No abstract

The transformation β→α in Mg‐substituted Ca3(PO4)2 was studied. The results obtained showed that, contrary to common belief, there is, in the system Mg3(PO4)2–Ca3(PO4)2, a binary phase field where β+α‐Ca3(PO4)2 solid solutions coexist. This binary field lies between the single‐phase fields of β‐ and α‐Ca3(PO4)2 solid solution in the Ca3(PO4)2‐rich zone of the mentioned system. In the light of the results and the Palatnik–Landau's Contact Rule of Phase Regions, a corrected phase equilibrium diagram has been proposed. The practical implications of these findings with regard to the synthesis of pure α‐ and β‐ Mg‐substituted Ca3(PO4)2 powders and to the sintering of related bioceramics with improved mechanical properties are pointed out.

Sorption of As(III) by calcite was investigated as a function of As(III) concentration, time and pH. The sorption isotherm, i.e. the log As(III) vs. log [As(OH)3 degrees / Assat] plot is S-shaped and has been modelled on an extended version of the surface precipitation model. At low concentrations, As(OH)3 degrees is adsorbed by complexation to surface Ca surface sites, as previously described by the X-ray standing wave technique. The inflexion point of the isotherm, where As(OH)3 degrees is limited by the amount of surface sites (ST), yields 6 sites nm-2 in good agreement with crystallographic data. Beyond this value, the amount of sorbed arsenic increases linearly with solution concentration, up to the saturation of arsenic with respect to the precipitation of CaHAsO3(s). The solid solutions formed in this concentration range were examined by X-ray and neutron diffraction. The doped calcite lattice parameters increase with arsenic content while c/a ratio remains constant. Our results made on bulk calcite on the atomic displacement of As atoms along [0001] direction extend those published by Cheng et al., (1999) on calcite surface. This study provides a molecular-level explanation for why As(III) is trapped by calcite in industrial treatments.Comment: 9 page

No abstract

No abstract

No abstract

The thermal decomposition mechanism of synthetic Al(OH) 3 (gibbsite) was studied in situ by neutron thermodiffractometry in an ambient atmosphere from room temperature to 6001C with 501C steps. Gibbsite decomposed to yield AlO . (OH) (boehmite) and then poorly crystallized v-Al 2 O 3 . Rietveld analysis was used to refine the cell parameters' variation of gibbsite and its thermal expansion coefficients were obtained: for the a-axis: 1571 Â 10 À6 K À1 , for b: 1072 Â 10 À6 K À1 , and for c: 1772 Â 10 À6 K À1 . J ournal

We report observations on the early hydration of tricalcium aluminate, the most reactive component of Portland cement, using rapid-energy dispersive diffraction on a high brilliance synchrotron source. In situ observations of the hydration process over short time scales, and through bulk samples, reveal an intermediate calcium aluminate hydrate appearing just prior to the formation of the final stable hydrate, demonstrating the nucleating role of this intermediate. The superior quality of the data is sufficient to yield concentration versus time plots for each phase over the whole hydration sequence. This improvement derives from being able to use smaller diffracting volumes and consequent removal of time smearing due to inhomogenetics, and thus now offers the possibility of extending the technique in terms of time resolution and diversity of system. ͓S0163-1829͑96͒51022-2͔Third generation synchrotron radiation sources are now providing intense beams of high-energy x-ray photons with collimation characteristics that can be exploited in the study of reactions within bulk solid state materials. For example, the European Synchrotron Radiation Facility ͑ESRF͒ has a source size of circa 120ϫ250 m, a very high brilliance and usable energies up to 200 keV for diffraction purposes. In the studies reported here the samples were exposed to fluxes of the order of 10 12 photons per second per mm 2 in a 0.1% bandwidth at 50 keV. This represents an increase of 50-100 compared to equivalent experimental fluxes using second generation sources.Given these features we can now use synchrotron methods to look at much faster solid-state transformations than previously. The hydration of Portland cement, the reactive binder in concretes 1 for construction and in grouts for oil field engineering, 2 at first appears to be slow, reacting with water on a time scale of hours and developing final strength over many months. 3,4 Nevertheless there is evidence, such as from adiabatic calorimetry 5 and environmental scanning electron microscopy, 6 of some intense chemical and microstructural activity over the first few minutes following the initial contact between water and the anhydrous cement. However the direct observation of critical rapid mineralogical changes in dense cement pastes on the time scale of seconds has only now become possible by use of the latest synchrotron technology.It has long been known 4,7 that the early hydration of Portland cement, i.e., the first few hours after mixing with water, is greatly influenced by the behavior of its most reactive component, tricalcium aluminate, or ''C 3 A'' for short. Its hydration reactions can be conveniently expressed using a cement chemistry shorthand ͑e.g., see Taylor 7,8 ͒ in which the various phases are written in terms of the oxides ͑CϭCaO; AϭAl 2 O 3 ; HϭH 2 O͒. Using this shorthand, we can summarize key parts of the hydration process in terms of the formation of several hydrates