I. BO[KOVI] et al.: CHARACTERIZATION OF RED MUD/METAKAOLIN-BASED GEOPOLYMERS ...
341–348
CHARACTERIZATION OF RED MUD/METAKAOLIN-BASED
GEOPOLYMERS AS MODIFIED BY Ca(OH)
2
KARAKTERIZACIJA GEOPOLIMEROV NA OSNOVI ZMESI
RDE^EGA BLATA IN METAKAOLINA, MODIFICIRANIH S
Ca(OH)
2
Ivana Bo{kovi}
1
*, Mira Vuk~evi}
1
, Sne`ana Nenadovi}
2
, Miljana Mirkovi}
2
,
Marija Stojmenovi}
2
, Vladimir Pavlovi}
3
and Ljiljana Kljajevi}
2
1
Faculty of Metallurgy and Technology, University of Montenegro, Cetinjski put bb, 81000 Podgorica, Montenegro
2
Laboratory for Materials Sciences, Institute of Nuclear Sciences Vin~a, Mike Petrovi}a Alasa 12-14, Vin~a, 11000 Belgrade, Serbia
3
Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35/IV, 11000 Belgrade, Serbia
Prejem rokopisa – received: 2018-06-29; sprejem za objavo – accepted for publication: 2018-12-13
doi: 10.17222/mit.2018.130
Geopolymers are an emerging class of materials that offer an alternative to the Portland cement as the binder of structural
concrete. One of the advantages is that the primary source of their production is waste alumosilicate materials from different
industries. One of the key issues in geopolymer synthesis is the low level of mechanical properties due to porosity as well as the
high activity of conductivity carriers. It can often lead to limited application possibilities, so the objective is to obtain an
enhanced strength as well as decreased cracking tendency through microstructure modification. The introduction of Ca(OH)2,
under certain pH conditions could lead to the filling-the-pores process and improving the mechanical properties. The aim was to
understand the role that calcium plays in the geopolymer synthesis, and to define which reaction prevails under the synthesis
conditions: formation of geopolymer gel or calcium silicate hydrate that contains aluminum substitution (CASH). The synthesis
was performed with different raw materials (with or without red mud) and different alkalinity conditions. Ca(OH)2 was the
obligatory supplement to both of the mixtures. Different techniques were performed for the testing of reaction products, as well
as to define the microstructural changes as the generator of improved mechanical properties and changed electrical conductivity.
The characteristics of the geopolymer’s macrostructure were defined by means of an SEM analysis. Compressive strength and
electrical conductivity are among the investigated product’s properties. X-ray diffraction (XRD) and Fourier transform infra-red
spectroscopy (FTIR) were used for the identification of various crystalline phases and an amorphous phase.
Keywords: geopolymers, compressive strength, calcium hydroxide, electrical conductivity
Geopolimeri so nova obetavna vrsta materialov, ki kot vezivo strukturnega betona predstavljajo alternativo Portlandskemu
cementu. Ena od njihovih prednosti je, da so primarni vir za njihovo proizvodnjo odpadni alumosilikatni materiali iz razli~nih
industrij. Klju~no pri sintezi geopolimerov je njihov nizek nivo mehanskih lastnosti zaradi poroznosti, kakor tudi visoka
aktivnost nosilcev prevodnosti. Visoka aktivnost lahko ~esto privede do omejitev pri njihovi uporabnosti. Tako je glavni cilj
sinteze geopolimerov visoka trdnost in majhna tendenca pokanja, kar naj bi dosegli z modifikacijo mikrostrukture. Uvajanje
Ca(OH)2 pri dolo~eni pH vrednosti lahko privede do procesa zapolnitve por in s tem izbolj{anja mehanskih lastnosti. Namen
pri~ujo~ega prispevka je bil razumeti vlogo kalcija v sintezi geopolimerov, kakor tudi definiranje reakcij, ki prevladujejo pri
danih pogojih sinteze: tvorba geopolimernega gela in, ali kalcijev-silikatni hidrat vsebuje nadomestek za aluminij (CASH).
Avtorji so sintezo izvedli z razli~nimi surovinami (z rde~im blatom, ali brez njega) in pri razli~nih pogojih bazi~nosti. Obvezno
so Ca(OH)2 dodajali obema me{anicama. Uporabili so razli~ne tehnike testiranja reakcijskih produktov, kakor tudi za definiranje
mikrostrukturnih sprememb, ki so generator izbolj{anja mehanskih lastnosti in spremenjene elektri~ne prevodnosti.
Makrostrukturo geopolimerov so ovrednotili z SEM analizo. Kot glavni lastnosti preiskovanih produktov so dolo~ili njihovo
tla~no trdnost in elektri~no prevodnost. Rentgensko strukturno analizo (XRD) in Furierjevo transformacijsko infrarde~o
spektroskopijo so uporabili za indentifikacijo razli~nih kristalini~nih in amorfnih faz.
Klju~ne besede: geopolimeri, tla~na trdnost, kalcijev hidroksid, elektri~na prevodnost
1 INTRODUCTION
Aluminosilicate inorganic polymers, also called geo-
polymers, are materials formed under high alkali con-
ditions from aluminosilicate solid and alkali silicate
solutions.
1
Geopolymers are produced from natural raw
alumino-silicate materials and unconventional materials
such as industrial and natural wastes.
1–8
Metakaolin as a
dehydroxylation product of the industrial mineral kaolin
as well as red mud from the alumina production process
are recognized as such. Red mud is a highly alkaline,
aluminosilicate-containing by-product of alumina pro-
duction. Potential reuse of this waste as the precursor for
new class of materials is highly welcomed as the
eco-friendly solution of the large open disposals. Both
metakaolin and red mud can be used in the process of
alkaline activation as the inorganic precursors mainly
constituted of silica, alumina and a low content of
calcium oxide. Unlike other industrial processes (e.g.,
sol-gel, clinkerization, sintering processes), the alkali
activation process does not require expensive chemical
reagents, use of carbonate-based raw materials or high
temperature thermal treatments.
9
Materiali in tehnologije / Materials and technology 53 (2019) 3, 341–348 341
UDK 620.1:543:678:549.52:666.321 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(3)341(2019)
*Corresponding author e-mail:
ivabo@ucg.ac.me

The basic part of the geopolymerization process is
hardening, which is the base for the polycondensation of
alkali precursors formed through the dissolution of active
silicates and aluminosilicate solid materials in alkali
hydroxide. The polymeric network as the result of poly-
condensation process hardens rapidly acting as the
gluing component.
1
These are materials with the cross-
linked long inorganic chain between tetrahedral AlO
4
and SiO
4
units built in three dimensional structures
having good compressive strength, corrosion resistance,
resistance to extreme temperatures. The potential weak-
ness lies in the increased porosity, which depends on
synthesis and curing conditions. Porosity can be lowered
by the modification of microstructure provoking the
filling-of-pores-mechanism by inorganic or organic
modificators of the microstructure. In the aluminosilicate
geoploymer materials, the M
+
, water molecule and hyd-
roxyl are the most important factors to influence
electrical conductivity and dielectric property at room
temperature. The existent way of alkali ion M
+
in the
molecular structure of geopolymer materials is not
theoretically clear. Usually, the accustomed viewpoint is
that the alkali metal ions play a charge-balancing role or
are actively bonded into the matrix; hence the typical
geopolymer composition is expressed as nM
2
O·Al
2
O
3
·
xSiO
2
·yH
2
O.
9–11
2 EXPERIMENTAL PART
The following materials have been used in the
process of geopolymer synthesis:
– red mud from the alumina factory KAP Podgorica,
Montenegro (whose chemical content is shown in
Table 1.
Table 1: Chemical composition of red mud from Alumina factory
KAP Podgorica
Oxide SiO
2
Fe
2
O
3
Al
2
O
3
TiO
2
Na
2
O CaO
wt % 11.28 40.78 17.91 10.20 6.9 1.5
– sodium hydroxide of analytical grade (Merck),
– metakaolin, obtained by the calcination of kaolin at
650 °C for 2 h (commercial product used in the
production of welding electrodes Factory FEP,
Montenegro), whose chemical content is shown in
Table 2.
Table 2: Chemical composition of metakaolin obtained by calcination
of kaolin
Oxide SiO
2
Fe
2
O
3
Al
2
O
3
MgO Na
2
O CaO K
2
OT i O
2
ZnO
w/% 52.26 1.01 42.83 0.09 0.29 0.02 1.56 0.13 < 0.01
– sodium-silicate solution (Merck Na
2
O: SiO
2
= 3.4,
Na
2
O 7.5–8.5 %, SiO
2
25.5–28.5 % and d = 1.347
gc
–3
).
Two different raw mixtures were used in this re-
search: metakaolin (80w/%), red mud (15 w/%) and
Ca(OH)
2
(5 w/%); Si/Al mass ratio = 1.48 (the sample
marked as GPRM) as well as metakaolin (95 w/%) and
Ca(OH)
2
(5 w/%); Si/Al mass ratio = 2.36 (the sample
marked as GPM).
The other relevant process parameters (concentration
of liquid-phase components) were as follows: c(NaOH)
= 4 mol dm
–3
and 8 mol dm
–3
; mass ratio Na
2
SiO
3
/NaOH
= 2.5; mass ratio S/L = 1.45.
The specimens were molded into the cylindrical
containers (d=33mm;H = 17 mm), keeping closed at
25 °C for a duration of 3 h, followed by drying of the
specimens at 60 °C for a duration of2haswell as air
aging of the closed-in-mold specimens for a duration of
24 h. The open-mold-specimen specimens were aged for
(7, 14, 21 and 28) d.
All the synthesized samples were characterized by
X-ray diffraction analysis (XRD) using an Ultima IV
Rigaku diffractometer, equipped with Cu-K 1,2
radiation,
with a generator voltage of 40.0 kV and a generator
current of 40.0 mA. The 2 range of 5–80° was used for
all powders in the continuous scan mode with a scanning
step of 0.02° at a rate of 5°/min.
The functional groups of all considered samples were
studied using FTIR spectroscopy. Samples were
powdered finely and dispersed evenly in anhydrous
potassium bromide (KBr) pellets (1.5 mg/150 mg KBr).
Spectra were taken at room temperature using a Bomem
(Hartmann & Braun) MB-100 spectrometer set to give
undeformed spectra.
The microstructure analysis was performed on
Au-coated samples using a JEOL JSM 6390 LV electron
microscope at 25 kV coupled with EDS (Oxford Instru-
ments X-Max
N
).
The HCl extraction was used to remove both geo-
polymer gel and CSH and to monitor the amount of
reaction products (both geopolymer and CSH gel).
Geopolymer was attacked with 37 % HCl solution (1:20,
by volume). For every1go fsample 250 cm
3
of HCl
solution was added. The mixture was stirred for three
hours and filtered. Insoluble residue was dried at 100 °C
for 24 h and weighed.
The compressive strength of the tested samples was
determined after 28 d air aging at room temperature. Test
was performed on a HPN400 type press (ZRMK-Ljub-
ljana).
The electrical properties of the sintered samples were
measured by complex impedance method, in a frequency
range 10 μHz to 1 MHz, using Interface 1000 Poten-
tiostat/Galvanostat / ZRA and EIS300 Electrochemical
Impedance Spectroscopy Software. The measurements
were conducted in air, in the temperature range 300–700
°C, with a 50 °C increment.
I. BO[KOVI] et al.: CHARACTERIZATION OF RED MUD/METAKAOLIN-BASED GEOPOLYMERS ...
342 Materiali in tehnologije / Materials and technology 53 (2019) 3, 341–348

3 RESULTS AND DISCUSSION
3.1 XRD analysis
The XRD analysis of red mud (Figure 1) shows the
presence of hematite Fe
2
O
3,
gibbsite Al(OH)
3
, akdalaite
(Al
2
O
3
)
4
·H
2
O, lepidocrocite FeO(OH) and calcite CaCO
3
.
Figure 2 shows the XRD patterns of the geopolymer
samples GPRM1 (a), GPRM2 (b), GPM1 (c), GPM2 (d)
(GPRM1-c(NaOH) = 4 mol dm
–3
, GPRM2-c(NaOH) =
8 mol dm
–3
, GPM1-c(NaOH) = 4 mol dm
–3
, GPM2-c
(NaOH) = 8 mol dm
–3
). According to the identified XRD
patterns the mineral phases identified in the GPRM1 are
muscovite, quartz, and grossular. Also, for sample
GPRM2 muscovite and quartz appeared, but new phases
gibbsite and acmite were also found. The difference in
the mineral composition of these two geopolymer
samples may be due to different composition of starting
materials, red mud or metakaolin. In these samples a
hump which is characteristic for geopolymer materials is
poorly expressed and only appeared for samples which
are aged for 7 d. The structure of the geopolymers sam-
ples GPRM1 and GPRM2 are ordered with time because
of the high presence of crystalline phases. The hump
which is an indicator of amorphousness disappears. The
intensity of other peaks with aging time is increased
(Figure 2a and Figure 2b).
According to the identified XRD patterns the mineral
phases identified in the GPM1 and GPM2 are muscovite
and quartz (Figure 2c and Figure 2d). XRD analysis of
these geopolymer samples revealed their amorphous-like
structure with the position of an amorphous halo in the
range 22°–35°, which indicate short range ordering of
both samples with crystalline admixture of SiO
2
( -quartz). The hump that appears in the XRD patterns
I. BO[KOVI] et al.: CHARACTERIZATION OF RED MUD/METAKAOLIN-BASED GEOPOLYMERS ...
Materiali in tehnologije / Materials and technology 53 (2019) 3, 341–348 343
Figure 2: XRD patterns of the geopolymer samples: a) GPRM1, b) GPRM2, c) GPM1 and d) GPM2
Figure 1: XRD analysis of red mud from KAP Podgorica

of the GPM sample is sharper when in the geopolymeri-
zation reaction there was a higher molarity of NaOH.
3.2 FTIR analysis
FTIR analysis was done for samples that were aged
at room temperature for 28 d in open molds. FTIR analy-
sis of aforementioned specimens is shown on Figure 3
and Figure 4.
The major bands were a broad band at 3000–3500 cm
–1
(in our case, the maximum band width ranges
 3450 cm
–1
) and 1638 cm
–1
, which represent the stretch-
ing and deformation vibration of OH and H-O-H groups
from water molecules. These absorptions have been
attributed to both atmospheric and bound water in
geopolymers.
10,11
The bands at 1424 cm
–1
and 878 cm
–1
point to carbonate presence.
12
This is probably caused by
the reaction of atmospheric CO
2
with calcium hydroxide
(band at  878 cm
–1
) (sample GPRM1 and sample
GPRM2). Some authors reported that geopolymers are
focused on the behavior of Si-O-Si and Si-O-Al
bands.
10–13
Taking a straightforward approach, the identi-
fication of the position of the main Si-O-T (T=Si or Al)
asymmetric stretching vibration (the strongest band) is
here defined as the point of maximum absorbance in the
region 1250–950 cm
–1
and at 420–500 cm
–1
. These peaks
will be referred to as the "main band" in the spectrum of
the geopolymers. The main broad band at 1030 cm
–1
corresponding to asymmetric stretching vibrations of
Si-O-Si and Al-O-Si shifts toward lower frequencies
related to precursor materials (red mud/metakaolin) as a
result of the formation of new reaction products asso-
ciated with ongoing alkali activation. The bands located
at 794 cm
–1
and 778 cm
–1
are ascribed to bending
vibrations of Si-O-Si and O-Si-O bonds, implying the
presence of quartz that is hardly affected by alkaline
activation of precursors. Moreover, in both GPRM
spectra, peak due to stretching vibrations of the Fe-O
(460–560 cm
–1
range) was also present.
14
Due to the
symmetric stretch of the Si-O-Al framework, an addi-
tional peak in the position at 692 cm
–1
(Figure 3)w a s
observed.
For the GPM1 and GPM2 samples there are differen-
ces in the characteristic band position in relation to the
previous samples (Figure 4). All the band positions
corresponding to the above are shifted to smaller wave
numbers. A shift of the Si-O-X stretching band towards
lower wave numbers indicates lengthening of the Si-O-X
bond, reduction in the bond angle, and thus a decrease of
the molecular vibrational force constant.
15
In our case the
shift of the Si-O-X stretching band is evident: its position
is 1030 cm
–1
for GPRM and 1014 cm
–1
for GPM. Fur-
thermore, the shifting of the wavenumber of the Si-O-Si
stretching to the right (to a lower wavenumber) is indica-
tive of the breaking of Si-O bonds and the formation of
new Si bonds in the process of geopolymerization.
16
In
addition, new vibration band appear at 1478 cm
–1
. There
are no registered vibration bands in the area 770–800 cm
–1
.
Changes in the positions and intensities of the vibrating
bands in these two systems confirm that new alumosili-
cate phases are generated and that there is a difference
between them as a function of applied precursors (red
mud/metakaolin and metakaolin).
3.3 SEM analysis
The microstructure of the obtained structures is
presented in Figure 5. The microstructure of the sample
GPRM1 (Figure 5a) reveals a relatively homogeneous
matrix with a grained structure of the unreacted particle
over the matrix surface. The sample obtained by acti-
vation with 8-M NaOH seems to have dispersed platy,
different-in-shape-and-size particles (probably residual
red mud or metakaolin) (Figure 5b).
Figure 5c and Figure 5d show SEM micrographs of
metakaolin-based geopolymers (4 mol dm
–3
NaOH and
8 mol dm
–3
NaOH respectively). The surface of the
GPM1 sample (Figure 5c) has a more homogeneous
structure. The smaller particles are grouped along the
surface of the agglomerated particles. The addition of
Ca(OH)
2
in the case with less molarity of NaOH,
allowed the formation of CSH gel. The GPM1 sample
has the finest contours and surfaces indicative of the
I. BO[KOVI] et al.: CHARACTERIZATION OF RED MUD/METAKAOLIN-BASED GEOPOLYMERS ...
344 Materiali in tehnologije / Materials and technology 53 (2019) 3, 341–348
Figure 4: FTIR spectra: c) GPM1, d) GPM2
Figure 3: FTIR spectra: a) GPRM1, b) GPRM2

extent of dissolution and polycondensation that occurred
during geopolymerization for this mixture.
The EDS spectra of all the geopolymer samples
showed the presence of Al, Si, Na, Ca, Mg, K, Fe and O.
There is more Fe in the first two samples, because of the
hematite in the red mud.
3.4 Chemical extraction
Hydrochloric acid extraction was used to dissolve the
geopolymers. This extraction is a quantitative procedure
in which the extraction measures the amount of reacted
material (both geopolymer and CSH gel) and unreacted
material in the sample.
17
The remaining residue is a pre-
cursor (metakaolin). This method can also be combined
with the extraction with salicylic acid/methanol extrac-
tion which removes the CSH phase.
17
The amount retained from the HCl extraction varied
with different Si/Al ratios and accordingly different pH.
It was 34.89 w/% when Si/Al ratio was 1.48 w/% and
7.14 w/%, when Si/Al was 2.36. These results show that
during HCl extraction more metakaolin formed geopoly-
mer gel as the Si/Al ratio increased, which corresponds
to the finding that different values of pH generate
different reactions (CSH forming under low alkalinity
conditions) and geopolymer gel forming under the
higher alkalinity conditions.
3.5 Compressive strength
Compressive strength values are shown in Table 3.It
is observed that the compressive strength is higher for
samples obtained with a lower concentration of NaOH.
In addition, the highest value is achieved in the case
when metakaolin is used as the precursor.
Table 3: Compressive strength values of obtained geopolymers
Specimen
Specific density
(g cm
–3
)
Compressive
strength (MPa)
GPRM1 2.507 13.26
GPRM2 2.503 7.17
GPM1 2.254 27.35
GPM2 2.217 9.02
The presence of calcium affects the structure change
as well as the compressive strength. It is assumed that in
the presence of calcium, CSH gel is produced as the
main product, resulting in a homogeneous structure and
therefore the value of compressive strength is higher. In
conditions of less alkalinity, higher compressive strength
is obtained. Decreasing of the compressive strength at
higher concentrations of NaOH is explained by the fact
that the pH is high enough to result in the dissolution of
silicon from the raw material and the dissociation of
calcium hydroxide is prevented. Under such conditions
there is not enough calcium in the system to form the
calcium-silicate-hydrate (CSH) phase, but a geopolymer
gel is formed as a major product.
Red mud does not affect favorably the compressive
strength. The strength of the samples based on the red
mud is smaller compared to the samples based on the
metakaolin due to the presence of Fe and excess Na
concentration in the red mud.
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Materiali in tehnologije / Materials and technology 53 (2019) 3, 341–348 345
Table 4: The temperature dependence of total ionic conductivity ( ) of the synthesized GPRM and GPM samples
Composition
 ( –1
cm
–1
)
500 °C
 ( –1
cm
–1
)
550 °C
 ( –1
cm
–1
)
600 °C
 ( –1
cm
–1
)
650 °C
 ( –1
cm
–1
)
700 °C
E a
(eV)
GPRM1 3.22·10
–3
5.81·10
–3
6.23·10
–3
8.39·10
–3
9.15·10
–4
0.31
GPRM2 4.31·10
–3
6.53·10
–3
7.22·10
–3
8.63·10
–2
9.98·10
–4
0.26
GPM1 2.06·10
–4
3.87·10
–4
6.56·10
–4
8.2·10
–4
1.22·10
–4
0.47
GPM2 1.16·10
–4
3.12·10
–4
5.29·10
–4
7.57·10
–4
1.13·10
–4
0.48
Figure 5: SEM micrographs and EDS analysis: a) sample GPRM1, b)
sample GPRM2, c) sample GPM1 and d) sample GPM2

3.6 Electrical conductivity
The complex impedance method was used to study
the electrical properties of solid electrolytes.
18–20
Gener-
ally, this method enables obtaining information about the
separate contribution of the bulk and the grain-boundary
resistance, as well as electrode process. A typical
Nyquist plot of electrolyte in the solid state consists of
three semicircles: one semicircle at high and one semi-
circle at intermediate frequency (ascribed to bulk and
grain boundary), and a third semicircle at low frequency
(ascribed the electrode process contribution).
21
For the
high and intermediate frequency the semicircles are
characteristic of two serially connected RC circuits (one
resistive and one capacitive element bonded in a parallel
arrangement), where was capacitive element being distri-
buted (frequency dependent) one. Such an equivalent
circuit with both constant
22,23
and distributed capacitive
elements was applied widely in the literature, related to
the sintered ceramics. The high-frequency semicircle
may be attributed to a parallel connection of the bulk
resistance (R
b
) of crystallite grains, and the geometric
capacitance (C
g
) of the sample. The low-frequency semi-
circle may be attributed to the grain-boundary resistance
(R
ig
) in parallel connection with the intergranular
capacitance (C
ig
). In this case, by means of the frequency
which refers to the semicircle maximum, the intergra-
nular capacitance can be calculated using the well-
known equation:
w
R
C
max,ig
ig
ig
=⋅
1
(1)
In our work, the original Nyquist plots recorded in
the available frequency range (1 Hz–100 kHz) are pre-
sented in Figure 6. For the potential application in
Intermediate-Temperature Solid-Oxide Fuel Cells
(IT-SOFCs) the measurements of ionic conductivity of
the electrolytes in solid state of the geopolymer based on
red mud-metakaolin-Ca(OH)
2
(GPRM) and metakaolin
and Ca(OH)
2
(GPM) were done in temperature range of
500–700 °C, with increments of 50 °C. As can be seen
(Figure 6a and Figure 6b), with increasing the tempe-
rature, high and intermediate frequency semicircles
disappear. At higher temperatures, the time constants
associated with the bulk and grain-boundary impedances
are much lower than those associated with the electrode
interface. As a result, semicircles due to the bulk and
grain-boundary disappear (Figure 6b) and only a single
semicircle due to the electrode interfacial processes can
be observed. In this case, only the whole sum R
b
+R
ig
became readable and the values of the total resistance
were estimated from the cross-section obtained semicir-
cles with the real component of impedance (Z
real
).
This intercept is marked by arrows in Figure 6
(insets). New semicircles the formed in a low-frequency
region, being particularly visible in temperature range
600–700 °C (Figure 6b), almost doubtless originates
from the oxygen electrode reactions, O
2
/O
2–
, which does
not belong to the scope of this study.
The values of the ionic conductivity of the synthe-
sized samples are shown in Table 4. By comparing the
obtained results, it can be noted that the highest values of
ionic conductivity show the GPRM2 synthesized sample.
The actual values of the total ionic conductivity of the
mentioned sample at 700 °C amount to 0.0112 –1
cm
–1
.
Additionally, comparing the obtained results of ionic
conductivity with the literature data,
24
it can be noted
that the measured values are higher for the entire order
of magnitude. More specifically, the values of ionic con-
ductivity observed at 700 °C in this paper were similar to
literature values obtained at 900 °C. In addition, the
conductivity of the samples measured in this study is
similar to the literature data on similar oxygen ion
conductor.
19,24
It is assumed that in these materials the
ionic conductivity occurs mainly via O
–
interstitials with
preferential c-axis conduction, similar to O
2–
vacancy
migration in fluorite and perovskite-based electrolytes.
The temperature dependence of total ionic conducti-
vity is given in Table 4.
I. BO[KOVI] et al.: CHARACTERIZATION OF RED MUD/METAKAOLIN-BASED GEOPOLYMERS ...
346 Materiali in tehnologije / Materials and technology 53 (2019) 3, 341–348
Figure 6: Complex impedance plots of the GPRM2 synthesized
sample, measured at temperatures: a) 500–550 °C and b) 600–700 °C,
in air atmosphere. The working temperatures are indicated in each
diagram. The arrows indicate the points on the real axis corresponding
to the readings R
b
and R
b
+R
ig
.

According to the results of total ionic conductivity
( ) of the synthesized samples listed in Table 4, the
dependence log  = f(1/T) of the synthesized sample
GPRM2 is presented in Figure 7. Activation energies
(E
a
) were calculated from Arrhenius plots according to
the derived Equation (2):
ln( ) ln TA
E
kT
=−
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ a
(2)
The activation energy (E
a
) for the total conduction
synthesized samples is presented in Table 4. The values
of E
a
presented in our work are very low compared with
the activation energy for similar ion conductors.
18–24
It
can be said that this is a consequence of a well-ordered
structure and better processing of the obtained powders,
which allows easier activation of conductivity carriers
and decrease E
a
.
Thus, it must be pointed out that the impedance
spectra measured at temperatures above 500 °C are
reproducible, but several factors could affect the impe-
dance measurements, such as incomplete contact bet-
ween the sample and electrode, a short circuit through a
less resistive path in the samples and the different
presence of moisture in the samples. Especially, it should
be emphasized that the significant improvements in
conductivity are likely, which would lead to the very real
possibility of the application of geopolymer-type electro-
lytes in fuel cell and other applications.
4 CONCLUSIONS
The presence of calcium affects the change of the
structure as well as the compressive strength. In the
presence of calcium in the geopolymerization product it
is likely to have CSH. The results of the HCl extraction
showed that different values of pH generate different
reactions (CSH forming under low alkalinity conditions)
and geopolymer gel forming under the higher alkalinity
conditions.
Under the lower alkalinity conditions, the increased
compressive strength is obtained. Higher alkalinity con-
ditions do not favor the formation of CSH, but the
geopolymer gel. The results of the SEM analysis showed
that the number of pores and cracks is much smaller for
samples with lower alkalinity concentrations.
The compressive strength of the red-mud-based spe-
cimens is lower in comparison with the metakaolin
specimens, and goes along with the weak evidence of the
geopolymer formation. The highest value of compressive
strength (27.35 MPa) is achieved in the case when
metakaolin is used as the precursor. The SEM analysis
showed that the microstructure of that sample is almost
completely homogeneous.
In post-curing aging treatment specimens with a lot
of crystalline phases undergo the crystalline arrangement
during this time. The amorphous response in the range of
2 = 20–35° does not show significant changes during
the aging process of 28 d. The addition of calcium has
been observed to accelerate the hardening process.
FTIR spectra of two types of geopolymer samples
(based on red mud and metakaolin and only on meta-
kaolin) confirmed the formation of a new aluminosilicate
phase and changes in the positions and intensities of
vibrating bands due to different applied precursors.
As is well known, red mud has a high ionic concen-
tration. In the case of the presence of moisture, conduc-
tion occurs through the spaces (pores and cracks) filled
with water. The presence of Na
+
,O H
+
,K
+
ions in red
mud which are highly conductive contribute to a higher
conductivity when the sample loses moisture. Another
factor is the higher porosity of the specimen containing
red mud. This kind of structure allows easier activation
of the conductivity carriers and decreases Ea. Without a
deep insight into the microstructural features and a
structural analysis that could shed light, the role of the
different crystalline and amorphous phases responsible
for the ionic conduction mechanism of ionic conductivity
cannot be completely explained, but hypothesized.
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348 Materiali in tehnologije / Materials and technology 53 (2019) 3, 341–348