Spinel Aggregates Research Articles

Enhanced corrosion resistance by simultaneous refinement of pore structures and phase composition: A novel lightweight microporous alumina-rich spinel (Mg0.4Al2.4O4)

A lightweight microporous alumina-rich spinel aggregate with a single phase (Mg0.4Al2.4O4) was prepared to fabricate lightweight refractories with guaranteed corrosion resistance. To explore the corrosion behaviour of slags with different compositions, four types of slags were tested and compared with commercial sintered alumina and lightweight corundum-spinel aggregates. The slag corrosion depth of the lightweight microporous alumina-rich spinel aggregates was significantly lower than that of the other two aggregates. The alumina aggregate formed a calcium aluminate isolation layer with a thickness of 100–150 µm at the interface. The lightweight corundum-spinel aggregate did not form any isolation layer; however, a 50 µm reaction layer, composing of spinel and calcium aluminate, exists between aggregate and slag. Nevertheless, at the contact interface between the lightweight microporous alumina-rich spinel aggregate and slag, a uniform isolation layer (approximately 50 µm thick) forms, preventing direct contact and ensuring excellent corrosion resistance.

Effect of hollow spheres on the properties of lightweight periclase‐magnesium aluminate spinel refractories

AbstractThis study presents new lightweight periclase‐magnesium alumina spinel refractories for the working lining of cement rotary kilns in which magnesium alumina spinel hollow spheres are used to replace conventional dense fused magnesia‐aluminate spinel aggregates. The effects of adding spinel hollow spheres on the physical properties, mechanical strength, thermal conductivity, and slag resistance of the samples were explored. The results showed that compared with the sample prepared with dense aggregates, the sample prepared with hollow spheres had a 10.3% higher cold compressive strength, 44.1% higher modulus of rupture (MOR), and lower bulk density. Additionally, with increasing hollow spheres content, the thermal conductivity decreased from 3.79 W/(m·K) to 2.53 W/(m·K), and the high‐temperature MOR increased from 2.82 to 4.09 MPa. The highest residual strength ratio was 90.73% (15 wt.% hollow spheres), which is 1.17 times that of the sample prepared without hollow spheres. Moreover, microstructure and energy dispersive spectroscopy of crucible specimens after corrosion by cement clinker showed that specimens with 15 wt.% hollow sphere additions had a better slag resistance. Introducing hollow spheres reduced the thermal conductivity of the refractories, providing a new strategy for improving the heat insulation performance of kiln linings.

Research on high‐temperature creep properties of Al2O3‐MgAl2O4 refractory

AbstractIn this study, the Al2O3‐MgAl2O4 refractory samples were prepared with tabular alumina and fused magnesia‐alumina spinel as the raw materials. A creep resistance test was performed at 1400°C and studied. The tested samples after the creep resistance test were characterized and analyzed by X‐ray diffraction, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy to investigate the effect of adding the amount and particle size of MgAl2O4 on high‐temperature creep resistance of Al2O3‐MgAl2O4 refractory, as well as its mechanism. As indicated by the results, under the same amount of spinel added, the sample with 3‐1 mm spinel had higher creep resistance than the sample with 1‐0 mm spinel. In the creep test, spinel aggregates would react with the alumina matrix to form a secondary spinel layer around the spinel particles, thereby connecting the matrix and the aggregates and improving the creep resistance of the specimens. As clearly indicated by the observed result of the microstructure, in the formation of the secondary spinel layer, Mg2+ had a higher migration rate and showed the secondary spinel layer's higher thickness ratio on both sides of the reaction interface, which would induce the Kirkendall effect. Accordingly, considerable pores generated and accumulating at the interface tended to reduce the creep resistance of the material, and this effect was highly dependent on the size of the spinel and the activation solid solution reaction intensity.

A strategy for controlling microstructure and mechanical properties of microporous spinel (MgAl2O4) aggregates from magnesite and Al(OH)3

In the present work, a novel strategy is proposed to control the microstructures of microporous spinel aggregates and improve their strengths. This strategy controls the in-situ decomposition behavior of raw material particles and the Kirkendall effect through preheating magnesite and Al(OH)3, then milling with balls and water. Thus, the expected microstructures of microporous spinel aggregates can be obtained. Four microporous magnesium aluminate spinel aggregates of stoichiometric ratio were prepared using magnesite, Al(OH)3, calcined magnesite, and calcined Al(OH)3. The microstructures and strengths of microporous spinel aggregates were analyzed by XRD, SEM, image analysis method, etc. The microporous spinel aggregates prepared from magnesite and Al(OH)3 had slightly lower apparent porosity (58.2%) than the one (59.2%) synthesized from Al(OH)3 and calcined magnesite. In addition, the median pore size significantly reduced from 9.5 to 6.8 µm, and the compressive strength dramatically increased from 7.3 to 15.2 MPa. These results verify the reliability of this strategy.

Effect of Nanocrystalline Magnesium Aluminate Precursor and Powder Granulometryon Spinellization and Densification to Develop Magnesium Aluminate Spinel Aggregates

Nanocrystalline spinel forming precursor spinel hydrate (PSH), prepared by co-precipitation technique with controlled concentration of mixed salt, pH, temperature and time, was used as additive in calcined alumina-dead burnt magnesia (DBM) admixture to develop spinel (MgAl2O4) aggregates. Powder granulometry, mixing and dispersion of additive PSH were controlled during milling. Three batches of varying particle size distribution with same proportion of additive were used for body making. Firing was conducted in 1500°-1650°C temperature range. True specific gravity (TSG) of alumina, DBM, spinel and individual fired pellets were used to estimate wt% spinel formation based on the derived mass balance equation and subsequently verified by X-ray diffraction (XRD) analysis. Fired properties, i.e. bulk density, apparent porosity, closed porosity, %linear shrinkage and densification rate were measured. XRD was conducted for phase analysis. SEM and HR-TEM were performed to study the grain/ pore morphology and elemental analysis was done to calculate the spinel stoichiometry. PSH was found to have positive effect both in spinellization and densification; the effect gradually decreased with finer particle size distribution and increasing firing temperatures. The maximum fired density achieved for three batches were 89.21, 92.84 and 96.2% ρth, respectively. Based on the requirements of specific powder granulometry, addition of PSH and firing schedule can be used for bulk production of spinel aggregates.

Features of the Geological Structure and Genesis of the Tebinbulak Deposit (Ridge Sultan-Uvais)

In this article, we want to draw attention to the hysteron-magmatic origin of titanium-magnetite ores of the Tibinbulak gabbro-peridotite complex located in the northwestern part of the Sultan-Uvais ridge. Which is the only outlet of the Phanerozoic foundation between the mountain structures of the Urals and the Southern Tien Shan. The article examines the characteristic features of petrochemical and structural features of ore minerals (ilmenite, magnetite) and the main rock-forming minerals of the gabbro-peridotite complex (peridotite, pyroxenite, tebinnite, pyroxene hornblendite and plagioclase) with clarification of the structural relationships. It was found that magnetite mineralization of the Tebinbulak intrusion was superimposed on ultrabasic rocks in the final stages of crystallization of the latter. Prolonged auto-metasomatic transformations led to the enlargement of magnetite crystals in places subject to decay. In this case, aggregates of spinel - hercynite with sphene rims were formed. Simultaneously with the formation of sphene, ilmenite crystals appeared in the form of radiant needle-shaped aggregates. In this case, the primary crystals of magnetite lost their faces and became shapeless. Relatively large precipitates of titano-magnetite formed at this stage with the filling of cracks in the rock and interstices between minerals. Based on the data presented, we note that the titano-magnetite ores of the Tebinbulak deposit arose in the late magmatic (hysteron-geneous) setting of auto-metamorphic replacement of the main carriers of iron and titanium pyroxene and plagioclase with hornblende. Also, the studied geochemical composition of ores damages that the titano-magnetite ores of the Tebinbulak deposit are a complex raw material, from which not only iron but also vanadium concentrate can be extracted.

Mechanisms of Partial Melting of Metasomatized Mantle Ultramafic Rocks beneath Avacha Volcano, Kamchatka, and the Growth of Minerals from a Gas Phase in Fractures

Data on xenoliths carried by lavas of Avacha volcano and ejected during eruptions are used in discussing the partial melting of metasomatized rocks of the lithospheric mantle (which were tectonically reformed by seismic events beneath the frontal zone of Avacha volcano) and the associated growth of minerals from a gas phase in open fractures above the magma chamber feeding the volcano. Based on a comprehensive study of representative mantle xenoliths from explosions of the volcano, we developed and numerically studied a mathematical model of convective heat and mass transfer in the permeable zone above the feeding magma chamber. The model enables analysis of the area of active seismic destruction of rocks at depths of 30–70 km beneath Avacha volcano. This analysis involves (1) current qualitative genetic hypotheses concerning the nature of the identified textural and structural relationships between the xenoliths and lavas transporting them, (2) data on the composition of mineral-hosted fluid and melt inclusions, (3) estimated homogenization temperatures of the inclusions, and (4) hypothetical melting mechanisms of metasomatized ultramafics and the associated growth mechanism of minerals in open fractures. It is demonstrated that fractured porous seismogenic regions above magma chambers are coupled with a set of convective processes of fluid-mediated heterophase heat and mass transfer in the lithosphere: infiltration metasomatism at the spinel depth facies and the sublimation and condensation of major components from a gas phase. It is hypothesized that local partial decompression melting of highly heated ultramafic and mafic rocks may be followed by major seismic events. It is shown that spinel-facies infiltration metasomatism in fractured mantle rocks may be associated with the growth of crystal crusts and clusters of clinopyroxene and amphibole from a gas phase in open fractures. There are two paths by which decompression partial melting takes place: (1) the melting of spinel aggregates with the host crystals of “metasomatic” orthopyroxene, which replaced olivine in harzburgites, and (2) the development of clinopyroxene, amphibole, and relict orthopyroxene vein fillings. To test these hypotheses, physical experiments were conducted with the use of convective heating of samples of natural xenoliths. The experiments were carried out in a flow-through reactor capable of reproducing the partial melting phenomena and in an electron gun with a power unit for a welding system with a dense electron beam. The latter experiments simulated the sublimation and deposition of major components from a gas phase over the solidus boundary for all petrographic types of mantle ultramafic rocks. Neither structural nor mineralogical indications of the infiltration of magmatic fluid were detected in the experiments, and no traces of diffusion–reaction zoning of the “melt–rock” type were found in the xenoliths.

Corrosion Mechanism of A Density-Reduced Steel Ladle Lining Containing Porous Spinel-Calcium Aluminate Aggregates

Refractory monolithics for steel ladle linings are typically products with low porosities and high bulk densities. They achieve high temperature, penetration, and corrosion resistance. Despite the high density of these products, which is due to the low porosity of the aggregates, their matrices still exhibit a high amount of pores. Since calcium magnesium aluminate (CMA) has already proven its resistance to penetration and corrosion as a binder in the matrix, this paper investigated if alumina spinel refractories containing microporous calcium magnesium aluminate aggregates can withstand conditions that occur in a steel ladle wall. The objective was to reduce the castable density with the advantage of a lower material requirement for a ladle lining and reduced heat and energy losses. This was achieved by replacing dense alumina aggregates by up to 38% of porous CMA aggregates (grains with 30 vol% porosity), which resulted in a bulk density reduction from 3.1 g/cm3 for the dense alumina castable to 2.8 g/cm3 for the 38% CMA aggregates containing castable. However, the despite the higher porosity, penetration, and corrosion resistance and thermomechanical properties were not impacted negatively for a model alumina spinel castable. A postmortem investigation was conducted on a newly developed dry-gunning mix that was installed in a steel ladle wall on top of a slag penetrated castable and that achieved a service life of 31 heats versus only 18 heats for the reference mix that contained dense alumina and spinel aggregates. This new repair mix contained the newly designed porous CMA aggregates, which in this case partly replaced the dense alumina and spinel aggregates. These porous aggregates consisted of magnesium aluminate and calcium aluminate micro-crystals. The postmortem study revealed two important phenomena that can explain the improved performance: at the hot face in contact with steel and slag, a thin densified zone was observed that blocked the slag penetration into the porous matrix and the porous aggregates. Iron oxides were almost completely blocked from penetration, and only some manganese oxide was observed in the penetrated zone together with some silica and lime from the slag. Clusters of calcium aluminate (CA6) and magnesium aluminate (MA) spinel build the refractory back-bone on the hot side of the material and gussets filled with mostly glassy calcium aluminum silicates close to the hot face and gehlenite further inside the penetrated zone. Alumina grains had a reaction rim consisting of CA2 or CA6 and a very intimate connection to the surrounding matrix unlike the CMA-free mix that showed micro cracks around the alumina grains. At the colder side, the gunning mix with CMA aggregates showed a very good connection to the substrate, supported by a hercynite formation in the gunning mix resulting from a cross-reaction with remains of iron oxide on the CMA containing repair mix. Furthermore, macroscopic observations of a CMA aggregate containing alumina magnesia castable in the metal zone of a steel ladle revealed that macro cracks developed only very slowly, which resulted in a superior service life.

Corrosion of MgO-C with Magnesium Aluminate Spinel Addition in A Steel Casting Simulator

For more than 20 years, the sidewalls and bottom of steel ladles have been lined with carbon-bonded magnesia (MgO-C) and magnesia-alumina bricks (MAC). The alumina raw materials react with magnesia forming a spinel, which decreases open porosity and slag infiltration. The amount, grain size, and chemistry of the added spinel impact the properties of spinel-containing MgO-C. Corrosion tests have been performed in a steel casting simulator at 1580 °C using 18CrNiMo7-6 steel and Fe-rich slag as corrosion medium. Digital light microscopy and SEM/ EDS (scanning electron microscope with energy dispersive spectroscopy) were used to evaluate the corrosion mechanisms. The metal casting simulator test showed that the addition of CaO-MgO-Al2O3 aggregates results in the highest corrosion resistance against molten steel and synthetic basic slag compared to alumina-rich spinel aggregates.

Effect of magnesium aluminate spinel content of porous aggregates on cement clinker corrosion and adherence properties of lightweight periclase-spinel refractories

The present study investigated five lightweight periclase-magnesium aluminate spinel refractories containing porous spinel aggregates with a varying spinel content. The cement clinker resistance and adherence properties of the lightweight refractories were analyzed by a static crucible test and a sandwich test, respectively. The effect of the spinel content of the aggregates (SCA) on the cement clinker resistance and adherence properties was investigated with SEM, EDS and with the thermochemical software FactSage®. The reaction between the spinel in the refractories with the cement clinker caused the formation of a liquid phase. This liquid phase determined the adherence properties of the refractories and also affected their cement clinker resistance. When the SCA was 0–25 wt%, the refractories had a high corrosion resistance and the cement clinker successfully adhered to the refractories. Yet, when the spinel content was between 50 wt% and 100 wt%, excessive liquid phase formation seriously impaired the refractories. Consequently, the optimized periclase-spinel refractories contained porous aggregates with 25 wt% spinel, which resulted in a high corrosion resistance together with an excellent adherence ability regarding the cement clinker.

Microstructure and phase evolution of Indian magnesite‐derived MgAl2O4 as a function of stoichiometry and ZrO2 doping

AbstractTo aid development of cost‐effective sintered spinel as a refractory raw material, this paper presents an extensive analysis of microstructure and complex phase evolution of Al‐rich, Mg‐rich, and stoichiometric spinel aggregates derived from Indian magnesite and calcined alumina. Pore morphology in Al‐rich spinel was transformed upon sintering at 1650°C and corundum laths embedded in porous Al‐rich spinel matrix was formed. Stoichiometric spinel sintered at 1600°C consisted of mostly direct bonded angular equiaxed spinel grains which incorporated the impurities in solid solution. Mg‐rich spinel was composed of spinel grains with reduced angularity along with intergranular amorphous phase, small round monticellite grains, and periclase. EDS line scan revealed impurity‐free joins existed between direct bonded spinel grains. Mg‐rich spinel containing 0.65 wt% ZrO2 formed cubic ZrO2‐CaO‐MgO solid solution located along spinel boundaries, which reduced both intergranular amorphous phase and monticellite. This increased SiO2 and MgO content in spinel solid solution triggering exsolution of metastable cubic forsterite manifested as split spinel peaks in XRD pattern. A 14.7% reduction in slag penetration was exhibited upon doping Mg‐rich spinel with 0.21% ZrO2. Stoichiometric and Mg‐rich spinels attained 0.35% and 0.54% apparent porosity at 1600°C, which is better than most commercial sintered refractory spinels.

Microstructural constraints on complex thermal histories of refractory CAI-like objects in an amoeboid olivine aggregate from the ALHA77307 CO3.0 chondrite

We have carried out a FIB/TEM study of refractory CAI-like objects in one AOA from the ALHA77307 CO3.0 chondrite. The CAI-like objects in the AOA consist of a zoned sequence with a spinel-rich core through an intergrowth layer of spinel and Al-Ti-rich diopside to a diopside rim. The spinel-rich core consists of polycrystalline aggregates of spinel and ±minor melilite showing equilibrated grain boundary textures. The intergrowth layer contains fine-grained diopside and spinel with minor anorthite with highly curved and embayed grain boundaries. The diopside rim consists of polycrystalline aggregates of diopside. The compositions of pyroxene change significantly outward from Al-Ti-rich diopside in contact with the spinel-rich core to Al-Ti-poor diopside next to the surrounding olivine of the AOA. Overall microstructural and chemical characteristics suggest that the spinel-rich core formed under equilibrium conditions whereas the intergrowth layer is the result of reactions that occurred under conditions that departed significantly from equilibrium. The remarkable changes in formation conditions of the CAI-like objects may have been achieved by transport and injection of refractory objects into a region of a partially-condensed, Ca,Ti-saturated gas which reacted with spinel and melilite to form Al-Ti-rich diopside.Crystallographically-oriented TiO2 nanoparticles decorate the grain boundaries between spinel grains and between spinel and Al-Ti-rich diopside grains. During the disequilibrium back-reaction of spinel with a partially-condensed, Ca,Ti-saturated gas, metastable TiO2 nanoparticles may have condensed by an epitaxial nucleation mechanism and grown on the surface of spinel. These TiO2 nanoparticles are disordered intergrowths of the two TiO2 polymorphs, anatase and rutile. These nanoparticles are inferred to have nucleated as anatase that underwent partial transformation into rutile. The local presence of the TiO2 nanoparticles and intergrowth of anatase and rutile imply that the disequilibrium back-reaction of spinel with the gas occurred on a short timescale, i.e., minutes to hours at maximum.

Immiscible melt droplets in garnet, as represented by ilmenite–magnetite–spinel spheroids in an eclogite-garnet peridotite association, Blanský les Granulite Massif, Czech Republic

![Figure][1] Interlayered eclogite and symplectitic garnet rock that is interpreted as former garnetite are found in the Gfohl Unit of the Bohemian Massif. They show unusual Fe–Ti-rich compositions, characterized by TiO2 contents up to 2.34 wt%, and Mg# of 59.8 and 51.6, respectively. Equilibration conditions of 1250 °C and 4.0 GPa are calculated for eclogite. The petrogenesis of this rock association can be best explained as high-temperature and ultrahigh-pressure magmatic cumulates. Highly decoupled Sr-Nd isotopic composition with nearly constant radiogenic 87Sr/86Sr values and a slightly negative e Nd value suggests interaction of aqueous fluid most likely derived from a subducting slab and/or from parental magmas. The garnetite contains large (up to 0.5 mm) Fe–Ti-rich spheroids of ilmenite–magnetite–spinel, interpreted as frozen droplets of a melt incorporated in the growing garnet. The interstices between these garnet crystals are filled by ilmenite–magnetite–spinel aggregates, with concave outer surfaces with trapped Fe–Ti-rich melt. These ilmenite–magnetite–spinel spheroids represent possibly the first record of such an oxidized assemblage in mantle rocks, and probably the first description of Fe–Ti-rich melt in eclogite-garnetite mantle rocks. A calculation based on mineral proportions in the spheroids and mineral composition indicates that the immiscible Fe–Ti-rich melt consisted of 28.7 TiO2, 3.7 Al2O3, 0.2 Cr2O3, 27.9 Fe2O3, 37.0 FeO, 0.8 MnO, and 1.7 MgO wt%. Petrology and geochemistry of the garnetite indicates an unusual composition for an upper mantle melt with a high oxygen fugacity and relatively high Fe content. [1]: pending:yes

Microstructural evidence for a disequilibrium condensation origin for hibonite‐spinel inclusions in the ALHA77307 CO3.0 chondrite

AbstractTwo hibonite‐spinel inclusions (CAIs 03 and 08) in the ALHA77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique combined with transmission electron microscopy. These hibonite‐spinel inclusions are irregularly shaped and porous objects and consist of randomly oriented hibonite laths enclosed by aggregates of spinel with fine‐grained perovskite inclusions finally surrounded by a partial rim of diopside. Melilite is an extremely rare phase in this type of CAI and occurs only in one inclusion (CAI 03) as interstitial grains between hibonite laths and on the exterior of the inclusion. The overall petrologic and mineralogical observations suggest that the hibonite‐spinel inclusions represent high‐temperature condensates from a cooling nebular gas. The textural relationships indicate that hibonite is the first phase to condense, followed by perovskite, spinel, and diopside. Texturally, melilite condensation appears to have occurred after spinel, suggesting that the condensation conditions were far from equilibrium. The crystallographic orientation relationships between hibonite and spinel provide evidence of epitaxial nucleation and growth of spinel on hibonite surfaces, which may have lowered the activation energy for spinel nucleation compared with that of melilite and consequently inhibited melilite condensation. Hibonite contains abundant stacking defects along the (001) plane consisting of different ratios of the spinel and Ca‐containing blocks within the ideal hexagonal hibonite structure. This modification of the stacking sequence is likely the result of accommodation of excess Al in the gas into hibonite due to incomplete condensation of corundum from a cooling gas under disequilibrium conditions. We therefore conclude that these two hibonite‐spinel inclusions in ALHA77307 formed by high‐temperature condensation under disequilibrium conditions.

Studies on densification, mechanical, micro-structural and structure–properties relationship of magnesium aluminate spinel refractory aggregates prepared from Indian magnesite

The present work intends to study the development of magnesium aluminate spinel aggregates from Indian magnesite in a single firing stage. The raw magnesite has been evaluated in terms of chemical analysis, differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, and X-ray diffraction. The experimental batch containing Indian magnesite and calcined alumina has been sintered in the temperature range of 1550°C–1700°C. The sintered material has been characterized in terms of physico-chemical properties like bulk density, apparent porosity, true density, relative density and thermo-mechanical/mechanical properties like hot modulus of rupture, thermal shock resistance, cold modulus of rupture and structural properties by X-ray diffraction in terms of phase identification and evaluation of crystal structure parameters of corresponding phases by Rietveld analysis. The microstructures developed at different temperatures have been analyzed by field emission scanning electron microscope study and compositional analysis of the developed phase has been carried out by energy dispersive X-ray study.

Direct evidence for upper mantle structure in the NW Pacific Plate: Microstructural analysis of a petit-spot peridotite xenolith

Petit-spots, the late Miocene alkali basaltic volcanoes on the Early Cretaceous NW Pacific Plate, originate at the base of the lithosphere. The petit-spot volcanic rocks enclose fragments of tholeiitic basalt, dolerite, gabbro, and mantle peridotite, providing a unique window into the entire section of subducting oceanic lithosphere. We provide here the first direct observations on the deep structure of the Pacific lithosphere using microstructural analyses of a petit-spot peridotite xenolith. The xenolith is a lherzolite that consists mainly of coarse- and medium-grained olivine, orthopyroxene, and clinopyroxene, as well as fine-grained aggregates of spinel and orthopyroxene that probably represent replaced pyrope-rich garnet. A strong deformational fabric is marked by a parallel alignment of millimeter-sized elongate minerals and their crystallographic preferred orientation. The olivine displays a [010] fiber pattern with a girdle of [100] axes and a maximum of [010] perpendicular to the foliation, a pattern which is consistent with a transpressional deformation in high temperature conditions at the base of oceanic lithosphere. Our microstructural observations and seismic data indicate that the lower part of the NW Pacific lithosphere possess an early stage structure of mantle flow at the asthenosphere. This interpretation is compatible with a conventional model in which oceanic lithosphere is thickened during cooling and plate convection. A discrepancy between the weak anisotropy in the petit-spot peridotite and the strong azimuthal anisotropy from the seismic data in the NW Pacific plate implies the existence of a highly anisotropic component in the deep oceanic lithosphere.

Influence of Microsilica Content on the Slag Resistance of Castables Containing Porous Corundum–Spinel Aggregates

Corrosion of five corundum–spinel castables containing the same porous aggregates and different matrices by converter slag (C/S=3) was conducted using the static crucible test through the counting pixels method, XRD, SEM, EDAX, and so on. It was found that the corrosion and penetration resistance depend on the microsilica content. Microsilica content strongly affects liquid phase formation in the matrix, changes the porosity, pore size distribution of matrix, and viscosity of slag penetrated, and thus affects its corrosion and penetration resistance. Increasing microsilica, decreasing the porosity, or increasing the viscosity of the penetrating slag improves, whereas increasing the pore size or liquid content reduces the penetration and corrosion resistance. The penetration resistance increased abruptly with an increase of the microsilica content from 0.65 to 1.95 wt%, but changed slightly with increase of the microsilica content from 1.95 to 3.90 wt%. The corrosion resistance increased with an increase of the microsilica content from 0 to 2.60 wt%, but slightly decreased with a further increase of the microsilica content to 3.90 wt%. 1.95–2.60 wt% microsilica leads to a compromise between low corrosion and low penetration.

Spatial correlation of deformation and mineral reaction in experimentally deformed plagioclase–olivine aggregates

Shear deformation of hot pressed plagioclase–olivine aggregates was studied in the presence and absence of mineral reaction. Experiments were performed at 900 °C, 1500 MPa, and a constant shear strain rate of ∼5×10 −5 s −1 in a solid medium apparatus. Whether the mineral reaction between plagioclase and olivine takes place or not is controlled by choosing the appropriate plagioclase composition; labradorite (An 60) does not react, anorthite (An 92) does. Labradorite–olivine aggregates deformed without reaction are very strong and show strain hardening throughout the experiment. Syndeformational reaction between olivine and anorthite causes a pronounced strain weakening. The reaction produces fine-grained opx–cpx–spinel aggregates, which accommodate a large fraction of the finite strain. Deformation and reaction are localised within a 0.5-mm-wide sample. Three representative samples were analysed for their fabric anisotropy R* and shape-preferred orientation α* (fabric angle with the shear plane) using the autocorrelation function (ACF). Fabric anisotropy can be calibrated to quantify strain variations across the sheared samples. In the deformed and reacted anorthite–olivine aggregate, there is a strong correlation between reaction progress and strain; regions of large shear strain correspond to regions of maximum reaction progress. Within the sample, the derived strain rate variations range up to almost one order of magnitude.

Synthesis of Magnesium Aluminate Spinels from Bauxites and Magnesias

The synthesis of refractory Mg‐Al spinel aggregates from bauxites and magnesias was addressed by applying the alumina–magnesia–silica ternary equilibrium phase diagram as the fundamental basis. Four different alumina sources (bauxites) and four different magnesia sources were investigated. These were fired in air through 1700°C and their reactions were monitored using X‐ray diffraction. Evolution of their microstructure was observed by scanning electron microscopy. Initially, the periclase reacts with the free corundum of the bauxite to produce Mg‐Al spinel. The periclase then reacts with the mullite in the bauxites to yield additional spinel and also some forsterite. Spinels were produced for all sixteen different raw‐material combinations.

Corrosion of high alumina and near stoichiometric spinels in iron-containing silicate slags

Dissolution of alumina-rich spinel aggregates in iron-containing silicate slag was studied using grain corrosion tests. High alumina spinels (AR90, 90% Al2O3) dissolve indirectly while dissolution of near stoichiometric (AR78, 78% Al2O3) spinel is entirely direct. This behaviour arises because higher alumina spinels can accommodate more cations from the slag due to the greater number of Mg2+ vacancies in their crystal structures. Removal of these cations increases the viscosity of the remaining silica-rich local slag so that alumina-rich spinels exhibit better penetration resistance. However, thermodynamic calculations predict that near stoichiometric spinel (AR78) is more corrosion resistant since more slag is needed to dissolve it completely.