Applied Clay Science 24 (2004) 237 – 243
www.elsevier.com/locate/clay
Effect of calcium hydroxide on slip casting behaviour
Aylin Sßakar-Deliormanlı a,*, Zeliha Yayla b
a
Chemical Engineering Department, Izmir Institute of Technology, Gülbahcße Köyü, Urla, Izmir, Turkey
b
Buca Education Faculty, Chemistry Department, Dokuz Eylul University, Buca, Izmir, Turkey
Received 1 October 2002; received in revised form 31 March 2003; accepted 22 April 2003
Abstract
The effect of calcium hydroxide addition on the casting performance of ceramic slips for sanitary ware was studied. Powder
composed of feldspar (24 wt.%), quartz (24 wt.%), kaolin (35 wt.%) and ball clay (17 wt.%) was mixed with water to contain 65
wt.% of solids (specific density 1800 g/l). Either Ca(OH)2 or Na2CO3 was added at concentrations ranging between 0.060 and
0.085 wt.% and the slurries were dispersed by the optimum addition of sodium silicate.
Calcium hydroxide in presence of sodium silicate improved the casting behavior of the slips, lowering the viscosity, and
water absorption, increasing bending strength and cake thickness, as compared to the addition of sodium carbonate.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Slip casting; Clays; Electrolytes; Ceramic processing
1. Introduction
The slip casting process is widely used to consolidate ceramic particles from aqueous suspensions. In
this process a porous mould is filled with a slip,
consisting of a ceramic powder mixed with water.
The capillary action due to the pores in the mould
withdraws the liquid medium from the slip. Excellent
fluidity, low water content, high stability, and high
water permeability are essential properties in ceramic
slip casting (Shan, 1990).
The main factor for successful wet processing of
slips that contain clay is the interactions between the
particles. Chemical additions strongly affect the sur* Corresponding author. Tel.: +90-232-4986246; fax: +90-2324986355.
E-mail address: aylindeliormanli@iyte.edu.tr
(A. Sßakar-Deliormanlı).
0169-1317/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.clay.2003.04.001
face chemistry of the ceramic powder by increasing or
decreasing the interaction forces between the particles
(Everest, 1988; Rosen, 1989; Reed, 1995).
The quality and strength of the final product
depends on the microstructure of the cast layer. Highly
porous bodies formed by a flocculated slip will usually
shrink unevenly during drying and sintering, resulting
in cracks and reduced strength. On the other hand a
dispersed slip has less particle aggregation and better
structural uniformity (Zhang and Binner, 2002).
Negative surface charge of clay particles is due to
isomorphous substitution in the crystal lattice and it is
neutralized by exchangeable cations. Edge charge
depends on adsorption of ions, e.g. OH ions on
edges of kaolinite particles may induce a negative
charge, whereas at pH < 7 it is positive and edge-toface attraction may increase the strength.
The charged surfaces of clay particles selectively
attract charged ions from the solution, resulting in the
A. Sßakar-Deliormanlı, Z. Yayla / Applied Clay Science 24 (2004) 237–243
238
formation of a double ionic layer on the particles.
Diffuse double layer is formed by exchangeable
cations due to their thermal motions. Interaction of
these layers of neighboring particles (if the distance is
short enough) results in diffuse double layer repulsion.
In the case of small clay particles (high specific
surface), highly dispersed, the viscosity may be high.
This process is of fundamental importance in forming
of ceramic shapes from slurries (wet forming), such as
slip casting (Shan, 1990; Williams, 1992; Luckham
and Rossi, 1999).
Many scientists have studied the effects of different
chemical agents such as sodium tripolyphosphate,
sodium polymethacrylates, ammonium phosphate, sodium citrate, polysulfonate on the dispersion behaviour of clay particles in aqueous systems (Papo et al.,
2002; Guler and Balci, 1998; Corradi et al., 1994).
Sodium carbonate and sodium silicate are well known
deflocculants used for sanitary ware ceramic slips.
Mixtures of these two chemical agents are usually the
most satisfactory. Sodium hydroxide is also as active
as sodium carbonate and sodium silicate, but it is not
used to a great extent because of its corrosiveness
(Mutsuddy, 1994).
A major disadvantage of slip casting is the time
required for casting which can take from hours to days
for large objects. Therefore, technologies are desired
which can increase the casting rate. Methods of
accelerating the consolidation of ceramic particles
have been studied by many researchers (Ching et
al., 1994; Zhang and Binner, 2002).
The aim of this study was to investigate the casting
behaviour of clay based ceramic slips in the presence
of calcium hydroxide. Effect of calcium hydroxide
was studied in sanitary ware slip containing sodium
silicate as dispersing agent. Similar experiments were
performed with slips containing sodium carbonate and
sodium silicate since these chemical agents are well
known in sanitary ware slip preparation. A comparison was made to observe the differences between two
systems based on casting properties.
2. Experimental
2.1. Materials
The ceramic powders used during the experiments,
potassium feldspar, quartz, ball clay and kaolin, were
supplied by a sanitary ware production company
(Serel-Turkey). Chemical analysis of the raw materials, which was reported by the manufacturer, is given
in Table 1. Chemical agents were obtained from
Merck Chemicals, Germany. Distilled water was used
for the suspension preparation.
Experiments were performed in two different sets.
The first set of suspensions was prepared by using
sodium carbonate at different concentrations ranging
from 0.06 to 0.085 wt.% and adequate amount of
sodium silicate to disperse the slurry. In the second
set, suspensions were prepared under the similar
conditions by using calcium hydroxide at the same
concentrations with sodium carbonate. Sodium silicate was also added to the system as dispersing agent.
The solid loading was adjusted at 65 wt.%. The
composition of the slurry used in both set of experiments, is shown in Table 2.
2.2. Method
For the slurry preparation powders and distilled
water was mixed for 30 min by a mechanical stirrer
(IKA, at speed 3). Either sodium carbonate solution or
calcium hydroxide solution (0.06 – 0.085 wt.%) was
added to the mixture. Then the slurry was poured in a
porcelain mill with alumina balls (1.5 cm diameter)
and milled for 16 h at 115 rpm (85% of the critical
speed). The density of the slurry was thereafter
adjusted to 1800 g/l and to pH 8 – 9. Particle size
was determined by sieve analysis, which indicates that
97.5% of the particles in the slip were below 45 Am.
To obtain better dispersion sodium silicate solution
was gradually added to the slurry prior to mixing. To
investigate the optimum amount of sodium silicate
Table 1
Chemical analysis of the raw materials
Materials
SiO2
Al2O3
Fe2O3
TiO2
CaO
MgO
Na2O
K2 O
SO3
LO.I
Clay
Kaolin
Feldspar
55.0
52.0
72.0
27.5
32.0
16.5
1.5
1.0
0.15
1.5
0.3
0.5
0.3
0.5
0.7
0.5
0.5
0.8
1.0
0.5
8.5
2.5
1.5
0.5
0.5
0.5
–
10.0
10.5
1.0
A. Sßakar-Deliormanlı, Z. Yayla / Applied Clay Science 24 (2004) 237–243
Table 2
The composition of the slurry
Material
wt.%
Feldspar
Quartz
Kaolin
Ball clay
24
24
35
17
that should be added to the system viscosity measurements were performed by using a Brookfield RV II
viscometer at 600 rpm (spindle number 3). A Gallencamp torsion viscometer was also used for measuring
the viscosity and thixotropy of the suspensions before
casting into the mould. The torsion type viscometer
contains a horizontal disc suspended by a long wire
that is immersed into the slip and spun through an
angle of 360j.
The prepared slip was casted into plaster of Paris
molds at room temperature and the temperature of the
slurry was fixed at 25 jC during the casting process.
After casting for a predetermined time, the excess slip
239
was poured away from the mould and cake thickness
data was collected in 5, 10, 20, 40 and 60 min
intervals. Test specimens were then removed form
the mould and allowed to dry at 40 jC for 24 h. The
green bodies were sintered at 1250 jC in an industrial
scale kiln (Riedhammer, 65 m length). A flow diagram of the processing steps is shown in Fig. 1.
Physical properties of the test specimens such as
shrinkage, bending strength and water absorption
were investigated in green state and after sintering.
Shrinkage was measured by a dilatometer (Netch,
Germany) and also calculated by measuring the
dimensions of the ceramic bars before and after
sintering.
Bending strength of the bar shape test specimens
(150 25 12 mm) were measured by applying a 40
N load to the cross sectional area using a mechanical
test equipment.
A water absorption test was accomplished in order
to determine the water absorption capacity. In this test
the samples were submerged in water for 24 h. Then
Fig. 1. Flow diagram of the processing steps.
240
A. Sßakar-Deliormanlı, Z. Yayla / Applied Clay Science 24 (2004) 237–243
they were removed from water and weighed. Water
absorption of each sample was calculated according to
the following formula:
Water Absorption% ¼ 100ðWs Wd Þ=Wd
where, Wd is the dry weight of the specimen; Ws is the
saturated weight of the specimen.
3. Results and discussion
3.1. Viscosity measurements
The optimum electrolyte amounts required to disperse the particles in the slip were determined by
Fig. 3. Optimum electrolyte amount graph for suspension containing Ca(OH)2; (a) 0.060%, (b) 0.080%.
Fig. 2. Optimum electrolyte amount graph for suspension containing Na2CO3; (a) 0.060%, (b) 0.080%.
means of viscosity measurements. Fig. 2 shows the
optimum amount of sodium silicate that should be
added to the slip containing sodium carbonate. It was
found that the slip already containing 0.06 wt.%
sodium carbonate exhibited its lowest viscosity (300
cp) at a sodium silicate concentration of 0.154%.
Above this concentration, addition of sodium silicate
caused an increase in the viscosity. Therefore, it is
possible to conclude that at high concentrations of
sodium silicate agglomeration starts due to an excess
amount of electrolyte. In the presence of 0.09%
sodium silicate viscosity of the slurry is about 800 cp.
Similarly Fig. 3 demonstrates the optimum amount
of sodium silicate that should be added to the suspension containing calcium hydroxide. The viscosity
A. Sßakar-Deliormanlı, Z. Yayla / Applied Clay Science 24 (2004) 237–243
241
Table 3
Results of the experiments that were carried out by using sodium carbonate
Experiment Number
1
2
3
4
5
Na2CO3 (%)
Na2SiO3 (%)
Density (g/l)
1st viscositya (j)
2nd viscositya (j)
Thixotropyb
Temperature (jC)
Shrinkage (%)-Green
Shrinkage (%)-Sintered
Total Shrinkage (%)
Strength (kg/cm2)-Green
Strength (kg/cm2)-Sintered
Water absorption (%)
0.060
0.164
1798
340
310
30
25
1.60
7.97
9.57
25.11
342.67
2.3
0.070
0.141
1802
340
308
32
26
2.10
7.20
9.30
26.87
351.14
1.91
0.075
0.143
1800
337
304
33
26
1.33
7.05
8.38
26.28
457.8
0.84
0.080
0.145
1802
335
302
33
25
1.95
8.45
10.6
26.95
484.9
0.78
0.085
0.160
1800
342
312
30
24
1.28
7.53
9.65
27.43
490.62
0.75
a
b
Values obtained by using Torsion type viscometer.
Thixotropy = 1st viscosity 2nd viscosity.
minimum is at 250 cp. There is a big difference
between the viscosity values of the suspensions in
the absence of sodium silicate containing only calcium hydroxide and sodium carbonate in the beginning.
3.2. Slip casting related properties
The physical properties of the green and sintered
test specimens and the properties of the slip before the
casting process are summarized in Tables 3 and 4 for
the Na2CO3 and Ca(OH)2 systems, respectively.
Viscosity values shown in Tables 3 and 4 were
obtained by using the Gallencamp torsion viscometer.
The term ‘‘second viscosity’’ was attributed to the
value that had been taken after a rest period (30 min)
by keeping the slurry undisturbed. Thixotropy coefficients of the both systems were determined as 30 –
33 in both sets by getting the difference between the
first and second viscosity values. These results suggest a high state of dispersion for the slip.
The thickness of the cast layer for similar suspensions but with different dispersants was determined by
the packing of the particles in the layer, which in turn
determines the permeability of the layer. The cast
layer thickness of the slurries with casting time
prepared by using sodium carbonate and calcium
hydroxide is illustrated in Figs. 4 and 5, respectively.
According to the results calcium hydroxide has a
Table 4
Results of the experiments that were carried out by using calcium hydroxide
Experiment Number
6
7
8
9
10
Ca(OH)2 (%)
Na2SiO3 (%)
Density (g/l)
1st viscositya (j)
2nd viscositya (j)
Thixotropyb
Temperature (jC)
Shrinkage (%)-Green
Shrinkage (%)-Sintered
Total shrinkage (%)
Strength (kg/cm2)-Green
Strength (kg/cm2)-Sintered
Water Absorption (%)
0.060
0.22
1800
339
309
30
26
1.45
8.35
9.8
25.34
496.8
0.74
0.070
0.23
1799
342
310
32
26
1.58
8.49
10.07
25.91
524.26
0.71
0.075
0.21
1802
341
309
32
25
1.15
6.61
7.76
29.16
544.36
0.70
0.080
0.20
1796
344
311
33
24
2.03
8.22
10.25
35.8
658.3
0.68
0.085
0.21
1802
320
290
30
25
1.85
7.98
9.83
37.94
650.52
0.69
a
b
Values obtained by using Torsion type viscometer.
Thixotropy = 1st viscosity 2nd viscosity.
242
A. Sßakar-Deliormanlı, Z. Yayla / Applied Clay Science 24 (2004) 237–243
Fig. 4. Cake thickness values of the slip containing sodium
carbonate at different concentrations with casting time.
positive effect on the casting rate of the ceramic slips.
In the case of sodium carbonate the slope of the graph
is 0.68 in Fig. 4, and the maximum cake thickness is
6.08 mm after 60 min drying time and with 0.08%
sodium carbonate content. On the other hand for
samples with calcium hydroxide (0.080%) the slope
reaches 0.79. The cake thickness is 7.2 mm after 60
min drying period.
For a dispersed suspension, the particles will be
densely packed in the cast layer and there will be a
limited secondary arrangement of the particles after
they have landed on the cast layer surface. However,
when the suspension is weakly flocculated, clusters of
particles exist in the suspension. Therefore, the cast
layer will have a greater degree of porosity (Zhang
and Binner, 2002).
Bending strength of the sintered products containing calcium hydroxide was higher than the samples
prepared by sodium carbonate. Higher strength values
may be attributed to better homogenization during slip
preparation.
Water absorption test results showed that the specimens which have a higher bending strength indicates
a lower water absorption capacity. Therefore minimum absorption values were obtained in the specimen
prepared by using 0.080% Ca(OH)2.
Results of the study shows that Ca(OH)2 might
serve the deflocculation process yielding a better casting rate to the system through similar mechanisms as
seen for sodium compounds such as Na(OH), Na2CO3.
The effects of sodium carbonate and sodium silicate are different in slip preparation. Sodium carbonate hydrolyses during the process to yield sodium
hydroxide and carbonic acid. On the other hand
sodium silicate gives free alkaline ions and silicic
acid after hydrolysis. The resultant silicic acid prevents the flocculation of the slip. The ball clay in the
slip casting contains some organic substances such as
humic acids. By addition of the sodium carbonate into
the slip an alkaline region is obtained. The alkaline
ions help this structure become colloidal, reacting
with the organic substances in the slip when sodium
silicate is added. As a result of precipitation of the
anions of the alkaline salts in the slip the clay
particles, which form the slip, become colloidal.
Ching et al. (1994) investigated the slip rheology
by the presence of calcium hydroxide. The sulphates
may accompany the ball clay or calcium sulphate may
be added to achieve desired rheology. The sulphate
can also contribute to glaze defects and adds sulphur
oxide emissions from the kiln. According to their
study, calcium slips can increase the casting rate by
as much as 65% and calcium hydroxide might serve
the same function with calcium sulphate and, at the
same time, eliminate the undesirable characteristics of
the sulphate anions.
Results of this current work are in good agreement
with those observed in the previous work carried out
by Ching et al. (1994).
Fig. 5. Cake thickness values of the slip containing calcium
hydroxide at different concentrations with casting time.
A. Sßakar-Deliormanlı, Z. Yayla / Applied Clay Science 24 (2004) 237–243
4. Conclusions
Results of this study indicated that the use of
calcium hydroxide might cause significant improvements in the casting behaviour of the slips. It has also
some positive effects on physical properties of the
green and sintered bodies.
The use of calcium hydroxide followed by addition
of sodium silicate lowered the viscosity of the
slips. Higher viscosity values were obtained by
using sodium carbonate.
The use of calcium hydroxide at 0.080 wt.%
caused an increase in the cake thickness by as
much as 30% within 60 min. Calcium hydroxide
accelerated the consolidation.
The dry and sintered strength of the final products
obtained by casting of the slips containing calcium
hydroxide was higher than the products containing
sodium carbonate.
Water absorption percentages of the specimens
containing calcium hydroxide were lower than the
specimens that contained sodium carbonate.
The addition of calcium hydroxide instead of
sodium carbonate required much higher amounts
of sodium silicate to deflocculate to similar
viscosity levels.
Acknowledgements
The authors are grateful to Dr. Ender Balci for his
valuable contributions and to Serel Seramik, Turkey
243
who kindly supplied raw materials and supported this
work.
References
Ching, H., Mancini, K., Gilbert, R., 1994. Improved casting slip
properties. Am. Ceram. Soc. Bull. 73, 70 – 74.
Corradi, A., Manfredini, T., Pellacani, G., Pozzi, P., 1994. Deflocculation of concentrated aqueous clay suspensions with sodium
polymethacrylates. J. Am. Ceram. Soc. 77, 509 – 513.
Everest, D.H., 1988. Basic Principles of Colloidal Science. Royal
Society of Chemistry, London.
Guler, C., Balci, E., 1998. Effect of some salts on the viscosity of
slip casting. Appl. Clay Sci. 13, 213 – 218.
Luckham, P.F., Rossi, S., 1999. The colloidal and rheological properties of bentonite suspensions. Adv. Colloid Interface Sci. 82,
43 – 92.
Mutsuddy, B.C., 1994. Rheology and mixing of ceramic mixtures
used in plastic molding. In: Lee, B.I., Pope, E.A. (Eds.), Chemical Processing of Ceramics, vol. 8. Marcel Dekker, New York.
Papo, A., Piani, L., Ricceri, R., 2002. Sodium tripolyphosphate and
polyphosphate as dispersing agents for kaolin suspensions:
rheological characterization. Colloids Surf., A Physicochem.
Eng. Asp. 201, 219 – 230.
Reed, J.S., 1995. Principles of Ceramic Processing, 2nd ed. Wiley,
Canada.
Rosen, J.M., 1989. Surfactants and Interfacial Phenomena, 2nd ed.
Wiley, Canada.
Shan, Y., 1990. The international training course on ceramic production technology. Lecture notes 2. Tangshan, The People’s
Rep. of China.
Williams, R.A., 1992. Colloid and Surface Engineering: Applications in the Process Industries. Butterworth – Heinemann, Oxford.
Zhang, Y., Binner, J., 2002. Enhanced casting rate by dynamic
heating during slip casting. J. Eur. Ceram. Soc. 22, 135 – 142.
Zeliha Yayla