Quantitative Aspects of 27Al MAS NMR of Calcium Aluminoferrites

²⁷Al MAS NMR spectra of synthetic calcium aluminoferrites, Ca₂Al_xFe_2−xO₅ with x = 0.93, 1, 1.33, reveal only a few percent of the expected intensity for the ²⁷Al central transition, indicating that the calcium aluminoferrite phase in Portland cements can barely be observed by ²⁷Al MAS NMR. This result supports the use of ²⁷Al MAS NMR for quantitative analysis of the tricalcium aluminate phase in Portland cements.     

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.     

Synthesis and hydration study of Portland cement components prepared by the organic steric entrapment method

The four components of Portland cement; dicalcium silicate (Ca₂SiO₄), tricalcium silicate (Ca₃SiO₅), tricalcium aluminate (Ca₃Al₂O₆), and tetracalcium aluminate iron oxide (Ca₄Al₂Fe₃O₁₀), were made by the PVA complexation process. Powders prepared by this new method can make relatively high yields of pure, synthetic, cement components of nano or sub-micron crystallite dimensions, high specific surface area, and extremely high reactivity at relatively low calcining temperatures in comparison with conventional methods. The above advantages can enhance setting speed, increase strength, and lead to other desirable characteristics of Portland cement. Optimum synthesis conditions, such as PVA content, degree of polymerization of the PVA, and calcination temperature, were determined for each component. Hydration speed and strength of the synthesized, mixed cement paste were also studied at room temperature (25°C). The powders and hydration behavior were characterized by microstructural examination (XRD, SEM) and specific surface areas were measured by nitrogen gas adsorption BET. DSC and Instron were used to study setting speed and compression strength.     

Characterization of surface oxide films on titanium and bioactivity

Biological properties of titanium implant depend on its surface oxide film. In the present study, the surface oxide films on titanium were characterized and the relationship between the characterization and bioactivity of titanium was studied. The surface oxide films on titanium were obtained by heat-treatment in different oxidation atmospheres, such as air, oxygen and water vapor. The bioactivity of heat-treated titanium plates was investigated by immersion test in a supersaturated calcium phosphate solution. The surface roughness, energy morphology, chemical composition and crystal structure were used to characterize the titanium surfaces. The characterization was performed using profilometer, scanning electronic microscopy, ssesile drop method, X-ray photoelectron spectroscopy, common Bragg X-ray diffraction and sample tilting X-ray diffraction. Percentage of surface hydroxyl groups was determined by X-ray photoelectron spectroscopy analysis for titanium plates and density of surface hydroxyl groups was measured by chemical method for titanium powders. The results indicated that heat-treatment uniformly roughened the titanium surface and increased surface energy. After heat-treatment the surface titanium oxide was predominantly rutile TiO₂, and crystal planes in the rutile films preferentially orientated in (1 1 0) plane with the highest density of titanium ions. Heat-treatment increased the amount of surface hydroxyl groups on titanium. The different oxidation atmospheres resulted in different percentages of oxygen species in TiO₂, in physisorbed water and acidic hydroxyl groups, and in basic hydroxyl groups on the titanium surfaces. The immersion test in the supersaturated calcium phosphate solution showed that apatite spontaneously formed on to the rutile films. This revealed that rutile could be bioactivated. The analyses for the apatite coatings confirmed that the surface characterization of titanium has strong effect on bioactivity of titanium. The bioactivity of the rutile films on titanium was related not only to their surface basic hydroxyl groups, but also to acidic hydroxyl groups, and surface energy. Heat-treatment endowed titanium with bioactivity by increasing the amount of surface hydroxyl groups on titanium and its surface energy.     

Carbonation of ternary building cementing materials

The carbonation processes of ettringite and calcium aluminate hydrates phases developed by hydration of calcium aluminate cement, fly ash and calcium sulphate ternary mixtures have been studied. The hydrated samples were submitted to 4% of CO₂ in a carbonation chamber, and were analysed, previous carbonation and after 14 and 90 days of carbonation time, by infrared spectroscopy and X-ray diffraction; the developed morphology was performed with the 14 days carbonated samples. The results evidenced that ettringite reacts with CO₂ after 14 days of exposition time and evolves totally at 90 days; the developed hydrated phases C₃AH₆ in samples with major CAC content, also reacts with CO₂. Due to carbonation, calcium carbonate – mainly vaterite but also aragonite-, depending on the initial formulation, aluminium hydroxide and gypsum were detected.     

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.     

Carbonation of ternary building cementing materials

The carbonation processes of ettringite and calcium aluminate hydrates phases developed by hydration of calcium aluminate cement, fly ash and calcium sulphate ternary mixtures have been studied. The hydrated samples were submitted to 4% of CO₂ in a carbonation chamber, and were analysed, previous carbonation and after 14 and 90 days of carbonation time, by infrared spectroscopy and X-ray diffraction; the developed morphology was performed with the 14 days carbonated samples. The results evidenced that ettringite reacts with CO₂ after 14 days of exposition time and evolves totally at 90 days; the developed hydrated phases C₃AH₆ in samples with major CAC content, also reacts with CO₂. Due to carbonation, calcium carbonate – mainly vaterite but also aragonite-, depending on the initial formulation, aluminium hydroxide and gypsum were detected.     

Dissolution and re-crystallization processes in multiphase silicon stabilized tricalcium phosphate

Ultrasonically accelerated dissolution of multiphase silicon stabilized tricalcium phosphate powders in water or Earle’s balanced salt solution transforms the powders into needle-like calcium deficient apatite crystals with the c-axis (001) oriented along the needle. Ion exchange with the solution occurs primarily in the first hours of immersion. The transformation is driven by an interaction between the crystal surface and adsorbed water leading to the growth of crystallites which have the most stable surface configuration. First principles calculations of the surface energies of various hydroxyapatite surfaces with and without adsorbed water shows that depending on the ion concentrations in the fluid that determine the chemical potential of tricalcium phosphate, either Ca-rich (010) or stoichiometric (001) layers are the dominant surfaces. The higher the chemical potential, the more elongated in the (001) direction the crystallites become to minimize the total surface energy. The loss of a calcium Ca²⁺ compensated by the addition of two H⁺ is strongly favoured energetically on the (001) and Ca-rich (010) surfaces. A high concentration of excess Si at grain boundaries may be partly responsible for the rapid transformation of multiphase Si-TCP.     

Preparation of Nanocrystalline Calcium Aluminate Powders

Nanocrystalline calcium aluminate (CaAl₂O₄and Ca₃Al₂O₆) powders have been prepared by pyrolysis of complex compound of aluminium and calcium with triethanolamine (TEA). The soluble metal ion-TEA complex forms the precursor material on complete dehydration of the mixtures of the complex of calcium-TEA and aluminium-TEA. The single-phase CaAl₂O₄and Ca₃Al₂O₆powders have resulted after heat treatment at 800° and 1000°C, respectively. The precursors and the heat-treated final powders have been characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis TG/DTA, and transmission electron microscopy (TEM). The average particle sizes as measured by transmission electron microscopy studies are around 30–40 nm and 25–30 nm for CaAl₂O₄and Ca₃Al₂O₆powders, respectively. These nano-sized powders are very suitable for making various nano-composites with polymers and inorganic compounds.     

Studies of early growth mechanisms of hydroxyapatite on single crystalline rutile: a model system for bioactive surfaces

Previous studies have shown that crystalline titanium oxide is in vitro bioactive and that there are differences in the HA formation mechanism depending on the crystalline direction of the titanium oxide surface. In the present study, the early adsorption of calcium and phosphate ions on three different surface directions of the single-crystal rutile TiO₂ substrate has been investigated. A crucial step in the nucleation of HA is believed to be the adsorption of Ca²⁺ and PO₄ ³⁻ from phosphate buffer solutions. The (001), (100) and (110) single crystalline rutile surfaces were soaked in phosphate buffer saline solution for 10 min, 1 h and 24 h at 37°C. The surfaces were then analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). The results show that the adsorption of Ca²⁺ and PO₄ ³⁻ is faster on the (001) and (100) surfaces than on the (110) surface. This study also shows that TOF-SIMS can be used as a tool to better understand the adsorption of calcium and phosphate ions and the growth mechanism of HA. This knowledge could be used to tailor new bioactive surfaces for better biological reaction.     

A Multiphase Model for the Early Stages of the Hydration of Retarded Oilwell Cement

Cement is used in the oil industry to line oil wells. The major components of oilwell cement are tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A) and calcium sulphate (CaSO₄). With the exception of C₂S, each of these plays an important role in the initial thickening of cement slurry. It is important to control the time that it takes for a slurry to thicken, and this is achieved by the addition of chemical retarders, which delay the onset of thickening. In this paper, the action of a retarder whose effects are firstly, to form a complex with calcium ions, and secondly, to inhibit the growth of ettringite crystals is investigated. Ettringite is a product of the hydration of C₃A and the subsequent reaction of the products with calcium sulphate. A modified version of a model for the hydration of C₃S previously investigated by Salhan, Billingham and King (J. Engng. Math. 45 (2003) 367), along with the chemical-kinetic scheme for the action of a retarder on ettringite proposed by Billingham and Coveney (J. Chem. Soc. Faraday Trans. 89 (1993) 3021) is used. The model distinguishes between liquid and solid phases, and treats water, which is significantly depleted by the formation of ettringite, as one of the chemical constituents. It is found that both of the chemical actions of the retarder contribute to slowing the initial reaction rate, and that the sudden crystallisation of ettringite as the effect of the retarder is overcome, investigated by Billingham and Coveney, occurs in successive layers around the surface of the cement grain A.C King: Professor King died on 14 January 2005     

Redox behaviour of copper mordenite zeolite

Fourier transform-infrared photoacoustic spectroscopy (FT-IR/PAS) and X-ray diffraction (XRD) techniques have been used to study the reduction of copper (II) oxide supported on mordenite zeolite, through the adsorption of carbon monoxide and hydrogen gases at 723 K. It was found that bands due to the bridged hydroxyl groups (3614–3630 cm^–1) and the Al-OH groups (3780–3787cm^–1) show significant changes upon carbon monoxide and hydrogen adsorption whereas the Si-OH band did not change after the adsorption. Two further bands were detected at 2156 and 2297 cm^–1, assigned to carbon monoxide adsorbed on Cu⁺ species and on the copper-aluminate species, respectively. XRD patterns have demonstrated the proposed interaction between Cu²⁺ with aluminium mordenite, to form aluminate, whereas FT-IR spectra have established an interaction between carbon monoxide molecules and the aluminate species.     

The corrosion of glass on the sea bed

Alkali-lime-silicate glasses, which have corroded on the sea bed for approximately 240 years, have been studied using scanning electron microscopy, electron-probe microanalysis, X-ray and electron diffraction, and infrared spectroscopy to quantify the nature of the decay processes. Leaching and dissolution of the glass network occurs, followed by reprecipitation of new phases to produce corrosion crusts consisting of discrete layers of striking regularity. Depth profiles of the elements in the crusts revealed them to be depleted of calcium and magnesium. Silicon was enriched at the glass-corrosion interface; aluminium was concentrated throughout the crusts; iron and sulphur were enriched at the exposed surfaces. Sodium and potassium showed less regular behaviour. Several crystalline phases were identified amongst the corrosion products, not all of which arose from the decomposition of the glass.     

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.     

Formation of OTS self-assembled monolayers at chemically treated titanium surfaces

Enhanced biocompatibility of titanium implants highly depends on the possibility of achieving high degrees of surface functionalization for a low immune response and/or enhanced mineralization of bioactive minerals, such as hydroxyapatite. In this respect, surface modification with Self Assembled Monolayers (SAMs) has a great potential in delivering artificial surfaces of improved biocompatibility. Herein, the effectiveness of common chemical pre-treatments, i.e. hydrogen peroxide (H₂O₂) and Piranha (H₂SO₄ + H₂O₂), in facilitating surface decontamination and hydroxylation of titanium surfaces to promote further surface functionalization by SAMs is investigated. The quality of the octadecyltrichlorosilane (OTS) based SAM appeared to strongly depend upon the surface morphology, the density and nature of surface hydroxyl sites resulting from the oxidative pre-treatments. Contrary to common belief, no further hydroxylation of the titanium substrate was observed after the selected chemical pre-treatments, but the number of hydroxyl groups available on the surface was decreased as a result of the formation of a titanium oxide layer with a gel-type structure. Further examinations by atomic force microscopy, infrared spectroscopy and X-ray photoelectron spectroscopy also revealed that mild oxidizing conditions were sufficient to remove surface contamination without detrimental effects on surface hydroxylation state and surface roughness. Furthermore, the adsorption of the alkylsiloxane molecules forming the SAM film is believed to proceed through hydrolysis at surface acidic hydroxyl groups rather than randomly. This site dependent adsorption process has significant implications for further functionalization of titanium based implants. It also highlights the difficulty of achieving an OTS based SAM at the surface of titanium and question the quality of SAMs reported at titanium surfaces so far.     

Chemical synthesis and stabilization of magnesium substituted brushite

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is the most ubiquitous calcium phosphate phase used in implant coatings and more recently in gene/drug delivery applications due to its chemical stability under normal physiological conditions (37 °C, pH ～ 7.5, 1 atm.). However, different calcium phosphate phases, such as brushite (CaH(PO₄)?2(H₂O)) and tricalcium phosphate (Ca₃(PO₄)₂) which are thermodynamically unstable under physiological conditions are also being explored for biomedical applications. One way of stabilizing these phases under physiological conditions is to introduce magnesium to substitute for calcium in the brushite lattice. The role of magnesium as a stabilizing agent for synthesizing brushite under physiological conditions at room temperature has been studied. Chemical analysis, Fourier transform infrared spectroscopy and X-ray diffraction have also been conducted to validate the formation of magnesium substituted brushite under physiological conditions.     

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.     

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.     

Aluminium oxide grafted on silica gel surface: Study of the thermal stability, structure and surface acidity

The synthesis of aluminium oxide grafted on silica gel surface was carried out by the reaction of a suitable aluminium precursor with the surface hydroxyl groups, SiOH, of the oxide support in non-aqueous solvent. The advantage of this preparation method, compared to the conventional ones (impregnation or precipitation and calcination), is that the oxide is highly dispersed on the surface (monolayer or submonolayer). The resulting material, SiO₂/Al₂O₃, was heat treated at temperature range of 423 to 1573 K. The Al/Si atomic ratios, determined by X-ray photoelectron spectroscopy (XPS), showed that aluminium is less mobile up to heat treatment of 1173 K and above this temperature part of it diffuses to the interior of the matrix. ²⁷Al solid state nuclear magnetic resonance spectroscopy (NMR) showed two different environments, tetrahedral and octahedral for sample calcined up to 1023 K and above this temperature, aluminium in a trigonal bipyramidal environment was also detected. Pyridine adsorbed on a Lewis acid sites were observed for samples calcined up to 1023 K, and above this temperature they were not detected.