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.     

Effect of Heat-Treatment on the Constitution and Mechanical Properties of Some Hydrated Aluminous Cements

The compound compositions of four aluminous cements were determined on anhydrous as well as hydrated specimens which had been heat-treated at temperatures between room temperature and 1400° C. Phases were identified by X-ray diffraction and differential thermal analysis. Specimens were also tested for transverse strength, dynamic modulus of elasticity, and thermal length change. A study of the dehydration characteristics of CaO - Al₂O₈ - 10H₂O₃ 3CaO.Al₂O₃. 6H₂O, and Al₂O₃. 3H₂O was included. The data indicated that CaO. Al₂O₃ 10H₂O was the primary crystalline hydrate formed in the cements at room temperature. At 50° C., 3 CaO Al₂O₃-6H₂O and Al₂O₃. 3H₂O were formed as by-products of the dehydration of CaO.Al₂O₃.10H₂O. When heated alone in an open system, CaO.Al₂O₃.10H₂O did not convert to 3CaO. Al₂O₃. 6H₂O and A1₂O₃. 3H₂O. A correlation between the mechanical properties and compound compositions was noted.     

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.     

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.     

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 Al2O3-Nd2O3 Phase Diagram

A tentative phase diagram for the system Al₂0₃-Nd₂O₃ is presented. Three compounds were obtained: a β -A1₂O₃-type compound, the perovskite NdAlO₃, and Nd₄Al₂O₉. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al₂O₃, mp 1900°C; Nd₄Al₂O₉, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al₂O₃, 1750°C; 23 mol% Al₂O₃,1800°C. Nd₄Al₂O9 decomposes in the solid state at 1780°C.     

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.     

Formation of Sm2+ lons in Sol-Gel-Derived Glasses of the System Na2 O-Al2O3-SiO2

Samarium ions (Sm²⁺) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na₂O-Al₂O₃SiO₂ synthesized by the hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, CH₃ COONa, and SmCl₃·6H₂O were heated in air at 500°C, then reacted with H₂ gas to form Sm²⁺ ions. Whereas Al³⁺ ions effectively dispersed the Sm³⁺ ions in the glass structure, Na⁺ ions were not effective. The Al₂O₃-SiO₂ glasses proved appropriate for reacting the Sm³⁺ ions with H₂ gas and exhibited the intense photoluminescence of Sm²⁺ ions. The reaction of Sm³⁺ ions with H₂ in the Al₂O₂-SiO₂ glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm²⁺ ions was achieved within 20 min.     

The System Al2O3-Cr2O3-Sio2

Liquidus phase equilibrium data are presented for the system Al₂O₃-Cr₂O₃-SiO₂. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al₂O₃-ICr₂O₃-93SiO₂) coexists with a mullite solid solution (61Al₂O₃-10Cr₂O₃-29SiO₂), a corundum solid solution (19Al₂O₃-81Cr₂O₃), and cristobalite (SO₂). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.     

Studies in Lithium Oxide Systems: II, Li2O Al2O3–Al2 O 3

Solid-state reactions between Li₂O and Al₂ O₃ were studied in the region between Li₂O.Al₂ O ₃ and Al₂ O ₃. The compound Li₂ O Al₂ O ₃ melts at 1610°± 15°C. and undergoes a rapid reversible inversion between 1200° and 1300°C. Vaporization of Li₂ O from compositions in the system proceeds at an appreciable rate at 1400°C, as shown by fluorescence. Lithium spinel, Li₂ O -5Al₂O₃, was the only other compound observed. The effect of Li₂ O on the sintering of alumina was investigated.     

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2 Liquids

The saturation surface of cassiterite, SnO₂, was determined for liquids in the system K₂O–Al₂O₃–SiO₂ as a function of bulk composition and temperature. At fixed K₂O/Al₂O₃ cassiterite solubility varies weakly with SiO₂ concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log ( f _O2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K₂O/Al₂O₃ lessthan equal to 1 (peraluminous liquids). As K₂O/Al₂O₃ increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K₂O in excess of Al₂O₃. It is proposed that potassium in excess of aluminum combines with Sn⁴⁺ to form quasi-molecular complexes with an effective stoichiometry of K₄SnO₄.     

Studies in Lithium Oxide Systems: XII, Li2O-B2O3-Al2O3

Tentative phase relations in the binary system BnOa-A1₂O₃ are presented as a prerequisite to the understanding of the system Li₂O-B₂O₃-Al₂O₃. Two binary compounds, 2A1₂O₃.B₂O₃ and 9A1₂O₃.-2B₂O₃, melted incongruently at 1030° f 7°C and about 144°C, respectively. Two ternary compounds were isolated, 2Li₂O.A1₂O₃.B₂O₃ and 2Li₂O. 2AI₂O₃. 3B₂0₃. The 2:1:1 compound gave a melting reaction by differential thermal analysis at 870°± 20° C, but the exact nature of the melting behavior was not determined. The 2:2:-3 compound melted at 790°± 20° C to Li_zO.-5Al₂O₃ and liquid. X-ray diffraction data for the compounds are presented and compatibility triangles are shown.     

The System CaO-Al2O3-P2O5-H2O at 200°C

The phase relations were established experimentally for the system CaO-Al₂O₃-P₂O₅-H₂O at 200°C and 1710 kPa. The quaternary compound, crandallite, CaAl₃(PO₄)₂(OH)₅· H₂O, was found to be stable. Compatibility joins in the system were determined. The phase relations are presented on the isothermal-isobaric 90 wt% water plane and by projecting the primary fields of the liquidus surface onto the same plane.     

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.     

Melting Phenomena in the System CaO–SiO2– H2O: I, The Join Ca2SiO,-Ca(OH)2

The liquidus-solidus relations along the join Ca₂SiO₄-Ca(OH), in the system CaO-SiO₂-H₂O have been determined at 1000 atm up to 1110°C. This join is binary and contains the calcium silicate hydrate, calciochondrodite, Ca5-(SiO₄(OH)₂. Calciochondrodite melts incongruently to Ca₂SiO₂+ liquid (composition 23 wt% Ca₂Si0₄) at 955°C. The eutectic between calcium hydroxide and calciochondrodite lies at 13% Ca₂Si0₄ and 822°C. Preliminary experiments, also at 1000 atm, in the ternary system CaO-Ca₂Si0₄-Ca(OH), indicate that the eutectic at which the fields of primary Ca(OH)₂, CaO, and Ca₂(Si0₄)₂(OH)₂ meet is close to the CaO-Ca. (OH), side of the triangle at approximately 805° C. The ternary reaction point Ca₂SiO_l+ liquid ⇌Ca₅(SiO₄)₂(OH)₂+ CaO + liquid is believed to lie in the low-CaO (<5%) high-Ca(OH)₂ (>70%) part of the system.     

Sc2O3 Nanopowders via Hydroxyl Precipitation: Effects of Sulfate Ions on Powder Properties

Hydroxyl-type Sc₂O₃ precursors have been synthesized via precipitation at 80°C with hexamethylenetetramine as the precipitant. The effects of starting salts (scandium nitrate and sulfate) on powder properties are investigated. Characterizations of the powders are achieved by elemental analysis, X-ray diffractometry (XRD), differential thermal analysis/thermogravimetry (DTA/TG), high-resolution scanning electron microscopy (HRSEM), and Brunauer-Emmett-Teller (BET) analysis. Hard-aggregated precursors (γ-ScOOH·0.6H₂O) are formed with scandium nitrate, which convert to Sc₂O₃ at temperatures ≥400°C, yielding nanocrystalline oxides of low surface area. The use of sulfate leads to a loosely agglomerated basic sulfate powder having an approximate composition of Sc(OH)_2.6(SO₄)_0.2·H₂O. The powder transforms to Sc₂O₃ via dehydroxylization and desulfurization at temperatures up to 1000°C. Well-dispersed Sc₂O₃ nanopowders (∼64.3 nm) of high purity have been obtained by calcining the basic sulfate at 1000°C for 4 h. The effects of SO₄²⁻ on powder properties are 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.     

Studies in Lithium Oxide Systems: IX, Li2o-Al2O3-TiO2

Compatible phases in the system Li₂O-Al₂O₃-TiO₂ at various temperature levels were determined mainly by solid-state reactions for the portion of the ternary system bounded by Li₂O Al₂O₂, Li₂O.TiO₂, Al₂O, and TiO₂. The existence of a ternary compound, Li₂O.Al₂O₃.4TiO₂, and nine joins was established. The ternary compound has a lower limit of stability at 1090°± 15°C. and dissociates and recombines rapidly at 1380°± 15°C.     

Phase Equilibria in the System Na2O – Nb2O5

Phase equilibria data, obtained both by differential thermal analysis and by quenching, are presented for the system Na₂O-Nb₂O₅. Five compounds corresponding to the formulas 3Na₂O.1Nb₂0₆, lNa₂O. 1Nb₂O₅, lNa₂O 4Nb₂O₆, lNa_zO.7Nb₂O₅, and lNa₂O. 10Nb₂O₆ have been found. The compound 3Na_z0.lNb₂O₅ melts congruently at 992°C. The compounds 1Na₂O. 4Nb₂O₆, lNa₂O.7Nb₂O, and 1Na₂O. 1Onb₂O₅ melt incongruently at 1265°, 1275°, and 1290°C., respectively. The well-known perovskite structure phase NaNbO₃ was found to melt congruently at 1412°C. The transition temperatures in NaNbO₅ were checked by thermal analysis and only the major structural changes at 368° and 640°C. could be detected. A new disordered form of NaNbO₃ could be preserved to room temperature by very rapid quenching.     

Mullite Crystallization from SiO2-Al2O3 Melts

SiO₂-Al₂O₃ melts containing 42 and 60 wt% A1₂O₃ were homogenized at 2090°C (∼10°) and crystallized by various heat treatment schedules in sealed molybdenum crucibles. Mullite containing ∼78 wt% A1₂O₃ precipitated from the 60 wt% A1₂O₃ melts at ∼1325°± 20°C, which is the boundary of a previously calculated liquid miscibility gap. When the homogenized melts were heat-treated within this gap, the A1₂O₃ in the mullite decreased with a corresponding increase in the Al₂O₃ content of the glass. A similar decrease of Al₂O₃ in mullite was observed when crystallized melts were reheated at 1725°± 10°C; the lowest A1₂O₃ content (∼73.5 wt%) was in melts that were reheated for 110 h. All melts indicated that the composition of the precipitating mullite was sensitive to the heat treatment of the melts.