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Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 °C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the ∼400–650 °C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121 °C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030 °C temperature interval; (c) growth of mullite crystals from ∼1136 °C, (d) high (or β) cristobalite crystallization at T > ∼1200 °C and (e) secondary mullite crystallization at T > ∼1300 °C. The calculated activation energy for the kaolinite dehydration was 115 kJ/mol; for the mullite nucleation was 278 kJ/mol and for the growth of mullite process was 87 kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481 kJ/mol.

In order to elucidate the mechanism of reaction during the cordierite synthesis a kinetic study has been carried out by means of X-ray diffraction and neutron thermo-diffraction. It was thus possible to elucidate between the reactions leading to the cordierite synthesis and those that gave rise to the undesired phases generated during this process. Depending on the reactants used these reactions are different. The present paper presents results obtained during the cordierite synthesis starting from talc, kaolin and hydromagnesite as reactants. This synthesis from natural raw materials takes place through a complex mechanism that has been described. In this kind of synthesis, at the end of the entire process the obtained cordierite is always accompanied by a second phase: spinel. This is the first time that the presence of the μ-cordierite phase has been observed when the synthesis is carried out in solid state from non-submicronic natural raw materials.

Publicación versión impresa: diciembre de 2008, In-situ monitoring of the changes that take place in the phase composition of cement pastes during accelerated carbonation (100% CO2) for different binders, has been carried out, by taking Neutron Diffraction patterns in parallel with the carbonation experiments. The variation of the intensity of chosen reflections for each phase along the experiment has been used to monitor concentration changes and has supplied data, in real time, for fractional conversion of different phases (Portlandite, Ettringite and CSH gel) of the hydrated cement pastes. Fitting of these results has allowed to make a qualitative approach to the kinetics of the carbonation of the different phases and extracting conclusions on the microstructural changes that takes place during the carbonation of cement pastes., The experiments reported here were done thanks to the beamtime (Exp 5-25-50) granted by the Institute Max von Laue- Paul Langevin (ILL). The authors are especially grateful to the staff of D20. The authors also acknowledge the funding provided by the Spanish MMA through the project n° 005/2006/2-3.1.

The hydration behaviour of Ca3Al2O6 ,C a 12Al14O33 and CaAl2O4 with added amorphous silica at 40, 65 and 901C has been studied for periods ranging from 1 to 31 days. In hydrated samples crystalline phases like katoite (Ca3Al2(SiO4)3� x(OH)4x) and gibbsite, Al(OH)3, were identified, likewise amorphous phases like Al(OH)x, calcium silicate hydrates, C–S–H, and calcium aluminosilicate hydrates, C–S–A–H, were identified. The stoichiometry of Ca3Al2(SiO4)3� x(OH)4x (0p3� xp0.334), which was the main crystalline product, was established by Rietveld refinement of X-ray and neutron diffraction data and by transmission electron microscopy.

The reaction process to obtain CaZrO3/MgO specimens in the ZrO2–CaO–MgO system from two mixtures of natural and synthetic raw materials were analyzed by neutron thermodiffractometry; differential thermal analysis–thermogravimetry (DTA-TG) also was used. The results from the different techniques provided a complete description of the decomposition and reaction process in the samples and proved that high-temperature neutron diffraction is a powerful tool for analyzing in situ reactions up to 1250°C in both mixtures. In the present work, the evolution of the crystalline phases, the mechanism and temperature of formation of CaZrO3, and the activation energy were properly established in terms of the neutron thermodiffraction study.

The reaction mechanism that takes place in ZrSiO4–Mg Ca(CO3)2 mixtures was studied in air up to 13001C by collecting neutron diffraction patterns during the heating ramp. Neutron diffraction intensities were used to monitor and establish the mechanism of reaction that occurs in successive stages. (a) MgCa(CO3)2 decomposition yielding MgO and CaCO3; (b) CaCO3 decomposition; (c) reactions between CaO, MgO, and ZrSiO4 involving the formation of phases such as: tetragonalZrO2, a-Ca2SiO4, and Ca3MgSi2O8, some of them acting as transitory phases; and (d) formation of CaZrO3. The results obtained by this technique agree with data obtained by differential thermal analysis and thermogravimetry. The final product has a porous structure, due to the release of CO2, with a very narrow pore size distribution (E1 lm). This open-porosity can be controlled by tailoring the reaction sintering process. # 2002 Elsevier Science (USA)

Zirconia is an important component material for several industrial applications. In these studies we have used intense radiation sources, primarily synchrotron radiation, to shed new light on the properties of zirconia and its synthesis via existing industrial routes. Certain “static” techniques have been used to probe the atomic structure of precursor compounds used in the synthesis of zirconia, whereas time-resolved techniques were required to dynamically capture the various calcination stages during the heating section of the synthesis cycle. Finally the important tetragonal to monoclinic zirconia transformation during cooling was studied in detail using time resolved energy dispersive diffraction. Altogether what emerges is a new unfolding story on the various stages of zirconia synthesis, in which synchrotron radiation has played a vital role.

Neutron [l-31 and X-ray [4,5] techniques have been used to study the formation of zirconia from zirconium hydroxide. Synchrotron radiation can now, using dynamic energy-dispersive diffraction (DEDD), shed light on the kinetics of both the hydroxide-to-oxide crystallization and tetragonal-to- monoclinic conversion. Furthermore, different rates of crystallization and conversion have been linked [5] to the preparative conditions of the hydroxide; in particular the final pH of the solution coexisting with the precipitate of zirconium hydroxide. The results here represent an important extension of our dynamic studies. The special advantage of neutron thermodiffractometr!~ (NTD) [6] is that the high scattering/diffraction background associated with incoherent scattering by hydrogen is an effec- tive monitor of the chemical composition of the hydroxide; the scattering background reflects the amount of hydrogen-containing species in the sam- ple. Zirconium hydroxide can be considered as an intermediate between a hydroxide and a hydrated oxide [7]. One possible model [H] depicts the hydroxide as an open polymer in which individual units are linked by double h!.droxy bridges (Fig. 1). Clearly, if such a model were essentially correct, NTD would be expected to monitor the combined loss of H,O and OH during calcination. Zirconium hydroxides were prepared under care- fully controlled conditions [9] at pH 8.37 and 10.35, since these preparations display extremes of crystal- lization behaviour [5]. Aqueous ammonia solution was added to an aqueous suspension of zirconium basic sulphate (Magnesium Elektron Ltd, Man- chester, UK) to the desired pH. the suspension was filtered and washed with distilled water. The sam- ples were dried at 130 "C to produce zirconium hydroxide. Thermal analysis was carried out in both samples at a heating rate of 2 "C min-' in static air. Synchro- tron experiments were undertaken at Daresbury Laboratory, station 9.7, using pellets of thickness approximately 0.5 mm. The pellets were heated at a rate of 20 "C min-' to 400 "C, and the ramp then reduced to 5

Partially deuterated Ca3Al2(SiO4)y(OH)12−4y–Al(OH)3 mixtures, prepared by hydration of Ca3Al2O6 (C3A), Ca12Al14O33 (C12A7) and CaAl2O4 (CA) phases in the presence of silica fume, have been characterized by 29Si and 27Al magic-angle spinning–nuclear magnetic resonance (MAS–NMR) spectroscopies. NMR spectroscopy was used to characterize anhydrous and fully hydrated samples. In hydrated compounds, Ca3Al2(OH)12 and Al(OH)3 phases were detected. From the quantitative analysis of 27Al NMR signals, the Al(OH)3/Ca3Al2(OH)12 ratio was deduced. The incorporation of Si into the katoite structure, Ca3Al2(SiO4)3−x(OH)4x, was followed by 27Al and 29Si NMR spectroscopies. Si/OH ratios were determined from the quantitative analysis of 27Al MAS–NMR components associated with Al(OH)6 and Al(OSi)(OH)5 environments. The 29Si NMR spectroscopy was also used to quantify the unreacted silica and amorphous calcium aluminosilicate hydrates formed, C–S–H and C–A–S–H for short. From 29Si NMR spectra, the amount of Si incorporated into different phases was estimated. Si and Al concentrations, deduced by NMR, transmission electron microscopy, energy dispersive spectrometry, and Rietveld analysis of both X-ray and neutron data, indicate that only a part of available Si is incorporated in katoite structures., This research was partially supported by MCYT under the project MAT-2006-12749-C02-01 and MAT-2004-04923-CO2-01. Neutron diffraction data were acquired at the SINQ (experiment number no II/025-18, Swiss Spallation Neutron Source-Paul Scherer Institute Villigen, Suisse). We thank Dr. D. Sheptiakov for the experimental assistance in collecting the ND data.

International audience; The ability of gypsum, a common sulfate mineral, to host arsenic atoms in its crystalline structure, is demonstrated through experimental structural studies of the solid solutions formed upon synthetic coprecipitation of gypsum (CaSO4 ·2H2O) and arsenic. Neutron and X-ray diffraction methods show an enlargement of the gypsum unit cell proportional to the concentration of arsenic in the solids and to the pH solution value. The substitution of sulfate ions (SO42-) by arsenate ions is shown to be more likely under alkaline conditions, where the HAsO42- species predominates. A theoretical Density Functional Theory model of the arsenicdoped gypsum structure reproduces the experimental volume expansion. Extended X-ray Absorption Fine Structure (EXAFS) measurements of the local structure around the arsenic atom in the coprecipitated solids confirm solid state substitution and allow some refinement of the local structure, corroborating the theoretical structure found in the simulations. The charge redistribution within the structure upon substitutions of either the protonated or the unprotonated arsenate species studied by means of Mulliken Population Analyses demonstrates an increase in the covalency in the interaction between Ca2+ and AsO4 3-, whereas the interaction between Ca2+ and HAsO4 2- remains predominantly ionic.

Hydrogarnet (Ca12Al2(OH)12) and katoite of composition Ca3Al2(SiO4)0.176(OH)11.3 were obtained by hydration of tricalcium aluminium oxide, and for katoite synthesis, by addition of amorphous silica. The thermal dehydration was monitored in situ, by neutron thermodiffractometry, from room temperature to 700 °C at atmospheric pressure and at a heating rate of 2 °C/min. On heating, powder neutron diffraction patterns were collected every 300 s. Cell parameters were fitted by the Pawley method. Hydrogarnet decomposed to yield Ca12Al14O32(OH)·mH2O and Ca(OH)2 that eventually transformed to CaO. For katoite, phases of formula Ca12Al14−ySiyO32(O,OH)1+y/2 related to mayenite were formed. Complementary annealing experiments, for 10 h, at higher temperatures and subsequent quenching lead to the formation of mayenite and traces of Ca3SiO5. Thermogravimetric curves were in agreement with the thermodiffractometry experiments. Also, neutron diffraction data allowed to measure thermal expansion coefficients, at atmospheric pressure, between 25 and 250 °C for hydrogarnet and katoite: 1.89 × 10−5 ± 0.09 and 1.63 × 10−5 ± 0.07 °C−1, respectively., This research was supported by MCYT under the projects MAT-2003-08331-CO2-01 and MAT2006-12749-C02-01-02. We thank ILL for beam time granted (experiment number 5-25-83) and Dr. J. Campo for his assistance in collecting ND data. We also thank to Dr. D. Sheptyakov for experimental assistance in collecting the neutron diffraction data at SINQ (experiment number II/02 S-18).

10 pages, 12 figures, 3 tables., Even though the electroosmotic flux through hardened cementitious materials during laboratory realkalisation trials had been previously noticed, it has never been in-situ monitored, analysing at the same time the establishment of the electroosmotic flux and the microstructure changes in the surroundings of the rebar. In this paper, two series of cement pastes, cast with CEM I and CEM I substituted in a 35% by fly ash, previously carbonated at 100% CO2, were submitted to realkalisation treatments followed on line by simultaneous acquisition of neutron diffraction data. As a result, it has been possible to confirm the electroosmosis as the driving force of carbonates towards the rebar and to determine the range of pH in the anolyte in which most of the relevant electroosmotic phenomena takes place. On the other hand, the behaviour of the main crystalline phases involved in the process has been monitored during the treatment, with the precipitation of portlandite as main result., The authors also acknowledge the funding provided by the Spanish MMA through the project n- 055/2004/3.

The influence of an external DC voltage in an aluminous cement paste has been studied by in situ neutron diffraction, by monitoring the evolution of CaAl2O4 and Ca3Al2(OH)12 at different distances from the electrodes. The microstructure of the whole aluminous paste becomes dramatically altered. Both phases have been more attacked in zones closer to the electrodes. Major changes took place in the zone next to the positive electrode, in which the anhydrous phase has disappeared completely and the Ca3Al2(OH)12, has decreased significantly. In the cathodic zone, a decrease is also noticed due to the aluminium amphoteric character. The noticeable background diffraction increase in the area close to the positive electrode has been attributed to the formation of an amorphous alumina-like-gel that incorporated water to its composition.

Dolomite powder from Coin (Spain) was heated in air at a constant rate of 2°C/min to 1000°C, while neutron diffraction patterns were collected every 150 s. Rietveld refinement was applied and raw intensity data were used to monitor decomposition. The full process happened in two stages: dolomite decomposition to give calcite and periclase, and calcite breakup. The first stage activation energy was 47 kcal·mol−1 from fitting to a contracting sphere model. The dolomite mean thermal expansion coefficients were (6.7 ± 0.4) × 10−6 and (2.7 ± 0.2) × 10−5 K−1 along the a and c axes, respectively. Changes in the Ca–O and Mg–O bond distances were also measured.