Feasibility of using fly ash admixtures in landfill bottom liners or vertical barriers at contaminated sites

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Feasibility of using fly ash admixturesin landfill bottom liners or verticalbarriers at contaminated sitesJ.P.A. Hettiaratchi a , G. Achari a , R. C. Joshi a & R.E. Okoli aa Department of Civil Engineering, University of Calgary, Calgary,Alberta, Canada, T2N 1N4Published online: 15 Dec 2008.

To cite this article: J.P.A. Hettiaratchi , G. Achari , R. C. Joshi & R.E. Okoli (1999) Feasibility ofusing fly ash admixtures in landfill bottom liners or vertical barriers at contaminated sites, Journalof Environmental Science and Health, Part A: Toxic/Hazardous Substances and EnvironmentalEngineering, 34:10, 1897-1917, DOI: 10.1080/10934529909376938

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J. ENVIRON. SCI. HEALTH, A34(10), 1897-1917 (1999)

FEASIBILITY OF USING FLY ASH ADMIXTURES IN LANDFILL BOTTOMLINERS OR VERTICAL BARRIERS AT CONTAMINATED SITES

Key Words: Fly ash, contaminants, landfills, bottom liners, vertical barriers,contaminated sites, pollution, hydraulic conductivity, strengthcharacteristics

J.P.A. Hettiaratchi, G. Achari, R. C. Joshi and R. E. Okoli

Department of Civil EngineeringUniversity of Calgary

Calgary, AlbertaCanada T2N 1N4

ABSTRACT

Results from a comprehensive experimental program conducted to examine the

feasibility of constructing landfill liners or vertical barriers to pollutant migration at

contaminated sites using fly ash based materials are described. The materials evaluated are

fly ash, lime-fly ash, poly vinyl alcohol (PVA) polymer-lime-fly ash and bentonite-fly ash.

The focus of this paper is to comparatively evaluate the engineering properties of fly ash

based materials in relation to potential use at landfills and/or at contaminated sites.

Experimental results indicate that Alberta fly ash itself can not be compacted to

achieve a hydraulic conductivity of less than 1×10-7 cms -1 , the "benchmark" requirement

for low permeable barriers, and therefore is not suitable as a construction material for landfill

bottom liners or vertical barriers. Fly ash amended with up to 20% lime satisfied the

hydraulic conductivity requirement of less than 1×10 - 7cms - 1 . However, lime-fly ash

becomes brittle with age. The resultant inflexibility will produce cracks when subjected to

differential loading and settlement conditions at landfill sites.

Fly ash mixed with Ca-bentonite (55% to 45%), or Na-bentonite (90% to 10%), and

compacted at wet-of-optimum moisture content exhibited hydraulic conductivity values less

than 1 x 10-7 cms -1. Therefore, fly ash-bentonite (or FAB) mixtures in a compacted state can

1897

Copyright © 1999 by Marcel Dekker, Inc. www.dekker.com

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1898 HETTIARATCHI ET AL.

be used as a landfill bottom liner material. However, utilization of FAB in the slurry form,

which is required for constructing vertical barriers, may not be preferable due to segregation.

Addition of 4 to 10% PVA polymer to lime-fly ash satisfied the hydraulic

conductivity requirement of less than lxKT'cms" ' . The PVA-lime-fly ash (or LFP)

samples are highly ductile and flexible. Hardened LFP samples exhibit compressive, tensile

and flexural strengths adequate to cause the material to remain in elastic equilibrium under

impact loads of landfills, and lateral loads due to soil pressures. Hardened LFP slurry is

highly likely to retain its structural integrity under differential settlement conditions better

than any other fly ash based material.

INTRODUCTION

Fly ash is a by-product of coal-fired power stations. The principal constituents of fly

ash are silica, alumina, iron and alkaline earth metals. High proportions of fly ash particles

are "cenospheres" or hollow particles, and are spherical in shape. Fly ash is an industrial

solid waste, and is usually disposed of in land-based disposal facilities at an added cost to the

utility industry and the environment. However, increasing environmental concerns associated

with land disposal sites have prompted investigations into alternative management routes for

fly ash.

Fly ash utilization in a variety of engineering applications has been reported in

literature. Gray and Lin (1972) reported that properly compacted and stabilized fly ash has

the requisite properties for use in load-bearing fills or highway sub-bases. Also, use of fly

ash in asphalt paving filler, lightweight sintered aggregate, Portland cement raw mix, and oil

well grouting have been reported by Gray and Lin (1972). Recent studies by Joshi and Lohtia

(1997) have shown that fly ash could be used as a partial replacement for cement or fine

aggregate in concrete works, as well as in mass concrete where the reduction of heat of

hydration is important.

Recently, research activities have been focused on fly ash utilization in novel

applications in the waste management industry (Sachdev and Amdurer, 1985; Vesperman et

a l , 1985; Edil et al., 1987; Joshi et a l , 1994; Moretti and Henke, 1987; Roy et al., 1991).

Joshi et al. (1994) and Edil et al. (1987) have studied fly ash use as a barrier to migration of

contaminants. In this case, the proportion of fly ash that is similar to clay soil and cement

plays a major role. Fly ash has been found useful in wastewater treatment and in water

pollution control, primarily due to its properties such as, small specific area, high cation

exchange capacity and adsorptivity (Banerjee et al., 1989). Temperature and pH play

important roles in determining the removal of cadmium by fly ash (Yadava et al., 1989 and

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FLY ASH ADMIXTURES IN LANDFILL BOTTOM LINERS 1899

Banerjee et al., 1989). Gangoli et al. (1975) concluded that the removal of hexavalent

chromium using fly ash involved a chemisorption mechanism associated with the bonding

between active alumina sites and the chromate anion. Li (1996) investigated the metal

removal capability of two Nova Scotia fly ashes, and suggested that the high pH values may

cause lead to precipitate. The high buffering capacity of fly ash enables the retardation of

metallic contaminants. A concern associated with the utilization of fly ash in low permeable

barrier construction is that metals present in fly ash may leach into solution and be

transported across the barrier. Leaching is due to the readily exchangeable and/or adsorbed

molecules on the surface of the inert glass that dissolves when in contact with water (Francis

and White, 1987).

Focus herein, however, is mainly on the uses of fly ash and modified fly ash in

specific waste management applications. This paper describes the results of studies

conducted to investigate the feasibility of constructing landfill liners or vertical barriers to

prevent pollutant migration using fly ash, lime blended fly ash, lime-fly ash modified with

poly vinyl alcohol (PVA) polymer, and fly ash stabilized with bentonite. Properties used to

judge the suitability of each material type included; low hydraulic conductivity, low

contaminant leachability, strength and flexibility of hardened composite, and resistance to

fracture under differential loading and/or settlement.

MATERIALS AND METHODS

The base-line materials used in this study are several types of Alberta fly ash, and fly

ash modifying agents such as lime, PVA polymer, and bentonite clay. The compositions and

important characteristics of these materials are presented below.

Alberta Fly Ash

Fly ashes used in this research program were those produced in the coal-fired power

plants of Sundance, Wabamun, and Forestburg. In Alberta, about 6 million tonnes of fly ash

are produced, and these account for 50% of the total fly ash production in Canada (Joshi et

al., 1994), but only about 0.9 million tonnes, or 15% of the total Alberta fly ash production,

is used in the construction industry. Alberta fly ashes are classified as marginally Class C as

they contain less than 15% of calcium oxide. The ashes have percent fineness in the 9.8 % to

32 % range which is comparable with the percent of fineness specified in ASTM for Class C-

type fly ash. The ashes have low carbon content which is preferable for maintaining proper

air entrainment in concrete preparation. The compositions and important characteristics of

Alberta fly ashes are presented in Table 1 and compared with ASTM-1 cement. The ashes

have high silica (SiO2) and alumina (A12O3) contents as compared with ASTM-1 cement. All

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TABLE 1Composition and Properties of Alberta Fly Ash and Cement

Composition and Properties

Specific Gravity

Specific Surface Area

Chemical Composition

SiO2

AI2O3

FeiC-3

CaO

MgO

SO3

Na2O

K2O

Loss on Ignition

Pozzolanic activity index

Unit

cm2/g

%

%

%

%

%

%

%

%

%

Sundance

2.04

3140

57.8

23.0

3.5

10.6

1.5

0.3

2.30.5

0.5

91.0

Material Type

Wabamun

2.01

3060

59.2

22.3

3.9

9.9

2.1

0.2

0.3

0.9

0.4

85.0

Forestburg

2.01

3690

56.3

21.7

4.9

9.0

1.2

0.4

4.2

1.0

0.4

93.0

ASTM-1

3.14

4300

20.8

4.4

2.6

62.7

4.4

2.5

0.2

0.8

1.1

three fly ashes contained small amounts of heavy metals, especially chromium (Cr),

cadmium (Cd), copper (Cu), nickel (Ni) and lead (Pb). These contaminants could pose

potential risks to humans and environment if they are leached from fly ash in significant

quantities (Wentz et al., 1988). However, the presence of amorphous iron and aluminum

oxides, manganese oxides and various types of organic materials, which possess high

affinities for trace metals, limit trace metal leachability (Theis and Wirth, 1977).

The average specific gravity of fly ash solids is about 2, 75% of that of quartz, the

major constituent of most soil. One explanation for this lower average specific gravity is the

fact that a high proportion of fly ash particles is cenospheres or hollow particles. When

examined under the microscope fly ash particles appear spherical in shape.

Lime and PVA Polymer

Chemical grade, high purity, hydrated lime meeting ASTM specifications was used

for preparing lime modified fly ash samples. A water-soluble polymer, poly vinyl alcohol

(PVA), obtained from BDH chemicals was used.

Bentonite

Two types of bentonite (Na-bentonite and Ca-bentonite) were used. Na-bentonite was

obtained from Avonlea Minerals, Saskatchewan, whereas Ca-bentonite was obtained from

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FLY ASH ADMIXTURES IN LANDFILL BOTTOM LINERS

TABLE 2Chemical and Physical Properties of Na and Ca-Bentonite

(Data supplied by American Colloid Company and Avonlea Minerals)

1901

Chemical Composition andPhysical Properties

Chemical Composition (%)

SiO2

A12O3

Fe2O3

FeOMgOCaO

Na2O and K2OTiO 2

Physical Properties

Moisture% passing 200 mesh

Specific gravity (g/cm3)Liquid limit (%)Plastic limit (%)

Na-Bentonite

58.6616.364.7

-2.112.0

1.96 and 0.10.2

8-1085-95

2.63407105

Ca-Bentonite

56-5918-215-8.5

0.47 - 0.653.0-3.31.2-3.5

0.84-1.250.80-0.86

10-1260

2.729861

American Colloid Company, Mississippi. The chemical and physical properties of bentonites

are presented in Table 2.

RESULTS AND DISCUSSION

Potential Use of Fly Ash and Modified Fly Ash as a Barrier Material

A distillation of studies on the properties and potential uses of fly ash and modified

fly ash for waste management applications is provided here. The data and results analyzed

herein are obtained from tests on: three Alberta fly ashes, lime treated fly ash, lime-fly ash

mixture modified with PVA (LFP) and fly ash treated with bentonite (FAB). The results are

examined in the light of field experience with compacted materials usually used as

contaminant control barriers. Factors used to judge the performance include hydraulic

conductivity, contaminant leachability, strength, and durability.

Alberta Flv Ash

Hydraulic Conductivity - The results from laboratory permeability tests on three

Alberta fly ashes are presented in Table 3, along with permeability data for modified fly

ashes for comparison.

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TABLE 3Hydraulic Conductivity of Fly Ash, Lime-Fly Ash and PVA modified Lime Fly Ash Test

Samples

]

Fly AshType

ForestburgForestburgForestburgForestburgForestburgSundanceWabamunSundanceSundanceSundanceSundanceWabamun

Vlaterial Properties

Curingperiod(days)

71011111014141616162828

PVA

(%)686810-

6810--

WaterContent

(%)100150200200200

--

200200200

--

Hydraulic Conductivity (cm s"')

FlyAsha

alone-----

4.2x10'6

3.7xlO"6

-

3.9xlO"6

2.8xlO"6

FlyAshy+10

% lime-----

4.7x10'6

-

-2.8x10'5

-

FlyAshr+20% lime

-----

9.4x10"7

-

--

6.5xlO-8

-

PVAmodified

fly ash8.3x10-"1.6xlO'8

7.2x10"7

9.9x108

4.3x10"8

-1.3xl0"7

1.6xl0'7

8.8xl0"8

--

a : All samples were prepared at water content of 28%.r: All samples were prepared at water content of 30%

The hydraulic conductivity (k) values ranged from 10~5to 10 6cm/s. The k values

correspond closely with the permeability characteristics of silts or silty-clays. The effect of

curing time on the hydraulic conductivity of the ashes was not significant. Following 28 days

of curing, a Sundance fly ash sample exhibited an average hydraulic conductivity of

3.9xlO~6cm/s and the corresponding Wabamun fly ash sample exhibited a value of

2.8xlO~6cm/s. Wabamun and Sundance fly ashes do not seem to possess significant

amounts of hydration products, and hence, are unable to self-harden significantly. Also,

characteristics of the porous matrix do not seem to change with time. Since the k value is a

function of the characteristics of the porous matrix, the time dependent effect on the k value

will be minimal.

Contaminant Leachability -The concentrations of cations in leachate from Alberta fly

ashes are presented in Table 4, along with data for fly ash modifying additives for

comparison. The leachate is alkaline with a pH of approximately 12. The amount of each

trace element leached constitutes a small fraction of what was initially present in the raw fly

ash. With the exception of lead, the concentrations of other trace elements are low in all

leachate samples. The amphoteric nature of lead contributes to the increase in lead

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Source

Tap Water

Fly Ash

Fly Ash+10%Lime

Metal Concentration

PH

7.3

11.6

12.0

Al

0.8

1.3

5.5

Metal

B

2.3

4.0

3.34

TABLE 4> in Leachate from Sundance Fly Ash Samples

Concentrations in Leachate (concentration

Ba

0.04

1.9

3.6

Cd

<lppb

<lppb

<lppb

Cr

<4ppb

0.07

0.22

Cu

0.8

0.08

0.34

Ni

<6ppb

<6ppb

<6ppb

in ppm except where noted)

Pb

<20ppb

0.07

0.94

Zn

0.01

0.13

1.18

Co

<2ppb

<2ppb

<2ppb

Ca

41

111

293.2

concentration. Lead and cadmium are amphoteric elements that in hydroxide form dissolve

in very high or very low pH environments (Cheng and Bishop, 1992).

Unconfined Compressive Strength - The compressive strength data for fly ash

specimens are presented in Fig. 1. The unconfined compressive strength of Alberta fly ashes

was quite low, as compared to that of lime-fly ash. The strength data of fly ash ranged from

120 kN/m2 to 660 kN/m2(Wabamun ash) and from 110 kN/m2 to 200 kN/m2 for

Sundance ash samples cured between 7 and 28 days, respectively. The fly ashes show age

hardening behavior or a time-dependent increase in strength. Strength increased by factors of

6 and 2 within 28 days in Wabamun and Sundance ashes, respectively. It is noted however,

that the response in strength gains with time in pure fly ash samples was marginal as

compared to that of lime stabilized fly ash. Age hardening behavior is correlated with the

presence of free lime in fly ash. The free lime contents are low in Alberta fly ashes, and

consequently, sufficient quantities of cementitious products are not formed during the first

days of hydration.

Overall Evaluation of Alberta Fly ash - Compacted fly ash has the requisite properties

for use in load-bearing fills or highway sub-bases, and its lower compacted density relative

to conventional earthfill is advantageous if a fill must be constructed over son, compressible

ground (Gray and Lin, 1972). Focus herein, however, is on the fly ash uses in relation to

waste management applications. A major concern associated with the utilization of fly ash in

landfill bottom liner construction is that undesirable trace elements present in fly ash may

leach into solution and be transported across the liner. Leaching is due to the readily

exchangeable and/or adsorbed molecules on the surface of the inert glass that dissolves when

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in contact with water (Francis and White, 1987). Materials used for construction of landfill

liners or vertical barriers to contaminant migration at contaminated sites should exhibit a low

k value of lxlO"7cms"' or less. Experimental evidences in Table 3 and in Gray and Lin,

(1972) have shown that fly ash can not be compacted into a dense mass adequate to achieve a

very low k. This single factor underscores the choice of fly ash as a low permeable barrier.

Lime Treated Fly Ash _J

Hydraulic Conductivity - The k values of Alberta fly ashes stabilized with lime are

presented in Table 3. Lime contents of the mixtures were kept at 10% and 20% by weight of

fly ash. Test specimens were cured for periods ranging between 7 and 28 days before testing.

The k values of lime-treated ash are less than the corresponding k values of un-modified fly

ash. The k value decreased with increasing lime content and curing age. The k value of fly

ash modified with 10% lime showed a data range of 4.7 xlO"6 cms"' (14 days) to

2.8xlO"6cms"' (28 days), whereas, the k values of 9.4xlO"7 cms"1 (14 days) to

6.5 x 10"8 cms"1 (28 days) described fly ash stabilized with 20% lime. The decrease of almost

two orders of magnitude when lime content increased from 0 % to 20 % is caused by the

formation of hydration products, which minimizes the voids available for fluid migration.

The concentration of most trace elements in leachate from lime modified fly ash samples

decreased with time except for lead.

Contaminant Leachability - Results of contaminant leachability tests performed on

fly ash and lime modified fly ash summarized in Table 4 show that only a small fraction of

trace elements originally present in fly ash had leached out. Although the concentrations of

elements, except lead, were low in all samples, in general, lime treated fly ash produced

leachate with higher metal levels.

Unconfined Compressive Strength - The effect of lime treatment on unconfined

compressive strength of Alberta fly ashes is shown in Fig. 1. The strength of lime-modified

fly ash samples exceeded the strength of fly ash samples at all ages. The lime-fly ash mixture

showed age hardening effect, i.e. time-dependent increase in strength. Compressive strength

also increased with increasing lime content. Addition of lime (up to 20% by weight)

increased the compressive strength of the ashes up to 6-fold after one month of curing. This

increase in compressive strength is accompanied with an increase in rigidity. The

inflexibility of the hardened lime-fly ash composite indicates that a barrier constructed of

this material is highly unlikely to withstand stresses induced by differential settlement, and

will fracture and crack. However, the cracks may heal with time as pozzolanic products

formed at the cracked surfaces cement the cracked pieces together. Fly ashes have been

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FLY ASH-ADMIXTURES IN LANDFILL BOTTOM LINERS 1905

7000

10 15 20

Lime content (%)

7000

10 15

Lime content (%)

FIGURE 1Compressive Strength of Fly Ash and Lime Modified Fly Ash Samples.

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known to heal cracks of significant dimensions (Ahlberg & Barenberg, 1965; Callahan et al.

1962). This property is known as autogenous healing. Despite autogenous healing, cracked

liners may not perform as effectively as un-cracked liners, in most situations.

Overall Evaluation of Lime Treated Fly Ash - Desiccation or other environmental

stresses should not affect barriers against pollutant migration significantly. A lime-fly ash

vertical barrier constructed by in-situ injection of lime-fly ash slurry may not provide a

continuous low permeability barrier because in a hardened state, the lime-fly ash mixture

may crack due to inflexibility and desiccation a phenomenon that may pave active paths for

the flow of contaminants.

Although, lime modified fly ash at a lime content greater than 10% satisfied the

hydraulic conductivity requirement of less than lxlO"7 cms"1, its usage is questionable on

account of brittleness and cracking with age of the mixture. The combined effects of higher

contaminant contents in leachate and the low flexibility of hardened lime-ash which aids

crack development suppresses the choice of using lime amended fly ash as a barrier to

contaminant migration either at landfills or at contaminated sites.

PVA Treated Lime-Fly Ash

The PVA treated lime-fly ash (LFP) was tested for the specific purpose of usage in

constructing vertical barriers to contaminant migration at contaminated sites.

Hydraulic Conductivity - The PVA-lime-fly ash samples were prepared using 10%

lime (based on fly ash weight), and 4 to 10% PVA (based on total solid weight). The water

content in test samples was maintained between 100% and 200%. Calgary tap water was

used as the permeant. The permeability test results are presented in Table 3. Experimental

results indicate the possibility of achieving a very low k value when PVA is mixed with

lime-fly ash. Mixtures with PVA content between 4 to 10% (by weight of lime and fly ash)

exhibited k values of the order of 10~7cm s"' and lower. The k values of the mixtures

decreased continuously as the PVA contents of the mixtures increased.

Compressive Strength - Unconfined compressive strength tests were conducted on 25

to 50 mm diameter and 50 mm long specimens using an INSTRON machine. The samples

were prepared using 10% of lime (based on fly ash weight), PVA contents up to 12% (based

on total solid weight), and a water content of 75% (based on total solid weight). The

resulting compressive strength results are presented in Table 5. The mode of failure of

specimens is brittle for PVA addition of up to 5%, but the failure mode changed from brittle

to ductile as the PVA content is increased beyond 5%. Significant increases in compressive

strength, modulus of elasticity and tensile strength are also noted as PVA content is

increased. The change of failure modes from brittle to ductile is analogous to a change from

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TABLE 5Compressive Strength and Young's Modulus of PVA Modified Lime Fly Ash Samples

(age = 14 days)

%PVA

345681012

Forestburg Fly Ash

C(kPa)

206497135263224450

E(MPa)

1.33.45.05.37.36.22.8

Modeof

Failure

BrittleBrittleBrittleDuctileDuctileDuctileDuctile

Sundance Fly Ash

C(kPa)

50

70100150

E(MPa)

2.4

4.63.01.3

Modeof

Failure

brittle

brittlebrittlebrittle

Wabamun Fly Ash

C(kPa)

12202052708783

E(Mpa)

1.74.02.02.22.02.92.9

Modeof

Failure

brittlebrittlebrittleductileductileductileductile

C = compressive strength, E =Young's modulus of elasticity

rigidity to flexibility. The tensile and flexural strength data indicate that LFP is indeed

flexible.

Durability - Durability of LFP samples was studied by conducting strength tests

(flexural and tensile) after subjecting the samples to soaking, drying, and wetting and drying

(ASTM-D559). Continuous soaking of LFP resulted in leaching out of lime and polymer

from the sample reducing its strength. Soaking of LFP specimens produced a thin layer of

white flaky material on the surface of water. X-ray diffraction tests revealed that the flaky

material mainly consisted of calcium carbonate. The leaching of lime from the specimen and

subsequent reaction with atmospheric carbon dioxide may produce calcium carbonate. The

results of durability tests (Tables 6-8) show that addition of polymer has a significant

influence on the flexibility of LFP, however, a reduction in material strength also occurred

concurrently during continuous wet curing.

Tensile strength tests were conducted as per ASTM C 190 and the extensions of the

specimens were measured. Flexural tests were conducted on 25 x 25 x 125 mm beam

specimens using third point loading method as per ASTM standards, D1635. The deflection

of the beam at mid-span was measured to assess the relative flexibility of specimens

prepared using different slurry mixtures. The 14 day tensile splitting strength data for

samples prepared with 10% lime, 75% water content and 2 to 8% PVA are presented in

Table 6. The specimens containing more than 5 to 6% PVA deformed continuously in a

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TABLE 6Tensile Splitting Strength of PVA Modified Lime Fly Ash Samples

(Age = 14 days)PVA in test sample

(%)

234568

Tensile Splitting Strength (kPa)

Forestburg Fly Ash7.710.220.9***• *** * *

Sundance Fly Ash6.16.63.18.7* • *

* * *

Wabamun Fly Ash5.14.15.16.1******

*** Tensile splitting strength values from these tests were not possible to assess because thespecimens deformed continuously in a ductile manner without splitting.

TABLE 7Strength Test Results on Soaked Forestburg Fly Ash Specimen

Age(days)

1428562856120

C(kPa)10414112615411491

E(kPa)365526260583900270

F(kPa)

444425-

* • *

* * *

Comments

Soaking after 7 days of curing in fog room

Soaking after 14 days of curing in fog room

F = flexural strength; C = compressive strength; E = Young's modulus of elasticity.

Age(wks)

56

TABLE 8Wetting and Drying Tests on the Forestburg Fly Ash Specimens

C(kPa)13.159.94

E(kPa)

122255

F(kPa)5.494.61

Comments

wetting and drying on 7 day cured samplesWetting and drying on 14 day cured samples

C = compressive strength, E = Young's modulus, F = flexural strength

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flexible manner without splitting. Forestburg fly ash had the highest tensile splitting strength

at 4% PVA.

The strength test results of soaked specimens for Forestburg fly ash containing 10%

lime (of fly ash weight), 6% PVA (of fly ash and lime weight) and 100% water content are

presented in Table 7. The results show that the compressive strength decreased after 28 days

of soaking whereas the flexibility of the test specimen decreased after 56 days of soaking.

Following 12 cycles of wetting and drying, the specimens were tested for compressive and

flexural strength, and the results are presented in Table 8. The specimens showed volume

shrinkages of up to 30%. This shrinkage caused continuous void formations at the center of

the specimen.

Overall Evaluation of PVA Treated Lime-Fly Ash - Lime-fly ash-PVA (LFP)

mixtures exhibit k values ranging between 8.8 xlO'8 cm s'1 and 4.3 xlO'8 cm s'1. Compressive

strength values varied from 20 to 224 kPa. The tensile and flexural strengths are 9% to 16%

and 20% to 28% of compressive strength, respectively. In slag-cement concrete, the percent

of flexural strength/compressive strength was reported to vary from 6% to 35% (Swamy and

Bouihni, 1990). It is therefore argued that LFP mixtures have tensile and flexural strength

developments compatible with those of compressive strength. The hardened LFP slurry has

sufficient tensile and flexural strengths to withstand typical earth stresses likely to occur

during the construction of a vertical barrier at a contaminated site. Also, it has adequate

compressive strength to remain in elastic equilibrium when subjected to the impact loads

expected at a landfill. The LFP barrier is highly flexible, and such flexibility will permit it to

retain it structural integrity under differential settlement conditions.

Bentonite Treated Fly Ash fFABt

Addition of bentonite to fly ash has important engineering implications in the

construction of landfill bottom liners. Fly ash stabilized bentonite generally have higher

maximum dry density, low hydraulic conductivity and decreased optimum moisture content

of the admixture (Achari 1995). Much lower hydraulic conductivity values (1 x 10"8cms"')

were obtained by mixing 10% bentonite with Nova Scotia fly ash (Li, 1996). Fly ash/sand

specimens have displayed tensile strengths slightly lower than the tensile strength of concrete

while being 10% as rigid as concrete (Edil et al, 1987). Fly ash and clay liners have fair

weathering resistance and compatibility with highly acidic leachate.

Hydraulic Conductivity - Results from laboratory studies are presented in relation to

potential uses of FAB as a landfill liner material. The FAB specimens were compacted both

dry and wet of optimum moisture contents (OMCs) with fly ash contents varying between

0% and 90%. It should be noted that FAB specimens for permeability testing were set up for

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saturation immediately after extruding from the compaction mold without being cured. The k

values of fly ash/Ca-bentonite samples (with ash content varying between 0% and 90%)

ranged from7.7x10"'cm/s to7xlO"6cm/s and from 4.2x10"' cm/s to 5.3x10"*cm/s for dry

of OMC and wet of OMC, respectively. Whereas, the k values of fly ash/Na-bentonite

mixtures varied between 1.7x10"'° cm/sand 3.9xlO"7 cm/s, and between 1x10"'° cm/sand

2.8x 10"8 cm/s for dry of OMC and wet of OMC, respectively. It was found that 55% fly ash

and 45% Ca-bentonite or 90% fly ash and 10% Na-bentonite compacted wet of OMC

achieved the k requirement of 1 x 10"7 cms"1. The significant reduction in k achieved in

FAB samples is associated with filling of the voids between fly ash particles by the finer

bentonite particles. It is also due to the formation of hydration products of fly ash such as

calcium silicate hydrate and calcium alumínate hydrate.

Compressive Strength - The increase in compressive strength of FAB is age-

dependent. The time dependency is caused by the slow pozzolanic reactions between calcium

rich compounds and silica and alumina present in fly ash and bentonite. There is an optimum

proportion of materials which corresponds with a maximum strength. This value is attained

when 80% fly ash is mixed with 20% Na-bentonite or 40% fly ash is mixed with 60% Ca-

bentonite. At these proportions, a strength of about 360 kPa was achieved at a 28-day curing

age. The FAB specimens exhibited some flexibility even after 28 days of curing. The degree

of flexibility of the hardened material was found to depend on the proportion of bentonite in

FAB. Samples with 60% fly ash and 40% Ca-bentonite compacted dry or wet of OMC

showed no surface cracks after prolonged drying at room temperatures. Similarly, 80% fly

ash and 20% Na-bentonite compacted dry or wet of OMC showed no surface cracks. As fly

ash content is decreased to less than 60% in fly ash-Ca bentonite mixtures and to less than

80% in fly ash-Na bentonite mixtures, hairline cracks appeared. These cracks increased

progressively as fly ash content is further decreased.

Contaminant Leachability - The results from FAB leachability studies are presented

in Figures 2 and 3. The results indicate that the leachability is low for most metals, generally

in the range of 10"' -10"2 ppm. The concentration of some metals was at or below the limit

of detection by the spectrophotometer. However, lead and chromium levels of about 0.2 ppm

were observed in FAB leachate. The leaching of non-metals was significantly higher than

that of metals. Addition of bentonite did not aggravate the leaching potential of fly ash for

most elements as indicated by a lower degree of leaching of some of the elements in FAB

samples (Figures 2 and 3). On the other hand, bentonite with a high cation exchange capacity

has a tendency to attenuate metals. Generally, metals in FAB leachates are low and they can

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FLY ASH-ADMIXTURES IN LANDFILL BOTTOM LINERS 1911

0.4

0.3

I 0.2ICDU

0.1 -

FA = Fly Ash and NB = Na-bentonite

£J80%FA+20%NB

• 60%FA+40%NB

• 20%FA+80%NB

• 100%NB

_^ Eh I Wen

Mn Zn Fe Cu Pb

Metals

Cd Cr Co

1400

1200 •

E 1000a.

800 -

= 600

400

200 •

0 -I

FA = Fly Ash and NB = Na-bentonite

Na

D80%FA+20%NB

• 60%FA+40%NB

02O%FA+8O%NB

ni00%NB

Ca Mg

Non-Metals

FIGURE 2Leaching of Fly Ash and Na-Bentonite Mixtures using Distilled Water.

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0.5

0.4

Q .CL

•E 0.3 -I«co

1oO

0.2 -

0.1 •

S -

Mn

FA = Fly Ash and CB = Ca-benton¡te

H100%FA

n80%FA+20%CB

• 60%FA+40%CB

• 20%FA+80%CB

Q100%CB

Zn Fe Cu Pb

Metals

Cd Cr Co

80 •

O .

« 60 -S

rati

• s

8 40-oO

2 0 •

0

FA = Fly Ash and CB =

H100%FA

n80%FA+20%CB

• 60%FA+40%CB

• 20%FA+80%CB

E3100%CB

1ii

Ca-bentonite

i1i1Li

71 ...i—^—i ._

Na Ca

Non-Metals

Mg

FIGURE 3Leaching of Fly Ash and Ca-Bentonite Mixtures using Distilled Water.

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be re-adsorbed by fly ash or by bentonite. Liskowitz et al (1983) has also concluded that fly

ash itself is capable of re-adsorbing most of what is leached out of the ash.

Overall Evaluation Bentonite Treated Fly Ash - Fly ash blended with bentonite

provides a better material for a landfill liner than fly ash or bentonite alone. This material has

a low hydraulic conductivity, high compressive strength, low leaching potential, some

flexibility and less susceptibility to cracking. Mixtures of fly ash and bentonite in suitable

proportions can be used as a landfill barrier material. However, utilization of fly ash-

bentonite in the slurry form, which is required for constructing vertical barriers, may not be

suitable because of thé segregation potential.

Comparative Evaluation of the Feasibility of Using Fly Ash or Modified Fly Ash in Bottom

Liners at Landfill Sites

Materials for constructing bottom liners in landfills should exhibit the following:

hydraulic conductivity less than lx 10'7 cm s"', very low potential to leach contaminants from

the construction material, sufficient compressive and flexural strength and compatibility with

constituents of landfill leachate.

A summarized evaluation of the four types of material is presented in Table 9.

According to Table 9, fly ash alone is not suitable as a bottom liner material because it can

not be compacted into a dense mass to achieve the "benchmark" hydraulic conductivity

requirement of 1 x 10'7 cm s"1. Lime-fly ash, with lime contents up to 20%, satisfied the

hydraulic conductivity requirement of less than lx 10"7 cms'1. However, lime-fly ash mixtures

are not suitable as construction materials at landfill sites because they not only become brittle

with age, but also exhibit high potential to mobilize amphoteric trace elements, such as lead

present in fly ash, thus causing a potential human health concern. The lime-fly ash barrier is

highly rigid, hence its structural integrity is questionable; it may fracture under differential

settlement conditions prevalent at landfill sites. Additionally, such a barrier may react with

acidic leachate and may lose its effectiveness. Fly ash blended with bentonite (or FAB) is a

better material for a landfill bottom liner than fly ash or bentonite alone. FAB has a low

hydraulic conductivity and a low cracking potential. Polymer (PVA) modified lime-fly ash

(or LFP) is also well suited for the construction of landfill bottom liners because of low

hydraulic conductivity and high flexibility. Mixing of 20% lime and 6% PVA with fly ash

bring the hydraulic conductivity value below 1x10"' cm s"'. Addition of PVA (5% or more)

produces hardened slurry of high ductility that is not very stiff. A LFP barrier has adequate

compressive strength to remain in elastic equilibrium when subjected to impact loads

expected at landfills. The LFP barrier is highly flexible, and such flexibility will permit it to

retain it structural integrity under differential settlement conditions expected at landfill sites.

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TABLE 9Comparative Evaluation of the Suitability of Fly Ash and Modified Fly Ash as Landfill Liners or Vertical Barriers to Contaminant

Migration at Contaminated SitesMaterial

Fly Ash

Lime - Fly Ash{5 - 20 % lime (by weight)}

PVA-Lime(10%)-Ash{4% PVA.(by weight)6% PVA (by weight)8% PVA (by weight)10% PVA (by weight)}

60% Fly ash - 40% Ca-bent.

< 60% Fly ash - >40%Ca-bent.

80% Fly ash - 20% Na-bent.

< 80% Fly ash - >20%Na -bent.

Natural Clay Soil (Compacted)Cement

PropertiesHydraulic

Conductivity

High

Medium

Low

Low

Low

Low

Low

LowMedium - Low

CompressiveStrength

Medium

High

Medium

Medium

Medium

Medium

Medium

MediumMedium-High

Flexibility

Brittle/rigid

Brittle/rigid

Ductile/flexible

Somewhatflexible

Somewhatflexible

Somewhatflexible

Somewhatflexible

Ductile/flexibleBrittle/rigid

SuitabilityAs a Bottom

LinerMaterial

Unsuitable

Unsuitable

Suitable

Suitable

Unsuitable

Suitable

Unsuitable

Fairly SuitableUnsuitable

For VerticalBarrier

Construction

Unsuitable

Unsuitable

Suitable

Fairly suitable

Unsuitable

Fairly suitable

Unsuitable

Fairly SuitableUnsuitable

Remarks

Prone to cracks.

Prone to cracks.

High defense to cracks.Potential for leaching ofPVA is a concern.

No surface cracks.

Surface hairline cracksprogressively increaseswith bentonite content.

No surface cracks.

Surface hairline cracksprogressively increaseswith bentonite content.

Suffers desiccation cracks.Prone to cracks.

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However, leaching of polymer under prolonged wet conditions may be a concern. Further

research is needed to verify the impact of polymer leaching on integrity of a barrier system

under long-term wet conditions.

Comparative Evaluation of the Feasibility of Using Flv Ash or Modified Flv Ash in Vertical

Barriers to Control Contaminant Migration at Contaminated Sites

Migration of pollutants from contaminated sites can be minimized by in-situ injection

of slurry into the soil along the periphery of contamination. The construction materials for

vertical barriers to pollutant migration at contaminated shall exhibit low hydraulic

conductivity, low leachability of contaminants and very high flexibility.

Lime-fly ash and bentonite-fly ash slurries, once hardened, satisfy the "benchmark"

hydraulic conductivity requirement of lxlO'7 cm s"1. However, bentonite and lime slurry

walls once constructed may not provide a continuous low permeability barrier. FAB slurries

may undergo segregation with most of the ash particles sinking to the bottom of the slurry.

Such barriers may also crack due to desiccation and/or chemical shrinkage. Furthermore, the

rigidity of the composite may lead to fracturing when its strata are subjected to differential

pressures at contaminated sites.

LFP may be used as a composite material for constructing impervious and flexible

vertical barrier to pollutant migration at contaminated sites. A hardened LFP slurry has a

hydraulic conductivity ranging between 8.8 xlO'8 cm s'1 and 4.3 xlO'8 cm s'1. The hardened

LFP slurry has sufficient tensile and flexural strengths to withstand typical earth pressures

likely to occur during the construction of a thin vertical wall. The hardened LFP slurries have

tensile and flexural strength developments compatible with those of compressive strength

data. It is noted that the hardened LFP slurry is very ductile and flexible, and is highly likely

that it will withstand differential lateral stresses expected along the excavated faces of thin

vertical walls at contaminated sites during construction.

CONCLUSIONS

Studies were conducted to investigate the technical feasibility of constructing landfill

liners or vertical barriers at contaminated sites with fly ash alone; lime-fly ash; PVA-lime-fly

ash; and bentonite-fly ash. A knowledge-based evaluation of the suitability of fly ash and

admixture modified fly ash was undertaken and presented in summarized form in Table 9.

Based on the experimental results, the PVA-lime-fly ash (or LFP) was found to be most

suitable for the construction of both low permeable, flexible landfill bottom liners and

vertical barriers to contaminant migration at contaminated sites. Others such as fly ash

bentonite mixtures are more applicable for bottom liners than for vertical barriers.

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ASTM-C618-96., "Standard Specification for Coal Fly Ash and Raw or Calcined NaturalPozzolan for use as a Mineral Admixture in Concrete", Philadelphia, U.S.A. (1996).

Banerjee Kashi, Horng P.Y., Cheremisinoff Paul N., Sheih M.S., and Cheng S.L., "Proc. of43rd Purdue University Industrial Waste Conf." (1989).

Callahan, J.P., Morrow, J., and Ahlberg, H.L., "Autogenous Healing in Lime-Pozzolan-Aggregate Mixture", University of Illinois, Theoretical and Applied Report #. 631, Urbana,Ill, (1962).

Cheng, K.Y., and Bishop, P.L., "Hazardous Waste and Hazardous Materials" 2, 289-296(1992).

Edil, T.B., Berthouex, P.M., and Vesperman, K.D., "Proc. of the Conf. on GeotechnicalPractice for Waste Disposal", Geotechnical Special Publication, 13 , ASCE, Ann Arbor,Michigan, pp. 447-461 (1987).

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Gray, D. H. and Lin, Y.K., "J. of Soil Mech. and Found. Div.", ASCE, 98 (SM4), 361-379(1972).

Joshi, R.C., and Lohtia, R.P., "Fly Ash in Concrete: Production, Properties and Uses", InAdvances in Concrete Technology Vol.2, Gordon and Breach Science Publishers, TheNetherlands, (1997).

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Liskowitz, J.W., Grow, J., Sheih, M. and Trattner, R., "Sorbate Characteristics of Fly Ash",Final Report, Vol. 1, New Jersey Institute of Technology, Newark, NJ (1983).

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Morettí, C.J., Henke, K.R., "Waste management", Final report to the US Department, ofEnergy, University of North Dakota Energy Research Center, Grand Forks, North Dakota(1987).

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Documents

Feasibility of using fly ash admixtures in landfill bottom liners or vertical barriers at contaminated sites