils after 28 days of curing (Miller and Azad, 2000)
2.3.4.6.2 Atterberg Limits
Miller and Azad (2000) investigated the change in Atterberg limits in different
soil samples treated with CKD. The results indicate that an increase in CKD increases
the plastic limit, decreases the liquid limit thus, significant PI reduction occurs with
CKD treatment, especially for soils with high PI.
Solid
Symbols
Open
Symbols
1: Circle
10 % CKD
2: Triangle
12 % CKD
3: Squares
13 % CKD
Untreated
Soils
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2.3.4.6.3 Compaction Characteristics
Compaction characteristics of the soils are affected by adding CKD. Miller
and Azad (2000) report that addition in CKD content increases the optimum water
content and decreases the maximum dry density of the treated soils.
2.3.4.6.4 Effect of CKD on pH
Experimental studies by Miller and Azad (2000) show that addition of CKD
increases soil pH, so the soil becomes more alkaline. The higher the pH, the higher is
the solubility of silica and alumina, which reacts with calcium ions released during
cement hydration to form secondary cementitious products and this is called
pozzolanic activity. CKD is a pozzolanic activator in low-strength materials.
2.3.4.7 Polymers Based Products
There are different types of polymers for the purpose of soil stabilization and
erosion control such as Soil-Sement, Curlex Net Free, Antiwash/Geojute, and
Slopetame2. Soil-Sement is an environmentally safe, advanced powerful polymer in
dust control, erosion control and soil stabilization. Results of the research by Little et
al. (2000) show the benefit of the polymer Soil-Sement on stabilizing Eolian and
Fluvial soils. Both types of soils are classified as poorly graded sand, based on
Unified Soil Classification System. Addition of this polymer to dry Eolian and
Fluvial soils increases their CBR values. The unconfined compressive strength of
silty sand treated with Soil-Sement also show a significant increase.
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Curlex Net Free erosion control blankets are made with softly barbed,
interlocking, curled wood fibers stitched together with thread (www.Curlex.com).
Antiwash/Geojute is a woven gird pattern, made of natural fiber, suitable for erosion
control of steep slopes (www.beltonindustries.com). Slopetame2 is a plastic grid
product designed for immediate erosion control of eroding slopes
(www.invisiblestructures.com).
2.3.4.8 Copolymer Based Products
There are different types of copolymer products for the purpose of soil
stabilization and erosion control such as Soiltac, Gorilla Snot, and Durasoil.
Soiltac is acopolymer non-toxic soil stabilizer, and dust control product used for
dust suppression, road base stabilization and soil stabilization worldwide. Powdered
Soiltac can be used by broadcasting the dry powder topically or mixing it in to the
treatment area and adding water to the site. Also Powdered Soiltac can be pre-
diluted into a liquid and applied in similar manner (www.soilworks.com).
Gorilla Snot is a copolymer which forms bonds between soil or aggregate
particles and is used as a soil stabilizer and dust control agent. Gorilla Snot is a
biodegradable product and environmentally safe to use (www.soilworks.com).
Durasoil is ultra-pure, synthetic organic fluid which is distinctively crystal clear,
odorless and is applied neat and simple with out dilution in water. Any equipment
capable of spraying water can be safely used to apply Gorilla Snot without any
damage to the equipment, even in freezing and wet conditions (www.soilworks.com).
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2.3.4.9 Fiber Reinforcement
The use of hair-sized polypropylene fibers in soil stabilization applications has
been popular in soil stabilization projects for its low cost compared with other
stabilization agents. These materials have a high resistance towards chemical and
biological degradation and do not cause leaching in the soil (Puppala and, Musenda
2000). Puppala and Musenda (2000) have conducted a series of tests to study the
engineering properties of clayey materials reinforced with randomly oriented fibers.
The study used polypropylene fibers of nominal size of one inch and two inches in
length. The physical and chemical properties of the fibers are shown in Table 2.
These fibers have high resistance to chemical reaction and can be applied in high
temperature conditions.
Table 2: Properties of polypropylene fibers (after Puppala and Musenda, 2000)
The results show that mixing soils with fibers increase uniaxial compressive
strength. The results also indicate that swelling and shrinkage are reduced (Puppala
and Musenda, 2000). Length and amount of fibers have an important effect on the
Value
Electrical conductivity LowAlkali resistance
0.91
551.6 to 758.45
3502.66
324
1100
None
High
High
Melting point,(F)
Ignition point, (F)
Absorption
Acid and saltresistance
Property
Specific gravityTensile strength,(MPa)
Modulus, (MPa)
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level of improvement. One of the advantages of fiber technology is that it can be
applied on the variety of soil types; it also does not need any special equipment or
skills.
2.3.4.10 Calcium Chloride
Calcium chloride is an inorganic salt, which is a by-product of sodium
carbonates. It is mainly used in highway constructions, dust control, and maintenance.
Calcium chloride has hygroscopic property. This means calcium chloride attracts and
absorbs water. This is a function of relative humidity and temperature. It can easily
liquefy in moisture of its own absorption. Calcium chloride is highly soluble and can
be dissolved easily so it can be easily washed away by rain and may require more
than one treatment in a single season to maintain its effectiveness (Sleeser, 1943). For
the same humidity and temperature the vapor pressure of calcium chloride is lower
than water (Ros, 1988; Shepard, 1991). Calcium chloride has a higher surface tension
and a lower freezing point compared to water (Shepard, 1991). In calcium chloride
treated pavement roads this property minimizes frost, heave, and reduces freeze-thaw
cycles, thus reducing maintenance cost (Wood, 1990; Ingles, 1973). Calcium chloride
is used as a dust palliative on unpaved roads as well as haul roads in mining and on
the earth-moving project. It is also used as a secondary additive to increase the
strength of the soils treated with cement or lime (Hausmann, 1990).
Addition of calcium chloride affects engineering properties of the treated
soils. Calcium chloride, depending on the soil type, may decrease the soil strength
(Kezdi, 1979) or increase it (Thornburn and Mura, 1969). Addition of calcium
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chloride has major effect on compaction characteristics of the treated soils. The
results will lead to an increase in dry density and a decrease in optimum water
content. Figure 22 shows the result of compaction tests on a gravely clay with and
without calcium chloride (Pacific Chemical Industries Pty.Ltd., 1983).
Figure 22: Compaction curves of gravelly clay with and without calcium chloride
(after Pacific Chemical Industries Pty.Ltd., 1983)
2.3.4.11 Sodium Chloride
Sodium chloride has the similar properties to calcium chloride. Singh and Das
(1999) have reported a major improvement in California Bearing Ration (CBR),
unconfined compressive strength, and indirect tensile strength of salt treated material.
The main application of sodium chloride is in long-term highway pavement subgrade
(Singh and Das, 1999).
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2.4 Environmental Issues of ChemicalStabilizers used for Erosion Control
When chemicals and reagents are used as means of soil stabilization or
erosion control, their chemical stability and environmental impacts must be evaluated
and well understood. If the treated area with chemical additives is not adequately
protected from surface runoff, the stabilized material can be washed onto surrounding
areas and damage the adjacent vegetation. Cement appears to have the least
environmental issues compared with lime or fly ash. Most of the fly ash products
have heavy metals in their compositions.Therefore, fly ash treated materials have the
potential to leach and contaminate water bodies. In case of lime treated soils there is a
potential for increasing pH on the surrounding areas.
CKD is not considered to be hazardous by Environmental Protection
Agencys RCRA (Resource Conservation and Recovery Act) regulations. However
CKD is not necessarily free of any environmental issues. CKD must be handled
properly to prevent environmental contamination and the toxicity of CKD must be
determined on a case-by-case basis (Haynes and Kramer, 1982). The finer particles
contain higher concentration of sulfates and alkalis, while coarser particles that are
collected closer to kiln have higher concentration of free lime.
The Environmental Protection Agency (EPA) has reviewed and studied the
impact of CKD on humans health and environment. It is concluded that the health
and environmental risks associated with CKD are low. However, there is a potential
danger to humans health and environment under particular circumstances. The data
collected by EPA shows that using CKD in different applications have caused and
may continue to cause, contamination of air and nearby surface water and ground
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water. That leads to potentially risks to humans health and environment. Material
safety data sheet of Roanoke cement corporation also reveals that CKD in contact
with moist in eyes or skin when mixed with water becomes caustic (pH>11) and may
damage or burn the skin (third degree burn), It also can cause irritation to the moist
mucous membrane of the nose, throat, and can lead to some respiratory problems
(www.titanamerica.com). In addition Eckert and Qizhong (1998) report that
substantial leaching from cement and CKD of specific metals, especially Cr and Ba
are below limits for hazardous waste defined in the Resource Conservation and
Recovery Act (RCRA).