Low-Temperature Synthesis of Calcium Hexa-Aluminate

Calcium hexa-aluminate (CaO·6Al₂O₃) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al₂(SO₄)₃·16H₂O for each mole of Ca(NO₃)₂·4H₂O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al₂O₃ and CaO·2Al₂O₃ were also formed as reaction intermediates in the reaction mix of CaO·6Al₂O₃. The kinetics of the formation of CaO·6Al₂O₃ have been studied using the phase-boundary-controlled equation 1 − (1 − x )^1/3= K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al₂O₃ was 40 kJ/mol.     

Retardation of Tricalcium Aluminate Hydration by Calcium Sulfate

The influence of calcium sulfate on the hydration of 3CaO· Al₂O₃ in the presence of Ca(OH)₂ was studied using conduction calorimetry, differential thermogravimetry, and X-ray diffraction. Sodium sulfate was also used instead of calcium sulfate. A substantial retardation of tricalcium aluminate hydration in the presence of sulfate occurs only when calcium sulfate is used and enough ettringite is formed. When ettringite disappears due to the consumption of gypsum, tricalcium aluminate hydration is renewed. Sodium sulfate does not significantly retard this hydration. The results confirm the hypothesis that ettringite formation is essential for coating 3CaO·Al₂O₃ grains and then retarding their hydration.     

Dissolution in Ceramic Systems: III, Boundary Layer Concentration Gradients

By use of electron microbeam probe analysis on quenched samples, the concentration distribution of CaO, A1₂O₃, and SiO₂ was determined across the boundary layer between molten calcium aluminum silicates and dissolving or growing sapphire and fused silica. A definite shift in the concentration ratios of the solvent components was found near the interface. Analysis of diffusion flux equations for a ternary system successfully related the shift in concentration ratio to the intrinsic diffusion coefficient for each component. For alumina dissolution in a melt rich in CaO, evidence of incongruent dissolution was observed with the formation of new phases, CaO· 6Al₂O₃ and CaO· 2A1₂O₃.     

Reaction Characteristics of Magnesia–Spinel Refractories with Cement Clinker

The adherence ability of cement clinker on magnesia–spinel refractories is investigated, using a sandwich test, at 1550°C for 30 min under a load of 5.3 kPa. Fractional factorial experiments determine that the silica ratio (SR)—SiO₂/(Al₂O₃+Fe₂O₃) and particle size of raw meal, as well as heating rate, have a significant effect on adherence ability. Microstructural analyses indicate that the adherence ability depends upon reactions between clinker and refractories at high temperature. Only spinel reacts with CaO and 3CaO·SiO₂ from clinker to form n -calcium aluminate (such as 3CaO·Al₂O₃, 12CaO·7Al₂O₃, CaO·Al₂O₃), but there is no reaction between MgO and the clinker. Fine crystalline spinel, evenly distributed in magnesia-based brick, is prone to reacting with lime-containing phases from clinker to form low melting phases and a belite-enriched zone at the clinker/brick interface. This reaction positively contributes to the high adherence on a magnesia−spinel brick. The high content of liquid in clinker with low SR accelerates reactions between spinel and clinker, while a limited reaction occurs at the brick/clinker interface with high silica.     

29Si and 27Al MASNMR Study of Stratlingite

Strätlingite (2CaO·Al₂O₃·SiO₂·8H₂O) is a complex calcium aluminosilicate hydrate commonly associated with the hydration of slag-containing cements or other cements enriched in alumina. Strätlingite can coexist with the hydrogarnet solid solution [hydrogarnet (3CaO·Al₂O₃·6H₂O)-katoite (3CaO·Al₂O₃·SiO₂·4H₂O)] and calcium silicate hydrate (C-S-H). Since Strätlingite is present in many blended cements, the knowledge of strätlingite's characteristic silicate anion structure and how aluminum is accommodated by the structure is important. Phase pure Strätlingite samples have been synthesized from oxides in the presence of excess water and from metakaolinite, calcium aluminate cement, CaO, NaOH, and water. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) and then further examined using ²⁹Si, with and without cross-polarization (CP), and ²⁷Al solid-state magic angle nuclear magnetic resonance spectroscopy (MASNMR). For the most part, NMR data for these strätlingites corroborate structural information available in the literature. The aluminum atoms are both tetrahedrally and octahedrally coordinated, and the silicon atoms exist predominantly as Q₂, Q₂(1Al), and Q₂(2Al) species. The presence of alkali affects the structure of strätlingite in subtle ways, significantly reducing the Al^IV/A1^VI ratio.     

Synthesis of AlN Powder by Carbothermal Reduction-Nitridation of Various Al2O3 Powders with CaF2

We investigated the effect of characteristics of raw Al₂O₃ powder on the synthesis of AlN by the carbothermal reduction-nitridation method, in which CaF₂ was added as a catalytic material. Four types of Al₂O₃ powders were selected. An Al₂O₃/C molar ratio of 0.29 was fixed, and the amount of CaF₂ was varied from 3 to 30 wt%. The carbothermal reduction-nitridation was conducted from 1350° to 1450°C in N₂ flow. The nitridation rate tended to increase with decreasing particle size of raw Al₂O₃ and was found to depend on the Al₂O₃ synthesizing method. The particle sizes of the synthesized AlN increased somewhat with increasing reaction temperatures. However, even though different particle sizes of Al₂O₃ powders were used, AlN powders synthesized under the same conditions exhibited almost the same particle size, round shape, and narrow size distribution. From XRD analysis, CaO·6Al₂O₃ and CaO·2Al₂O₃ were identified as intermediate compounds during these reactions. The above phenomena suggest that the synthesis mechanism of AlN powder by carbothermal reduction-nitridation of Al₂O₃ with CaF₂ addition was the nitridation of the intermediate compounds through the liquid phase of the system CaF₂-CaO·6Al₂O₃-CaO·2Al₂O₃.     

Formation of Ettringite from Monosubstituted Calcium Sulfoaluminate Hydrate and Gypsum

The formation of ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) from monosulfate (3CaO·Al₂O₃·CaSO₄·12H₂O) and gypsum (CaSO₄·2H₂O) was investigated by isothermal calorimetry and X-ray diffraction (XRD) analyses. Hydration was carried out at constant temperatures from 30° to 80°C using deionized water and 0.2 M , 0.5 M , and 1.0 M sodium hydroxide (NaOH) solutions. Ettringite was found to be the dominant crystalline phase over the entire temperature range and at all sodium hydroxide concentrations. A sodium-substituted monosulfate phase was formed as a hydration product in the 1.0 M sodium hydroxide solution regardless of temperature. XRD and calorimetry demonstrate that hydration in increasing sodium hydroxide concentrations decreases the amount of ettringite formed and retards the rate of reaction. The apparent activation energy for the conversion of the monosulfate/gypsum mixture to ettringite was observed to vary depending on the sodium hydroxide concentration. Ettringite formation was observed to depend upon the concentration of calcium in solution; thus the formation of calcium hydroxide and sodium-substituted monosulfate phase competes with ettringite formation.     

Influence of Citric Acid on Reactions in the System 3CaO·Al2O3-CaSO4·2H2O-CaO-H2O

The influence of citric acid on paste hydration of 3CaO· Al₂O₃ in the presence of CaSO₄·2H₂O and Ca(OH)₂ was studied using X-ray diffraction, scanning electron microscopy, and conduction calorimetry. The time at which the citric acid is added (either prior to or with the mixing water) determines how it affects the reactivity of the aluminate. Immediately after the paste is gaged citric acid promotes a more rapid reaction, but later reactions are retarded. Hexagonal calcium aluminate hydrates, ettringite, and monosulfate were all detected as early hydration products. The influence of citric acid on the hydration of 3CaO·Al₂O₃ slabs immersed in saturated CaSO₄·2H₂O solutions was also studied and a reaction scheme proposed.     

The System CaO-Al2O3-H2O

Phase equilibria have been determined in the system CaO-Al₂O₃-H₂O in the temperature range 100° to 1000°C. under water pressures of up to 3000 atmospheres. Only three hydrated phases are formed stably in the system: Ca(OH)₂, 3CaO·Al₂O₃·6H₂O, and 4CaO·3Al₂O₃-3H₂O. Pressure-temperature curves delineating the equilibrium decomposition of each of these phases have been determined, and some ther-mochemical data have been deduced therefrom. It has been established that both the compounds CaO·Al₂O₃ and 3CaO·Al₂O₃ have a minimum temperature of stability which is above 1000°C. The relevance of the new data to some aspects of cement chemistry is discussed.     

Crystallization of Pseudo-orthorhombic Anorthite on Basal Sapphire

Anorthite-glass films were grown on basal Al₂O₃ substrates using pulsed-laser deposition. The substrates were cleaned and annealed in air at 1400°C to produce crystallographically flat (0001) terraces. The films were deposited in an oxidizing environment. X-ray microanalysis confirmed the composition of the glass films to be close to that of anorthite (CaO·Al₂O₃·2SiO₂). Although anorthite usually has triclinic symmetry, subsequent crystallization of these films in air at 1200°C resulted in the formation of pseudo-orthorhombic CaAl₂Si₂O₈ ( o -anorthite), a known metastable form of the mineral. Microstructural characterization was performed using visible-light microscopy, scanning electron microscopy, and transmission electron microscopy. The films dewetted the substrate either before or after crystallization to form o -anorthite islands which had strong orientation relationships to the Al₂O₃ substrate. The epitaxy of the o -anorthite islands was accompanied by a small lattice mismatch parallel to the substrate plane. The formation of three orientational variants is consistent with the symmetry of the basal Al₂O₃ surface. The dislocation network observed at the o -anorthite/Al₂O₃ interface indicates that nucleation and growth of the anorthite occurs directly on the substrate surface without an intervening interfacial amorphous layer. The study of anorthite-glass films is important because they are present in liquid-phase-sintered Al₂O₃, and may be devitrified by postsintering heat treatments.     

Phase Equilibrium Relationships in a Portion of the System MgO–Al2O3–2CaO·SiO2

The system MgO–Al₂O₃–2CaO·SiO₂ comprises a plane through the tetrahedron CaO–MgO–Al₂O₃–SiO₂. A total of 108 compositions were prepared having an alumina content below the line joining 2CaO·Al₂O₃SiO₂ (gehlenite) and MgO·Al₂O₃ (spinel). Quenching experiments were carried out on 96 of these compositions at temperatures up to 1590°C. Three binary eutectic systems and two ternary eutectic systems are described. Compositions on this plane are of significance in an investigation of the constitution of basic refractory clinkers made from Canadian dolomitic magnesites. They also concern the compositions of certain blast furnace slags.     

Equilibrium Amorphous Silicon–Calcium–Oxygen Films at Interfaces in Copper–Alumina Composites Prepared by Melt Infiltration

The microstructure of copper–alumina (Cu-Al₂O₃) composites that have been prepared via the melt infiltration of liquid copper into porous alumina preforms was studied in detail, using various transmission electron microscopy (TEM) techniques. Two different samples—with open pore diameters of 0.2 and 0.8 μm—were investigated. For both specimens, a single crystalline copper network that extended throughout the open porosity of the alumina preform was observed. An amorphous glass phase that contained silicon and calcium was observed at the Al₂O₃/Cu/Al₂O₃ triple junctions. The diameters of these amorphous pockets, which were strongly faceted along the Al₂O₃ grains, were up to 20 and 100 nm for the initial pore sizes of 0.2 and 0.8 μm, respectively. A glass phase that contained silicon and calcium also was present at the Cu/Al₂O₃ interfaces, whereas the Al₂O₃ boundaries remained dry. Detailed high-resolution transmission electron microscopy investigations have shown that the interfacial glass phase at the Cu/Al₂O₃ interfaces exhibited a uniform equilibrium film thickness along the interface region. However, the interfacial film thickness was dependent on the orientation of the Al₂O₃ grain, and its value varied from 0.4 nm for Al₂O₃ rhombohedral-plane termination ((1¯012)) up to 1 nm for Al₂O₃ basal-plane termination ((0001)).     

Synthesis of α-Alumina Hexagonal Platelets Using a Mixture of Boehmite and Potassium Sulfate

Single-crystal α-alumina (Al₂O₃) hexagonal platelets with a diameter of about 200 nm and 25 nm in thickness were synthesized by heating a mixture of boehmite and potassium sulfate at 1000°C for 2 h and washing with water. The potassium sulfate addition effects on the Al₂O₃ phase and morphology were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It was found that potassium sulfate addition helps in the formation of single-crystal α-Al₂O₃ hexagonal platelets and promotes phase transformation from intermediate γ-Al₂O₃ to α-Al₂O₃.     

Determination of 1600° and 1700°C. Liquidus Lines in CaO-2Al2O3 and AI2O3 Stability Fields of the System CaO—Al2O3—SiO2

The 1600° and 1700°C. liquidus lines in the CaO·2Al₂O₃ and A1₂O₃ stability fields of the system CaO-Al₂O₃-SiO₂ are determined from the chemical analyses of saturated slags at these temperatures.     

Formation Kinetics of Calcium Aluminates

The kinetics of formation of calcium aluminates was studied by firing the reaction mixes in the temperature range 12000° to 1460°C for reaction times from 15 to 360 min. Phases formed were determined by taking X-ray diffractograms of the samples. It was observed that all stable calcium aluminates were formed and that monocalcium aluminate (CA) grew with calcium dialuminate (CA₂) in a 1:2 reaction mix of CaO and Al₂O₃. CA reacted further with Al₂O₃ to form CA₂. The formation of CA₂ obeyed the rate law equation 1 - (1 - x )^1/3= Kt / r ². The activation energy for the system (140 kJ·mol⁻¹ (33.4 kcal · mol⁻¹)) was determined by the Arrhenius equation.     

The Effects of Prehydration on the Liquid Hydration of 3CaO·Al2O3 with CaSO 4·2H2O

Liquid hydration and water-vapor hydration of 3CaO·Al₂O₃, were studied. Variable parameters were hydration time, temperature, relative humidity, and amount of gypsum. The hydration products (gel, ettringite, hexagonal hydrates, and 3CaO·Al₂O₃·6H₂O) were studied by electron microscopy, X-ray diffractometry, and thermal analysis. A reaction scheme is proposed. The degree of water-vapor hydration influenced the sequence of the subsequent liquid hydration which, however, was independent of the composition of the water-vapor hydration products. Below a critical degree of water-vapor hydration (≊3% combined water) the reaction with liquid water occurred as if no water-vapor hydration had taken place. Above this value the reaction gave hydration products suggesting a change of the 3CaO·Al²O³ reactivity. A possible correlation with the retardation of strength development of prehydrated cement is suggested.     

Phase Relations in the System CaO-Ta2O5-SiO2

The system CaO-Ta₂O₃-SiO₂ was studied using a combination of hot-stage microscopy and the quenching technique. Primary crystallization fields were defined for the various calcium silicates, and for the calcium tantalates: CaO·2Ta₂O₅, CaO·Ta₂O₅, 2CaO·Ta₂O₅, and 4CaO·Ta₂O₅. A congruently melting ternary compound 10CaO·Ta₂O₅·6SiO₂, isostructural with the mineral niocalite, is the only ternary phase in the system. A large twoliquid region extends across the system from the CaO-SiO₂ binary to within 1 wt% of the Ta₂O₃-SiO₂ binary but does not cut it, in marked contrast to the analogous CaO-Nb₂O₅-SiO₂ system. Other somewhat unexpected differences were noted between these systems.     

Precise Determination of the 3CaO·SiO2 Cells and Interpretation of Their X-Ray Diffraction Patterns

The cell dimensions of pure triclinic 3CaO·SiO₂ and monoclinic 3CaO·SiO₂ solid solution (54CaO·16SiO₂·Al₂O₃·MgO) were determined and the powder diffraction patterns were indexed by the method of precise measurement of the spacings. The lattice constants are expressed in terms of triclinic or monoclinic cells corresponding to pseudo-orthorhombic cells derived from Jeffery's trigonal cell. The apparent lattice constants for pure 3CaO·SiO₂ are a = 12.195 a.u., b = 7.104 au., c = 25.096 a.u., α= 90°, β= 89°44'γ= 89°44'; for 54CaO·16SiO₂.-Al₂O₃MgO, a = 12.246 a.u., b = 7.045 a.u., c = 24.985 a.u., β= 90°04'. Precise lattice constants of Jeffery's monoclinic lattice for 54CaO.-16SiO₂-Al₂O₃·MgO are derived as a = 33.091 a.u., b = 7.045 a.u., c = 18.546 a.u., β= 94°08'. High-temperature X-ray patterns showed that pure triclinic 3CaO·SiO₂ transformed to a monoclinic form at about 920°C. and then to a trigonal form at about 970°C. Monoclinic 54CaO.16SiO₂·Al₂O₃–MgO transformed to trigonal at about 830°C. These transitions were reversible and reproducible and were accompanied by only slight deformation of the structure forms.     

Conversion of CaO·Al2O3·10H2O to 3CaO·Al2O3·6H2O

The morphological changes accompanying the conversion of the hexagonal CaO·Al₂O₃·10H₂O phase to the cubic 3CaO·Al₂O₃·6H₂O phase were studied by scanning electron microscopy. The hydration and conversion reactions were monitored by X-ray diffraction analysis. From the micrographs, it was inferred that changes in the pore structure and the presence of large cubic crystals of questionable adhesive value were probably the principal factors responsible for the loss of strength in converted calcium aluminate cement pastes.     

Effect of Y2O3 Additions on the Densification of an Al2O3–TiC Composite

The densification of Al₂O₃–30TiC (in weight percent) composite is investigated as a function of Y₂O₃ additions. It is observed that very small amounts of Y₂O₃ are effective in aiding the densification. The density was observed to pass through a maximum at 0.35 wt% of Y₂O₃. The gas-generating reaction of Al₂O₃ with TiC is likely to be suppressed by the addition of Y₂O₃.