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.     

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).     

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.     

Sintering of biphasic calcium phosphates

Biphasic calcium phosphate (BCP) discs were fabricated and then sintered using two different sintering programs to establish whether the phases present could be controlled at low and high sintering temperatures. X-ray diffraction (XRD) was used to establish the phases present after sintering and scanning electron microscopy (SEM) determined the microstructure. Sintering program 1 involved a simple heating and cooling schedule and temperatures of 1100, 1250, 1275 and 1300°C. It produced samples containing an additional alpha-tricalcium phosphate (α-TCP) phase at temperatures above 1100°C. The original ratio of hydroxyapatite/beta-tricalcium phosphate (HA/β-TCP) could not be maintained above this temperature. Sintering program 2 combined the heating and cooling schedules of the first program with a 900°C hold stage to allow α-TCP to β-TCP conversion to take place. At temperatures of 1250 and 1275°C, this program was successful in completely removing the α-TCP phase and preserving the HA:β-TCP ratio. The SEM results show that the surface morphology of the discs was not greatly affected by choice of sintering program.     

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.     

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.     

Preparation of calcium aluminate matrix composites by combustion synthesis

CaO–Al₂O₃–TiB₂ composites have been produced by the Combustion Synthesis technique. These materials have matrices based on binary calcium-aluminate compounds, i.e., Ca₃Al₂O₆ (C₃A), Ca₁₂Al₁₄O₃₃ (C₁₂A₇), CaAl₂O₄ (CA), CaAl₄O₇ (CA₂) and CaAl₁₂O₁₉ (CA₆). Except for samples with the matrix composition of C₃A, the combustion synthesis reactions can be characterized as stable self-propagating waves with combustion temperatures ranging from 2125 K to 2717 K and combustion wave velocity from 4.0 mm/s to 10.6 mm/s. For samples with a matrix composition of C₁₂A₇, CA, and CA₂, predominantly equilibrium compound phase was formed, while for samples with a matrix composition of C₃A, non-equilibrium phases were also present. There was no evidence of CA₆ formation for samples with a matrix composition corresponding to CA₆.     

Surface composition of anhydrous tricalcium aluminate and calcium aluminoferrite

Electron spectroscopy and optoacoustic spectroscopy (OAS) have been used to study the surfaces of synthetic tricalcium aluminate, Ca₃Al₂O₆ and calcium aluminoferrite, Ca₂AIFeO₅. The surfaces of these compounds have compositions which differ markedly from those of the bulk. The surface of tricalcium aluminate is depleted in calcium and enriched in aluminium and also carries relatively stable hydroxyl groups, which can be detected by OAS. Calcium aluminoferrite has a surface enriched in aluminium and depleted in calcium and iron. Again, some hydroxyl groups are present on the surface. These differences in surface composition are discussed with particular reference to the early stages of reaction with water.     

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.     

Chemical activation of calcium aluminate cement composites cured at elevated temperature

The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH₁₀ and C₂AH₈ to form the strätlingite (C₂ASH₈). The later reactions prevent the strength loss by preventing the conversion of CAH₁₀ and C₂AH₈ to the cubic C₃AH₆ phase. The acceleration effect of Na₂SO₄ on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH₁₀ and C₂AH₈ phases.     

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.     

Chemical activation of calcium aluminate cement composites cured at elevated temperature

The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH₁₀ and C₂AH₈ to form the strätlingite (C₂ASH₈). The later reactions prevent the strength loss by preventing the conversion of CAH₁₀ and C₂AH₈ to the cubic C₃AH₆ phase. The acceleration effect of Na₂SO₄ on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH₁₀ and C₂AH₈ phases.     

Microstructure and properties of silicon nitride ceramics with calcium aluminate additions

Silicon nitride ceramics containing calcium aluminates as sintering aids have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and the effect of sintering aid content on their microstructure, phase composition, mechanical strength, and air oxidation resistance has been studied. The results demonstrate that the Si₃N₄ ceramic containing 10 wt % calcium aluminates has a uniform distribution of intergranular multicomponent oxide phases and consists of densely packed silicon nitride grains. Owing to this, it offers the maximum mechanical strength (850 MPa) and is stable to air oxidation up to 1300°C.     

An investigation of the hydration of calcium aluminate through ultrasonic wave propagation

The hydration process of a high alumina cement was investigated through ultrasonic wave propagation techniques. A correlation between wave amplitude and velocity and breaking strength of the cement is demonstrated. The wave behaviour through hydrating cements was found to be a function of the cement-to-water ratio between values of 2 and 4. Changes in the temperature of the hydrating samples were found to exhibit a similar dependence.