1. 23rd
RegionalSymposiumonChemicalEngineering(RSCE2016)
Innovation in Chemical Engineering towards the linkages among
education, academia, industry
27-28 Oct. 2016 in Vung Tau City, Vietnam
Effects of Sintering Temperature on Phase Formation
and Microstructure Development of a Porous
Cordierite Ceramic from Rice Husk
Nguyen Luong The Thinh, Vu Thi Ngoc Minh, Mai Van Vo
Department of Silicate Materials Technology, Hanoi University of Science and Technology
Hanoi, Vietnam
Abstract—Porous cordierite ceramic samples were synthesized
by firing a powder compact from clay, talc, alumina and rice husk
charcoal (RHC). Gasses released by the combustion of the RHC
and decomposition of the hydrates in the raw materials during
firing created a matrix of interconnected channels throughout the
samples. Cordierite (Mg2Al4Si5O18) was formed at 1160 degree
Celsius, but became a major phase only when the firing
temperature reached 1220 degree Celsius and above. The
apparent porosity of the samples decreased from 49.5 percent to
42.7 percent as the firing temperature increased from 1140 degree
Celsius to 1240 degree Celsius. At all firing temperatures, their
compressive strength were approximate 50 mega Pascal, and their
apparent densities were around 1.5 gram per cubic centimeter.
Keywords— porous ceramic, cordierite, sintering temperature,
phase formation, microstructure.
I. INTRODUCTION
Cordierite (2MgO.2Al2O3.5SiO2) was characterized by a
very low thermal expansion, which was followed by a
significantly higher thermal shock resistance in comparison to
other ceramic materials [1]. In the porous form, cordierite has
been proven effective as a thermal insulator in high temperature
applications, and catalyst support in automotive exhaust
treatment [2, 3].
Porous cordierite ceramic has been produced by a number of
processing methods, including solid-state reaction, sol-gel, gel-
casting, and foam replication [4-6]. Among these methods,
solid-state sintering of a mixture of oxide precursors with one or
more pore-forming agents allowed a better control on the shape,
size and structure of the final product.
The present work synthesized porous cordierite ceramic at
low sintering temperatures by utilizing rice husk, which is an
agricultural by-product of the rice milling process, and contains
high amount of silica and potassium oxides after ignition [7].
The influence of the sintering temperature on the porosity and
mineral phase formation was studied.
II. MATERIALS AND METHOD
The present work used Truc Thon white clay, YFA talc
powder, Almatis CT9FG alumina powder, and Tien Hai rice
husk (RH) as the starting materials. The RH was heat treated to
form rice husk charcoal (RHC) for a better grindability
according to a previous study [7], and then ground in a planetary
ballmill until passed the 0.063mm sieve. Talc and clay were
crushed until passed the 0.5mm sieve, and dried at 110o
C until
constant mass is attained.
A powder mixture, comprised 23% by weight (wt%) of the
clay, 29 wt% talc, 17 wt% alumina, and 30 wt% RHC was
prepared. The chemical compositions of these materials are
presented in Table 1. The mixture was ground in 15 minutes to
ensure efficient mixing. To estimate the behavior of the powder
mixture upon heating, thermal analysis was performed in air at
a heating rate of 10o
C/min. Distilled water was added to the
powder mixture to form a paste having a water content of
30wt%. The mixture was kept in a plastic bag for 48 hours before
shaping.
After aging, the paste was pressed to form cylindrical pellets
with a dimension of  20mm x H 20mm. They were then dried
at 110ºC overnight, and fired at different sintering temperatures:
1140ºC, 1160ºC, 1180ºC, 1200ºC, 1220ºC and 1240ºC. The
heating rate was kept at 125ºC/h from room temperature to
600ºC, and at 180ºC/h from 600o
C to the sintering temperature.
A dwell time of 60 minutes at the sintering temperature was
applied to all samples.

2. 23rd
RegionalSymposiumonChemicalEngineering(RSCE2016)
Innovation in Chemical Engineering towards the linkages among
education, academia, industry
27-28 Oct. 2016 in Vung Tau City, Vietnam
TABLE I. CHEMICAL COMPOSITION OF THE STARTING MATERIALS.
Material
Chemical composition (wt%)
SiO2 Al2O3 TiO2 CaO MgO Fe2O3 FeO K2O Na2O LOI
Clay 59.14 26.90 0.95 0.41 0.61 1.30 - 3.05 0.43 7.19
Talc 59.40 3.27 0.15 0.07 31.95 3.27 0.02 0.08 0.09 3.36
Alumina 0.06 99.50 - - - 0.06 - - 0.30 -
Heat-treated rice husk 16.10 0.14 0.01 0.44 0.17 - 0.08 0.88 0.05 82.14
III. RESULTS AND DISCUSION
A. Thermal Analysis of the Rice Husk and Raw Mix
Thermal analysis of the RH was presented in Figure 1 and of
the raw mix was presented in Figure 2. In case of the RH, below
120o
C was the evolution of the absorbed water with an
endothermic peak presented at 105o
C. There was almost no
weight change in the range of temperatures from 105o
C to
240o
C. A rapid outgassing occurred between 240o
C and 490o
C
with two exothermic peaks presented at 355o
C and 425o
C.
Further changes in the TG and DTA curves were not significant
after the heating temperature crossed 490o
C. This outgassing
was expected to contribute to the formation of interconnected
pores in the sintered ceramic.
The DTA curve of the raw mix behaved similarly to that of
the RH with two partially overlapped exothermic peaks, one
appeared at around 350o
C and the other at around 680o
C. It was
possible that gasses released by RHC decompostion and firing
were increased in concentration but partially blocked by the
inorganics in the raw mix. As a result, kinetics of the reaction
was reduced, and the exothermic effect on the DTA curve was
postponed to higher temperatures. An endothermic peak appear
immediately after the 680o
C could be because of the
decomposition and phase transition of the raw mix minerals.
After crossing 700ºC, the exothermic effect appeared again
while the weight change was negligible. This significant
increase of exothermic effect above 700ºC majorly attributed to
the combustion of RHC, which was previously hindered.
B. Effect of the Sintering Temperature on the Physical
Properties
The physical properties of samples fired at six different
temperatures were shown in Table 2. They all reached a
compressive strength of around 50MPa and an apparent density
of around 1.5g/cm3
. As the temperature increased, the apparent
porosity decreased slightly from 49.5% at 1140ºC to 42.7% at
1240ºC, the water absorption decreased from 34.1% to 28.2%,
and the total shrinkage increased from 22.9% to 30%.
Explanation for these observations could be that either void
spaces were filled with glass phase or a better sintering occurred
at elevated temperature.
The microstructure of the polished sample fired at 1240ºC
was shown in Figure 3. The x200 SEM image indicated that the
sample fired at 1240ºC had a high porosity, with interconnected
void spaces. The smaller holes, which had a diameter of 10μm,
was seen on the surface of the bigger ones, which had diameter
of 100μm. The x1000 image showed that the grains were
connected by a glassy phase, instead of a direct bonding.
Fig. 1. DTA and TG curves of rice husk. Fig. 2. DTA and TG of the raw mix.
-50
0
50
100
150
0
20
40
60
80
100
0 200 400 600 800 1000
Heatflow(mW)
Weight(%)
Temperature (oC)
TG
DTA
0
10
20
30
40
50
0
20
40
60
80
100
0 200 400 600 800 1000
Heatflow(µV)
Weight(%)
Temperature (oC)
TG
DTA

3. 23rd
RegionalSymposiumonChemicalEngineering(RSCE2016)
Innovation in Chemical Engineering towards the linkages among
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TABLE II. PHYSICAL PROPERTIES OF SAMPLES FIRED AT DIFFERENT TEMPERATURES.
Firing
temperature
(ºC)
Water
absorption
(wt%)
Apparent
density
(g/cm3
)
Apparent
porosity
(%)
Linear
shrinkage
(%)
Compressive
strength
(MPa)
1140 34.10 1.452 49.49 22.90 52.83
1160 30.58 1.462 44.74 23.79 51.42
1180 29.90 1.535 45.83 24.81 48.58
1200 31.03 1.500 46.51 29.97 56.91
1220 29.71 1.535 45.59 29.34 52.23
1240 28.22 1.512 42.69 29.97 50.40
C. Effect of the Sintering Temperature on Phase Formation
Figure 4 presents the XRD patterns of samples fired at six
different temperatures after being cooled down to room
temperature. The presence of the crystalline phases became
more complicated as the firing temperature increased. The
typical diffraction peaks of mullite, a phase that was usually
formed when synthesizing cordierite, were not seen in all cases.
The diffraction peaks of cordierite were seen on sample fired at
1160o
C, but very weak. They became more obvious only on
samples fired at 1220o
C and above. Corundum, quartz and
cristobalite was seen on all samples.
-cristobalite, which is stable at high temperatures, was seen
on samples fired at under 1220o
C. α-cristobalite, which is stable
at room temperature, was seen on samples fired at 1220ºC and
above. These cristobalite phases were formed as the products of
RHC combustion. The distribution of minor elements in the
cristobalite matrix induced stresses and prevented the
transformation of -cristobalite to α-cristobalite. A higher
sintering temperature and a longer dwell time led to the
recrystallization of silica and forced the impurities out toward
the grain boundaries. Therefore, the stress was released and α-
cristobalite was seen on samples fired at 1220ºC and above.
The presence of spinel could be explained by the reaction
between the products of clay and talc decomposition as follow:
Kaolinite, an important mineral in clay, dehydrated to form
metakaolinite at temperatures above 450o
C:
Al2O3. 2SiO2. 2H2O
450oC
→ Al2O3. 2SiO2 + 2H2O
Talc decomposed at temperature above 950o
C to form
protoenstatite:
3MgO. 4SiO2. H2O
950oC
→ 3(MgO. SiO2) + SiO2 + H2O
Metakaolinite and protoenstatite reacted to form spinel and
release SiO2:
Al2O3. 2SiO2 + MgO. SiO2 → MgO. Al2O3 + 2SiO2
Fig. 3. SEM images of the sample fired at 1240o
: a) at x200 magnification, and b) at x1000 magnification.
a) b)

4. 23rd
RegionalSymposiumonChemicalEngineering(RSCE2016)
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education, academia, industry
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The transformation of metakaolinite at temperatures above
1000o
C could form mullite as follow:
3(Al2O3. 2SiO2)
>1000 𝑜 𝐶
→ 3Al2O3. 2SiO2 + 4SiO2
The transformation of metakaolinite at temperatures above
1000o
C could form mullite as follow:
3(Al2O3. 2SiO2)
>1000 𝑜 𝐶
→ 3Al2O3. 2SiO2 + 4SiO2
However, it was possible that its size was small and its structure
was defective, causing mullite peaks to be unclear and easily be
recognized as the background.
The typical diffraction peaks of cordierite were observed
from 1160ºC, but its intensity increased slowly as the sintering
temperature increased to 1200o
C. Only after reaching 1220ºC
did cordierite perfect its structure and crystalize in a large
enough amount to become a major phase.
Cordierite was possibly formed by the reaction between
spinel and cristobalite and/or between mullite, cristobalite and
protoenstatite as follow:
2(MgO. Al2O3) + 5SiO2 → 2MgO. 2Al2O3. 5SiO2
and
2(3Al2O3. 2SiO2 + 5SiO2 + 6(MgO. SiO2)
→ 3(2MgO. 2Al2O3. 5SiO2)
Other major phases that presented on all sample was
corundum and quartz with high intensity diffraction peaks. It
was likely that these minerals were relatively inactive and did
not participate much in cordierite formation.
Fig. 4. X-ray diffraction patterns of the sintered samples.

5. 23rd
RegionalSymposiumonChemicalEngineering(RSCE2016)
Innovation in Chemical Engineering towards the linkages among
education, academia, industry
27-28 Oct. 2016 in Vung Tau City, Vietnam
IV. SUMMARY
Rice husk charcoal played the role of a pore-forming agent
in a porous cordierite ceramic fabrication. Its decomposition
created a matrix of interconnected pores. The microstructure,
crystalline phases and other properties including water
absorption, and apparent porosity of the porous cordierite
ceramic were strongly related to the firing temperature. As the
temperature increased, the apparent porosity and water
absorption decreased slightly while the apparent density and
compressive strength remained at relatively stable values.
MgO-containing mineral phases presented as spinel and
cordierite. Spinel was formed quite early while cordierite was
formed later. The strongest peak of the cordierite phase (at
2θ=10.5º) first appeared on the sample fired at 1160ºC with the
intensity increased slowly until a firing temperature of 1200ºC.
Cordierite became a major phase only on samples fired at
1220ºC and 1240o
C. Corundum presented on all studied samples
as a major phase while quartz and cristobalite presented as the
minor ones.
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