351
BILJANA R. ILIĆ
ALEKSANDRA A. MITROVIĆ
LJILJANA R. MILIČIĆ
Institute for Testing of Materials,
Belgrade, Serbia
SCIENTIFIC PAPER
UDC 553.612:66.094.32
DOI: 10.2298/HEMIND100322014I
THERMAL TRE
A
TMENT OF KAOLIN CLAY
TO OBTAIN METAKAOLIN
The metakaolin was produced by thermal treatment (calcination) of the starting high-
-quality kaolin clay from Serbia. The optimal calcination parameters, for which nearly
complete dehydroxylation of the material was achieved, are: temperature 650
°
C and
heating time of 90 min. The conversion of the kaolinite to metakaolinite was confirmed
by XRD and IR analyses of the starting and thermally treated kaolin samples. The poz-
z
olanic activity was determined by Chapelle test. The obtained value 0.65 g Ca(OH)
2
/g
of metakaolin indicates that produced metakaolin may be used as supplementary ce-
mentitious material.
Development of construction materials which of-
fers technical and environmental benefits is the main
challenge of the new millennium. One of such materials
is metakaolin (MK), pozzolanic addition, which is clas-
sified as a new generation of supplementary cementitous
material. Supplementary cementitious materials (SCMs)
are finely ground solid materials that are used to replace
part of the clinker in a cement or cement in a concrete
mixture. Use of metakaolin in cement-based systems,
provides, beside technical [1,2], significant environmen-
tal benefits [3]. Metakaolin is unique in that it is not the
by-product of an industrial process nor is it entirely na-
tural; it is derived from a naturally occurring mineral,
and is manufactured specifically for cementing applica-
tions. Metakaolin is usually produced by thermal treat-
ment, i.e., calcination of kaolin clays within a definite
temperature range.
Serbia has high-quality kaolin clay deposits at Aran-
djelovac and Kolubara basin, and thus good potential to
produce metakaolin from them.
The main process important for production high
reactivity pozzolana from kaolin clay is calcination. The
heating process drives off water from the mineral kao-
linite (Al
2
O
3
⋅2SiO
2
⋅2H
2
O), the main constituent of kao-
lin clay, and collapses the material structure, resulting in
an amorphous aluminosilicate (Al
2
O
3
⋅2SiO
2
), metakao-
linite. The process is known as dehydroxylation [4], and
may be presented by simple equation:
Al
2
O
3
⋅2SiO
2
⋅2H
2
O → Al
2
O
3
⋅2SiO
2
+ 2H
2
O↑ (1)
The thermal transformation of kaolinite, which has
been the subject of a large number of investigations [5–
–8], has shown that the heating parameters such as tem-
perature, heating rate and time, as well as cooling para-
meters (cooling rate and ambient conditions), signifi-
cantly influence the dehydroxylation process.
The major quantitative criterion for evaluating the
performance of kaolinite by thermal treatment is a de-
gree of the dehydroxylation, D
tg
[9]:
Corresponding author: B. Ilić, Institute for Testing of Materials, Bel-
grade, Bulevar vojvode Mišića 43, Belgrade, Serbia.
E-mail: biljana.ilic@institutims.rs
Paper received: 22 March 2010
Paper accepted: 13 April 2010
D
tg
= 1 − (M/M
max
) (2)
where M and M
max
are residual and maximum sample
mass loss, respectively. The dehydroxylation of pure
kaolinite (39.5% Al
2
O, 46.5% SiO
2
and 14% H
2
O) in
ambient atmosphere results in mass loss of about 14%
and D
tg
= 1, which corresponds to mass in bound hyd-
roxyl ions in kaolinite.
The development of pozzolanic properties in fired
clays mainly depends on nature and abundance of clay
minerals in raw materials, on calcination conditions and
on the fineness of the final product [10–12]. The calci-
nation temperature producing the reactive state is usual-
ly in the range of 600–800 °C, and plays a central role
in the reactivity of the resulting MK product. On
prolonged heating, recrystallization and mullite
(3Al
2
O
3
⋅2SiO
2
) formation take place resulting in the
decline of material reactivity.
The main characteristic of produced metakaolin,
for the use in cement based systems, is its pozzolanic
activity, which is defined as ability of material to react
with calcium hydroxide in the presence of water to form
compounds that have cementitious properties.
Pozzola-
nic activity may be determined by direct [13–15] and
indirect methods [16]. Direct methods are based on the
measurement of the amount of lime reacted, which is
determined by instrumental techniques such as thermo-
gravimetry (TG), differential thermal analysis (DTA),
X-ray diffraction (XRD) and calorimetric analysis. Indi-
rect methods are based on the strength development oc-
curring with reaction time. The values of pozzolanic ac-
tivity indicate how good and effective pozzolan is.
The goal of this study is to determine optimal calci-
nation parameters for obtaining metakaolin from domes-
tic high-quality kaolin clay, for the use as supplemen-
tary cementitous material.
EXPERIMENTAL
Properties of the starting clay
Our previous study [17] showed that starting clay
is high quality kaolin clay, with kaolinite content of
about 80% and loss of ignitation (LOI) on the tempera-
ture 950±25 °C of 12.33%. Kaolin clay was collected
from the location Vrbica (Arandjelovac basin). Repre-
B.R. ILIĆ, A.A. MITROVIĆ, Lj.R. MILIČIĆ: THERMAL TREATMENT OF KAOLIN CLAY... Hem. ind. 64 (4) 351–356 (2010)
352
sentative 5-kg samples of kaolin clay were taken from
the initial materials using the quartering method. Before
being characterized, the samples were dried to less than
0.5% moisture content, crushed and milled (to the par-
ticle size of less than 43 μm).
Chemical composition, determined by silicate me-
thod, and physical characteristics such as specific gra-
vity, γ
sr
, specific surface area, S
p
, and fineness, R (given
as residue on the sieve), determined according to stan-
dards SRPS EN 196-3 and SRPS EN 196-6, are given in
the Table 1.
Table 1. Chemical composition and physical characteristics of
the starting clay
Component Content, mass%
SiO
2
48.00
Al
2
O
3
31.75
Fe
2
O
3
4.38
CaO 1.00
MgO 0.48
N
a
2
O 0.16
K
2
O 1.50
LOI 12.33
Physical characteristics
γ
sr
, g/cm
3
2.60
S
p
, cm
2
/g 9180
R
, % (sieve, 0.043 μm) 4.90
Applied experimental techniques
Mineralogical composition of the starting and ther-
mally treated clays was determined using a Siemens
D5000 diffractometer (CuKα radiation, Ni filter).
Thermal behavior of the starting clay was investi-
gated using a Netzsch STA 409EP instrument. The
sample was heated from 20 to 1000 °C at a constant rate
of 10 °C/min in air.
In order to confirm the characteristic bands of kao-
linite in raw sample and the absence of these bands in
thermally treated samples, a FTIR spectrophotometer,
Nicolet 6700 Thermo Scientific, was used.
The pozzolanic activity of the thermally treated
samples (metakaolin) was evaluated according to Cha-
pelle test [18]. Metakaolin of a mass of 1 g was mixed
with 1 g Ca(OH)
2
and 200 mL boiled water. The sus-
pension was subsequently boiled for 16 h and the free
Ca(OH)
2
was determined by means of sucrose extrac-
tion and titration with HCl solution.
Calcination/dehydroxylation procedure
Samples of about 50 g were heat treated in the la-
boratory furnace at different temperatures (550, 600,
650 and 700 °C) and at different heating times (30, 60,
90, 120, 150 and 180 min). After heating, the samples
were “quenched” to room temperature at ambient condi-
tions to avoid crystallization of amorphous metakaolin.
The weight of the samples before and after the
thermal treatment was measured in order to determine
weight loss during calcinations process.
RESULTS AND DISCUSSION
Thermal behavior of the starting clay is presented
in the Figure 1. The main changes revealed by TG and
DTA analysis are as follows [14]. At temperatures be-
low about 200 °C release of water absorbed in pores and
on the surfaces occurs. Between 200 and 450 °C, mass
F
igure 1. DTA/TG curve of the startin
g
clay.
B.R. ILIĆ, A.A. MITROVIĆ, Lj.R. MILIČIĆ: THERMAL TREATMENT OF KAOLIN CLAY... Hem. ind. 64 (4) 351–356 (2010)
353
loss attributed to the pre-dehydration process takes pla-
ce, as a result of the reorganization in the octahedral
layer. In the temperature range 450–650 °C, dehydroxy-
lation of kaolinite and formation of metakaolinite takes
place, while at about 1000 °C, mullite was formed, as
indicated by an exothermic peak. The observed endo-
thermic peak with a maximum at 552 °C may be attri-
buted to dehydroxylation process.
In order to obtain optimal calcination parameters,
the clay was subjected to thermal treatment at different
heating temperatures and times. The mass loss of start-
ing clay for given calcinations parameters is given in
Table 2.
As can be seen, for calcination temperatures of
550, 600 and 650 °C, mass loss increases up to 90 min,
while prolonged heating has a negligible effect on the
mass loss. For all applied heating times at temperature
700 °C obtained values for mass loss are nearly the same
≈12%. It is evident that at calcination temperature 650
°C and heating time 90 min, mass loss is almost iden-
tical with the values obtained at 700 °C. If we take eco-
nomic factors into consideration, the optimal parameters
for calcination are temperature 650 °C and heating time
of 90 min.
Using mass loss values during calcination, and LOI
obtained by chemical analysis (M
max
), the degree of de-
hydrohylation calculated by Eq. (2) are presented in
Figure 2.
As can be seen, nearly complete dehydroxylation
was achieved after 90 min for temperature of 650 and
700 °C, for which the degree of dehydroxylation, D
tg
, is
0.97.
In order to confirm disappearance of kaolinite peaks,
after thermal treatment, the XRD patterns of starting
and calcined clay were compared. The results are pre-
sented in Figure 3. It is evident (Figure 3a) that the ma-
jor mineral constituents of the starting clay are kaolinite
and quartz. The results of XRD measurements of the
calcined clays, selected on the base on their degree of
dehydraxylation, are given in Figures 3b–3d. After ther-
mal treatment of clays at temperatures 600, 650 and 700
°C and heating time 90 min, characteristic peaks for kao-
linite (2θ 12.41, 20.21 and 25.49°) [19] disappear, while
peaks assigned to quartz (2θ 21.22 and 27.45°) remains
unchanged.
Besides XRD measurements, IR spectroscopy was
applied to confirm kaolinite transformation during calci-
nation. IR spectra obtained for starting clay and ther-
Table 2. Mass loss (%) of kaolin clay for different calcination temperatures and times
Heating time, min
Temperature, °C
550 600 650 700
15 – – 10.72 11.90
30 9.26 10.56 11.22 12.00
60 10.49 11.42 11.73 12.05
90 11.11 11.80 12.00 12.07
120 11.24 11.80 12.03 12.15
150 11.33 11.74 11.92 12.11
180 11.51 11.76 – –
F
igure 2. Dependence of degree of the dehydrohylation, D
t
g
, on calcination time for different temperatures of thermal treatment.
B.R. ILIĆ, A.A. MITROVIĆ, Lj.R. MILIČIĆ: THERMAL TREATMENT OF KAOLIN CLAY... Hem. ind. 64 (4) 351–356 (2010)
354
mally treated samples are presented in Figure 4. The re-
sults of IR spectroscopy of starting clay (Figure 4a)
show the characteristic bands of kaolinite [20,21]: OH
−
at 3700, 3650, 3620 cm
−1
; Al–OH at 913 cm
−1
; Si–O at
1032, 1008, 469 cm
−1
and Si–O–Al
VI
at 538 cm
−1
.
Absence of the detectable Al–O–H bands at 913
cm
−1
, and the doublet at 3700 and 3620 cm
−1
, is evident
from Figure 4b–4d. Absence of the band at 539 and 913
cm
−1
and the appearance of a new band at 800 cm
−1
can
be related to the change from octahedral coordination of
Al
3+
in kaolinite to tetrahedral coordination in metakao-
linite. The bands at 1100 and 1200 cm
−1
are assigned to
amorphous SiO
2
.
For the optimal calcination parameters (tempera-
ture 650 °C and heating time 90 min) pozzolanic acti-
vity is 0.65 g Ca(OH)
2
/g of metakaolin. The obtained
value is in agreement with the results obtained by other
authors (values from 0,55 to 0,82 g Ca(OH)
2
/g of meta-
kaolin) [14].
CONCLUSION
Metakaolin, pozzolanic additive, may be obtained
by calcination of kaolin clay. The optimal conditions of
the thermal treatment are: calcination temperature of
650 °C and heating time of 90 min. Calcination results
in dehydroxylation degree of 0.97. The produced
metakaolin has pozzolanic activity 0.65 g Ca(OH)
2
/ g
of metakaolin.
Acknowledgement
This research has been supported by the Serbian
Ministry of Science and Technological Development
(Project No. TR 19206A).
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0 20406080
500
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K
Q
Q
Q
L
I(Impulsi)
2θ
M
F
(a) (b)
0 20406080
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igure 3. XRD Patterns of starting kaolin clay (a) and thermally treated (calcinated) at 600
°
C for 90 min (b); at 650
°
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(c) and at 700
°
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Si
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(OH)
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O
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(OH)
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(Si
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⋅
nH
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(a) (b)
(c) (d)
F
igure 4. IR spectra of starting kaolin clay (a) and thermally treated (calcinated) at 550
°
C for 90 min (b); at 600
°
C for 90 min (c)
and at 650
°
C for 90 min (d).
