Calcium Hexaluminate Research Articles

Effects of the initial CaO-Al2O3 ratio on the microstructure development and mechanical properties of porous calcium hexaluminate

This paper addresses an evaluation of the effects of variation in the CaCO3-Al2O3 ratio on the physical properties of in situ calcium hexaluminate (CaAl12O19 or CA6) porous structures during sintering. Combinations of calcined alumina (α-Al2O3) and different amounts of precipitated CaCO3 particles were thermally treated between 500°C and 1500°C. Their physical properties (Total Porosity, Flexural Strength, Flexural Elastic Modulus and Thermal Linear Variation), microstructure (Scanning Electron Microscopy), and phase composition (X-Ray Diffraction) were assessed. For thermal treatments conducted at 500–1100°C, the addition of higher contents of CaCO3 increased total porosity, strength and rigidity levels due to the simultaneous generation of pores (from CaCO3 decomposition) and low-melting-point compounds (such as C12A7) that helped the sintering of particles. A competition between the maintenance of pores resulting from the expansive formation of CA6 and the densification of the structure was observed at 1300–1500°C. Compositions whose CaO fraction was equal to or larger than the stoichiometric content for the production of CA6 showed the highest porosity levels (up to 58% after sintering at 1500°C, for 3h) and lowest densification rates.

Revisiting CA6 formation in cement-bonded alumina-spinel refractory castables

This work revisits the proposed mechanisms presented in the literature for CA6 formation in Al2O3-MgO and Al2O3-MgAl2O4 castables bonded with calcium aluminate cement. New experimental tests, thermodynamic simulations and re-evaluation of the chemical composition and microstructural aspects observed for samples fired in the temperature range of 1150°C to 1500°C were carried out. Based on these data, a new interpretation of the CA6 generation process, as well as the features which influence the location and morphology of this phase were proposed. CA6 formation via solid and liquid states are suggested to take place in all evaluated compositions, where the former (solid-state) is the main reaction predicted for the silica-free refractories (AM0MS and AS0MS), whereas the liquid-state one prevails in the AM1MS and AS1MS materials. The CA6 crystal morphology should be affected by these different reaction mechanisms. According to the experimental results, it was also discussed the role of the calcium hexaluminate features in the overall corrosion behavior of the designed refractories when they were placed in contact with molten slag at high temperatures. Such aspects have not been previously reported in published papers related to this subject.

Study on Hydration and Strengthening of High Alumina Cements

Abstract Refractory concretes based on aluminous cements are used with great success in areas where high temperatures are required. The mineralogical composition of the high alumina cement is the main factor which gives the physical and mechanical properties at high temperatures of refractory monolithic materials. It is therefore desirable to use high alumina cements based on mineralogical compounds with high refractoriness, because in the end those beneficial properties can be found in the final product - refractory concrete. The aim of this paper is to design, realize and characterize different compositions of high alumina cements based on mineralogical compounds with the highest refractory from the CaO-Al2O3 binary system (i.e. CA, CA2, and CA6), and to find ways of hydraulic activation of calcium hexa aluminate, also.

Effects of rare earth oxides additions on microstructure and properties of alumina-magnesia refractory castables

To accommodate the glaring requirements of castables with high performances for the iron and steel making industry, effects of rare earth oxides additions on microstructure and properties of alumina-magnesia refractory castables were investigated. The properties of castables including phase compositions, apparent porosity, permanent linear change and strength were compared. It was demonstrated that yttria and ceria presented varying degrees of influence on in-situ reactions and volumetric stability of castables. The solid-soluted content of spinel and calcium hexaluminate (CA6) was both dependent on the additions.

Microstructure and physical properties of steel-ladle purging plug refractory materials

Three different castables were prepared as steel-ladle purging-plug refractory materials: corundum-based low-cement castable (C-LCC), corundum–spinel-based low-cement castable (C-S-LCC), and no-cement corundum–spinel castable (C-S-NCC) (hydratable alumina ρ-Al2O3 bonded). The properties of these castables were characterized with regard to water demand/flow ability, cold crushing strength (CCS), cold modulus of rupture (CMoR), permanent linear change (PLC), apparent porosity, and hot modulus of rupture (HMoR). The results show the CCS/CMoR and HMoR of C-LCC and C-S-LCC are greater than those of the castable C-S-NCC. According to the microstructure analysis, the sintering effect and the bonding type of the matrix material differ among the three castables. The calcium hexaluminate (CA6) phase in the matrix of C-LCC enhances the cold and hot mechanical strengths. In the case of C-S-LCC, the CA6 and 2CaO·2MgO·14Al2O3 (C2M2A14) ternary phases generated from the matrix can greatly increase the cold and hot mechanical strengths. In the case of the no-cement castable, sintering becomes difficult, resulting in a lower mechanical strength.

Effect of Y2O3 addition on the densification, microstructure and mechanical properties of MgAl2O4[sbnd]CaAl4O7[sbnd]CaAl12O19 composites

With the aim to obtain dense composites containing spinel (MgAl2O4) and calcium hexaluminate (CaAl12O19), the effect of Y2O3 addition on the sintering, microstructure and mechanical properties of MgAl2O4CaAl4O7CaAl12O19 composite was investigated by solid state reaction sintering at 1500–1600 °C. The results showed that the Y2O3 addition had significant effects on the densification and microstructure at sintering temperature of 1600 °C. For the composites with addition of 2 and 3 wt% Y2O3, the aspect ratios of CA6 grains were dramatically decreased. This change of the CA6 grain morphology indirectly promoted the development of the MA and CA2 grains. Accordingly, highly uniform and dense microstructures were formed, with the apparent porosity decreased from 18.3% to 4.8% and 4.2%, respectively. Significant improvements in the mechanical properties were also achieved for the Y2O3 added composites after heating at this temperature. The optimum flexural strength was obtained by adding 2 wt% Y2O3, achieving an increase from 186 to 292 MPa, while the optimum fracture toughness was obtained by adding 3 wt% Y2O3, with the value increased from 2.3 to 3.7 MPa m1/2.

Improvement of densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite by addition of MnO

Composites containing spinel (MgAl2O4) and calcium hexaluminate (CaAl12O19) can excellently combine the properties of these two phases. However, their fabrication in dense composites for structural applications is limited on account of the poor sinterability. Considering this aspect, the effect of MnO addition on the sintering, microstructure and mechanical properties of MgAl2O4-CaAl4O7-CaAl12O19 composite was investigated by solid state reaction sintering at 1500–1600°C. The results showed that the grain growth of MA was significantly promoted due to the formation of Mg(Mn)Al2O4 by introducing MnO. The addition of MnO can also weaken the barrier effect of the elongated CA6 grains, indirectly promoting the development of MA and CA2 grains. Accordingly, highly uniform and dense microstructures were formed, with the apparent porosity decreased from 18.3% to 3.4% when adding 6wt% MnO at 1600°C. Significant improvements in the mechanical properties were also achieved for the MnO added composites sintered at this temperature. The optimum flexural strength was obtained by adding 6wt% MnO, achieving an increase from 186 to 308MPa, while the optimum fracture toughness was obtained by adding 4wt% MnO, with the value increased from 2.3 to 3.5MPam1/2.

Formation mechanism of calcium hexaluminate

To investigate the formation mechanism of calcium hexaluminate (CaAl12O19, CA6), the analytically pure alumina and calcia used as raw materials were mixed in CaO/Al2O3 ratio of 12.57:137.43 by mass. The raw materials were ball-milled and shaped into green specimens, and fired at 1300–1600°C. Then, the phase composition and microstructure evolution of the fired specimen were studied, and a first principle calculation was performed. The results show that in the reaction system of CaO and Al2O3, a small amount of CA6 forms at 1300°C, and greater amounts are formed at 1400°C and higher temperatures. The reaction is as follows: CaO·2Al2O3 (CA2) + 4Al2O3 → CA6. The diffusions of Ca2+ in CA2 towards Al2O3 and Al3+ in Al2O3 towards CA2 change the structures in different degrees of difficulty. Compared with the difficulty of structural change and the corresponding lattice energy change, it is deduced that the main formation mechanism is the diffusion of Ca2+ in CA2 towards Al2O3.

Substitution of Ba for Ca in the Structure of CaAl12O19

The effect of Ba2+ on the crystallization thickness of calcium hexaaluminate (CA6) along the c axis was studied. The results show that after introduction of Ba2+, the CA6 flake crystals fired at 1600°C become thicker obviously. The structure refinement shows that the structure formula of the doped CA6 is (Ca0.973Ba0.027)O·6Al2O3, indicating Ba2+ has entered the lattices of CA6 and substituted Ca2+ in the mirror layers. The introduction of Ba2+ results in more defects VAl in spinel block layers in CA6, making ions easier to diffuse along c axis and fastening the growing of CA6 crystals along c axis. Thus, the introduction of Ba2+ is feasible and advantageous to increase the crystallization thickness of CA6 along c axis.

Development of alumina microspheres with controlled size and shape by vibrational droplet coagulation

Monodisperse alumina microspheres were shaped by making use of the vibrational droplet coagulation technique. A combination of both physical and chemical parameters provided a flexible basis to widely regulate the size of the microspheres. The controlled granulation is based on a stable alumina suspension containing sodium alginate. The sodium alginate binder is used to coagulate the microspheres jetted into a bath of CaCl2. Several parameters have been varied to study the impact on the suspension’s rheology. This in turn dictates the physical parameters required for controlled shaping by the vibrational droplet coagulation technique. The decomposition of the calcium alginate in the alumina microspheres was monitored during the thermal post processing. In this step, the formation of CaCO3 was revealed, resulting in the presence of CaO during the calcination process. Subsequently, this existence of CaO in the alumina matrix leads to the formation of calcium hexaluminate (CaO·6Al2O3) during sintering.

Preparation and Properties of High-Purity Porous Calcium Hexaluminate Material

In this paper, high-purity porous calcium hexaluminate materials were prepared with α-alumina, nano-meter calcium carbonate and ρ-alumina as raw materials. The properties of the prepared porous materials, such as bulk density, true density, thermal conductivity, and cold crushing strength, were investigated. The increase in sintering temperature led to the increase of true density. Bulk density and cold crushing strength of the specimen fired at 1500°C had the lowest values. As ρ-alumina content increased, bulk density, true density, and cold crushing strength had no obvious changes, but the thermal conductivity decreased at first and then increased. The calcium hexaluminate formation was intense from 1450°C to 1500°C and finished at 1500°C. Above 1500°C, the main phase of specimens was calcium hexaluminate (CA6). And CA6 grains were regular hexagonal plates morphology and widely spread after firing at 1550°C.

Mechanism of pore generation in calcium hexaluminate (CA6) ceramics formed in situ from calcined alumina and calcium carbonate aggregates

Porous structures of calcium hexaluminate (CaAl12O19 or CA6) are resistant to densification, chemical attack and thermal shock and have been widely employed in thermal insulators for high temperatures (above 1400°C). Despite its technological importance, little is known corcerning the in situ formation of CA6 from the early calcination temperatures (500–900°C) up to full sintering (above 1000°C). This study investigated the mechanisms of pores generation in in situ CA6 obtained from a mixture of calcined alumina (α-Al2O3) and chemically precipitated calcium carbonate (CaCO3) aggregates. The evolution of the microstructure was followed by scanning electron microscopy, X-ray diffraction, dilatometric analysis, and measurements of physical properties, in the 500–1500°C temperature range. Samples of 50-40% total porosity and different degrees of linear expansion were obtained after sintering at 1100–1500°C, respectively. The crystallization of liquid phases of eutectic composition played a major role in the formation and maintenance of pores after CaCO3 decomposition.

Thermodynamic evaluation and properties of refractory materials for steel ladle purging plugs in the system Al2O3-MgO-CaO

The influence of iron oxide in the phase relationships of Al2O3-MgO-CaO and Al2O3-MgO purging plug refractory material has been studied by the thermodynamic analysis tool FactSage. Results showed that the system without CaO to be advantageous on the onset temperature of liquid phase, and on the liquid content at defined chemical composition and temperature. Therefore, CaO negatively influences the iron-rich slag resistance of the purging plug material. Analysis of used purging plugs proved the thermodynamic results. They showed that spinel solid solution and calcium hexaluminate as stable phases were formed in the reaction with iron oxide slag.Corundum-spinel castables with and without calcium aluminate cement were investigated in the laboratory to compare the relevant technical properties. In the cement bonded castable, the curing and drying strength are increased by increasing the cement content. However high cement addition requires higher water demand, which results in higher open porosity and a lower hot modulus of rupture (HMoR). In the CaO-free, no-cement castable with hydratable alumina binder, the water demand is slightly higher when compared to the ultra-low cement castable. However, the curing and drying strength are still slightly higher for the no-cement castable. As there is no significant difference in HMoR with various hydratable alumina binder additions, low dosage of such binder in the range of two to four percent is normally recommended to avoid excessive water addition. Cement bonded castables (with CaO) show some advantages to the no-cement castable (non-CaO) regarding HMoR, however the no-cement castable could have advantages regarding the iron-rich slag resistance and thermal shock resistance.

Effect of BaO Addition on Densification and Mechanical Properties of Al2O3-MgO-CaO Refractories

Considering the requirement for a reduction of refractory consumption, the present work investigated the fabrication of Al2O3-MgO-CaO-based refractory with BaO addition by means of solid-state reaction sintering. The effect of BaO addition on densification and the properties of the refractory were also discussed. Results indicated that the formation of calcium hexaluminate (CaO·6Al2O3, or CA6) grains with a high aspect ratio in the alumina-rich zone depressed the densification of the sample without BaO addition, resulting in a higher apparent porosity of 21.2%. When 6 wt. % BaO was added, a new phase of Ba2Mg6Al28O50 (BAM) with a lower aspect ratio was formed and the densification of the sample with an apparent porosity of 5.52% was promoted. In addition, mechanical performance was significantly improved due to an increase in compactness and modification of the microstructure. The cold compressive strength increased from 348 MPa to 569 MPa and the flexural strength increased from 178 MPa to 243 MPa by addition of 6 wt. % BaO. Meanwhile, the breadth of the widest crack after the thermal shock test decreased from 7 μm to 1 μm in the refractory.

Effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite

Materials designed in the high-alumina region of Al2O3–MgO–CaO system have been widely used in many technological fields. However, their further applications are limited by the high sintering temperatures necessary to achieve densification due to the poor sintering ability of calcium hexaluminate (CaAl12O19) and spinel (MgAl2O4). Considering this aspect, the present work investigated the effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite by solid state reaction sintering. The results showed that the CA6 grains presented a more equiaxed morphology instead of platelet structure by incorporating Ti4+ into its structure, which greatly improved the densification after heating at 1600°C. The flexural strength was greatly enhanced with increasing addition of TiO2 due to the significant decrease in porosity and improvement in uniformity of grain size as well as the absence of microcracks in the presence of Al2TiO5. The increased content of TiO2 also played an active role in toughening this composite attributed to the increase in resistance to crack initiation and propagation.

Interfacial Reactions Between Anorthite (CaAl2Si2O8) and Al 7075 Alloy at 850°C and 1150°C

The present work reports an investigation of the interactions of Al 7075 alloy and anorthite at 850°C (150 h) and 1150°C (24 h). Transmission electron microscopy, electron probe microanalysis, X‐ray diffraction, and scanning electron microscopy coupled with energy‐dispersive spectroscopy were used to identify the mineralogical and microstructural changes at the metal–ceramic interface. At 850°C, the phase formation mechanisms were (a) Si4+–Al3+ interdiffusion between the Al alloy and anorthite to form calcium dialuminate (CA2) and Ca2+–Mg2+ interdiffusion between the Al alloy and calcium dialuminate to form spinel. At 1150°C, spinel + Al2O3 and calcium hexaluminate (CA6) + CA2 were the major and minor phase mixtures, respectively in the corroded area. A thin layer of calcium monoaluminate (CA), gehlenite, and Si was present in the immediate vicinity of anorthite. The early stages of corrosion at 1150°C and 850°C were identical. However, due to thickening of the corroded region (viz., spinel formation) and enhanced evaporation of Mg at the higher temperature, the interdiffusion path evolves from Si4+–Al3+ + Ca2+–Mg2+ to Si4+–Al3+ + Ca2+–Al3+, thus establishing the following phase evolution path at the interface:urn:x-wiley:00027820:media:jace14091:jace14091-math-0001

Effect of Sodium on Microstructures and Thermoelastic Properties of Calcium Aluminate Cement–Bonded Refractories

The effect of sodium on refractory phase formation in a model Calcium Aluminate Cement–bonded refractory was investigated from 700°C to 1500°C. Sodium reacts with α‐alumina to form sodium β‐alumina (β‐Al2O3) via the intermediate NaAlO2. Formation of β‐Al2O3 disrupts the reaction path of calcia with alumina, delaying crystallization of calcium hexaluminate, CaO·6Al2O3, from 1350°C to 1500°C. β‐Al2O3 is also shown to reduce Young's modulus and delay sintering. The presence of NaAlO2 and β‐Al2O3 result in an increase in internal friction. Increased linear expansion of up to 47% is observed when 1 wt% Na is added. The expansion is shown to scale with the amount of dopant with only 0.3 wt% Na leading to an additional 31% linear expansion. On cooling, the presence of β‐Al2O3 can be demonstrated by a peak in internal friction between 1200°C and 1000°C which could be caused by Na+ ion hopping along the spinel‐like planes.

Effect of ZrO2 Addition on Densification and Mechanical Properties of MgAl2O4–CaAl4O7–CaAl12O19 Composite

Although the platelet structure of calcium hexaluminate (CaAl12O19, or CA6) grains can strengthen and toughen the Al2O3–MgO–CaO system materials designed in the high‐alumina region, it also results in poor densification and subsequent accelerated slag penetration for refractory application. Considering this aspect, MgAl2O4–CaAl4O7–CaAl12O19 composite was fabricated by solid‐state reaction sintering in this work, and the effect of ZrO2 addition on densification and mechanical properties was investigated. The results showed that the CA6 grains presented a more equiaxed morphology by addition of ZrO2, contributing to form highly dense microstructures after heating at 1600°C without evident grain coarsening. The compressive strength and flexural strength were greatly enhanced mainly due to the significant decrease in porosity and pore sizes. Besides, the increased content of ZrO2 plays an active role in toughening this composite attributed to the dense microstructure and strong bonding with higher strength, as well as considerable t‐ZrO2 transformability.

Calcium hexaaluminate synthesis and its influence on the properties of CA2–Al2O3-based refractories

The investigations of low thermal expansion coefficient refractories, made of calcium dialuminate (CA2) and corundum, have been presented. Classifying them into a group of reactive materials means that there is a chemical reaction at the contact area of CA2 and Al2O3 grains, accompanied by a formation another compound CaAl12O19 (CA6) during firing. The conversion degree of the reaction as a function of temperature and time has been determined, including the formulation of kinetic equations, the assessment of apparent activation energy and a presumable mechanism of CA6 formation has been proposed. It has been proved that the formation of plate-shaped CA6 crystals at the contact area of Al2O3 grains with the surrounding CA2 matrix changes pore size distribution and total porosity of the materials. It is also related with local changes of thermal expansion coefficients, which in turn influences total expansion of the tested samples.

Production of porous ceramic materialusing different sources of alumina and calcia

Numerous papers and publications report the use of microporous calcium hexaluminate (CaO.6Al2O3; CA6) as a key raw material for high temperature insulating materials. This material has unique properties with respect to chemical purity and mineral composition. Another important property of CA6 is its structure, which consists of platelet-shaped crystals that interlock. The free distance between the crystals defines the microporous structure. The low density in combination with the micropores hampers heat transfer by radiation at temperatures exceeding 1000 oC and results in a low thermal conductivity. Given the advantages presented by this material, it is necessary to understand the formation mechanism of CA6 grains in order to better develop the potential applications of this material. CA6 can be fabricated using organic binders to consolidate the Al2O3-CaCO3 powder mixture and to provide green strength so that a green body can be formed and retains the desired shape before heating. However, these organic binders must be completely thermally decomposed so that they do not remain in the sintered body as carbon or ash. Moreover, the use of organic binders releases large volumes of gases such as carbon dioxide from the green body during heating. Therefore, an eco-friendly ceramic fabrication process has been developed that employs an inorganic binder (hydraulic alumina). The aim of the present work was to study the synthesis of porous calcium-hexaluminate ceramics using calcined alumina or hydraulic alumina combined with different sources of calcia (CaCO3 and Ca(OH)2) at different temperatures. The materials produced were characterized by X-ray diffraction, scanning electron microscopy, apparent porosity and mercury intrusion porosimetry. The materials produced by hydraulic alumina presented higher porosity and larger pores compared to those produced from calcined alumina