K. D. T. KIEN et al.: SYNTHESIS OF CALCIUM SILICATE MATERIALS FROM THE RESIDUAL WASTE SLUDGE ...
53–60
SYNTHESIS OF CALCIUM SILICATE MATERIALS FROM THE
RESIDUAL WASTE SLUDGE OF A WATER-PURIFICATION
PLANT
SINTEZA KALCIJ-SILIKATNIH MATERIALOV IZ OSTANKOV ODPADNEGA
BLATA, NASTALEGA V NAPRAVI ZA ^I[^ENJE ODPADNIH VOD
Kieu Do Trung Kien
1,2
, Nguyen Hoc Thang
3
, Nguyen Vu Uyen Nhi
1,2
,
Do Quang Minh
1,2*
1
Department of Silicate Materials's, Faculty of Materials Technology, Ho Chi Minh City University of Technology (HCMUT),
268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Vietnam
2
Vietnam National University Ho Chi Minh City, Linh Trung Ward, Thu Duc City, Ho Chi Minh City, Vietnam
3
Faculty of Chemical Technology, Ho Chi Minh City University of Industry and Trade (HUIT), Ho Chi Minh City, 70000, Vietnam
Prejem rokopisa – received: 2023-05-15; sprejem za objavo – accepted for publication: 2023-12-15
doi:10.17222/mit.2023.876
Calcium silicate is produced from a mixture of silica sand powder, lime, paper pulp, and Portland cement hydrothermally
steamed at 180 °C for about 16 h. This material is considered environmentally friendly and is popular in countries around the
world. In this study, quartz sand was replaced with residual waste sludge from water-filtration plants to produce calcium silicate
materials. Nowadays, the residual waste sludge from water-filtration plants is an environmental problem that needs to be treated.
The results of determining the properties showed that a sample using 10 w/% residual waste sludge gave the best replacement.
This sample had a bending strength of 10.95 MPa, a volumetric density of 1.57 g/cm
3
, and water absorption of 23.67 %. The re-
sults of the analysis of the mineral composition (by X-ray diffraction analysis and Fourier-transform infrared spectroscopy) and
microstructure (by scanning electron microscopy) showed that all samples formed tobermorite and xonotlite minerals. The
tobermorite and the xonotlite are hydro-silicate-calcium minerals characteristic of calcium silicate materials, which are the syn-
thesis products of chemical reactions of SiO2, CaO, and H2O under hydrothermal conditions. Samples using 5–10 w/% of waste
residual sludge have even higher mechanical strength than samples without. Therefore, using waste residual sludge from wa-
ter-filtration plants to replace part of the sand in producing calcium silicate materials can be considered an effective method to
treat environmental problems caused by waste residual sludge.
Keywords: residual waste sludge, water-purification plant, hydrothermal reactions, tobermorite and xonotlite, Al-tobermorite
Kalcij-silikatne materiale se obi~ajno proizvaja iz me{anice kremenovega (SiO2)peska oz. prahu, kalcijevega hidroksida
(Ca(OH)2), papirne pulpe oz. ka{e in hidrotermalno pri 180 °C in 16 urah s paro obdelanega Portland cementa. Tako izdelan ma-
terial se ocenjuje kot zelo okolju prijazen in popularen material v ve~ini dr`av tega sveta. V tem ~lanku avtorji opisujejo
postopek izdelave kalcij-silikatnega materiala pri katerem so kremen~ev prah zamenjali z odpadnim blatom nastalim kot ostanek
pri filtraciji odpadne vode. Dandanes odpadno blato, ki nastaja v obratih za filtracijo vode oz. odplak postaja okoljski problem,
ki ga je potrebno re{iti. Rezultati raziskave so pokazali, da je optimalni dodatek odpadnega blata 10 w/%. Vzorec iz tak{ne
me{anice je imel upogibno trdnost 10,95 MPa pri volumski gostoti 1,57 g/cm
3
in absorpciji vode 23,67 %. Rezultati analiz
mineralne sestave in mikrostrukture s pomo~jo rentgenskega difraktometra (XRD), Fourierjeve transformacijske infrarde~e
spektroskopije (FTIR) ter vrsti~ne elektronske mikroskopije (SEM) so pokazali, da vsi vzorci vsebujejo tobermoritne in
ksonotlitne minerale. Tobermorit in ksonotlit sta hidro-silikatno-kalcijeva minerala zna~ilna za silikatne materale, ki nastajajo
kot produkti kemijskih reakcij med SiO2, CaO in H2O v hidrotermalnih pogojih. Vzorcis5do10w/% odpadnega blata so imeli
celo vi{je mehansko trdnost kot vzorci brez le-tega. Zato avtorji v ~lanku povdarjajo, da preostanek odpadnega blata iz obratov
za filtracijo vode lahko predstavlja delni nadomestk za kremen~ev prah. Tak{en na~in izdelave Ca-Si materialov se lahko smatra
kot u~inkovita metoda za re{itev okoljskih problemov zaradi ostankov odpadnega blata.
Klju~ne besede: ostanek odpadnega blata, obrat oziroma tovarna za ~i{~enje vode, hidrotermalne reakcije, tobermorit in
ksonotlit, Al-tobermorit
1 INTRODUCTION
Residual sewage sludge (RWS) is a byproduct of wa-
ter-filtration plants. It is not hazardous waste, but it occu-
pies a large area of urban land for storage and landfill.
Therefore, this waste needs to be treated. If handled
properly, it will provide better economic and environ-
mental performance. The RWS is aluminosilicate with
fine particle size and consists of minerals like kaolinite,
quartz, and hematite, which can be suitable raw materials
for producing building materials. Many studies have
looked at reusing RWS, such as unburnt construction
materials
1
or treatment by polymerization.
2
In particular,
the reuse of industrial waste as an additive for calcium
silicate by hydrothermal autoclaving technology has
been studied.
3
Building materials made in hydrothermal
conditions are relatively lightweight, have lower thermal
conductivity, higher heat resistance, lower shrinkage, and
structure faster than conventional concrete.
The raw materials for hydrothermal solidification are
usually natural aluminosilicate resources such as sand,
felspar, clay minerals, Ca(OH)
2
, and Portland cement.
Materiali in tehnologije / Materials and technology 58 (2024) 1, 53–60 53
UDK 62-73:666.965 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(1)53(2024)
*Corresponding author's e-mail:
mnh_doquang@hcmut.edu.vn (Do Quang Minh)

Hydrothermal solidification is carried out at 140–180 °C
for 2–40 h. Under the hydrothermal reaction conditions,
minerals like xonotlite and tobermorite will be formed.
The solids’ flexural strength will increase as the process-
ing temperature and processing time increase, reaching
values up to 20 MPa.
4
The temperature and pressure conditions considered
optimal for the hydrothermal process to fabricate indus-
trial products such as autoclaved aerated concrete (AAC)
and calcium silicate sheets are 180 °C, corresponding to
a pressure of about 1 MPa.
5
In terms of microstructure,
the tobermorite mineral (Ca
5
Si
6
O
16
(OH)
2
.4H
2
O) is a sig-
nificant contributor to the strength of these materials.
The tobermorite is formed in the hydrothermal and
strongly alkaline environment of the CaO–SiO
2
–
Al
2
O
3
–H
2
O system.
6
Co-forming with the tobermorite
under hydrothermal conditions is xonotlite
(Ca
6
Si
6
O
17
(OH)
2
).
6
However, tobermorite does not pro-
duce as high strength as xonotlite.
Hydrothermal materials are becoming more and more
attractive due to their excellent properties and environ-
mental friendliness. AAC is a combination of silica sand,
Portland cement, gypsum, lime, water, and an expander.
To improve the mechanical properties and reduce pro-
duction costs, waste materials were used as raw materi-
als.
7
With AAC bricks, part of the raw gypsum is replaced
with waste up to 30 % without affecting the functional
and structural properties of the final product. Physical
properties and sound absorption data confirmed that the
addition of waste did not significantly alter the typical
porosity of the AAC. It is even suggested that the opti-
mum ratio of the raw-material mix consists of 55–65 %
of construction waste.
8
The formation of tobermorite
when Ca(OH)
2
reacts with kaolinite has also been inves-
tigated. Adding Al-containing compounds such as
kaolinite to the mix also changed the reaction. Al
3+
ions
can replace Si
4+
and produce Al-tobermorite. The forma-
tion of tobermorite will be increased, thus preventing the
appearance of xonotlite.
9
Some other studies also indi-
cated that the presence of Al
3+
accelerates the tober-
morite formation from CSH and inhibits its conversion to
xonotlite,
10
even forming at lower temperatures.
11
Thus,
wastes containing many Al
3+
ions, such as the residual
waste sludge, can enhance the mechanical strength of the
material’s alkaline Ca(OH)
2
activities under hydrother-
mal conditions.
In this study, the RWS from the Thu Duc water-filtra-
tion plant was used to partially replace the silica sand to
produce calcium silicate sheets – the material for ceiling
and partition panels. The method used to synthesize the
calcium silicate is the hydrothermal method in a stain-
less-steel autoclave. The tobermorite crystals are formed
from a mixture of Portland cement, lime, sand, paper fi-
ber, and hydrothermal reaction in the autoclave at 180 °C
and 1 MPa pressure for 16 h.
2 EXPERIMENTAL METHODOLOGY
The raw materials were used, including RWS, sand,
lime, Portland cement (PC), paper fiber (PF), and water.
The prototyping process is shown in Figure 1.T h e y
were mixed in different proportions, in which the resid-
ual waste sludge partially replaced sand in the range
0–25 w/%.
Samples were mixed with a moisture content of 30 %
by mass at room temperature. The mixing equipment is a
planetary machine model JJ-5. The mixing process is
carried out according to ISO 0674:1984E standards. The
mixture compounds were shaped in a steel mold with di-
mensions of 4 cm×4c m×1 6c m .T h epost-formed
samples were cured in the air for 24 h. Finally, the sam-
ples were hydrothermally autoclaved to form calcium sil-
icate at 180 °C and 1 MPa for 16 hours. These parame-
ters, such as the processing temperatures and times,
correspond to the production of calcium silicate sheets
today. After hydrothermal treatment processing, the sam-
ples were determined to have various properties, includ-
ing bending strength, volumetric density, and water ab-
sorption. The microstructure was determined through
phase composition, functional group composition, and
morphology.
The bending strength was determined on the Matest
E183N device by the 3-point bending method with a
loading speed of 25 N/s. Volumetric mass and water ab-
sorption were determined according to the ASTM C830
standard.
The phase composition was determined by X-ray dif-
fraction (XRD) on the D8 Advance – Bruker instrument
and using Cu–K
 radiation ( = 0.154 nm). The analyti-
cal sample was a powder sample, scanning angle 2 from
10° to 60°, scanning step 0.019°. An XRD pattern was
also used to calculate the formed crystal composition.
The calculation expression is shown in Equation (1),
where, %C is the crystal content, S
c
is the total area on
the XRD pattern of the crystal, and S is the total area of
the XRD pattern.
%C = 100%·S
c
/ S (1)
The functional group composition was determined by
Fourier-transform infrared spectroscopy (FT-IR) on a
Nicolet 6700 – Thermo Scientific instrument. Analytical
conditions include KBr binder, scan angle from 450 cm
–1
to 4000 cm
–1
, and scan step at 0.964 cm
–1
.
K. D. T. KIEN et al.: SYNTHESIS OF CALCIUM SILICATE MATERIALS FROM THE RESIDUAL WASTE SLUDGE ...
54 Materiali in tehnologije / Materials and technology 58 (2024) 1, 53–60
Figure 1: Process of Prototyping

The morphology was determined by scanning elec-
tron microscopy (SEM) on a Hitachi S-4800 instrument.
Analytical conditions were a voltage of 10 kV, low elec-
tronic mode SE(M), and magnification of 20,000 times.
3 RESULTS AND DISCUSSION
3.1 Raw materials and the mix compositions of the
samples
The waste used to make the calcium silicate materials
in this study was RWS. The RWS was taken from the
Thu Duc water-purification plant in Ho Chi Minh City,
Vietnam. The particle size of RWS was in the range of
6.720–77.339 μm. The average diameter was 14.259 μm.
The chemical compositions (w/ % )w e r e3 2 . 8%S i O
2
,
26.3 % Al
2
O
3
,33.7%Fe
2
O
3
,0.22%P
2
O
3
,1.91%K
2
O,
0.38 % CaO, 0.59 % TiO
2
, and 6.38% lost on ignition.
Besides RWS, the remaining raw materials used to make
the calcium silicate included sand, lime, and water. The
mixed compositions of the samples are shown in Ta-
ble 1. These samples were denoted by S0, S5, S10, S15,
S20, and S25, corresponding to the amount of sand re-
placement by RWS from 0 w/% to 25 w/%.
The XRD pattern in Figure 2 shows that the mineral
compositions of the RWS were nacrite
(Al
2
Si
2
O
5
(OH)
4
),
12
quartz (SiO
2
),
13
diaspore
(AlO(OH)),
14
and kaolinite (Al
2
Si
2
O
5
(OH)
4
).
15
Nacrite
and kaolinite have the same chemical formula, but
nacrite crystallizes in the monoclinic, while kaolinite is
in the triclinic system. These are minerals commonly
found in the composition of clay mud. In addition, the
background of the XRD pattern in Figure 2 also shows
that the RWS pattern contains several components in the
amorphous state. This composition may contain amor-
phous SiO
2
, which enhances the RWS activity.
16
Asare -
sult, using RWS can increase the ability to form calcium
silicate hydrate minerals, which helps to increase the
strength of the calcium silicate materials after the hydro-
thermal autoclave.
17
Table 1: Compositions of raw materials
Raw materials Samples (w/%)
S0 S5 S10 S15 S20 S25
R W S –51 01 52 02 5
PC 39 39 39 39 39 39
Sand 56 51 46 41 36 31
Lime Ca(OH)2 555555
PF
(*)
4.76 4.76 4.76 4.76 4.76 4.76
(*)
Paper fibre was used for 4.76% of the solid material mass.
Some other materials were used in the experiments,
such as sand with a composition of 99 % SiO
2
, crushed
to a particle size of less than 45 μm, and the Portland ce-
ment used was PC40.
3.2. Mechanical and physical characteristics
The mechanical and physical characteristics, such as
bending strength, volumetric density, and water absorp-
tion, are presented in Figure 3 and Figure 4.
The results of Figure 3 show that the volumetric den-
sity decreased, and the water absorption increased when
replacing the sand with RWS. Since sand and sludge
K. D. T. KIEN et al.: SYNTHESIS OF CALCIUM SILICATE MATERIALS FROM THE RESIDUAL WASTE SLUDGE ...
Materiali in tehnologije / Materials and technology 58 (2024) 1, 53–60 55
Figure 3: a) Volumetric density and b) water absorption of the sam-
ples
Figure 2: XRD pattern of RWS

have different particle sizes, the replacement of sand
with RWS also contributes to a change in the density of
the particles in the aggregate composition. Therefore, the
volumetric mass of the calcium silicate will also change.
With an average sand grain size of 45 μm and an average
sludge particle size of 14.259 μm, when replacing sand
with RWS, the samples would have a tighter arrange-
ment. As a result, the volumetric density of the sample
increases, and the water absorption decreases. However,
Figure 3 shows that the volumetric density of the sample
decreased, and the water absorption of the sample in-
creased. As the results of the XRD pattern in Figure 2
show, the composition of RWS contains a quantity of
clay mud and impurities. When the amount of RWS used
was too much, clay mud and fine impurities would inter-
fere with the bonding between the aggregate particles.
Therefore, in the calcium silicate samples after
autoclaving, the clay particles and impurities on the out-
side would be washed away in water when determining
the volumetric weight and water absorption according to
ASTM C830. This decreased the volumetric density and
increased the water absorption of the samples.
Figure 4 shows that the bending strength increased
with the S5 and S10 samples, corresponding to the
amount of sludge from0%to10%byweight. However,
if the amount of used RWS was more than 10 %, the
bending strength would be reduced compared to the S0
sample. The results show that the strength of the calcium
silicate samples increased when replacing the sand with
the RWS using a reasonable amount. The bending
strength of the samples achieved is similar to the re-
search conducted by M. Takagi et al.
18
As shown in Figure 2, the composition of RWS can
contain SiO
2
, which is amorphous. This amorphous SiO
2
reacts more readily with lime than the crystalline SiO
2
of
sand in a hydrothermal reaction. These reactions helped
to form minerals that strengthen the calcium silicate.
However, besides amorphous SiO
2
, the sludge contains
many clay mud and other fine impurities.
19
When using
too much RWS, fine particles interfered with the compo-
nents’ bonding. Therefore, the strength of the sample af-
ter the hydrothermal process would be reduced when us-
ing RWS of more than 10 %.
3.3 The XRD and FT-IR
Figure 5 shows the FT-IR analysis results of the cal-
cium silicate samples. The analysis of the samples
showed the appearance of two groups of wave numbers.
The wave-number group characterized the molecular vi-
brations of the O-H group. The other group represented
the molecular vibrations of the silicate tetrahedra.
K. D. T. KIEN et al.: SYNTHESIS OF CALCIUM SILICATE MATERIALS FROM THE RESIDUAL WASTE SLUDGE ...
56 Materiali in tehnologije / Materials and technology 58 (2024) 1, 53–60
Figure 5: FT-IR spectra of the samples
Figure 4: Bending strength of the samples

The peak at 1640 cm
–1
was attributed to the -OH vi-
bration. It is the free-water molecules present in the sam-
ple. The second vibration from 3400 cm
–1
to 3600 cm
–1
is
characteristic of the chemical water that existed in the
structure of the mineral.
20
Combined with the bending
vibration of the CaO-H group at wavenumber of
629–631 cm
–1
, the chemical water formed was the
hydroxyl group existing in the structure of the calcium
silicate hydrate mineral.
21
The vibration at 675 cm
–1
was
also a characteristic of the bonding oxygen bridge be-
tween the silicate tetrahedra and the CaO polyhedron
(Q2).
21
With the silicate tetrahedra, the FT-IR spectra of all
the samples showed the characteristic vibrations of
[SiO4]
4–
in the structure of calcium silicate hydrate. In
Figure 5, all the samples appeared to vibrate at
wavenumber 1200–1220 cm
–1
. This band is typical for
the stretching vibration of Si–O at the junction between
two silicate tetrahedra through an oxygen bridge (Q3)
22
.
The stretching vibration of Si–O in the range
900–1000 cm
–1
, the bending vibration of O–Si–O in the
range 451 533 cm
–1
, the bending vibration of Si–O–Si in
the range 675–790 cm
–1
, and the stretching vibrations of
Si–O of the last tetrahedron in the bond chain [SiO4]
4–
in
the range 800–830 cm
–1
(Q1)
22
also showed the appear-
ance of the silicate tetrahedra.
23
The results of the FT-IR spectra showed that there
was structure formation of calcium silicate hydrate in the
samples after the hydrothermal autoclave. The XRD re-
sults in Figure 6 indicate the formation of these calcium
silicate hydrate minerals.
The results of the XRD patterns analysis showed the
appearance of peaks characteristic of quartz, tobermorite,
and xonolile minerals. Quartz
23
was the mineral in the
material remaining in the samples after the reaction. The
results also showed that crystalline SiO
2
hardly reacted
with CaO to form calcium silicate hydrate under hydro-
thermal conditions. Meanwhile, tobermorite
24
and
xonotlite
25
were two minerals formed after the hydrother-
mal treatment. These two minerals were the calcium sili-
cate hydrate minerals shown on the FT-IR spectrum.
Tobermotite and xonotlite would help increase the me-
chanical strength of the product. The XRD results in Fig-
ure 6 also showed that the diffraction peaks of tober-
morite and xonotlite on the S5–S25 samples appear more
obvious than those of the S0 sample. This result proves
that the sludge contains amorphous SiO
2
. Amorphous
SiO
2
readily reacts with CaO to form tobermorite and
xonotlite. The highly developed tobermorite and
xonotlite crystals make the diffraction peaks of the S5
and S10 samples more obvious than those of the S0 sam-
ple.
K. D. T. KIEN et al.: SYNTHESIS OF CALCIUM SILICATE MATERIALS FROM THE RESIDUAL WASTE SLUDGE ...
Materiali in tehnologije / Materials and technology 58 (2024) 1, 53–60 57
Figure 7: Chemical composition of the samples
Figure 6: XRD patterns of the samples

The formation of minerals was also calculated from
the XRD patterns (Figure 6) and Equation (1). Figure 7
is the result of calculating the mineral composition. The
decrease in quartz content shows that the quartz is
mainly located in sand. When the sand content in the
mixture is reduced, the quartz in the product is also re-
duced. The calcium silicate minerals (including xonotlite
and tobermorite) in the product gradually increase as the
RWS increases. For a sample containing 25 % RWS, the
content of xonotlite and tobermorite accounts for
63.73 %, an increase of 112.93 % compared to the sam-
ple without RWS. This result shows the role of RWS in
forming calcium silicate minerals by hydrothermal
method. The RWS contains active substances that help
make forming the calcium silicate easier. In addition, the
tobermorite content was also higher than the xonotlite in
all samples. This result is similar to the conclusions in
previous studies. M. Diez-Garcia et al. also demonstrated
that the main mineral formed would be tobermorite when
the calcium silicate material is formed by the hydrother-
mal method at 170–200 °C and 1 MPa.
26
Besides, the
composition of other minerals also increases when the
amount of RWS increases. These minerals are usually
minerals of clay mud. They reduce the strength of cal-
cium silicate materials (Figure 4). The existence of clay
minerals was shown on the XRD patterns in Figure 8.
Because the intensity of quartz mineral at position
26.62° is too high, it can mask other diffraction peaks.
Therefore, Figure 8 shows the samples’XRD result after
cutting this diffraction peak. Figure 8 showed that in the
samples using RWS, besides quartz, tobermorite, and
xonotlite, minerals such as nacrite, diaspore, and
K. D. T. KIEN et al.: SYNTHESIS OF CALCIUM SILICATE MATERIALS FROM THE RESIDUAL WASTE SLUDGE ...
58 Materiali in tehnologije / Materials and technology 58 (2024) 1, 53–60
Figure 8: XRD patterns of the samples at 5°–20° and 27°–60°
Figure 9: SEM images of the S0 and S10 samples

kaolinite also appeared. Nacrite, diaspore, and kaolinite
were components of the clay mud present in RWS (XRD
pattern in Figure 2). These minerals did not participate
in forming calcium silicate hydrate and remained in the
sample after the hydrothermal process. The existence of
nacrite, diaspore, and kaolinite in the XRD results of the
S5–S25 samples also explained the decreased bending
strength (S15–S25 samples), decreased volumetric den-
sity, and increased water absorption, as shown in Fig-
ure 3 and Figure 4.
3.4 Microstructure by SEM
The microstructures of the samples were also ob-
served through scanning electron microscopy. Figure 9
shows the SEM results of the samples.
The SEM images show that the samples were typical
tobermorite and xonotlite crystals. Crystalline tober-
morite was plate-shaped, and crystalline xonotlite was
needle-shaped.
27
The crystal shapes of tobermorite and
xonotlite are similar to those shown in the study of K.
Luke et al. However, at the same magnification (20,000
times), the size and density of the tobermorite and
xonotlite crystals of sample S10 were clearer and larger
than the S0 sample. It again proved that the mineraliza-
tion capacity of the RWS was higher than sand.
5 CONCLUSIONS
The results show that using 5–10 % of the RWS was
an appropriate ratio to create calcium silicate materials in
hydrothermal at 180 °C, 1 MPa for 16 h. The formed
sample had a bending strength of 10.95 MPa, a volumet-
ric density of 1.57 g/cm
3
, and a water absorption of
23.67 %. The results of the mineral composition analysis
by XRD and FT-IR showed that the primary minerals
formed were quartz, tobermorite, and xonotlite. The
tobermorite and xonotlite contributed to the mechanical
strength of the calcium silicate materials. In addition, the
XRD patterns of the samples that used RWS appeared as
nacrite, diaspore, and kaolinite minerals. They were
components of the clay mud present in the RWS. There-
fore, if the amount of residual waste sludge were used
too much (over 10 w/%), clay mud and fine impurities in
the sludge would hinder the association between parti-
cles and reduce the mechanical strength of the product.
SEM images also demonstrated that the sample using the
RWS has a larger size and density of the tobermorite and
xonotlite crystals than the untreated sample. The results
show that RWS can enhance the formation of tober-
morite and xonotlite minerals under hydrothermal condi-
tions. Therefore, a hydrothermal process can be consid-
ered an effective method to treat the RWS of
water-filtration plants.
Acknowledgment
We acknowledge Ho Chi Minh City University of
Technology (HCMUT), VNU – HCM for supporting this
study.
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