N. H. THANG et al.: SYNTHESES AND CHARACTERISTICS OF CALCIUM-BASED GEOPOLYMER FROM SOLAR-CELL ...
467–475
SYNTHESES AND CHARACTERISTICS OF CALCIUM-BASED
GEOPOLYMER FROM SOLAR-CELL PANEL-GLASS WASTE BY
HYDROTHERMAL METHOD
SINTEZE IN ZNA^ILNOSTI GEOPOLIMERJA NA OSNOVI
KALCIJA IZ STEKLENIH ODPADKOV SOLARNIH CELIC S
HIDROTERMALNO METODO
Nguyen Hoc Thang
1*
, Nguyen Quang Binh
1
, Nguyen Van Phuc
1
,
Pham Trung Kien
2,3,4
1
Faculty of Chemical Technology, Ho Chi Minh City University of Industry and Trade (HUIT), Ho Chi Minh City, 70000, Vietnam
2
Faculty of Materials Technology, Ho Chi Minh City University of Technology (HCMUT), 268 Ly Thuong Kiet Street,
District 10, Ho Chi Minh City, Vietnam
3
Vietnam National University Ho Chi Minh City, Linh Trung Ward, Thu Duc City, Ho Chi Minh City, Vietnam
4
Polymer Research Center, Ho Chi Minh City University of Technology (HCMUT), 268 Ly Thuong Kiet Street,
District 10, Ho Chi Minh City, Vietnam
Prejem rokopisa – received: 2024-04-13; sprejem za objavo – accepted for publication: 2024-06-18
doi:10.17222/mit.2024.1153
A calcium-based geopolymer was synthesized using a blend of recycled glass powder from solar panels (PV glass waste), lime-
stone, and a sodium silicate solution, which underwent hydrothermal autoclaving at 180 °C for varying durations. This material
is regarded as environmentally friendly and has garnered research attention worldwide. In this investigation, limestone served as
the calcium source, while recycled glass from solar panels provided the SiO2 necessary for producing calcium-based
geopolymer materials. Currently, solar-panel waste poses a significant environmental challenge that requires attention. The ob-
jective of this research was to develop a sustainable and high-performance calcium-based geopolymer using waste materials,
specifically recycled glass from solar panels and limestone. The study aimed to evaluate the effects of hydrothermal autoclaving
on the compressive strength, volume weight, porosity, and water absorption of the synthesized geopolymer, as well as to under-
stand the microstructural transformations involved. The results revealed a remarkable 430 % increase in the compressive
strength of the specimens following hydrothermal autoclaving for 24 to 96 hours compared to the unautoclaved samples. Con-
currently, the volumetric weight had a substantial rise from 1.54 % to 2.31 g/cm
3
, with corresponding decreases in the porosity
and water absorption from 38.1 % to 16.9 % and 10.26 % to 5.08 %, respectively. An X-ray diffraction (XRD) analysis of the
mineral composition and a scanning electron microscope (SEM) examination of the microstructure demonstrated a transforma-
tion of all samples from an amorphous gel structure to needle- or spike-shaped fibrous, and then sheet-like CSH structures.
These new structures exhibited dimensions of less than 10 μm in length, less than 2 μm in width, and less than 400 nm in thick-
ness. The resulting CSH products constitute calcium silicate hydrate minerals, characteristic of calcium silicate materials, which
are synthetic products arising from chemical reactions among SiO2, CaO, and H2O under hydrothermal conditions. This study
underscores the potential of using recycled materials to produced high-strength geopolymers, offering a promising solution to
the environmental challenge of solar-panel waste.
Keywords: calcium-based geopolymer, solar photovoltaic panel, glass waste, hydrothermal method, calcium silicate
Avtorji so geopolimere sintetizirali z uporabo recikliranega odpadnega steklenega prahu iz son~nih fotovoltai~nih panelov,
apnenca in raztopine natrijevega silikata. Me{anico so izpostavili v avtoklavih hidrotermalni obdelavi pri 180°C. Te vrste stekla
mnogi raziskovalci smatrajo kot okoljsko prijazen material in zato ga po svetu tudi na~rtno zbirajo. V raziskavi, ki jo opisuje ta
~lanek so avtorji uporabili apnenec kot vir kalcija, medtem ko je reciklirano steklo iz son~nih panelov slu`ilo kot vir SiO2,kise
potrebuje za izdelavo geopolimernih materialov. V tem trenutku odpadni son~ni paneli predstavljajo dolo~en okoljski izziv
glede njihove nadaljne uporabe oziroma reckliranja. Predmet te raziskave je torej bil: razviti trajen in visoko kakovosten
geopolimer na osnovi kalcija (Ca) iz recikliranega odpadnega stekla solarnih panelov in apnenca (CaCO3). [tudija je bila
osredoto~ena na u~inek hidrotermalne obdelave v avtoklavu na tla~no trdnost, masni volumen, poroznost in absorpcijo vode
sintetiziranih geopolimerov, kakor tudi razumeti in pojasniti mikrostrukturne transformacije, ki potekajo med hidrotermalno
obdelavo v avtoklavu. Rezultati analiz so pokazali, da je pri{lo do znatnega, postopoma tudi do 430 %-nega pove~anja tla~ne
trdnosti biopolimernih vzorcev, ki so bili avtoklavirani od 24 do 96 ur v primerjavi z neavtoklaviranimi vzorci. Nadalje se je tudi
masni volumen vzorcev izrazito pove~al z 1,54 g/cm
3
na 2,31 g/cm
3
. Skladno s tem sta se zmanj{ali poroznost vzorcev z 38,1%
na 16,9% in absorpcija vode z 10,26% na 5,08%. Rentgenska strukturna analiza (XRD; angl.: X-ray diffraction) mineralne
sestave in morfolo{ke preiskave z vrsti~nim elektronskim mikroskopom (SEM) so pokazale transformacijo amorfne gelne
strukture vseh vzorcev v igli~asto ali koni~asto vlaknasto oblikovano in nato plo{~i~asto C-S-H strukturo. Te nove strukture so
izredno majhne, dol`ine pod 10 μm, {irine manj kot 2 μm in debeline manj kot 400 nm. Rezultirajo~i C-S-H produkti so
sestavljeni iz Ca-Si hidratnih mineralov ter karakteristi~nih Ca-Si materialov, ki so sinteti~ni produkti Ca-Si materialov in
izvirajo iz kemi~nih reakcij med SiO2, CaO, in H2O v pogojih hidrotermalne obdelave. S to {tudijo so avtorji potrdili potencial
recikliranih materialov iz odpadnih solarnih panelov za okolju prijazno izdelavo geopolimerov z visoko trdnostjo.
Klju~ne besede: geopolimer na osnovi kalcija, solarni fotovoltai~ni paneli, stekleni odpadki, hidrotermalna obdelava, kalcijev
silikat
Materiali in tehnologije / Materials and technology 58 (2024) 4, 467–475 467
UDK 66.095.26:621.311.243 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(4)467(2024)
*Corresponding author's e-mail:
thangnh@huit.edu.vn (Nguyen Hoc Thang)

1 INTRODUCTION
The solar panels available on the market, commonly
referred to as photovoltaic (PV) panels, are crafted from
crystalline silicon (c-Si), constituting more than 95 % of
the total production as of 2023 and holding nearly 90 %
of the market share over the past two decades. At the end
of their operational lifespan, c-Si photovoltaic panels are
anticipated to become a significant source of photovol-
taic waste.
1,2
These PV panels typically have a lifespan
estimated at 25–30 years.
3
Projections suggest that by
2030, the waste generated by PV panels may amount to
approximately 1.7–8.0 million tons, with an anticipated
escalation to 60–78 million tons by 2050.
4
According to
reports, each solar panel weighs approximately
21,036 kg, with the tempered glass panel constituting
13.2 kg, equivalent to 62.75 % of the total weight. Con-
sequently, the projected glass waste from solar panels by
2030 range from 1.07–5.02 million tons, with a global
increase to 37.65–48.95 million tons by 2050. This sub-
stantial waste output poses significant environmental and
economic challenges worldwide, prompting researchers
to devise technology systems aimed at efficiently recy-
cling photovoltaic panels.
5
The studies on glass recycling
showed that the tempered glass waste in solar panels
contains amorphous SiO
2
, exceeding 70 %, presenting a
promising avenue for research into recycling this type of
waste glass as a raw material for producing products
such as geopolymer or calcium silicate for applications
in construction materials.
6
Presently, there are alarming concerns regarding the
environmental pollution stemming from waste, emis-
sions, and global warming induced by the greenhouse ef-
fect.
7
One significant contributor to the elevated levels of
CO
2
in the atmosphere is the conventional cement pro-
duction industry.
8
The combustion of fossil fuels such as
coal, oil, or natural gas for the calcination of limestone
and other raw materials results in the emission of CO
2
,as
depicted in the following chemical reactions.
7–9
:
Burning coal: C + O
2
=> CO
2
(1)
Burning petroleum and gas:
2C
x
H
y
O
z
N
t
+ (2x+y/2-z)O
2
=> 2xCO
2
+yH
2
O+tN
2
(2)
Limestone calcination: CaCO
3
=> CaO + CO
2
(3)
The amount of CO
2
discharged into the atmosphere
has surged in recent decades, exacerbating global warm-
ing. Consequently, the imperative to mitigate CO
2
emis-
sions as a response to environmental challenges has in-
tensified.
10
A primary source of CO
2
emissions within
the construction materials sector, particularly in cement
production, stems from the high-temperature sintering of
limestone to synthesize CaO and others, as outlined ear-
lier.
11
In efforts to curtail CO
2
emissions within the con-
struction materials industry, numerous studies have ex-
plored the geopolymerization method.
12,13
This technique
involves utilizing alumino-silicate or calcium silicate in
an alkaline environment to solidify raw powder at lower
temperatures.
12,14
Furthermore, some studies have dem-
onstrated the feasibility of effectively curing calcium
carbonate powder through hydrothermal autoclaving.
15
Geopolymer is an environmentally friendly material
with many superior properties to replace cement-based
materials. These properties include high mechanical
strength, excellent heat resistance, chemical durability,
and the capacity to immobilize heavy metal ions.
14,16,17
Moreover, geopolymer offers the advantage of utilizing
recycled raw materials sourced from inorganic waste rich
in SiO
2
,A l
2
O
3
, and CaO, facilitating a straightforward
manufacturing process with minimal energy consump-
tion and negligible CO
2
emissions.
18
Various studies have
explored hydrothermal autoclaving at temperatures be-
low 200 degrees Celsius and the curing properties within
the xCaO·ySiO
2
·nH
2
O system.
19,20
This process instigates
the formation of new phases, such as tobermorite gel and
calcium silicate hydrate (CxSy·nH gel), which augment
the durability of hydrothermally cured materials during
the formation of xCaO·ySiO
2
·nH
2
O.
21,22
The hydrother-
mal reaction process, conducted at relatively low temper-
atures, proves to be an effective method for curing calcite
powder without triggering a decomposition into CO
2
at
this temperature.
23
This research conducted geopolymerization of a mix-
ture of PV glass waste from solar panels and limestone,
with the addition of sodium silicate binders. Then, speci-
mens were cured under hydrothermal autoclaving condi-
tions for different reaction times. Notably, there have
been no other studies testing the production of construc-
tion materials from limestone combined with PV glass
waste from solar panels using both the geopoly-
merization method and hydrothermal method during the
curing of specimens.
2 EXPERIMENTAL PART
Tempered glass was separated from solar panels from
Solar Power Vietnam Technology Joint Stock Company,
Ho Chi Minh City, Vietnam. Initially, a heat gun was em-
ployed to heat the solar panel surface, facilitating the
manual separation of glass from the EVA adhesive layer.
The separated materials encompassed fragments of PV
glass waste, solar cells, wires, and aluminum frames.
Subsequently, the glass waste fragments were being
ground in a ball mill for 6 hours. After grinding, the PV
glass waste was sieved through a 90 μm mesh to ensure a
uniform particle size of the PV glass waste powder while
eliminating EVA glue residue as shown in Figure 1 (on
the left).
Limestone was from FICO Tay Tinh Cement Joint
Stock Company (Tay Ninh province, Vietnam) and was
preliminarily crushed to a size of less than 5 mm, then
dried at 110 °C for 24 h. Finally, limestone powder was
obtained via grinding in a ball mill for 5 h and passing
through a 90 μm sieve as shown in Figure 1 (at the cen-
tre).
N. H. THANG et al.: SYNTHESES AND CHARACTERISTICS OF CALCIUM-BASED GEOPOLYMER FROM SOLAR-CELL ...
468 Materiali in tehnologije / Materials and technology 58 (2024) 4, 467–475

Sodium silicate solution or water glass solution is an
inorganic binder, which was used to increase the shaping
ability. It had a silica modulus of 2.41, and a density of
1.47 g/cm
3
, and was provided by New Ha Viet Silicate
Joint Stock Company (Binh Duong province, Vietnam).
The powders of limestone and PV glass waste were
tested for physico-chemical properties such as density,
volumetric weight, moisture, loss on ignition (L.O.I),
chemical composition, and mineral composition. The
raw materials were mixed dry in a 1:1 ratio (in weight)
of limestone and PV glass waste for 10 min and then this
mixture was mixed with a 10 % water glass solution for
15 min; 30 % water was added and mixed for 15 min to
get a paste. The paste mixture was cast into a mold mea-
suring (20 × 20 × 20) mm, kept there for 24 h to harden
and dry before removing the mold as shown in Figure 1
(on the right). After de-molding, the samples were classi-
fied in two groups, including non-autoclaved specimens
(sample M0) and autoclaved specimens. The autoclaved
samples were autoclaved in a hydrothermal reactor at a
temperature of 180 °C for periods of 24 h (sample M24),
48 h (sample M48), 72 h (sample M72) and 96 h (sample
96). After the process in hydrothermal conditions, all the
specimens (samples M0, M24, M48, M72, and M96)
were tested for engineering properties such as compres-
sive strength, volumetric weight, water absorption, po-
rosity and characterized microstructural properties using
XRD (Brucker D2 Phaser X-ray diffractometer) and
SEM (JEOL JSM-IT200 Instrument). The D2 Phaser
X-ray diffractometer was set at the analytical conditions
as follows: an operating temperature of 25 °C, a mea-
surement time per step of 38.4 s, a diffraction angel of
2-Theta of 5–80 degrees, using a copper (Cu) anode ma-
terial with  K
 of 0.15406 nm and others. The JEOL
JSM-IT200 scanning electron microscope operated at a
beam voltage of 10 kV, using a secondary electron detec-
tor.
3 RESULTS
3.1 Physico-chemical properties of raw materials
Raw materials including PV glass waste and lime-
stone were tested for physical and chemical properties
such as density, volumetric weight, moisture content,
loss in ignition (L.O.I), chemical composition, and min-
eral composition, and the results are shown in Table 1,
Table 2, and Figure 2. The chemical composition of the
materials was analyzed by X-ray fluorescence (XRF) us-
ing a Brucker S2 PUMA device, while the mineral com-
position was characterized by X-ray diffraction (XRD)
using a D2 Phaser device from Brucker. Limestone had a
moisture content of 2.73 %, volume weight and density
of 1.84 g/cm
3
and 2.72 g/cm
3
, respectively. Meanwhile,
PV glass waste had a moisture content of 1.2 %, volu-
metric weight of 1.58 g/cm
3
, and density of 2.83 g/cm
3
as
shown in Table 1.
Table 1: Engineering properties of limestone and PV glass waste
No. Engineering properties Limestone PV glass waste
01 Density (g/cm
3
) 2.72 2.83
02
Volumetric weight
(g/cm
3
)
1.84 1.58
03 Moisture content (%) 2.73 1.21
04 Mean particle size (μm) 36.1 52.8
In Table 2, the results of the chemical composition
analysis by X-ray fluorescence show that the PV glass
waste powder included 70.1 % SiO
2
, 11.3 % Na
2
O,
11.4 % CaO, 3.2 % MgO, 1.9 % Fe
2
O
3
,1.6%Al
2
O
3
and
N. H. THANG et al.: SYNTHESES AND CHARACTERISTICS OF CALCIUM-BASED GEOPOLYMER FROM SOLAR-CELL ...
Materiali in tehnologije / Materials and technology 58 (2024) 4, 467–475 469
Figure 1: Raw materials including PV glass waste (a) and limestone (b), and calcium-based geopolymer (c)
Figure 2: XRD patterns of limestone and PV glass waste

a small amount of about 0.3 % loss on ignition (LOI).
The LOI test was performed at a temperature of 1000 °C
and maintained for 2 h. The heating rate from room tem-
perature to 1000 °C was 5 °C/min, with the cooling pro-
cess occurring naturally in accordance with the furnace’s
normal temperature decrease. The LOI value of the recy-
cled PV glass was 0.3 %, attributable to the residual glue
left over from pre-processing and grinding of the solar
panels. Additionally, the recycled PV glass contained up
to 11.3 % Na
2
O alkalinity. When calcined at 1000 °C, a
portion of this alkalinity also partially evaporated. Mean-
while, limestone exhibited a CaO content of 55.6 % and
loss on ignition of up to 43.9 % due to carbonate decom-
position, emitting CO
2
. The results of this analysis show
that both PV glass waste and limestone had a high cal-
cium silicate content, thus meeting the conditions for the
formation of calcium silicate hydrate phases during the
geopolymerization process combined with hydrothermal
reactions in an autoclave. When limestone and PV glass
waste powder were mixed in a ratio of 1:1 and 10 % so-
dium silicate solution was added, the mixture included
33.5 % CaO and 38.2 % SiO
2
, which was almost equiva-
lent to the 1:1 ratio of CaO and SiO
2
. This is the ideal
condition for synthesizing the wollastonite mineral
(CaSiO
3
) according to the CaO-SiO
2
phase diagram sys-
tem.
7,24
XRD patterns show that the PV glass waste powder
contained mainly an amorphous phase with a noise spec-
trum and a broadened peak at the bottom in a diffraction
angle range of 10–30 degrees. Meanwhile, limestone
contained the main crystal mineral of calcite (CaCO
3
)
with diffraction peaks at 23.5°, 30.0°, 36.4°, 40.0°,
43.7°, 48.0°, 49.0° and other positions as shown in Fig-
ure 2. The analytical results of the raw materials are con-
sistent with the previous reports related to PV glass
waste and limestone.
3,4,6,8
3.2 Engineering properties of the calcium-based
geopolymer
The calcium-based geopolymer samples were tested
for compressive strength, water absorption, volumetric
weight and porosity before and after hydrothermal auto-
claving, and the results are shown in Figures 3 and 4.
Based on Figure 3, the compressive strength of the spec-
imens increased intensely after the hydrothermal reaction
at 180 °C. The non-autoclaved sample (M0) had a com-
pressive strength of 16.2 MPa. The compressive strength
of the sample exposed to the hydrothermal reaction for
24 h (M24) increased 2.2 times, to 35.8 MPa. The hydro-
thermal reaction sample after 48 h (M48) exhibited a
compressive strength that was up to 4 times higher,
65.4 MPa. The compressive strength of the samples ex-
posed to steaming for 72 h (M72) and 96 h (M96) in-
creased slightly compared to the sample steamed for
48 h, i.e., the compressive strength of sample M72 was
68.5 MPa, and the compressive strength of sample M96
was 69.7 MPa. The values of the compressive strength in
this study are higher than those in the previous investiga-
tions.
7,12,14,17,22
This proves that when autoclaved in the hydrothermal
reactor, the geopolymer-based materials continued react-
ing to form a new structure causing their compressive
strength to become higher than that of the non-auto-
claved sample. However, when autoclaved for too long,
these structures were relatively saturated so their strength
N. H. THANG et al.: SYNTHESES AND CHARACTERISTICS OF CALCIUM-BASED GEOPOLYMER FROM SOLAR-CELL ...
470 Materiali in tehnologije / Materials and technology 58 (2024) 4, 467–475
Table 2: Chemical compositions of limestone, PV glass waste, and sodium silicate solution
Raw materials
Chemical composition of oxides (w/%)
SiO2
Na
2
O CaO Al
2
O
3
MgO Fe
2
O
3
Other L.O.I
PV glass waste 70.1 11.3 11.4 1.6 3.2 1.9 0.2 0.3
Limestone – – 55.6 – 0.3 – 0.2 43.9
Sodium silicate solution 31.8 12.6 –––– 55.6
(*)
–
(*) This value is the amount of water added to sodium silicate to get a solution.
Figure 4: Changes in the volumetric weight and porosity after various
hydrothermal reaction times for calcium-based geopolymer synthe-
sized from limestone and PV glass waste
Figure 3: Changes in the compressive strength and water absorption
after various hydrothermal reaction times for calcium-based
geopolymer synthesized from limestone and PV glass waste

did not increase as significantly as before. This is also
shown through the density of the samples, with an in-
crease in the volumetric weight from 1.54 g/cm
3
up to
2.31 g/cm
3
, and a decrease in the porosity from 38.1 %
(M0) to only 16.9 % (M72). The water absorption of the
specimens also improved from 10.26 % (M0) to 5.08 %
(M72). This shows the reasonableness of the results. It is
noted that when increasing the hydrothermal reaction
time from 72 h to 96 h, there was a slight increase in the
porosity from 16.9 % to 17.4 % and water absorption
also increased from 5.08 % to 5.22 %. These results are
explained in detail through a microstructural analysis us-
ing imaging results of SEM and X-ray diffraction
(XRD). The porosity of calcium-based geopolymer in
this research changed similarly to what was mentioned in
the study by Maeda et al. (2011).
25
3.3 Microstructural characteristics of calcium-based
geopolymer
3.3.1 Effect of hydrothermal reaction times on the
structure formation of the products observed with
scanning electron microscopy (SEM)
Figure 5 shows SEM images of sample M0 (the
non-autoclaved sample) with a geopolymeric matrix
made of particles of PV glass waste powder and lime-
stone. There is very tight adhesion between them, with
no clear boundaries between the particles. Simulta-
neously, there are very small and short spike- or nee-
dle-shaped structures with a width of about 50–100 nm
and a length of about 1 μm. This is the result of the dis-
solution and reaction between the powders of PV glass
and limestone in the sodium silicate solution.
Figure 6 shows SEM images of sample M24 (the
sample was reacted in hydrothermal conditions at 180 °C
for 24 h) with a difference in the structure of the cal-
cium-based geopolymer after hydrothermal autoclaving
at 180 °C for 24 h compared to the same sample without
the hydrothermal reaction (sample M0). The spike- or
needle-shaped structures grew in size very quickly with a
width of about 200–400 nm and a length greater than 3
μm (the structural size increased 3 times compared to
that of sample M0). These structures changed into a fi-
ber-like shape with length and width but they are rela-
tively thin (their thickness is really small and difficult to
observe). This explains why sample M24 had a 2.2 times
higher compressive strength compared to that of sample
M0. However, the fiber-like shape structures were still
distributed sparsely and unevenly as shown in Figure 6.
N. H. THANG et al.: SYNTHESES AND CHARACTERISTICS OF CALCIUM-BASED GEOPOLYMER FROM SOLAR-CELL ...
Materiali in tehnologije / Materials and technology 58 (2024) 4, 467–475 471
Figure 6: Fiber-like shaped structures of calcium-based geopolymer
(sample M24) at magnifications of: a) 5000× and b) 10000×
Figure 5: Needle-shaped structures of calcium-based geopolymer
(sample M0) at magnifications of: a) 5000× and b) 10000×

Figure 7 shows SEM images of sample M48 (the
specimen was reacted in hydrothermal conditions at
180 °C for 48 h) with a fibrous structure. It developed a
denser and more uniform structure than that of sample
M24. The width of this structure increased to
500–700 nm and its length increased to about 5–6 μm.
Thus, when increasing the hydrothermal reaction time,
the dissolution and reaction processes were more vigor-
ous. Many structures allowing an increase in the me-
chanical strength were formed and stabilized. Therefore,
specimen M48 became denser (reduced porosity) and its
compressive strength was 65.4 MPa, much higher than
that of samples (M0) or (M24).
Figure 8 shows SEM images of sample M72 (the
specimen was autoclaved in hydrothermal conditions at
180 °C for 72 h) with a growth of fibrous structures.
These became sheet- or plate-like structures with a
length of about 8 μm, a width of about 1–2 μm, and a
thickness of about 100–200 nm. They are more numer-
ous, dense, and uniform, occupying most of the struc-
tural space of the sample (it is difficult to recognize
where the original matrix of material particles are). Thus,
when increasing the hydrothermal reaction time, the dis-
solution and reaction processes were more vigorous. The
plate-like structures play a role in stabilizing the
mechanical strength, and also show the void space that
appears among them. Therefore, the specimen exhibited
a slight increase in the volume weight and a slight de-
crease in the porosity compared to sample M48, but
these values did not change much in comparison with the
change among samples M0, M24 and M48. The com-
pressive strength of sample M72, which was 68.5 MPa,
increased slightly compared to that of sample M48,
which was 65.4 MPa.
Figure 9 shows SEM images of sample M96 (the
sample was autoclaved in hydrothermal conditions at
180 °C for 96 h) with sheet-like or plate-like structures
that developed very quickly, overlapping and alternating
into layers. The sheets became more uniform with a
length of about 10 μm, a width of about 1–2 μm, and a
thickness of about 300–400 nm. Some structures have a
triangular spike at the tip indicating the direction of their
growth. These structures also occupy most of the struc-
tural space of the sample (it is difficult to recognize the
original matrix of material particles). It is easy to see that
the sheet-like or plate-like structures became stable after
autoclaving at 180 °C for 96 h. This is also evidence for
the research group to stop the hydrothermal reaction time
N. H. THANG et al.: SYNTHESES AND CHARACTERISTICS OF CALCIUM-BASED GEOPOLYMER FROM SOLAR-CELL ...
472 Materiali in tehnologije / Materials and technology 58 (2024) 4, 467–475
Figure 7: Growth of fiber-like shaped structures of calcium-based
geopolymeric matrix (sample M48) at magnifications of: a) 5000× and
b) 10000×
Figure 8: Growth of plate-like structures of calcium-based
geopolymeric matrix (sample M72) at magnifications of: a) 5000× and
b) 10000×

from exceeding 96 h without further investigation. The
plate-like structures play a role in stabilizing the me-
chanical strength, and also show the void space that ap-
pears among them. Therefore, the specimen exhibited a
slight increase in the volume weight and a slight increase
in the porosity compared to sample M72. The compres-
sive strength of sample M96, which was 69.7 MPa, in-
creased slightly compared to that of sample M72, which
was 68.5 MPa.
3.3.2 Formation of calcium silicate hydrate (CSH)
structures and an amorphous phase in the calcium-
based geopolymer detected with XRD
Figure 10 shows XRD patterns of calcium-based
geopolymer specimens (M0, M24, M48, M72, and
M96). Compared to the XRD patterns of the raw materi-
als (limestone and PV glass waste) from Figure 2, the
diffraction intensity of calcite crystal peaks of sample
M0 sharply decreased from nearly 5000 cps (limestone
material) to about 2500 cps at a diffraction angle
(2-Theta) of around 30 degrees. This reduction is attrib-
uted to the addition of 50 % PV glass waste (in weight),
which contains a predominantly amorphous phase as
shown in Figure 2. Furthermore, dissolution and reac-
tion processes between the limestone powder and PV
glass powder resulted in the formation of a new gel
phase in the shape of needles or spikes, as indicated in
the SEM images in Figure 5. These new gel phases are
also amorphous, as no diffraction peaks of any new crys-
tal appear on the XRD pattern.
On the XRD pattern of sample M24, under hydro-
thermal conditions, the calcite crystals grew stronger, in-
creasing the diffraction intensity to about 2800 cps
N. H. THANG et al.: SYNTHESES AND CHARACTERISTICS OF CALCIUM-BASED GEOPOLYMER FROM SOLAR-CELL ...
Materiali in tehnologije / Materials and technology 58 (2024) 4, 467–475 473
Figure 9: Overlapping of the plate-like structures in calcium-based
geopolymeric matrix (sample M96) at magnifications of: (a) 5000×
and (b) 10000×
Figure 10: XRD patterns of the calcium-based geopolymer specimens before and after their reaction in hydrothermal conditions

(higher than that of sample M0 of about 2500 cps) at 2 of around 30°. Additionally, a new diffraction peak ap-
peared at 2 of 14° on the XRD pattern of sample M24,
corresponding to the formation of a new crystalline
phase of calcium silicate hydrate (CSH). Although the
diffraction intensity of this peak is low, it indicates the
structural transformation of the spike-shaped amorphous
gel to a fibrous form, as observed in the SEM images
from Figure 6.
When the hydrothermal reaction time increased to
48 h, the dissolution and reaction processes became
more effective. The XRD pattern of sample M48 shows a
gradual decrease in the diffraction intensity to 1700 cps
at 2 of around 30° (similar to calcite crystals). Simulta-
neously, the XRD pattern of sample M48 still exhibits
the diffraction peak at 2 of 14°, corresponding to the
new crystal phase of CSH, similar to that of sample
M24.
For the samples with the hydrothermal reaction time
of up to 72 h, the diffraction peak of calcite at 2 of
around 30° had a diffraction intensity of less than
1000 cps, much less than those of samples M0, M24, and
M48. Concurrently, CSH crystals became more pro-
nounced at 2 of 14 degrees, with new diffraction peaks
appearing at 2 of 8°, 15°, 16°, 19°, 25°, and 31°. This is
the results of the hydrothermal reaction and transforma-
tion of the amorphous gel structure into crystals, evolv-
ing from spikes to fibers and then into sheets with denser
structures, as observed in the SEM images in Figure 8.
For the sample subjected to the hydrothermal reac-
tion for up to 96 h, the diffraction peak of calcite at 2 of
around 30° had an intensity of less than 200 cps, the low-
est among all the samples, including the initial raw mate-
rials. The intensity of the CSH phase peaks increased at
diffraction angles (2 ) of 8° and 31°, with additional dif-
fraction peaks appearing at 2 of 32° and 32.5°. This in-
dicates the development of sheet-like or plate-like struc-
tures that grew rapidly, overlapping and alternating into
layers, as observed in the SEM images in Figure 9.
The remainder of a sample consists of an amorphous
phase, easily identifiable through the XRD patterns of
samples M96 and M72. These patterns display broad dif-
fraction peak widths, heavily disturbed or noisy spec-
trum lines, and very low diffraction intensities (200 cps
for sample M96 and less than 1000 cps for sample M72).
The CSH structures were reported in the previous studies
by Maeda et al. (2011),
25
Ji et al. (2024),
26
and others.
7,27
4 CONCLUSIONS
This research successfully recycled glass waste from
solar panels to produce calcium-based geopolymer mate-
rials using geopolymerization and hydrothermal method.
The hydrothermal reaction process demonstrated that
limestone and PV glass waste dissolved and reacted to
form an amorphous gel structure. As the hydrothermal
reaction time increased from 24 h to 96 h, the amorphous
gel structures progressively transformed from needle or
spike shapes to fibrous shapes and eventually to sheet- or
plate-like shapes of calcium silicate hydrate. This struc-
tural development was observed via a scanning electron
microscope, while the amorphous and crystalline struc-
tures of CSH were identified through X-ray diffraction
patterns.
The calcium-based geopolymer samples were evalu-
ated in terms of compressive strength, volumetric
weight, porosity, and water absorption. It was found that
with longer hydrothermal reaction times, the volumetric
weight of the materials increased, while their porosity
and water absorption decreased, resulting in a significant
increase in the mechanical strength. However, an exces-
sively long hydrothermal reaction time did not necessar-
ily enhance the properties of the materials substantially.
This study recommends an optimal hydrothermal reac-
tion time of approximately 48 h at 180 °C. Future re-
search should explore different temperature conditions in
controlled alkaline solutions and test other raw materials
besides limestone. Additionally, application properties
such as heat resistance, chemical resistance, and other
relevant characteristics should be investigated.
Acknowledgment
This work was financially supported by the Ho Chi
Minh City University of Industry and Trade under con-
tract no 91/HÐ-DCT dated 15 Aug 2023.
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