Effect of mechanical activation on mullite
formation in an alumina-silica ceramics

system at lower temperature
David O. Obada

Department of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria

David Dodoo-Arhin
Department of Materials Science and Engineering, University of Ghana, Legon, Ghana

Muhammad Dauda and Fatai O. Anafi
Department of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria

Abdulkarim S. Ahmed and Olusegun A. Ajayi
Department of Chemical Engineering, Ahmadu Bello University, Zaria, Nigeria, and

Ibraheem A. Samotu
Department of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria

Abstract
Purpose – This work aims to analyze the effect of mechanical activation on structural disordering (amorphization) in an alumina-silica ceramics
system and formation of mullite most notably at a lower temperature using X-ray diffraction (XRD). Also, an objective of this work is to focus on
a low-temperature fabrication route for the production of mullite powders.
Design/methodology/approach – A batch composition of kaolin, alumina and silica was manually pre-milled and then mechanically activated in
a ball mill for 30 and 60 min. The activated samples were sintered at 1,150°C for a soaking period of 2 h. Mullite formation was characterized by
XRD and scanning electron microscopy (SEM).
Findings – It was determined that the mechanical activation increased the quantity of the mullite phase. SEM results revealed that short milling
times only helped in mixing of the precursor powders and caused partial agglomeration, while longer milling times, however, resulted in greater
agglomeration.
Originality/value – It is noted that, a manual pre-milling of approximately 20 min and a ball milling approach of 60 min milling time can be
suggested as the optimum milling time for the temperature decrease succeeded for the production of mullite from the specific stoichiometric batch
formed.

Keywords Mullite, Mechanical activation, Amorphization, Agglomeration

Paper type Research paper

1. Introduction
Mullite (3Al2O3 · 2SiO2) is an important ceramic phase in
conventional ceramics (such as tableware, construction
ceramics and refractories), advanced high-temperature
structural materials, heat exchangers, catalyst converters,
filters, optical devices and electronic packaging materials
(Elmas et al., 2013). Mullite is an alumino-silicate compound
that is used extensively in traditional refractory applications.
Most traditional ceramic products have mullite as part of their
final phase composition, as they usually contain some clay and
silicon as starting materials.

Mullite is becoming increasingly important in electronic,
optical and high-temperature structural applications, because
of its low dielectric constant, good transparency for
mid-infrared light and excellent creep resistance. In
conventional processing methods, mullite powders are
shape-formed and sintered. This mullite is designated
“sinter-mullite”. The term sinter mullite describes a mullite
which has been produced from its starting materials essentially
by solid-state diffusion controlled reactions. Oxides,
hydroxides, salts and silicates can be used as the starting
materials. The Al2O3 content of sinter mullite is influenced by
the sintering temperature, the duration of heat treatment, the
initial bulk composition and the nature, grain size and
efficiency of mixing of the starting materials. Mullitization
takes place by solid-solid or transient liquid-phase reactions of
the starting materials by aluminum, silicon and oxygen atom
interdiffusion (Souto et al., 2009).

Mechanical activation of starting materials is a promising
method for precursor preparation. The particle size reduction,
which increases the contact surfaces between the particles, is a
direct consequence of milling. Also, the energy of the system

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increases, resulting in a decrease in the reaction temperature
(Behmanesh et al., 2008). Different processes can remarkably
influence the reactivity of the solids. Particularly, the
mechanical treatments are important as long as they can help
to produce the changes in the texture and structure of the
solids. In many cases, these alterations in the structure cause
certain modifications in the phases formed with a thermal
treatment of the solids that were mechano-chemically treated
(Tamborenea et al., 2004; Mazzoni et al., 1991).

Mullite has been synthesized in many ways, such as simple
sintering of alumina and silica powders, sol-gel method,
co-precipitation, hydrothermal and chemical-vapor-deposition
processes. The mullitization temperature is as high as 1,600°C
for the conventional fabrication method, i.e. the solid-state
reaction of high-purity alumina and quartz (Souto et al., 2009;
Dong et al., 2008; Viswabaskaran et al., 2002). These findings
are obviously without mechanical milling. In their work,
Elmas et al. (2013) mechanically activated powder mixtures of
alumina and quartz in a planetary mill for 2 h. Both
non-activated and activated samples were sintered at different
temperatures (1,250, 1,300, 1,325, 1,350 and 1,375°C) for 1,
2, 3 and 5 h, and the formation of a mullite phase was
examined with an X-ray diffraction (XRD) analysis. It was
determined that the mechanical activation increased the
quantity of the mullite phase. For activated precursors milled
for 60 min using a planetary ball mill as reported, it was found
that at a temperature of 1,375°C, the mullite phases increased
significantly.

Fotoohi and Blackburn (2013) investigated three milling
methods (ball, ring and planetary) and the effects of
mechanical action and milling times on the formation of
cordierite powders at different temperatures. Sintering at
1,000°C for 2 h resulted in the development of significant
mechanically assisted phase transformations specifically
related to formation of mullite in the most highly activated
materials. Mullite transformation to pre-cordierite mineralogy
and the formation of magnesium aluminum silicate were also
found to be dependent on the degrees and methods of
mechanical processing. Planetary milling resulted in the
highest densification of the material, which when sintered at
1,300°C for 2 h produced a ceramic body rich in cordierite
composition.

Wu et al. (2013) reported a homogeneous coating on
mullite particles where the mullitization occurs completely as
low as 1,250°C. Homogeneously coated sol transforms to
liquid binder during sintering to form an interlocking
structure, which leads to a remarkable improvement in
mechanical properties and thermal alkali corrosion resistance.
The authors were of the opinion that producing porous
ceramics by this method brings a lot of advantages including
pure phase product and simple technique. Moreover, it can be
widely used in an industrial exhaust filter area. The
mullitization temperature is of interest in this present study.

Kirsever et al. (2015) showed the XRD patterns of sintered
samples heated at 1,200, 1,250, 1,300, 1,350, 1,400 and
1,450°C for 1 h. When the samples were sintered at 1,200°C
for 1 h, the quartz and alumina have been noted to be the
major phases. The mullite phase appeared as the sample was
sintered at 1,300°C for 1 h. When sintering temperatures were
elevated to between 1,300 and 1,450°C, the intensity

increased and the much sharper X-ray reactions of mullite
were observed. It was found that quartz and alumina reacted
completely and had mainly converted to mullite phase at
1,450°C. It is worthy of note that in their study, no mechanical
activation was to the best of our knowledge reported to
disorder the structure of the materials, hence the high
temperatures reported in main conversion to mullite.

Baitalik et al. (2015) in their work observed the role of
mullite source on the properties of final ceramics by using two
sources of mullite (commercial mullite and synthesized
mullite through sol-gel route). Porous SiC ceramics were
prepared by sintering powder compact of mullite coated SiC/
SiC � mullite particles in air at 1,300°C for 3 h keeping the
mullite content same. Notably, sintering at this temperature of
1,300°C kept the mullite content intact.

In this work, the effect of mechanical activation on
structural disordering (amorphization) in an alumina-silica
ceramics system and formation of mullite most notably at a
lower temperature were analyzed using XRD and SEM.
Another objective was to focus on a low-temperature
fabrication route for the production of mullite powders.

2. Materials and methods
The starting raw materials used were kaolin (Kankara,
Nigeria), alumina (Sigma Aldrich: analytical grade), alumina
hydroxide (Sigma Aldrich: analytical grade) and silica gel
(Sigma Aldrich). These materials were wet-mixed during the
applied milling processes based on weight ratios that satisfy
the stoichiometric compositions in formation of mullite
according to equation (1):

3Al2O3 � 2 SiO2 � 4.07 H2O ¡ Mullite � Water (1)

The stoichiometric mullite (3Al2O3 · 2SiO2) was dispersed
(equal percentage in weight) in distilled water, pre-milled
manually in a ceramic mortar for approximately 20 min before
wet milling with ceramic balls (Ø10 mm) for varying periods
(30 and 60 min). The green bodies were compacted without
using additives or binders. The rectangular compacts were
then sintered at 1,150°C. Heating and cooling cycles in the
furnace were set at 5°C/min before and after a 2-h dwell time.
The temperature range (1,150°C) covered the area where
most of the decomposition, phase transformation and
reactions were expected to occur (Fotoohi and Blackburn,
2013).

Table I gives the details of the milling experiments as well as
sample coding for the ground material. Sample CDO
(unmilled reference) was prepared with no tumbling action in
the ball mill to avoid any intensive mechanical action on the
powders. Samples with codes CD1 and CD2 were ball-milled
(tumbling action) at 30 and 60 min, respectively, and fired at
1,150°C.

The as-milled sintered samples were examined using XRD
analysis (Philips X’Pert, PAN analytical B.V., Almelo, The
Netherlands) to investigate mullite formations. The chemical
composition of starting materials was carried out using the
X-ray florescence (XRF) technique. Scanning electron
microscopy (SEM) analysis was conducted using an ultra-high
vacuum and high-resolution FEI, XL-30 scanning electron
microscope for the morphological analysis of activated and

Effect of mechanical activation on mullite formation

David O. Obada et al.

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non-activated mixed powders. Table II gives the batch
formulation of stoichiometric mullite (100 Wt.%).

3. Results and discussion
The XRF and XRD of starting materials used in this study
are presented. Beneficiated kaolin, aluminum hydroxide,
alumina and silica gel were batch-formulated according to
stoichiometric equations as stated earlier for mullite-based
composite ceramics. Table III gives the elemental
composition of starting materials used in this study.

The XRD patterns for all starting materials are as shown in
Figures 1–5. Figure 1 shows the reduction in the intensity of
the quartz at Braggs’ angle of 26.65 for the beneficiated kaolin.
This is attributed to reduction in the concentration due to
washing away during the beneficiation process.

Table I Mechano-chemical milling test conditions and sample coding

Machine type Grinding media
Approximate grams of powder

used in one test
Milling time and
relevant sample coding Sintering temperature

Ball mill Ceramic balls (Ø10 mm) 100 0 min, CD0 NA
Ball mill Ceramic balls (Ø10 mm) 100 30 min, CD1 1,150°C
Ball mill Ceramic balls (Ø10 mm) 100 60 min, CD2 1,150°C

Table II Batch formulations of stoichiometric mullite

Raw material (mullite) Amount in composition (Wt.%)

Kaolin, Al2(Si2O5)(OH)4 31.55
Alumina, Al2O3 21.04
Aluminum hydroxide, Al(OH)3 27.61
Silica, SiO2 19.80
Total 100

Table III Chemical composition of starting raw materials

Element
wt %

(Kaolin)
wt %

(Alumina)

wt %
(Alumina

hydroxide)
wt %

(Silica gel)

Si 59.5886 0.1125 ND 99.102
Na 0.0618 0.055 0.1721 ND
Mg 0.0759 ND ND ND
Al 38.528 99.425 99.823 0.1125
P ND ND ND ND
S 0.0043 ND ND ND
Cl ND ND ND 0.5081
K 1.4389 ND ND ND
Ca 0.0414 ND ND ND
Ti 0.0283 ND ND ND
Mn 0.0343 ND ND 0.0522
Fe 0.1495 ND ND 0.0146
Zn ND ND ND ND
Rb 0.0084 ND ND ND
Zr 0.0073 ND ND ND
Ba 0.0296 ND ND ND
Co ND ND ND 0.1971
Ni ND ND ND 0.0134
Ga 0.0037 ND 0.0044 ND

Note: ND–not detected

Figure 1 XRD pattern of beneficiated Kankara kaolin

Figure 2 XRD pattern of aluminium hydroxide

Figure 3 XRD pattern of alumina

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Figures 2 and 3 show the characteristic XRD peaks of mixed
phases of aluminum hydroxide and pure alumina which could
be observed at Bragg’s angles of 20.4, 27.9, 38.0, 40.3, 48.7
and 64.80. The peaks at Bragg’s angles 27.9, 38.0, 48.7 and
64.80 were the characteristic peaks for the boehmite phase of
aluminum hydroxide and alumina, while peaks at 20.4, and
40.3 were due to the bayerite phase of aluminum hydroxide
(Du et al., 2009). It can be observed that peaks were not
noticeable between Braggs’ angles of 30-35 in Figure 2 in
comparison to Figure 3; this can be attributed to the
percentage composition in terms of alumina content in the two
samples.

Figure 4 shows XRD patterns of silica gel. As expected, the
characteristic peak of quartz is observed at Braggs’ angle of
26.65. The XRD pattern of the silica gel shows the powder is
mostly amorphous in nature. However, the broad peak around
Bragg’s angle 22.8 may be attributed to crystalline silica.

The XRD pattern of pure mullite-based ceramic batch
composition sintered at 1,150°C for 2 h has been shown in
Figure 5, where CD1 and CD2 represent the samples with
100 per cent mullite composition ball milled for 30 min and
60 min and fired at 1,150°C for 2 h, respectively. All the
samples show the presence of mainly mullite phases only. No
other impurity could be detected in the XRD analysis.

A comparison of the peaks from the two diffraction patterns
shows that all the diffraction peaks change after mechanical
activation. This reflects the partial amorphization and

structural disordering in alumina and silica (quartz).
Mechanical activation has already been reported to amorphize
materials (Balaz, 2008). Without sufficient milling, the
precursor powders would remain largely unmixed, and as a
result of inefficient contact between particles, the intermediate
phase transitions could largely be inhibited. Differences in
reaction products did develop during the sintering process,
indicating that the milling method induced different degrees
of damage to the starting powder system. The reaction
sintering of kaolinite, alumina, aluminum hydroxide and silica
is a complex process, with various intermediates being created
and consumed before the final structure is generated. These
transformations occur at different temperatures depending on
the grinding method and the duration of grinding. When the
heat treatment was undertaken at 1,150°C, powder which had
been milled with greater intensity showed higher levels of
alumina-quartz decomposition and corresponding growth of
mullite. It is assumed that a better mechanically induced
transformation after sintering the powders will be the
formation and subsequent removal of mullite, suggesting that
the mullite reaction should occur at lower temperatures in
powders, which is the case in this study. It is obvious that if
ball milling is used, then mullite remains under all conditions.
This finding is a further confirmation of mechanically
promoted phase transformation. Mullite as a decomposition
product of kaolinite is expected to form above 1,000°C (Chen
et al., 2003), and mechanical activation seems to have
improved mullite formation after the material is sintered at
1,150°C for a 2-h dwell time.

Comparatively, with the results obtained in this study at
lower temperature, Wu et al. (2013) reported a homogeneous
coating on mullite particles where the mullitization occurs
completely at as low as 1,250°C. The result in this present
study showed mullitization at a lower temperature of 1,150°C,
which is due to the intensive milling applied to the precursors
of starting materials. Elmas et al. (2013) showed the XRD
patterns of non-activated and activated quartz-alumina
mixture powders fired at 1,250°C and 1,375°C for 60 min. It
was found that the intensity of the mullite phase in the
activated sample increases. On the other hand, when they
were sintered at 1,375°C for 60 min, the mullite content
increased. The mullitization temperature recorded occurred at
even higher temperature as compared to the mullitization
temperature, which was as low as 1,150°C, as achieved in this
study. Their study adopted the planetary ball milling in
activating precursors of starting materials at the milling time of
60 min. The present study used a pre-milling approach before
adopting the ceramic ball milling approach using the tumbling
action to activate precursors of starting materials for 30 and 60
min, respectively. This may have systematically induced to a
larger extent the structural disordering of the starting
materials, hence enhancing their surface areas and reducing
significantly their particle sizes. This in turn increases their
reactivity and hence reduces the mullitization temperature.
According to Kirsever et al. (2015), when the samples were
sintered at 1,200°C for 1 h, the quartz and alumina were noted
to be the major phases. The mullite phase appeared as the
sample was sintered at 1,300°C for 1 h. When sintering
temperatures were elevated to between 1,300 and 1,450°C,
the intensity increased and the much sharper X-ray reactions

Figure 4 XRD pattern of silica gel

Figure 5 XRD patterns of sintered mullite batch precursors

Effect of mechanical activation on mullite formation

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of mullite were observed. It was found that quartz and alumina
reacted completely and had mainly converted to mullite phase
at 1,450°C. It is worthy of note that in their study, no
mechanical activation was performed to disorder the structure
of the materials, hence the high temperatures reported in
conversion to mullite. Hence, mechanical activation as
reported in this study is key to achieving a much lower
mullitization temperature. Baitalik et al. (2015) in their study
sought to keep mullite content same in an SiC/SiC � mullite
composite structure in air at 1,300°C for 3 h. Notably,
sintering at this temperature of 1,300°C kept the mullite
content intact. No information of intense milling was recorded
for this study, hence the relatively high temperature needed to
keep the mullite content in the composite intact.

From the foregoing, it is interesting to note that the
pre-milling and ball milling approach using the tumbling
action as employed in this study ensured to a large extent the
structural disordering of starting raw materials, thereby
lowering significantly the mullitization temperature.

Fotoohi and Blackburn (2013) reported shorter and longer
milling times than the ones presented in this study. For the
ball mill approach, they considered 60, 300 and 600 min; for
the ring mill approach, 2, 5 and 10 min; and for the planetary
mill, 10, 20 and 50 min for three batch-formulated samples.
The ball milling adopted for a milling period of 60 min, as
investigated by Fotoohi and Blakburn (2013) after sintering at
1,000°C, led to a body rich in enstatite and amorphous silica
phases. It was apparent that the milling route and the duration

of milling strongly influenced the phase development at this
sintering temperature. Ring milling for short periods resulted
in mullite formation but when milled for longer, magnesium
aluminosilicate (MAS) dominated the traces mainly because it
was a magnesia-alumina-quartz ceramic system. A slightly
different phase composition was observed when applying each
milling. In the planetary-milled samples as reported, for
sample milled for 50 min, mullite and spinel phases were still
found in relatively high proportions associated with the MAS
phase. Comparatively, this present study which considered a
ball milling approach with milling times of 30 and 60 min,
respectively, at a temperature of 1150°C produced mainly
mullite phases when analyzed by the XRD. This is in
agreement with findings of Fotoohi and Blackburn (2013)
when the ring milling approach was used. However, at our
sintering temperature and milling times, results show mainly
mullite phase formation.

The morphology and size distribution of the precursor as
well as the ground powders were evaluated with SEM.
Differences in shape and size of the individual powders and
relatively high degree of agglomeration were found to be major
obstacles in achieving accurate size measurements. Different
milling times as expected resulted in powders with different
particle sizes, shapes and, most importantly, degrees of
agglomeration.

As illustrated in Figures 6 and 7, short milling times only
helped in mixing of the precursor powders and caused partial
agglomeration; this was observed in sample CD1. Longer
milling times, however, resulted in greater agglomeration in
CD2. When the milling times are extended, agglomeration
was observed to increase possibly as a result of increases in
surface energies and the development of defects and
amorphization. Complete amorphization was not achieved
using the applied milling method and times; however,
differences in microstructure and reaction products did
develop during the sintering process, indicating that the
milling method induced different degrees of damage to the
starting powder system. The reaction sintering of kaolinite and
alumina is a complex process, with various intermediates
being created and consumed before the final structure is
generated. These transformations occur at different
temperatures depending on the grinding method and the
duration of grinding.

Figure 6 SEM image of starting raw materials (CD0) at 200�

Figure 7 SEM images of mullite powders (CD1 and CD2) at 200�

Effect of mechanical activation on mullite formation

David O. Obada et al.

World Journal of Engineering

Volume 13 · Number 4 · 2016 · 288–293

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In general, the more intense the milling, the fewer phases
were identifiable in the sintered samples. Occasionally, large
grains were seen to emerge from the background, and these
had elongated rectangular shapes.

After a successful low-temperature route fabrication of
mullite powders, it is critical to note that for this study, a
manual pre-milling of approximately 20 min and a ball milling
approach of 60 min milling time can be suggested as the
optimum milling time for the temperature decrease succeeded
for the specific batches. It is noted that the tumbling action of
the ball mill and the obvious reduction in sizes and
agglomeration of particles of precursor materials during
intensive milling are significant variables which affect both the
milling time and sintering temperatures toward mullite
production.

4. Conclusion
The effect of mechanical activation on mullite formation in a
batch mix system was studied by using XRD and SEM. The
mechanical activation caused amorphization and structural
disordering of the silica-alumina mixture. The application of
energy milling allowed a change in the structure performance
of batch mix, so the content of mullite was increased with
mechanical activation. It is noted that, a manual pre-milling of
approximately 20 min and a ball milling approach of 60 min
milling time can be suggested as the optimum milling time for
the temperature decrease succeeded for the production of
mullite from the specific stoichiometric batch formed.

References

Baitalik, S., Kayal, N. and Chakrabarti, O. (2015),
“Fabrication of mullite bonded porous sic ceramics via
sol-gel coated precursor”, Transactions of the Indian Ceramic
Society, Vol. 74 No. 3, pp. 181-185.

Balaz, P. (2008), Mechanochemistry in Nanoscience and
Minerals Engineering, Springer-Verlag, Berlin.

Behmanesh, N., Heshmati-Manesh, S. and Ataie, A. (2008),
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processing of nano-structured mullite phase”, Journal of
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Chen, Y.F., Wang, M.C. and Hon, M.H. (2003),
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Dong, Y., Feng, X., Feng, X., Ding, Y., Liu, X. and Meng, G.
(2008), “Preparation of low-cost mullite ceramics from
natural bauxite and industrial waste fly ash”, Journal of
Alloys and Compounds, Vol. 460 No. 1, pp. 599-606.

Du, X., Wang, Y., Su, X. and Li, J. (2009), “Influences of pH
value on the microstructure and phase transformation of
aluminium hydroxide”, Powder Technology, Vol. 192 No. 1,
pp. 40-46.

Elmas, E., Yildiz, K., Toplan, N. and Toplan, H.O. (2013),
“Effect of mechanical activation on mullite formation in an
alumina-quartz ceramics system”, Materiali in Tehnologije,
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Fotoohi, B. and Blackburn, S. (2012), “Study of phase
transformation and microstructure in sintering of mechanically
activated cordierite precursors”, Journal of the American
Ceramic Society, Vol. 95 No. 8, pp. 2640-2646.

Kirsever, D., Karabulut, N.K., Toplan, N. and Toplan, H.O.
(2015), “Recycling of secondary aluminium production
waste in processing of mullite based refractory ceramics”,
Acta Physica Polonica A, Vol. 127 No. 4, pp. 1035-1037.

Mazzoni, A., Aglietti, E.F. and Pereira, E. (1991),
“Preparation of spinel powders (MgAl2O4) at low
temperature by mechanical activation of hydroxide
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Souto, P.M., Menezes, R.R. and Kiminami, R. (2009),
“Sintering of commercial mullite powder: effect of MgO
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Tamborenea, S., Mazzoni, A.D. and Aglietti, E.F. (2004),
“Mechanochemical activation of minerals on the cordierite
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Viswabaskaran, V., Gnanam, F.D. and Balasubramanian, M.
(2002), “Mullitisation behaviour of South Indian clays”,
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Wu, D., Mao, F., Wang, S. and Zhou, Z. (2013), “A
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Further reading

Lee, J.S., Kim, H.S., Park, N., Lee, T.J. and Kang, M.
(2013), “Low temperature synthesis of �-alumina from
aluminium hydroxide hydrothermally synthesized using
[Al(C2O4)x (OH)y] complexes”, Chemical Engineering
Journal, Vol. 230, pp. 351-360.

Corresponding author
David O. Obada can be contacted at: obadavid4@gmail.
com

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	Effect of mechanical activation on mullite formation in an alumina-silica ceramics system at low ...
	1. Introduction
	2. Materials and methods
	3. Results and discussion
	4. Conclusion
	References