Journal
J. Am. Ceram. Soc., 82 [8] 2193–203 (1999)
Ternary System Al2O3–MgO–CaO: I, Primary Phase Field of
Crystallization of Spinel in the Subsystem MgAl2O4–CaAl4O7–CaO–MgO
Antonio H. De Aza, Pilar Pena, and Salvador De Aza*
Instituto de Cerámica y Vidrio, CSIC, 28500 Arganda del Rey, Madrid, Spain
published,16 and, more recently, a study on the subsolidus relations in the high-alumina region of this ternary system also
has been reported. According to those studies, two new ternary
phases—Ca2Mg2Al28O46 and CaMg2Al16O27, both with limited solid-solution ranges—have been established.17,18
The purpose of part I of the present investigation was to
establish the solid-state compatibility relations in the subsystem MgAl2O4–CaAl4O7–CaO–MgO and the melting relationships in the subsystems CaAl2O4–MgAl2O4–MgO and
CaAl2O4–MgAl2O4–CaAl4O7. The primary phase field of crystallization of spinel in the above-mentioned subsystem,
MgAl2O4–CaAl4O7–CaO–MgO, can be established subsequently.
Using a novel experimental procedure in the field of ceramics/materials science that is based on the precise microanalysis (scanning electron microscopy, coupled with wavelength-dispersive spectroscopy) of the phases that are
present in equilibrated specimens, the solid-state compatibility relations in the subsystem MgAl2O4–CaAl4O7–CaO–
MgO and the melting relationships in the subsystems
MgAl 2 O 4 –CaAl 2 O 4 –MgO and MgAl 2 O 4 –CaAl 2 O 4 –
CaAl4O7 were established. The primary phase field of crystallization of spinel in the above-mentioned subsystem,
MgAl2O4–CaAl4O7–CaO–MgO, was determined subsequently. The temperature, composition, and character of
the ternary invariant points of the subsystem were established, and the ranges of the solid solutions in periclase,
spinel, monocalcium aluminate, and dicalcium aluminate
also were studied and determined, up to 1725°C.
I.
II.
Literature
The first study of the Al2O3–MgO–CaO system was conducted in 1916 by Rankin and Merwin,11 who studied the
melting relationships in the low-fusion zone of the system,
which surrounds the primary phase fields of crystallization of
Ca3Al2O6 and Ca12Al14O33 within the system. Their findings
have been confirmed essentially, although some changes in
regard to the calcium aluminates have been made since that
confirmation. These workers did not identify any ternary compounds in this system.
A ternary compound with a chemical composition of
Ca3MgAl4O10 was first reported by Welch,13 who also found a
metastable ternary phase (Ca7MgAl10O23) in this system.
These two phases and their melting relations were investigated
thoroughly by Majumdar,14 who established the primary phase
field of crystallization of the compound Ca3MgAl4O10 and its
solid-state compatibilities. Although the presence of the binary
compound Ca12Al14O33 in the Al2O3–CaO system remains
controversial, it was included in the ternary diagram that was
proposed by Majumdar.14 However, a year later, based on a
Introduction
T
ternary system Al2O3–MgO–CaO is remarkably important in the field of nuclear waste storage,1 catalysis,2 new
synthetic slags for secondary steel refining,3 and refractories.4
Currently, spinel (MgAl2O4, or MA†) is used as a component
in high-alumina refractory concrete where improvement in the
corrosion and penetration ratios (wear rate), as well as resistance to thermal shock (spalling), is required.5,6 On the other
hand, calcium hexaluminate (CaAl12O19, or CA6) is formed in
such high-alumina concretes during their use at high temperature. This material also presents a wide range of solid solutions
with iron oxides and slags that contain transition cations,7 adequate chemical resistance in alkaline environments, high stability in reducing atmospheres,8 and low solubility in several
multicomponent systems.6–9 Further applications of CA6 (hibonite) can be found in the field of solid-state ionic conductors,
because of its ␤-alumina defect structure.10 To properly understand the role of both phases in the behavior of the abovementioned concretes and to improve their design and properties, a good knowledge of the solid-state compatibilities and
melting relationships in the high-alumina region of the system
is required.
However, until recently, a search of the literature revealed
only a few studies on this ternary system,11–14 in which most of
the information involves the quaternary system Al2O3–CaO–
MgO–SiO2.15 In 1995, a summary of previous investigations
on the system Al2O3–MgO–CaO, together with a thermodynamic assessment of the above-mentioned system, has been
HE
Table I.
Selected Compositions in the Isopletal Section
(CaOⴢMgO)–Al2O3 (76.04 wt%)
Composition (wt%)
CaO⭈MgO
R. S. Roth—contributing editor
Manuscript No. 189973. Received August 3, 1998; approved January 15, 1999.
Supported by Plan Nacional de Materiales CICYT and European Union, under
Project Nos. MAT97-0728 and BRPR-CT97-0427, respectively.
*Member, American Ceramic Society.
†
For simplicity, solid phases given in the figures of this paper are described by
abbreviated formulas: C ⳱ CaO, M ⳱ MgO, and A ⳱ Al2O3 (i.e., CaAl12O19 ⳱
CaO⭈6Al2O3 ⳱ CA6).
2193
Designation
CaO
MgO
Al2O3
44
46
48
50
52
54
56
58
60
62
64
66
68
70
71
73
75
32.5810
31.4174
30.2538
29.0902
27.9265
26.7629
25.5993
24.4357
23.2721
22.1085
20.9449
19.7813
18.6177
17.4541
16.8723
15.7087
14.5451
23.4190
22.5826
21.7462
20.9098
20.0735
19.2371
18.4007
17.5643
16.7279
15.8915
15.0551
14.2187
13.3823
12.5459
12.1277
11.2913
10.4549
44
46
48
50
52
54
56
58
60
62
64
66
68
70
71
73
75
2194
Journal of the American Ceramic Society—De Aza et al.
Fig. 1. High-alumina region of the Al2O3–MgO–CaO system.
Sketch shows the 1650°C isothermal section and the situation along
the doloma (CaO⭈MgO)–Al2O3 line of samples 1–7, which are located
within different three-phase and two-phase coexisting areas.
thorough study of the Ca12Al14O33 phase, Nurse et al.19 concluded that this compound is not stable under strictly anhydrous conditions but is stabilized by the presence of moisture.
In a recent study of the Ca-Al-O system, Srikanth et al.20
claimed that Ca12Al14O33 is, in fact, a stable phase in the CaO–
Al2O3 system. However, the previously mentioned study by
Nurse et al.19 still is considered to be the most exhaustive and
reliable. Since water is present in many technological and geological processes, the compound Ca12Al14O33 has been considered to be a stable phase under ambient conditions in the present study.
Ramakrishna Rao15 conducted a study of the quaternary subsystem CaAl2O4–CaAl4O7–Ca2Al2SiO7–MgAl2O4. In this
Fig. 2.
Vol. 82, No. 8
study, he established the binary systems CaAl2O4–MgAl2O4
and CaAl4O7–MgAl2O4 and the tentative liquidus surface of
the ternary subsystem CaAl2O4–CaAl4O7–MgAl2O4.
Recently, more-accurate versions of each constituent binary
system of the ternary system Al2O3–MgO–CaO have been reported.21–23 The systems were assessed using the CALPHAD
(Calculation of Phase Diagrams) technique,24 using a computerized optimization of parameters in thermodynamic models
called PARROT.
Recently, a thermodynamic assessment of the system
Al2O3–MgO–CaO was performed by Hallstedt,16 using the
CALPHAD technique.24 Although this assessment gives a
reasonably good description of the available experimental
data, the author, however, concluded that it should be regarded as provisional and reassessment should be considered
when more data, especially on solid-phase relations, become
available.
Lastly, two new phases within the system Al2O3–MgO–CaO
have been identified by Göbbels et al.17 and Iyi et al.18 These
new phases lie on the join that connects calcium hexaaluminate
(CaAl12O19) and spinel (MgAl2O4). Their stoichiometric compositions are given as Ca2Mg2Al28O46 and CaMg2Al16O27 and
show limited solid-solubility ranges. However, this work does
not provide any detailed information on the melting relationships of the new crystalline phases.
III.
Experimental Procedure
Seventeen selected compositions were prepared and studied
(Table I); these compositions are located in the doloma–
alumina ((CaO⭈MgO)–Al2O3 (76.04 wt%)) isopletal section
(which will be described later). Additional compositions that
were located inside the fields MgAl 2 O 4 –MgO–liquid,
MgAl2O4–CaAl2O4–liquid, and MgAl2O4–CaAl4O7–liquid
within the Al2O3–MgO–CaO system also were studied at different temperatures, to establish the solid-solution ranges of
spinel and the corresponding tie triangles. Calculated batches
were weighed from dry powders at 110°C/24 h; these powders
Solid-state compatibilities in the system Al2O3–MgO–CaO without solid solutions.
August 1999
Ternary System Al2O3–MgO–CaO: I
included 99.99 wt% pure Al2O3 (Fluka AG, Buchs SG, Switzerland), 99.5 wt% pure CaCO3 (E. Merck, Darmstadt, Germany), and 99.9 wt% pure MgO (E. Merck). All the materials
had an average particle size of <5 ␮m and specific surface
areas in the range of 5–8 m2/g.
The compositions were mixed in acetone and dried in air.
Green bars (100 mm long and 5 mm in diameter) were obtained
via cold isostatic pressing at 200 MPa. Cylinders 5 mm in
diameter and 6 mm long were diamond-machined from the
composition blocks, loaded into small platinum-foil crucibles,
and prefired at a temperature of 800°C to decompose the
CaCO3. Then, these cylinders were fired in air at selected temperatures in a high-temperature furnace that was equipped with
an electronic temperature controller (with an accuracy of
±1°C). The platinum crucibles, with the samples inside, were
suspended in the hot zone of the electrical furnace by a platinum wire. A calibrated Pt /6Rh-Pt /30Rh thermocouple, with its
tip in contact with the samples, was attached to the crucibles.
The time period that was required to attain equilibrium was
3–52 h. After heat treatment, the samples were air quenched.
Sometimes, the samples were reground after quenching and
2195
then pressed and fired again, to ascertain the attainment of
equilibrium. After quenching, the specimens were removed
from the platinum crucibles and mounted in an epoxy resin.
Then, the mounted samples were polished using different
grades of diamond finishing with a 1 ␮m diamond suspension.
The first phase analysis of the equilibrated specimens was
performed using reflected-light microscopy (Model HP 1, Carl
Zeiss, Oberköchen and Jena GmbH, Germany) on the polished
and chemically etched surfaces of the samples. Different etching solutions were used to distinguish between phases. For
instance, CaAl2O4 when etched with steam for 15–30 s yielded
a blue or brown color, depending on the crystal orientation.
However, CaAl4O7 that was etched with HF (5 vol%) for 20–
30 s became a bluish-pink color. Qualitative phase analysis
also was performed using X-ray diffractometry (XRD) (Model
D5000-Kristalloflex 710, Siemens, Karlsruhe, Germany) with
nickel-filtered CuK␣ radiation. The microstructure of the
samples also were studied via scanning electron microscopy
(SEM), and the individual grains and liquid phases were analyzed using a microprobe analyzer to determine the phase composition and to establish the solid-solution limits. The micro-
Fig. 3. (CaO⭈MgO)–Al2O3 (76.04 wt%) experimental isopletal section. Dots represent samples that were studied. (See Tables I, II, and VI for
details.)
2196
Journal of the American Ceramic Society—De Aza et al.
Vol. 82, No. 8
Fig. 4. SEM micrographs of typical samples within different crystallization fields ((a) sample 48 at 1450° ± 2°C/(23+19) h, (b) sample 66 at
1700° ± 2°C/(5+14) h, (c) sample 64 at 1450° ± 2°C/(23+29) h, and (d) sample 75 at 1700° ± 2°C/(5+14) h. (See Table II for sample identification.)
analyses were performed on polished sections using SEM
(Model ISI-DS 130, International Scientific Instruments, Milpitas, CA) coupled with energy-dispersive spectroscopy (EDS)
and wavelength-dispersive spectroscopy (WDS) (Model
WDX-3PC, Microspec Corp., Fremont, CA). Proper calibration of the instrument with standards of each phase was used to
obtain accurate analyses. For better comparison of the chemical
composition, microanalyses were conducted under the same
conditions for each specimen. Quantitative analysis was made
using the ZAF (atomic number, absorption, fluorescence) correction software, following the Microspec WDX-3PC program.
Exact quantitative determination of magnesium and aluminum via EDS was very difficult, even using standards. This
difficulty was caused by overlapping of the X-ray signals of
both magnesium and aluminum. On the other hand, both X-ray
signals appear where the background signal is greater, giving
additional errors. This problem was solved using WDS, where
the magnesium and aluminum signals were clearly separated.
In each sample, at least five microanalyses were performed
for each solid phase that was present and at lest ten microanalyses were performed for the liquid phase. The results obtained were treated statistically. (Note: The experimental procedure that has been used in the present investigation, for the
purpose of determining the melting relationship in the subsystems CaAl2O4–MgAl2O4–MgO and CaAl2O4–MgAl2O4–
CaAl4O7, is novel in the field of ceramics/materials science.)
One example of this procedure is shown in Fig. 1. This
diagram represents a hypothetical isothermal section at 1650°C
within the system Al2O3–MgO–CaO. Samples 1, 2, and 3,
which are located in the doloma–Al2O3 isopletal section, are
within the MgAl2O4ss‡–MgOss–liquid A tie triangle. Consequently, the three coexisting phases must have constant composition in the three samples. If the compositions of all the
phases are determined, the tie triangle can be established and,
consequently, the location of the MgAl2O4ss, MgOss, and liquid
A. This liquid will be located at the boundary line that separates the primary phase fields of crystallization of MgAl2O4
and MgO. Samples 4 and 5 (Fig. 1) are located at the two-phase
field MgAl2O4ss+liquid. Determination of the phase composition in both samples will give the composition of liquids G and
H and the spinel (MgAl2O4) solid solutions I and J that are in
equilibrium, respectively, with the corresponding liquid. On
the other hand, compositions 6 and 7 are located within the
MgAl2O4–CaAl4O7–liquid C tie triangle. Equally, determination of the phase composition in these samples will fix the
MgAl2O4ss (point D in Fig. 1)–CaAl4O7–liquid C tie triangle.
This liquid will be located at the boundary line that separates
the primary phase fields of crystallization of MgAl2O4ss and
CaAl4O7.
The same procedure has been used to determine the aforementioned tie triangles at different temperatures. Thus, this
procedure makes it possible to establish all the tie triangles at
different temperatures, where MgAl2O4 and a liquid coexist
with MgO, CaAl2O4, and CaAl4O7, respectively. After the tie
triangles are determined, the boundary lines, which delimit the
fields of primary crystallization of MgAl2O4 with MgO,
‡
The subscript “ss” denotes a limited solid solution.
August 1999
Ternary System Al2O3–MgO–CaO: I
CaAl2O4, and CaAl4O7 respectively, can be drawn. Therefore,
the primary phase field of MgAl 2 O 4 in the subsystem
MgAl2O4–CaAl4O7–CaO–MgO is established; consequently,
the melting relationships in the subsystems CaAl 2 O 4 –
MgAl2O4–MgO and CaAl2O4–MgAl2O4–CaAl4O7 are determined, as well as the extension of the solid solutions.
IV.
Results and Discussion
(1) Solid-State Phase Diagram
The solid-state compatibility relations that exist in the subsystem MgAl2O4–CaAl4O7–CaO–MgO are shown in Fig. 2. In
this preliminary diagram, the solid solubilities have been ignored. The diagram was constructed using data that has been
published previously,11–18 considering the free energy of formation of the different compounds25 (mainly on ⌬G°MgO,
⌬G°CA, ⌬G°MA, ⌬G°CA2, and ⌬G°C12A7 values, as well as the experimental results that have been obtained in the next part of
the present investigation in the study of the (CaO⭈MgO)–Al2O3
(76.04 wt%) isopletal section).
The entire diagram differs from that of Hallstedt,16 who
did not include the Ca12Al14O33 phase in the existence of
the compatibility join CaAl2O4–MgO, which is consistent
with the results of Majumdar.14 This phase also has been confirmed, along with the presence of two new compounds,
Table II. Sample Analysis
via SEM–WDS
Composition
(wt% Al2O3)
2197
Ca2Mg2Al28O46 and CaMg2Al16O27, as reported by Göbbels et
al.17 and Iyi et al.18 in the alumina-rich portion of the ternary
system Al2O3–MgO–CaO.
The solid-state compatibilities in the very-high-alumina region of the system Al2O3–MgAl2O4–CaAl4O7 have been
drawn tentatively. These compatibilities will be established in
part II of this work.26
(2) (CaOⴢMgO)–Al2O3 (76.04 wt%) Isopletal Section
The (CaO⭈MgO)–Al2O3 (76.04 wt%) experimental isopletal
section that was plotted with the results of selected compositions that have been obtained after heat treatment at different
temperatures is shown in Fig. 3. Figures 4(a)–(d) show typical
SEM microstructures, within different fields of crystallization,
of the samples after quenching from different annealing
temperatures.
These results show that the peritectic nature and temperature
of the invariant points of the subsystems MgAl2O4–CaAl2O4–
MgO and MgAl2O4–CaAl2O4–CaAl4O7 are established at temperatures of 1372° ± 2°C and 1567° ± 2°C, respectively. The
binary subsystem CaAl4O7–MgAl2O4 presents an eutectic
point at a temperature of 1737° ± 12°C. On the other hand, the
results clearly confirm the compatibility join CaAl2O4–MgO,
which is consistent with the data that was previously published
by Majumdar14 but not with the data of Hallstedt.16
(3) Primary Phase Field of Crystallization of Spinel in
the Subsystem MgAl2O4 –CaAl4O7 –CaO–MgO
To determine the melting relationships in the subsystems
MgAl2O4–CaAl2O4–MgO and MgAl2O4–CaAl2O4–CaAl4O7,
respectively, the primary phase field of spinel (MgAl2O4) in
Phases in equilibrium
1400°C
48
50
MgO + MgAl2O4 + liquid
MgO + MgAl2O4 + liquid
48
50
62
64
P†
MgO + MgAl2O4 + liquid
MgO + MgAl2O4 + liquid
MgAl2O4 + liquid
MgAl2O4 + CaAl2O4 + liquid
MgAl2O4 + CaAl2O4 + liquid
52
54
62
66
P†
MgO + MgAl2O4 + liquid
MgO + MgAl2O4 + liquid
MgAl2O4 + liquid
MgAl2O4 + CaAl2O4 + liquid
MgAl2O4 + CaAl2O4 + liquid
66
70
MgAl2O4 + liquid
MgAl2O4 + CaAl4O7 + liquid
54
58
71
73
75
MgO + MgAl2O4 + liquid
MgAl2O4 + liquid
MgAl2O4 + CaAl4O7 + liquid
MgAl2O4 + CaAl4O7 + liquid
MgAl2O4 + CaAl4O7 + liquid
50
60
66
73
75
MgO + MgAl2O4 + liquid
MgAl2O4 + liquid
MgAl2O4 + liquid
MgAl2O4 + CaAl4O7 + liquid
MgAl2O4 + CaAl4O7 + liquid
50
70
73
77
MgO + MgAl2O4 + liquid
MgAl2O4 + liquid
MgAl2O4 + CaAl4O7 + liquid
MgAl2O4 + CaAl4O7 + liquid
1450°C
1550°C
1570°C
1650°C
1700°C
1725°C
†
P is an additional sample with the following composition: 63 wt% Al2O3, 6 wt% MgO, and 31 wt% CaO. This
composition is located within the MgAl2O4–CaAl2O4–
liquid tie triangle but outside the isopletal section.
Fig. 5. Compositional variation with temperature in the MgAl2O4–
MgO–liquid tie triangles for (a, b, and c) periclase and (d, e, and f )
spinel. Dotted lines in Figs. 5(d) and (e) represent the stoichiometric
content of MgO and Al2O3 in the spinel.
2198
Journal of the American Ceramic Society—De Aza et al.
Table III. Liquid Compositions in Equilibrium with MgO
and MgAl2O4 at Different Temperatures
Temperature (°C)
Al2O3
Composition (wt%)
MgO
CaO
1400
1450
1550
1650
1700
1725
52.9 ± 1.3
51.5 ± 0.5
51.5 ± 1.0
50.8 ± 2.1
50.0 ± 1.2
50.5 ± 1.6
7.9 ± 0.5
9.1 ± 0.7
10.5 ± 2.1
16.0 ± 1.7
18.5 ± 1.2
19.9 ± 1.0
39.2 ± 1.3
39.4 ± 0.7
38.0 ± 1.2
33.2 ± 2.0
31.5 ± 0.6
30.3 ± 1.4
the subsystem MgAl2O4–CaAl4O7–CaO–MgO first was established. For this purpose, the compositions of different phases
that coexist at equilibrium in selected samples, within the
(CaO⭈MgO)–Al2O3 (76.04 wt%) isopletal section (Fig. 3) at
different temperatures, were quantitatively determined via
SEM–WDS.
Several samples that are located within the isopletal section
at different temperatures in the coexisting fields MgAl2O4–
MgO–liquid, MgAl 2 O 4 –CaAl 2 O 4 –liquid, and MgAl 2 O 4 –
CaAl4O7–liquid (Fig. 3) were selected, and the composition of
all the coexisting phases were determined via SEM–WDS.
Table II shows the selected compositions, their treatment temperatures, and the coexisting phases.
(A) MgAl2O4 –MgO–Liquid Tie Triangles: Figure 5 represents the compositional variation of MgO and spinel
(MgAl2O4), with temperature, in the MgAl2O4–MgO–liquid tie
triangles, plotted from the results that have been obtained. In
addition, Table III gives the liquid compositions that are in
equilibrium with both phases in the tie triangles at different
temperatures. Figure 6 was drawn using these data. This figure
shows the MgAl2O4–MgO–liquid tie triangles at 1400°, 1450°,
Vol. 82, No. 8
1550°, 1650°, 1700°, and 1725°C, as well as the boundary line
that separates the primary phase fields of crystallization of
MgAl2O4 and MgO.
The invariant point “P3” in Fig. 6, which is the peritectic
point of the subsystem MgAl2O4–MgO–CaAl2O4, has been
located in the composition by projection from the MgO and
MgAl2O4 compositions. These points are projections in the
isopletal section of this invariant point (denoted as points ␾ and
␪ in Fig. 3).
From these data, the extent of substitution of Al3+ ions for
Mg2+ ions in the cubic close-packed oxygen lattice of periclase
(MgO) was established over a temperature range of 1400°–
1725°C within these tie triangles. Figure 5(b) clearly shows a
linear variation in the weight percent of Al2O3 in the periclase
crystals as the temperature increases. These variations are
caused by three larger, divalent magnesium ions (ionic radius of
0.72 Å) being replaced by two smaller trivalent aluminum ions
(ionic radius of 0.53 Å) and a vacancy (Va). This configuration
can be represented by the formula Mg1−3xAl2xVaxO, where 0 ⱕ
x ⱕ 0.016.
On the other hand, the amount of CaO that is determined
(ⱕ0.2 wt%) is close to the detection limit of the instrumental
method that is used, and the relative error is great. Therefore,
the amount of CaO can be considered to be negligible.
Similarly, the extent of solid solubility of MgO in spinel was
determined over a temperature range of 1400°–1725°C within
the MgAl2O4–MgO–liquid tie triangles. Figure 5(d) shows that
the solid solution of MgO in spinel is negligible at temperatures
below ∼1650°C and only significant amounts in solid solution
can be detected at temperatures above ∼1650°C. In the spinel
structure, the oxygen ions form a face-centered cubic (fcc)
lattice. In stoichiometric spinel, the cations occupy one-eighth
of the tetrahedral sites and one-half of the octahedral interstitial
sites, whereas in natural spinel, the tetrahedral sites are occupied by Mg2+ ions and the octahedral sites are occupied by Al3+
Fig. 6. MgAl2O4–MgO–liquid tie triangles at 1400°, 1450°, 1550°, 1650°, 1700°, and 1725°C; the boundary line (e5–P3) that delimits the primary
phase fields of crystallization of MgAl2O4 and MgO also is shown.
August 1999
Ternary System Al2O3–MgO–CaO: I
ions. This arrangement is the so-called “normal distribution;”
however, at high temperature, different degrees of inversion
can be obtained and some tetrahedral sites are occupied by Al3+
ions.
Substitution of Mg2+ ions for Al3+ ions in the spinel structure
involves the introduction of 3⁄2Mg2+ ions for each Al3+ ion that
is removed, and the extra 1⁄2Mg2+ ion is assumed to enter into
octahedral sites that normally are not occupied. These sites are
modeled as an extra interstitial sublattice. This distribution can
be written as follows: (Al3+,Mg2+)1[Al3+,Mg2+]2[Va0,Mg2+]2(O2−)4, where the parentheses denote the tetrahedral sites, the
brackets indicate the octahedral sites, and Va represents the
empty sites. This solid solution, measured up to a temperature
of 1725°C, has the formula Mg1+3xAl2−2xO4 for 0 ⱕ x ⱗ 0.04.
On the other hand, the amount of CaO that is determined as
solid solution in spinel, up to a temperature of 1650°C (Fig.
5(f )), can be considered to be negligible (ⱕ0.3 wt%). Above
this temperature, the amount of CaO that is detected increases,
up to 0.9 wt%. These results supports the existence of a real
CaO solid solution in spinel. However, despite this finding, the
absolute values are not reliable, because of the previously mentioned reasons.
(B) MgAl2O4 –CaAl2O4 –Liquid Tie Triangles: Figure 7
represents the compositional variation with temperature of
spinel (MgAl2O4) and CaAl2O4 in the MgAl2O4–CaAl2O4–
liquid tie triangles, and Table IV gives the liquid compositions
that are in equilibrium with both phases in the aforementioned
tie triangle at different temperatures. Figure 8 was constructed
with these data. This diagram shows the MgAl2O4–CaAl2O4–
liquid tie triangles at 1450° and 1550°C, as well as the bound-
2199
Table IV. Liquid Compositions in Equilibrium with
MgAl2O4 and CaAl2O4 at Different Temperatures
Composition (wt%)
Temperature (°C)
Al2O3
MgO
CaO
1450
1550
55.4 ± 4.5
59.1 ± 2.6
4.6 ± 1.7
3.7 ± 1.3
40.0 ± 7.2
35.2 ± 3.1
ary line that separates the primary phase fields of crystallization of MgAl2O4 and CaAl2O4.
There are no appreciable changes in the spinel and CaAl2O4
stoichiometric compositions in the temperature range of
1450°–1550°C. Therefore, one can state that, in both phases,
there is no detectable solid solutions at these temperatures
within the system Al2O3–MgO–CaO.
(C) MgAl2O4 –CaAl4O7 –Liquid Tie Triangles: Figure 9
represents the compositional variation with temperature of
spinel (MgAl2O4) and CaAl4O7 in the MgAl2O4–CaAl4O7–
liquid tie triangles, and Table V gives the liquid compositions
that are in equilibrium with both phases in the aforementioned
tie triangle at different temperatures. Figure 10 was drawn
using these data. This figure shows the MgAl2O4–CaAl4O7–
liquid tie triangles at 1570°, 1650°, 1700°, and 1725°C, as well
as the boundary line that separates the primary phase fields of
crystallization of MgAl2O4 and CaAl4O7.
From these data, the extent of solid solution of Al2O3 in
spinel was established over a temperature range of 1570°–
1725°C within these tie triangles. Figure 9 shows that Al2O3 is
very soluble in spinal at high temperature, mainly above
∼1650°C. When excess Al3+ ions are introduced into the spinel
structure, vacant sites also are formed, to maintain electroneutrality. These vacancies form on either tetrahedral or octahedral
sites or both. If it is assumed, for convenience,16 that vacancies
form on octahedral sites only, the formula for spinel is then
represented by the following expression: (Al 3+ ,Mg 2+ ) 1 [Al3+,Mg2+,Va0]2(O2−)4, where the parentheses denote the tetrahedral sites, the brackets indicate the octahedral sites, and Va
represents the vacancies, as mentioned previously. This solidsolution series, up to 1725°C, also can be represented by the
formula Mg1−3zAl2+2zVazO4, with 0 ⱕ z ⱗ 0.027.
Bearing in mind the previously mentioned considerations
about the detection limit of the instrumental method that is
used, the amount of CaO recorded as a solid solution in spinel
within the MgAl2O4–CaAl4O7–liquid tie triangles, up to
1725°C, can be considered to be negligible (ⱕ0.26 wt%).
There are no measurable changes in the CaAl4O7 stoichiometric composition over the temperature range of 1570°–1725°C;
therefore, it can be stated that there are no meaningful solid
solutions in calcium dialuminate at this range of temperatures
within the system Al2O3–MgO–CaO.
Integration of the experimental data that are shown in Figs.
6, 8, and 10 yields a determination of the primary phase field
of crystallization of spinel (MgAl2O4) within the subsystem
MgAl2O4–CaAl4O7–CaO–MgO (Fig. 11). The broad extension
of the spinel primary phase field of crystallization within the
aforementioned subsystem is noted.
The projection of the liquidus surface of the subsystem
MgAl2O4–CaAl4O7–CaO–MgO was established from accumulation of all the information (bibliographic and experimental).
Table V. Liquid Compositions in Equilibrium with
MgAl2O4 and CaAl4O7 at Different Temperatures
Fig. 7. Compositional variation with temperature in the MgAl2O4–
CaAl2O4–liquid tie triangles for (a, b, and c) spinel and (d, e, and f)
calcium monoaluminate. Dotted lines represent the stoichiometric content of MgO, Al2O3, and CaO in both phases.
Temperature (°C)
Al2O3
1570
1650
1700
1725
64.3 ± 3.6
68.1 ± 1.5
69.7 ± 1.4
70.6 ± 0.7
Composition (wt%)
MgO
3.5 ± 1.8
3.5 ± 1.4
4.3 ± 1.7
3.2 ± 1.7
CaO
32.2 ± 2.9
28.4 ± 1.68
26.0 ± 1.4
26.2 ± 2.1
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Journal of the American Ceramic Society—De Aza et al.
Vol. 82, No. 8
Fig. 8. MgAl2O4–CaAl2O4–liquid tie triangles at 1450° and 1550°C; the boundary line that delimits the primary phase fields of crystallization of
MgAl2O4 and CaAl2O4 also is shown.
Fig. 9. Composition variation with temperature in the MgAl2O4–CaAl4O7–liquid tie triangles for (a, b, and c) spinel and (d, e, and f ) calcium
dialuminate. Dotted lines represent the stoichiometric content of MgO, Al2O3, and CaO in both phases.
August 1999
Ternary System Al2O3–MgO–CaO: I
2201
Fig. 10. MgAl2O4–CaAl4O7–liquid tie triangles at 1570°, 1650°, 1700°, and 1725°C; the boundary line (P4–3) that delimits the primary phase
fields of crystallization of MgAl2O4 and CaAl4O7 also is shown.
Fig. 11.
Primary phase field of crystallization for spinel (MgAl2O4) in the subsystem MgAl2O4–CaAl4O7–CaO–MgO.
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Fig. 12.
Journal of the American Ceramic Society—De Aza et al.
Vol. 82, No. 8
Projection of the liquidus surface of the subsystem MgAl2O4–CaAl4O7–CaO–MgO; the solid-state compatibility lines also are shown.
Table VI.
Composition, Temperature, and Nature of Characteristic Points within the Subsystem
MgAl2O4–CaAl4O7–CaO–MgO
Composition (wt%)
Point
Temperature (°C)
Al2O3
CaO
MgO
Comments
Reference
P1
E1
E2
E3
P2
P3
P4
1
2
3
1450
1321 ± 3
1346
1344
1350
1372
1567 ± 2
1593
1635 ± 2
1737 ± 12
42.3
47.85
50
50.74
51.11
52.5
63.2
65
65.3
77.3
51.5
46.40
44.26
43.88
42.59
40.55
33.3
33.4
31.1
18.9
6.20
5.75
5.74
5.38
6.30
6.95
3.50
1.60
3.6
3.8
Peritectic point
Eutectic point
Eutectic point
Eutectic point
Peritectic point
Peritectic point
Peritectic point
CA–MA system
CA–MA system
Eutectic point; CA–MA system
Rankin and Merwin11
Majundar14
Majundar14
Majundar14
Majundar14
Present investigation
Present investigation
Ramakrishna Rao15
Present investigation
Present investigation
August 1999
Ternary System Al2O3–MgO–CaO: I
This projection is shown in Fig. 12, where the solid-state compatibility lines between the different phases also have been
drawn without considering the solid solutions. The composition, temperature, and nature of the different invariant points
within the subsystem are indicated in Table VI.
The subsystem Al2O3–MgAl2O4–CaAl4O7, which is shown
in Fig. 12, is only tentative and will be the subject of the part
II of this work. This uncertainty is due to the high temperatures
that are involved (>1750°C), the complexities of the solid-state
compatibilities, and the melting relationships in this region of
the diagram.
V.
Conclusions
The solid-state compatibility relations in the subsystem
MgAl2O4–CaAl4O7–CaO–MgO and the melting relationships
in the subsystems CaAl2O4–MgAl2O4–MgO and CaAl2O4–
MgAl2O4–CaAl4O7 were established. The primary phase field
of crystallization of spinel (MgAl2O4) in the above-mentioned
subsystem (MgAl2O4–CaAl4O7–CaO–MgO), and the temperature, composition, and character of the ternary invariant points
of the subsystem were determined. In addition, the solid solutions in periclase, spinel, monocalcium aluminate, and dicalcium aluminate also were established, up to 1725°C.
The important features of the subsystem that has been studied are as follows:
(1) There is a broad extension of the primary phase field of
crystallization of spinel, in comparison with those of calcium
aluminates. This result confirms its low solubility in slags that
contain CaO and MgO and justifies the improvement in corrosion that is obtained when spinel is added to the matrix of
high-alumina concretes.
(2) New low-melting synthetic slags that contain
Ca12Al14O33 have been obtained for use in secondary steel
refining. These slags can be obtained saturated in CaO and
MgO or in MgO and spinel, depending of which type or refractory is used in the slag-line of the steel ladle.
(3) Spinel-containing calcium aluminate cements can be
obtained within the subsystem MgAl2O4–CaAl2O4–CaAl4O7
via the reaction sintering of appropriate mixtures of dolomite
and alumina.
Acknowledgments:
The authors wish to thank Dr. A. P. Tomsia and
Dr. Eduardo Saiz (Lawrence Berkeley Laboratory, Berkeley, CA) for the use of
the Materials Science Division facilities and their valuable help and assistance
during the SEM–WDS analyses.
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䊐
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