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Soil Stabilization by Using Alkaline-Activated
Ground Bottom Ash Coupled with Red Mud
My Quoc Dang1, Young-sang Kim2(&), and Tan Manh Do2
Department of Civil Engineering, Nha Trang University,
Nha Trang, Khanh Hoa, Vietnam
Department of Civil and Environment Engineering,
Chonnam National University, Yeosu, South Korea,
Abstract. This study evaluates the feasibility of incorporating bottom ash and
red mud into a binder to stabilize soil. In this study, the bottom ash collected
from Honam Thermal Power Plant in South Korea was ground to decrease
particle size. It was then coupled with red mud to form a new binder based on
the geopolymer synthesis theory. Sodium silicate solution (Na2SiO3) in terms of
alkaline-activator was added into mixture to enhance the activity of binder.
Weathered granite soil which is classified as SM in USCS is the target of
stabilization. Unconfined compressive strength of stabilized soil and heavy
metal content of leachate were examined. Experimental results showed that
ground bottom ash coupled with red mud can be used to stabilize weathered
granite soil at the ambient curing condition. The highest compressive strength of
stabilized soil was 4.1 MPa. Red mud in certain limits has contributed to the
increment of soil strength, however, the long-term strength decreased with the
increase of red mud content. In addition, based on the results obtained with
leaching test, it can be concluded that leachate from the stabilized soil is not
harmful to the environment.
Keywords: Soil stabilization
Ground bottom ash Red mud
1 Introduction
With urbanization, more and more residences, high-rise buildings, and transport systems (e.g. highways, parking lots, railways, stations) have been constructed. As a
result, the demand of land for infrastructure construction increases rapidly. Nevertheless, the construction sites with favorable geotechnical conditions become less available. This makes soil stabilization become an important part of geotechnical practice.
Currently, Portland cement is being widely used as a stabilizer to improve the
engineering properties of soil. According to statistics, the global cement production
increased from 3310 million to 4100 million tons in the period from 2010 to 2015 (Kim
et al. 2016). However, the production of cement raises some environmental issues
especially CO2 emissions causing the greenhouse effects. It is estimated that corresponding to one ton of cement produced, one ton of CO2 is emitted into the atmosphere
© Springer Nature Singapore Pte Ltd. 2018
H. Tran-Nguyen et al. (eds.), Proceedings of the 4th Congrès International
de Géotechnique - Ouvrages -Structures, Lecture Notes in Civil Engineering 8,
DOI 10.1007/978-981-10-6713-6_79
Soil Stabilization by Using Alkaline-Activated Ground Bottom Ash
(Daviddovits 2002). The cement production contributes about 7% of the total greenhouse gas emissions on over the world (Shi et al. 2012). In the efforts to solve these
issues, many studies that try to find out the alternative material to substitute cement
have been published (Higgins 2005; Chindaprasirt et al. 2009). One of the new
alternative materials which can be used to replace cement is geopolymer based
The term “geopolymer” was first introduced by Joseph Davidovits in 1978
(Davidovits et al. 1990). Geopolymers are formed by reaction between aluminosilicate
materials (contain a high amount of silica and alumina) with alkaline solutions,
resulting in a mixture of gels and crystalline compounds that ultimately harden into a
new strong matrix (Verdolotti et al. 2008). Using geopolymer materials has great
significance for the environment. It not only reduces greenhouse gas emissions but also
consumes vast volumes of industrial wastes (Majidi 2009). A source of aluminosilicate
binder is coal ash - a waste material generated from coal-fired thermal power plants.
The main components of coal ash are fly ash (65–95%) and bottom ash (5–35%)
(Wang et al. 2005). According to American Coal Ash Association, around 83% of
recycling coal ash in U.S is fly ash (ACAA 2016). It has been an effective material
successfully used in many projects to improve the strength characteristics of soils.
Opposed to fly ash, only a small amount of bottom ash has been recycled. Most of it
has been discharged into the environment by mixing with water and pumping to the ash
pond or compacting into landfill (Asokan et al. 2005). Coal ash disposal is a big
problem of the world due to its possible adverse environmental impacts as well as due
to its high volume of the generation which requires a large land area for disposal. Coal
ash contains heavy metals and metalloids, these elements can be leached out under
acidic conditions and can contaminate the surrounding soils, surface water, and
groundwater sources. In recent years, a new approach to the recycling of the bottom ash
has been pointed out in some studies of Cheriaf et al. (1999), and Jaturapitakkul and
Cheerarot (2003). In that, bottom ash was ground to be used as a partial replacement of
cement due to its pozzolanic reaction.
Red mud is a solid waste material produced during the physical and chemical
processing of bauxite (Bayer process) in the industrial production of aluminum. The
disposal of red mud is nearly similar with coal ash. It is usually disposed in mud
impoundment (for slurry state) or heap in the pond (for dry state). The disposal of
massive amount of red mud with high alkalinity (pH 10–12.5) has caused serious
environmental issues such as groundwater pollution, soil contamination. Furthermore,
due to its fine particle, red mud stored in the pond can emit dust into the air and threaten
the health of people living around that area (Liu and Zhang 2011). To recycle red mud,
there are some studies have succeeded in improving the compressive strength of
concrete by using red mud to substitute a part of cement (Ashok and Sureshkumar
2014; Metilda et al. 2015).
This study proposes a new binder that can be used to replace cement in soil
stabilization. Herein, ground bottom ash and red mud were activated by alkaline
activator to form a binder based on geopolymer synthesis. The specimen was cured at
the ambient curing condition which makes it possible to apply in the field. This can turn
the bottom ash and red mud into a valuable resource for sustainable infrastructure
construction instead of treating it as a waste material.
M.Q. Dang et al.
2 Experimental Program
In the present study, bottom ash was collected from Honam Thermal Power Plant in
South Korea. The particles of collected bottom ash were angular, amorphous, irregular
and had high porosity with a lot of craters and pores on the surface. In order to be used
as a binder, bottom ash was dried at a temperature of 105 °C for 2 days to ensure that
moisture content was completely evaporated. Subsequently, a laboratory ball mill
machine (UBM-100L, RAMT) was used. After grinding, fineness of ground bottom
ash (GBA) was checked complying with standard ASTM C204-16, the test methods for
the fineness of hydraulic cement by air-permeability apparatus. The test result indicated
that after 3 h of grinding the fineness of ground bottom ash was 2,000 cm2/g. Figure 1
shows the particle size distribution of bottom ash before and after grinding. It can be
observed that the particle size of raw bottom ash ranges from 0.075 mm to 10 mm,
whereas that of ground bottom ash varies from 0.004 mm to 0.25 mm. In order to
determine the chemical compositions of bottom ash, the X-ray Fluorescence test
(XRF) was performed. The analyzing result was given in Table 1.
Raw BA
Ground BA
Red mud
Weathered soil
Percent passing (%)
Grain size (mm)
Fig. 1. Particle size distributions of materials
Table 1. Chemical composition of ground bottom ash and red mud (% by weight)
SiO2 Al2O3 CaO MgO Fe2O3 TiO2 K2O Na2O LOI
GBA 62.53 20.91 1.80 0.69 8.70 1.28 1.44 0.39 1.85
RM 15.12 19.87 7.10 0.37 22.21 5.24 0.11 14.92 13.68
Red mud (RM) was collected from an alumina plant in Naju, South Korea, which
was dried and pulverized to obtain fine powders. Thereafter, the particle size analysis
Soil Stabilization by Using Alkaline-Activated Ground Bottom Ash
and XRF were performed. The test results of the red mud were shown in Fig. 1 and
Table 1 along with those of ground bottom ash for comparison. It can be observed that
red mud has particle size ranges from 0.0004 mm to 0.05 mm which is much smaller
than that of ground bottom ash. In addition, it contains 15.12% of SiO2, 19.87% of
Al2O3 which have a major role in forming geopolymer.
To activate ground bottom ash sodium silicate solution (Na2SiO3) were used.
Sodium silicate solution known as water glass was purchased directly from the company with a SiO2/Na2O molar ratio of 3.1 and the percentages by weight are 10% of
Na2O, 30% of SiO2 and 60% of water. The other characteristics of the sodium silicate
solution are specific gravity = 1.41, viscosity at 20 °C = 400 cp.
Weathered soil was the object of stabilization. After being collected from the field,
some components in soil such as roots, leaves were discarded. Thereafter, the soil was
dried in the oven to remove the water content. Geotechnical properties of soils
including specific gravity, liquid limit, plastic limit, plasticity index, particle size were
determined complying to the procedure of American Society of Testing and Materials
(ASTM). Test results were summarized in Table 2.
Table 2. Geotechnical properties of materials
Type of materials Gs % passing No. 200 Cu Cc LL
Weathered soil
2.62 15.05
3.22 0.86 27.8
2.50 19.77
1.81 0.92 –
3.15 100
2.98 0.89 –
Remark: NP (Non-plastic)
Sample Preparation and Test Procedure
The mixture proportion was designed to evaluate the effects of red mud content and the
amount of Na2SiO3 on compressive strength of stabilized soil. Herein, the ratio of
water/binder and binder/soil were selected as 1.2 and 0.33, respectively. To evaluate
the effects of red mud content, the ratio of RM/GBA was varied from 0 to 0.4 and
Na2SiO3/binder was kept at 0.5. To evaluate the effect of activator content, the ratio of
Na2SiO3/binder was varied from 0.3 to 0.6, whereas, RM/GBA ratio was kept at 0.2.
The mixing of mixtures was performed by using a 10-litres mixer. At first, soil and
binder were mixed at the setting of 150 rpm for 3 min. Subsequently,
alkaline-activators and extra water were slowly added to the mixture and mixed for
10 min. The mixture was then cast into the plastic cylindrical molds (50 mm in
diameter and 100 mm in height) by dividing into three layers and tamping 25 times for
each layer. For each case study, 10 specimens were prepared to measure unconfined
compressive strength as well as the concertation of toxic heavy metals. The specimen
was kept in the molds for 3 days, thereafter, it was demolded and cured at the ambient
condition with the temperature of 25 °C and 50% of humidity.
The unconfined compressive strength of stabilized soil was determined at 7, 14 and
28 days of curing age and complied a procedure of standard ASTM D2166-16.
M.Q. Dang et al.
Moreover, in order to measure the toxic heavy metal content of leachate, a procedure
based on a research of Razak et al. (2009) on industrial waste bottom ash has been
applied. At the age of 7 days, the specimen was immersed in a tank filled with distilled
water. The amount water filled in the tank was eight times the volume of the specimen
(liquid/solid = 8:1). The tank was kept closed at room temperature. At 28 days, water
in the tank was collected to determine the toxic heavy metal content. The experiment
was performed by using inductively coupled plasma mass spectrometer (ICP-MS) and
complied standard ASTM D5673-16.
3 Experimental Results
Unconfined Compressive Strength
The effects of RM/GBA ratio on the unconfined compressive strength of stabilized soil
is shown in Fig. 2. For the first case that without using of red mud, the compressive
strengths obtained at 7, 14, and 28 days are 0.5, 2.4 and 3.1 MPa, respectively. The
replacement of ground bottom ash by red mud in certain limit has a positive influence
on the development of soil strength. In particular, the compressive strengths increase
and get maximum value at the RM/GBA ratio of 0.1 for all curing times. After that
point, the compressive strength continuously decreases with the addition of red mud
into mixture. This indicates that 0.1 is the optimum RM/GBA ratio for compressive
strengths in soil stabilized with ground bottom ash and red mud. This result is consistent with some previous studies (Ashok and Sureshkumar 2014; Metilda et al. 2015).
The compressive strength of stabilized soil increases to a certain limit with the use of
red mud in the mixture can be explained by the characteristics of red mud. Firstly, red
mud has high alkalinity (pH = 11.6) so it can accelerate the dissolve of ions from
aluminosilicate materials. Secondly, red mud has fine particles that make specimens
become denser and low porosity (Liu and Zhang 2011). Finally, as can be seen in
Table 1, red mud contains 35% of SiO2 and Al2O3. It can offset a part of reactants
which were lost when ground bottom ash is replaced. However, when the amount of
red mud exceeds the optimal value, the compressive strength decreases due to the
7 days
14 days
28 days
Compressive strength (MPa)
Fig. 2. Effects of RM/GBA ratio on unconfined compressive strength of stabilized soil
Soil Stabilization by Using Alkaline-Activated Ground Bottom Ash
7 days
14 days
28 days
Compressive strength (MPa)
Na2S iO 3 / (GBA+RM)
Fig. 3. Effects of Na2SiO3/(GBA + RM) ratio on unconfined compressive strength of stabilized
reduction of ground bottom ash amount – the main ingredient which provides Al and Si
for geopolymerization.
Figure 3 shows the effect of Na2SiO3/(GBA + RM) ratio on the unconfined
compressive strength of the stabilized. It can be seen that the 7-day compressive
strengths of all cases are almost the same. However, the 14-day and 28-day compressive strength show a rapid increase with the increase of Na2SiO3 amount. In
particular, strength increases 1.2 times for case of Na2SiO3/binder ratio of 0.3 and 1.6
times for final case which the ratio of Na2SiO3/binder is 0.6. The reason for this
phenomenon is that a higher Na2SiO3/binder ratio provides more reactive silica and
hence promotes a larger extent of geopolymerization of the soil mixture. The higher
concentration of soluble silica in the mixture induces an increase of silicon in the
structure of geopolymer which typically leads to higher compressive strength (Duxson
et al. 2007).
Concentration of Toxic Heavy Metals
The term toxic heavy metal is generally used to refer to a group of metals and metalloids that have a relatively high density (>5 g/cm3) and potential toxicity (Alloway
1995). The most common contaminants of toxic heavy metal are arsenic (As), cadmium
(Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) (Lambert et al.
2000). The accumulation of large amounts of toxic heavy metals in soil reduces the
number and activity of soil microorganisms that conducive to the development of the
plants. Moreover, toxic heavy metals can enter food chains from polluted soil and
water, consequently, cause food contamination threatening human and animal health.
In this study, the toxicity of the stabilized soils was studied by measuring the concentration of toxic heavy metals in the leachate. The results were compared with contamination levels suggested by the United States Environmental Protection Agency
(CFR 2016) and the Ministry of Environment – South Korea (MOE 2010). As can be
observed from Table 3, the concentrations of toxic heavy metals are below the limitation.
Therefore, soils stabilized by ground bottom ash are non-hazardous. The toxic heavy
metals arranged in descending order of concentration are Cu, As, Cr, Zn, Ni, Pb, and Cd.
M.Q. Dang et al.
Table 3. Concentration of toxic heavy metals in leachate of stabilized soil (mg/L)
R0.0-Si0.5 0.0080 0.0000
R0.1-Si0.5 0.0428 0.0002
R0.2-Si0.5 0.0599 0.0000
R0.3-Si0.5 0.0725 0.0000
R0.4-Si0.5 0.0869 0.0001
R0.2-Si0.3 0.0662 0.0000
R0.2-Si0.4 0.0797 0.0000
R0.2-Si0.5 0.0599 0.0000
R0.2-Si0.6 0.0825 0.0000
Contamination levels
- U.S.
- South
0.0012 - R0.0 represents for
0.0094 the RM/GBA of 0.0.
0.0149 - Si0.5 represents for
0.0207 the Na2SiO3/
(RM + GBA) of 0.5
4 Conclusions
This study evaluates the feasibility of utilization of ground bottom ash and red mud as a
binder for soil stabilization. A series of experiment has been performed on various
mixtures to find out the element affecting the compressive strength of stabilized soils
and the impacts of binder to the environment. Based on the obtained data, the following
conclusions could be derived.
1. Ground bottom ash and red mud activated by Na2SiO3 solution can be used as a
binder to stabilize soil. The compressive strength of soil increase with the activator
2. The presence of red mud in the mixture has contributed to the development of
strength at early-age as well as later-age. At all curing time, the optimum RM/GBA
ratio is 0.1.
3. Soil stabilized by ground bottom ash and red mud is non-hazardous. The concentration of toxic heavy metals is within the allowable ranges.
Acknowledgments. This research was supported by a grant (No. 16-RDRP-B076564-03) from
Regional Development Research Program funded by Ministry of Land, Infrastructure, and
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