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.     

Chemical Preparation of the Binary Compounds in the Calcia-Alumina System by Self-Propagating Combustion Synthesis

The binary compounds Ca₃Al₂O₆ (C₃A), Ca₁₂Al₁₄O₃₃ (C₁₂A₇), CaAl₂O₄ (CA), CaAl₄O₇ (CA₂), and CaAl₁₂O₁₉ (CA₆) in the CaO-Al₂O₃ system have been synthesized as high-compound-purity ceramic powders by using the self-propagating combustion synthesis (SPCS) method. Materials characterization of the above-mentioned phases was performed via powder X-ray diffractometry (XRD), Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The structural characterization of the C₁₂A₇ phase has been performed via Rietveld analysis on the powdered XRD samples. It has hereby been shown that, by using this synthesis procedure, it should be possible to manufacture high-purity ceramic powders of CA, CA₂, and C₁₂A₇ at 850°C, C₃A at 1050°C, and CA₆ at 1200°C in a dry-air atmosphere.     

Preparation of Portland Cement Components by Poly(vinyl alcohol) Solution Polymerization

The four components portland cement-dicalcium silicate, C₂S (Ca₂SiO₄); tricalcium silicate, C₃S (Ca₃SiO₅); tricalcium aluminate, C₃A (Ca₃Al₂O₆); and tetracalcium aluminate iron oxide, C₄AF (Ca₄Al₂Fe₃O₁₀)-were formed using a solution-polymerization route based on poly(vinyl alcohol) (PVA) as the polymer carrier. The powders were characterized using X-ray diffraction techniques, BET specific surface area measurements, and scanning electron microscopy. This method produced relatively pure, synthetic cement components of submicrometer or nanometer crystallite dimensions, high specific surface areas, as well as extremely high reactivity at relatively low calcining temperatures. The PVA content and its degree of polymerization had a significant influence on the homogeneity of the final powders. Two types of degree of polymerization (DP) PVA were used. Lower crystallization temperatures and smaller particle size powders were obtained from the low-DP-type PVA at optimum content.     

Influence of SO3 on the Phase Relationship in the System CaO–SiO2–Al2O3–Fe2O3

The influence of the additive SO₃ on the phase relationships in the quaternary system CaO-SiO₂-Al₂O₃-Fe₂O₃ was investigated by observing the change of volume ratio of 3CaOSiO₂ (C₃S) to 2CaOSiO₂ (C₂S) + CaO (C) in the sintered material with the increase of SO₃ content. The primary phase volume of C₃S in the quaternary phase diagram shrank with the increase of SO₃ and disappeared when the SO₃ content exceeded 2.6 wt% in the sintered material. Changes in the peritectic reaction relationship between CaO (C), 2CaOSiO₂ (C₂S), 3CaOSiO₂ (C₃S), 3CaOAl₂O₃ (C₃A), 4CaOAl₂O₃Fe₂O₃ (C₄AF), and liquid were also observed and discussed.     

Character of Hydration of 3CaO.Al2O3

The C₃A compacts were hydrated and the reaction was studied by DTA, X-ray diffraction, mercury porosimetry, and volume change analysis. The hexagonal hydroaluminates C₂AH₈ and C₄AH₁₉ formed at 2°, 12°, and 23°C by a direct mechanism between C₃A and H₂O. The hydration reaction at 52° and 80°C was stopped by formation of C₃AH₆ around the C₃A grains. The rate of conversion of the hexagonal hydrates to cubic C₃AH₆ increased with temperature. Volume change analysis confirmed that C₃AH₆ grows epitaxially on the surface of the C₃A grain. The reaction at this surface and the passage of water through the layer of hexagonal hydroaluminates control the overall reaction rate. The conversion of the hexagonal hydrates to C₃AH₆ accelerates the reaction by removing the layer of products from around the C₃A grain by a solution mechanism. At 52° and 80°C, C₃AH₆ may form without the intermediate formation of the hexagonal hydrate.     

Effect of Organic Compounds on the Interconversions of Calcium Aluminate Hydrates: Hydration of Tricalcium Aluminate

The paste hydration of tricalcium aluminate (C₃A) in the presence of organic compounds was investigated at several temperatures up to 75°C. The results confirm earlier hypotheses that the hexagonal calcium aluminate hydrates (principally C₄AH₁₃) which are first formed create a protective barrier around the remaining C₃A and severely restrict further hydration. Above 30°C, conversion to C₃AH₆ breaks down this barrier and causes rapid hydration of C₃A. Organic compounds retard the hydration of C₃A by inhibiting the conversion reaction. Experiments with synthetic C₄AH₁₃ showed that organic molecules can form interlayer compounds, and it is considered that random sorption into the C₃AH₁₃ structure restricts the transformation to C₃AH₆. Other aspects of C₃A hydration and of the reactivity of C₄AH₁₃ are also discussed.     

Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

The phase stability in part of the P₂O₅-bearing pseudoquaternary system CaO–SiO₂–Al₂O₃–Fe₂O₃ has been studied by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. At 1973–1653 K, the α-Ca₂SiO₄ solid solution [α-C₂S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C₂S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt-differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca₃Al₂O₆ solid solution [C₃A(ss)]. When the α-C₂S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al₂O₃ with respect to Fe₂O₃. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C₃A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.     

Calcium Phosphate Cements Prepared by Acid–Base Reaction

We investigated the characteristics of calcium phosphate cements (CPC) prepared by an exothermic acid–base reaction between NH₄H₂PO₄-based fertilizer (Poly-N) and calcium aluminate compounds (CAC), such as 3CaO · Al₂O₃ (C₃A), CaO · Al₂O₃ (CA), and CaO · 2Al₂O₃ (CA₂), in a series of integrated studies of reaction kinetics, interfacial reactions, in-situ phase transformations, and microstructure development. Two groups were compared: untreated and hydrothermally treated CPC specimens. The extent of reactivity of CAC with Poly-N at 25°C was in the following order: CA > C₃A ≫ CA₂. The formation of a NH₄CaPO₄· x H₂O salt during this reaction was responsible for the development of strength in the CPC specimens. The in-situ phase transformation of amorphous NH₄CaPO₄· x H₂O into crystalline Ca₅(PO₄)₃(OH) and the conversion of hydrous Al₂O₃ gel →γ-AIOOH occur in cement bodies during exposure in an autoclave to temperatures up to 300°C. This phase transformation significantly improved mechanical strength.     

Single-Crystal Elastic Constants of Yttria-Stabilized Zirconia in the Range 20° to 700°C

Elastic constants of single crystals of yttria-stabilized zirconia were determined through the temperature range 20° to 700°C. Crystals containing 8.1, 11.1,12.1, 15.5, and 17.9 mol% Y₂0₃were measured. The elastic constant C₁₁ was found to decrease and C₁₂ and C₄₄ to increase with increasing Y₂O₃ content; this appears to be due to decreasing coulombic interaction as Y³⁺ replaces Zr⁴⁺. Except for the 8.1 mol% Y₂O₃ crystal, the conventional elastic constants all showed normal monotonic decreases with increasing temperature. In the case of the 8.1 mol% Y₂O₃ crystal, measurements as a function of temperature were not reproducible, and it is likely that this composition at room temperature is below the composition limit of thermodynamic stability of the cubic fluorite phase.     

Kinetics of Tricalcium Aluminate and Tetracalcium Aluminoferrite Hydration in the Presence of Calcium Sulfate

Hydration reactions of C₃A and C₄AF with calcium sulfate hemihydrate and gypsum were investigated and the kinetics of the reactions compared. The rates of C₃A and C₄AF hydration, as determined by heat evolution, vary depending on whether the sulfate-containing reactant is gypsum or calcium sulfate hemihydrate. The following sequence of reactions involving C₄AF occurs when hemihydrate is the reactant: gypsum formation during the first hour, ettringite formation between 20 and 36 hours, and the conversion of ettringite to monosulfate over a period of about 12 hours. Monosulfate formation initiates prior to the complete consumption of gypsum. The onset of this conversion occurs at a shorter hydration time when hemihydrate is a reactant and the total amount of heat evolved is lower. The hydration reactions in saturated calcium hydroxide solution occur more slowly than those in water. Based on heat liberation, C₄AF reacts at a much higher rate than C₃A. Ettringite formation occurs during the first 8 to 9 days of C₃A hydration. Once the gypsum is consumed, ettringite converts to monosulfate during two additional days. Compared to gypsum, hemihydrate decreases the rates of hydration of both C₃A and C₄AF. The effects on the hydration characteristics of C₄AF are significant. The hydration of C₃A with gypsum in water, in saturated Ca(OH)₂ solution, and in 0.3 M NaOH solution were compared. Heat evolution is the lowest for hydration in 0.3 M NaOH. The onset of monosulfate formation occurs prior to the complete reaction between gypsum and C₃A in the NaOH solution.     

Reaction Kinetics of Polycrystalline Yttrium Iron Garnet

The formation of yttrium iron garnet, Y₃Fe₂-(FeO₄)₃, starting with (1) Fe₂O₃ and Y₂O₃ and (2) Fe₃O₄ and Y₂O₃, was studied as a function of temperature and time by means of magnetic moment and X-ray measurements. The reaction began at 600°C. and was completed at 1200°C. The perovskite phase appeared only between 600° and 800°C. Above 1200°C. only the garnet phase was present. The microwave line width and g -factor at 9303 mc. per second were also measured and related to the preparation variables.     

Thermal Decomposition of Some Substituted Barium Titanyl Oxalates and Its Effect on the Semiconducting Properties of the Doped Materials

Thermal analysis has been performed on BaTiO(C₂O₄)₂.4H₂O, Ba_0.6Sr_0.4TiO(C₂O₄)₂.4H₂O, Sr(TiO(C₂O₄)₂.4H₂O, Ba_0.9Pb_0.1TiO(C₂O₄)₂.4H₂O, and BaTi_0.9Zr_0.1O(C₂O₄)₂.4H₂O. It was observed that the strontium compound decomposes differently than the others. Previous investigators have proposed conflicting mechanisms for the pyrolysis of the barium salt and these results are discussed in comparison with this work. The electrical resistivity and temperature coefficient of fired lanthanum-doped materials were found to vary with the calcination temperature. Maximum conductivity was observed in samples calcined at 900°C whereas maximum positive temperature coefficient was observed for materials calcined at 1050°C. Particle sizes of the calcined material were compared with grain sizes in the fired pieces and correlated with the electrical properties. A cursory examination was made on the effects of fabrication pressure, 1.25 to 15 tsi, on the electrical conductivity. Both the conductivity and positive temperature coefficient were found to increase with decreasing fabrication pressure.     

Kinetics of the α-to-α'H Polymorphic Phase Transition of Ca2SiO4 Solid Solutions

The α-to-α'_H transition of Ca₂SiO₄ solid solutions (C₂S(ss)) is a nucleation and growth process. This process was shown on time–temperature–transformation (TTT) diagrams for C₂S(ss) with different concentrations of foreign oxides (Na₂O, Al₂O₃, and Fe₂O₃). The kinetic cutoff temperature and the activation energy for growth of the α'_H phase increase steadily with increasing concentration of impurities. The activation energy for nucleation also increases above 950°C. The α'_H phase, which exists in equilibrium with the α phase at 1280°C, is formed at a maximum rate at around 1100°C regardless of the chemical composition. The TTT diagrams enable us to predict, as a function of cooling rate, the phase constitution of C₂S(ss) at ambient temperature.     

Synthesis of AlN Nanopowder from γ-Al2O3 by Reduction–Nitridation in a Mixture of NH3–C3H8

Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al₂O₃ using NH₃ and C₃H₈ as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH₃–C₃H₈ gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al₂O₃. The products synthesized at 1100°C for 120 min contained unreacted γ-Al₂O₃. The ²⁷A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO₄ resonance decreasing prior to the disappearance of the octahedral AlO₆ resonance. This suggests that the tetrahedral AlO₄ sites of the γ-Al₂O₃ are preferentially nitrided than the AlO₆ sites. AlN nanoparticles were directly formed from γ-Al₂O₃ at low temperature because of this preferred nitridation of AlO₄ sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al₂O₃ not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al₂O₃ contains both AlO₄ and AlO₆ sites, by contrast with α-Al₂O₃ which contains only AlO₆.     

The System Sodium Fluoride—Alumina Investigated by Quenching Methods

Alumina was found to react with sodium fluoride on fusion to produce sodium aluminate and cryolite according to the reaction 6NaF + 2Al₂2c₃= 3NaA10₂+ Na₃A1F₆. An insoluble sodium aluminate phase was observed under the polarizing microscope in samples quenched from as high as 1400°C. The equilibrium crystallization temperature of sodium fluoride in the presence of solid sodium aluminate was found to be slightly depressed with added alumina. A maximum lowering of 6°C was found for a starting alumina content of 5.4%. Further alumina additions resulted in the secondary precipitation of β-Al₂O₃. The shallow depression of the sodium fluoride crystallization temperature and the observed limited alumina solubility are attributed to the formation of cryolite. The composition of the liquid in equilibrium with sodium aluminate and sodium fluoride or sodium aluminate and β-alumina is represented in terms of the pseudo-ternary system NaF-Al₂O₃-Na₃A1F₆.     

Stability Domains of TiO2–Ti3O5–Ti2O3–TiC/Ti(CN) Systems During Reduction Process

The phase domain of Ti₃O₅–Ti₂O₃–Ti(CO) at 1580 K was determined from the formation energies of Ti(C _x O _y ), as calculated via the Gibbs–Duhem equation. An extensive Ti(CO) domain is attributed to the high affinity between TiC and TiO. The phase domain of Ti₃O₅–Ti₂O₃–Ti(CN) was obtained at 1673 K using the formation energies of Ti(C _x N _y ). This study shows that the stable region for Ti₂O₃ is significantly small in the Ti₃O₅–Ti₂O₃–Ti(CN) phase domain. It demonstrates the absence of TiO and Ti₂O₃ in the normal syntheses of TiC and Ti(CN) from TiO₂, respectively.     

Effect of Phosphorus Impurity on Tricalcium Silicate T1: From Synthesis to Structural Characterization

Alite is the major compound of anhydrous Portland cement: it is composed of tricalcium silicate Ca₃SiO₅ (C₃S) modified in composition and crystal structure by ionic substitutions. Alite is also the main hydraulic phase of cement and the most important for subsequent strength development. Using raw meals (rich in Ca₃P₂O₈) as alternative fuels in cement plants raises the question about the effect of phosphorus on C₃S and its consequences on reactivity with water. This paper deals with a systematic study of C₃S triclinic T₁ polymorph doped with P₂O₅ in the range 0–0.9 wt%. All the samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electron-microprobe analysis. The appearance of a phase rich in phosphorus is shown. It displays a structure derivative of the α'_H–Ca₂SiO₄ polymorph, noted α'_H–C₂S(P). As phosphorus content increases, C₃S is more and more decomposed into free lime and α'_H–C₂S(P). The α'_H phase was detected from 0.1 wt% P₂O₅ and located at the interfaces of C₃S grains. Two identification keys are proposed in order to highlight the α'_H–C₂S(P) phase: the XRD angular window at 2θ_Cu=32.8°–33.2° and a smooth aspect on SEM micrographs.     

Precipitation and Magnetic Behavior of Beta NaFeO2 in Glasses Along the Na2SiO3-Fe2O3 Join

The magnetic behavior of β-NaFeO₂ precipitated in compositions along the Na₂SiO₃-Fe₂O₃ join of the Na₂O-Fe₂O₃-SiO₂ system is reported from magnetic susceptibility data and Moessbauer studies. The anomalous magnetic properties observed are discussed in terms of the possibility of the presence of super-paramagnetism.     

Synthesis and Electrical Properties of Stabilized Manganese Dioxide (α-MnO2) Thin-Film Electrodes

Manganese dioxide (α-MnO₂) thin films have been explored as a cathode material for reliable glass capacitors. Conducting α-MnO₂ thin films were deposited on a borosilicate glass substrate by a chemical solution deposition technique. High carbon activities originated from manganese acetate precursor, (Mn(C₂H₃O₂)₂·4H₂O) and acetic acid solvent (C₂H₄O₂), which substantially reduced MnO₂ phase stability, and resulted in Mn₂O₃ formation at pyrolysis temperature in air. The α-MnO₂ structure was stabilized by Ba²⁺ insertion into a (2 × 2) oxygen tunnel frame to form a hollandite structure. With 15–20 mol% Ba addition, a conducting α-MnO₂ thin film was obtained after annealing at 600–650°C, exhibiting low electrical resistivity (∼1 Ω·cm), which enables application as a cathode material for capacitors. The hollandite α-MnO₂ phase was stable at 850°C, and thermally reduced to the insulating bixbyte (Mn₂O₃) phase after annealing at 900°C. The phase transition temperature of Ba containing α-MnO₂ was substantially higher than the reported transition temperature for pure MnO₂ (∼500°C).     

Effects of Transition-Metal Substitution on the Catalytic Properties of Barium Hexaaluminogallate

The effects of the substitution of transition-metal ions and/or reductant gases on the catalytic properties of barium hexaaluminogallate were investigated. Transition-metal-substituted hexaaluminogallates (BaM(Al,Ga)₁₁O₁₉, M = transition metal, Al/Ga = 9/3) were synthesized from aqueous metal nitrates and ammonium carbonate by the coprecipitation followed by crystallization at 1100°C. The direct NO _x reduction was observed over BaM(Al,Ga)₁₁O₁₉ to be around 10%. The NO _x removal activity of BaM(Al,Ga)₁₁O₁₉ powders was improved by addition of C₃H₆ as a reductant gas. Co-, Ni- and Cu-substituted BaM(Al,Ga)₁₁O₁₉ catalysts exhibited about 40% NO _x reduction with C₃H₆ in excess oxygen at a high space velocity of 10 000 h⁻¹. The NO _x reduction on Mn- and Fe-substituted BaM(Al,Ga)₁₁O₁₉ catalysts was less than 10% even in the presence of C₃H₆. The temperature of the effective NO _x reduction on BaM(Al,Ga)₁₁O₁₉ catalysts could be adjusted from 350° to 500°C by the selection of the transition-metal substitution in the catalysts. The catalysts hold high activities for NO _x reduction even at 500°C in water vapor produced in the combustion system of reductant gases.