HYDRAULIC CONDUCTIVITY AND LEACHATE CHARACTERISTICS OF LIME STABILIZED FLYASH

HYDRAULIC CONDUCTIVITY AND LEACHATE CHARACTERISTICS OF LIME STABILIZED FLYASH
HYDRAULIC CONDUCTIVITY AND LEACHATE
CHARACTERISTICS OF LIME STABILIZED FLYASH
A THESIS
Submitted towards partial fulfillment
of the requirements for the degree of
MASTER OF TECHNOLOGY (RESEARCH)
IN
CIVIL ENGINEERING
WITH SPECIALIZATION IN
GEOTECHNICAL ENGINEERING
BY
Ms. SUSHREE SANGITA
Roll No. 612CE3008
Under the Guidance
of
Prof. S. P. SINGH
&
Prof. N. ROY
DEPARTMENT OF CIVIL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA,
ROURKELA – 769008
June 2015
CERTIFICATE
This is to certify that the thesis entitled, “Hydraulic Conductivity and Leachate Characteristics
of Lime Stabilized Fly Ash” submitted by Sushree Sangita, Roll No: 612CE3008 in partial
fulfillment of the requirement for the award of Master of Technology (Research) degree in Civil
Engineering with specialization in Geotechnical Engineering at National Institute of Technology
Rourkela is an authentic work carried out by her under our supervision and guidance. To the best of
our knowledge, the matter embodied in the thesis has not been submitted to any other
University/Institute for the award of any degree or diploma.
Place: Rourkela
Date:
Prof. N. Roy
Prof. S. P. Singh
Department of Civil Engineering
National Institute of Technology
Rourkela-769008
DECLARATION
This is to certify that project entitled “Hydraulic Conductivity and Leachate Characteristics of
Lime Stabilized Fly Ash” which is submitted by me in partial fulfillment of the requirement for the
award of Masters of Technology (Research) in Civil Engineering, National Institute of
Technology, Rourkela, Odisha , comprises only my original work and due acknowledgement has
been made in the text to all other materials used. It has not been previously presented in this
institution or any other institution to the best of my knowledge.
Name: Ms. Sushree Sangita
Roll No: 612CE3008
Place: Rourkela
Date:
Department of Civil Engineering
NIT Rourkela, Odisha
ACKNOWLEDGEMENT
First and foremost, I would like to express my deep sense of gratitude and indebtedness to
my supervisors Prof. S. P. Singh and Prof. N. Roy for their guidance, valuable advice and
suggestions from the initial stage of this programme and providing me a golden opportunity to
amass extraordinary experiences and honed my skill as a research scholar through this research
work. Their persuasion, inspiration and support in various ways have always enriched my growth as
a protégé as well as a researcher.
I am grateful to Prof S.K Sarangi Director and Prof S. K. Sahu, Head of the Department of
Civil Engineering, NIT Rourkela for their kind support and concern regarding my academic
requirements.
I would like to thank all the faculties of Civil Engineering Department Prof. M.R. Barik, Prof
K.C. Biswal, Prof K.K.Paul, Prof C.R. Patra, Prof K.C. Patra, Prof S.K Das, Prof A.V.Asha for their
direct/ indirect influence , wholehearted suggestions at various stages of work.
I am thankful to all the staff members of Civil Enginerring Department especially Mr. A.K.
Nanda, Mr C. Sunaiani, Mr H.M. Garnaik, Mr. P. Pandit for their kind co-opertation during the
research work.
I express my inbebtedness to the faculties of other departments namely Prof. Smrutirekha
Bal, Prof. B.C.Roy, Prof. D.K. Bisoyi for allowing me to work in their laboratory and all the
laboratory assistants of their respective lab for the help rendered to me during the progress of my
research work.
I would never forget to thank the co-workers of Geotechnical lab specially Narayan
Mohanty and Dilip Das for their continuous help which made this formidable task engrossing and
riveting.
Loads of love to my seniors , juniors as well as my batch-mates who made this busy and
monotonous life phenomenal and worth living and with whom I could enjoy these 2 years of stay at
NIT without missing my home.
I am eternally thankful to my beloved parents for their endless encouragement, support, love
and prayers throughout the research period. They are the persons who made me realize the approach
towards life being a moral booster for me at the time of critical juncture.
Last but not the least, I thank the one above all of us, the Almighty God for bestowing upon
me the showers of blessings as well as strength, courage and patience enough to endure and
counteract the obstacles that came across during the course of the this career..
Sushree Sangita
Abstract
Coal based thermal power plants have been the backbone of a country due to its major contribution
in electricity generation for the developmental purposes. Due to indomitable rise in demand of the
electricity, the increase in generation of fly ash has become inevitable now-a-days. Moreover, the
disposal of fly ash has become a major issue for coal based thermal power plants as it requires a vast
disposal area and gives rise to a lot of problems like shortage of useful land, increase in disposal cost
and dusting of atmospheric air. Fly ash generated from the coal fired thermal power plants is mostly
sluiced into ash ponds by wet disposal method. This fly ash contains a number of soluble major and
trace elements such as As, Fe, Cd, Hg, Zn, Pb and Cu, etc. There is possibility that leachate
emanating from this fly ash bed may contaminate the ground and pose a threat to the human as well
as aquatic life. Therefore, stabilization of fly ash by some chemical additives like lime/cement is an
excellent method to mitigate the leachate characteristics of fly ash in addition to improve the
strength and stability of the structure.
The leaching of metals mainly depends on two factor such as pH and hydraulic conductivity. The pH
plays a pivotal role in reducing the concentration of the elements. Hydraulic conductivity also has a
major effect in preventing the leachate from contaminating the ground water. If the material can be
made less permeable, the leachate can be confined at the source of generation and thus the ground
water can be protected from being contaminated.
A thorough study of the previous research works reveals that lime stabilization is an effective means
of reducing the permeability and concentration of metals in the leachate emanating from ash ponds.
In the present research work an effort has been taken to study the effect of lime on hydraulic
conductivity and leaching characteristics of fly ash by varying the mix-proportion and curing period.
i
In addition to this, an experimental set up has been developed to investigate the efficacy of lime
column in mitigating the leachate characteristics of compacted and sedimented ash beds.
The experiments were performed in two phases. At first, the compaction characteristics of fly ash
mixed with different lime content such as 0%, 2%, 4%, 8%, 12% and 15% were found out from light
and heavy compaction tests. The hydraulic conductivity and leachate characteristics of compacted
fly ash-lime mixes were determined after 0, 7, 15, 30, 60 and 90 days of curing. All these samples
were prepared corresponding to their respective MDD and OMC and cured for the specified curing
periods as mentioned above. The concentration of the major and trace elements like Cu, Fe, Zn, Ca,
Ni, Pb and Cr were found out by atomic absorption spectrometer. The effects of lime content and
curing period on microstructure, morphology and hydration products in the stabilized specimens
were studied by various microanalyses such as XRD and SEM tests.
Further, large scale laboratory models of sediment and compacted fly ash beds were prepared with a
centrally installed lime column simulating a field condition as closely as possible. The samples were
collected from various radial distances and depths after 7, 30, 90 and 180 days of curing period and
subjected to different tests such as pH, and leachate analysis of different elements like Ca, Ni, Pb,
Zn, Cu, Cr, and Fe. In addition to this, the hydraulic conductivity of treated ash deposit was
measured by collecting undisturbed specimens from different radial distances and depths.
From compaction test results it is found that for light compaction test, with increase in lime content
the OMC value increases up to 4% and thereafter, it decreases whereas in case of heavy compaction
test, the OMC increases up to 2% lime addition and thereafter, it decreases. Similarly, the MDD in
case of light compaction test decreases with increase in lime content up to 4% and thereafter it
ii
increases whereas in case of heavy compaction test the same value decreases up to 2% lime addition
and thereafter, it increases.
The hydraulic conductivity value is found to depend on the lime content, compaction effort, and
curing period. The samples containing higher doses of lime shows significant decrease in hydraulic
conductivity value. It was found that at 90 days curing, it reduces about 10 times for samples
compacted with light compaction energy whereas in case of heavy compaction, it decreases about
100 times than the unstabilized specimen. However, sample with no lime content showed marginal
change in hydraulic conductivity value with curing period. XRD analysis shows the presence of
compounds like ettringite, C-S-H and C-A-H gel which blocks the pore space and reduces the
capillary voids. SEM analysis also confirms an interlocking network of hydration products which is
responsible in reducing the hydraulic conductivity.
From the leachate analysis, it was observed that concentration of all elements was less than that of
leachate sample of raw fly ash collected from acid digestion and extraction method. At 0 days curing
the concentration of each metal (except Ca) is approximately same for different lime contents
whereas at higher curing period, as the lime content increases, the concentration of metals in the
leachate follows a decreasing trend. This is due to presence of alkaline medium which is unfavorable
for metal precipitation and also due to encapsulation of metals by the hydration products. It is also
observed that the concentration of all the metals was below the threshold limit of IS-10500 and
WHO water quality standard.
The pH test results of the sample collected from sediment and compacted ash deposits show that the
value is more in the sample collected adjacent to the lime column than that of the sample collected at
a remote area from the lime column and also it increases with increase in depth. This is due to
iii
migration of lime to the periphery and downward direction of the tank. Moreover, it is observed that
the pH value increases with curing period up to 180 days and thereafter, it decreases. Because with
longer curing period lime is consumed in pozzolanic reaction which results in reduction of the pH
value.
The permeability test result of lime column stabilized ash bed shows that during the early period of
stabilization (30 days of curing) no significant variation in hydraulic conductivity value is observed
in specimens collected at different locations (different radial distances and depths). However, as the
curing period increases, the hydraulic conductivity follows an increasing trend with increase in
radial distance whereas the same decreases with depth. In addition, it is also observed that as the
curing period increases, a significant reduction in hydraulic conductivity occurs in all the layers of
sedimented pond ash deposit.
The leachate analysis result shows that concentration of elements in the leachate sample collected
from the test tank is much lower than the leachate sample extracted from raw fly ash. At early stage
of curing the concentration of Ca is found to be more than that in the virgin fly ash, however at
longer curing period, i.e at 365 days, the concentration of Ca is found to be decreased due to
participation of lime in pozzolanic reaction. It is also observed from the results that the concentration
of major and trace elements in the leachate sample collected adjacent to the lime column is lesser
than that of the sample collected at the periphery of the test tank. This is due to higher pH value
adjacent to the lime column as compared to remote areas. Moreover, the concentration of other
elements in the leachate collected on 365 days curing is less than that of the sample collected on 90
and 180 days.
This is due to the formation of hydration product such as C-S-H gel which
encapsulates the elements and prevents leaching. The concentration of elements is found to be less
than the threshold limit of WHO and IS-10500 water quality standard.
iv
Thus, it is concluded that lime treatment is an effective means of reducing the hydraulic conductivity
and concentration of metals in the leachate emanating from compacted as well as sedimented fly ash
specimens.
v
Table of Contents
Title
Page No.
Abstract…………………………………………………………………………………i
List of Figures…………………………………………………………………………..x
List of Tables……………………………………………………………………………xiii
List of abbreviations and symbols………………………………………………………xv
CHAPTER-1 INTRODUCTION………………………………………………………..1
1.1.AN OVERVIEW ON FLY ASH GENERATION………………………………….1
1.2.FLY ASH UTILIZATION………………………………………………………….1
1.3.ENVIRONMENTAL PROBLEMS ASSOCIATED WITH FLY ASH…………….3
1.4.ORGANIZATION OF THE THESIS………………………………………………7
CHAPTER-2 LITERATURE REVIEW………………………………………………...9
2.1. INTRODUCTION………………………………………………………………….9
2.2. REVIEW OF LITERATURES……………………………………………………..9
2.2.1. Physical Properties……………………………………………………………….10
2.2.2. Chemical Properties………………………………………………………………11
2.2.3. Leachate Characteristics………………………………………………………….14
2.3. CRITICAL OBSERVATION……………………………………………………..16
2.4. OBJECTIVE AND SCOPE OF THE PRESENT WORK…………………………16
2.4.1. Stabilization of Fly Ash by Lime……………………………………………….16
2.4.2. Stabilization of Sediment and Compacted Fly Ash Bed
by Lime Column Technique………………………………………………………17
CHAPTER-3 METHODOLOGY………………………………………………………..18
3. 1. INTRODUCTION…………………………………………………………………..18
3.2. DETAILS OF TEST CONDUCTED………………………………………………..18
3.3. MATERIALS………………………………………………………………………..19
3.3.1 Fly Ash………………………………………………………………………………19
3.3.2 Lime…………………………………………………………………………………19
vi
3.4. PROPERTIES OF RAW FLY ASH………………………………………………..22
3.4.1 Index Properties……………………………………………………………………22
3.4.1.1 Specific gravity………………………………………………………………….22
3.4.1.2 Particle size distribution………………………………………………………...22
3.4.1.3 Liquid limit……………………………………………………………………..22
3.4.2 Chemical Properties………………………………………………………………..23
3.4.2.1 pH value………………………………………………………………………….23
3.4.2.2 Total metal concentration and
concentration of metals in leachate sample……………………………………………..23
3.4.3 Engineering Properties of Fly ash………………………………………………….23
3.4.3.1 Compaction characteristics ………………………………………………………23
3.4.3.2 Unconfined compressive strength (UCS Value)……………………………….....24
3.4.3.3 Shear parameters …………………………………………………………………24
3.5. EXPERIMENTAL STUDY…………………………………………………………25
3.5.1 Procedure for Lime-Mixed Fly Ash………………………………………………..25
3.5.1.1 Laboratory compaction test………………………………………………………25
3.5.1.2 Hydraulic conductivity…………………………………………………………..26
3.5.1.3 Leachate analysis…………………………………………………………………27
3.5.1.4 Hydration products and microstructure analysis………………………………….28
3.5.2 Procedure for Lime Column Experiments…………………………………………..29
3.5.2.1 Preparation of sediment and compacted fly ash beds……………………………..29
3.5.2.2 Installation of lime column………………………………………………………..30
3.5.2.3 Installation of temperature sensors………………………………………………..30
3.5.2.4 Sampling program…………………………………………………………………30
3.5.2.5. Details of test conducted………………………………………………………….34
3.5.2.5.1 pH test……………………………………………………………………………34
3.5.2.5.2. Leachate analysis………………………………………………………………..34
vii
3.5.2.5.3. Hydraulic conductivity……………………………………………………………35
CHAPTER- 4 RESULT AND DISCUSSION-1…………………………………………….38
4.1. INTRODUCTION………………………………………………………………………38
4.2. GEOTECHNICAL PROPERTIES OF LIME –FLY ASH MIXES…………………….38
4.2.1. Water Content- Dry Density Relationship for Lime-Fly Ash Mixes………………….38
4.2.2. Hydraulic conductivity………………………………………………………………...40
4.2.3. pH Value……………………………………………………………………………….42
4.2.4. Leachate Analysis……………………………………………………………………...42
4.2.4.1. Leachate–Load Ratio………………………………………………………………..44
4.2.5. Hydration products and Microstructure ……………………………………………....45
CHAPTER -5 RESULT AND DISCUSSION-II…………………………………………….48
5.1. INTRODUCTION……………………………………………………………………….48
5.2. SEDIMENT POND ASH BED TREATED WITH LIME COLUMN………………….48
5.2.1. pH test…………………………………………………………………………………48
5.2.2. Hydraulic Conductivity………………………………………………………………50
5.2.2. Leachate Analysis…………………………………………………………………….53
5.3. COMPACTED FLY ASH BED TREATED WITH LIME COLUMN………………..54
5.3.1 pH Test………………………………………………………………………………….54
5.3.2 Hydraulic Conductivity…………………………………………………………….......58
5.3.3. Leachate Analysis……………………………………………………………………...61
5.3. HYDRAULIC CONDUCTIVITY AND LEACHATE LOAD RATIO OF SEDIMENT AND
COMPACTED FLY ASH BED………………………………………………….........64
5.4.1. Hydraulic Conductivity………………………………………………………………..64
5.4.1. Leachate Load Ratio…………………………………………………………………..64
CHAPTER- 6 SUMMARY AND CONCLUSIONS………………………………………..66
6.1. SUMMARY…………………………………………………………………………….66
6.2. CONCLUSIONS………………………………………………………………………..66
viii
6.3. SCOPE FOR FUTURE WORK……………………………………………………69
REFERENCES………………………………………………………………………….70
LIST OF PUBLICATIONS……………………………………………………………..76
ix
List of Figures
Title
Page No
Figure 1.1. Progressive generation and utilization of fly ash in India………………..2
Figure 1.2. Utilization of Indian fly ash in different sectors (2012-2013)…………....2
Figure 3.1. SEM image of RSP fly ash……………………………………………....20
Figure 3.2. SEM image of lime………………………………………………………20
Figure 3.3 XRD analysis of raw fly ash……………………………………………...21
Figure 3.4 XRD analysis of lime…………………………………………………….21
Figure 3.5 Grain size distribution curve of fly ash…………………………………..22
Figure 3.6 Compaction curve of RSP fly ash………………………………………..24
Figure 3.5 Permeability test set up…………………………………………………..27
Figure 3.6 Constant head permeameter……………………………………………...27
Figure 3.7 Acid digestion in fume hood…………………………………………….. 27
Figure 3.8 Atomic absorption spectrometer……………………………………….….27
Figure 3.9 Specimens used for XRD ………………………………………………. 28
Figure 3.10 Rikagu Ultima-IV X-ray diffractometer ………………………………28
Figure 3.11 JEOL-JSM-6480 LV SEM……………………………………….. …… 29
Figure 3.12 Instrument used for coating the sample…………………………………..29
Figure 3.13 Placing of fly ash slurry in test tank………………………………….… 31
Figure 3.14 Sedimentation of fly ash………………………………………………..31
Figure 3.15 Sampling locations in the test tank for pH and leachate analysis………31.
Figure 3.16. Details of test tank and its components………………………..………..32
Figure 3.17. Plan of the test tank showing locations
for collection of leachate samples………………………………………….………….32
Figure 3.18. Elevation of the test tank showing locations for collection of samples for determination
of hydraulic conductivity……………………………………………………..……….33
Figure 3.19. Plan of the test tank showing locations for collection of samples for determination of
hydraulic conductivity………………………………………………………..……….33
x
Figure 3.20. pH meter used for measuring the pH of the samples…………………….34
Figure 3.21 Filtration of sample for AAS test……………………………..…………..35
Figure 3.22. Sample used for AAS test…………………………………..………….35
Figure 4.1. Variation of OMC with lime content
at light and heavy compaction energies………………………………………….……39
Figure 4.2. Variation of MDD with lime content
at light and heavy compaction energies……………………………………………….39
Figure 4.3. Variation of hydraulic conductivity
with lime content for light compaction……………………………………….………..41
Figure 4.4. Variation of hydraulic conductivity
with lime content for heavy compaction……………………………………..………..41
Figure 4.5. Variation of pH with lime content on different days of curing…….…….42
Figure 4.6. Variation in concentration of Ca
in the leachate sample with lime content………………………………..…………….43
Figure 4.4 XRD analyses of specimens containing
different doses of lime on 90 days curing……………………………………………..46
Figure 4.5 XRD analysis of FA+4%L specimen on different days of curing………...46
Figure 4.6. SEM image of FA+4%L specimen on 0 day curing……………...............47
Figure 4.7. SEM image of FA+4%L specimen on 7 days curing ……………….. ..47
Figure 4.8. SEM image of FA+4%L specimen on 90 days curing…………………47
Figure 4.9. SEM image of FA+15%L specimen on 90 days curing …………….….47
Figure 5.1.Variation of pH value with radial distance……………………………….49
Figure 5.2. Variation of pH value with depth…………………………………….….50
Figure 5.3.Variation of hydraulic conductivity
with radial distance on 90 days curing……………………………………………….51
Figure 5.4.Variation of hydraulic conductivity
with radial distance on 180 days curing…………………………………………….51
xi
Figure 5.5.Variation of hydraulic conductivity
with radial distance on 365 days curing………………………………………………52
Figure 5.6.Variation of hydraulic conductivity
with curing periods at different depths……………………………………………….52
Figure 5.7.Variation of pH value with radial distance………………………………..57
Figure 5.8.Variation of pH value with depth…………………………………………57
Figure 5.9.Variation of hydraulic conductivity
with radial distance on 90 days curing……………………………………………….59
Figure 5.10.Variation of hydraulic conductivity
with radial distance on 180 days curing……………………………………………..59
Figure 5.11.Variation of hydraulic conductivity
with radial distance on 365 days curing…………………………………………….60
Figure 5.12.Variation of hydraulic conductivity
with curing periods at different depths……………………………………………...60
Figure 5.13 Variation of hydraulic conductivity with depth in the specimens collected from 5cm
radial distance in the ash beds on 365 days curing…………………………………..64
Figure 5.13 Variation of leachate load ratio of Cu with depth in the specimens collected at 25cm
radial distance in the ash beds on 365 days curing………………………………….65
xii
List of Tables
Title
Page No.
Table 1.1. Concentration of trace elements in different types of Indian coal………….4
Table 1.2. Average concentration of trace elements
in Indian, USA and Australian coal…………………………………………………...5
Table 1.3. Allowable and threshold limits of
concentration of metals in drinking water…………………………………………….5
Table 1.4. Adverse effect of trace elements on environment…………………….……6
Table 3.1 Properties of RSP fly ash…………………………………………………..25
Table 3.2 OMC and MDD values of fly ash-lime mixes…………………………..…26
Table 3.3. Hydraulic conductivity of sediment
pond ash bed (in cm/sec) on 90 days curing……………………………………….…36
Table 3.4. Hydraulic conductivity of sediment
pond ash bed (in cm/sec) on 180 days curing………………………………….……36
Table 3.5. Hydraulic conductivity sediment
pond ash bed (in cm/sec) on 365 days curing………………………………….……36
Table 3.6. Hydraulic conductivity of compacted
pond ash bed (in cm/sec) on 90 days curing…………………………………………37
Table 3.7. Hydraulic conductivity of compacted
pond ash bed (in cm/sec) on 180 days curing………………………………………37
Table 3.8. Hydraulic conductivity of compacted
pond ash bed (in cm/sec) on 365 days curing………………………………………37
Table 4.1 Concentration of metals in leachate sample of raw fly ash………………43
Table 4.2. Concentration of elements in leachate sample
after 0 and 90 days of curing………………………………………………………44
Table 4.3. Leachate load ratio values for different metals
in samples cured for 90 days……………………………………………………….45
xiii
Table 5.1. Concentration of metals in leachate on 90 days curing………………….54
Table 5.2. Concentration of metals in leachate on 180 days curing………………...55
Table 5.3. Concentration of metals in leachate on 365 days………………………..55
Table 5.4. Concentration of metals in leachate on 90 days curing………………….62
Table 5.5. Concentration of metals in leachate on 180 days curing…………………62
Table 5.6. Concentration of metals in leachate on 365 days curing…………………63
xiv
List of Abbreviation and Symbols
Abbreviations
Particular
Description
AAS
Atomic Absorption Spectrometer
C-A-H
Calcium Aluminate Hydrate
C-S-H
Calcium Silicate Hydrate
C-A-S-H
Calcium Aluminium Silicate Hydrate
DSC
Differential Scanning Calorimetry
EPA
Environmental Protection Agency
FA
Fly Ash
IS
Indian Standard
L/S
Liquid-to Solid Ratio
MDD
Maximum Dry Density
OMC
Optimum Moisture Content
SEM
Scanning Electron Microscope
TCLP
Toxicity Characteristics of Leaching Procedure
TGA
Thermogravimetric Analysis
UCS
Unconfined Compressive Strength
WHO
World Health Organization
XRD
X-Ray Diffraction
Symbols
Cc
Coefficient of Curvature
Cu
Coefficient of Uniformity
k
Hydraulic Conductivity
L
Lime
R
Leachate- Load Ratio
xv
CHAPTER-1
INTRODUCTION
1.1 AN OVERVIEW ON FLY ASH GENERATION
Coal based thermal power plants (TPPs) have been the backbone of a country due to its major
contribution in electricity generation for the developmental purposes. With a stock of 70 billion tons
of fossil fuel reserve, majority of TPPs (84%) are run on coal. About 260 million tons (MT) of coal
(65% of annual coal produced in India) is being used by TPPs which ultimately results in generation
of enormous quantity of fly ash in the country. At present, over 165 MT of fly ash is being generated
by TPPs as a by- product of coal combustion and is predicted to cross 225 million tons by the year
2017. With the increase in generation of fly ash, its disposal has become a major issue for thermal
power plants as it creates a lot of problems like shortage of usable land, increase in disposal cost,
leaching of noxious heavy metals and dusting of atmospheric air. Generally, the fly ash is disposed
of by using either dry or wet disposal method. Most of the power plants in India adopt wet disposal
system where fly ash is being converted to slurry form and dumped in large settling pond called as
ash pond which ultimately strikes negative impact on the environment. At present around 50,000
acres of land surface is occupied by ash ponds and continuously creating disposal and environmental
problems. So the best way-out in order to get rid of these problems is its right application at the
appropriate place by using available technologies.
1.2 FLY ASH UTILIZATION
Utilization of fly ash will not only help in mitigating the environmental problems, but also in
preserving the conventional earth materials. Fly ash can be used in various sectors such as concrete
manufacturing industries, structural fills for low lying areas, embankment and subgrade for
1
highways, backfill in retaining structures, mine stowing, dam construction and dyke construction etc.
Figure 1.1 shows the progressive generation and utilization of fly ash during the period from 199697 to 2012-13 and Figure 1.2 shows the quantity of fly ash utilization in various sectors during the
year 2012-13.
Figure 1.1. Progressive generation and utilization of fly ash in India
Source: (www.cea.nic.in)
7
5%
6
12%
8
1%
1
47%
1 cement manufacturing
5
7%
2 Cement substitution
3 Road embankments
4 Low lying area filling
5 Ash bund raising
6 Mine fill
4
10%
7 Brick manufacturing
3
9%
8 Agriculture & others
2
9%
Figure 1.2. Utilization of Indian fly ash in different sectors (2012-2013)
Source: Central Electrical Authority (CEA) annual report
2
1.3 ENVIRONMENTAL PROBLEMS ASSOCIATED WITH FLY ASH
Generation of huge quantity of fly ash creates a lot of problems such as leaching and dusting. Due to
the presence of finer particles and being light weight, it has the potential to be carried by air easily
and pollute the environment which results in pulmonary diseases, including asthma and silicosis.
Moreover, fly ash contains a lot of toxic heavy metals such as As, Be, Cd, Cr, Cu, Pb, Hg, Mo, Ni,
V and Zn etc. [Adriano et al .( 1980)]. These metals can leach out by the effect of rain and
contaminate the ground water and pose a serious threat to the human as well as aquatic life. The
concentration of metals in the leachate may vary according to the type and source of coal. Table 1.1
shows the concentration of trace elements in different types of coal in Indian context and Table 1.2
shows the average concentration of trace elements in Indian, USA and Australian coal.
However, it is observed from Table 1.3 that the concentration of major and trace element is quite
more than the allowable and threshold limit of WHO and IS-10500 water quality standard. There is
possibility that leaching of these metals to ground water may violate the statutory regulation for
ground water pollution and can cause detrimental health problems to the human being as shown in
Table 1.4. So, a major focus should be imparted on the environmental impact of fly ash utilization.
Once the environmental impacts can be confidently predicted, a larger market can be encouraged to
productively reuse fly ash.
The leaching of metals mainly depends on two factors such as pH and hydraulic conductivity. The
pH plays a pivotal role in reducing the concentration of the elements. Hydraulic conductivity also
has a major effect in preventing the leachate from contaminating the ground water. Fly ash
stabilization by certain chemical additives may be an effective way-out to
mitigate the effect of
leaching and thus may help to keep the concentration of toxic metals within threshold limit of WHO
and IS-10500 water quality standard.
3
Table 1.1. Concentration of trace elements in different types of Indian coal
Trace
elements
Anthracite
High
Bituminous
volatile Low
Volatile Medium
Volatile Lignite
Bituminous
Bituminous
Max
ppm
Min
ppm
Av
ppm.
Max
ppm
Min
ppm
Av.
ppm
Max
Ppm
Min
ppm
Av
ppm
Max
Ppm
Min
ppm
Av
ppm
Max
Ppm
Min
ppm
Av.
ppm
B
Co
130
165
63
10
90
81
2800
305
90
12
770
64
180
440
76
26
123
172
123
290
74
10
218
105
85
40
10
2.1
28
11
Cr
395
210
304
315
74
193
490
120
221
230
36
169
90.8
5
70
Ga
71
30
42
98
17
40
135
10
41
52
10
-
16
0
6.7
Ge
20
20
-
285
20
-
20
20
-
20
20
-
10
0
3
Cu
540
96
405
770
30
293
850
76
379
560
130
313
67.1
1.6
20
Mn
365
58
270
700
31
170
780
40
280
4400 125
1432 228.4
10.5
100
Ni
320
125
220
610
45
154
350
61
141
440
20
263
0
100
45
Pb
120
41
81
1500 32
183
170
23
89
210
52
96
46.5
0
15
Sn
19
962
825
10
171
230
10
92
160
29
75
35
0
12
V
425
0
310
210
248
840
60
249
480
115
278
860
170
390
278
0
86
Zn
350
155
-
1200 50
310
550
62
231
460
50
195
100
6
40
Reference: Vacovic (1983) and Chadwick (1987)
4
Table 1.2. Average concentration of trace elements in Indian, USA and Australian coal
Sl No
Element
Indian
Average
(ppm)
1
2
3
4
5
6
7
8
9
10
11
As
Hg
Cd
Ni
Co
Cr
Cu
Zn
Mn
V
Pb
5.0
0.35
1.3
45
11
70
20
40
100
86
15
USA
Average
(ppm)
Australian
Average
(ppm)
Worldwide
Average
(ppm)
15
0.18
1.3
15
7
15
19
39
100
20
16
3
0.10
0.10
15
6.0
1.5
25
20
10
5
0.012
15
5
10
15
50
50
25
25
Reference: Vacovic (1983) and Chadwick (1987)
Table 1.3. Allowable and threshold limits of concentration of metals in drinking water
Sl no
Elements
Content
ranges in
Indian fly
ash (mg/kg)
WHO(1993)
Allowable Threshold
limit
limit (mg/l)
(mg/l)
0.01
1.00
IS-10500(1992)
Allowable
limit (mg/l)
Threshold
limit (mg/l)
0.05
5.00
1
As
2.3-6300
2
Ca
200
20000.00
200
20000.00
3
Cr
338177,100
10-1000
0.05
5.00
0.05
5.00
4
Cu
14-2800
2.0
100.00
1.5
150
5
Fe
36-1333
0.3
30
0.3
30
6
Pb
3.1-50000
0.01
10.00
0.1
10
7
8
Hg
Ni
0.002-1.
6.3-4300
0.001
0.02
0.1
2.00
0.001
0.02
0.1
2
10
Zn
10-3500
3
300.00
5
500
11
Mg
116-60,800
150
15000.00
100
10000
12
Al
4615-24200
0.2
20.00
0.2
20
5
Table 1.4 Adverse effect of trace elements on environment
Sl No.
Trace
elements
Health hazards
1
Pb
2
Hg
Tiredness, abdominal discomfort, irritability , anemia, brain
swelling, kidney disease, cardio-vascular problems, nervous system
damage
Itai-itai disease, hypertension, kidney damage, sterility among males
developmental defects like reduced IQ and mental retardation
3
Cr
4
As
5
Cu
Stomach and intestinal ulcers, anemia, stomach cancer, lung cancer,
asthma
Lung cancer, skin cancer, cardio-vascular and neurological
disorders, affect the gastrointestinal tract respiratory tract, urinary
tract,
headache, weakness Nausea, vomiting epigastric pain, diarrhea,
6
Ni
Cancers of nose and lungs
7
Zn
Vomiting and diarrhea, affect digestive mucous membrane
8
Se
Neurological disorder, impaired vision, paralysis
9
Sb
Eye, skin irritation, stomach pain, ulcers ,lung cancer
110
Cd
Kidney disease, Hypertension and lung cancer
Reference: Gottlieb et al. (2010)
By a thorough study of previous research works, it is perceived that most of the researchers have
adopted lime stabilization technique to reduce the concentration of elements in the leachate but a
limited attempt has been made to study the efficacy of lime column in mitigating the leachate
characteristics of pond ash. In addition to this, the role of hydraulic conductivity and hydration
products on leachability of elements are not investigated in detail. Moreover, mixing of lime in ash
pond is practically not a feasible process. Keeping these aspects in mind an effort has been made to
study the effect of lime column in mitigating the problems of leaching from the ash pond deposits
through large scale laboratory model tests.
6
1.4 ORGANIZATION OF THE THESIS
The whole research work has been presented in six chapters. A brief description about the chapters
is given below:
Chapter 1 describes some background knowledge on the fly ash generation, utilization and its effect
on the environment. A brief description on the leachate characteristics of fly ash and the remedies to
mitigate its effect is given.
Chapter 2 represents the critical review of relevant literatures including the effect of compaction
effort, amount of lime, curing period on the hydraulic conductivity and leachate characteristics of fly
ash. This also includes the efficacy of lime column in reducing the hydraulic conductivity and
leachate characteristics of sediment pond ash deposits.
Chapter 3 present in details about the materials used, test procedure adopted for sample preparation,
sampling scheme and details of experimental studies undertaken.
Chapter 4 delineates the effect of lime on the hydraulic conductivity and leachate characteristics of
fly ash. The compaction characteristics of fly ash mixed with different lime content along with the
hydraulic conductivity and leachate characteristics of the compacted fly ash specimens were
determined after specified curing periods. The effect of lime and curing period on hydration
products, microstructure and morphology in the stabilized specimens were studied by various
microanalyses such as XRD, and SEM tests. A correlation is established between the hydraulic
conductivity, leachate characteristics and the hydration products, microstructure.
Chapter 5 highlights the efficacy of lime column in reducing the leachate characteristics of
sedimented and compacted fly ash bed. A lime column of 10cm diameter and 100cm depth was
7
installed at the centre of the ash beds to study the effect of lime on leachate characteristics of pond
ash. Radial and vertical migration of lime from the lime column was investigated by collecting
leachate samples from different radial distances and depths at specified curing periods. At a given
depth and radial distance, migration was studied by comparing the pH value of the treated fly ash
with that of virgin fly ash specimen. The effect of lime was studied by performing leachate analysis
and hydraulic conductivity test and comparing the values with that of the untreated fly ash.
Chapter 6 presents the experimental findings and future scope of the work.
8
CHAPTER 2
LITERATURE REVIEW
2.1. INTRODUCTION
Fly ash is enriched with a number of toxic metals such as cadmium, chromium, nickel, lead, zinc,
aluminum, iron, manganese, magnesium and silicon etc. There is a possibility that these undesirable
components present in the fly ash leach out by the effect of rain and contaminate the ground as well
as surface water. The leaching phenomenon is predominantly governed by two factors such as pH
and hydraulic conductivity. Metal solubility generally decreases with increase in pH. Hydraulic
conductivity also has a major effect in preventing the leachate from contaminating the ground water.
If the material can be made impermeable, the leachate can be confined at the source of generation
and thus, the ground water can be protected from being contaminated. Therefore, suitable chemical
stabilization technique is adopted in order to control the migration of heavy metals to the
surrounding. A good number of relevant literatures are available on the leaching behavior of major
and trace elements which are cited in this chapter.
2.2. REVIEW OF LITERATURES
A good number of literatures are available on engineering properties of lime treated fly ash
including the effect of lime on the leachate characteristics of fly ash deposits. The available literature
are reviewed and presented in the following headings:
 Physical properties
 Chemical properties
 Leachate characteristics
9
2.2.1. Physical Properties
Bowders et al. (1990) investigated the effects of lime, cement and bentonite on the permeability and
leaching of metals from the fly ash and found a reduced value of permeability (1.0E-08 cm/sec) by
substituting lime or cement with bentonite. There was also a significant reduction in the
concentration of trace elements due to addition of lime or cement.
Ghosh and Subbarao (1998) reported that the hydraulic conductivity of the fly ash specimens
reduced substantially due to addition of lime and the concentrations of in the leachate were below
threshold limits of water quality standard.
Ghosh and Subbarao (2001) used XRD, SEM and EDX techniques to gain information on the fly
ash-lime interaction and reported that the reduction of permeability is due to the development of
hydration products which blocks the capillary voids.
Kalinski and Yerra (2005) reported the hydraulic conductivity of compacted cement stabilized fly
ash is affected compaction effort and curing.
Chand and Subbarao (2007) studied the efficacy of lime column in reducing the permeability and
concentration of heavy metals emanating from pond ash deposits and reported that the lime column
method was an effective means of reducing hydraulic conductivity and concentration of heavy
metals emanating from pond ash deposits in addition to modifying other geotechnical parameters.
Pal and Ghosh (2011) studied the effect of compaction effort on hydraulic conductivity of class F
fly ashes by preparing the samples corresponding to their light and heavy compaction energy. From
the test results it was found that with increase in compaction effort, MDD increased and OMC of the
10
sample decreased. A decreasing trend in hydraulic conductivity values were with the rise in MDD of
the samples.
Cuisinier et al. (2011) reported that hydraulic conductivity of a soil was reduced significantly due to
addition of lime.
Kishan et al. (2012) studied the leaching of metals by stabilizing fly ash with lime and/or gypsum
and found a reduced value of hydraulic conductivity due to addition of these chemical additives.
Amiralian et al. (2012) studied effects of lime and fly ash in compaction properties of sand based
on the seven specimens (i.e. 1 sand, 2 lime, 2 fly ashes and 4 mixture of lime-fly ash). The given
result of lime and fly ash specimens illustrated that fly ash stabilization was more effective than lime
treatment alone. However, utilization of combination of additives leads to optimum effect on
compaction characteristics of sand.
Tran et al. (2014) studied the effects of lime treatment on the microstructure and hydraulic
conductivity of compacted expansive clay and reported that the decrease in hydraulic conductivity in
the cured specimens was due to the formation of cementitious compounds which clog the pores.
2.2.2. Chemical Properties
Shively et al. (1986) conducted sequential extraction test on Portland cement-based products
containing metal sludge and reported that leaching of metals is dependent upon the pH.
Luxa’n et al. (1989) used several techniques like X-ray diffraction and infrared absorption
spectroscopy and identified the formation of hydration products (C-A-H gel) due to fly ash - lime
reaction.
11
Gould et al. (1989) reported that metal solubility generally decreases with increase in pH.
Xu and Sarkar (1991) used various techniques like XRD, SEM, and EDX to study the effect of
lime and cement on the strength properties of fly ash and reported that the hydration products are
formed due to fly ash-lime reactions and these are responsible for gaining of strength.
Cheng and Bishop (1992) performed sequential leaching extraction test on fly ash and found that
leaching of metals occurs at low pH.
Webster and Loehr (1996) investigated on the leaching of elements like Cd and Pb from concrete
products and reported that the reduced concentration of metals is due to presence of calcium matrix
within the concrete which was responsible for reducing the concentration of cadmium and lead.
Fleming et al. (1996) studied the leach-ability of metals like Cd, Zn, Pb, Cr, Ag and Hg from fly ash
generated by thermal power plant and a municipal waste incinerator by conducting column leaching
experiments under acidic conditions and reported that extraction of metals depends upon pH value.
Bishop (1988) performed sequential extraction test to study the leaching of metals like chromium,
cadmium and lead and reported that the reduction in concentration of metals was due to the
encapsulation of metal ions by hydration products.
Sengupta and Miller (1999) investigated on the contaminants leaching out from the scrap tire
material treated with a synthetic solution of varying pH and reported that the concentration of
contaminants that leach out depended on the pH of the environment
Lau and Wong (2001) reported that leaching behavior of different element differs due to
differences in elemental properties and pH of the solution.
12
Bin-Shafique et al. (2006) conducted various types of leaching tests on the fly ashes to study the
leaching of chromium, cadmium, Selenium and silver. From the results it was found that the
concentration of elements in the fly ash is greatly dependent upon pH of the material.
Jankowski et al. (2006) studied the leaching of metals like As, B, Mo and Se in fly ash generated
from Australian power stations by conducting batch leaching test and reported that the pH of the
leaching solution was the major factor which affects the migration of these trace elements in the fly
ashes.
Ghosh and Subbarao (2006) reported that the leaching of metals reduces due to addition of
chemical additives like lime and/or gypsum.
Wang et al. (2007) studied the effect of Calcium on arsenic adsorption onto coal fly ash by
conducting batch leaching test and reported that addition of calcium significantly reduced the
soluble arsenic ratio in the alkaline pH range.
Prasad and Mondol (2008) investigated on the heavy metals leaching in Indian fly ash and reported
that maximum leachibility of all elements was found at pH of 2.
Sridevi et al. (2010) studied the leaching of lime from fly ash bed stabilized with varying
percentage of lime 2 to 10% with 2% increment of lime and reported that the cumulative percentage
of lime leached was negligibly small. For 10% lime content in the FAC, only 0.11% of lime was
found to leach out. For lower lime contents the cumulative percentage of lime leached was even
smaller than 0.11% and lime-stabilized FACs could be used in civil engineering applications such as
pavements, foundations, embankments etc.
13
Vítková et al. (2010) studied the leaching characteristics of fly ash generated from a cobalt smelter
using the pH-static leaching test (CEN/TS 14997, pH range 5-12) coupled with mineralogical
investigation and speciation-solubility modelling. It was observed that the maximum leaching
occurred at pH 5 and minimum at pH 12. The main solubility-controlling phases in this system were
CaCO3 and CaSO4·2H2O.
Behera and Mishra (2012) studied the microstructural characteristics of low lime fly ash stabilized
with lime by conducting XRD as well as SEM tests.
The leachate analysis shows that the
concentration of Ni, Cr and Pb in the leachate effluents were below threshold limits. The reduced
concentration of elements is due to the development of hydration products.
Izquierdo and Querol (2012) reported that the amount of calcium plays a pivotal role on the pH of
the fly ash. The mobility of most elements is greatly dependent upon the pH of the medium. The
leaching of elements reduces due to encapsulation of metals by hydration products.
Guleria and Dutta (2013) studied the effect of treated tire chips on leaching characteristics of fly
ash-lime-gypsum composite and found a reduced concentration of leaching metals.
Ozkok et al. (2013) studied the leaching behavior of arsenic, chromium, and copper from highcarbon fly ash–soil mixtures and reported that As and Cr (VI) are retained at acidic pH and leach
more at pH >7. Significant leaching of Cu was only observed at pH<7.
2.2.3. Leachate Characteristics
Hajarnavis et al. (2000) studied the environmental impact of fly ash by collecting 32 number of fly
ash samples from different thermal power plants located in various states and reported that total
14
metal concentration was very high in most of the fly ashes. The wide variation in heavy metals was
attributed to the quality of coal used in thermal power plant.
Choi et al. (2002) performed batch leaching tests to study the leaching characteristics of different
types of fly ashes. It was found that the concentration of Si, Al and K was higher in anthracite coals
than the sub-bituminous coals.
Sushil and Batra (2006) analyzed the content ranges of heavy metals like Pb, Cr, Cu, Ni, Zn,Co,
and Mn in fly ash collected from different thermal power plants in India and from the test results, it
was found that the concentration of Chromium and Zinc were highest whereas the concentration of
Cobalt was lowest.
Sarode et al. (2010) studied the effect of cement on the leach-ability of heavy metals fly ash, bottom
ash by toxicity characteristic leaching procedure (TCLP) and from the test results it was found that
all the elements except Ni and Pb were slightly higher than the WHO water quality standard.
Lokeshappa and Dikshit (2012a) worked on single step extractions of metals in coal fly ash The
evaluation of the optimum time for leaching of the toxic metals and metalloids present in the three
fly ashes was determined using the single step extraction procedure.
Lokeshappa and Dikshit (2012b) studied on the leachate characteristics of both class c and class F
laboratory scale ash ponds and found out that the concentration of metals like arsenic, chromium
increased with time in case of class F ash whereas the concentration of all metals deceased in ash
pond containing class C ash.
15
2.3. CRITICAL OBSERVATION
Scanning thorough the previous research works, it is perceived that most of the researchers have
adopted lime stabilization technique to reduce the concentration of elements in the leachate but a
limited attempt has been made to study the efficacy of lime column in mitigating the leachate
characteristics of pond ash. In addition to this the role of hydraulic conductivity and hydration
products on leachability of elements are not investigated in detail. Moreover, mixing of lime in ash
pond is practically not a feasible process. Keeping these aspects in mind an effort has been made to
study the effect of lime column in mitigating the problems of leaching from the ash pond deposits
through large scale laboratory model tests.
2.4. OBJECTIVE AND SCOPE OF THE PRESENT WORK
The objective of the present research work is
 To reduce the concentration of major and trace elements in the leachate generated from fly
ash deposit /construction site below threshold limit so as not to pollute the surrounding area
and ground water.
 To confine the leachate to the source i.e. not allowing it to migrate from the point of
disposal/in the area where fly ash is used as construction material to the surrounding.
The above objective is achieved by following means
2.4.1. Stabilization of Fly Ash by Lime
 Preparation of specimens by varying the fly ash and lime proportions (0%, 2%, 4%, 8%,
12%, and15%) and compacting them to their respective MDD and OMC obtained from light
compaction and heavy compaction tests.
16
 Evaluation of hydraulic conductivity of specimens after curing periods of 0, 7,15,30,60 and
90 days.
 Analysis of major and trace elements by collecting leachate samples from permeability test
specimens after specified days of curing.
 Analysis of cured sample by XRD and SEM techniques to study hydration products
microstructure and morphology and to correlate with the hydraulic conductivity and leachate
analysis results.
2.4.2. Stabilization of Sediment and Compacted Fly Ash Bed by Lime Column
Technique
 Preparation of tank for sedimentation of fly ash slurry and installation of lime column.
 Determination of pH value and analysis of major as well as trace element concentration by
collecting leachate samples from different radial distances( 5cm, 15cm, 25cm and 35 cm and
depths (10cm ,30cm, 50 cm, 70cm and 90cm ) after curing periods of 30,90,180 and 365
days.
 Collection of undisturbed samples from different radial distances and depths and Evaluation
of hydraulic conductivity after curing periods of 30, 90, 180 and 365 days.
 Analysis of test results.
17
CHAPTER 3
METHODOLOGY
3. 1. INTRODUCTION
The primary objective of this study is to reduce the concentration of metals in the leachate
emanating from the fly ash bed and also to prevent the leachate effluents from contaminating the
ground water so that a high level confidence can be built to reuse the fly ash as a sustainable and
eco-friendly construction material in various geotechnical applications conserving the conventional
construction metal. This chapter presents in details about the materials used test procedure adopted
for sample preparation sampling scheme and details of experimental studies undertaken.
3.2. DETAILS OF TEST CONDUCTED
The experiments were performed in two phases. At first, the compaction characteristics of fly ash
mixed with different percentage of lime content such as 0%, 2%, 4%, 8%, 12% and 15% were found
out from light and heavy compaction tests. The hydraulic conductivity and leachate characteristics of
compacted fly ash specimens were determined after 0, 7, 15, 30, 60 and 90 days of curing. All these
samples were prepared corresponding to their respective MDD and OMC values and cured for the
specified curing periods. The concentration of the major and trace elements like Cu, Fe, Zn, Ca, Ni,
Pb and Cr in the leachate sample; collected on the above mentioned curing periods were found out
by atomic absorption spectrometer. The hydration products and microstructure of the stabilized
specimens were studied by using XRD and SEM analysis. Further, large scale laboratory models of
sediment and compacted fly ash beds were prepared with a centrally installed lime column
simulating a field condition as closely as possible. The samples were collected from various radial
distances as well as depths after 7, 30, 90, 180 and 365 days of curing period and subjected to
18
different tests such pH, and leachate analysis of different elements like Ca, Ni, Pb, Zn, Cu, Cr, and
Fe. In addition to this, hydraulic conductivity of treated ash deposit was measured by collecting
undisturbed specimens from different radial distances and depths.
3.3. MATERIALS
3.3.1 Fly Ash
Fly ash used in the experimental work was procured from RSP Rourkela. This is collected directly
from hopper which in turn receives the fly ash from the electrostatic precipitator. The procured fly
ash was stored in large bins before it was used for preparation of the test beds. It was completely in
dry state. Its physical properties and chemical composition are given in Table3.1. The SEM image of
fly ash is shown in Figure 3.1 which reveals that most of the particles are spherical structure with
few irregular particles. The XRD analysis result of fly ash is shown in Figure 3.3 from which it is
observed that it’s major constituents of are silica, alumina and iron oxide. From energy dispersive
X-ray analysis, it was found that calcium present in the fly ash is less than 20%. So, according to
ASTM specification C 618-89 (1992), this fly ash belongs to a Class F category.
3.3.2 Lime
The lime used in this experimental investigation was procured from local market. From the SEM
image as shown in Figure 3.2, it is observed that the particles of lime are irregular in shape. The
XRD test result of lime sample is shown in Figure 3.4. From the figure it is observed that it’s major
constituents are CaO and CaCO3. The purity of lime was found to be 90.2%. It also contains silica
and other earth material in small quantity.
19
Figure 3.1. SEM image of RSP fly ash
Figure 3.2. SEM image of lime
20
Figure 3.3 XRD analysis of raw fly ash
Figure 3.4 XRD analysis of lime
21
3.4. PROPERTIES OF RAW FLY ASH
3.4.1 Index Properties
3.4.1.1 Specific gravity
The specific gravity of fly ash was determined using density bottle method as per IS: 2720-1980
(Part 3) and was found to be 2.44.
3.4.1.2 Particle size distribution
Particle size distribution of fly ash was determined using both mechanical sieve and hydrometer
analysis in accordance with IS: 2720-1985 (Part 4). Hydrometer analysis was conducted for the
portions of fly ash passing through 75μm sieve and the mechanical sieve analysis for larger size
particles. The gradation curve is shown in Figure 3.5. The uniformity coefficient (Cu) and
coefficient of curvature (Cc) for Fly ash were found to be 8.34 & 2.08 respectively, indicating
uniform gradation of samples.
100
Percentage Finer, (%)
90
80
70
60
50
40
30
20
10
0
0.001
0.01
0.1
1
Diameter (mm)
Figure 3.5 Grain size distribution curve of fly ash
3.4.1.3 Liquid limit
The liquid limit was determined by cone penetrometer method IS: 2720-1985 (Part 5) and was found
to be 56.8%.
22
3.4.2 Chemical Properties
3.4.2.1 pH value
The pH of the virgin fly ash was determined according to IS: 2720- 1987 (Part 26). For this test,
oven dried raw fly ash passing through 425 micron sieve was taken, mixed with water in liquid to
solid ratio (L/S) = 2.5 and it was stirred in magnetic stirrer for 24 hours. Then the solution was
filtered with Whatman 42 filter paper in order to make the sample free from suspended particles and
the test was performed in a calibrated pH meter.
3.4.2.2 Total metal concentration and concentration of metals in leachate sample
The total concentration of major and trace elements present in the fly ash is determined by acid
analysis (Figure 3.7) according to Environmental Protection Agency (EPA 3050B method). The
leachate characteristics of raw fly ash are determined by extraction method (Toxicity Characteristic
Leaching Procedure 1311 method). For this test, oven dried raw fly ash was taken and mixed with
water in liquid to solid ratio (L/S) = 10 and it was stirred in electrical stirrer for 24 hours. Then it
was filtered with Whatman 42 filter paper in order to make the sample free from suspended particles
and then subjected to leachate analysis in atomic absorption spectrometer (Figure 3.8)
3.4.3 Engineering Properties of Fly ash
3.4.3.1 Compaction characteristics
Compaction curves of fly ash were obtained for both light and heavy compaction energies (Figure
3.6). The water content-dry density relationship for raw fly ash was determined from light and heavy
compaction test according to the procedure prescribed in IS: 2720-1980(Part VII) and IS: 27201983(Part 8) respectively. The OMC values for light and heavy compaction test were found to be
38.74% and 32.29% respectively. The MDD values were found to be 1.16 and 1.28g/cc respectively.
23
1.8
Light Compaction
Dry Density (gm/cc)
1.6
Zero air void line
1.4
Heavy Compaction
1.2
1
0.8
0.6
10
20
30
40
Water Content (%)
50
60
Figure 3.6 Compaction curve of RSP fly ash
3.4.3.2 Unconfined compressive strength (UCS Value)
The UCS tests were conducted according to IS: 2720-1991(Part 10). Cylindrical specimens of 100
mm in height and 50 mm diameter were prepared corresponding to MDD at OMC by static
compaction method. For this test required quantity of dry fly ash was added with water to bring it to
OMC and compacted in a constant volume mold. The samples were ejected from the mold and
tested under an axial strain rate of 1.2mm/min. The UCS value was found to be 180kPa.
3.4.3.3 Shear parameters
The shear strength parameters of fly ash specimens were determined as per IS: 2720-1986 (Part 13).
The undisturbed samples were collected from a specimen which was pre-compacted to MDD at
OMC
by inserting sampling device of size 60mm×60mm×25mm and the extra portions are
trimmed. All the specimens were sheared at a rate of 0.2 mm/ min in a motorized direct shear
machine. The unit undrained cohesion and angle of internal friction of fly ash were found to be 0.04
kg/cm 2 and 440 respectively.
24
Table 3.1 Properties of RSP fly ash
Physical Properties of Fly Ash
Chemical Composition
Properties
Value
Constituents
Percentage
Specific gravity
Dry density
OMC
Liquid limit
Cu and Cc
Cohesion
Angle of internal friction
2.44
1.16 gm/cc
38.7%
56.8%
8.34and 2.08
0.04kg/cm2
440
SiO2
AL2O3
Fe2O3
CaO
MgO
SO4
Unburnt carbon
59.2
17.9
9.5
3.2
1.3
1.2
7.0
Hydraulic conductivity
4.25X10-5
cm/sec
Others
0.7
3.5. EXPERIMENTAL STUDY
3.5.1 Procedure for Lime-Mixed Fly Ash
3.5.1.1 Laboratory compaction test
In this study both light compaction and heavy compaction tests were performed with different
combination of fly ash and lime to determine OMC and MDD values of the fly ash lime mixes. In
total 12 numbers of samples (six each for light and heavy compaction) were prepared by varying
lime content as 0%, 2%, 4%, 8%, 12% and 15% of the dry mass of fly ash. In order to conduct
compaction test on fly ash-lime mixes dry fly ash and lime were mixed in the mechanical mixture
for a period of 3 minutes to bring homogeneity of sample and then required amount of water was
added and mixing operation continued by hand to ensure uniform distribution of water. The samples
were subjected to compaction tests without any further delay. The OMC and MDD values for all the
samples were determined from the compaction curves and given in Table 3.2. For this IS: 4332(Part
III) 1967 was followed instead of IS: 2720 (Part VII or Part VIII). As fly ash particles are
cenosphere its compacted dry density was found to be much lower than the conventional earth
material.
25
Table 3.2 OMC and MDD values of fly ash-lime mixes
Mix Proportions
FA+0%L
FA+2%L
FA+4%L
FA+8%L
FA+12%L
FA+15%L
Light compaction test
OMC
MDD
(%)
(gm/cc)
38.74
1.160
40.38
1.130
41.20
1.128
38.62
1.140
38.00
1.160
37.92
1.170
Heavy compaction test
OMC
MDD
(%)
(gm/cc)
31.00
1.284
36.88
1.259
36.50
1.250
32.57
1.290
30.80
1.324
30.50
1.330
3.5.1.2 Hydraulic conductivity
In total twelve numbers of permeability samples (6 numbers of samples compacted with light
compaction and rest 6 numbers of samples compacted in heavy compaction energies) were prepared
with different combination of fly ash and lime and compacted by means of hydraulic jack to their
respective MDD at OMC value obtained from light and heavy compaction tests. The permeability
tests were performed according to the procedure prescribed in IS: 2720-1986 (Part 17) in a constant
head permeameter (Figure 3.5 and 3.6). The specimens were then saturated and the same was
allowed to cure in ambient temperature (average value of 270C) for specified curing periods. The
coefficient of permeability of fly ash specimens treated with different lime contents were evaluated
after curing periods of 0, 7, 15, 30, 60 and 90 days. Further, the effluents coming out from the
permeability mold at 0 and 90 days of curing were collected in sampling bottles and were tested for
the concentration of different elements in an atomic absorption spectrometer. The pH value of the
leachate samples collected from the permeability test on 0 and 90 days of curing were also
determined and reported.
26
Figure 3.5 Permeability test set up
Figure 3.6 Constant head permeameter
3.5.1.3 Leachate analysis
The leachate samples were collected from the permeability molds after curing periods of 0 and 90
days in order to study the variation of leachate concentration in fly ash specimens. The samples are
filtered with whatman 42 No. filter paper in order to make the sample free from suspended particles
and then subjected to leachate analysis. Before filtration the sample the funnel, beaker and the
sample storing bottles are washed with dilute nitric acid and later with distilled water.
Figure 3.7 Acid digestion in fume hood
Figure 3.8 Atomic absorption spectrometer
27
3.5.1.4 Hydration products and microstructure analysis
The formation of hydration products and microstructure in cured specimens were studied by
different methods likes X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM)
respectively. The XRD tests were done to find out the different phases of hydration peaks that
appeared in the specimens after specified days of curing. This is performed with the help of X-ray
diffractometer (model Rigaku, Ultima IV) and image shown in Figure 3.10. After the specified
curing periods the samples are collected and soaked in acetone to stop further hydration. These
samples (Figure 3.9) were ground to size less than 75µm before the tests. The analysis was done
between the scanning range of 7-70o with a, scanning rate 5o/min.
Figure 3.9 Specimens used for XRD
Figure 3.10 Rikagu Ultima-IV X-ray diffractometer
The microstructure of the cured specimens was studied with the help of SEM analysis. The test was
performed by using JEOL JSM-6480LV SEM, equipped with an energy dispersive detector of
Oxford data reference system as shown in Figure 3.11. Before the test the samples are coated in a
coating instrument (Figure 3.12). The coating was done at the rate of 20mA/120sec. Micrographs
were taken at accelerating voltage of 20kV for the best possible resolution from the specimen.
28
Figure 3.11 JEOL-JSM-6480 LV SEM
Figure 3.12 Instrument used for coating the sample
3.5.2 Procedure for Lime Column Experiments
3.5.2.1 Preparation of sediment and compacted fly ash beds
Figure 3.14 and 3.16 shows the test set up for sedimentation of ash slurry which consists of a large
circular galvanized iron tank of 105cm diameter and 120cm height open at the top with a drainage
arrangement at the base. About 1 ton of fly ash sample was used and the amount of water required
for the flow-able fly ash slurry was determined from step-by-step water addition, and mixing of fly
ash. The optimum moisture content without bleeding of water from fly ash was based on eye
judgment and it was found to be 75%. The slurry was prepared at this moisture content and placed in
the tank. Figure 3.13 shows the placing of fly ash slurry in the test tank. Before placing slurry in the
test tank, a cylindrical steel casing of 10cm dia and 100cm height, wrapped with fiber mesh of small
aperture was placed at the middle of test tank. In addition to this fly ash beds were also prepared at
MDD and OMC value. For this about 1 ton of fly ash sample was taken and compacted to
maximum dry density (1.16gm/cc) at optimum moisture content (38.7%). After mixing, the sample
was placed in the tank by 10 equal layers and tamped with a large hammer so that the compacted fly
ash sample could be placed uniformly throughout the tank. Similar arrangements were also made
here for placing the lime column.
29
3.5.2.2 Installation of lime column
At the end of the initial sedimentation period of one month, the lime column was installed at the
center of the ash beds. The quantity of lime required for installing the lime column was 10kg. The
required quantity of lime was taken and it was divided into10 equal parts after placing each part, the
layers are tamped with a small hammer. Thus, a lime column of 10 diameter and 100cm height was
installed at the middle of the ash beds.
3.5.2.3 Installation of temperature sensors
Temperature sensors are located at a depth of 0.5m from top of the fly ash bed in radial direction
with c/c spacing of 0.1 m. In total five numbers of temperature sensors were placed in each testing
tank. A multi-channel temperature recorder was used to note the temperature of the ash bed at
different times and locations. These sensors were used in order to observe the difference in ambient
temperature and temperatures at different locations inside the tank. This also measures the variation
of temperature at various locations inside the tank after the installation of lime column. The sensors
reveal that the temperature is higher at the locations near to the lime column as compared to the
remote locations inside the fly ash bed initially after the installation of lime column. However, with
passage of time no significant variation of temperature at different locations was observed.
3.5.2.4 Sampling program
Stabilized fly ash samples were collected after 30 days, 90 days, 180 and 365 days curing from
different radial distances and depths and subjected to various tests. The samples were collected from
four radial distances, i.e. 5cm, 15 cm, 25cm and 35cm and at 5 different depths i.e. 10cm, 30cm,
50cm, 70cm, and 90cm (Figure 3.18 and 3.19). For leachate analysis, 5 nos of steel hollow pipes of
1cm diameter, length varying from 10 to 90 cm are inserted with c/c spacing of 8 cm at a radial
distance of 25 cm from the center of the test tank (Figure 3.17). In addition, another 4 numbers of
30
similar pipes having length 50cm are inserted at four different radial distances for collecting leachate
samples from a depth of 50cm. The detail positions for leachate sample collection are shown in
Figure 3.15.
Figure 3.13 Placing of fly ash slurry in test tank Figure 3.14 Sedimentation of fly ash
Figure 3.15 Sampling locations in the test tank for pH and leachate analysis
31
1-Temperature sensors, 2-Lime column, 3-Base plate, 4-Sandbed, 5-Stand pipe, 6-Fly ash bed
Figure 3.16. Details of test tank and its components
Figure 3.17. Plan of the test tank showing locations for collection of leachate samples
All dimensions are in m
32
Figure 3.18. Elevation of the test tank showing locations for collection of samples for determination
of hydraulic conductivity
Figure 3.19. Plan of the test tank showing locations for collection of samples for determination of
hydraulic conductivity
33
3.5.2.5. Details of test conducted
3.5.2.5.1 pH test
In order to know the variation of pH in the test tank, water samples are collected from different
radial distances and depth of the sediment ash deposits after specified periods and stored in bottles
for pH test. Then it was filtered with Whatman 42 filter paper and tested in a calibrated pH meter
(Sony µ pH system 361) as shown in Fig. 3.20. During the test care is taken to see that the electrode
of pH meter is immersed in the solution.
Figure 3.20. pH meter used for measuring the pH of the samples
3.5.2.5.2. Leachate analysis
The total concentration of major and trace elements present in the fly ash is determined by acid
analysis according to Environmental Protection Agency (EPA 3050B method). The leachate
characteristics of raw fly ash are determined by extraction method (Toxicity Characteristic Leaching
Procedure 1311 method). In this method oven dried raw fly ash was taken with liquid to solid ratio
(L/S) = 10 and it was stirred in magnetic stirrer for 24 hours. Then it was filtered with Whatman 42
filter paper in order to make the sample free from suspended particles (Figure 3.21) and then
34
subjected to atomic absorption spectrometer (AAS) test. Before filtration of the sample all the glass
wares such as funnels, beaker as well as the sample storing bottles (Figure 3.22) are rinsed with
dilute nitric acid and later with distilled water. In order to know the leachate effluent characteristics
of lime column treated ash, samples are collected from the test tank at various radial distances as
well as depths after specified curing periods and the concentration of the elements like Cu, Fe, Ca,
Ni, Pb, Cr and Zn were found out by AAS (Perkin Elmer).
Figure 3.21 Filtration of sample for AAS test
Figure 3.22. Sample used for AAS test
3.5.2.5.3. Hydraulic conductivity
The permeability of test specimens was performed according to the procedure prescribed IS: 27201987 (Part 36) using a constant head permeameter. In order to know the hydraulic conductivity of
stabilized fly ash specimens, the samples were collected from different radial distances as well as
different depths after specified days of curing with the help of the sampling tube. Then it was
transferred into the permeability mold. Before transferring the sample into the permeability mold,
sampling tubes are leveled at both the ends and the bottom of the permeability mold is covered with
filter paper. Then the sample is transferred carefully without any disturbance. Another filter paper is
35
placed at the top. Then the molds were fitted well and placed in position. Before conducting the test,
it is ensured that the flow pipe is air free. If not, the air is removed by pulling the air vent. Tap water
was allowed to flow though the sample and saturate it well and after some time effluents coming
from the outlets of hydraulic conductivity molds were collected in sampling bottles and time taken
in collecting the sample was noted.
Table 3.3. Hydraulic conductivity of sediment pond ash bed (in cm/sec) on 90 days curing
Depth (cm)
Radial distance (cm)
15cm
25cm
-5
2.4X10
2.6X10-5
10
5 cm
1.5X10-5
35cm
2.68X10-5
30
4.8 X10-5
6X10-5
6.9X10-5
7.6X10-5
50
4X10-5
3.7X10-5
4.1X10-5
5.9X10-5
70
2.2X10-5
2.8X10-5
2.9X10-5
3.14X10-5
Table 3.4. Hydraulic conductivity of sediment pond ash bed (in cm/sec) on 180 days curing
10
5 cm
1.1X10-5
Radial distance (cm)
15cm
25cm
-5
1.5X10
1.7X10-5
30
1.73X10-5
1.9X10-5
2.5X10-5
2.7X10-5
50
1.36X10-5
1.8X10-5
2.1X10-5
2.5X10-5
70
1.21X10-5
1.5X10-5
1.9X10-5
2.3X10-5
Depth (cm)
35cm
2X10-5
Table 3.5. Hydraulic conductivity sediment pond ash bed (in cm/sec) on 365 days curing
10
5 cm
2.26X10-6
Radial distance (cm)
15cm
25cm
-6
2.33X10
2.49X10-6
30
2.56X10-6
2.6X10-6
2.7X10-6
2.96X10-6
50
2.49X10-6
2.58X10-6
2.64X10-6
2.91X10-6
70
2.44X10-6
2.51X10-6
2.59X10-6
2.88X10-6
Depth (cm)
36
35cm
2.7X10-6
Table 3.6. Hydraulic conductivity of compacted pond ash bed (in cm/sec) on 90 days curing
Depth (cm)
5 cm
Radial distance (cm)
15cm
25cm
35cm
10
2.43X10-5
2.68X10-5
2.79X10-5
2.92X10-5
30
2.15X10-5
2.40X10-5
2.60X10-5
2.66X10-5
50
1.75X10-5
2.05X10-5
2.21X10-5
2.56X10-5
70
1.50X10-5
1.66X10-5
1.95X10-5
2.21X10-5
Table 3.7. Hydraulic conductivity of compacted pond ash bed (in cm/sec) on 180 days curing
Depth (cm)
5 cm
Radial distance (cm)
15cm
25cm
35cm
10
2.21X10-5
2.32X10-5
2.48X10-5
2.64X10-5
30
1.83X10-5
1.96X10-5
2.21X10-5
2.32X10-5
50
1.50X10-5
1.68X10-5
1.70X10-5
1.85X10-5
70
1.21X10-5
1.39X10-5
1.46X10-5
1.71X10-5
Table 3.8. Hydraulic conductivity of compacted pond ash bed (in cm/sec) on 365 days curing
Depth (cm)
5 cm
Radial distance (cm)
15cm
25cm
35cm
10
1.96X10-6
1.85X10-6
2.84X10-6
3.72X10-6
30
1.50X10-6
1.77X10-6
2.43X10-6
2.92X10-6
50
1.38X10-6
1.49X10-6
2.00X10-6
2.44X10-6
70
1.09X10-6
1.31X10-6
1.74X10-6
1.87X10-6
37
CHAPTER 4
RESULT AND DISCUSSION I
(STABILIZED FLY ASH-LIME MIXES)
4.1. INTRODUCTION
This chapter delineates the effect of lime on the hydraulic conductivity and leachate characteristics
of fly ash. The compaction characteristics of fly ash mixed with different lime content along with the
hydraulic conductivity and leachate characteristics of the compacted fly ash specimens were
determined after specified curing periods. The effect of lime and curing period on hydration
products, and microstructure in the stabilized specimens were studied by various microanalyses such
as XRD, and SEM tests. The leachability of different elements is expressed in terms of leachate load
ratio. Further the leachate load ratio of different elements in the leachate sample is correlated to the
hydration products, pH value and hydraulic conductivity.
4.2. GEOTECHNICAL PROPERTIES OF LIME –FLY ASH MIXES
4.2.1. Water Content- Dry Density Relationship for Lime-Fly Ash Mixes
The light compaction and heavy compaction tests on different fly ash-lime mixes were performed
according to IS: 4332(Part III) 1967. The optimum moisture content (OMC) and maximum dry
density (MDD) values obtained for different fly ash-lime mixes are presented in Table 4.1. From
light compaction test, it was observed that the optimum moisture content varied from 37.92 % to
40.38%, whereas maximum dry density (MDD) ranged from 1.128 to 1.17g/cc. But in case of heavy
compaction test it was found that OMC was varied from 30.5 to 36.88 % and MDD from 1.29 to
1.33g/cc.
38
42
40
OMC (%)
38
36
Heavy Compaction
34
Light Compaction
32
30
0
2
4
6
8
10
12
14
16
Lime Content (%)
Figure 4.1. Variation of OMC with lime content at light and heavy compaction energies
MDD (g/cc)
1.35
1.3
1.25
Heavy Compaction
Light Compaction
1.2
1.15
1.1
0
2
4
6
8
10
12
14
16
Lime Content(%)
Figure 4.2. Variation of MDD with lime content at light and heavy compaction energies
39
From Fig. 4.1, it was found that for light compaction test, with increase in lime content the OMC
value increases up to 4% and thereafter, it decreases whereas in case of heavy compaction test, the
OMC increases up to 2% lime and thereafter, it decreases. Similarly, Fig. 4.2 shows that the MDD in
case of light compaction test decreases with increase in lime content up to 4% and thereafter it
increases whereas in case of heavy compaction test the same value decreases up to 2% lime addition
and thereafter, it increases. Addition of lime to fly ash specimen brings about the colloidal type of
reaction in which particles flocculate. As flocculated structure is more resistance to applied force,
the particles do not slide to a denser state. However, with further increase in lime the MDD value is
found to increase. This may be attributed to higher specific gravity of lime compared to fly ash
particles. An increase in OMC value at low lime content is attributed to the flocculated structure of
the compacted mass with higher internal void space, which accommodates more moisture. As the
formation of cementitious gel products in lime stabilized material is rather a slow phenomenon, it is
presumed that the above observed trend is a consequence of colloidal type of reaction rather than the
pozzolanic reaction.
4.2.2. Hydraulic Conductivity
Figure 4.3 and Figure 4.4 shows the variation of hydraulic conductivity with lime content for
specimens compacted at light as well as heavy compaction energies and cured for different days. It
is observed that the hydraulic conductivity values follow a decreasing trend with increase in lime
content and it also depends on the compaction effort. Samples having more compaction show less
value of permeability. With increase in curing period the permeability of the specimens decreases
many fold. This is due to the formation of C-S-H gel which clogs the capillary pore space.
However, sample with no lime content showed marginal change in hydraulic conductivity value with
curing period. The samples containing higher doses of lime shows significant decrease in hydraulic
40
conductivity value. It was found that at 90 days curing, it reduces about 10 times for samples
compacted with light compaction energywhereas in case of heavy compaction, it decreases about
100 times than that of the unstabilized specimen. In a nutshell, permeability depends on lime
content, compaction effort and curing period.
4.51E-05
Hydraulic Conductivity (cm/sec)
4.01E-05
0 Days
7 Days
15 Days
30 Days
60 Days
90 Days
3.51E-05
3.01E-05
2.51E-05
2.01E-05
1.51E-05
1.01E-05
5.10E-06
1.00E-07
0
2
4
6
8
10
Lime Content (%)
12
14
16
Figure 4.3. Variation of hydraulic conductivity with lime content for light compaction
Hydraulic Conductivity (cm/sec)
3.50E-05
3.00E-05
0 Days
7 Days
2.50E-05
15 Days
30 Days
2.00E-05
60 Days
90 Days
1.50E-05
1.00E-05
5.01E-06
1.00E-08
0
2
4
6
8
10
Lime Content(%)
12
14
16
Figure 4.4. Variation of hydraulic conductivity with lime content for heavy compaction
41
4.2.3. pH Value
Figure 4.5 shows that the pH value of the leachate sample collected from permeability test of
compacted fly ash-lime mixes increases with increase in doses of lime. Also it is observed that with
increase in curing period i.e. at 90 days the pH value is less than that of 0 days curing period. This is
because the pozzolanic reaction in lime treated fly ash continues for a longer period. So at early days
of curing the unreacted lime comes out with the leachate. However, with increase in curing period,
the lime present in the specimens participate in the pozzolanic reaction, so the amount of lime left in
the specimens eventually decreases. Thus, the pH value of the specimens decreases.
14
0 days
12
90days
pH Value
10
8
6
4
2
0
0
2
4
8
12
15
Lime Content (%)
Figure 4.5. Variation of pH with lime content on different days of curing
4.2.4. Leachate Analysis
From the leachate analysis it was observed that at 0 days curing with increase in lime content in the
specimens the concentration of Ca increases. However, with increase in curing period the
concentration of Ca gradually decreases in all the samples. This is because at early stage of curing
the pozzolanic reaction is slow and the unreacted lime leaches out. As the curing period increases,
pozzolanic reaction becomes stronger and less amount of lime could leach out. It is also observed
42
that at 90 days curing the concentration of Ca is higher for the specimen containing higher doses of
lime. For specimens having low lime content, most of lime added reacts with reactive silica and
only a little amount of lime left unreacted which leach out with the percolating water. With increase
in lime content, the required amount of lime participates in pozzolanic reaction and rest unreacted
lime leaches out with the percolating water. Fig. 4.6 represents the variation of calcium
concentration in the leachate sample with lime contents for different curing periods. For a particular
lime content the concentration of Ca decreases with higher curing period. This indicates that as the
curing period increases the added lime in the fly ash mix takes part continuously in the pozzolanic
reaction and is consumed.
Concentration of Ca (mg/l)
160
140
120
100
80
60
0 days curing
40
90 days curing
20
0
0
2
4
6
8
10
Lime Content (%)
Figure 4.6. Variation in concentration of Ca in the leachate sample with lime content
Table 4.1 Concentration of metals in leachate sample of raw fly ash
Sample ID
Concentration of metals (mg/l)
Ca
Cu
Fe
Pb
Cr
Ni
Zn
N1
46.409
1.543
23.350
1.699
2.464
1.48
2.172
N2
35.219
0.06
0.057
0.325
1.875
0.159
0.315
N1 denotes the sample prepared from acid digestion of raw fly ash and N2 denotes the extracted leachate sample of raw
fly ash (L/S=10).
43
Table 4.2. Concentration of elements in leachate sample after 0 and 90 days of curing
Concentration of elements in leachate on
Concentration of elements in leachate on
0 day curing(mg/l)
90days curing(mg/l)
Elements
FA+0%L FA+2%L FA+4%L FA+8%L FA+0%L FA+2%L FA+4%L FA+8%L
Cu
0.05
0.046
0.037
0.031
0.05
0.02
0.015
0.01
Zn
0.283
0.258
0.242
0.217
0.282
0.044
0.037
0.021
Ca
35.119
78.721
116.7
145
35.116
52.262
64.548
75.321
Pb
0.256
0.254
0.242
0.239
0.256
0.172
0.139
0.11
Cr
1.252
1.248
1.244
1.243
1.251
0.162
0.153
0.106
Fe
0.05
0.049
0.048
0.047
0.05
0.032
0.025
0.017
Ni
0.151
0.149
0.148
0.144
0.151
0.057
0.043
0.038
Table 4.2 shows the concentration of metals in the leachate collected from permeability mold after 0
days and 90 days curing. From the test it was observed that the concentration of all the metals was
less than that of leachate sample of raw fly ash obtained from acid digestion and extraction method
(Table 4.1). At 0 days curing the concentration of all the metals expect Ca were approximately same
in specimens having different lime contents whereas with increase in curing period the concentration
of metals was found to be decreased. This is due to increase in the alkalinity of the medium which is
unfavorable for metal precipitation and also due to encapsulation of metals by the hydration
products. It is also observed that after 90 days of curing the concentration of all the metals is below
the threshold limit of IS-10500 and WHO water quality standard (Table 1.3).
4.2.4.1. Leachate–load ratio
The total amount of a metal in the leachate depends on the hydraulic conductivity of the material and
on the concentration of the metal in the leachate. The effect of lime stabilization in mitigating the
leachate characteristics of fly ash is studied through the term leachate load ratio which is defined as
44
the ratio of the total metal emanating from an unstabilized specimen per unit time to that of the total
metal emanating from a stabilized specimen for same time. The total metal emanating from the
specimen is the product of hydraulic conductivity and concentration of the metal under study. When
the leachate-load ratio value is greater than 1, it indicates that the total metal coming out of the
stabilized specimen per day is less than the total metal emanating from unstabilized specimen. Table
4.3 shows that the leachate load ratio for all the elements are greater than 1. Therefore, the total
metal coming out of the stabilized specimen is less than the total metal emanating from unstabilized
specimen. It also shows that with increase in lime content the leachate-load ratio of each metal
follows an increasing trend.
Table 4.3. Leachate load ratio values for different metals in samples cured for 90 days
Mix
Proportion
Leachate load ratio of of metals
Cu
Zn
Ca
Pb
Cr
Fe
Ni
FA+0%L
1
1
1
1
1
1
1
FA+2%L
7.66
19.64
2.06
19.98
23.67
4.79
8.12
FA+4%L
13.01
29.76
2.12
31.49
31.93
7.81
13.71
FA+8%L
23.46
63.02
2.18
47.82
55.38
13.80
18.62
4.2.5. Hydration products and Morphology
The hydration products, morphology and microstructure formed during hydration process were
studied using XRD and SEM analysis. The XRD patterns of fly ash specimens containing different
doses of lime on 90 days curing are shown in Fig. 4.4. A series of compounds such as quartz (Q),
calcite (C), hematite, ettringite (E), and calcium silicate hydrate (B), calcium aluminiumate hydrate
(A) are found in hydrated specimens. As the curing period increases, (Fig.4.5) hydration products or
phases are intensified and the peaks of calcite diminishes. The diminished intensity of calcite peaks
45
with an increased curing time is an indication of participation of lime in hydration process and
formation of more amount of C-S-H gel.
Figure 4.4 XRD analyses of specimens containing different doses of lime on 90 days curing
Figure 4.5 XRD analysis of FA+4%L specimen on different days of curing
46
The microstructure and hydration products of specimens cured for different periods are studied
using scanning electron microscope. Figure 4.8 and 4.9 shows the hydration products in
specimens containing 4% and 15% of lime respectively and cured for 90days. Abundance of
needle-like structures of ettringite is found in the specimen cured for 7 days (Figure 4.7). Usually
needle like crystals appeared during the early period of hydration. As curing proceeds the needle
shaped crystals are seen wrapped with gel like substances of calcium silicate hydrate. A further
increase in the curing period resulted in formation more amount of C-S-H gel (Fig. 4.8 and 4.9).
This results in reduction of capillary voids and decrease in hydraulic conductivity value. The
SEM analysis shows the compounds that are identified earlier from XRD analysis.
Figure 4.6. SEM image of FA+4%L specimen Figure 4.7. SEM image of FA+4%L specimen
on 0 day curing
on 7 days curing
Figure 4.8. SEM image of FA+4%L specimen Figure 4.9. SEM image of FA+15%L specimen
on 90 days curing
on 90 days curing
47
CHAPTER 5
RESULT AND DISCUSSION-II
(FLY ASH BED TREATED WITH LIME COLUMN)
5.1. INTRODUCTION
Lime columns are often used to stabilize soft saturated cohesive soil deposits. It is one of the
promising and cost effective in-situ stabilization method practised for cohesive soils. However a
limited literature is avalable on stabilization of fly ash bed using this technique. The present
work used lime column method to reduce the hydraulic conductivity and mitigate the leachate
characteristics of sedimented and compacted fly ash deposits. All the results of the above
investigations and their corresponding analyses have been presented in the following sections.
5.2. SEDIMENT POND ASH BED TREATED WITH LIME COLUMN
5.2.1. pH test
Figure 5.1 and 5.2 represent the pH test results of the samples collected from different
radial
distances and depths of the test tank after 90 days , 180 and 365 days curing respectively. It is
observed that pH value follows a decreasing trend with an increase in radial distance and
follows an increasing trend with increase in depth from the top surface of the pond ash deposit.
This is due to migration of lime to the periphery and bottom of the tank. As there is much
concentration of lime at the location near to the lime column, so pH value is higher for the
samples collected adjacent to the lime column. Similarly an increasing trend of pH value with
48
increase in depth from the surface of the tank is due to migration of lime towards downward
direction. Moreover, it is also observed that the pH value increases with curing period (upto 180
days). This indicates that the migration of lime continues even upto 180 days, and the amount of
lime migrated is higher than the amount of lime consumed in pozzolanic reaction. This leads to a
gradual increase in the pH value. However, beyond 180 days of curing, the pH value is found to
be reduced due to participation of more lime in pozzolanic reaction. Further, the migration of
lime from lime column towards the peripheral region reduces with time as the hydration products
clogs the capilarry voids. The pH value of the pore water always remains above 7.0, indicating
an alkaline environment. The alkaline medium of the pore fluid reduces the solubility of metal
ions and thus help in controlling the migration of metals from the sedimented fly ash bed.
8.8
pH value
8.6
8.4
8.2
8.0
7.8
90 days
7.6
180 days
7.4
365 days
7.2
7.0
0
10
20
30
40
Radial distance (cm)
Figure 5.1.Variation of pH value with radial distance
49
50
8.8
8.6
8.4
pH value
8.2
8.0
90 days
7.8
180 days
7.6
365 days
7.4
7.2
7.0
0
20
40
60
80
100
Depth (cm)
Figure 5.2. Variation of pH value with depth
5.3.2. Hydraulic Conductivity
Figure 5.3, 5.4 and 5.5 represent the hydraulic conductivity values of pond ash specimens
collected after 90, 180 and 365 days of curing period respectively. From the test results, it is
found that the hydraulic conductivity follows a decreasing trend with increase in depth from the
top surface of the fly ash bed and also a reduced value is obtained in the samples collected
adjacent to the lime column compared to the remote areas. As the lime migrates from the central
lime column toward the periphery it gets distributed over an larger area and thus the
concentration get reduced. As there is much concentration of lime at the location near to the
lime column, so hydraulic conductivity is lesser for the samples collected adjacent to the
column whereas the hydraulic conductivity is more for the samples collected at a remote area
from the lime column.
50
Hydarulic Conductivity (cm/sec)
8.00E-05
7.00E-05
10 cm
30 cm
50 cm
70 cm
6.00E-05
5.00E-05
4.00E-05
3.00E-05
2.00E-05
1.00E-05
0
10
20
30
40
Radial Distance (cm)
Figure 5.3.Variation of hydraulic conductivity with radial distance on 90 days curing
Hydraulic Conductivity (cm/sec)
2.80E-05
2.60E-05
2.40E-05
2.20E-05
2.00E-05
1.80E-05
1.60E-05
10 cm
1.40E-05
30 cm
1.20E-05
50 cm
70 cm
1.00E-05
0
10
20
30
40
Radial Distance (cm)
Figure 5.4.Variation of hydraulic conductivity with radial distance on 180 days curing
51
Hydarulic Conductivity (cm/sec)
3.20E-06
3.00E-06
10 cm
30 cm
50 cm
70 cm
2.80E-06
2.60E-06
2.40E-06
2.20E-06
2.00E-06
0
10
20
30
40
Radial Distance (cm)
Figure 5.5.Variation of hydraulic conductivity with radial distance on 365 days curing
Hydraulic Conductivity (cm/sec)
5.05E-05
10 cm depth
30 cm depth
4.05E-05
50 cm depth
3.05E-05
70 cm depth
2.05E-05
1.05E-05
5.00E-07
90
180
365
Curing Periods (days)
Figure 5.6.Variation of hydraulic conductivity with curing periods at different depths
The reduced value of hydraulic conductivity is due to the formation of hydration products like CS-H , C-A-H, and C-A-S-H
gels which causes a reduction of void space and the
52
interconnectivity of pore channel gets reduced. The test results show that hydraulic conductivity
at depth 10cm is less than depth 90. This indicates the presence of finer size particles on the top
layer and coarser size at the bottom layer. Another possible reason for decrease in permeability
on the top layer may be due to the evaporation of lime added water from the surface of the pond
ash deposit and participation of efflorescent lime in the hydration reaction. It is also observed
from Fig. 5.6 that as the curing period increases, significant reduction in hydraulic conductivity
occurs in all the layers of sediment pond ash deposit. This indicates that the hydration reaction
becomes even more stronger with a higher curing period, which causes the generation of more
amount of hydration products and hence reduction in hydraulic conductivity.
5.3.3. Leachate Analysis
The leachate analysis results of sample collected on 90 , 180 days and 365 days curing are given
in Table 5.1, 5.2 and 5.3 respectively. It shows that concentration of elements in the leachte
sample collected from the test tank is much lower than the leachte sample extracted from raw fly
ash (Table 4.1). It also shows that at early period of curing the concentration of Ca is more
whereas in the longer curing period, i.e at 365 days, the concentration of Ca gradually reduces.
This is because during initial stage of the curing period the pozzolanic reaction is slow and the
unreacted lime leached out very easily. However, at the longer curing period the concentration of
calcium decreases due to participation of lime in pozzolanic reaction. It is also observed that the
concentration of Ca in sample follows an increasing trend in the sample collected at same radial
distances but varying depth in the order of S1<S2<S3<S4<S5 due to migration of lime in
downward direction, whereas the concentration of Ca in samples follows a decreasing trend in
the samples collected at the same depth but different radial distance in the order of
S6>S7>S3>S8>S9 due to lesser migration of lime at greater radial distance from the lime
53
column. It is also observed from the results that the concentration of major and trace elements in
the leachate sample collected adjacent to the lime column is lesser than that of the sample
collected at the periphery of the test tank. This is due to the migration of lime from the lime
column towards the periphery resulting in higher pH value near the lime column and lower pH
value at a remote area from lime column which provides an unfavorable alkaline medium for
metal precipitation. Similarly, as with increase in depth from the top surface of the pond ash
deposit , the concentration of the element decreases due to downward movement of the lime
from the lime column.
Table 5.1. Concentration of metals in leachate on 90 days curing
Samples
Concentration of metals (mg/l)
Ca
Cu
Fe
Pb
Cr
Ni
Zn
S1
78.211
0.059
0.324
0.251
1.369
0.126
0.259
S2
93.102
0.052
0.314
0.183
1.354
0.118
0.253
S3
93.699
0.051
0.296
0.181
1.132
0.108
0.145
S4
97.390
0.049
0.173
0.175
1.203
0.097
0.068
S5
98.235
0.024
0.107
0.151
1.047
0.052
0.065
S6
96.805
0.047
0.049
0.164
1.423
0.1
0.050
S7
96.463
0.050
0.112
0.169
1.493
0.104
0.099
S8
90.467
0.054
0.180
0.210
1.529
0.113
0.165
S9
89.446
0.056
0.332
0.223
1.587
0.114
0.236
Note: S1, S2, S3, S4, S5 are the leachate samples collected from same radial distance that is at 25cm but
with varying depth of 10cm, 30cm, 50cm, 70 cm and 90cm respectively, whereas S6, S7, S8, S9 are the
samples collected from same depth ,that is 50cm but at different radial distances of 5cm, 15cm, 35cm
and 45cm respectively.
54
Table 5.2. Concentration of metals in leachate on 180 days curing
Samples
Concentration of metals in leachate (mg/l)
Ca
Cu
Fe
Pb
Cr
Ni
Zn
S1
71.481
0.028
0.052
0.194
1.003
0.124
0.115
S2
71.790
0.025
0.029
0.172
0.940
0.044
0.095
S3
72.365
0.024
0.026
0.139
0.811
0.035
0.081
S4
76.628
0.021
0.016
0.083
0.784
0.028
0.071
S5
77.275
0.019
0.013
0.011
0.692
0.022
0.036
S6
75.426
0.013
0.011
0.112
0.527
0.010
0.044
S7
72.795
0.023
0.012
0.136
0.537
0.024
0.073
S8
69.925
0.027
0.029
0.163
1.168
0.052
0.113
S9
68.231
0.030
0.036
0.204
1.271
0.071
0.174
Table 5.3. Concentration of metals in leachate on 365 days
Samples
Concentration of metals in leachate (mg/l)
Ca
Cu
Fe
Pb
Cr
Ni
Zn
S1
47.995
0.021
0.026
ND
0.587
0.041
ND
S2
48.267
0.018
0.025
ND
0.579
0.039
ND
S3
48.581
0.016
0.017
ND
0.571
0.033
ND
S4
48.897
0.014
0.014
ND
0.564
0.024
ND
S5
49.686
0.011
0.012
ND
0.491
0.006
ND
S6
49.183
0.010
0.007
ND
0.427
0.009
ND
S7
48.886
0.013
0.01
ND
0.521
0.012
ND
S8
48.232
0.017
0.017
ND
0.601
0.48
ND
S9
48.02
0.028
0.032
ND
0.632
0.051
ND
55
Moreover, it is observed with increase in curing period, the concentration of elements in the
leachate decreases. This is due to the formation of hydration product such as C-S-H gel which
encapsulates the elements and prevents leaching. So this confirms that addition of lime plays a
pivotal role in reducing the concentration of elements and with higher curing period the
concentration of element reduces even more. The concentration of all the elements was found to
be less than threshold limit of WHO and IS-10500 (Table 1.3) water quality standard.
5.4. COMPACTED FLY ASH BED TREATED WITH LIME COLUMN
5.3.1 pH test
Figure 5.7 and 5.8 represent the pH test results of the samples collected from different depths and
radial distances of the test tank after 90, 180 and 365 days of curing. It is observed that the pH
value is more at the locations adjacent to the lime column and less in the sample collected at a
remote area from the lime column. The value follows an increasing trend with increase in depth
from the top surface of the pond ash deposit. This is due to migration of lime to the surrounding
and downward direction of the tank. As there is much concentration of lime at the location near
to the lime column, the pH value is more and as there is less concentration of lime at the remote
area from the lime column the same value is less. Moreover, it is also observed that the pH value
increases with increase in curing period (up to 180 days). This indicates that the amount of lime
migrated is higher than the amount of lime consumed in pozzolanic reaction. This leads to a
gradual increase in the pH value. However, with further curing that is after 180 days the pH
value decreases due to participation of lime in pozzolanic reaction. The pH value of the pore
fluid of sedimented and compacted ash beds are found to be almost equal for comparable
conditions that is at same location and same curing period. This shows that the degree of
compaction has not much influence on the migration of lime from the central column towards the
peripheral region.
56
pH Value
9.0
8.8
90 days
8.6
180 days
8.4
365 days
8.2
8.0
7.8
7.6
7.4
7.2
0
10
20
30
40
50
Radial Distance (cm)
Figure 5.7.Variation of pH value with radial distance
8.6
90 days
8.4
180 days
pH Value
8.2
365 days
8.0
7.8
7.6
7.4
7.2
0
20
40
60
80
Depth (cm)
Figure 5.8.Variation of pH value with depth
57
100
5.3.2 Hydraulic Conductivity
The hydraulic conductivity fly ash specimens were determined after 90, 180 and 365 days
curing period and the results are presented in Fig. 5.9, 5.10 and 5.11 respectively. It is observed
that the hydraulic conductivity follows a decreasing trend as with increase in depth and also a
reduced value is obtained in the samples collected adjacent to the lime column.This is due to the
uneven concentration of lime in different parts of the compacted bed. As
there is much
concentration of lime at the locations near to the lime column, so hydraulic conductivity is
lesser for the samples collected adjacent to the column whereas the hydraulic conductivity is
more for the samples collected at a remote area from the lime column. The reduced value of
hydraulic conductivity is obtained due to the participation of lime in hydration reaction and
formation of hydration products like C-S-H, C-A-S and C-A-S-H gels which cause a reduction of
void space and interconnectivity of pore channel. In addition, it is also observed from Fig. 5.12
that as the curing period increases, a significant reduction in hydraulic conductivity occurs in all
the locations of compacted fly ash bed. This reduction in the hydraulic conductivity is more
pronounced in locations near to the lime column than farther points. This indicates that the
hydration reaction becomes even more stronger with a higher curing period, which causes the
generation of more amount of hydration products and hence reduction in hydraulic conductivity.
The hydraulic conductivity of untreated compacted fly ash was 4.25x10-5 cm/sec which reduced
to 1.09 x10-6 cm/sec after a curing period of 365 days and at a radial distace of 5cm from lime
column and depth of 70cm. This is of a reduction of about 22 times. This reduction in hydraulic
conductivity of the fly ash bed helps in mitigating the migration of heavy and trace elements
from the bed towards the surrounding areas.
58
3.50E-05
Hydraulic Conductivity (cm/sec)
10cm depth
30cm depth
3.00E-05
50cm depth
70cm depth
2.50E-05
2.00E-05
1.50E-05
1.00E-05
0
10
20
30
40
Radial Distance (cm)
Figure 5.9.Variation of hydraulic conductivity with radial distance on 90 days curing
2.80E-05
10cm depth
Hydraulic Conductivity (cm/sec)
2.60E-05
30cm depth
2.40E-05
50cm depth
2.20E-05
70cm depth
2.00E-05
1.80E-05
1.60E-05
1.40E-05
1.20E-05
1.00E-05
0
10
20
30
40
Radial Distance (cm)
Figure 5.10.Variation of hydraulic conductivity with radial distance on 180 days curing
59
Hydraulic Conductivity (cm/sec)
4.00E-06
10 cm depth
3.50E-06
30 cm depth
3.00E-06
50 cm depth
70 cm depth
2.50E-06
2.00E-06
1.50E-06
1.00E-06
0
10
20
30
40
Radial Distance (cm)
Figure 5.11.Variation of hydraulic conductivity with radial distance on 365 days curing
Hydraulic Conducivity (cm/sec)
2.60E-05
10 cm depth
2.10E-05
30 cm depth
50 cm depth
1.60E-05
70 cm depth
1.10E-05
6.00E-06
1.00E-06
90
180
365
Curing Periods (days)
Figure 5.12.Variation of hydraulic conductivity with curing periods at different depths
60
5.3.3. Leachate Analysis
The leachate analysis results of sample collected on 90, 180 days and 365 days curing are given
in Table 5.4, 5.5 and 5.6 respectively. It shows that concentration of all elements in the leachate
samples collected from the test tank are much lower than that extracted from raw fly ash (Table
4.1). It also shows that at early period of curing the concentration of Ca in the leachate sample
collected from all the locations of the tank is more than that in the virgin fly ash. The
concentration of Ca in the leachate sample continues to increase up to a curing period of 180
days thereafter the same follows a decreasing trend. During initial period there is profuse
migration of lime from the lime column towards the surrounding. As curing period increases the
migrated lime takes part in pozzolanic reaction producing the C-S-H gel. This gel blocks the
capillary pore space of thee ash bed thus prohibiting further migration of lime which results in a
decrease in the concentration of Ca in the leachate sample with longer curing period. Further, it
is observed that the concentration of Ca in sample follows an increasing trend with increase in
depth whereas the same follows a decreasing trend with increase in radial distance. This is due to
the migration of lime to the surrounding and distribution of migrated lime over a wider area. It is
also observed from the results that the concentration of other major and trace elements in the
leachate sample collected adjacent to the lime column is lesser than that of the sample collected
at the periphery of the test tank. This is due to the presence of higher concentration of lime at the
location adjacent to the lime column which results in higher pH value near the lime column and
lower at a remote area from lime column thus, providing an unfavorable alkaline medium for
metal precipitation. In addition to this, the higher concentration of lime results in the formation
of more amount of C-S-H gel which encapsulates the metal ions and thus, prevents leaching of
elements.
61
Table 5.4. Concentration of metals in leachate on 90 days curing
Samples
Concentration of Metals (mg/l)
Fe
Pb
Cr
Ca
Cu
Ni
Zn
S1
91.339
0.049
0.025
0.190
ND
0.124
0.259
S2
91.520
0.044
0.024
0.170
ND
0.118
0.253
S3
91.693
0.038
0.016
0.140
ND
0.108
0.145
S4
91.886
0.034
0.011
0.081
ND
0.097
0.068
S5
92.700
0.026
0.013
0.012
ND
0.052
0.065
S6
91.762
0.019
0.008
0.113
ND
0.1
0.044
S7
91.717
0.027
0.02
0.134
ND
0.104
0.099
S8
89.594
0.041
0.017
0.165
ND
0.113
0.165
S9
89.125
0.047
0.032
0.207
ND
0.114
0.174
Table 5.5. Concentration of metals in leachate on 180 days curing
Concentration of Metals (mg/l)
Samples
Ca
Cu
Fe
Pb
Cr
Ni
Zn
S1
87.995
0.029
0.017
0.094
ND
0.044
ND
S2
88.02
0.028
0.015
0.077
ND
0.043
ND
S3
88.686
0.025
0.008
0.074
ND
0.037
ND
S4
88.581
0.021
0.006
ND
ND
0.029
ND
S5
88.886
0.014
ND
ND
ND
0.01
ND
S6
89.183
0.011
ND
ND
ND
0.02
ND
S7
88.897
0.023
0.001
ND
ND
0.033
ND
S8
88.267
0.032
0.014
ND
ND
0.059
ND
S9
88.232
0.034
0.017
0.073
ND
0.08
ND
62
Table 5.6. Concentration of metals in leachate on 365 days curing
Samples
Concentration of Metals (mg/l)
Ca
Cu
Fe
Pb
Cr
Ni
Zn
S1
41.535
0.007
ND
ND
ND
0.027
ND
S2
40.270
0.005
ND
ND
ND
0.022
ND
S3
39.767
0.004
ND
ND
ND
0.019
ND
S4
38.172
0.003
ND
ND
ND
0.011
ND
S5
32.688
0.002
ND
ND
ND
ND
ND
S6
35.113
0.001
ND
ND
ND
0.015
ND
S7
38.155
0.002
ND
ND
ND
0.018
ND
S8
39.012
0.005
ND
ND
ND
0.037
ND
S9
40.724
0.006
ND
ND
ND
0.05
ND
Note: ND denotes these elements are below detection level.
Moreover, it is observed with increase in curing period, the concentration of elements in the
leachate decreases. This is due to the formation of hydration products such as C-S-H, C-A-S and
C-A-S-H gels which encapsulates the elements and prevents leaching. So this confirms that
addition of lime plays a pivotal role in reducing the concentration of elements and with higher
curing period the concentration of element reduces even more. The concentration of all the
elements was found to be less than threshold limit of WHO and IS-10500 water quality standard
(Table 1.5). Particularly the concentration of metals like Fe, Pb, Cr, and Zn are found to be
below detection level.
63
5.5. HYDRAULIC CONDUCTIVITY AND LEACHATE LOAD RATIO OF SEDIMENT
AND COMPACTED FLY ASH BED
5.4.1. Hydraulic Conductivity
It is observed from Figure 5.7 that in all the layers the hydraulic conductivity value of the
specimens collected from compacted fly ash bed is smaller than that of sediment pond ash
deposits. This is due to the presence of more dense layer in case of compacted fly ash bed than
that of sedimented pond ash bed which leads to the reduction of voids and hence, reduction in
hydraulic conductivity
Hydraulic Conductivity (cm/sec)
3.10E-06
Sediment
2.60E-06
Compacted
2.10E-06
1.60E-06
1.10E-06
6.00E-07
1.00E-07
10
30
50
70
Depth (cm)
Figure 5.7 Variation of hydraulic conductivity with depth in the specimens collected from
5cm radial distance in the ash beds on 365 days curing
5.4.1. Leachate Load Ratio
Leachate-load ratio of an element indicates the total amount of metal coming out of the
unstabilized specimen to that of a stabilized specimen at comparable conditions. It primarily
depends upon the concentration of element in the leachate sample as well as the hydraulic
64
conductivity of the sample. As the stabilization process continues the hydraulic conductivity of
the specimen decreases and the element gets encapsulated in the hydration products formed
during pozzolanic reaction and hence, the leachate-load ratio increases. A higher value of
leachate-load ratio indicates lesser migration of an element. The leachate-load ratio of different
elements after different curing periods has been evaluated. Figure 5.7 shows a typical values of
leachate-load ratio of Cu at locations S1, S2, S3 and S4 for sediment and compacted ash beds
after curing period of 365 days. In general, the leachate-load ratio obtained from compacted bed
is higher than the sedimented bed at comparable conditions (same location and curing period).
This is due to the lower hydraulic conductivity of compacted ash bed than the sedimented one. In
addition to this, the lower value of concentration of this element in case of compacted ash bed on
365 days curing (Table 5.6) compared to sedimented bed (Table 5.3) also confirms the higher
encapsulation of elements by hydration products.
Leachte-Load Ratio of Cu
600
500
sedimented
400
Compacted
300
200
100
0
10
30
50
70
Depth (cm)
Figure 5.8 Variation of leachate load ratio of Cu with depth in the specimens collected at 25cm
radial distance in the ash beds on 365 days curing
65
CHAPTER 6
SUMMARY AND CONCLUSIONS
6.1. SUMMARY
Disposal of fly ash is a major issue faced by the coal based thermal power plants. It requires a
huge disposal area and creates environmental problem like leaching and dusting. Stabilization of
fly ash by chemical additives is one of the promising methods to transform the waste material
into a safe construction material. The primary objective of this study is to reduce the
concentration of metals in the leachate emanating from the fly ash bed and also to prevent the
leachate effluents from contaminating the ground water. Based on this, the scope of the present
study is defined and the same has been summarized in Chapter 2. The details of procedure
adopted for sample preparation and details of experimental studies undertaken are presented in
Chapter 3. Chapter 4 delineates the effect of lime on the hydraulic conductivity and leachate
characteristics of fly ash while Chapter 5 highlights the efficacy of lime column in reducing the
hydraulic conductivity and leachate characteristics of sediment and compacted fly ash bed. In the
present chapter the conclusions drawn from the test results are presented.
6.2. CONCLUSIONS
Based on test results the following conclusions can be drawn.
1. The concentration of metals in leachate majorly depends on two factors, pH and
hydraulic conductivity. With increase in lime content, compaction effort, and curing
period, the hydraulic conductivity value was found to be decreased. At higher curing
66
period the reduction in hydraulic conductivity is due to the formation of C-S-H, C-A-H
and C-A-S-H gels which clogs the pores and decreases the capillary voids.
2. The pH value of the leachate sample collected after permeability test increases with
increase in doses of lime. With increase in curing period the pH value of leachate
collected from the permeability samples decreases due to
participation
of lime in
pozzolanic reaction
3. From leachate analysis it was found that at higher curing period with increase in lime
content the concentration of metals in the leachate decreases. The leachate load ratio
values of all the metals are greater than 1.Therefore, the total metal coming out from the
stabilized specimen is less than the total metal coming out from the unstabilized
specimen. The reduction in concentration of all the metals is due to presence of alkaline
medium which is unfavorable for metal precipitation and also due to encapsulation of
metals by the hydration products.
4. It is also observed that the concentration of other metals is below the threshold limit of
IS-10500 and WHO water quality standard.
5. From XRD analysis it is found that a series of compounds such as quartz, calcite,
hematite, calcium silicate hydrate, calcium aluminium silicate hydrate and calcium
aluminium silicate hydrate are formed in hydrated specimens. As the curing period
increases, hydration products or phases are intensified and the peaks of calcite
diminishes. The diminished intensity of calcite peaks with an increased curing time in the
specimen is an indication of participation of lime in hydration process and formation of
more amount of C-S-H gel.
67
6. SEM analysis shows that Abundance of needle-like structures are found in the specimen
at initial stage of curing. However, at later stage of curing, common fibrous type of
irregular grains forming a reticular network of calcium-silicate-hydrate gel is found. The
presence of hydration products result in reduction of hydraulic conductivity value.
7. Thus, lime treatment is an effective means of reducing the hydraulic conductivity and
concentration of metals in the leachate emanating from compacted fly ash specimens.
Samples were collected from different depths as well as radial distances of sediment and
compacted fly ash bed and subjected to different tests such as pH, leachate analysis and
hydraulic conductivity. Based on the experimental findings, the following conclusions are drawn.
1. The pH value of the samples was found to be decreased radially and increased vertically
due to migration of lime and thus provides an alkaline medium in reducing the solubility
of the toxic metals.
2. With higher curing period the hydraulic conductivity value was found to be decreased
vertically due to migration of lime and formation of C-S-H gel which clogs the pores and
decreases the capillary voids.
3. From the leachate analysis, it is observed that the concentration of Ca increases at the
early period of stabilization due to the migration of lime from lime column and decreases
at later period of stabilization due to participation of Ca in pozzolanic reaction. There is
also a reduced concentration of other metals due to presence of unfavorable pH medium
for metal precipitation and also due to encapsulation of metals by the hydration products.
4. It is also observed that the concentration of other metals is below the threshold limit of
IS-10500 and WHO water quality standard.
68
5. A comparative study between sediment and compacted fly ash bed shows that the
hydraulic conductivity values of the specimens of compacted fly ash bed is less than that
of the sediment fly ash bed.
6. Based on the experimental findings it can be adjudged that pond ash treated with lime
column can be implemented as a reliable and ecofriendly construction material in the
Geotechnical applications.
6.3. SCOPE FOR FUTURE WORK
The investigation has certain limitation and hence all the factors that could not be addressed in
time. So the future research should incorporate the following aspects in detail.
1. The migration of lime in the ash bed needs to be simulated numerically. Same should be
checked in field conditions.
2. Leachate analysis should be done for the elements like As, Hg, Cd, Se and Mn etc. which
could not be performed due to non-working of the instruments.
3. Microanalysis of the cured samples should be done by DSC, TGA and mercury intrusion
porosity meter.
69
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75
LIST OF PUBLICATIONS
1. Singh, S. P., and Sangita, S. (2015). “Influence of Lime on Hydraulic Conductivity and
Leachate Characteristics of Fly Ash.”, Journal of Civil engineering and Environmental
Technology, Vol. 2, No. 2, pp. 126-130, Jan-March 2015
2. Singh, S. P., and Sangita, S. (2014). “Studies on the leachate characteristics of compacted
fly ash bed treated with lime column.”, I- Manager’s Journal on Structural Engineering,
Vol. 3, No. 2, pp. 21-28, June-August 2014.
3. Singh, S. P.,and Sangita, S. (2015). “Influence of lime on hydraulic conductivity and
leachate characteristics of fly ash.” International Conference on Innovative Trends in
Civil Engineering, Architecture and Environmental Engineering for Sustainable
Infrastructure Development-2015.
4. Singh, S. P., Sangita, S., and Ganesh, R. (2014). “Hydraulic Conductivity and Leachate
Characteristics of Pond Ash Deposits Treated with Lime Column.” Indian Geotechnical
Conference-2014, pp. 912-920.
5. Singh, S. P., Sangita, S. and Sahoo, S.P. (2014). “Studies on the Leachate Characteristics
of Compacted Fly Ash Bed Treated with Lime Column.” National Conference on
Geotechnical Engineering Practice and Sustainable Infrastructure Development-2014,
pp. 197-206.
6. Singh, S. P., Roy, N., Sangita, S., and Samantasinghar S. (2015). “A Comparative Study
On Geo-Technical Characteristics of Sedimented and Compacted Fly Ash Bed Treated
with Lime Column.” Indian Geotechnical Conference-2015. (Communicated).
7. Singh, S. P., Roy, N., and Sangita, S. (2015). “Mitigation of Major and Trace Elements
Leaching from Pond Ash Deposits by Lime Column Treatment.” International Journal of
Waste Management, (Communicated).
76
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