Surface composition of anhydrous monocalcium aluminate,CaAl2O4

Electron spectroscopy (ESCA) and electron spin resonance (ESR) have been used to study the surface of synthetic calcium monoaluminate, CaAl₂O₄. There are differences between annealed and quenched samples; the surface of the annealed material has the expected composition, while that of the quenched specimen has an apparently non-stoichiometric surface with high aluminium levels. Aluminium-oxygen groups are probably associated with unpaired electrons which make up the deficiency in negative charge. Suggestions are made regarding substitution processes by various impurity ions.     

Effect of calcium lignosulfonate on tricalcium aluminate and its hydration products

In the tricalcium aluminate—calcium lignosulfonate—water system, evidence has been obtained for the formation of a complex and precipitation of a highly basic calcium lignosulfonate. The hexagonal calcium aluminate hydrate as well as the cubic aluminate hydrate irreversibly adsorb calcium lignosulfonate. Adsorption results in changes of surface area, morphology, thermal behaviour and rate of interconversions.     

The hydration of doped tricalcium aluminate

The limit of solid solution of iron in tricalcium aluminate prepared in an oxygen atmosphere increases with increasing temperature to give a solid solution of composition Ca₃Al_1.85Fe_0.15O₆ at 1325° C. The lattice parameter of the cubic unit cell increases from 1.5263 to 1.5289 nm. Iron-doping causes a significant retardation in the reactivity of the Ca₃A_ss hydrated in the presence of gypsum. Not only does the rate of disappearance of the C₃A_ss decrease as the iron content increases, but also the setting time of the paste, the time taken for the gypsum to be totally consumed, and the time when rapid conversion of the AFt phase into the AFm phase takes place all increase. These observations apply even more to samples with iron contents greater than the solid solution limit, due to the presence of CaO and C₄AF_ss in addition to C₃A_ss.Abbreviations C CaO - A Al₂O₃ - F Fe₂O₃ - ¯S SO₃ - H H₂O - ss solid solution     

Predicted calcium titanate solution mechanisms in calcium aluminoferrite and related phases

Computer simulation, using an ionic, Born-like model, is used to investigate the accommodation of titanium impurities in Ca-Al-Fe-O phases. Specifically, calcium titanate solution in Al₂O₃, Fe₂O₃, CF, C₂F, CA, C₂A, C₃A, and C₄AF (where C denotes CaO) is considered. The simulations predict that titanium impurities are found preferentially in the ferrite phases. At sufficiently high concentrations, the solution involves the formation of clusters containing 3 to 6 ions that strongly resemble the structure of the calcium titanate phase. The calculations reveal that the compensation mechanism varies appreciably over the range of compounds considered.     

Morphology and composition of strontium calcium aluminate matrix doped with Dy3+

Abstract

The aim of the present study was to determine the influence of doping rare earth ion on strontium calcium aluminate (CaO–SrO–SiO₂–Al₂O₃). Therefore, the authors have manufactured luminescent material consisting of 40CaO–5SrO–5SiO₂–50Al₂O₃ doped with Dy³⁺. The compositions have been selected on the basis of chemical stability. Five pellets were prepared with different calcination temperatures and times, namely 400 and 600°C for 1 and 2 h, in order to shed light on their luminescence behaviour. X-ray diffraction, energy dispersive X-ray analysis and scanning electron microscopy elaborate and characterise the formation of small particle of photoluminescent material in the phosphor matrix host material.     

Vinylphosphonic acid-modified calcium aluminate and calcium silicate cements

Cementitious materials in terms of calcium phosphate cements (CPC) were prepared through the acid-base reaction between vinylphosphonic acid (VPA) and calcium aluminate cement (CAC) reactants or calcium silicate cement (CSC) reactants at 25 °C. Using CAC, two factors were responsible for the development of strength in the cements: one is the formation of an amorphous calcium-complexed vinylphosphonate (CCVP) salt phase as the reaction product, and the other was the high exothermic reaction energy. Because the formation of CCVP depletes the calcium in the CAC reactants, Al₂O₃·xH₂O gel was precipitated as a by-product. CCVP amorphous calcium pyrophosphate hydrate (CPPH) and Al₂O₃·xH₂O -AlOOH phase transitions occurred in the CPC body autoclaved at 100 °C. Increasing the temperature to 200 °C promoted the transformation of CPPH into crystalline hydroxyapatite (HOAp). In the VPA-CSC system, the strong alkalinity of CSC reactant with its high CaO content served in forming the CPPH reaction product which led to a quick setting of the CPC at 25 °C. Hydrothermal treatment at 100 °C resulted in the CPPH HOAp phase transition, which was completed at 300 °C for both the VPA-CAC and VPA-CSC systems, and also precipitated the silica gel as by-product. Although the porosity of the specimens was one of the important factors governing the improvement of strength, a moderately mixed phase of amorphous CPPH and crystalline HOAp as the matrix layers contributed significantly to strengthening of the CPC specimens.     

Sintering study of calcium aluminate

Measurement of the initial sintering shrinkage of CaAl₂O₄ at temperatures of 1300, 1325 and 1350 °C are reported. The particle sizes chosen were –53 + 45, –63 + 53 and –75 + 63 microns and the soaking periods were from 15 to 360 min. A time dependence of the shrinkage has shown that volume diffusion is the dominant mechanism of sintering. At any given time and temperature, the per cent shrinkage was found to be a decreasing function of particle size. The activation energy for the sintering of CaAl₂O₄ was found to be 766.38 KJ mol^–1.     

Three-dimensional printing of flash-setting calcium aluminate cement

Three-dimensional indirect printing of flash-setting calcium aluminate cement (CAC) was investigated. Upon water injection into a biphasic mixture of tricalcium aluminate (3CaO·Al₂O₃) and dodecacalcium heptaaluminate (12CaO·7Al₂O₃) (phase ratio 0.56/0.44) initially a gel formed acting as a bonding phase which stabilizes the printed object geometry. Post-exposure in water finally resulted in the formation of 2CaO·Al₂O₃·8H₂O and 4CaO·Al₂O₃·19H₂O reaction phases as confirmed by SEM, X-ray diffraction, and FTIR analyses. Reduction of porosity by volume expansion upon hydrolysis reaction from 50% after printing to 20% after post-treatment gave rise for an increase of compressive strength from 5 to 20 MPa, respectively. A bone regenerating scaffold for a micro-vascular loop model was fabricated by 3D printing and hydraulic reaction bonding to demonstrate the potential of using flash-setting calcium aluminate cement powder for biomedical ceramic applications.     

Crystallization behaviour of calcium aluminate glass fibres

Crystallization behaviour of as-spun and fully-nucleated calcium aluminate (CA) glass fibres produced via inviscid melt spinning (IMS), was studied. Differential thermal analysis (DTA) scans on the as-spun and fully-nucleated CA fibres were performed at different heating rates. By applying the Kissinger method to the DTA scan data the activation energy values for crystallization were determined to be 569 and 546 kJ mol–1, respectively for the as-spun and fully-nucleated CA fibres. The Ozawa analysis on the DTA scan data gave the Avrami parameters at 2.7 and 2.4, respectively, for the as-spun and fully-nucleated CA fibres, which indicates high tendency of bulk crystallization mode. The formation of equilibrium phases of Ca12Al14O33 and CaAl2O4 in the crystallized CA fibres was identified by using X-ray diffraction (XRD).     

Electron spin resonance and thermoluminescence of tricalcium aluminate,Ca3Al2O6

Electron spin resonance (ESR) spectra and thermoluminescence (TL) curves and have been obtained for tricalciurn aluminate, Ca₃Al₂O₆. The ESR spectra and one peak in the TL spectrum can be rationalized in arms of electron trapping at Fe(III), present as impurity, in all of the possible tetrahedral sites, with holes on non-bridging oxygen atoms. Manganese (II), which is also present as an impurity, probably replaces calcium in only two out of six possible sites.     

Optical properties of hollow calcium aluminate glass waveguides

Calcium aluminate glass has a refractive index less than 1 at 10.6 μ, and therefore it is a good candidate for a hollow fiber for the transmission of CO(2) laser energy. We have drawn hollow calcium aluminate glass fibers with inner diameters ranging from 380 to 500 μ. The loss for our 500-μm inner-diameter hollow glass fibers measured at 10.6 μm is 8.6 dB/m.     

Macro-defect-free processing and low-frequency dielectric response of calcium aluminate cements

Eight commercial grade calcium aluminate cements were prepared macro-defect free by high shear mixing, lowering the water/cement ratio and using polyvinyl alcohol as a plasticizer. Samples for dielectric measurements were prepared by die pressing to form disks. Relative dielectric permittivity and dissipation factor were measured over the frequency range of 100 Hz to 1 MHz. Variations in frequency response and loss mechanism between the cements are related to bulk chemistry.     

Air-foamed calcium aluminate phosphate cement for geothermal wells

Air-foamed low-density calcium aluminate phosphate (CaP) cement slurry was prepared by mixing it with chemical foaming reagent at room temperature without any pressure, followed by autoclaving at 200 °C. When the porosity, compressive strength, and water permeability of the autoclaved CaP foam cement made from a 1.22 g/cc slurry density was compared with those of N₂ gas-foamed Class G cement made from a slurry of similar density under high pressure and hydrothermal temperature at 288 °C, the CaP cement revealed some advanced properties, such as a higher compressive strength and lower porosity. These advanced properties were due to the hybrid formation of three crystalline hydrothermal reaction products; hydroxyapatite, boehmite, and hydrogarnet phases. However, one shortcoming was an increase in water permeability because of the formation of an undesirable continuous porous structure caused by coalesced air bubble cells, suggesting that an appropriate lesser amount of foaming reagent be used to create a conformation in which fine discrete air-bubble cells are uniformly dispersed throughout the slurry. For non-foamed cement, three major factors contributed to protecting carbon steel against corrosion: (1) good adherence to steel, reflecting a high extent of coverage by the cement layer over the steel’s surfaces; (2) retardation of cathodic corrosion reactions; and, (3) minimum conductivity of corrosive ionic electrolytes. However, adding an excessive amount of foaming reagent did not offer as effective corrosion protection as that of non-foamed cement.     

A Mössbauer study of the high temperature reactions of iron oxides with tricalcium aluminate

Mössbauer spectroscopy and X-ray diffraction techniques were used to study the solid state reactions between tricalcium aluminate and both Fe₂O₃ and FeO (0.5 to 5.0 wt% iron oxide) in both oxidizing and reducing atmospheres. The iron oxides interact with the aluminate both by forming solid solutions (as evidenced by changes in the aluminate lattice parameter) and by the formation of separate ferrite and/or aluminoferrite phases. Under oxidizing conditions, solid solution of Fe³⁺ increases the aluminate lattice parameter, whereas under reducing conditions, the lattice parameter is decreased, suggesting some solid solution of Fe²⁺. The Mössbauer spectra of oxidized samples are typical of tetracalcium aluminoferrite and related ferrite phases, while under reducing conditions the spectra all show a pair of Fe³⁺ doublets characteristic of ferrite phases, and a pair of Fe²⁺ doublets suggesting four-fold co-ordination in distinctly different sites. Consideration of the possible ferrous sites in the aluminate and aluminoferrite systems suggests the ferrous ions might replace Al in the aluminate.     

Thermoluminescence dosimetry of rare earth doped calcium aluminate phosphors

The thermoluminescence (TL) properties of calcium aluminate (CaAl²O⁴) doped with different rare earth ions have been studied and their suitability for radiation dosimetry applications is discussed. It is observed that monocalcium aluminate doped with cerium is a good dosimeter having linear response up to about 4 kGy of radiation doses. Dopant concentration of 0.25 mol% cerium gives maximum TL emission. The well-defined single peak observed at 295°C can be advantageously used for high temperature dosimetry applications.