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Coal Combustion and Gasification Products is an international, peer-reviewed on-line journal that provides free access to full-text papers, research communications and supplementary data. Submission details and contact information are available at the web site.
© 2013 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association
Web: www.coalcgp-journal.org
ISSN# 1946-0198
Volume# 5 (2013)
Editor-in-chief: Dr. Jim Hower, University of Kentucky Center for Applied Energy Research CCGP Journal is collaboratively published by the University of Kentucky Center for Applied Energy Research (UK CAER) and the American Coal Ash Association (ACAA). All rights reserved.
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Suggested Citation format for this article:
Mallick, S.R., Mishra, M.K., 2013, Geotechnical Characterization of Clinker-Stabilized Fly Ash - Coal Mine Overburden Mixes for Subbase of Mine Haul Road. Coal Combustion and Gasification Products 5, 49-56, doi: 10.4177/CCGP-D-12-00011.1
I S S N 1 9 4 6 - 0 1 9 8
j o u r n a l h o m e p a g e : w w w . c o a l c g p - j o u r n a l . o r g
Geotechnical Characterization of Clinker-Stabilized Fly Ash–Coal Mine OverburdenMixes for Subbase of Mine Haul Road
Soumya Ranjan Mallick*, Manoj Kumar Mishra
Department of Mining Engineering, National Institute of Technology, Rourkela, India
A B S T R A C T
Fly ash is a major by-product of thermal power plants that adversely affects land, water, and air. Its gainful utilization in many
areas is being continuously explored. Use of fly ash in opencast coal mine haul road construction is one such option. This
article reports detailed laboratory investigations carried out on clinker-stabilized fly ash–mine overburden mixes to evaluate
their suitability for subbase of mine haul roads. Composite materials were prepared from fly ash, mine overburden, and clinker
at different proportions. Proctor compaction test, unconfined compressive strength (UCS), and ultrasonic pulse velocity tests
were carried out at different curing periods. UCS values were found to be strongly dependent on clinker content as well as
curing period. Ultrasonic pulse velocity tests confirmed the obtained UCS results. The composite with 62% fly ash and 8%
clinker content showed adequate mechanical strength suitable for the subbase of a mine haul road.
f 2013 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association
All rights reserved.
A R T I C L E I N F O
Article history: Received 30 November 2012; Received in revised form 5 February 2013; Accepted 16 May 2013
Keywords: UCS; ultrasonic pulse velocity; clinker; coal mine overburden; fly ash
1. Introduction
Coal is the most abundant fossil fuel available in India to
produce thermal power, comprising about 75% of the total power
generation. Currently in India, about 83 thermal power stations use
bituminous and subbituminous coal, generating more than 180 Mt
of fly ash per year. This generation of fly ash will reach 225 Mt a
year by 2017. Previously, it was considered as an industrial waste
and was disposed in ash ponds near the power plants. Now it is
considered as a resource material and is being widely used in
various sectors. However, all those applications do not accommo-
date the huge quantities of fly ash being generated and, hence, new
avenues are continuously being explored.
There are about 170 surface coal mines in India, and many are
near thermal power plants. A typical surface coal mine has about
4–5 km of permanent haul road in addition to 10–12 km of branch
roads. Large-capacity haul trucks are used to run over these roads.
Haul roads with inadequate material adversely affect mining by
reducing productivity, as well as by increasing costs due to
repeated road and dumper maintenance. Typically, haulage cost is
about 30–50% of total cost incurred by a surface coal mine
(Thompson and Visser, 2003).
The haul road construction materials are sourced from
overburden dump. These materials are mudstone, sandstone,
siltstone, crushed gravel, and clay. These materials do not offer
any ground stability. Potholes, sinking, rutting, and uneven
surfaces are major symptoms observed in almost all mines
(Tannant and Regensburg, 2001). The grain sizes of coal mine
overburden material vary from fine to coarse particles, with
variable dimensions, and often create dump instability as well as
environmental problems.
Fly ash, being very fine and reactive, is more suitable for road
construction compared with other materials. Bulk utilization of fly
ash alone or fly ash stabilized with soil and additive has been
reported previously (Maser et al., 1975; Srivastava, 1995; Grice,* Corresponding author. Tel.: 91 661 246 4609, 91 9437545820(M). Fax: 91
661 2462999. E-mail: [email protected]
doi: 10.4177/CCGP-D-12-00011.1
f 2013 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association. All rights reserved.
1998). The use of soil stabilized with fly ash in haul roads has
multiple benefits, including diverting fly ash from landfills to haul
road construction to increase mine productivity (Tannant and
Kumar, 2000; Mallick and Mishra, 2011).
There are many reports on bulk utilization of fly ash. Fly ash has
been extensively used for soil stabilization (Chu et al., 1955), as
embankment material (Raymond, 1961), structural fill (DiGioia
and Nuzzo, 1972), for injecting grouting (Joshi et al., 1981), as a
replacement to cement (Gopalan and Haque, 1986; Xu and Sarkar,
1994), and in roads and embankments (Singh and Kumar, 2005),
etc.
Class F fly ash mainly consists of siliceous (SiO2) and aluminous
(Al2O3) compounds that lack self-cementing nature, but in
presence of water, they react with calcium oxide to form
cementitious gels (Cockrell and Leonard, 1970). Its low specific
gravity, ease of compaction, frictional resistance, free draining
nature, and insensitivity toward change in moisture content can be
used for road and embankment construction (Pandian, 2004).
In previous studies, addition of fly ash decreased the plasticity
index and increased California bearing ratio (CBR) values of all
types of soil and improved their suitability for construction of road
base and subbase (Sahu, 2005). Fly ash, kiln dust, and mine spoil or
coal partings attained compressive strength of 1 MPa with elastic
moduli of 350 MPa at 14–28 days and were found to be suitable for
constructing haul road base and subbase layers (Tannant and
Kumar, 2000).
The addition of fly ash reduced the dry density of the soil due to
low specific gravity and unit weight (Prabakar et al., 2004). Class F
fly ash achieved a compressive strength of 6.3 MPa at 90 days of
curing and CBR of 172% at 28 days of curing when mixed with
10% lime and 1% gypsum (Ghosh and Subbarao, 2006).
Fly ash exhibited some cohesion when moist, which was
influenced by the size and number of void spaces, as well as by
degree of saturation (Ramasamy and Kaushik, 2001). Butalia (2007)
reported that fly ash filled voids in the granular pulverized
pavement mix, reducing permeability of the full-depth reclamation
stabilized base layer. CBR values of soil stabilized with 10% and
20% fly ash and 2.5% and 5% lime-kiln dust (LKD) were 69–142%
at 7 days of curing and .164% at 28 days of curing (Cetin et al.,
2010).
Ultrasonic pulse velocity measurement is a popular nondestruc-
tive method extensively used to test the properties of concrete
mixtures that is based on measurement of travel time of
longitudinal ultrasonic waves through the sample (Lin et al.,
2007). Limited studies have been carried out to evaluate its
application on stabilized mixes (Ferreira and Camarini, 2001;
Yesiller et al., 2001).
Overall, the higher the CBR values, the better is the suitability of
the material for haul road construction. This article reports on an
investigation conducted to develop an alternative subbase material
with mine overburden, fly ash, and clinker. The respective CBR
values of the developed composite materials have been confirmed
through ultrasonic pulse velocity test.
2. Materials and Methods
Class F type fly ash was collected by an electrostatic precipitator
from a thermal power unit of the Rourkela Steel Plant (RSP). The
mine overburden used in the study was collected from Basundhara
surface coal mine, Mahanadi Coalfields Limited, Orissa. The
additive as clinker selected for the study was collected from
nearby Cement Plant OCL India Limited, Rajgangpur. The Atterberg
limits, specific gravity, particle size distribution, pH, compaction
characteristics, CBR, and ultrasonic pulse velocity were tested as
per the prescribed Indian standards (IS).
The specific gravity of mine overburden and fly ash was
determined using volumetric flask method as per IS: 2720 part III.
Free swelling index was determined as per IS: 2720 part 40. Grain
size distributions were carried out through a standard set of sieves
as per IS: 2720 part IV. The material passing through the 75-mm
size was collected carefully, and grain size distribution analysis
was performed using the hydrometer test.
The Atterberg limits of mine overburden and fly ash were
determined as per IS: 2720 part V and part VI. Liquid limit was
determined using standard liquid limit apparatus designed by
Casagrande. Liquid limit is the minimum water content at which
a part of the material cut by a standard groove will flow together
for a distance of 12 mm under an impact of 25 blows. Plastic
limit is the minimum water content at which soil will begin to
crumble when rolled into a ,3-mm-diameter thread. Shrinkage
limit is the maximum water content at which reduction in water
content will not decrease the volume of soil. Plasticity index is
the difference between the liquid limit and plastic limit.
The pH value was determined as per IS: 2720 part 26 to identify
acidic or alkaline behavior of mine overburden and fly ash. The
measurement of pH was carried out (Systronics scale pH meter,
India) with an accuracy up to 60.02 units. The instrument was
Table 1
Various proportions of fly ash, mine overburden, and clinker
Fly
ash (%)
Overburden
(%)
Clinker
(%)
Fly ash
(%)
Overburden
(%)
Clinker
(%)
90 10 0 70 30 0
88 10 2 68 30 2
86 10 4 66 30 4
84 10 6 64 30 6
82 10 8 62 30 8
80 20 0 60 40 0
78 20 2 58 40 2
76 20 4 56 40 4
74 20 6 54 40 6
72 20 8 52 40 8
Fig. 1. Grain size distribution curve of fly ash and mine overburden.
50 Mallick and Mishra / Coal Combustion and Gasification Products 5 (2013)
standardized with three standard buffer solutions of pH 7.0, 4.0,
and 10.0 at 25uC. Loss on ignition (LOI) of mine overburden, fly
ash, and clinker was determined as per IS: 1760 part I. Chemical
compositions of mine overburden, fly ash, and clinker were
determined using the energy dispersive X-ray (EDX) technique.
The optimum moisture content (OMC) and maximum dry density
(MDD) of different compositions of fly ash–overburden–cement
clinker were determined by modified Proctor test as per IS: 2720
part VIII. The prepared samples were compacted in five layers in
the Proctor mold. A rammer of weight 4.5 kg dropped from a
height of 0.45 m was used for compaction. Each layer was given 25
blows.
2.1. Sample preparation
The fly ash–overburden–clinker composite materials were
prepared at their respective OMC and MDD obtained from the
modified Proctor compaction test for determination of unconfined
compressive strength (UCS) and P-wave velocity. The raw materials
were blended in the required proportions in a dry state. The
required amount of water was added to respective mixtures and
mixed thoroughly. Wet mixtures were then compacted in the mold
as per guidelines. The aim of the investigation was to develop and
evaluate performance of the fly ash major composite materials for
haul road application. Hence, the fly ash amount was kept more
than 50% (Table 1).
2.2. Unconfined compressive strength
A split mold 38 mm in diameter and 76 mm in length was used
for preparation of the UCS test samples as per IS: 2720 part 10
(1991). Samples were prepared with uniform tamping. Two circular
metal spacer discs 5 mm in height and 37.5 mm in diameter, each
with a base 7 mm in height and 50 mm in diameter, were used at
the top and bottom ends of the mold to compact the sample such
that the length of the specimen was maintained at 76 mm. Then,
the discs were removed, and another spacer disc 100 mm in height
and 37.5 mm in diameter, with a base 7 mm in height and 50 mm
in diameter, was used to remove the sample from the mold. The
final prepared specimen had a length-to-diameter ratio of 2.
2.3. Ultrasonic pulse velocity test
The ultrasonic P-wave velocity test was carried out as per ASTM
D2845-08. All pulse velocity measurements were determined using an
ultrasonic velocity measurement system (GCTS, Tempe, AZ, USA).
This system includes a 10-MHz bandwidth receiver pulse with a raise
time of ,5 ns, 20-MHz acquisition rate with 12-bit resolution
digitizing board, transducer platens with 200-kHz compression mode,
and 200-kHz shears mode. The test was conducted by placing two
sensors on opposite surfaces of the prepared sample. Honey was used
to increase the surface contact between two sensors and the sample.
3. Results and Discussion
The aim of the investigation was to develop and evaluate fly
ash–based composite material to replace the common subbase
Table 2
Physical properties of fly ash and mine overburden (O/B)
Property Fly ash O/B
Specific gravity 2.10 2.63
Atterberg limits
Liquid limit (%) 31.57 26.90
Plastic limit (%) Nonplastic 17.10
Shrinkage limit (%) – 16.02
Plasticity index (%) – 9.80
Sieve analysis (%)
Gravel (.4.75 mm) – 8
Sand (4.75–0.075 mm) 18 27
(A) Coarse sand 0 13
(B) Medium sand 0 6
(C) Fine sand 18 8
Silt (0.075–0.002 mm) 79.8 57
(A) Coarse silt 52 46
(B) Medium silt 16 10
(C) Fine silt 11.8 1
Clay (,0.002 mm) 2.2 8
Coefficient of uniformity (Cu 5 D60/D10) 4.47 4.25
Coefficient of curvature (Cc 5 [D30]2/D10 3 D60) 1.82 0.94
pH value 7.10 5.5
Free swell index Negligible 18.18
Table 3
Chemical composition (% by weight) of fly ash, mine overburden (O/B), and clinker
Constituents SiO2 Al2O3 Fe2O3 CaO K2O MgO TiO2 Na2O SO3 LOI
Mine O/B 48.24 29.18 8.36 1.10 0.40 1.30 0.69 – – 10.73
Fly ash 53.11 33.64 6.44 0.55 1.45 0.83 2.05 0.13 – 1.8
Clinker 20.46 4.52 3.57 66.38 0.68 2.01 – 0.16 1.39 0.75
Note: LOI 5 loss on ignition.
Fig. 2. Moisture content–dry density relationship of fly ash and mine
overburden.
Mallick and Mishra / Coal Combustion and Gasification Products 5 (2013) 51
material typically used in the haul road of a surface coal mine. The
experiments and their results are reported next.
3.1. Physical and chemical properties
Particle size distribution reflects whether the material is poorly,
medium, or well graded (Figure 1). Fly ashes are predominantly silt
sized, with some sand-sized fractions. The fly ash contains more
than 50% coarse-grained silts (0.020 mm , particle size ,
0.075 mm) and so belongs to the nonplastic inorganic coarse-sized
fraction, i.e., MLN group according to the geotechnical classifica-
tion system developed by Sridharan and Prakash (2007) (Table 2).
Mine overburden contains a mixture of poorly graded sand and silt
and hence belongs to the SM group (Sridharan and Prakash, 2007).
Coefficient of uniformity (Cu) and the coefficient of curvature (Cc)
values of fly ash and mine overburden represent that both fly ash
and mine overburden are poorly graded (Pandian, 2004).
The specific gravity of fly ash is less than that of mine
overburden, as it contains a large number of cenospheres and less
iron content (Sridharan and Prakash, 2007). The pH values indicate
that fly ash is slightly alkaline and mine overburden is acidic in
nature due to the presence of free lime content and alkaline oxide
content. Carbon content is typically assessed by measuring LOI
as 90% of LOI value (Sear, 2001). The carbon content of the
overburden material and fly ash is 9.65% and 1.6%, respectively.
High carbon content adversely affects material properties. Chem-
ical composition suggests the possible applications of fly ash. The
amount of SiO2 or (SiO2+Al2O3) in fly ash influences the pozzolanic
activity (Throne and Watt, 1965). EDX analysis confirms that fly
ash satisfies the chemical requirements for use as a pozzolana
(Table 3).
3.2. Compaction characteristics
Modified Proctor compaction was carried out to consider higher
standards of compaction. MDD of the composites decreased with
an increase in fly ash percentage. The OMC of all the composites
was between 14% and 20%, with highest OMC being 22.3% for fly
ash only. The highest MDD obtained was 1941 kg/m3 for mine
overburden only, whereas lowest MDD was 1296 kg/m3 for fly ash
only due to its noncohesive nature (Figure 2).
Fig. 4. Moisture content–dry density relationship of fly ash (FA)–mine
overburden (O/B)–clinker (CL) mixes containing 10% and 20% mine O/B.
Fig. 3. Moisture content–dry density relationship of fly ash (FA)–mine
overburden (O/B) mixes without additive.
Fig. 5. Moisture content–dry density relationship of fly ash (FA)–mine
overburden (O/B)–clinker (CL) mixes containing 30% and 40% mine O/B.
Fig. 6. Variation of maximum dry density with clinker content. O/B 5 mine
overburden.
52 Mallick and Mishra / Coal Combustion and Gasification Products 5 (2013)
The variation of dry density and moisture content of composites
without clinker is shown in Figure 3. Addition of clinker to fly
ash–overburden mixes resulted in an increase in MDD and decrease
in OMC. The variation of dry density and moisture content of
composite with clinker is shown in Figures 4 and 5. As specific
gravity of clinker is higher than that of fly ash and mine
overburden, replacement of fly ash or mine overburden by clinker
resulted in increased MDD (Figure 6).
3.3. Unconfined compressive strength
The UCS test is one of the common laboratory tests in pavement
design and soil stabilization applications and is often used as an
index to quantify the strength enhancement of materials due
to treatment. The UCS values of untreated fly ash and overburden
composites immediately after preparation could not be obtained as
they failed immediately after loading. Marginal increase in UCS
values was observed at different curing periods (Figure 7).
The compressive strength values changed significantly with the
addition of clinker. The composites achieved UCS values between
0.15 to 1.1 MPa, which were significantly dependent on clinker
Fig. 7. Unconfined compressive strength (UCS) values of fly ash (FA)–mine
overburden (O/B) mixes without additive at 7, 14, and 28 days of curing.
Fig. 8. (A–D) Unconfined compressive strength (UCS) values of fly ash (FA)–mine overburden (O/B)–clinker (CL) mixes.
Mallick and Mishra / Coal Combustion and Gasification Products 5 (2013) 53
content as well as on curing period. The composite 70% fly ash +30% overburden with 2–8% clinker showed the highest strength
(0.32–1.09 MPa) as compared with other composites at 7 days of
curing (Figure 8A). The composite 62% fly ash + 30% overburden
stabilized with 8% clinker achieved a UCS value of 1.4 MPa at
28 days of curing (Figure 8C).
The availability of additional clinker produced enhanced
bonding between reactive elements. Each composition exhibited
higher strength values with an increase in clinker content and
curing period. These values are far above the minimum values
suggested for subgrade (Das, 1994).
Table 4
Unconfined compressive strength gain of fly ash–mine overburden–clinker
composites
FA
(%)
O/B
(%)
CL
(%)
Curing
period (days)
UCS
(MPa)
UCS
gain
90 10 0 28 0.2 1
88 10 2 28 0.35 1.75
86 10 4 28 0.48 2.4
84 10 6 28 0.63 3.15
82 10 8 28 0.99 4.95
80 20 0 28 0.22 1
78 20 2 28 0.41 1.86
76 20 4 28 0.55 2.5
74 20 6 28 0.7 3.18
72 20 8 28 1.12 5.09
70 30 0 28 0.27 1
68 30 2 28 0.52 1.92
66 30 4 28 0.71 2.62
64 30 6 28 0.9 3.33
62 30 8 28 1.4 5.18
60 40 0 28 0.25 1
58 40 2 28 0.47 1.88
56 40 4 28 0.62 2.52
54 40 6 28 0.81 3.24
52 40 8 28 1.29 5.16
Note: FA 5 fly ash; O/B 5 overburden; CL 5 clinker; UCS 5 unconfined compressive
strength.
Fig. 9. Failure of a few unconfined compressive strength samples.
Fig. 10. (A–C) Young’s modulus values of fly ash–mine overburden (O/B)–
clinker mixes.
54 Mallick and Mishra / Coal Combustion and Gasification Products 5 (2013)
The composite containing 62% fly ash and 30% mine overburden
with 8% clinker exhibited maximum compressive strength as
compared with other composites at 7 and 28 days of curing.
Typically, the stress values at the base/subbase layers of a mine haul
road for 35–170 t dumpers are 300–650 kPa, respectively (Tannant
and Regensburg, 2001). The strength achieved by almost all the
mixes in this study is above these values after curing and hence
suitable for mine haul road construction.
UCS gain is the ratio of UCS value of clinker-treated composite
to untreated composite. The UCS gain values were between 1.75
and 5.18 for 28-day cured composites (Table 4). Optimum
quantities of CaO, Al2O3, and SiO2 react among themselves and
exhibit maximum UCS values. More availability of CaO, Al2O3, and
SiO2 do not add to strength gain (Sivapullaiah et al., 1995).
3.4. Young’s modulus
Young’s modulus values were obtained from the UCS test. All
the samples in unconfined compressive loading conditions exhib-
ited shear type failure (Figure 9). All but a few samples failed by
shear, reflecting the combined influence of sample and machine
characteristics (Singh and Ghosh, 2006). Load-bearing capacity
and longitudinal-displacement recording were done until failure,
i.e., peak strength of all the samples. The axial strain values could
not be recorded for postfailure investigation, because the weakened
sample disintegrated soon after its peak strength. The Young’s
modulus values (stress/strain) were calculated for every sample.
Maximum Young’s modulus value was achieved by 64% fly ash +30% overburden + 6% clinker, i.e., around 150 MPa at 28 days of
curing (Figure 10C). Young’s modulus values of different compo-
sitions are shown in Figures 10A, 10B, and 10C.
3.5. Ultrasonic pulse velocity
The ultrasonic pulse velocity method, a nondestructive method
to evaluate the quality of the composite materials (Yesiller et al.,
2001), is influenced by factors such as direction, travel distance,
diameter of sensors, and material properties. Laboratory ultrasonic
velocity measurements have been used to study the elastic
behavior of geologic materials (Dimter et al., 2011).
P-wave velocities varied between 550 and 1650 m/s, with the
maximum being with 62% fly ash and 8% clinker content at 7 days
of curing (Figure 11A). The trend is the same at 14- and 28-day
curing periods with an increase in P-wave velocities (Figures 11B
and 11C). P-wave velocities increased with clinker content as well
as curing periods. The results compare favorably to those for
material with fly ash and cement binder (Lav et al., 2006). At initial
stages of curing, the composites had high moisture content before
the onset of hydration, which caused instability in the specimen
and allowed pulse velocity to pass through the shortest possible
path. As hydration progressed with higher clinker content, the P-
wave velocity value increased, resulting in higher UCS and
Young’s modulus values.
4. Conclusions
Our investigation evaluated the geotechnical characteristics of
20 different composite materials with fly ash in major percentages as
a replacement for conventional material in the subbase of a surface
coal mine haul road. The results obtained were encouraging, and the
following conclusions are drawn from the investigation.
Fig. 11. (A–C) P-wave velocity values of fly ash-mine overburden (O/B)-clinker
mixes.
Mallick and Mishra / Coal Combustion and Gasification Products 5 (2013) 55
1. Mine overburden material does not exhibit suitable strength
values for haul road application.
2. Different mixtures of fly ash–mine overburden without additives
do not have sufficient strength to be used as subbase material.
3. Addition of clinker improves strength values significantly.
4. The curing period, as well as the clinker percentage, has a strong
influence on the strength behavior of composites.
5. At 2% clinker content, the curing period is the dominant factor
for suitability of the composite material.
6. At 8% clinker content, most of the composites achieved the
desired UCS strength values.
7. The UCS values of the optimum composite exceed the minimum
required values for use in subbase of a haul road. The P-wave
velocity results confirmed the observations.
8. The composite with 62% fly ash and 8% clinker content exhibits
the best result for haul road application as a subbase material.
9. The fly ash–based composite materials would facilitate the use
of a high percentage of fly ash in haul road construction.
Acknowledgments
The authors acknowledge the financial assistance provided by
the Council of Scientific and Industrial Research (CSIR)–New Delhi
under EMR-II Scheme Vide Letter No. 22/0474/09/EMR-II dated
12-02-2009.
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