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30 de maio a 2 de junho de 2001 - Florianópolis – SC.
GELCASTING OF HIGH-ALUMINA REFRACTORY CASTABLES
A.R. Studart, F.S. Ortega, V.C. Pandolfelli
Via Washington Luís, km 235, C.P. 676, CEP 13565-905, São Carlos-SP
E-mail: pars@iris.ufscar.br ou vicpando@power.ufscar.br
Universidade Federal de São Carlos, DEMa
ABSTRACT
The recent development of refractory castables free of hydraulic binders, known
as zero-cement castables, has led to a marked improvement in the high-temperature
performance of refractory materials, retaining the castable mechanical strength before
firing. Nevertheless, the elimination of hydraulic binder may result in the formation of
large flaws in the castable, most probably due to migration of fine particles during the
drying process. This indicates that the presence of a coagulant/gelling agent that could
play the role of cement is still necessary to set the castables at room temperature,
giving a homogeneous macrostructure. The objective of this paper is to apply the
gelcasting forming method to promote the consolidation of high-alumina zero-cement
castables. Such method is based on an “in-situ” polymerization of monomers that
originates a gel network throughout the castable, preventing particle migration during
drying. This approach enabled the production of highly homogeneous refractory
castables, exhibiting pre-firing mechanical strength comparable to that obtained with
cement-containing compositions.
Key words: alumina, refractory, castable, gelcasting, zero-cement.
INTRODUCTION
A substantial increase in the working life of high-alumina castables has been
observed in steel-making industries during the latest decades, due mainly to the
gradual reduction of the cement content of such refractories. Inferior contents of
calcium aluminate cement, and consequently CaO, in the castable usually results in
remarkable gains in refractoriness and mechanical properties at high temperature, as
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a consequence of the reduction of low melting point phases amount in Al 2O3-SiO2CaO ternary system. Low-cement (LC), ultra-low-cement (ULC) and CaO-free
compositions (containing -Al2O3 as hydraulic binder) have been developed taking
this aspect into account.
Such developments have been extended with the recent advent of castables
free of hydraulic binders, known as zero-cement refractory castables
(1,2).
The minor
content of liquid phase formed at high temperature in these castables leads to a
marked increase in the refractory creep strength in comparison to the thermo
mechanical behavior of other categories of castable
(2).
In spite of their acceptable pre-firing mechanical strength and outstanding high
temperature behavior, it has been observed that the absence of a setting agent in
zero-cement compositions may result in the formation of large flaws in the castable
during the drying process
(2).
Such heterogeneities, illustrated in Figure 1, are
probably originated due to migration of fine particles inside the castable, driven by
the action of capillary forces developed during drying
(2).
This phenomenon most
likely occurs in castables displaying high flowability (i.e., self-flowing castables), since
a considerable amount of fine particles is usually present in these compositions.
Such drawback has been overcome through the use of additives that promote
particle agglomeration as a function of time, by means of an ionic strength increase
of the castable liquid medium. Urea and aluminum powder have been used with this
purpose
(2),
since these additives release ions when heated up in an aqueous
medium. However, the concurrent relief of gases that display reduced solubility in
water over the temperature range from 60 to 100ºC usually limits the applicability of
such additives. Small temperature oscillations during drying/coagulation of castables
prepared with these components may lead to the formation of a considerable amount
of micro-bubbles that can degrade the castable mechanical properties.
Although they have not been applied to refractory castables yet, other
coagulation/gelation mechanisms are currently used to consolidate ceramic parts
from colloidal suspensions
Casting) methods
(5,6,7),
(3,4),
including the gelcasting and DCC (Direct Coagulation
among others. As they rely on organic reagents to impart
coagulation/gelation, these methods have as major advantage the fact that they do
not incorporate any metallic ion that could deteriorate the castable mechanical
properties after firing.
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The main objective of this paper is to apply the gelcasting technique to
consolidate high-alumina zero-cement castables, as a means to obtain refractory
materials displaying homogeneous macrostructure and high-performance at high
temperatures.
Figure 1: Zero-cement refractory castables cross-sections, illustrating examples of
flaws originated during drying in the absence of coagulant agents.
EXPERIMENTAL PROCEDURE
(a) Gelcasting reagents
The gelcasting technique is based on the in-situ polymerization of organic
monomers dissolved in the suspension liquid medium. The consolidation of the
ceramic body is accomplished by simultaneously adding mono and bifunctional
monomers that could promote the formation of a relatively rigid cross-linked gel
network between particles. The polymerization can be activated either by increasing
temperature or through the addition of chemical initiators. These two activating
mechanisms are usually combined with the use of catalysts to control the gelling
kinetics.
Methacrylamide (MAM) and N,N´-methylene bisacrylamide (MBAM) were the
mono and bifunctional monomers, respectively, used in this work to promote castable
gelation (Table I). Methacrylamide was chosen in order to avoid the toxicity
associated with original gelcasting compositions
(5,6).
An additional advantage of
methacrylamide is that it is a powder that can be mixed with the castable raw
materials prior to water addition. The polymerization was initiated through the
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addition of fixed contents of ammonium persulfate (APS), followed by a thermal
treatment of castables at 100ºC.
Table I: Reagents used to produce different compositions of gelcasting refractory
castables.
Compositions
Reagents
M19
M32
M46
Methacrylamide
(wt-% based on dry castable)
0.19
0.32
0.46
(MAM)
(wt-% based on water)
4
7
10
N,N´-methylene bisacrylamide (wt-% based on MAM)
15
15
15
Ammonium persulfate (wt-% based on MAM)
0.33
0.33
0.33
(b) Castable preparation
Castables were prepared using white fused alumina (EK8R, Alcoa-Brazil) as
aggregates and the calcined aluminas A-1000 SG and A-3000 FL (Alcoa Chemicals,
USA) as the matrix constituents, as shown in Figure 2. An optimum content (0.26
mg/m2) of anhydrous citric acid (Labsynth, Brazil) was used to promote the
dispersion of fine particles (1).
The particle size distribution of castables was adjusted to a theoretical curve
based on Andreasen packing model (q = 0.21), in order to obtain potentially self-flow
compositions (Figure 2).
Castables with varying contents of methacrylamide (0.19 to 0.46 wt-%) were
prepared (Table I) using an optimum mixing procedure previously described by the
authors (8).
The initiator (ammonium persulfate - APS) and the cross-linking agent (N,N´methylene bisacrylamide - MBAM) had their addition based on the amount of
methacrylamide added (Table I), using ratios typically used for gelcasting
consolidation (5,6).
The castable water content varied according to the methacrylamide amount,
so that the total solution content (monomer + water) of all compositions could be
fixed at 15.5 vol-%. In order to assure full dissolution of monomers/initiator, all
reagents were dissolved in the admixing water just before the beginning of the mixing
process.
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100
Zero-cement castable
CPFT (%)
Andreasen - q = 0.21
10
Raw materials (wt.%)
ZC
White fused
Mesh # range:
alumina
4 - 200
Calcined
A-1000 SG
9.9
Aluminas
A-3000 FL
15.1
75.0
1
0.1
1
10
100
1000
10000
Diameter (m)
Figure 2: Particle size distribution and content of raw materials of the high-alumina
castables evaluated in this work.
Zero-cement castables without coagulating/gelling agents (ZC) and ultra-lowcement castables (ULC) containing 1 wt% of calcium aluminate cement (CA 270,
Alcoa Chemicals, USA) were produced for comparative purposes. In the case of ULC
castables, a slightly higher content of citric acid was used (0.36 mg/m 2) in order to
properly control cement setting time (2).
Castable flowability was evaluated just after the mixing process employing the
free-flow test adapted from ASTM 860 standard
(1).
Subsequently, castables were
molded into prismatic (2525150 and 3838160 mm3) or cylindrical ( = 40 mm and
height = 55 mm) shapes and submitted to a thermal treatment at 100ºC (15 hours),
where polymerization and the drying process proceeded concurrently. In order to
guarantee full hydration of cement particles, an additional batch of ULC samples was
first cured at 30ºC (98% of humidity, 24 hours) and then dried at 100ºC (24 hours).
(c) Macrostructure and properties
The mechanical strength of dried castables (2525150 mm3 specimens) was
determined through modulus of rupture measurements (MTS, model 810) using a
three-point bending device (span = 110 mm, displacement rate = 10 mm/min).
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Dried castables were characterized in terms of apparent porosity using the
Archimedes immersion method.
Thermogravimetric analysis was accomplished in dried cylindrical specimens
in order to investigate the burnout of organic materials during the firing process.
These tests were conducted in a TGA equipment, specially designed to evaluate the
weight loss of large, heavy samples (~ 200 g).
In an attempt to observe the macrostructural homogeneity of castables, the
3838160 mm3 specimens were transversely cut in several pieces, rendering cross
sections approximately 5 mm thick. In order to facilitate the machining process,
samples were previously fired at 1100ºC for 2 hours. Taking into account the large
size of flaws (Figure 1), the chemical composition of the castable and the short
exposure time to moderately high temperature, this firing step is not expected to
significantly alter the castable macrostructure in the length scale of interest.
RESULTS AND DISCUSSION
(a) Flow behavior
The free-flow results (Figure 3) revealed that castables prepared with
monomer solutions exhibited flowability slightly superior than that of compositions
either containing cement (ULC) or without any hydraulic binder (ZC). Such higher
free-flow values for gelcasting castables were obtained with inferior water content, as
part of the water was substituted by monomer in order to keep a total liquid content of
15.5 vol% in all compositions. Even though a reasonable explanation for these
results still remains unclear, Young et al.
(5)
observed a similar behavior when the
rheological properties of alumina suspensions containing only water were compared
to those of suspensions prepared with monomer solutions.
Although the lower water content would not affect the total weight loss of
castables during firing, this can be considered as an advantage of the gelcasting
compositions, since an inferior amount of “free” water would be eliminated in this
case. It has been observed that evaporation of water at 100ºC increases the
probability of thermal shock failure during castable heating (9).
Another important difference between cement-containing and gelcasting
castables is that the former usually display a specific setting time established by the
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cement dissociation rate (45 to 90 minutes to completely loose flowability), while the
latter may present considerably high free-flow values (> 60%) under prolonged
periods of time (~ 100 minutes). This feature of gelcasting compositions provides
higher flexibility during castable installation, and is due to the fact that gelation
process is activated only by temperature increase.
120
16
Free-flow (%)
80
15
60
40
14
Free-flow
20
Water content (vol.%)
Self-flow behavior
100
Water-%
0
13
ULC
ZC
M19
M32
M46
Composition
Figure 3: Initial free-flow values and water content of castables with different
monomer amounts (M19, M32, M46) in comparison to those of zero-cement (ZC) and
ultra-low cement (ULC) compositions. The figure also points out the free-flow range
where self-flow behavior is expected (1).
Although a thorough investigation about this matter has not been undertaken
yet, it was observed that polymerization does not occur at room temperature even
with the addition of considerable amounts of initiator and catalyst. This is believed to
be due to the high concentration of oxygen incorporated into the castable during the
mixing operation. Oxygen is known to prevent gelation in this system by inhibiting the
formation of free radicals in the monomers.
(b) Macrostructure and green strength
The green strength of castables has a significant impact on the refractory final
performance, since it determines the ability of the refractory lining to support its own
weight and its resistance to failure under the mechanical stresses developed during
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drying. In spite of its deleterious effect at high temperature, the addition of cement
has been currently the only alternative to render castables with moderate to high
green strength.
Figure 4 presents the modulus of rupture of unfired castables containing
varying concentrations of methacrylamide as the strengthening agent in substitution
to cement. Results show that the castable green strength increases as a function of
the monomer concentration, achieving values similar or superior to the strength level
obtained with ULC castables cured at 30ºC. Such increase in mechanical strength
may be attributed to the formation of a crosslinked gel network around particles,
which has its volume fraction increased for higher contents of monomer added.
Green strength (MPa)
6
5
ULC - 100ºC
4
ULC - 30ºC
3
2
1
0
-0.1
0
0.1
0.2
0.3
0.4
0.5
Methacrylamide content (wt-%)
Figure 4: Green strength of refractory castables as a function of their
methacrylamide content, in comparison to data obtained for ultra-low cement
compositions (ULC) either cured at 30ºC or cured/dried directly at 100ºC. Three
samples were tested for each composition to render the average and standard
deviation data presented in the figure.
The superior strength level exhibited by castables cured and dried directly at
100ºC may be related to the higher dissociation rate of cement at this temperature,
which might have resulted in a superior amount of hydrated strengthening phases in
the castable.
The mechanical evaluation reveals that the use of gel-forming monomers can
provide castables displaying high mechanical strength, without incorporating in the
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composition any oxide that could be deleterious to the refractory performance at high
temperatures.
Furthermore, gelcasting castables exhibited macrostructure as homogeneous
as that obtained with the cement-containing composition (ULC), as illustrated in
Figure 5. Therefore, the initial purpose of eliminating the large flaws observed in
zero-cement castables was successfully accomplished.
Figure 5: Cross sections of castables containing (a) 1 wt% of cement (ULC), (b) 0.19
wt% MAM, (c) 0.32 wt% MAM and (d) 0.46 wt% MAM.
Gelcasting and cement-containing castables were also compared in terms of
apparent porosity before firing (Table II). Surprisingly, gelcasting compositions
exhibited apparent porosity significantly higher than the values obtained for ULC and
ZC compositions. Such superior porosity may have been originated by the release of
O2 from ammonium persulfate (APS) decomposition during castable drying at 100ºC
(10).
It must be noted that the high temperature and APS content used in this work
were chosen to ensure the occurrence of polymerization. No further attempts were
made to establish an optimum temperature and APS content that could reduce or
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even avoid gas formation. Also worthy mentioning is that even at the
conditions used in this work, the APS is likely to generate an amount of gas markedly
lower than that expected from the thermal decomposition of urea. APS may evolve
O2 upon heating in quantities up to 28 wt-% of its own weight, while in the case of
urea the percentage of CO2 released can be as high as 73 wt-%. The superior
solubility of CO2 in water in comparison to that of O2
(11)
is not sufficient to reverse
these considerations, specially if one takes into account the fact that the amount of
urea necessary to promote coagulation by thermal decomposition (0.23 – 0.33 wt%
based on castable)
(2)
is significantly higher than the content of APS needed to
accomplish gelation (refer to Table I).
Table II: Pre-firing apparent porosity of the refractory castables evaluated. Three
samples were tested for each composition to render the average and standard
deviation data presented.
Apparent porosity (%)
Cured at 30ºC and dried at 100ºC
13.2  0.5
Cured and dried at 100ºC
13.5  0.7
ULC
Zero-cement (0% MAM)
Gelcasting (% MAM)
13.7  0.3
0.19
17.1  0.3
0.32
17.3  0.9
0.46
15.5  0.7
(c) Polymer burnout
After playing its role as a strengthening agent at lower temperatures (30 –
200ºC), the polymeric network formed in the gelcasting castable must be eliminated
during the firing process.
Figure 6 shows the thermogravimetric analysis performed to evaluate the
pyrolysis of the polymeric network from castables prepared with distinct monomer
contents. It can be observed that all compositions presented minimum weight loss
due to polymer pyrolysis (< 0.7 wt%), as a result of the very low monomer contents
required to obtain gelcasting castables with suitable pre-firing mechanical strength.
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The results indicate that most of the polymer is eliminated at 400-500ºC, which
agrees with the data recently obtained by Janney et al. for the same combination of
monomers
(6).
However, a substantial weight loss is also observed at 200-300ºC for
castables containing the highest amount of monomer evaluated (0.46 wt%). This
might be an indicative that part of the methacrylamide introduced in this composition
did not polymerize during the thermal treatment and was only burnt out during the
thermogravimetric analysis. Such incomplete reaction may be the reason for the
slight increase in mechanical strength observed when the monomer content was
elevated from 0.32 wt% to 0.46 wt%, in contrast to the sharp linear increase verified
up to 0.32 wt% (Figure 4).
100
Weight loss (%)
M19
99.8
M32
99.6
99.4
M46
99.2
0
200
400
600
800
1000
Temperature (ºC)
Figure 6: Thermogravimetric analysis of gelcasting castables prepared with 0.19
(M19), 0.32 (M32) and 0.46 wt% (M46) of monomer.
The minimum amount of polymer present in gelcasting compositions suggests
that no significant adjustments in typical firing schedules would be required for
heating up refractory parts lined with these castables.
CONCLUSIONS
A forming technique well known in colloidal ceramic processing was applied to
consolidate refractory materials, enabling the development of a novel class of highalumina zero-cement castables.
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The so-called gelcasting castables described in this work presented
macrostructural homogeneity and pre-firing mechanical strength similar to that
obtained with cement-containing castables, with the major advantage that it does not
incorporate any oxide that could deteriorate the refractory mechanical properties at
high temperature. Additionally, a slightly higher flowability was attained with
gelcasting compositions in comparison to that of castables containing cement at the
same overall solid loading.
The minimum amount of polymer necessary to consolidate castables using the
gelcasting method is expected to dispense the use of special firing schedules to
accomplish pyrolysis of the polymeric network.
Further investigations would be necessary to establish contents of monomers,
initiator and drying/gelling temperatures that could optimize the processing of
gelcasting castables.
ACKNOWLEDGEMENTS
The authors would like to acknowledge FAPESP and Alcoa / Brazil for the financial
support to this work.
REFERENCES
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