Paul W. Brown

The stabilities of calcium arsenates at 23±1°C

Abstract The stabilities of calcium arsenate compounds were established by analysis of suspensions made with varying molar Ca/As ratios. Solution chemistry analyses determined the concentrations of calcium and arsenic and pH. The phases that were shown to form in order of descending pH were Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O, Ca 5 (AsO 4 ) 3 OH (arsenate–apatite), Ca 3 (AsO 4 ) 2 ·3 2 3 H 2 O, Ca 3 (AsO 4 ) 2 ·4 1 4 H 2 O, Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O — ferrarisite, Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O — guerinite and CaHAsO 4 ·H 2 O. The analytical concentrations of calcium and arsenic and pH were used in estimating solubility products. The estimated values were then refined through the comparison of the analytical data with calculated K sp values using the computer program PhreeqC. From the refined solubility products, the free energies of formation of the calcium arsenate hydrates were calculated as follows: Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O (−4941 kJ/mol), Ca 5 (AsO 4 ) 3 OH (−5087 kJ/mol), Ca 3 (AsO 4 ) 2 ·3 2 3 H 2 O (−3945 kJ/mol), Ca 3 (AsO 4 ) 2 ·4 1 4 H 2 O (−4085 kJ/mol), Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O — ferrarisite (−7808 kJ/mol), Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O — guerinite (−7803 kJ/mol), and CaHAsO 4 ·H 2 O (−1533 kJ/mol). Unlike other solubility studies on arsenate immobilization, this study was the first to consider the complete array of calcium arsenate hydrates that can form and to use the associated ions, CaAsO 4 − , CaHAsO 4 0 and CaH 2 AsO 4 + in determining their solubility products.

Mechanical properties of hydroxyapatite formed at physiological temperature

The mechanical properties of monoliths of calcium-deficient and carbonated hydroxyapatite formed by dissolution-precipitation reactions at 38°C have been determined. Particulate solid reactants were mixed at liquid-to-solid weight ratios of 0.11 and 0.2 and pressed into various configurations on which mechanical tests were carried out. Testing was performed on wet had formed. Calcium-deficient hydroxyapatite produced at a liquid-to-solids ratio of 0.11 exhibited a tensile strength as high as 18 MPa, an average compressive strength of 174 MPa and a Youngs modulus of 6 GPa. These values were lower when a larger proportion of water (liquid-to-solid 0.2) was used in sample preparation. However, the compressive strengths of calcium-deficient hydroxyapatite prepared at 38°C are comparable to the compressive strengths of sintered hydroxyapatite containing an equivalent total porosity. Carbonated hydroxyapatite showed mechanical properties inferior to those exhibited by calcium-deficient material. These differences appear to be related to the microstructural variations between these compositions.

alpha-Tricalcium phosphate hydrolysis to hydroxyapatite at and near physiological temperature.

The kinetics of hydroxyapatite (HAp) formation by direct hydrolysis of α-tricalcium phosphate (α-TCP) [α-Ca3(PO4)2] have been investigated. Transformation kinetics were examined for reactions at 37 °C, 45 °C and 56 °C by isothermal calorimetric analysis. Setting times and morphologies of the resultant HAp were found to be strongly dependent on reaction temperature. XRD analysis accompanied by FTIR confirmed that phase pure calcium-deficient hydroxyapatite (CDHAp) [Ca10-x(HPO4)x(PO4)6-x(OH)2-x] was formed. Complete reaction occurs within 18, 11, 6.5 h at 37, 45 and 56 °C, respectively. The extent of HAp formation differs for particulate slurries and pre-shaped forms of reactant α-TCP. Formation of hydroxyapatite in pre-formed pellets was hindered due to limited water penetration, but enhanced with the presence of NaCl as a pore generator. Regardless of the precursor characteristics and temperature, HAp formation is characterized by an initial period of wetting of the α-TCP precursor, an induction period and a growth period during which the bulk transformation to HAp occurs. The microstructures of the resultant HAp at all temperatures were generally similar and are characterized by the formation porous flake-like morphology. Microstructural coarsening was observed for the CDHAp formed above the physiological temperature. The hardening generated by the hydrolysis reaction was demonstrated using diametrical compression tests. The original tensile strength of 56% dense α-TCP increased from 0.70±0.1 MPa to 9.36±0.4 MPa after hydrolysis to CDHAp at 37 °C, corresponding to a density of 70%. ©2000 Kluwer Academic Publishers

Chemical changes in concrete due to the ingress of aggressive species

Chemical changes in field concretes due to the ingress of sodium, magnesium, sulfate, chloride, and carbonate are analyzed. Each of these species participated in the deterioration of these concretes. Magnesium compounds formed were brucite and magnesium silicate hydrates. Sulfate form gypsum, ettringite, and thaumasite. Chloride intrusion permitted the formation of Friedels salt. Finally, sodium carbonate was formed. The mechanisms by which these compounds were formed are discussed.

Low temperature formation of calcium-deficient hydroxyapatite-PLA/PLGA composites.

Hydroxyapatite-biodegradable polymer composites have been formed by a low temperature chemical route. Precomposite structures were prepared by combining alpha-Ca(3)(PO(4))(2) (alpha-tricalcium phosphate or alpha-TCP) with poly(L-lactic) acid and poly(DL-lactide-co-glycolide) copolymers. The final composite structure was achieved by in situ hydrolysis of alpha-TCP to Ca(9)(HPO(4))(PO(4))(5)OH (calcium deficient hydroxyapatite or CDHAp) either in solvent cast or pressed precomposites. Hydrolysis was performed at 56 degrees C-a temperature slightly above the glass transition of the polymers. The effects of polymer chemistry, composite formation technique, and porosity on hydrolysis kinetics and degree of transformation were examined with isothermal calorimetry, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, and scanning electron microscopy. Calorimetric data and XRD analyses revealed that hydrolysis reactions were inhibited in the presence of the polymers. Isothermal calorimetry indicated the extent of the alpha-TCP to CDHAp transformation in 24 h to be 85% in the solvent cast composites containing PLGA (85:15) copolymer; however, XRD analyses suggested almost complete reaction. The CDHAp formation extent was 26% for the pressed composites containing the same polymer. In the presence of NaCl as a pore generator, 81% transformation was observed for the pressed composites. This transformation occurred without any chemical reaction between the polymer-inorganic components, as determined by Fourier transform infrared spectroscopy. Minimal transformation to CDHAp occurred in composites containing poly(L-lactic) acid.

Hydrothermal reactions of fly ash with Ca(OH)2 and CaSO4·2H2O

Hydrothermal reactions of fly ash were investigated. Variations in reactivity depended on the presence of added Ca(OH){sub 2} or CaSO{sub 4}{center_dot}2H{sub 2}O. Isothermal calorimetry determined the kinetics of the reactions between fly ash and these compounds. Fly ash was activated by these calcium salts and activation influenced hydration rates as determined by the rates of heat evolution. X-ray diffraction analysis determined the phases formed as a result of hydrothermal treatment. Calcium silicate hydrate, tricalcium aluminate hydrate and ettringite were observed. The mechanical properties developed by hydrothermal treatment demonstrates that hazardous ashes could be consolidated to be handled. Alternatively, non-hazardous ashes can be reacted to form useful products.

Hydrolysis of dicalcium phosphate dihydrate to hydroxyapatite

Dicalcium phosphate dihydrate (DCPD) was hydrolysed in water and in 1 M Na2HPO4 solution at temperatures from 25–60°C. Hydrolysis was incomplete in water. At 25 °C, DCPD partially hydrolysed to hydroxyapatite (HAp). Formation of HAp is indicative of incongruent DCPD dissolution. At the higher temperatures, hydrolysis to HAp was more extensive and was accompanied by the formation of anhydrous dicalcium phosphate (DCP). Both of these processes are endothermic. When hydrolysis was carried out in 1 M Na2HPO4 solution, heat absorption was greater at any given temperature than for hydrolysis in water. Complete hydrolysis to HAp occurred in this solution. The hydrolysis of DCPD to HAp in sodium phosphate solution was also endothermic. The complete conversion of DCPD to HAp in sodium phosphate solution would not be expected if the only effect of this solution was to cause DCPD dissolution to become congruent. Because of the buffering capacity of a dibasic sodium phosphate solution, DCPD hydrolysed completely to HAp. Complete conversion to HAp was accompanied by the conversion of dibasic sodium phosphate to monobasic sodium phosphate. The formation of DCP was not observed indicating that the sodium phosphate solution precluded the DCPD-to-DCP dehydration reaction. In addition to affecting the extent of hydrolysis, reaction in the sodium phosphate solution also caused a morphological change in the HAp which formed. HAp formed by hydrolysis in water was needle-like to globular while that formed in the sodium phosphate solution exhibited a florette-like morphology.

Calcium‐deficient hydroxyapatite‐PLGA composites: Mechanical and microstructural investigation

The microstructural and mechanical properties of composites composed of calcium deficient hydroxyapatite (CDHAp) and poly(lactide-co-glycolide) (PLGA) have been investigated. The composites were formed by hydrolysis of alpha-tricalcium phosphate (alpha-TCP) to CDHAp in pressed precomposite compacts of alpha-TCP-PLGA-NaCl. The differences in hydrolysis of alpha-TCP-PLGA-NaCl for two compositions of 80:10:10 wt % and 60:20:20 wt %. were monitored by isothermal calorimetry and X-ray diffraction. The microstructural evolution and variance in final composite microstructure after hydrolysis at 37 degrees C, 45 degrees C, and 56 degrees C were examined by scanning electron microscopy. HAp-PLGA composite formed from the alpha-TCP-PLGA-NaCl (80:10:10) precomposites at 37 degrees C developed a tensile strength of 13.3 +/- 0.9 MPa, a flexural strength of 24.8 +/- 1.7 MPa, and Youngs modulus of 2.8 +/- 0.3 GPa. These values were 12.00 +/- 0.2 MPa, 36.1 +/- 2.1 MPa, and 5.5 +/- 0.8 GPa for the precomposite composition 60:20:20. All these mechanical properties showed a variation with hydrolysis temperature and composition. The differences in mechanical properties were related to the final microstructures of the composites, which are governed by the morphological changes in the polymer structure at its glass transition temperature and the extent of cement-type formation of CDHAp by hydrolysis of alpha-TCP.

Effects of magnesium on the formation of calcium-deficient hydroxyapatite from CaHPO4.2H2O and Ca4(PO4)2O.

Calcium-deficient hydroxyapatite (HA) with a Ca/P molar ratio of 1.50 was synthesized in various concentrations (0.01-75 mM) of MgCl2 at 37.4 degrees C by reaction between particulate CaHPO4.2H2O and Ca4(PO4)2O. The effects of magnesium on the kinetics of HA formation were determined using isothermal calorimetry. All reactions completely consumed the precursor phases as indicated by X-ray diffraction analysis and a constant enthalpy of reaction (240 kJ/mol). Magnesium concentrations below 1 mM had no effect on the kinetics of HA formation. Magnesium concentrations between 1 and 2.5 mM affected the reaction path but did not affect the time required for complete reaction. Higher concentrations extended the times of complete reaction due to magnesium adsorption on the precursor phase(s) and HA nuclei, and stabilization of a noncrystalline calcium phosphate (NCP). HA formation in the presence of magnesium resulted in separation of the following two events: initial formation of HA nuclei and NCP, and consumption of CaHPO4.2H2O. This was indicated by the appearance of an additional calorimetric peak. Variations in calcium, magnesium, and phosphate concentrations and pH with time were determined. Increasing the magnesium concentration resulted in elevated calcium concentrations. After an initial decrease in magnesium owing to its adsorption onto HA nuclei and precursor(s), a period of slow reaction at constant magnesium concentration was observed. Both the magnesium concentration in solution and the proportions of precursors present decreased prior to any evidence of a crystalline product phase. This is attributed to the formation of NCP capable of incorporating magnesium. This noncrystalline phase persisted for more than 1 year for reactions in magnesium concentrations about 2.5 mM. Its conversion to HA resulted in the release of magnesium to the solution.

Analyses of the aqueous phase during early C3S hydration

Abstract The concentrations of calcium and silica in solution during the first 4 hours of C 3 S hydration were measured. The results of these analyses indicate that a solid calcium silicate hydrate forms within 30 seconds of the start of hydration and that an equilibrium between the solution and the solid hydrate is rapidly established. A strong dependence of the rate of early hydration on the w:C 3 S ratio was observed, while the dependence on the surface area of the C 3 S was minimal.