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.     

Properties of anti-washout-type calcium silicate bone cements containing gelatin

Novel washout-resistant bone substitute materials consisting of gelatin-containing calcium silicate cements (CSCs) were developed. The washout resistance, setting time, diametral tensile strength (DTS), morphology, and phase composition of the hybrid cements were evaluated. The results indicated that the dominant phase of β-Ca₂SiO₄ for the SiO₂–CaO powders increased with an increase in the CaO content of the sols. After mixing with water, the setting times of the CSCs ranged from 10 to 29 min, increasing with a decrease in the amount of CaO in the sols. Addition of gelatin into the CSC significantly prolonged (P < 0.05) the setting time by about 2 and 8 times, respectively, for 5% and 10% gelatin. However, the presence of gelatin appreciably improved the anti-washout and brittle properties of the cements without adversely affecting mechanical strength. It was concluded that 5% gelatin-containing CSC may be useful as bioactive bone repair materials.     

In vitro studies of calcium phosphate silicate bone cements

A novel calcium phosphate silicate bone cement (CPSC) was synthesized in a process, in which nanocomposite forms in situ between calcium silicate hydrate (C–S–H) gel and hydroxyapatite (HAP). The cement powder consists of tricalcium silicate (C₃S) and calcium phosphate monobasic (CPM). During cement setting, C₃S hydrates to produce C–S–H and calcium hydroxide (CH); CPM reacts with the CH to precipitate HAP in situ within C–S–H. This process, largely removing CH from the set cement, enhances its biocompatibility and bioactivity. The testing results of cell culture confirmed that the biocompatibility of CPSC was improved as compared to pure C₃S. The results of XRD and SEM characterizations showed that CPSC paste induced formation of HAP layer after immersion in simulated body fluid for 7 days, suggesting that CPSC was bioactive in vitro. CPSC cement, which has good biocompatibility and low/no cytotoxicity, could be a promising candidate as biomedical cement.     

Ceramic joining through reactive wetting of alumina with calcium aluminate refractory cements

Compositions in CaO-Al₂O₃ system have been prepared by gel-to-crystallite conversion method. Reactive powders of 1 : 2, 1 : 1, 2 : 1 and 3 : 1 of CaO and₂O₃ compositions were obtained by calcining the product at 800–1200°C. Fine grained powders were used as refractory cement for joining alumina ceramics. An optimum temperature of 1450°C for 4 h produced joints of satisfactory strength. The microstructure and X-ray phase analysis of the fractured joint surface clearly indicate reactive wetting of the alumina ceramics. This wetting enhances the joining of alumina substrates and can be attributed to the formation of Ca₁₂Al₁₄O₃₃ liquid phase. The results are explained by using CaO-Al₂O₃ phase diagram.     

Alkali carbonation of calcium aluminate cements: influence of set-retarding admixtures under hydrothermal conditions

The preferential uptake of aluminium ions by lactone and carboxylic acid groups in glucuronic-6,3-lactone and gluconic acid suggested that these organic admixtures have a high potential as set-retarding admixtures of high-temperature calcium aluminate cement slurry. However, the liberation of abundant free calcium ions caused by the adsorption of aluminium ions by the admixtures, increased the carbonation rate of hydrated cement pastes after exposure to Na₂CO₃-laden water at 300 C. Using inorganic acid admixtures, such as boric acid and sodium tetraborate decahydrate, the retarding ability of colloidal Ca(BO₂)₂ ·nH₂O and aluminium hydroxide yielded by the reaction between admixture and cement was less than that of the reaction products derived from organic acid admixtures. Although Ca(BO₂)₂ ·nH₂O in hot Na₂CO₃ solution was converted into CaCO₃, the rate of alkali carbonation was almost the same as that of admixture-free calcium aluminate cement pastes.     

Formation of chloroaluminates in calcium aluminate cements cured at high temperatures and exposed to chloride solutions

The formation of hexagonal chloroaluminates in mortar specimens pre-cured at 20, 40 and 60°C for two weeks and stored in a 0.5 M NaCl solution for up to 255 days has been studied. The appearance of this phase as a function of time has been monitored by X-ray diffraction. In addition, its microstructure has been observed by means of backscattering electron microscopy. The chemical composition was studied by X-ray microanalysis. The formation of chloroaluminate phases in reinforced concrete is related to the immobilization of chloride ions penetrating through the concrete to the reinforcement. Thus the formation of stable chloroaluminates could lower the risk of corrosion. In order to check this point, corrosion rate measurements were performed throughout the experiment. In spite of the high capacity of aluminous hydrates to react with chloride ions to form chloroaluminates, the remaining chloride ions in the pore solution leads over time to reinforcement corrosion. The presence of hexagonal phases in the cement paste ensure a better resistance against the penetration of chloride ions than when cubic phases are present. This effect was attributed to the denser microstructure exhibited, by samples containing the hexagonal phases. This revised version was published online in November 2006 with corrections to the Cover Date.     

Water uptake and loss in silicate cements

The water absorption of a dental silicate cement was measured both in de-ionized water and in pH controlled phosphate buffers of several ionic strengths. Uptake is found to be related to ionic strength. Water loss was studied using thermogravimetry. Water loss is seen to be a continuous process from ambient temperatures to in excess of 500° C. The implications of the findings are considered not only from a clinical viewpoint but also in relation to past reports of solubility of this material which were uncorrected for water uptake. The usefulness of the concepts of evaporable and non-evaporable water are considered and the results are examined to assess the importance of drying temperature in carrying out solubility measurements.     

Performance of sewer pipe concrete mixtures with portland and calcium aluminate cements subject to mineral and biogenic acid attack

The paper reports on the performance of a series of sewer pipe concrete mixtures and cementitious lining mixtures in acid environments. Binder types based on ordinary portland cement (OPC) and calcium aluminate cement (CAC) were used, with both acid-soluble and acid-insoluble aggregates and various supplementary cementitious materials (SCM). One series of tests subjected the mixtures to pure mineral acid (hydrochloric acid, pH = 1), using a specially designed dynamic test rig. The other series of tests involved monitoring specimens placed in a live sewer under very aggressive conditions induced by acid-generating bacteria. Under mineral acid attack on concretes with conventional dolomite aggregates, OPC/silica fume concretes displayed best performance, attributed to their densified microstructure coupled with substantially improved ITZ. CAC concretes with dolomite aggregate did not perform any better than similar OPC specimens under these conditions, primarily because of their higher porosity. However, with concretes using synthetic alag^TM aggregates in mineral acid testing, CAC/alag^TM mixtures performed exceptionally well due to their homogeneous microstructure, inferred absence of an ITZ, and slower dissolution and finer size of alag^TM aggregate particles. The dynamic acid test was able to reveal differences in physical and chemical interactions between constituents in concrete mixes. Under biogenic acid conditions in the sewer, CAC concretes clearly outperformed OPC concretes. This is ascribed to the ability of CAC to stifle the metabolism of the acid-generating bacteria, thereby reducing acid generation. Thus the effects of neutralisation capacity and stifling of bacterial activity need to be distinguished in designing concrete mixtures to provide good acid resistance. Relative rates of dissolution of binder and aggregates are also important in overall performance, with uniform rates preferable in order to avoid aggregate fallout.     

The formation and microstructure of dental silicate cements

The chemistry of the cement-forming reaction between phosphoric acid solutions and fluorine-containing aluminosilicate glasses has been studied using a variety of physicochemical methods. These materials are the dental silicate cements and the microstructures of a number of these cements have been examined by optical, electron and scanning electron microscopy. The element distribution between partially reacted glass particles and the gel matrix which binds them has been determined by electron probe microanalysis. The chemical stability and variations in the mechanical strength between different cements is discussed in terms of a new hypothesis on the constitution of the gel matrix. Consideration is given to the possible strengthening role of microcrystallinity which has been shown by X-ray and electron diffraction to develop in the matrix. Attempts to strengthen the cement by incorporation of reinforcing agents is described.     

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.     

Microsphere-filled lightweight calcium phosphate cements

The incorporation of inorganic and organic microsphere fillers into calcium phosphate cement (CPC) to produce lightweight cementitious materials that could be used under hydrothermal conditions at high temperatures between 200 and 1000 °C was investigated. An aluminosilicate based hollow microsphere, with a density of 0.67 gcm^–3 and a particle size of 75–200 m, was the most suitable having a low slurry density of 1.3 gcm^–3, and a compressive strength greater than 6.89 M Pa. This microsphere-filled lightweight CPC exhibited the following characteristics: 1. after autoclaving at 200 °C, amorphous ammonium calcium orthophosphate (AmCOP) salt and Al₂O₃·xH₂0 gel phases, formed by the reaction between calcium aluminate cement and an NH₄H₂P0₄ based fertilizer, were primarily responsible for the development of strength; 2. at a hydrothermal temperature of 300 °C, the microsphere shell moderately reacted with the CPC to form an intermediate reaction product, epistilbite (EP), while crystalline hydroxyapatite (HOAp) and boehmite (BO) were yielded by the phase transformations of AmCOP and Al₂O₃·xH₂O, respectively; 3. at an annealing temperature of 600 °C, the HOAp phase remained in the cement body, even though an EP anorthite (AN) phase transition occurred; 4. at 1000 °C, the phase conversion of HOAp into whitlockite was completed, while the AN phase was eliminated; and 5. the microsphere demonstrated excellent thermal stability up to temperatures of 1000 °C.This work was performed under the auspices of the US Department of Energy, Washington, DC, under Contract No. DE-AC02-76CH00016.     

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).     

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.     

Growth and scintillation properties of Yb doped aluminate, vanadate and silicate single crystals

For the purpose of quick screening for charge transfer (CT) transitions of Yb³⁺ in various hosts, (Lu_1−xYb_x)₃Al₅O₁₂ (Yb:LuAG) with x=0.05, 0.15, 0.30 and (Y_1−xYb_x)AlO₃ (Yb:YAP) with x=0.05, 0.10, 0.30 were grown by the micro-pulling-down method. (Y,Yb)VO₄ with strong wetting was grown by edge defined film-fed growth method and materials, which require moderate temperature gradient, such as Ca₈(La,Yb)₂(PO₄)₆O₂ and (Gd,Yb)₂SiO₅ were grown by Czochralski method. Strong dependence of the CT luminescence decay time and intensity on temperature was observed for Yb-doped LuAG and YAP. Super fast decay with 0.85 ns decay time was observed in Yb(30%) doped YAP at room temperature. Though the emission intensity is weak at room temperature, it exceeds several times that of PbWO₄. In addition, CT luminescence of Yb:YAP occurs at longer wavelength than in BaF₂, which enables the usage of glass-based photomultiplier for the detection. In addition, higher stopping power will be expected due to the higher density host compared with BaF₂.