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.     

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.     

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.     

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.     

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.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Ternary System Al2O3—MgO—CaO: Part II, Phase Relationships in the Subsystem Al2O3—MgAl2O4—CaAl4O7

Solid-state compatibility and melting relationships in the subsystem Al₂O₃—MgAl₂O₄—CaAl₄O₇ were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al₂O₃. The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca₂Mg₂Al₂₈O₄₆ and CaMg₂Al₁₆O₂₇) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl₄O₇, CaAl₁₂O₁₉, MgAl₂O₄, Ca₂Mg₂Al₂₈O₄₆, and CaMg₂Al₁₆O₂₇ were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al₂O₃—MgO—CaO.     

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.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.     

Influence of CaCO3 on the Hydration of 3CaO·Al2O3

The influence of CaCO₃ on the hydration of 3CaO Al₂O₃ containing about 90% C₃A is studied with compacts of their mixtures proportioned to 0, 2.5, 10, 20, and 50 wt% CaCO₃. The compacts are exposed to H₂O in liquid and in vapor phase, the attendant expansion being measured as a function of time. Differential thermal analysis, infrared absorption, and X–ray diffraction are used in conjunction with this technique. It is shown that the hydration reaction of 3 CaO · Al₂O₃ is suppressed by CaCO₃ additions and that this is due primarily to the formation of the low form of calcium carboaluminate on the surface of the C₃A grains. Expansion measurements of the compacts during the reactions show that this technique indicates the progress of hydration and detects the formation of the carboaluminate at an early stage of the reaction. Using the same technique, a study at 50% relative humidity provides the basis for the conclusion that C₃A reacts with CaCO₃ under these conditions by a direct mechanism.     

Thermodynamic Assessment of the CaO–MgO–Al2O3 System

The system CaO–MgO–Al₃O₃ has been assessed with the Calphad technique using a computerized optimization procedure called parrot . The rather meager experimental information, mainly on liquidus relations, is described reasonably well, but the lack of data, especially on solid-phase relations, implies that the present assessment should be regarded as provisional. The system contains one stable ternary phase with the stoichiometry 3CaO·2Al₂O₃·MgO.     

Growth of α-Al2O3 Crystals by Na2O Evaporation

A technique for growing α-Al₂O₃ crystals is described in which Na₂O·11Al₂O₃ is dissolved in a liquid of composition Na₂O·4TiO₂·3Al₂O₃. Alpha Al₂O₃ is precipitated as Na₂O evaporates from the system; Na₂O·11Al₂O₃ serves as a source of Al₂O₃, and Na₂O in the liquid. The content of solids in the mixture is always such that it does not melt completely. The size of the α-Al₂O₃ crystals grown is related to the Na₂O content of the composition. Crystals as large as 4000 by 3000 μm in the α-axis direction and 500 μm in the c -axis direction have been grown.     

Quaternary System Al2O3–CaO–MgO–SiO2: II Study of the Crystallization Volume of MgAl2O4

Solid-state compatibility and melting relations of MgAl₂O₄ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching selected samples located in the 65 wt% MgAl₂O₄, plane followed by microstructural and energy dispersive X-ray analysis. A projection of the liquidus surface of the primary crystallization volume of MgAl₂O₄ was constructed from CaO, SiO₂ and exceeding Al₂O₃, not involved in stoichiometric MgAl₂O₄ formation; those three amounts were recalculated to 100 wt%. The temperature and character of six invariant points, where four solids co-exist with a liquid phase, were defined. One maximum point was localized and the positions of the isotherms were tentatively established. The effect of CaO, SiO₂, and Al₂O₃ impurities on the high temperature behavior of spinel materials was also discussed.     

Phase Formation in the System Na2O · Al2O3-CaO · A12O3-Al2O3 at 1200°C in Air

Phase relations in the system Na₂O_· Al₂O₃-CaO· Al₂O₃-Al₂O₃ at 1200°C in air were determined using the quenching method and high-temperature X-ray diffraction. The compound 2Na₂O · 3CaO · 5Al₂O₃, known from the literature, was reformulated as Na₂O · CaO · 2Al₂O₃. A new compound with the probable composition Na₂O · 3CaO · 8Al₂O₃ was found. Cell parameters of both compounds were determined. The compound Na₂O · CaO-2Al₂O₃ is tetragonal with a = 1.04348(24) and c = 0.72539(31) nm; it forms solid solutions with Na₂O · Al₂O₃ up to 38 mol% Na₂O at 1200°C. The compound Na₂O · 3CaO · 8Al₂O₃ is hexagonal with) a = 0.98436(4) and c = 0.69415(4) nm. The compound CaO · 6Al₂O₃ is not initially formed from oxide components at 1200°C but behaves as an equilibrium phase when it is formed separately at higher temperatures. The very slow transformation kinetics between β and β "-Al₂O₃ make it very difficult to determine equilibrium phase relations in the high-Al₂O₃ part of the diagram. Conclusions as to lifetime processes in high-pressure sodium discharge lamps can be drawn from the phase diagram.     

Preparation of Al2O3–AlN–Ni Composites

A method is proposed to prepare Al₂O₃-AlN-Ni composites. The composites are prepared by sintering Al₂O₃/NiAl powder mixtures at 1600°C in a mixture of nitrogen and carbon monoxide. The presence of NiAl particles raises the green density of Al₂O₃/NiAl powder compacts. During sintering, NiAl reacts with nitrogen to form AlN and Ni inclusions. A volume expansion accompanies the reaction. Because of the high green density and the reaction, the volume shrinkage of the Al₂O₃-AlN-Ni composite decreases with the increase of added NiAl content.     

Assessment of the CaO-Al2O3 System

A thermodynamic assessment of the quasi-binary system CaO-Al₂O₃ has been made using a computerized CALPHAD (calculation of phase diagrams) technique. The actual optimization was carried out with a computer program for the optimization of parameters in thermodynamic models called PARROT. The liquid phase is described by a simple two-sublattice subregular solution model with Ca²⁺ and Al³⁺ as cationic species and O^2- as anionic species. All solid phases are treated as stoichiometric compounds. A consistent set of parameters describing the system is presented, and numerous comparisons with experimental data are made. There is a lack of accurate data in the high-alumina part of the system, and the importance of a better knowledge of the CaO·6Al₂O₃ phase is stressed.     

Environment-Sensitive Hardness and Machinability of Al2O3

The influences of water, toluene, and n -alcohols on the pendulum hardness of Al₂O₃ monocrystals and on the ease of drilling of mono- and polycrystalline Al₂O₃ were studied. Adsorption-induced increases in pendulum hardness were produced by certain alcohol environments; these increases resulted in rates of drilling with diamond-studded core bits of up to 10 times those measured under water. A correlation between pendulum hardness, machinability, and surface charge (as indicated by ζ-potential measurements) was established and revealed that Al₂O₃ is hardest and most effectively drilled with a diamond core bit at its zero point of charge. The possible use of this correlation in developing more cost-effective and ecologically viable cutting fluids for the machining of Al₂O₃ is discussed.     

Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3–Cr2O3 and V2O3–Cr2O3 Systems

The phase diagrams in the Al₂O₃–Cr₂O₃ and V₂O₃–Cr₂O₃ systems have been assessed by thermodynamic modeling with existing data from the literature. While the regular and subregular solution models were used in the Al₂O₃–Cr₂O₃ system to represent the Gibbs free energies of the liquid and solid phases, respectively, the regular solution model was applied to both phases in the V₂O₃–Cr₂O₃ system. By using the liquidus, solidus, and/or miscibility gap data, the interaction parameters of the liquid and solid phases were optimized through a multiple linear regression method. The phase diagrams calculated from these models are in good agreement with experimental data. Also, the solid miscibility gap and chemical spinodal in the V₂O₃–Cr₂O₃ system were estimated.     

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.