Chemical Soil Stabilization

8/11/2019 Chemical Soil Stabilization

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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


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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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



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Figure19: Relationship between the optimum water content and tar content


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


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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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).

Documents

Chemical Soil Stabilization