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.     

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 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.     

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.     

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.     

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.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Kinetics of Growth of Al2O3 Whiskers

The kinetics of the growth of Al₂O₃ whiskers by the reaction of Al with a trace amount of H₂O in an H₂ atmosphere were studied. The activation energy in the region of constant growth rate was ∼77 kcal mol⁻¹. This activation energy value, in the light of thermodynamic calculations and other theoretical considerations, suggests that the rate-determining step in the growth of these whiskers is the dissociation of H₂O molecules on the surface of Al₂O₃.     

Leaching of Glasses with Molar Composition 20Na2O · 10RO ·x Al2O3· (70-x)SiO2

Sodium depth profiles were determined in water-leached glass samples with molar composition 20Na₂O · 10RO · x Al₂O₃· (70 - x)SiO₂ (RO = CaO, MgO, and ZnO) using secondary ion mass spectrometry. The leaching of sodium ions decreases with increasing Al₂O₃ content in all three glass systems. For x = 0 the leaching is hardly affected by the nature of the divalent cation. For x = 5 and 10 the corrosion resistance is highest for the glass containing ZnO, and for the glasses containing ZnO and MgO, respectively. These glasses correspond to those with the smallest fraction of NBOs. From all these results it is concluded that the nonbridging oxygen atoms play an important role in promoting the leaching of a glass.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Hydration Study of the System Ca3Al2O6–CaSO4· 2H2O–Ca(OH)2–H2O with and without Citric Acid

Hydration occurring in the system Ca₃Al₂O₆–CaSO₄· 2H₂O–Ca(OH)₂–H₂O has been studied at different temperatures and it was found that the reactions are diffusion controlled. The kinetic data obeyed Jander's equation and the rate of reaction increased with increasing temperature. X-ray diffraction studies and calorimetric measurements show that when gypsum is consumed, ettringite is converted into monosulfate. The rate of this conversion also increased with the increasing temperature and decreased in the presence of citric acid. Spectroscopic studies showed that there was some interaction between citric acid and the cement and that the product of hydration is of colloidal nature. Zeta potential measurements show that retardation of Ca₃Al₂O₆ hydration in the presence of gypsum and Ca(OH)₂ is not due to SO²⁻₄ adsorption. Electrical conductivity and thermoelectric potential measurements of solid Ca₃Al₂O₆ show that Ca₃Al₂O₆ is an n -type semiconductor and contains defects. The retardation of Ca₃Al₂O₆ may be due to poisoning of reaction sites by gypsum and Ca(OH)_2.     

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.     

Chemical Vapor Deposition of Polycrystalline Al2O3

Polycrystalline Al₂O₃ was chemically vapor-deposited onto sintered Al₂O₃ substrates by reaction of AlCl₃ with (1) H₂O, (2) CO:H₂, and (3) O₂ at 1000° and 1500°C and 0.5 and 5.0 torr. Although the thermodynamics of all these reactions predict the formation of solid Al₂O₃, the deposition rate of the first reaction was considerably greater than that of the second. The third reaction was so slow that no measurable deposit was formed in 6 h at 1500°C. Formation of dense deposits of α-Al₂O₃ was favored by increasing temperature and decreasing pressure. Microstructural examination of the dense deposits showed long columnar grains, the largest of which extended through the deposit from the substrate to the surface.     

High-Toughness Ce-TZP/Al2O3 Ceramics with Improved Hardness and Strength

Simulataneous additions of SrO and Al₂O₃ to ZrO₂ (12 mol% CeO₂) lead to the in situ formation of strontium aluminate (SrO · 6Al₂O₃) platelets (∼0.5 μm in width and 5 to 10 μm in length) within the Ce-TZP matrix. These platelet-containing Ce-TZP ceramics have the strength (500 to 700 MPa) and hardness (13 to 14 GPa) of Ce-TZP/Al₂O₃ while maintaining the high toughness (14 to 15 MPa ± m^1/2) of Ce-TZP. Optimum room-temperature properties are obtained at SrO/Al₂O₃ molar ratios between 0.025 and 0.1 for ZrO₂ (12 mol% CeO₂) with starting Al₂O₃ contents ranging between 15 and 60 vol%. The role of various toughening mechanisms is discussed for these composite ceramics.     

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: 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.     

Reactions in the System Li2O–MgO–Al2O3–SiO2: I, The Cordierite-Spodumene Join

Phase equilibria along the nonbinary join between cordierite (2MgO · 2Al₂O₃· 5SiO₂) and spodumene (Li₂O · Al₂O₃· 4SiO₂) were investigated in the temperature range 800° to 1550°C. using the quench technique on fourteen compositions. The phase diagram at high temperatures is characterized by a very small region of solid solution on the cordierite side, appreciable solid solution on the spodumene side, and regions of three and four phases toward the center of the system, including liquid, α-cordierite, mullite, spinel, corundum, and β-spodumene and its solid solutions. The liquidus has a flat minimum between 40 and 50% cordierite at 1347°, and rises on one side to the congruent melting point of β-spodumene (1421°) and on the other side to the temperature of complete melting of cordierite (1530°). The lowest temperature at which liquid appears is 1325°. At low temperatures a complete series of metastable solid solutions exists between μ-cordierite and β-spodumene. The significance of the data in the preparation of thermal-shock-resisting bodies is discussed.     

The System MgO─P2O5─H2O at 25°C

The phase diagram for the ternary system MgO─P₂O₅─H₂O at 25°C has been constructed. The magnesium phosphates represented are Mg(H₂PO₄)₂· n H₂O ( n = 4, 2, 0), MgHPO₄·3H₂O, and Mg₃(PO₄)₂· m H₂O ( m = 8, 22). Because of the large differences in the solubilities of these compounds, the technique which involves plotting the mole fractions of MgO and P₂O₅ as their 10th roots has been employed. With the exception of MgHPO₄·3H₂O, the magnesium phosphates are incongruently soluble. Because incongruency is associated with a peritectic-like reaction, the phase Mg₂(PO₄)₃· 8H₂O persists metastably for an extended period.