1 Study of new laboratory prepared periclase- Magnesium ...

Study of new laboratory prepared periclase- Magnesium aluminate spinel
refractories from sintered dead burned magnesite and various presynthesized
spinel- based compositions (II):Compositional variation between coexisting
spinel, periclase, Ca silicate and Ca-aluminate phases in magnesia spinel
refractories and in their spinel- based precursors. .
P.G. Lampropoulou*, C.G. Katagas, I. Iliopoulos
Department of Geology, University of Patras, 26500 Patras, Greece
*Corresponding author: P.G. Lampropoulou Tel: +30-2610997599, Fax: +30-
2610997560, E-mail address: p.lampropoulou@upatras.gr
Abstract
The chemistry and distribution of phases formed in a set of six laboratory prepared
magnesia-magnesium aluminate spinel ceramics and in three of their precursor spinel
based compositions, sintered at 1600 o C and 1760 o C respectively, are examined and
compared.
Despite the differences in stoichiometry of the spinel phases formed in each type of
the materials and in their firing temperatures, the spinels have a strong preference for
normal structure. Microtextures and microanalyses of cracked and non cracked
domains in periclase crystals from spinel based compositions suggest that the
development of fractures is probably facilitated by differences in the thermal
expansion coefficient of periclase crystals having domains differing in their Al 2 O 3
contents. A comparison of coexisting phases from both types of materials indicate that
heat treatment of periclase and spinel mixtures at lower temperatures (1600 o C)
involved reactions leading to the formation of different periclase s.s and spinel s.s in
the ceramics, which contrary to the theoretically predicted compositions, depart
clearly from equilibrium assemblages.
Minor amounts of stoichiometric Ca-silicate and Ca-aluminate phases were formed in
both types of the materials and are encountered as small particles in the siliceous
bonds; C 3 S 2 , the most abundant of the low melting metastable phases, occurs in
microdomains of only few microns wide, and cannot be augmented with Ca-aluminate
phases to a migrated liquid, which could result in extensive negative effects on the
properties of the refractory ceramics.
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Introduction
Most of the studies concerned with magnesia-spinel refractories focuse mainly to their
physical and technological properties as related to their microstructures. Such studies
on the physical properties of various Magnesia-spinel based compositions and their
microstructures in relation to their densification parameters and refractory resistance
as well as on different synthesis routes (1-5) and on the effects of different additives
on the development of spinel based compositions with improved sintered properties
have been published by many workers [e.g. 6-10].
It is now well established that Magnesia-Spinel refractories derived from pure raw
materials with a high degree of direct bonding of MgO-MgO and MgO-spinel grains,
and with low amounts of low-melting silicate phases, exhibit high hot temperature
strength, an improved resistance to slag attack, and dimensional stability at high
temperatures [11, 12].
Nowadays, the need for a detailed study of magnesia spinel materials is further
triggered in order to promote their uses in many other fields such as in catalysis,
optical ceramics, humidity sensors [13-17] under the framework of “green policies”
which are widely sought for by organizations and governments. Newly developed
magnesium-aluminate spinel ceramics, for instance, appear to meet the requirements
for geological time-period disposal of high level nuclear and hazardous wastes [18,
19]. In a companion paper to this study [20] we report details on the laboratory
synthesis, mineralogical composition, microstructure and property evaluation of a set
of six rebonded magnesia-magnesium aluminate spinel refractories containing various
amounts of Al 2 O 3 , synthesized from three previously prepared spinel-based
compositions and high purity MgO. The objective was to contribute to the
development of more environmentally friendly refractories, which could substitute for
the magnesia-chromite bricks, or find applications to some advanced usage.
Unfortunately, relatively few studies have presented detailed systematic data on the
chemistry of coexisting phases in spinel based compositions and on their significance
on the growth of refractory periclase-spinel bearing technological materials.
However, understanding chemical and mineralogical compatibilities among
coexisting periclase, magnesium aluminate spinel, Ca-silicate and Ca-aluminate
phases in various bulk compositions and temperatures is an essential prerequisite for
the experimental design of magnesia-spinel refractories and for the evaluation and
characterization of these materials before their use as refractories or in other
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applications. In this article we present results of investigations on variations in the
chemical composition of coexisting phases from the six laboratory synthesized
magnesia spinel refractories of various compositions as well as from their respective
spinel based precursors, and examine and compare the chemical mineralogy and some
microstructural characteristics of these materials in relation to their bulk chemical
composition and firing temperatures.
Experimental procedures, materials and methods
We present here only a brief account of the synthesis procedures and the analytical
methods used for the chemical, mineralogical and microstructural characterization of
the materials. More detailed information on these issues is presented in the
companion paper to this study. [20].
Pure magnesia (MgO>96%), and Alumina (Al 2 O 3 >99.5%) powders, mixed in
different proportions, were used as raw materials for the laboratory synthesis of three
spinel based compositions. The compacted raw materials mixtures were initially
calcined to 1760 o C, followed by regrinding to powder, recompaction and refiring at
the same temperature. The spinel based materials were termed (a) Sp55z : made of
alumina and magnesia in about 1:1 ratio and 0.5 wt% zirconium silicate (ZrSiO 4 ) as
additive; (b) Sp73z: made of alumina and magnesia in about 2.3:1 ratio and 1wt%
ZrSiO 4 as additive; and (c) Sp73chr: made of alumina and magnesia in about 2.3:1
ratio and 3wt% chromite (Mg 0.48 Fe 0.54 )(Cr 1.18 Al 0.57 Fe 0.22 )O 4 . as additive.
Using mixtures of ground powders of the presynthesized spinel-based compositions
and pure dead-burned magnesia, six new magnesia-magnesium aluminate spinel
refractory materials, containing various amounts of Al 2 O 3 were laboratory prepared
(see 19, 20). After mixing, the powders of various grain sizes were pressed, dried and
sintered up to 1600 o C, remained at this firing temperature for four hours and then the
samples were furnaced-cooled for twelve hours. Three of the powder mixtures were
selected to result to rebonded magnesia-magnesium aluminate spinel refractories (M-
Sp-L) containing low (L) amounts of Al 2 O 3 (~8-11wt%) while the rest three samples
(M-Sp-H) have a high (H) Al 2 O 3 content (~19-21wt%)
ICP –MS bulk chemical analyses of the six new refractory materials and their three
precursors are summarized in Table 1. Phase identification was performed through X-
Ray powder diffraction, SEM imaging and SEM EDS-WDS microanalyses on
polished, carbon coated thin sections of all samples. Micro-Raman spectroscopy has
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een tentatively used to study the degree of order in spinel crystals and for the
identification of minor phases, in one of the samples employing a T6400 Jobin Yvon
monochromator and Ar + laser tube
Phase composition of the synthesized materials.
Detailed results on the mineralogical composition and microstructure, of the
synthesized materials were reported in the companion paper of this article[19];
Periclase and various amounts of spinel, dependent on the Al 2 O 3 wt% contents of the
starting materials, are major phases of the spinel based materials (20-21).
Minor Ca-silicate and Ca-aluminate phases are confined to small domains of the
microstructures and many of them are probably not equilibrium phases, because they
were not expected to form on the basis of known phase equilibria for bricks of these
compositions [22, 23].
Back scattered electron images from the magnesia spinel bricks suggested the growth
of spinel crystals with a characteristic shape, showing different micro-structural and
possibly different compositional characteristics from those in their precursor materials
as well as secondary spinel precipitates, visible along magnesia-magnesia or
magnesia-spinel grain boundaries or as small exsolution blobs precipitates within
periclace. Secondary Ca-aluminate and Ca-silicate phases are also present in low
amounts in tiny domains of a few microns wide.
Results and discussion
Chemical compositions of phases
Spinel
Knowledge of the chemical composition of the spinel is of great importance because
variations in lattice parameters, occurring as the stoichiometry of the spinel changes
among spinel grains, must considerably affect the mechanical stability of the material.
According to Nestola et al [24] and references therein, an excess of Al in the spinel
structure, accompanied by the formation of cation vacancies mostly at the octahedral
site, strongly affects its thermal expansion.
Spinels with the general chemical formula AB 2 O 4 , have a unit cell capable of holding
a large number of cations occupying octahedral and tetrahedral sites in different ways.
The cation distribution is said to be normal if all the A cations (Mg, Fe 2+ , Zn, Mn) are
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on tetrahedral sites with all B cations (Al, Cr, Fe 3+ ) on octahedral sites or inverse if it
is characterized by occupation of one of B-sites by a divalent cation with one trivalent
cation taking its place on the A-site. Thus, normal spinel has a general formula
A 2+ B 3+ 2 O 4 in which the A-cations exhibit 4-coordination and B-cations exhibit 6-
coordination, and in inverse spinels, with the general formula B 3+ A 2+ B 3+ O 4 , the A
cations are 6-coordinated and half of the B cations are 4-coordinated and half are 6-
coordinated.
The MgAl 2 O 4 spinel formed in the Periclase-spinel refractories and their precursor
materials, is assumed to have a strong preference for the normal structure, since
simple radius ratio arguments suggest that smaller cations would prefer to occupy
tetrahedral sites and Mg +2 is the smallest (r=0.66Å). However, it is known that by heat
treating natural or synthetic spinel to high temperatures, aluminum and magnesium
ions start to change sites, giving rise to more random distributions of the cations
leading to different degrees of inversion. This change in the distribution of the cations
is accompanied by changes in the thermodynamic properties of the spinel and can be
retained depending on the temperature of the heat treatment and cooling rate (25, 26,
27). XRD and SEM/EDS-WDS analyses were used to examine the degree of
inversion and stoichiometry of the spinel phases formed at 1760 o C in the preprepared
spinel-based compositions and of those formed at 1600 o C in the Periclasemagnesium
aluminate spinel refractories. Raman spectroscopy has been applied on
one of the nearly stoichiometric spinel based compositions to test the distribution of
the cations in the spinel structure.
The spinel phase in spinel based compositions
More than thirty spot microanalyses (spot size

usually accompanied by the formation of cation vacancies. Microanalyses of crystals
from the sample with chromite additive (Sp73chr) show lower mean Αl 2 Ο 3 wt%
values than those from samples with zirconium silicate additive. This is thought to be
a result of mainly Cr and Fe 3+ ions substituting for Al in the spinel lattice. As Cr 2 O 3,
Fe 2 O 3 and FeO enter the spinel by solid solution, its lattice parameter and crystal
density increase as a function of the ionic radii of the divalent and trivalent ions (See
Table 2a, Sp73chr sample) and the direct diffusion bonding should be stronger [18].
The lattice parameters and the mean density values of spinel crystals from each
sample have been calculated according to Cullity [28] and Deer et al [29] using the
general empirical formula: ρ=1.66020*MW of the spinel/α 3 ;
α(Å)=5.790+0.95R 2+ +2.79R 3+ . In the present study we see that the α cell parameter
values of the spinels formed in the spinel based materials, as derived from their X-Ray
diffraction patterns are much closer to the ideal lattice parameter values reported in
the literature than those obtained using their chemical analyses and the above cited
empirical formula. The latter α lattice parameter values of spinels from both types of
materials are always lower than the expected by about 3.0-3.3%. This discrepancy
needs further investigation.
It has been suggested that when a single crystal of Al 2 O 3 is converted to spinel
through a reaction with periclase, a concentration gradient is developed by counter
diffusion of Mg 2+ inward and Al 3+ outward, therefore compositional differences are
expected to occur in various microdomains. In the backscattered - micrographs of
spinel based composition with chromite additive for instance, spinel exhibits very
often different shades of gray. Line scan elemental profiles performed on such a
spinel crystal demonstrated that such variations are common and reflect variations in
the atomic distribution, especially of Cr and Fe 3+ , in micro domains of the spinel
crystal (Fig 1). Recalculations of spot analyses from the sample sp73chr spinels
(Table 2a), indicate that the spinels may differ slightly in their Fe 2+ and Fe 3+ contents,
departing thus from the spinel-hercynite -chromite-magnesiochromite plane of the
normal spinels, towards the magnesioferrite-magnetite (or ulvospinel) compositions.
Study of the XRD patterns of the spinel crystals, however, indicate that there is not a
significant degree of inversion since most of the stronger odd reflections like (311)
etc, show higher intensities compared to the standard list of the fully ordered spinel
peaks recorded in the ICDD 21-1152 pattern, while the stronger even reflections e.g.
(400) do not show an increase but present lower, similar intensities or are absent [29].
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The use of Raman spectroscopy, which has not yet found a wide application in the
study of the mineral phases, was experimentally tested on the Sp73z sample and a
representative Raman spectrum for the MgOAl 2 O 3 is presented in Fig. 2. According
to Barpanda et al [30] a peak near 723cm -1 indicates the occupancy of some Al ions in
tetrahedral sites making it a disordered structure while the intensity of this peak
decreases or entirely collapses with increasing calcination temperature. The absence
of such a peak from the Raman spectrum obtained from the spinel of Sp73z suggests
the formation of an ordered spinel structure, after sintering of the spinel based
composition at 1760 o C.
The spinel phase in Magnesia-Magnesium Aluminate spinel refractories.
Though some original spinel crystals seem to have been inheritted, apparently
unchanged, from the precursor spinel based compositions, careful examination by
analytical scanning electron microscopy revealed that with MgO addition to
presynthesized materials, and subsequent sintering most of the primary spinel reacted
with periclase to form secondary spinels (and periclase) of different compositions. It
is of interest that at the calcination temperature of 1600 o C used for their synthesis, a
temperature lower than the firing temperature of the precursor spinel based
compositions (1760 o C), there appear to be marked compositional (and
microstructural) changes in the spinel particles of the six magnesia-magnesium
aluminate spinel refractories, indicating thus that spinel is not an inert component in
the system at this lower temperature. Secondary spinel is also found in the form of
exsolved precipitates which have been formed by exsolution from periclase solid
solution on cooling, as particles with no characteristic euhedral shape, or as white
streaks occurring mostly in periclase -periclase grain boundaries (Figs. 3, 4).
The abundance of the secondary spinel in the various magnesia-magnesium aluminate
spinel refractories is affected by the proportions of the raw materials in the mixtures
and as expected, increases with increasing the proportions of the presynthesized spinel
based material. The amount of the grain boundary spinel is negligible as compared
with that of the large primary supposed spinels in the matrix of the refractories. The
micrographs in Fig. 3 show various periclase and spinel particles.
Representative combined SEM EDS-WDS analyses performed on primary supposed
spinel crystals from magnesia-spinel refractory polished thin sections specimens are
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summarized in Table 2b. The stoichiometries of the spot analyses indicate that the
spinels formed obey to the general formula A 2+ B 3+ 2O 4 of the normal spinel.
There was a difficulty in identifying and obtaining fully quantitative chemical
analyses of minute exsolved secondary spinels and of spinel streaks confined to small
bond regions. Admissible analyses of these phases were accomplished only in the
samples M-Sp55z-L, M-Sp73z-H and M-Sp73chr-H and were plotted in the Mg-
Fe(tot)-Al+Cr ternary diagram (Fig. 5a) .
The projection of the chemical composition of the spinel crystals from the six
Magnesia-Spinel refractories and of those from their respective pre-synthesized spinel
based compositions in the Mg-Fe(tot)-Al+Cr ternary diagram (Fig. 5a) revealed a Mg
enrichment in the spinel crystals of the former in respect to those in their counterpart
precursor compositions (with the exception of the M-Sp73chr-H sample).
Moreover, as is illustrated in the same ternary diagram (Fig. 5a), the secondary spinel
Type II in the refractory materials M-Sp55z-L and M-Sp73z-H (both involving
primary spinel that was synthesized with zirconium silicate as additive) is enriched in
Mg and Fe and impoverished in Al compared to the secondary spinel crystals of Type
I. A similar trend has been recorded in the spinels of the refractory material M-
Sp73chr-H, synthesized employing a pre-prepared, chromite added, magnesiumaluminate
spinel based material. Such variations have been attributed to the
combination of the eutectic iron-calcium-aluminate silicate solutions that formed at
the sintering temperatures at the boundaries of the periclase and spinel grains [31],
and to the vicinity of the precipitated secondary spinel of Type II with periclase
grains/crystals. A perforated texture has been occasionally observed in the bonding
secondary spinel of the magnesia spinel refractories (Fig. 4) which could be attributed
to the presence of limited amounts of spinel phase in these micro domains, in
conjuction with the remoteness of the alterated silicate liquid during the cooling
processes .
Structural formulae recalculations of the spinel crystals (Table 2) provide evidence
that iron ions (Fe +2 and Fe +3 ) may occupy both octahedral and/or tetrahedral sites in
the spinel lattice of the spinel based compositions, whereas the majority of the spinel
microanalyses from the Magnesia-Spinel refractories, indicate that iron is mainly
octahedrally coordinated. According to the melting points of the MgFe 2 O 4 (1750 o C)
and FeAl 2 O 4 (1440 o C) end members of spinel series, one can predict that the trivalent
iron oxide in the MgOAl 2 O 3 lattice plays a more critical role under high temperatures.
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Periclase
Representative combined EDS-WDS SEM analyses performed on periclase crystals
from all specimens are summarized in Table 3a.
The periclase in spinel based compositions
The periclase crystals in spinel based compositions exhibit intra-individual variation
in their FeO content within sample, as well as variation among the different samples.
Although individual analyses reveal that periclase can contain up to 2.1wt% FeO in
solid solution as wustite, the mean values of FeO wt% contents from each sample are
significantly lower (1.32, 0.64 and 0.74wt% for samples Sp55z, Sp73z and Sp73chr,
respectively).
Periclase crystals from the chromite added sample (Sp73chr), incorporate small
amounts of chromium (not exceeding 0.002 a.p.f.u.) in their structure as a solid
solution, while in the case of the samples with zirconium silicate additive, periclase
crystals contain only trace amounts of ZrO 2 .
The solid solubility of Al 2 O 3 in the periclase crystals is low; the maximum Al 2 O 3
content recorded in the analyses was only 1.1wt% (max. mean value: 0.34wt% in
Sp73z sample). Although, the periclase crystals of the raw pure magnesia can contain
up to ~2.0%CaO in solid solution [32], the crystals of the spinel based compositions
contain only trace amounts of CaO. The negligible CaO component of periclase in the
spinel based compositions is probably due to longer cooling time compared to that of
the feeding material, leading to the destruction of the Cao-MgO solid solution.
Periclase crystals in spinel based compositions often exhibit cracks (See in
companion paper [20], Fig. 2a) that maybe attributed to their higher thermal
coefficient factor compared to that of the surrounding spinel. The latter inhibits
probably the maximum expansion of periclase crystals resulting to microcracks. It is
worthy to note, that microanalyses of cracked and no cracked regions of periclase (Fig
6) reveal higher Al 2 O 3 wt% contents in the fractured domains. It has been suggested
that thermal expansion coefficient values are probably higher in periclase crystals
having more appreciable Al 2 O 3 contents. Expansion of such periclase crystals may
lead to the development of micro-fractures and the formation of smaller
microdomains. Wilson et al [33] and Aksel et al [34] argued that microcracks in free
periclase bearing spinel based compositions improve the resistance of the materials to
thermal shocks. However, periclase hydration to brucite due to a long atmospheric
9

exposure of the materials prior to their examination, could also lead to cracks
manifestation in the microstructure.
The periclase in Magnesia-Spinel refractory materials
Micro analyses of periclase crystals from the magnesia-spinel refractory materials and
their respective spinel based compositions (Table 3b), plotted in the Mg-Fe(tot)-
Al+Cr ternary diagram (Fig. 5b) illustrate an Mg enrichment of the periclase formed
in the former materials. Iron is found in periclase crystals in all samples as wustite,
but it never exceeds 0.01 a.p.f.u and does not affect the melting point of this phase.
The maximum mean values of aluminium and chromium, are recorded in periclase
crystals of the M-Sp73z-L (0.003 a.p.f.u) and M-Sp73chr-H (0.002 a.p.f.u)
refractories, respectively.
As noted earlier, an intracrystalline distribution of spherical shaped microscopic
pockets (up to ca. 5μm in diameter) filled by exsolved spinel grains and a calciumaluminate
phase has been recognised in the periclase crystals. Fig. 7 is a
representative SEM/EDS elemental mapping of a periclase crystal from the M-
Sp73z-H sample, illustrating the distribution of magnesium, iron, calcium, aluminium
and silicon.
Coexisting periclase-spinel pairs
Various studies have established that periclase and spinel solid solutions can stably
coexist at temperatures above 1500 o C.
According to the phase diagram of the binary MgO-Al 2 O 3 system [35] and the
compositional range of the studied materials, periclase is expected to shift to more
Al 2 O 3 rich compositions whereas the coexisting magnesium aluminate spinel becomes
slightly depleted in Al 2 O 3 and enriched in MgO, with increasing temperatures,
following the solvus limbs.
Contrary to the theoretically predicted compositions, our results indicate that coexisting
periclase and spinel crystals from the magnesia-magnesium aluminate spinel
refractories are both more MgO rich and Al 2 O 3 poor than periclase-spinel pairs from
the precursor spinel based materials, despite the lower firing temperature (1600 o C) of
the former.
Yet, whereas the chemical compositions of most of the coexisting periclase-spinel
pairs from the pre-synthesized materials seem to have been equilibrated, though at
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temperatures lower than the expected (~1650 o C instead of 1760 o C), periclase-spinel
pairs from the refractory materials depart clearly from equilibrium. As is evident from
Fig. 8 (M-Sp73z), various MgO-rich spinel solid solution crystals in the refractories
(for example in M-Sp73z) indicate temperatures higher than those actually prevailed
during their formation, whereas a variety of coexisting periclase s.s. crystals indicate
temperatures close to or only little higher than the theoretically predicted by the phase
diagram.
We have not an obvious explanation for the observed discrepancies however, several
factors could conceivably account for the lack of equilibrium between coexisting
periclase and spinel after the firing of the refractory mixtures at 1600 o C.
Heating of periclase and spinel bearing mixtures at this subsolidus temperature could
produce a more random distribution of aluminium and magnesium cations, which,
depending on other components present in the system, may lead to the formation of
different MgO and spinel solid solutions. Compositional differences in various
microdomains due to inhomogeneous distribution and differences in grain size of the
precursor constituents, could also produce random distribution of the cations,
principally on the basis of lack of short diffusion paths between spinel and magnesia
particles [36]. The presence of other oxides as impurities (CaO, SiO 2 , Fe 2 O 3 , TiO 2 ) or
additives (chromite, zirconium silicate) may contribute to the inhomogeneous
distribution of the magnesium and aluminium atoms. For example, the addition of
chromite enchances the solid solution reaction of Al 2 O 3 in the spinel, as Cr +3
substitute for Al +3 in the spinel lattice and spinel is therefore not a strictly MgO-Al 2 O 3
solid solution phase. The periclase crystal structure permits also substantial solid
solution of Cr 2 O 3, Al 2 O 3 and FeO/Fe 2 O 3 at this temperature. Furthermore, the soaking
time of 4hr at 1600 o C might not be so effective as to permit a strong interaction
between the larger spinel and periclase grains and a rapid attainment of equilibrium,
while the after heating slow furnace cooling treatment of the samples could also
contribute to the development of a variety of compositions, depending on the other
matrix phases present. SEM observations suggest that periclase-spinel crystals in the
spinel based compositions may have been cooled quickly enough to suppress
exsolution textures, whereas in the refractory materials, in which the lower
temperature along solvus limbs reactions are expected to be more sluggish, exsolution
of various spinels in the exsolving host periclase crystals are developed.
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Ca-silicate and ca-aluminate bonding phases
Small amounts of SiO 2 , CaO and iron oxides present in the starting materials are
combined after heating with the major components of the system resulting in various
calcium silicate and calcium aluminate phases, among others. According to Landy
[37] the CaO/SiO 2 wt% ratio controls the kinds of the phases that will form and also
affects the thermal, chemical and mechanical properties of the materials at high
temperatures through the CAS liquid formation and the distribution of the low melting
phases in the bond [38].
Minor calcium-silicate and / or calcium-aluminate phases are usually found as small
particles in the bonding regions of the examined samples impeding accurate
quantitative analyses. Thus, only the few representative micro-analyses which either
satisfy stoichiometric constraints of the phases occurring in the siliceous bond regions
of the precursor and refractory materials or approximate them very closely, are
presented in Tables 4.
Occasionally the C 2 S crystals exhibit a relatively little replacement of Ca by Fe,
reaching up to ~1wt% FeO (in Sp55z sample) and a more extended replacement of Ca
by Mg up to ~2 wt% MgO (in M-Sp73z-H sample).
A limited substitution of Ca by Mg in the lime structure seems to be possible,
considering the data presented in Table 4. It is also worth noting that although Zr +4
can be accommodated in the octahedrally coordinated sites of the MgAl 2 O 4 spinel
forming a solid solution (16) (reference in the originally submitted) the stabilizing
additive of zirconium silicate favoured the formation of the high melting point phase
of CaZrO 3 in the relevant samples. An additional characteristic is the formation of the
2CaOSiO 2 -3CaOP 2 O 5 solid solution phase due to the use of a low amount (0.5wt%)
of hexaphosphate as stabilizer in the refractory materials. The presence of these s.s.
phases was proved by a number of microanalyses in which, when they were
recalculated on the basis of 12(O), the number of cations Ca:Si:P ratio remain stable
to approximately 5:1:2
The majority of the C 12 A 7 crystals analyses show an almost stoichiometric formula,
although crystals with Si up to 0.98 a.p.f.u. (in Sp73chr sample ) or with Fe up to 0.04
a.p.f.u. (in Sp55z sample ) have been analyzed.
Electron microanalyses of C 3 S, C 3 S 2 and C 3 A exhibit approximately stoichiometric
proportions and only traces of Mg and Fe or Si, participate in their structures
respectively.
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The non equilibrium metastable phases C 12 A 7 and C 3 S 2 have melting points (1455 and
1475 o C, respectively) lower than the temperature conditions usually prevailed during
the use of their host as refractory materials. The presence of a liquid phase at
temperatures lower than ~1500 o C is certainty a disadvantage for the stability of the
refractory, however, these phases appear usually in low amounts. The most abundant
of them, C 3 S 2 , occurs in microdomains of only a few microns wide, and does not
coexist with other calcium-aluminate or calcium silicate phases, preventing in this
way a liquid augmentation and extensive liquid migration [38].
It is also worth noting that although Zr 4+ can be accommodated in the octahedrally
coordinated sites of the MgAl 2 O 4 spinel cell forming a solid solution [23], in the
ZrSiO 4 added samples M-Sp55z-L, M-Sp73z-L, M-Sp55z-H, M-Sp73z-H, the
stabilizing additive of zirconium silicate favoured the formation of the high melting
point phase CaZrO 3 by tying up Ca 2+ from the system.
According to Perepelitsym and Sivash [39] satisfying high energy structure criteria
(energy density and energy strength) among the secondary phases of the materials,
CaZrO 3 and C 2 S could be theoretically predicted as high wear resistant compounds
under extreme thermodynamic conditions (high temperature, high pressure, high
concentrations of corrosive agents).
Of particular interest are the results obtained from the application of Raman
spectroscopy to sample Sp73z. The Raman spectra of Fig. 9a confirm the coexistence
of the non equilibrium phase C 3 S 2 with spinel and Fig.9b affirms the presence of the
C 2 S phase in equilibrium with spinel.
Thus, despite of the difficulties that Raman spectroscopy encounters in detecting
minor phases in such microstructures, the results obtained hitherto make its use
advisable and advantageous for the identification of phases even when they occur in
microdomains of up to 2 μm wide.
Conclusions
The study of spinel phases formed at 1760 o C in the pre-prepared spinel-based
compositions and of those formed at 1600 o C in the periclase-magnesium
aluminate spinel refractories revealed that there is not a significant degree of
inversion, it is therefore suggested that the observed differences in the
stoichiometry of the spinel phases formed in each of the materials or among spinel
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grains of the same sample, could not considerably affect the mechanical stability
of the refractories.
Microanalyses of cracked and non cracked domains of periclase crystals from
spinel based compositions revealed higher Al 2 O 3 wt% contents in the fractured
domains, suggesting that the development of different thermal expansion
coefficient values within a periclase crystal scale, is probably responsible for this
fracturing.
The study of the chemical composition of coexisting phases in magnesiamagnesium
aluminate refractories indicate that heat treatment at 1600 o C of
compacted powder mixtures of presynthesized at 1760 o C magnesium aluminate
spinel based compositions and periclase, involved reactions of primary spinels
with periclase to form spinels and periclase of different compositions, indicating
that spinel is not an inert component in the system when this is fired at lower
temperatures. However, contrary to the theoretically predicted compositions, our
results indicate that coexisting periclase s.s and spinel s.s crystals from the six
synthesized refractories have compositions which depart clearly from equilibrium.
Several factors have been considered which may all contribute to a random
distribution of Al +3 and Mg +2 cations in microdomains of the refractories, leading
thus to the development of a variety of coexisting periclase and spinel solid
solutions which are not in fact equilibrium assemblages.
The calcium silicate and calcium aluminate phases formed in the magnesia-spinel
refractories are chemically fairly similar to those found in their counterpart spinel
based compositions. Most of them are usually encountered as small particles in the
siliceous bond. Microanalyses of the bonding phases C 3 S, C 3 S 2 and C 3 A are
almost stoichiometric and reveal the presence of trace amounts of Mg, Fe or Si in
their structures. Occasionally, the C 2 S crystals exhibit relatively little replacement
of Ca by Fe and a more extended of Ca by Mg; appreciable substitution of Al by
Si in the C 12 A 7 structure seems to be possible. C 3 S 2 , the most abundant of the low
melting non-equilibrium metastable phases (C 3 S 2 and C 12 A 7 ) occurs in
microdomains of only few microns wide and does not coexist with other calcium
aluminate phases, preventing in this way a liquid augmentation which could result
in extensive liquid migration and negative effects on the properties of the
refractories.
14

The results obtained from the application of Raman spectroscopy, (a method
which has not yet had wide application in the study of mineral phases) to one of
the spinel based composition samples are promising for a wider implementation of
the method for the determination of the structural characteristics of spinel as well
as for the identification of minor calcium/aluminate/silicate phases occurring in
microdomains with a diameter of up to 2μm.
Acknowledgments
The authors wish to thank Mr. V. Kotsopoulos of the laboratory of Electron
Microscopy and Microanalysis, University of Patras, for his help with the
Microanalyses and SEM photomicrographs as well as to Mr G. Voyiatzis, Principal
Researcher of Institute of Chemical Engineering and High Temperature Chemical
Processes of University of Patras, for his assistance on carrying on Micro-Raman
analyses. Special thanks are due to Prof. P. Tsolis-Katagas and Dr. Ch.Rathossi for
their precious comments on improving an early version of the manuscript.
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17

Table 1: Chemical composition (ICP-MS) and bulk density of the spinel based compositions and the synthesized Magnesia-Spinel
refractories.
Sample Sp55z Sp73z Sp73chr M-Sp55z-L M-Sp55z-H M-Sp73z-L M-Sp73z-H M-Sp73chr-L M-Sp73chr-H
%wt
SiO 2
Al 2 O 3
Fe 2 O 3 (tot)
MnO
MgO
CaO
Na 2 O
K 2 O
TiO 2
P 2 O 5
Cr 2 O 3
ZrO 2
0.55
46.28
2.51
0.03
48.34
1.76
0.02
0.06

Table 2a: Representative microanalyses, lattice parameter and mean density of spinel crystals in spinel based compositions.
samples Sp55z Sp73z Sp73chr
wt% 1 2 Range
Mean
1 2 Range
Mean
1 2 Range
Mean
Al 2 O 3 70.85 69.76 69.76-71.74
71.08
70.47 70.56 69.36-71.25
70.44
70.91 68.50 68.20-70.91
69.54
TiO 2

Table 2b: Representative microanalyses and lattice parameters of spinel crystals in Magnesia-Spinel refractories.
samples M-Sp55z-L M-Sp73z-L M-Sp73chr - -L
wt% 1 2 Range
Mean
1 2 Range
Mean
1 2 Range
Mean
Al 2 O 3 69.62 71.06 69.61-71.33
70.73
69.81 69.88 69.07-70.22
69.96
69.33 68.34 67.34-69.33
68.41
TiO 2

Table 2b (cont.)
samples M-Sp55z - -H M-Sp73z-H M-Sp73chr - H
wt% 1 2 Range
Mean
1 2 Range
Mean
1 2 Range
Mean
Al 2 O 3 67.94 70.06 67.93-70.97
69.77
70.42 70.57 69.71-70.71
70.40
71.11 70.86 69.07-71.59
70.88
TiO 2

Table 3a: Representative microanalyses of periclase crystals in spinel based compositions
samples Sp55z Sp73z Sp73chr
wt% 1 2 Range
Mean
1 2 Range
Mean
1 2 Range
Mean
Al 2 O 3 0.25 0.13 0.11-1.01
0.26
0.18 0.09 0.09-1.10
0.34
0.18 0.18 0.11-0.18
0.17
TiO 2

Table 3b: Representative microanalyses of periclase crystals in Magnesia-Spinel refractories
samples M-Sp55z-L M-Sp73z-L M-Sp73chr - L
wt% 1 2 Range
Mean
1 2 Range
Mean
1 2 Range
Mean
Al 2 O 3 0.28 0.20 0.18-0.43
0.25
0.25 0.27 0.27-0.51
0.33
0.23 0.14 0.11-0.29
0.20
TiO 2

Table 3b (cont.)
samples Magnesia-Sp 55z- H Magnesia-Sp 73z -H Magnesia-Sp 73chr- H
wt% 1 2 Range
Mean
1 2 Range
Mean
1 2 Range
Mean
Al 2 O 3 0.24

Table 4: Microanalyses of C/A-silicates phases in spinel based compositions and Magnesia-Spinel refractories.
phases C 2 S C 3 S C 3 S 2 C 3 A C C 2 S-C 3 P C 12 A 7 CZ
samples M-Sp55z -L M-Sp73z-H M-Sp73chr- Sp55z M-Sp73z-l M-Sp73chr- M-Sp73chr-H Sp73z
H
L
wt%
Al 2 O 3

FIGURE CAPTIONS
Figure 1: Line scan elemental profiles in a spinel crystal of the Sp 73chr spinel based
composition.
Figure 2: Raman spectrum of spinel in the Sp 73z sample.
Figure 3: Periclase and spinel particles in M-Sp73chr-H. (M)= periclase; (PMA)=
primary supposed spinel (SMA)= secondary spinel; (C/A-S)= Ca-silicate or
aluminate;(P)= pore.
Figure 4: Perforated texture of the secondary spinel in the magnesia spinel
refractories.
Figure 5: (a): Mg-Fe(tot)-Al+Cr ternary plot of the primary supposed spinel (PMA)
in the Magnesia-Spinel refractories and in their respective raw spinel based
compositions, and plot of the secondary spinel (SMA) in M-Sp55z-L, M-Sp73z-H and
M-Sp73chr-H.
(b):Mg-Fe-(Al+Cr) ternary plot of the periclase in the Magnesia-Spinel refractories
and in their respective raw spinel based compositions.
Figure 6: Histogram of Al 2 O 3 (wt%) contents in cracked and no cracked periclase
crystals in the spinel based compositions.
Figure 7: Elemental mapping (SEM) in a periclase crystal of the M-Sp73z-H sample,
using the distribution of magnesium, iron, calcium, aluminium and silicon and
SEM/EDS spectra of the exsolved phases.
Figure 8: Plots of coexisting spinel and periclace chemical compositions in the Sp73z
and M-Sp73z-H samples on the MgO-Al 2 O 3 phase diagram [35].
Figure 9a: Raman spectrum of spinel and C 3 S 2 in the Sp73z sample; b: Raman
spectrum of spinel and C 2 S in the Sp73z sample.
26

Fig. 1
50
spinel(=sp)
sp(380cm -1 - )
45
Relative Intensity
40
35
sp(650.02cm -1 )
sp(702.5cm -1 )
sp(822.5cm -1 - )
30
500 1000
Raman Shift (cm -1 )
Fig 2
Fig 3
28

Fig 4
Fe2 + Fe3
Mg
b
a
Al + Cr
Sp55z
Sp73z
Sp73chr
M-Sp55z-L
M-Sp73z-L
M-Sp73chr-L
M-Sp55z-H
M-Sp73z-H
M-Sp73chr-H
b a
M-Sp55z-L
(SMA)
M-Sp73z-H
(SMA)
M-Sp73chr-H
(SMA)
Mg
Fig 5
Al + Cr
mean values of wt%Al 2
O 3
in cracked and no cracked periclase crystals
0,8
0,6
0,4
0,2
0,0
cracked periclase
no cracked periclase
1 2 3
spinel based compositions*
Fig 6
*1:sp 55z ; 2:sp 73z ; 3:sp 73chr
29

cps
cps
cps
50 0
Mg
60 0
Mg
15 0
Mg
Al
Ca
30 0
Al
40 0
10 0
O
O
10 0 C
0
0
20 0
O
C
Ca
Fe Fe
0
2 4 6 8 Energy (keV) 0
C
50
Al
Ca Fe Fe
0
2 4 6 8 Energy (keV) 0
Ca
Fe
Fe
2 4 6 8
Energy (keV)
Fig 7
Temperature C
o
2800
2400
2000
1600
0
1200
0
MgO
Mole Al 2
O 3
0,2 0,4 0,6 0,8 1,0
Periclase
solid solution
+
Liquid
Periclase
solid solution
Liquid
Periclase
solid solution
+
Spinel
20 40 60 80 100
Al 2
O 3
Wt (% )
Spinel +
Liquid
Spinel
B, b
Spinel
+
Corundium
a
0
Mole Al O
0,2 0,4 0,6 0,8 1,0
Temperature C
o
1600
Periclase
solid solution
Periclase + Spinel
Spinel
2 3
b
Spinel +
Corundium
Fig 8
1200
0
MgO
20 40 60 80 100
Wt (% )
Al 2 O 3
Spinel crystals in equilibrium with periclase crystals in Sp73z
Spinel crystals in no equilibrium with periclase crystals in M-Sp73z-H
30

70
60
50
spinel+C 3
S 2
sp+C 3
S 2
(825cm -1 )
Relative Intensity
40
30
20
10
sp+C 3
S 2
(395.65cm -1 )
0
-10
200 300 400 500 600 700 800 900 1000
Raman Shift (cm -1 )
Fig 9a
Relative Intensity
70
65
60
55
50
spinel+C 2
S
sp
C 2
S(491.1cm -1 )
C 2
S(961.1cm -1 )
C 2
S(577.78cm -1 )
sp C C 2
S(1122.2cm -1 2
S(855.56cm -1 )
)
45
Fig 9b
40
300 400 500 600 700 800 900 1000 1100 1200
Raman shift (cm -1 )
31

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