MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 63, 1, (2011), 74-79
www.ptcer.pl/mccm
Inuence of Firing Conditions on Properties
of Ceramic Materials Made of Carbon Slate
JÓZEF STOLECKI*, PAWE MURZYN
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Kraków, Poland
e-mail: [email protected]
Abstract
The results of investigations on ceramic materials properties are presented in this paper. The ceramics were obtained by mixing carbon
slate from Lublin Coal Basin (LCB) and clays with addition of melaphyre (M) and quartz sand (QS). Subsequently the samples were red
according to three different ring cycles. The maximum temperature of ring was 1100°C and the cycles differed in the heating rate and
soaking time. The ring cycles were modied in order to obtain a ceramic material with the limited number or even without defects, aiming
also to meet the quality characteristics of clinker ceramics. The results show that the best properties were obtained for the ceramics red
according to the cycle with the longest duration. An addition of melaphyre and other plastic clays to the carbon slate ceramic masses improves
the properties of the so-obtained ceramics. In most cases, the ceramics from carbon slate are characterized by satisfactory compression
strength (about 40-65 MPa), required for clinker ceramics, low water absorption (about 8.5-12 %) and full resistance in the freeze-thaw test.
Keywords: Carbon slate, Clay, Melaphyre, Quartz sand, Clinker brick, Ferruginous stain, Black core
WPYW WARUNKÓW WYPALANIA NA WACIWOCI CERAMIKI WYKONANEJ
Z UYCIEM UPKÓW WGLOWYCH
W artykule przedstawiono wyniki bada dotyczcych waciwoci materiaów ceramicznych. Badan ceramik wytworzono drog mieszania upku wglowego pochodzcego z Lubelskiego Zagbia Wglowego (LZW) z glin z dodatkiem melaru (M) i piasku kwarcowego (QS). Nastpnie próbki wypalono stosujc trzy róne cykle ogrzewania. Maksymalna temperatura wypalania wynosia 1100°C, a cykle
róniy si szybkoci ogrzewania i czasem wytrzymania. Cykle wypalania modykowano, aby otrzyma materia ceramiczny z ograniczon liczb lub pozbawionego wad, dc do spenienia wymaga jakociowych ceramiki klinkierowej. Uzyskane wyniki pokazuj, e najlepsze waciwoci uzyskano w przypadku ceramiki wypalanej w najduszym cyklu ogrzewania. Dodatek melaru i innych glin plastycznych do mas ceramicznych zawierajcych upek wglowy poprawia waciwoci otrzymanej ceramiki. W wikszoci przypadków ceramika z upku wglowego charakteryzowaa si zadowalajc wytrzymaoci na ciskanie (okoo 40-65 MPa), wymagan w przypadku ceramiki klinkierowej, nisk nasikliwoci wody (okoo 8,5-12 %) i cakowit odpornoci w tecie zamraania-odmraania.
Sowa kluczowe: upek wglowy, glina, melar, piasek kwarcowy, cega klinkierowa, elazista plama, czarny rdze
74
1. Introduction
2. Experimental
In comparison to traditional clay raw materials (loams and
clays) carbon slates (from LCB) can be characterized by the
increased content of organic substance (coal inclusions) that
is visible on TG curve as an increased loss on ignition. This
organic substance makes difcult to obtain strongly sintered
ceramic products (clinker bricks) with low water absorption
[1]. Moreover, it causes the formation of low-melting ferruginous, reductive stains on a shell of manufactured bricks
which worsen the outward appearance of the products and
the presence of reductive black core inside the ceramic body.
The volume of reductive core depends on the content of
organic substance in the natural slate and on its oxidations
degree during ring. The less the organic substance the
slate contains and the more this substance is oxidized while
ring, the less the mentioned phenomena are noted [2, 3].
2.1. Raw materials
In this investigation, the mentioned below raw materials
were used to obtain ceramic masses and ceramic materials
on their basis:
– carbon slate (L) – waste material in hard coal mining
in LCB,
– tertiary miocene clay (Im),
– quaternary pliocene clay (Ip),
– quartz sand (QS),
– melaphyre (M).
The research program of the raw materials included the
following tests: particle size distribution, mineral and chemical
composition, thermal analysis. The raw materials were used
to make ceramic masses, then samples were formed, which
INFLUENCE OF FIRING CONDITIONS ON PROPERTIES OF CERAMIC MATERIALS MADE OF CARBON SLATE
after drying were red and tested in order to determine their
properties. Additional raw materials were used in order to:
– increase the porosity of the green body to facilitate oxidation of the organic matter: M, QS;
– improve the rheological properties of the ceramic masses:
Im, Ip;
– increase the liquid phase while sintering: M;
– enhance the properties of the ceramics: M.
Another way to improve the properties of the ceramics
was to work out suitable ring programs. Three ring cycles
were used in this research, differing in the heating rate as well
as in the holding time in given temperature. The main goals
of the temperature program modications were to achieve
oxidation of the majority of the organic substance (coal inclusions), included in ceramic masses as part of carbon slate,
before the sintering process occurred (at 900ºC) and to obtain
while sintering the crystalline and amorphous phases giving
high mechanical strength and low water absorption [4, 5].
Table 1. Chemical composition of raw materials [wt%].
Component
Slate (L)
Pliocene
clay (Ip)
Miocene
clay (Im)
Melaphyre
SiO2
46.80
59.31
52.60
53.80
Al2O3
23.90
14.52
12.85
14.00
FeO+Fe2O3
4.55
8.36
4.85
9.60
TiO2
1.02
0.78
0.64
1.35
CaO
2.00
1.97
9.15
6.50
MgO
0.15
1.24
2.80
2.80
Na2O
0.33
0.61
2.00
3.65
K2 O
1.78
2.42
2.30
2.40
SO3
0.22
0.09
0.02
0.12
L. ign.
18.40
10.22
12.79
5.10
Total
99.17
99.52
100.30
99.34
Cat
6.5
-
-
-
Corg
5.4
-
-
-
2.2. Properties of the raw materials
Carbon slate (CS) is a waste material which occurs in
ceiling and oor of hard coal beds and often makes up also
vain insertions in mere carbon rock with thickness of the
laminas up to 10 cm. It is mined during exploitation of hard
coal-beds. Figs. 1 and 2 show the carbon slate in its natural
and ground form, respectively.
From the petrographic point of view these rocks were
found to be of sedimentary origin and composed of clay
carbon slates with different admixtures of coal matter (shales
Fig. 1. Carbon slate in natural form.
Table 2. Particle size distribution of the raw materials.
Share of fraction [wt%]
Fraction
[mm]
L
Qs
Ip
Im
Mz
Mm
>2.0
0.0
0.0
0.0
0.0
0.3
0.0
1.02-2
1.5
0.0
3.2
0.1
21.0
0.1
0.49-1.02
20.0
4.3
26.4
21.7
27.6
0.4
0.25-0.49
26.5
56.6
22.8
21.8
22.6
14.6
0.12-0.25
15.5
34.4
14.1
11.9
12.6
21.9
0.063-0.12
32.0
3.5
5.4
4.6
4.3
10.0
<0.063
4.5
1.2
28.1
39.9
11.6
52.6
and claystones), siltstones, seldom of sandstones and clayey
siderites. Mineral composition of slates contains clay and
non-clay minerals. The latter are the major components.
Amongst clay minerals a principal role is played by kaolinite;
the typical shape of DTA curve is shown in Fig. 3. Illite is
present as an admixture. In addition, there are also minerals
of the montmorillonite group. Non-clay minerals are predominated by quartz and coal substance. Feldspars, pyrite,
siderite and dolomite are accessory minerals [6].
From the chemical point of view the slates are rich in Al2O3
and indicate considerable burning loss (TG curve in Fig. 3).
From the point of view of building ceramic materials technology, claystone and clay slates are interesting because of similar mineralogical composition (kaolinite, illite, montmorillonite,
quartz, ferruginous minerals) to the tertiary and quaternary
clays that are commonly used in the production of building
ceramic materials [7, 8]. In comparison to typical clay raw
materials, carbon slates contain a larger amount of organic
substances, especially coal substance. The quantity of the
coal substance amounts about 5 wt%, and loss on ignition
is about 20 wt% at 1000ºC (Table 1, Fig. 3).
It is characteristic for the slates to be strongly packed in
its natural form (Fig. 1), and their plasticity depends on their
granularity; therefore in industrial practice the slate has to be
milled. The slate is usually milled to granulation lower than
1 mm. The ner the slate is milled, the more water is neces-
Fig. 2. Carbon slate ground to granulation below 1.0 mm.
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 63, 1, (2011)
75
J. STOLECKI, P. MURZYN
Fig. 3. DTA, TG curves of the carbon slate.
1000
Q
M
900
800
a)
700
600
K
500
400
M
M
M
K
200
M
M
100
M
K
M
K
M
300
M
M
K
K
M M
MM
M
K
M
M
K
M M
K
K
M
Q
0
5 6 7 8 9 101112131415161718 192021222324252627 28293031323334353637 383940414243444546 474849505152535455 5657585960
Fig. 4. XRD pattern of the carbon slate; K – kaolinite, Q – quartz,
I – illite, M – montmorillonite.
sary to be added to raw materials and the better plasticity
possesses the ceramic mass. The plasticity, another technological property of the slate, depends also on the kind and
quantity of minerals contained in it. Particle size distribution
of the carbon slate used in the study is shown in Table 2.
In industrial practice, ground slate is used as a basic
raw material (80 vol.%), together with other additional raw
materials. Quartz sand and plastic clay are the most often
used as the additional raw materials. However, the products
obtained from this kind of ceramic masses have many defects
like low-melting ferruginous phases, discoloration on shell
(Figs. 5a and 6a), the reductive black core (Fig. 5b) and
ferrous chips (Fig. 6b).
The defects visible in Figs. 5 and 6 are probably caused
during ring by reducing atmosphere around the samples
due to a large content of organic substance in the slate [1-3].
The mineral and chemical composition of the slate in
uences its behaviour while heating (ring) as it can be seen by
the dilatometric method. The dilatometric curves presented
in Fig. 7 show the differences in changes of length of the
samples made of the slate only (L100) and with addition of
quartz sand (L90Qs10). Both curves indicate small increase
of the length to 900ºC (higher for sample L90Qs10 with Qs).
Next, the curves indicate a signicant shrinkage which is
caused by the sintering of the samples. The shrinkage of
sample L100 is larger (-10.3 %) than the one with addition
of Qs (L90Qs10).
Quartz sand (Qs) used in the research originates from
the bed placed near the factory. It contains 92.0 wt% of
quartz, 0.5 wt% of illite/smectite, about 1.0 wt% of muscovite/
76
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 63, 1, (2011)
b)
Fig. 5. Defects on the laboratory samples made of carbon slate:
a) low-melting ferruginous phases, b) black core.
chlorite, 0.5 wt% hematite/goethite and about 2.0 wt% of
feldspar. It is yellow colored and is free of organic substance
contamination. 95 wt% of quartz sand particles is smaller
than 0.5 mm (Table 2).
Melaphyres (coarse grained marked Mz, ne grained
- Mm) belong to the group of magmatic irruptive rocks.
The minerals of plagioclases, pyroxenes and olivines predominate in their mineralogical composition. The melaphyres
contain about 20 wt% uxing oxides numbered to uxes with
7 wt% of alkaline oxides. Such oxides in ceramic masses
intensify the process of sintering, contributing in the development of the liquid phase [4, 5]. The chemical composition of
melaphyre is shown in Table 1.
INFLUENCE OF FIRING CONDITIONS ON PROPERTIES OF CERAMIC MATERIALS MADE OF CARBON SLATE
Fig. 7. Dilatometric curves of samples L100 and L90Qs10.
a)
Table 4. Properties of the ceramic masses: Wz – batched water,
SD – drying shrinkage.
Mass no.
Notation
WZ
[wt%]
SD
[%]
0
L100
15.7
5.52
1
L95 Qs5
15.1
5.23
2
L90Qs10
13.5
5.19
3
L95 Mm5
14.1
5.15
4
L90 Mm10
14.1
4.82
5
L95 Mz5
14.1
4.70
6
L90 Mz10
14.0
4.68
7
L85 Im10 Mm5
14.7
5.10
8
L85 Ip10 Mm5
15.8
6.35
b)
Fig. 6. Defects on the products made of carbon slate: a) reductive
stains, b) ferrous chips.
Tertiary Miocene clay (Im) contains mainly illite accompanied by smectites and kaolinite. Moreover, it contains
carbonates (calcite, dolomite), giving the CaO and MgO
content of 9 wt% and 3 wt%, respectively (Table 1). This raw
material contains a signicant quantity of alkalis, about 6 wt%.
The above mentioned oxides create difculties while ring
because of their low melting point and very short sintering
interval (40ºC). This raw material shows the thermal softening
and thermal expansion [7, 9].
Quaternary Pliocene clay (Ip). The main mineral component of this raw material is quartz and clay minerals in various
contents. Clay minerals components are predominated by
illite and montmorillonite. Kaolinite is present in minor levels.
Feldspars, pyrite, siderite and dolomite are accessory minerals. In this kind of clays, illite often turns into montmorillonite
or individuals are formed with alternating illite-montmorillonite
packets, i.e., interstratied clay minerals. In building ceramics industry these kinds of clays are used in production of
all kinds of stock [7].
2.3. Preparation of ceramic masses and ceramic
materials
Ceramics masses were made of carbon slate with
additions of quartz sand (Qs), two qualities of melaphyre
differing in the particle size distribution (ne grained – Mm,
coarse grained – Mz) and two kind of clays. Compositions
Table 3. Raw material composition of ceramic masses.
Mass
no.
Notation
Share of components [wt%]
L
Qs
Mm
0
L100
100
1
L95 Qs5
95
5
2
L90Qs10
90
10
3
L95 Mm5
95
5
4
L90 Mm10
90
10
Mz
Im
Ip
-
5
L95 Mz5
95
5
6
L90 Mz10
90
10
7
L85 Im10 Mm5
85
5
8
L85 Ip10 Mm5
85
5
10
10
of ceramic masses made of carbon slate with additions are
shown in Table 3.
It was necessary to add about 14-16 % of water to the
dry raw material mixes to obtain plastic masses. This small
amount of water is suitable for low and middle plasticity ceramic masses. The least amount of water was required for
the masses with quartz sand and melaphyre (14 %).
Drying shrinkage of the ceramic masses depends on
properties and quantity of added raw materials and amounts in
the range from 4.70 %, for masses with melaphyre, to 6.35 %
for masses with melaphyre and Pliocene clay (Table 4). The
samples in green body state are presented in Fig. 8.
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 63, 1, (2011)
77
J. STOLECKI, P. MURZYN
Table 5. Parameters of the red samples: Ng – boiling water absorption, PW – open porosity, RC – compressive strength.
Mass no.
0
1
Fig. 8. Samples in green body state.
2
The samples were red in the laboratory furnace according to three different temperature programs. The maximum
ring temperature was 1100°C. The particular ring programs
differ in terms of heating rate, maximum ring temperature,
holding (retention) time and total ring time. The course of
the ring cycle in details is presented below.
The ring program no. 1:
20°C to 500°C - 100°C/h
500°C to 700°C - 50°C/h
700°C - isothermal step
700°C to 1100°C - 100°C/h
1100°C - soaking for 2h
Total ring time 17h
The ring program no. 2:
20°C to700°C - 100°C/h
700°C - isothermal step
700°C to 1100°C - 100°C/h
1100°C - soaking for 2h
Total ring time
3
4
5
5h
4h
2h
4h
2h
6
7
8
7h
4h
4h
2h
Maximum ring temperature = 1100°C
Parameter
Prog-1
Prog-2
Prog-2
Ng [%]
9.8
10.0
9.4
20.1
20.5
19.4
PW [%]
55.0
49.6
65.0
RC [MPa]
Ng [%]
11.9
12.0
11.5
24.0
24.1
23.4
PW [%]
48.1
47.2
48.4
RC [MPa]
Ng [%]
11.8
11.8
11.7
PW [%]
23.9
23.9
23.5
42.3
43.1
43.4
RC [MPa]
Ng [%]
9.5
9.7
8.8
PW [%]
19.5
19.9
18.3
52.5
53.8
55.9
RC [MPa]
Ng [%]
8.5
8.5
8.2
PW [%]
17.8
17.7
17.2
58.2
52.6
62.3
RC [MPa]
Ng [%]
11.3
11.1
10.6
PW [%]
22.9
22.5
21.7
59.1
59.2
60.1
RC [MPa]
Ng [%]
10.5
10.7
9.9
PW [%]
21.7
22.0
20.5
RC [MPa]
49.6
65.0
63.3
Ng [%]
8.9
8.5
8.4
PW [%]
18.4
17.7
17.4
44.9
57.3
50.6
RC [MPa]
Ng [%]
11.1
11.2
10.6
PW [%]
22.8
22.7
21.7
RC [MPa]
48.9
47.1
50.3
17h
The ring program no. 3:
20°C to700°C - 50°C/h
700°C - isothermal step
700°C to 1100°C - 100°C /h
1100°C - soaking for 2h
Total ring time 22h.
14h
2h
4h
2h
Cooling of the samples was free and uncontrolled.
3. Results and discussion
The ring temperature programs were modied in order to
decrease the amount of organic substance before the beginning of sintering process(i.e., high densication) which according to dilatometric tests occurs at the temperature 900°C.
For the red samples, the tests of water absorption,
compressive strength, sulphates content, freeze-thaw were
conducted. The properties of the ceramic materials obtained
in accordance to the three ring programs are presented in
Table 5. The appearance of the obtained ceramic materials
is shown in Figs. 9 and 10.
The materials red according to the third ring program
with the longest total ring time of 22 hours have the best
properties. The differences in properties between the samples red according to third ring program (22 h) and shorter
78
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 63, 1, (2011)
Fig. 9. Samples red according to the temperature program 1.
Fig. 10. Samples red according to the temperature program 3.
INFLUENCE OF FIRING CONDITIONS ON PROPERTIES OF CERAMIC MATERIALS MADE OF CARBON SLATE
ring programs 1 (17 h) and 2 (17 h) are not particularly large.
Nevertheless, the samples with ne grained melaphyre Mm
(L90 Mm10) and with ne grained melaphyre Mm together
with Miocene clay (Im) (L85 Im10 Mm5) present nal properties closer to the requirements for the dense ceramic material
(clinker bricks, stoneware). Generally, the addition of quartz
sand has negative in
uence on the properties of ceramic
materials and their properties are even worse than the samples made merely of carbon slate. The test for the presence
of soluble sulphates showed their lack in the material. The
samples were also checked by the freeze-thaw resistance
test (25 cycles from the temperature of +20°C to -25°C, and
back). After the test, there were no damages or defects in any
of the samples. Probably, a ring program with the 30-50°C
higher maximum ring temperature than the temperatures
used in this research would be better to obtain more dense
(better sintered) ceramic materials.
4. Conclusions
The samples red according to the curve III possess
better utility properties than those red according to curve
I and II. The ring curve III is characterized by the slowest
heating rate at the beginning of the ring process to the
temperature 700°C.
The type of the additional raw materials used in
uences
the properties of red materials. Ceramics made with addition of melaphyre have lower water absorption and higher
compressive strength compared to the ones made with the
addition of quartz sand.
Ceramics obtained with the addition of ne grained melaphyre have lower porosity and water absorption than those
obtained with the addition of coarse grained melaphyre [4].
The increase of melaphyre addition from 5 wt% to 10 wt%
improves the properties of the ceramics, such as water absorption and compressive strength.
Due to the lack of signicant correlation between compressive strength and ring temperature program, each
mixture of raw materials should be considered individually.
Referring to the results of compressive strength, the
optimal ring program for each individual ceramic mass can
be suggested: masses 5 and 8 – program 1; masses 2, 6,
7 – program 2; masses 0, 1, 3, 4 – program 3.
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Acknowledgements
The research work has been executed within the framework of research project of the Polish Ministry of Science
and Higher Education no. N N508 381835.
i
Received 1 March 2010; accepted 8 May 2010
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 63, 1, (2011)
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