Vibrational spectroscopic studies on crystallisation of sol–gel derived thin films of calcia–alumina binary compound

An optimized sol–gel process has been developed to produce homogeneous thin films of calcium aluminate binary (12CaO·7Al₂O₃) compound, on magnesium oxide substrates via spin coating. Fourier transform infrared and Raman spectroscopies have been employed to investigate the effect of annealing temperature and duration on the phase transformations in the films. Heat treatment at 1,300 °C under air atmosphere for 2 h produced single-phase 12CaO·7Al₂O₃ films. However, annealing at a lower temperature of 1,100 °C in air for a period of 4 h in total resulted in the crystallization of 5CaO·3Al₂O₃ rather than 12CaO·7Al₂O₃. The X-ray photoelectron spectrum of the thin film annealed at 1,300 °C corresponds to the binding energies of C12A7 compound. The annealing temperature of 1,300 °C for 2 h is found to be sufficient for formulating single phase calcia–alumina binary films in correct stoichiometric ratio of 12:7.     

Phase relation of ternary system ZrO2CaOAl2O3

The phase relation in the ternary system ZrO₂CaOAl₂O₃ has been studied at 1,380°C in the composition range below 50 mol% CaO. The mixed oxides with various compositions have been prepared from their acidic solutions through evaporation, calcination, and heat-treatment. Nine regions have been found. In the stabilized zirconia with fluorite structure, 4 mol% Al₂O₃ is soluble. The fluorite phase also exists in equilibrium in two-phase regions (fluorite + ZrO₂, and fluorite + CaO·ZrO₂) and in three-phase regions (fluorite + ZrO₂ + CaO·2Al₂O₃, and fluorite + CaO·ZrO₂ + CaO·2Al₂O₃).     

Effect of barium hydrosilicates on the early hydration rate of Portland cement

We have studied the effect of a barium hydrosilicate-based modifier on the phase composition of Portland cement hydration products. The results demonstrate that the addition of a modifier containing barium hydrosilicates, silicic acid, and calcium carbonate makes it possible to reduce the nucleation rate of crystals of new phases during the induction period of the hydration process, increase alite (3СaO · SiO₂) hydration, and reduce the rate of aluminate phase (3CaO · Al₂O₃) hydration. The use of such a modifier increases the degree of cement hydration and the amount of calcium hydrosilicates and reduces the amount of forming portlandite and ettringite, thereby improving the mechanical properties and durability of the set cement.     

Novels patterns of hydration and new compounds in the ternary system of CaO-Al2O3-P2O5

 The patterns of hydrating and solidifying with the compositional variation of phosphorus-rich, phosphorus-calcium-rich and aluminum-calcium rich regions in ternary system CaO-Al₂O₃-P₂O₅ has been studied in detail, and two new ternary compounds L and H have been synthesized here. The results indicate that the region of 48–56% P₂O₅ doesn’t present cementitiousness, which contains mainly crystal phases of β-C₂P(2CaO·P₂O₅), α-C₃P(3CaO·P₂O₅) and AlPO₄; the phosphorus-calcium-rich region of 21–35% P₂O₅ exhibits substantial cementitiousness, which contains mainly crystal phase of α-C₃P and certain amount of CA(CaO·Al₂O₃) and new phases L/H; and the aluminum-calcium-rich region of 8–18% P₂O₅ is full of promise for cementitiousness. It contains mainly new crystal phase L and certain amount of α-C₃P and CA. The hydration and solidification mechanisms have been preliminarily analyzed by means of XRD, XPS and DTA. It appears that crystal phase CA might hydrate directly to the stable phase of C₃A·6H₂O in the phosphorus-rich case of 21–35% P₂O₅; new phase H has the behavior of rapid setting; and L, being a dominant phase, can prevent cement pastes from significant strength loss in long curing cycles. Received: 12 March 1998 / Accepted: 29 July 1998     

The effects of different precipitant agents on the formation of alumina-magnesia composite powders as the magnesium aluminate spinel precursor

Al₂O₃/MgO composite powders were synthesized via a partially wet chemical method. The effects of precipitant agent on the morphology, size and chemical composition of the resultant powders were investigated. The structures of rod-like with polygonal prism surface, platelet-like and uniform spherical Mg-compound particles were successfully prepared by using ammonium hydrogen carbonate, sodium hydroxide and ammonia water as precipitant agents, respectively. Analysis results proved that using ammonium hydrogen carbonate as precipitant agent produced nesquehonite (MgCO₃·3H₂O) Mg-compounds but in the case of other two precipitant agents, Mg-compound particles with magnesium hydroxide (Mg(OH)₂) chemical composition were obtained. The morphological features of MgO particles in Al₂O₃/MgO composite powders were the same as individual Mg-compound particles. Furthermore, conversion of the Al₂O₃/Mg-compound precursor synthesized with ammonia water to pure magnesium aluminate spinel particles was studied. The precursor converted to pure magnesium aluminate spinel phase with 220?nm particle size at 1200?°C.     

Doped calcium–aluminium–phosphate cements for biomedical applications

Calcium–aluminium–phosphate cements (CAPCs) for biomedical applications, mainly intended for applications in the dental field as non-resorbable fillers, were obtained by reacting Ca-aluminates compounds, i.e. CaO·Al₂O₃ (CA) and CaO·2 Al₂O₃ (CA₂), with Al(H₂PO₄)₃ aqueous solution. Hydroxyapatite was also introduced as a bioactive dispersed phase. Suitable elements like Sr and La were used to increase the radiopacity of the set yielded pastes towards X-ray wavelength used in clinical diagnostic radiographic equipments. La and Sr doped Ca-aluminates powders have been synthesized by solid state reaction at 1,400°C from a mixture of CaCO₃, Al₂O₃, La₂O₃ and SrCO₃. The characteristics of the obtained powders were analyzed and related to the starting compositions and synthesis procedures. The microstructure, setting time, radiopacity and compressive strength of the CAPCs have been investigated and discussed.     

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.     

Secondary electron emission and glow discharge properties of 12CaO·7Al2O3 electride for fluorescent lamp applications

Abstract

12CaO·7Al₂O₃ electride, a sub-nanoporous compound having a work function of 2.4 eV, was examined as a candidate cathode material in fluorescent lamps. The electron emission yield was higher and the discharge voltage was lower for 12CaO·7Al₂O₃ than for existing cathode materials such as Ni, Mo or W; therefore, the energy consumption of the fluorescent lamps can be improved using 12CaO·7Al₂O₃ cathodes. Prototype glow-discharge lamps using 12CaO·7Al₂O₃ were constructed and exhibited reasonable durability.     

Tricalcium aluminate hydration: Microstructural observations by in-situ electron microscopy

Environmental scanning electron microscopy (ESEM) was used to study the microstructural changes that take place during the hydration of tricalcium aluminate (C₃A) in the absence and presence of gypsum (CS¯H₂; where A = Al₂O₃, C = CaO, H = H₂O, S¯ = SO₃). The ESEM proves to be a valuable tool in the observation of cement hydration and no specialised equipment other than the ESEM is required. The hydration process can be observed at any time without the need to halt the hydration process prior to specimen preparation. Subsequently, artefacts associated with specimen preparation, such as water loss and desiccation, are now avoided. In the absence of sulphate, amorphous gel, poorly crystalline hexagonal calcium aluminate hydrate (? C₄AH₁₉) and cubic calcium aluminate hydrate (C₃AH₆) are observed on the surface of C₃A grains. When small amounts of sulphate (2% gypsum) are present the same phases are observed. If larger amounts of sulphate (25% gypsum) are added to the system amorphous gel products, crystalline ettringite (C₆AS¯₃H₃₂) and monosulphate (C₄AS¯H₁₂) are observed. The crystalline products grow both within the amorphous gel and, where space allows, in interstices suggesting a through solution mechanism of transport.     

Calcium aluminate cements in fly ash/calcium aluminate blend phosphate cement systems: Their role in inhibiting carbonation and acid corrosion at a low hydrothermal temperature of 90°C

Study was focused upon formulating sodium polyphosphate-modified fly ash/calcium aluminate blend (SFCB) geothermal well cements with advanced anti-carbonation and anti-acid corrosive properties. At a low hydrothermal temperature of 90°C, to improve these properties, we investigated the effectiveness of various calcium aluminate cement (CAC) reactants in minimizing the rate of carbonation and in abating the attack of H₂SO₄ (pH 1.6). We found that the most effective CAC had two major phases, monocalcium aluminate (CA) and calcium bialuminate (CA₂), and a moderate CaO/Al₂O₃ ratio of 0.4. The reaction between sodium polyphosphate (NaP) and CA or CA₂ at room temperature led to the formation of amorphous dibasic calcium phosphate hydrate and anionic aluminum hydroxide caused by the decalcification of CA and CA₂. When SFCB cement made with this CAC was exposed to 4% NaHCO₃-laden water at 90°C, some carbonation of the cement occurred, forming calcite that was susceptible to the reaction with H₂SO₄. This reaction resulted in the deposition of gypsum gel scales as the acid corrosion product on the cement surfaces. The scale layer clinging to the cement protected it from further corrosion. Under such protection, the amorphous dibasic calcium phosphate hydrate crystal hydroxyapatite and anionic aluminum hydroxide crystal boehmite phase transitions were completed in acid solution. Meanwhile, the further chemical and hydration reactions of NaP with fly ash led to the formation of additional crystalline Na-P type zeolite phases. Thus, we propose that passivation of the surface of the cement by deposition of gypsum, following the formation of these reaction products, which are relatively inert to acid, are the acid corrosion-inhibiting mechanisms of the SFCB cements.     

Sintering behaviour of hydroxyapatite reinforced with 20 wt % Al2O3

Hydroxyapatite reinforced with 20 wt% Al₂O₃ was sintered at 1200, 1300 and 1400°C. The sintering behaviour was investigated using X-ray diffraction and electron microscopy. Calcium aluminate phases were observed to be produced by the solid-state reaction between Al₂O₃ and hydroxyapatite. As a hard phase these phases influence the densification of hydroxyapatite with 20 wt% Al₂O₃ due to different sintering rates between the matrix and calcium aluminate.     

The effect of fillers on chloropolyester blend CPB 4 and inhibition of double-base propellants

A direct bonding inhibition system based on chloropolyester blend CPB 4 has recently been developed for inhibition of double-base rocket propellants. The effect of alumina trihydrate (Al₂O₃ · 3H₂O) and antimony trioxide (Sb₂O₃) on various properties of CPB 4 has been studied, such as effect on gel time, exotherm peak temperature, tensile strength, per cent elongation, water absorption, nitroglycerine absorption, heat resistance, and flame retardant characteristics. The data on flame retardance and other characteristics indicate that CPB 4 with 30% Al₂O₃ · 3H₂O and CPB 4 with 20% Sb₂O₃ are suitable for inhibition of double-base propellants. The double-base propellant sustainers (containing 2-NDPA) have been inhibited with CPB 4 containing 30% Al₂O₃ · 3H₂O and CPB 4 containing 20% Sb₂O₃ separately (without the application of any barrier coating), and statically fired at ambient, sub-zero (after conditioning at ?40°C for 18 h) and high (after conditioning at +50°C for 18 h) temperatures. The pressure—time profiles were found to be smooth and flat in all cases, indicating that the pressure level was the same throughout the combustion duration, inferring the suitability of these systems for inhibition of double-base propellants without the application of any barrier coating.     

Characterization of interfacial reaction products in alumina fiber/barium zirconate coating/alumina matrix composite

Barium zirconate (BaZrO₃) is a candidate material for interface engineering of alumina fiber/alumina matrix composites. Al₂O₃ and BaZrO₃ react at high temperatures to form a series of reaction products. The objective of this work was to characterize these reaction products and to investigate their effect on crack propagation. BaZrO₃ coating was applied on Al₂O₃ fibers via a sol–gel route. The characterization was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The reaction products between BaZrO₃ coating and Al₂O₃ (fibers and/or matrix) included ZrO₂, barium monoaluminate (BaO·Al₂O₃) and Ba β-Al₂O₃ (BaO·7.3Al₂O₃). In hot-pressed composites, barium aluminate was mainly present in the form of Ba β-Al₂O₃. The Ba β-Al₂O₃ is known to have a layer-type structure, which is likely to be propitious for crack deflection. The interfacial reactions are diffusion-controlled solid-state processes, mainly depending on the diffusion of Ba cations. The Ba β-Al₂O₃ phase can form either through direct reaction between the BaZrO₃ coating and Al₂O₃ or through indirect reaction between a BaO·Al₂O₃ intermediate phase and Al₂O₃. The BaZrO₃ coating and the reaction products between the coating and fiber/matrix provide multiple weak interfaces, which are likely to result in crack deflection and thus toughness enhancement in an all oxide composite.     

Effects of crystallinity and silica content on the hydration kinetics of 12CaO · 7Al2O3

The effects of 12CaO₇ · Al₂O₃ crystallinity on the kinetics of hydration reaction were studied by measuring the heat liberated using a conduction calorimeter. The crystallinity of the sample was modified by changing the cooling rate of the sample after synthesis reaction. In addition, the effects of silica addition to 12CaO₇ · Al₂O₃ glass on kinetics were also investigated. The results indicated that the glass underwent a faster initial kinetic hydration reaction compared with that of crystalline calcium aluminate. The addition of silica, on the other hand, decreased the reaction rates. The results are discussed in terms of the solvation rate of the calcium aluminate phases and the precipitation of the hydrate phases.     

Optimization of material characteristics of macro-defect free cement

Macro-defect free (MDF) cement is a high-strength cement-polymer composite produced by mixing cement (commonly calcium aluminate cement) with small amounts of polymer (commonly polyvinyl alcohol acetate) and water, applying high shear, and finally applying relatively low pressure (about 5 MPa) and modest temperature (about 80-100 °C). However, MDF cements lose considerable strength when exposed to water. The objective of this study was to explore the effects of cement and polymer compositions on flexural strength and water sensitivity. Calcium aluminate cements were used with Al₂O₃ contents between 42% and 79%. Production of MDF cement was successful with all cements, but the highest strength (268 MPa) was obtained with 70% Al₂O₃ cement. Secondly, PVAs were used that differed in their degree of hydrolysis between 73% and 99%. Of these, the one with a moderate degree of hydrolysis produced the highest strength (228 MPa). All mixtures had strength loss on exposure to water, but PVAs with moderate degrees of hydrolysis exhibited the lowest strength losses (50-60%).     

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.     

Aluminium-27 solid state NMR spectroscopic studies of chloride binding in Portland cement and blends

Alumina-rich pozzolanic and latent hydraulic binders such as pulverised fuel ash, metakaolin, and ground granulated blast furnace slag, together with silica fume, are frequently added to Portland cement concrete to improve performance and to retard chloride ingress and thereby inhibit chloride-induced corrosion of the carbon steel reinforcement. ²⁷Al{¹H} MAS and CP/MAS NMR spectroscopies have been used to follow both the hydration processes of the cement blends and the interactions of chloride ion with the hydrated aluminium species. The spectra of the hydrated aluminate phases were interpretable on the basis that the AFt (Aluminate Ferrite tri-) phase ettringite, C ₆ A H ₃₂(3CaO·Al₂O₃·3CaSO₄·32H₂O, or C ₃ A·3CaSO₄·32H₂O), and the AFm (Aluminate Ferrite mono-) phases calcium mono-sulphoaluminate, C ₄ A H ₁₂ (3CaO·Al₂O₃·CaSO₄·12H₂O, or C ₃ A·CaSO₄·12H₂O), and the lamellar tetracalcium aluminate hydrate, C ₄ AH ₁₃ (3CaO·Al₂O₃·Ca(OH)₂·12H₂O, or C ₃ A·Ca(OH)₂·xH₂O) were present as the only hydrated species containing octahedrally-coordinated aluminium. In all cases, only the AFm phase Friedel's salt (3CaO·Al₂O₃·CaCl₂·10H₂O, or C ₃ A·CaCl₂·10H₂O) could be identified as the major chloroaluminate phase produced by the interactions of the cement pastes with chloride ion.     

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.     

Preparation of lithium aluminosilicate glass-ceramic monolith from metal-alkoxide solution

Crack-free dried gel monoliths of the composition Li₂O · Al₂O₃ · 4SiO₂ have been prepared as a precursor of lithium aluminosilicate glass-ceramics by the hydrolysis and polycondensation of mixed metal alkoxides in solutions containing LiOCH₃, Al(O-secC₄H₉)₃, Si(OC₂H₅)₄, alcohols and water. Higher concentrations of water in the starting solution, close control of reaction conditions and slow drying rates produced crack-free gel monoliths of cylinder shape, 37 mm in diameter and 35 mm in height. Properties of the gel monolith and their change with heating have also been examined.     

AlPO4 solid solutions containing Ca and Zn phosphates

Portions of an Al₂O₃·3H₂OH₃PO₄CaO compositional triangle were examined at 300°C for a solid solution of Ca (presumably in the form of Ca(H₂PO₄)₂) in the Cristobalite AlPO₄ crystal structure. X-ray diffraction patterns of the various compositions showed the existence of such a solid solution including a lattice parameter change. The limit of the solid solution was approximated at composition 2CaO, Al₂O₃·3H₂O, 6H₃PO₄. This solid solution, however, proved to be unstable at 800°C, but was stabilized by the addition of $\text{1}\text{4}$ mole ZnO at the limit composition.Three points in an Al₂O₃·3H₂OH₃PO₄ZnO compositional triangle were also examined and evidence of solid solution found including a lattice parameter change.