Glass-ceramic foams produced from zeolite-poor rock (Tokaj)

Abstract

This study evaluated the possibility of producing innovative glass-ceramic foams from zeolite-poor rock (Tokaj, Hungary) using alkali-activation and reactive sintering techniques. The composition and morphology of the samples were studied using X-ray diffraction, X-ray fluorescence, scanning electron microscope, and computed tomography techniques. The influence of various sintering temperatures on glass-ceramic foams was examined. It has been observed that zeolite-poor rock has a self-foaming capability. The heat treatment temperature affects the pore size and distribution as well as the technical characteristics of the obtained samples. The resulting glass-ceramic foams possess moderate thermal conductivity ranging from 0.11 to 0.17 W mK1 and good compressive strength (1.5–4.4 MPa). The produced samples might be utilized for thermal insulation, which would have both economic and environmental advantages.

Full text

Glass-ceramic foams produced from zeolite-poor
rock (Tokaj)
Jamal Eldin F. M. Ibrahim
1
p, Mohammed Tihtih
1
,
Emese Kurovics
1
, Ethem _
Ilhan S
¸ahin
2
, László A. Gömze
1
and
István Kocserha
1
1
Institute of Ceramics and Polymer Engineering, University of Miskolc, Miskolc-Egyetemváros,
Hungary
2
Advanced Technology Research and Application Center, Adana Alparslan Türkes
¸Science and
Technology University, Adana, Turkey
Received: March 28, 2022 •Revised manuscript received: July 15, 2022 •Accepted: August 1, 2022
ABSTRACT
This study evaluated the possibility of producing innovative glass-ceramic foams from zeolite-poor rock
(Tokaj, Hungary) using alkali-activation and reactive sintering techniques. The composition and
morphology of the samples were studied using X-ray diffraction, X-ray fluorescence, scanning electron
microscope, and computed tomography techniques. The influence of various sintering temperatures on
glass-ceramic foams was examined. It has been observed that zeolite-poor rock has a self-foaming
capability. The heat treatment temperature affects the pore size and distribution as well as the technical
characteristics of the obtained samples. The resulting glass-ceramic foams possess moderate thermal
conductivity ranging from 0.11 to 0.17 W mK
1
and good compressive strength (1.5–4.4 MPa). The
produced samples might be utilized for thermal insulation, which would have both economic and
environmental advantages.
KEYWORDS
zeolite-poor rock, ceramic foams, alkali activation, thermal conductivity
1. INTRODUCTION
Global energy consumption is predicted to jump in the upcoming decades due to the rapid
growth in population and urbanization [1]. Presently, 80% of the energy consumption is
supplied by fossil fuels which have harmful environmental implications like CO
2
emission,
global warming, and climate change [2,3]. Buildings are indeed the highest energy consumer,
accounting for around 40% of the overall global energy usage. Most of this energy is used for
heating and cooling the building to obtain internal thermal comfort in the buildings [4]. The
energy requirement for heating or cooling in the buildings could be lowered by employing the
passive thermal insulation method using materials like glass-ceramic foams [5]. Glass-
ceramic foams is a heterogeneous system comprising both solid and gas phases. The solid
matrix is made of ceramic material, whereas the gas fills the interior pores [6]. They are
typically manufactured by fusing tiny powdered ceramics or glass and foaming agents at
temperatures ranging from 800 to 1,000 8C[7]. Upon the sintering and when the temperature
exceeds the glass transition temperature, the mixture starts to soften, making a viscoelastic
material associated with the emission of the gas resulting from the foaming agent. As the heat
treatment temperature rises, the viscosity reduces, and the volume of gas grows under the
influence of increased internal pressure leading to a high degree of foam-ability [8]. However,
when the pressure reaches the critical value, the gas escapes from the pores leading to
emerging of neighboring pores in the bulk of the ceramic foams. Therefore, adjusting the
sintering parameters (heating rate, sintering temperature, residence time), particle size, and
Pollack Periodica •
An International Journal
for Engineering and
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DOI:
10.1556/606.2022.00641
© 2022 The Author(s)
ORIGINAL RESEARCH
PAPER
pCorresponding author.
E-mail: qkojamal@uni-miskolc.hu
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the foaming agent is of great importance to achieve the
product with the required technical properties [9]. The
glass-ceramic foams normally possess fascinating charac-
teristics, including high durability, low thermal conductivity,
lightweight, high corrosion resistivity, sound insulation, and
flame resistance. All these properties make glass-ceramic
foams a recommended choice over other thermal insulation
materials like polymeric materials [10–12]. However,
manufacturing glass-ceramic foams from primary glass is a
highly expensive operation since multiple heating periods
are necessary in this case: Firstly, sintering at 1,450–1,500 8C
is required to produce primary glass. Secondly, the glass and
foaming agent mixture is sintered at 800–900 8C to achieve
the desired cellular structure [13,14]. Therefore, the
manufacturing of new eco-friendly glass-ceramic foams that
are both highly effective and low-cost is gaining a lot of
attention lately. Cost-effective glass-ceramic foams can be
obtained by either recycling waste glass or using natural
materials, for instance, fly ash, red mud, natural zeolite, and
other waste [15–19].
Zeolite-poor rocks are volcanic rocks that comprise a low
proportion of zeolite (less than 10%) and additional com-
ponents like cristobalite, quartz, montmorillonite, and
possibly feldspar [20]. However, the low zeolite content of
these minerals limits their classical uses. Yet, zeolite-poor
rocks are great construction materials that may be utilized to
make bricks due to their good mechanical qualities [21–23].
Furthermore, it is a great alternative for making glass-
ceramic foam as it is readily available, offers a high silica
concentration, and is easy to mine. According to the liter-
ature, only a few research explored making glass-ceramic
foam exclusively from zeolite rocks. The majority of these
researchers, however, employed zeolite-rich rocks, which
possess lower silica content [17,24]. The availability of large
zeolite-poor rock resources in Hungary (Tokaj) prompted
this study to look into the possibility of using zeolite-poor
rocks to make glass-ceramic foams. As far as we can tell, no
previous work has been done on the production of glass-
ceramic foams using Tokaj zeolite poor-rock. It is highly
believed that the results of this paper will have a value-added
to the literature.
The goal of this study is to evaluate the possibilities of
making glass-ceramic foams entirely from zeolite-poor rock
(Tokaj) through alkali-activation and reactive sintering
techniques. The alkali-activated samples were sintered at a
temperature range of 850–950 8C and examined for their
mechanical, physical, and thermal properties. The ability to
produce glass-ceramic foams at a lower cost while still
meeting quality standards is especially compelling. This
might lower manufacturing costs and boost competitiveness.
2. MATERIALS AND METHODOLOGIES
2.1. Characterization methods
Zeolite-poor rock is mined from Tokaj Hill in northeastern
Hungary, which is utilized as a basic material for the
manufacturing of glass-ceramic foam. X-Ray Fluorescence
(XRF) spectroscopy was used to analyze the chemical
constitution. The X-Ray Diffractometer (XRD, Rigaku,
Miniflex II, Japan, 150 mA and 40 kV, CuKαradiation with
wavelength 1.5418 Å, step size of 0.018) was utilized to
investigate the qualitative and quantitative phases compo-
sition of the basic raw material and the manufactured glass
ceramic foams at 2θ55–908and scan speed of 108min
1
.
X’Pert HighScore Plus software was employed for the
computer-aided phase evaluation. The thermal characteris-
tics of alkali-activated zeolite-poor rock were studied using
ThermoGravimetry/Differential Thermal Analysis (TG/
DTA), (1750 SETARAM, Sestys evolution) from ambient
temperature to 1,200 8C with a heating speed of 5 8Cmin
1
at oxygen environment. The foaming capability of the alkali-
activated samples was examined using a heating microscope
(Camar Elettronica). Scanning Electron Microscopy (SEM,
EVO MA10, Carl Zeiss) and Energy Dispersive Analysis
X-Ray (EDAX Genesis) were used to investigate the
microstructure and elemental content of the fracture surface
of the prepared glass-ceramic foams. Moreover, the pore
structure of the produced sample was determined using
X-ray computed tomography (CT, YXLON FF35, Geminy).
The densities of the produced sample were calculated using
Archimedes approach. The thermal conductivity is evaluated
using a thermal conductivity analyzer (C-Therm TCi), and
the compressive strength is determined using a hydraulic
testing machine (INSTRON 5566).
2.2. Preparation method of the glass-ceramic foams
Glass-ceramic foams were made entirely from zeolite-poor
rock by means of the alkali activation and reactive sintering
procedures. Zeolite-poor rock powder was mixed with 15 wt
% NaOH dissolved in distilled water and oven-dried at
200 8C for 2 days. Next, the alkali-activated material was
milled for 20 min at 150 rpm in a planetary ball mill. The
milled powders were then compressed using uniaxial com-
pacting equipment under 18 MPa pressure to produce
cylindrical discs having a diameter of around 20 mm and a
height of around 10 mm. The manufactured pellets were
Fig. 1. XRD diffraction spectra of zeolite-poor rock
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fired in a high-temperature controlled kiln for 10 min at
temperatures ranging from 850 to 950 8C and 5 8C min
1
heating rate. The kiln was then switched off, and the sample
inside the kiln was left to cool to room temperature.
3. RESULTS AND DISCUSSION
3.1. Characterization findings of the raw materials
3.1.1. XRD and XRF investigations. The XRD pattern of
zeolite-poor rock powder is shown in Fig. 1. Different
minerals like cristobalite, quartz, montmorillonite, calcite,
and clinoptilolite are found in zeolite-poor rock powder
(Tokaj), as confirmed in Fig. 1. The quantitative phase
composition of the samples was determined by the full
profile fitting method.
Tables 1 and 2indicate the phase constituent and oxide
compositions of zeolite-poor rock resulted from XRD and
XRF, respectively. Silica is found to be the basic oxide with
alumina and other oxides in small amounts.
3.1.2. Thermal characteristics of alkali activated zeolite-
poor rock. Figure 2 shows the ThermoGravimetric and
Differential Thermal Analysis (TG/DTA) experimental
investigation of alkali-activated materials. A total weight loss
of 14.88% was detected that might be broken into three
weight loss periods. The first weight loss in the temperature
40–192 8C was 6.9% is coincided with the DTA curve’s
endothermic peak at 121 8C. The elimination of free water
from montmorillonite and adsorbed water from zeolite are
connected to this weight loss. At a temperature range of
190–385.2 8C, a second weight loss of 5.12 is recorded, which
can be attributed to the vaporization of the combined water
in the alkali-activated material. Finally, at temperatures
ranging from 380 to 803 8C, a weight reduction of 3.66% is
achieved, which may be ascribed to the combustion of
organic content and disintegration of the aluminosilicate
framework.
Figure 3 show the foaming capability of alkali-activated
zeolite-poor rock, which is carried out via heating micro-
scope in a temperature range of 25–950 8C. The maximum
expansion of the samples was inspected in the temperatures
Table 1. Phases constituent of Tokaj’s zeolite-poor rock as determined by XRD examination
Raw material
Phase composition
Total
Clinoptilolite Cristobalite Montmorillonite Quartz Calcite
Zeolite-poor rock 10.00 50.00 30.00 8.00 2.00 100
Table 2. Oxide composition of zeolite-poor rock acquired from XRF analysis
Raw material
Oxide content
Total
SiO
2
Al
2
O
3
MgO Na
2
O CaO CO
2
H
2
O LOI
Zeolite-poor rock 82.92 5.95 3.21 1.31 1.12 0.88 4.47 5.50 100
Fig. 2. DTA/TG profile of alkali-activated zeolite-poor rock
Fig. 3. Heating microscope photos of the alkali-activated zeolite-
poor rock before sintering and after sintering at different
temperatures
Fig. 4. Glass-ceramic foams showing the samples before sintering
and after sintering at different temperatures
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between 850 and 950 8C. At this temperature, the alkali-
activated materials decompose, corresponding to a low-vis-
cosity state, and are transferred into a semi-liquid phase
associated with the decomposition of calcite that produces
gas. The emitted gas pushed the viscous material and
resulted in the formation of cellular porous ceramics with
lightweight and good thermal insulation properties.
3.2. Characterization findings of the sintered
glass-ceramic foams
3.2.1. Dimensional behavior after heat treatment. The
dimensional variations of the several samples burnt at
various temperatures are depicted in Fig. 4. As the sintering
temperature rises from 850 to 950 8C, the pores expand and
merge with the microscopic pores that surround them,
lowering the surface energy of the system. The increase in
the pore size could affect the technical characteristics of the
samples.
Fig. 5. XRD analysis of zeolite-poor rock powder, alkali-activated
powder, and sintered samples (Z: zeolite-poor rock, ZS: zeolite-
poor þsodium hydroxide, M: montmorillonite; CL: clinoptilolite;
C: cristobalite; Q: quartz; CA: calcite; H: heulandite A: anorthite)
Fig. 6. SEM images of the fracture surface of a) alkali-activated, and sintered samples at b) 850 8C, c) 900 8C and d) 950 8C
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3.2.2. XRD investigations. Figure 5 shows the XRD spectra
of samples heated at different temperatures. The reaction of
zeolite-poor rock components with NaOH resulted in the
creation of heulandite (Si
36
Al
1.21
Na
1.52
O
97.84
, PDF# 96-900-
2185) after alkali activation. Firing at 850 8C produced an
amorphous phase and anorthite (Na
1.92
Ca
2.08
Si
10
A
l6
O
32
,
PDF# 96-100-8758). At a temperature of 950 8C, anorthite
decomposes into a completely amorphous phase.
3.2.3. SEM investigation. SEM images of the alkali-acti-
vated and sintered samples are shown in Fig. 6. The alkali-
activated samples confirm the formation of a whiskers-like
structure. All heat-treated samples demonstrate porosity,
which is uneven in size and distribution. The pores seen are
essentially round-like in form and depicted in the photos by
various color differences. The detected different pore size
and distribution is thought to be caused by interconnected
cells. In general, porosity becomes larger with increasing
temperature. This happens due to the reduction in viscosity
and increase in the internal pressure of the produced gas
that induces the pore coalescence.
3.2.4. CT scan analysis and pore structure characteris-
tics. Figure 7 shows the CT scan images of the glass-
ceramic foam sintered at 850 8C. The 3D picture was created
by overlaying 1,000 binary photos along the z-axis. Because
X-rays are absorbed differently by materials of various den-
sities, the darker area in the 3D image represents the solid
portion of the sample, whereas the bright spots indicate the
pores. In contrary, the darker area in the top view image
represents the pores, while the lighter areas indicate the solid
material. It can be seen clearly the formation of semi-spherical
pores of different sizes. Most of the large pores are sur-
rounded by smaller pores, and the cell walls can be identified.
Pore size and wall thickness have a substantial influence on
mechanical properties; smaller pore sizes and broader cell
walls are desirable for greater mechanical strength.
3.2.5. Technical properties. The connection among
compressive strength, density, and thermal conductivity of
the generated foams is depicted in Fig. 8. Because the
compact material has fewer pores, increasing the density
increases thermal conductivity and compressive strength. In
contrast, increasing the porosity decreases thermal conduc-
tivity since the gaps are generally filled with gases that
function as thermal insulators.
4. CONCLUSION
This research revealed that zeolite-poor rock (Tokaj) might
be used as raw material to make glass-ceramic foams
through alkali-activation and reactive sintering process. The
calcite that exists in zeolite-poor rock composition acts as a
Fig. 7. CT scan images of the foam heat-treated at 850 8C a) top view image and b) 3D image
Fig. 8. The correlation between bulk density, thermal conductivity
and compressive strength of the glass-ceramic foams sintered at
variable temperatures
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foaming agent, which decomposes at higher temperatures
and produces gas leading to cellular structure formation.
The alkali-activation leads to the formation of low-temper-
ature melting compounds. The XRD and SEM analysis
confirmed the formation of heulandite in a whiskers-like
structure. The sintered samples showed good foamability in
the temperature range of 850–950 8C. Increasing the sin-
tering temperature resulted in large pores formation and
transformation of crystalline phases to completely amor-
phous structure, which lowered the compressive strength.
The obtained samples have a low density (0.6–0.7 g cm
3
),
moderate thermal conductivity (0.11–0.17 W mK
1
), and
good compressive strength (1.5–4.4 MPa). These satisfying
technical characteristics confirm the potential use of the
produced glass-ceramic foams for thermal and sound
insulation.
ACKNOWLEDGMENTS
The described study was carried out as part of the EFOP-
3.6.1-16-2016-00011 “Younger and Renewing University –
Innovative Knowledge City –institutional development of
the University of Miskolc aiming at intelligent specialisa-
tion”project implemented in the framework of the Szeche-
nyi 2020 program. The realization of this project is
supported by the European Union, co-financed by the Eu-
ropean Social Fund.
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