This study investigates the improvement of soft clay through the deep mixing method with an alkaline substance and the production of geopolymer materials through the reaction between clay and alkaline substance.

1. Investigation of Physicochemical Changes of Soft Clay
around Deep Geopolymer Columns
MAJID BAGHERINIA*
NEŞE IŞIK
Department of Civil Engineering, University of Ataturk, Erzurum 25240, Turkey
Key Terms: Deep Mixing, Geopolymer, Soft Clays,
Full-Scale Tests, Ground Improvement
ABSTRACT
Geopolymers show good promise for soft clay sta-
bilization. This paper aimed to improve soft clay with
geopolymer and evaluate the effects of geopolymer-
stabilized columns on the mechanical and chemical be-
havior of the surrounding clay. Ranges of tests were
conducted on both stabilized and unstabilized speci-
mens to determine the impacts of geopolymer on the
clay structure. The results showed that increasing the
binder content and curing time significantly increased
the unconfined compressive strength of stabilized sam-
ples. Microstructure and mineralogy analyses revealed
that hardened materials were formed within the geopoly-
mer matrix from the amorphous clay phases. In ad-
dition, the formation of the effective area around the
geopolymer-stabilized columns resulted in the water
content of soft clay decreasing while the undrained shear
strength, pH, and electric conductivity values increased.
Furthermore, the bearing capacity of soft clay dramat-
ically increased (30-fold) due to an increase in column
area.
INTRODUCTION
Soft soils are a major concern in geotechnical en-
gineering because of their low shear strength and
large deformation under light loads (Cristelo et al.,
2013; Yaghoubi et al., 2020). In ground-improvement
projects, the deep mixing (DM) method is an alterna-
tive to the usual ground-improvement techniques in
soft ground, such as stone columns and sand com-
paction piles. In DM, binders in a dry or wet form
are injected into the ground with a hollow auger and
mixed by the rotation of cutting tools. As a result, the
ground particles are ultimately blended, and stabilized
columnar elements are formed (Bruce and Bruce, 2003;
Kitazume and Terashi, 2013). The stabilized area has
*Corresponding author email: majid.bagherinia13@ogr.atauni.edu.tr
high strength and stiffness, low permeability, and low
settlement (Kitazume and Terashi, 2013; Jamsawanga
et al., 2017). This method is suitable for providing
an appropriate bearing capacity for light to medium
loads and is superior to other methods, both economi-
cally and practically (Bruce et al., 2013; Kitazume and
Terashi, 2013).
Lime and cement are commonly used as traditional
binders in DM, and they stabilize weak soils with a
binder content of up to 30 percent by weight relative
to the soil (Rogers et al., 2000; Shen et al., 2003a;
Horpibulsuk et al., 2011; Pakbaz and Alipour, 2012;
and Bruce et al., 2013). These binders, especially Port-
land cement (PC), have a negative impact on the atmo-
sphere and cause serious environmental problems due
to the emission of carbon dioxide (CO2) during com-
bustion and chemical calcination (Zhang et al., 2013;
Du et al., 2016; and Yaghoubi et al., 2018). In addi-
tion, the durability of cement is unacceptable, result-
ing in excessive cost and time required for re-fixation.
Due to these challenges, research into new materi-
als is essential. Therefore, this study aimed to inves-
tigate an alternative material to PC for use in the DM
application.
Geopolymers are inorganic materials that have
recently received special attention from researchers. In
addition to the mechanical performance of geopoly-
mers, compared to traditional binders, their environ-
mental advantages make them an attractive alternative
(Yaghoubi et al., 2019). Previous studies have reported
that geopolymers are an alternative to PC due to their
preferable compressive and flexural strength, higher
ductility, lower shrinkage, and superior durability
(such as fire and acid resistance) (Gao et al., 2013;
Zhang et al., 2013; Phoo-Ngernkham et al., 2015,
2016; Du et al., 2016, 2017; Nath and Sarker, 2017;
and Rios et al., 2017).
A geopolymer is a type of inorganic material that
appears as an analog of thermoset organic cross-
linking agents (Sukmak et al., 2013b). Geopoly-
merization (alkaline activation) occurs when alumi-
nosilicate and alkaline materials react with each
other. During the reaction, silica and alumina units
blend in conjunction and connect the oxygen ions;
this is known as the polycondensation reaction
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2. Bagherinia and Işik
(Davidovits, 1991; Gao et al., 2013; and Pourakbar
et al., 2016). Davidovits (2013) concluded that kaoli-
nite (with a high content of silica and alumina) is poly-
merized by alkalis to form a hard, concrete-like mate-
rial. In the literature, different types of waste materials
(e.g., fly ash, blast furnace slag, and volcanic ash) have
been activated with various alkalies (e.g., sodium ox-
ide, sodium hydroxide, sodium silicate, potassium hy-
droxide, and calcium hydroxide) to improve soft clay
soils (Cristelo et al., 2013; Sargent et al., 2013; and
Miao et al., 2017). The results indicate that the com-
pressive strength of the stabilized soft clay increased
significantly, while the swelling potential and plastic-
ity index decreased. Other researchers indicated that
soil type, binder type and ratio, and mixing properties
are important factors affecting the performance of sta-
bilized soils (Huseien et al., 2016; Olivia and Nikraz,
2012; Pacheco-Torgal et al., 2008; Phoo-Ngernkham
et al., 2015, 2016; Nath and Sarker, 2017; and Wu
et al., 2019). Geopolymers can be used in geotechni-
cal applications such as soil stabilization in ground-
improvement (Phetchuay et al., 2016; Pourakbar et al.,
2016; Singhi et al., 2016; Du et al., 2017; and Yaghoubi
et al., 2018) and DM projects (Arulrajah et al., 2018;
Mohammadinia et al., 2019).
After the installation of DM columns, the mechan-
ical and chemical properties of the surrounding soft
clay are affected by ion migration. Many studies have
already reported the migration of ions from lime or
lime-cement columns into soft clay, and their effects
on the adjacent ground have been investigated. Nev-
ertheless, knowledge of ion migration related to the
behavior of geopolymeric compounds and their effect
on soil properties, such as undrained shear strength,
bearing capacity, settlement, etc., is limited. In a re-
cent study, Bagherinia and Zaimoğlu (2021) installed
DM columns with potassium hydroxide as a binder
and found that the properties of soft clay improved due
to ion migration.
This paper presents an experimental study on the
stabilization of soft clay with geopolymers. Ranges of
tests were conducted on both stabilized and unstabi-
lized specimens to evaluate the effects of binder con-
tent, curing time, and condition during the geopoly-
merization process, including unconfined compressive
strength (UCS), X-ray diffraction pattern (XRD), and
scanning electron microscopy (SEM) analyses. The
main objective was to increase the bearing capacity of
soft clay and reduce settlement by installing geopoly-
mer DM columns. To determine the effective area
around the columns due to ion migration and the
distance of this area from the column, experiments
were performed to determine pH, electric conductiv-
ity (EC), water content, and undrained shear strength
(St) in the surrounding clay.
Table 1. Index properties of clay.
Property Clay
Clay content, <0.002 mm (%) 17
Finer content, <0.075 mm (%) 80
Specific gravity, Gs 2.65
Liquid limit, LL (%) 44
Plastic limit, PL (%) 26
Plasticity index, PI (%) 18
Optimum water content (%) 16
Maximum dry unit weight, γdmax (kN/m3
) 18.5
Hydraulic conductivity, k (cm/s) 6.980 × 10−7
MATERIALS AND METHOD
Materials
Clay soil was collected from the field (a site in
Kırşehir, Turkey), from about 3.5 m below the ground
surface, and dried in an oven at 105°C for 1 day; it
was then pulverized and passed through a 1 mm sieve.
The soil type selected for the study was alluvial soil,
which is classified as low-plasticity clay according to
the Unified Soil Classification System (USCS). Dif-
ferent properties of clay are shown in Table 1. The
XRD and X-ray fluorescence (XRF) patterns of the
clay illustrated the presence of kaolinite (46.65 per-
cent), quartz (27.14 percent), aluminum oxide silicate
(5.24 percent), nacrite (7.6 percent), and alunite (4.39
percent) as common minerals in this soil. The remain-
ing minerals (8.98 percent) were amorphous in na-
ture. The soil also contained alumina (Al2O3 = 26 per-
cent) and silica (SiO2 = 69 percent), which conveyed
geopolymeric properties on the clay (Gao et al., 2013;
Pourakbar, 2016).
Sodium hydroxide (NaOH) in the form of beads
(purity: 97 percent) was obtained from local suppli-
ers in Turkey. White NaOH beads, with a pH of 12,
were used in the experiments as an alkaline additive
that is soluble in water, ethanol, and methanol. Other
characteristics of NaOH are shown in Table 2. NaOH
solution with higher molarities is dangerous to work-
ers because of its corrosive nature (Phoo-Ngernkham
et al., 2015; Phetchuay et al., 2016; and Subekti et al.,
Table 2. Chemical properties of sodium hydroxide.
Property Sodium Hydroxide
Chemical formula NaOH
Molecular weight 39.9771 g/mol
Melting point 318°C
Boiling point 1,388°C
pH 12
Density 2.13 g/cm3
Solubility Water, ethanol, methanol
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3. Clay Changes around Geopolymer Columns
Table 3. Summary of experimental program.
Materials (%)
Sample Name Clay at 44% Water Content NaOH Total Curing Time (days) Number of Samples
Stage 1: Unconfined compression strength test (38 mm × 76 mm)
S1 100 — 100 7, 14, 28, 56, 112 15
S2 99 1 100 7, 14, 28, 56, 112 15
S3 97 3 100 7, 14, 28, 56, 112 15
S4 95 5 100 7, 14, 28, 56, 112 15
S5 93 7 100 7, 14, 28, 56, 112 15
S6* 91 9 100 7, 14, 28, 56, 112 15
S7 89 11 100 7, 14, 28, 56, 112 15
Total = 105
Stage 2: Small-scale model test (water content, pH, EC, and St)
SC1 100 — 100 7, 14, 28, 56, 112 5
SC2 99 1 100 7, 14, 28, 56, 112 5
SC3 97 3 100 7, 14, 28, 56, 112 5
SC4 95 5 100 7, 14, 28, 56, 112 5
SC5 93 7 100 7, 14, 28, 56, 112 5
SC6 91 9 100 7, 14, 28, 56, 112 5
Total = 30
Stage 3: Large-scale model test (load-bearing capacity)
LS 91 9 100 56 1, 3, and 7 columns
*S6 is the sample used for XRD and SEM analyses at 28, 56, and 112 days.
2017). Therefore, it was used in dry form in the experi-
ments to reduce its harmful effects.
Sample Preparation and Testing
The laboratory testing program was planned in three
stages: (1) UCS test, (2) small-scale test, and (3) large-
scale test (Table 3). An optimum binder content of 20
to 25 percent (Horpibulsuk et al., 2011; Bushra and
Robinson, 2013) up to 30 percent (Bruce et al., 2013;
Kitazume and Terashi, 2013) has been suggested for
the stabilization of clayey soils using the DM method.
In the experiments, the NaOH content was selected as
1, 3, 5, 7, 9, and 11 percent (by mass of soil), which is
very low compared to the literature values.
Previous studies (Lorenzo and Bergado, 2004; Hor-
pibulsuk et al., 2011; Cristelo et al., 2013; Phetchuay
et al., 2016; and Arulrajah et al., 2018) indicated that
the optimum water content for the improvement of
clayey soils with high water content is 1.0 liquid limit
(LL) of the soil. The same value was chosen for this
study. The natural water content and liquid limit of
the in situ clay was 38 to 51 percent and 44 percent,
respectively.
For the UCS test, the dried clay and water were
placed in a mechanical stirrer and mixed for approxi-
mately 5 minutes. The mixture was poured into a plas-
tic bag and kept for 1 day to be homogenized. Then,
the NaOH beads were added to the mixture, and the
mixing process was repeated at 150 rpm. The mixture
was collected from the mechanical stirrer, and three
layers were poured into a cylindrical metal mold with
a length of 76 mm and a diameter of 38 mm. The in-
ner surfaces of the molds were lubricated with a thin
layer (Grease oil) to facilitate the extrusion of the test
specimens. The edge of the mold was lightly tapped
by hand to reduce air bubbles. The prepared speci-
mens were kept in a curing room at 20 ± 3°C am-
bient temperature and 90 ± 5 percent humidity for
7, 14, 28, 56, and 112 days. Alkali-stabilized sam-
ples need to be cured at a high temperature (but be-
low 100°C) to complete the geopolymerization pro-
cess (Davidovits, 2017). Since curing at high temper-
atures is not possible in the field, a curing temperature
suitable for field conditions was chosen. Therefore, the
ambient temperature was selected to detect the forma-
tion of the geopolymeric products in the clay struc-
ture. The preparation of the specimens and the cur-
ing conditions were performed according to Japanese
Geotechnical Society (2000), Euro Soil Stab (2002),
and Arasan et al. (2017). At the end of the curing pe-
riod, the UCS test was conducted on the specimens ac-
cording to ASTM D2166/D2166M (2016) (Figure 1).
The testing load was applied at a rate of 1.0 mm/min.
For each NaOH ratio and curing time, three specimens
were prepared, and the UCS values were determined
by averaging three measurements.
Following the UCS test, sample S6 was evaluated
using XRD and SEM analyses to assess soft clay
stabilization. The XRD patterns were recorded with
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4. Bagherinia and Işik
Figure 1. Unconfined compressive strength test after (a) 7, (b) 14, (c) 28, (d) 56, and (e) 112 days.
an EMPYREAN diffractometer and analyzed with
the PANalytical measurement program. The measure-
ments were conducted on treated and untreated clay
soil at λ = 1.54060 Å wavelength from the Cu-Ku
source, at a scanning range of 5° to 75°, a scanning
speed of 2 degrees/min, and a screening step of 0.0260
degrees (Bagherinia and Zaimoğlu, 2021).
The SEM images were taken of the samples to evalu-
ate the interactions between NaOH and clay minerals.
The sample in the powdered form was put on the sam-
ple plate with a carbon band adhered and coated with
gold (5 nm in thickness), and vacuum treatment was
carried out for 3 minutes. Finally, it was placed in the
sample holder and analyzed using a Zeiss Sigma 300
(Bagherinia and Zaimoğlu, 2021).
Installation of DM Columns (Small-Scale Test)
At this stage, the end-bearing geopolymer DM
columns were installed into the clay to investigate the
effects of ion migration on the mechanical and chemi-
cal properties of the surrounding soft clay.
A clay mixture with optimum moisture content (and
LL = 44 percent) was prepared in the same way
as that done for the UCS test. A cylindrical mold
made of polyvinyl chloride (PVC), with a height of
110 mm and a diameter of 100 mm, was used to pre-
pare clay beds. The clay mixture was filled into the
mold and compacted into three layers using a wooden
tamper (Malarvizhi and Ilamparuthi, 2004; Şengör,
2011; Demir et al., 2013; and Malekpoor and Poore-
brahim, 2014). Then, the columns were installed using
a dry mix method for DM (Figure 2). The dry mix
method refers to the state of the binder as it is dis-
tributed in the mixer and the soil as a dry powder. The
NaOH powder was cast into the soft clay as follows,
to install columns with 30 mm diameter and 110 mm
height.
A tube with an inner diameter of 30 mm was ver-
tically pushed into the soft clay. Another tube with
an inner diameter of 10 mm was then placed inside
a rod with an outer diameter of 30 mm, and the rod
was plugged at one end. The smaller tube and the rod
were then placed within the larger tube and driven ver-
tically down to the desired depth. The rod, plugged at
the end, was then removed. The smaller tube (inner
diameter of 10 mm) was filled with a pre-determined
amount of NaOH. After a while, the 10 mm tube was
pulled out, and the NaOH remained in the clay. A
mixing tool with a shaft-tipped blade, 25 mm in di-
ameter, was rotated and inserted through the soft clay,
down to the intended depth and withdrawn at a rate of
2 m/min and rotational velocity of 320 rpm (Larsson
et al., 2009; Bagherinia and Zaimoğlu, 2021). The mix-
Figure 2. Images of installation of small-scale geopolymer DM col-
umn: (a) plan view and (b) cross-section view.
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5. Clay Changes around Geopolymer Columns
Figure 3. Schematic view of installation of DM column in soft clay.
ing process was repeated in triplicate for every column.
Finally, the 30-mm-diameter tube was pulled out. A
schematic view of the installation of the DM column
is displayed in Figure 3. The PVC molds were sealed
with a plastic cover to prevent the loss of the clay’s
water content. The samples were kept in the curing
room at 20 ± 3°C ambient temperature and 90 ±
5 percent humidity for 7, 14, 28, 56, and 112 days.
At the end of the curing times, water content, pH,
EC, and St tests were conducted at 5, 10, 15, 20, and
25 mm distances from the column edge and 20 mm
depth from the surface, according to the following
experiments.
r Water content test: The samples were taken from
the specified intervals and depths using a very thin
metal ruler, and the tests were conducted according
to ASTM D2216 (2019).
r pH and EC test: The sampling procedure was per-
formed as in the water content test. Subsequently,
air-dried soil passing through a no. 10 sieve was
collected and placed in a glass container, followed
by distilled water, and the mixture was stirred ev-
ery 10 minutes for 1 hour. Then, the pH and EC of
the mixture were read by a pH and EC meter, re-
spectively. The ratio of distilled water was 1:1 and
1:2.5 for the pH and EC tests, respectively. The pH
and EC tests were performed according to ASTM
D4972 (2019) and Rayment and Higginson (1992),
respectively.
r St test: The St test was performed around the
columns according to ISO/TS 17892-6: (CSN EN
ISO 17892-6, 2004) to determine the strength de-
velopment of the soft clay. This test measures the
cone penetration depth, which is correlated with the
undrained shear strength. The St values were evalu-
ated according to Eq. 1 (Hansbo, 1957).
St = Cg
m
i2
. (1)
St is undrained shear strength (kPa), C is a constant
determined by the angle between the ground and the
cone (C = 0.8 at 30°C), g is the acceleration due to
gravity (9.81 cm/s2
), m is the mass of the cone (g), and
i is the cone penetration distance (mm).
Installation of DM Columns (Large-Scale Test)
A large-scale model test was performed to investi-
gate the load-settlement behavior of the DM columns
in the soft clay and simulate their behavior in practice.
Moreover, a single column was produced to examine
load capacity during settlement in the soft clay, and
the result was compared to the clay bed containing
no column (untreated soil). To measure the effect of
column groups on the load-settlement behavior of the
soft clay, two different area replacement ratios (triple
columns, as = 0.0748, and seven columns, as = 0.1745)
were used in this study.
In the experiment, a cylindrical metal tank (1,000
mm in diameter, 400 mm in height, and 20 mm in
thickness) was used to install the end-bearing columns
(30 mm in diameter and 200 mm in height). Accord-
ingly, the clay bed and columns were prepared as de-
scribed in the small-scale model test using a 300-mm-
long auger (Fig. 4a). The tank was then sealed with a
plastic cover for 56 days at 20 ± 3°C to control humid-
ity (90 ± 5 percent). It should be noted that the highest
compressive strength value obtained in the UCS test
was with 9 percent NaOH, so this amount was used
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6. Bagherinia and Işik
Figure 4. Large-scale model test: (a) preparation of clay bed and installation of geopolymer DM columns, (b) attachment LVDTs, and (c)
testing columns.
in the column installation, and the spacing between
columns in the column groups was chosen based on
the most effective area from the small-scale test results
(distance column periphery = 25 mm).
At the end of the curing period, the columns were
subjected to vertical loading at a constant strain rate of
1.0 mm/min using a loading frame. The vertical load
was applied to the columns via a rigid circular plate
with a diameter of 80 mm and 190 mm for the single
and column groups, respectively. All plates had a thick-
ness of 20 mm. An LVDT (linear variable differential
transformer) was attached to measure the settlement
of the plate during loading (Figure 4b and c).
RESULTS AND DISCUSSION
UCS Test Results
The effects of the NaOH ratio and curing time on
the UCS of the samples are presented below. The
UCS results of the stabilized specimens were compared
with untreated specimens and other studies from the
literature.
Figure 5 shows the relationship between the NaOH
ratio and the UCS value at different curing ages. Gen-
erally, the strength of all treated specimens increased
with the increase of NaOH ratio from 1 percent
Figure 5. Relationship between the UCS-NaOH ratio and curing
time of treated and untreated soft clay.
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7. Clay Changes around Geopolymer Columns
to 9 percent. Between 1 percent and 3 percent, there
was a negligible increase in UCS values, indicat-
ing that the low binder content did not sufficiently
complete geopolymerization in the soil. The UCS
values increased steadily at 7 percent and then in-
creased dramatically when the NaOH content reached
9 percent. Previous studies have also reported a
significant strength improvement for mixtures pre-
pared with NaOH (Nematollahi and Sanjayan, 2014;
Phoo-Ngernkham et al., 2015; and Yaghoubi et al.,
2018). Moreover, NaOH has a noticeable potential
for strength development in the soft clay compared to
KOH (Bagherinia and Zaimoğlu, 2021). This implies
a possible difference in ionic diameter between sodium
and potassium.
Based on Figure 5, the UCS values dramatically de-
creased when more than 9 percent NaOH was added
to the soil. This could have been due to the excessive
NaOH content, which, when it penetrates the clay lay-
ers, separates them instead of sticking together. Sim-
ilar results were noted by other authors who showed
that the compressive strength of the specimens de-
creased when the binder content was above 10 percent
(Consoli et al., 2009; Sabet et al., 2013; and Valipour
et al., 2014).
In addition, increasing the curing period had no ma-
jor effect on the UCS values of samples S2 (1 percent),
S3 (3 percent), and S4 (5 percent). The UCS values
of sample S5 (7 percent) showed a gradual increase
after 14 days but remain unchanged until 112 days.
Otherwise, the curing ages were more effective for S6
(9 percent). Here, UCS values suddenly increased after
7 days, and a dramatic increase was observed up to 112
days, indicating the critical role of the NaOH ratio in
geopolymerization at early stages and thereafter. The
highest UCS value was reached using 9 percent NaOH
after 56 curing days. The UCS values of the specimens
at this ratio for the curing periods of 7,14, 28, 56, and
112 days were 923, 1,421, 2,345, 3,120, and 3,115 kPa,
respectively. The highest UCS value (9 percent NaOH
and 3,120 kPa) of the treated sample was 680 percent
higher than that of the untreated specimens (0 percent
NaOH and 400 kPa) for the same curing time. Nev-
ertheless, the lowest UCS value obtained from the 9
percent NaOH results was higher than the lower UCS
limit for cohesive soil (200 kPa) for the DM method
(Bruce et al., 1998; Bruce and Bruce, 2003). This re-
sult indicates that NaOH is an acceptable binder in the
DM application.
The curing state of the specimens stabilized with al-
kaline materials is crucial in geopolymerization; low
temperatures lead to low strength, while high tempera-
tures lead to high strength (Abdeldjouad et al., 2019).
The results demonstrated that soft clay with high alu-
mina and silica contents can be geopolymerized at
Figure 6. XRD analysis of (a) untreated clay, (b) clay + 9 percent
NaOH after 28 days, (c) clay + 9 percent NaOH after 56 days, and
(d) clay + 9 percent NaOH after 112 days.
room temperature (20 ± 3°C), and the strength values
of the stabilized samples are acceptable, according to
the literature.
It is worth mentioning that the UCS values of the
samples prepared with 7 percent and 9 percent NaOH
slightly decrease in 112 days compared to that at 56
days. This was probably related to the time required for
the formation of the nucleation phase. The same result
was previously observed for soil stabilization with lime
(Cristelo et al., 2009) and alkaline materials (Cristelo
et al., 2013).
XRD Results
The XRD analysis focused on the specimens that ex-
hibited the highest compressive strength from the UCS
test. The effect of the curing period on the XRD of
the treated specimens was investigated, and the results
were compared with those of the untreated clay.
Figure 6 displays the XRD results of the untreated
clay and the stabilized specimens of S6 after 28, 56,
and 112 days. The untreated clay was composed of
amorphous to significantly crystalline materials. The
peak crystalline phases were reduced between 28 and
56 days by the addition of 9 percent NaOH and re-
mained unchanged until 112 days. The reduction in
XRD peaks after soft clay treatment should increase
the strength of the samples (Villa et al., 2010; Cristelo
et al., 2013; and Phoo-Ngernkham et al., 2015). Af-
ter 28 days, new amorphous phases were found, some
of which overlapped with the main peaks. These ob-
servations suggest that compounds such as sodium
aluminosilicate hydrate (NASH), sodium hydrogen
aluminosilicate, and hydrogen sodium aluminosilicate
formed by the reaction of NaOH and clay minerals
(silica and alumina) in the presence of water. On the
other hand, an additional semi-crystalline peak of hy-
drosodalite (θ = 14°) and NASH (θ = 32.5°) increased
after 28 and 56 days, respectively. The alumina-silicate
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8. Bagherinia and Işik
in the kaolinite mineral reacts with NaOH to form hy-
drosodalite (Marsh et al., 2018). Hydrosodalite causes
some geopolymerization, which leads to an increase in
strength (Somna et al., 2011; Phoo-Ngernkham et al.,
2015), and so the formation of NASH gels in the sys-
tem may have enhanced the strength of the samples
(Ismail et al., 2014; Phoo-Ngernkham et al., 2015; and
Phetchuay et al., 2016).
SEM Results
SEM images provide useful information for moni-
toring structural changes in clay as a result of chemical
reactions. Figure 7 shows the SEM micrographs of un-
treated clay, NaOH, and S6 after 28, 56, and 112 days
of curing. The SEM of clay illustrates a porous tex-
ture with widely spaced plates and loose and scattered
layers (Figure 7a and b). NaOH has an angular and
filled structure. The particle size of NaOH is generally
smaller than that of clay (Figure 7c and d). In Fig-
ure 7e and f, soft clay stabilized with 9 percent NaOH
shows geopolymerization and bonding between lay-
ers after 28 days, with strength increasing with time
(from 400 kPa to 2,345 kPa); however, some microp-
ores are still present between clay layers. At this age,
tiny particles of less than 0.1 μm are detected in the
compacted mass (Figure 7f), indicating geopolymer-
ized particles (Sukmak et al., 2013a; Miao et al., 2017).
Figure 7g and h shows that the geopolymerized com-
pounds adhere to most clay particles after 56 days and,
subsequently, cause significant agglomeration in the
soil structure, enhancing the strength by decreasing the
micropore volume (3,120 kPa). The formation of hy-
drosodalite and NASH gels within the clay structure
significantly increased the strength of soft clay over
time (Somna et al., 2011; Ismail et al., 2014; Phoo-
Ngernkham et al., 2015; and Phetchuay et al., 2016).
This can be attributed to an increase in the Si/Al ratio
in the system. Similar conclusions were drawn in previ-
ous studies, when NaOH was used as an alkali to pre-
pare the geopolymeric products, and the mechanical
improvement was observed via microstructure analysis
(Wang et al., 2005; Komljenović et al., 2010; Cristelo
et al., 2013; Singhi et al., 2016; and Yaghoubi et al.,
2018). In addition, the SEM analysis results displayed
the geopolymerization of soft clay at room tempera-
ture, which was also supported by the XRD and UCS
test results.
Small-Scale Test Results
To determine the migration of geopolymers into the
surrounding soft clay, tests were performed for wa-
ter content (percent), pH, electrical conductivity (EC),
and undrained shear strength (St) from the distance
column periphery (DCP). The negligible result of the
DM column preparation with 1 percent NaOH is not
included in the test results. After DCP = 25 mm (0.83d,
d = column diameter), the values of the aforemen-
tioned tests dropped drastically (similar to the soft clay
values) and were, therefore, excluded from the results.
Water Content Test Results
Figure 8 displays the results of the changes in
water content around the geopolymer-stabilized DM
columns at different points of DCP and curing age.
For all additive ratios, the water content of soft clay
decreased near the column and increased far from the
column edge (at high DCP). This demonstrates that in-
teraction between the geopolymer column and the clay
particles is more effective near the column than farther
away. In other words, ion migration is high near the
column.
Furthermore, the water content, especially near the
columns, decreased slightly with increasing NaOH ra-
tio, and this can be attributed to the migration of the
hydroxyl groups (Bagherinia and Zaimoğlu, 2021) and
sodium ions. These reductions were even more pro-
nounced with increasing aging time (up to 56 days)
and then reached a plateau within 112 days. A similar
trend was observed for the improvement of soft clay
soils with lime columns (Tonoz et al., 2003). The min-
imum water content (42.7 percent) was obtained for
SC6 in 56 days, which was 3.0 percent lower than the
initial value (untreated clay = 44 percent).
pH Test Results
The pH test was performed on the soft clay around
the geopolymer DM columns to determine the changes
in the alkalinity of the soil, which is an important fac-
tor in stabilizing clay soils. As shown in Figure 9, the
pH simultaneously increased near the column when
the NaOH ratio increased, and it decreased with dis-
tance from the column. It is expected that the pH of
the soil near the columns will be high due to the in-
tense ion transfer; hydroxyl ions can migrate within the
clay and cause highly alkaline conditions (Diamond
and Kinter, 1966; Bagherinia and Zaimoğlu, 2021). In
the clay-water system, a high alkaline status indicates a
high pH, which leads to effective ground stabilization
(Vural, 2012; Ghobadi et al., 2014; and Bagherinia and
Zaimoğlu, 2021). Accordingly, the highest pH value il-
lustrates stiff and firm ground, while the lowest value
describes weak and loose ground. According to the re-
sults and literature review, the increased pH of soft
clay near the column indicates improved clay proper-
ties (Shen et al., 2003a, 2003b, 2008; Tonoz et al., 2003;
Larsson et al., 2009).
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9. Clay Changes around Geopolymer Columns
Figure 7. SEM micrographs at different magnifications of samples: (a–b) untreated clay, (c–d) NaOH, (e–f) clay + 9 percent NaOH after 28
days, (g) clay + 9 percent NaOH after 56 days, and (h) clay + 9 percent NaOH after 112 days.
Environmental & Engineering Geoscience, Vol. XXVIII, No. 4, November 2022, pp. 371–386 379

10. Bagherinia and Işik
Figure 8. Water content values around the geopolymer DM columns.
Moreover, the pH values showed an upward trend
with greater aging time. For all NaOH ratios, soil pH
increased from 7 to 56 days and remained constant un-
til 112 days. The maximum pH of 8.78 was reached at
DCP = 5 mm, 9 percent NaOH (SC6), and a curing
period of 56 days. This value is 35 percent higher than
that for raw clay (6.5).
Electric Conductivity Test Results
The electrical conductivity of the clay-water system
depends on the type and number of ions in the mix-
ture. When ions on the clay surface are dissolved into
the solution, the conductivity of the cation under the
electric field and the movement of the colloidal parti-
cles increase the conductivity of the solution.
Figure 10 displays the relationship between the elec-
trical conductivity of the clay soil and the distance
from the edge of the column at different curing times.
The EC values of the surrounding soft clay increased
with the increase of the NaOH content in the geopoly-
mer DM columns (from 3 percent to 9 percent) and
the curing time. On the other hand, similar to the wa-
ter content and pH tests, the EC of the surrounding
clay increased steadily, peaking within 56 days (at all
NaOH ratios) and then remaining constant or decreas-
ing up to 112 days. The increase of the EC was more
efficient near the columns, particularly at 7 percent
and 9 percent NaOH, indicating migration of Na+
and
OH−
ions into the surrounding soft clay and increased
colloidal particle and ion concentration activity. The
maximum EC value of 1.78 mS/cm was reached at
DCP = 5 mm, 9 percent NaOH, and 56 days of cur-
ing, which is about 88 percent higher than the original
value (untreated clay = 0.943 mS/cm). It should be
noted that the EC value did not increase after 56 days.
These trends could be due to decreased ion transfer
from the geopolymer DM columns to the surrounding
soft clay, probably leading to NaOH consumption. As
reported earlier (Salimi and Ghorbani, 2020), the EC
value of the geopolymer-stabilized samples decreased
with aging after 56 days, which could be due to lower
ion concentration and additive consumption.
Undrained Shear Strength Test Results
The undrained shear strength (St) test was per-
formed around the columns to measure the mechan-
ical improvement of the soft clay and to investigate the
effects of the geopolymer DM columns on the shear
380 Environmental & Engineering Geoscience, Vol. XXVIII, No. 4, November 2022, pp. 371–386

11. Clay Changes around Geopolymer Columns
Figure 9. The pH values of soft clay around the geopolymer DM columns.
strength of the soil. The effects of curing times and
NaOH ratios on the strength development of the soft
clay were investigated at different DCPs. The results of
St tests are shown in Figure 11.
Figure 11 shows that the strength of the clay soil in-
creased with an increase in NaOH ratio from 3 to 9
percent. Due to the higher ion concentrations, the in-
creases were greater near the periphery of the columns
than at greater distances from the edge of the columns.
A trend similar to that described above was also ob-
served during the long aging process. The St values in-
creased with increasing curing time (7 to 56 days) for
all NaOH ratios and then remained unchanged un-
til 112 days. The effect of curing time was more pro-
nounced for 9 percent NaOH and close distances from
the column. As with the tests for water content, pH,
and EC, the St of the surrounding soft clay remained
unchanged after 56 days. As described in the previous
section, such trends could be due to decreased ion mi-
gration that results in NaOH consumption.
The maximum St value of 5.68 kPa was reached
for SC6 at DCP = 5 mm and 56 days of curing,
which is about eight times higher than the value for
the untreated clay (0.63 kPa). According to Bagherinia
and Zaimoğlu (2021), the undrained shear strength of
soft clay can be improved near DM columns prepared
with alkaline materials. They reported that the DM
columns contain an effective area or transition zone
due to the migration of potassium and hydroxyl ions,
and that the stabilization of the soil occurs due to the
reaction between the clay particles and the migrating
ions. Ghobadi et al. (2014) reported that the undrained
shear strength of lime-stabilized specimens increased
with increasing pH. They indicated that alkalinity is
a factor affecting the stabilization of clay soils. Other
studies focused on ion migration and its effects on the
undrained shear strength of soft clay around lime or
cement columns (Shen et al., 2003a, 2003b, 2008; Lars-
son et al., 2009).
Large-Scale Test Results
Figure 12 shows the load-settlement behavior of
the clay bed, a single column, and three- and seven-
column groups of end-bearing columns after 56 days.
As shown, the bearing capacity of the clay bed
with no column increased from 0.05 to 0.20 kN af-
ter the installation of the single column. In other
words, the geopolymer DM column noticeably im-
proved the bearing capacity of the soft clay. The effec-
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12. Bagherinia and Işik
Figure 10. The EC values around the geopolymer DM columns.
tive area (transition zone) increased when the number
of columns was increased, which further improved the
bearing capacity of the soft clay. The bearing capacity
of three and seven columns was 0.6 and 1.7 kN, respec-
tively.
The ultimate bearing capacity of 1, 3, and 7 end-
bearing columns was approximately 275 percent, 1,018
percent and 2,990 percent higher than the ultimate
bearing capacity of the clay bed with no columns, re-
spectively. These values suggest that the DM columns
were successfully installed into the soft clay and signif-
icantly improved the mechanical properties of the sur-
rounding soft clay by increasing the area ratio (num-
ber of columns). Nevertheless, the results of this study
could be more beneficial than other studies on deep
mixed or stone columns in the literature (Malarvizhi
and Ilamparuthi, 2004; Ali et al., 2010; Malekpoor and
Poorebrahim, 2014).
CONCLUSIONS
In this study, series of unconfined compressive tests
and small- and large-scale model tests were conducted
to investigate the effects of geopolymer DM columns
on the mechanical strength and chemical changes of
the surrounding soft clay. The following conclusions
and recommendations can be drawn from the test
results.
r The maximum UCS value obtained in this study was
approximately 56 percent greater than the accepted
limit value (i.e., 2,000 kPa) for deep mixed clay soils
with cement reported in the literature.
r Clay soils that are rich in silica and alumina can be
geopolymerized with NaOH beads at room temper-
ature, which was confirmed by the XRD and SEM
analyses.
r The transition zone (DCP = 25 mm) formed around
the geopolymer DM columns due to the migration
of ions.
r The properties of the soft clay can vary in the transi-
tion zone. While the undrained shear strength, pH,
and EC of the soft clay increased, the water content
decreased. The transition of ions depended on the
ratio of additive, curing time, and distance from the
edge of the column.
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13. Clay Changes around Geopolymer Columns
Figure 11. The St values around the geopolymer DM columns.
r The end-bearing geopolymer DM column increased
the load-bearing capacity of the soft clay, and in-
creasing the number of columns increased the load-
bearing capacity of the soft clay.
r The load-bearing capacity of the soft clay after in-
stalling seven groups of end-bearing columns was
approximately 30 times higher than that of soft clay
without any columns.
Figure 12. Load-settlement curves for end-bearing geopolymer DM
columns.
Considering the results of this study and the liter-
ature review, NaOH can be recommended as a new
binder for stabilizing soft clay soils using the deep mix-
ing method. Field studies are needed to confirm the
results of this study.
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