Philippe Blanchart

The influence of heating rate on the thermal behaviour and mullite formation from a kaolin raw material

Abstract The thermal behaviour of a kaolin raw material (Bio) containing mainly well-crystallised kaolinite and muscovite minerals was studied in relation to heating rate, from 3 to 20°C min −1 . For comparison, reference raw materials, which were very rich in kaolinite or muscovite, were also analysed. The transformation heats during heating were quantified from DTA measurements and phase changes were followed by X-ray diffraction analyses. Results indicated that a fast heating rate increases the transformation heats associated with the endothermic dehydroxylation transformation of kaolinite and suggests the formation of a more disordered metakaolin. A similar trend was observed in the exothermic phenomenon. This suggests that Bio kaolin has a complex nature that changes the mechanisms involved in the structural transition. In Bio kaolin, mullite crystallises only from the kaolin-derived phase, which underlines the specific role of clay mineral interfaces in a natural mineral mix. In this case, the crystallisation of mullite occurs via a direct route without any spinel intermediate phase. The amount of mullite between 1050 and 1150°C increases with the heating rate and is comparatively higher than from a pure kaolinite phase.

Local Structure and Charge Distribution in Mixed Uranium–Americium Oxides: Effects of Oxygen Potential and Am Content

Partitioning and transmutation (P&T) of minor actinides (MA) is currently studied to reduce the nuclear waste inventory. In this context, the fabrication of MA bearing materials is of great interest to achieve an effective recycling of these highly radioactive elements. To ensure the in-pile behavior, nuclear oxide fuels have to respect several criteria including preservation of the fluorite structure and defined oxygen to metal ratio (O/M). In the case of Am bearing materials, such as U(1-y)Am(y)O(2±x) (y = 0.10, 0.15, 0.20), the O/M determination is quite challenging using conventional methods (TGA, XRD) because of the particular thermodynamic properties of Am. Despite the lack of experimental data in the U-Am-O system, thermodynamical models are currently developed to effectively assess the O/M ratio. In this work, the O/M ratios were calculated for various oxygen potentials using the cation molar fraction determined by XAS measurements. These results are an important addition to the experimental data available for the U-Am-O system. Moreover, XRD and XAS indicated that the fabrication of fluorite U(1-y)Am(y)O(2±x) solid solution was achieved for all Am content and oxygen potentials investigated. On the basis of the molar fraction, a description of the solid solution was proposed depending on the considered sintering conditions. Finally, the occurrence of an unexpected charge compensation mechanism was pointed out.

Low temperature sintering of a pottery clay from Burkina Faso

The sintering at 1000°C of a clay raw material for pottery mixed with calcite was studied to increase the mechanical characteristics of the fired product. The kaolinitic clay used comes from a large pottery production area in Burkina Faso. The experiments of this study were conducted as similar as possible to the traditional practices of potters to ensure a future technological transfer of results. An increase of the flexural strength from 7 to 17 mPa was obtained by adding 15% of calcite. To explain the mechanisms involved, the sintering behaviour was initially observed by dilatometry. Results indicated that at 1000°C, the maximum material densification and properties were obtained after about 15 min of dwell time. Therefore, a quantitative study of crystalline phases nucleated during this period was realised by X-ray diffraction methods. It revealed that anorthite is the most important phase formed during the firing time. Therefore, the nucleation mechanism, mostly involving quartz, but also gehlenite consumption was discussed. At the low firing temperature, it appears that the firing process is characterised by the absence of a liquid phase, drastically limiting the diffusion effect. As a consequence, the sample microstructure, as observed by SEM, shows a network of small dense zones, including quartz grains, interconnected by recrystallized porous phases. The comparison of material containing the natural kaolinitic clay to material obtained from pure reference minerals underlined the important role of iron impurities in anorthite formation.

Shape, size and composition of mullite nanocrystals from a rapidly sintered kaolin

The characteristics of mullite crystallites from a kaolin raw material were determined at the early stage of crystallisation by X-ray diffractometry. The equipment was a Debye–Scherrer system equipped with a curved position sensitive detector. We measured the size and shape of mullite crystallites in relation to the temperature (1100–1150°C) and the temperature rate (3–20°C/min). In particular, a temperature rate influence was found on crystallites anisotropy. This result was satisfactorily correlated with TEM observations of sintered materials. The mullite cells constants were also measured, they vary significantly with sintering conditions. Cell constants were correlated with mullite stœchiometry using the Cameron relationship. Results indicated a narrow composition domain (62–65% Al2O3) for all thermal cycles. In each case, the orthorhombic phase was identified, in spite of the low temperature range studied and the relatively high alumina content.

In situ study of the solid-state formation of U(1-x)Am(x)O(2±δ) solid solution.

In order to reduce the nuclear waste inventory and radiotoxicity, U(1-x)Am(x)O(2±δ) materials are promising fuels for heterogeneous transmutation. In this context, they are generally fabricated from UO(2+δ) and AmO(2-δ) dioxide powders. In the subsequent solid solution, americium is assumed to be trivalent whereas uranium exhibits a mixed-valence (+IV/+V) state. However, no formation mechanisms were ever evidenced and, more particularly, it was not possible to know whether the reduction of Am(IV) to Am(III) occurs before the solid-solution formation, or only once it is established. In this study, we used high-temperature X-ray diffraction on a UO(2±δ)/AmO(2-δ) (15 mol %) mixture to observe in situ the formation of the U(1-x)Am(x)O(2±δ) solid solution. We show that UO(2+δ) is, at relatively low temperature (<700 K), oxidized to U(4)O(9-δ), which is likely to be caused by oxygen release from the simultaneous AmO(2-δ) reduction to cubic Am(2)O(3±δ). Cubic Am(2)O(3+δ) then transforms to hexagonal Am(2)O(3) at 1300 K. Thus, the initial Am(IV) is fully reduced to Am(III) before the solid solution starts forming at 1740 K. The UO(2) fluorite phase vanishes after 4 h at 1970 K, indicating that the formation of the solid solution is completed, which proves that this solid solution is formed after the complete reduction of Am(IV) to Am(III).

Mineralogy of clay raw material from Burkina Faso and Niger used for ceramic wares

Abstract Some clay deposits in West Africa provide raw materials for the local and traditional manufacture of ceramic products. We selected three clays in Burkina Faso, with the references ROG, GRI and POT and a fourth one, AKN1 in Niger. These clays have been mined for many years. There is growing demand for such materials and the reserves are sufficient to meet this demand. The ROG and GRI clays are used as construction materials in both fired and unfired bricks. The POT and AKN1 clays are used for producing the porous ware of fired pottery. A study was carried out to look at some of the physico–chemical characteristics of these raw materials in order to enhance their use. This is a preliminary study which will be followed by further investigations into the production of different ceramic wares. An analysis of the chemical and mineralogical composition of the materials and some investigations of their fired properties during a firing cycle up to 1000°C has demonstrated that, taken as a whole, they possess satisfactory characteristics for the described applications.

Electrical characterization of aluminous cement at the early age in the 10 Hz–1 GHz frequency range

Abstract A CFA cement from Lafarge prepared at 20°C, 100% relative humidity, and with water-to-cement weight ratio (W/C)=0.4, has been tested electrically between 10 Hz–1 MHz and 1 MHz–1 GHz. The samples have been measured at 20°C and 50% relative humidity from 8 h up to 150 h after mixing the water with the cement. For the first frequency range, the results presented here focus on the effect of the chemical nature of the electrode on the electrical response of the cement. Different electrode materials, namely, silver, gold or an indium–gallium mix, have been investigated. Silver leads to a major contribution of the cement–electrode interface to the global electrical response. This effect is less pronounced with gold and indium–gallium. Concerning the higher frequency range, measurements between 1 MHz and 1 GHz enable the evolution of the interface between cement and water during hydration to follow. An analysis of electrical data, namely, the permittivity and the conductivity, has been carried out using a procedure based on a continuous distribution of time constants. The corresponding relaxation frequencies are presented and related to the state of water inside the cement material. A comparison of the electrical behaviour between a cement and a silico-aluminous ceramic, in which water is not as reactive as in a cement, is also discussed.

Desorption of water during the drying of clay minerals. Enthalpy and entropy variation

Abstract The purpose of this paper is to characterize the energy associated to the drying of three based clay raw materials containing mostly the minerals kaolinite, illite, and smectite, respectively. These natural clays, are commonly employed in many ceramic processes. Shaped samples were dried in a convection dryer at 55°C and 70°C, with an air humidity ratio ranging between 0.99 and 0.2. From the desorption isotherms, we have determined the enthalpy and entropy variations corresponding to the water desorption phenomena. They were calculated using the interpretation of the Gibbs free energy of the desorption mechanism, H 2 O adsorbed →H 2 O free.liq.water . The behavior observed for shaped raw material containing kaolinite is similar to what is described in literature for kaolinite mineral. A maximum in binding heat of 17 kJ mol −1 is observed for a complete monolayer coverage located between the layers of kaolinite clay. For shaped raw material containing smectite the behavior is fairly similar and the maximum in binding heat is about 18 kJ mol −1 . It corresponds to a situation for which 2/3 of the interlayer water is present. For shaped raw material containing illite, it was only possible to observe the beginning of the strong variation of Δ H ° and Δ S ° when the number of water layers on clay surface is low.

Sintering of a clay from Burkina Faso by dilatometry Influence of the applied load and the pre-sintering heating rate

The sintering of a pottery clay from Burkina Faso was studied as a function of the heating rate, at 3 or 10°C/min. The experimental method used was loading dilatometry in isothermal conditions at 1120°C. In these conditions, we found that the densification rate of the material is low, but tend to a limiting value after 2 h at 1120°C, depending on the pre-sintering heating rate and the load used. The relationship between the pre-sintering heating rate and the densification rate indicated the existence of a weakly organised material at higher heating rates. Nevertheless, higher values of shrinkage were observed when the temperature increased continuously. It is, therefore, proposed that the material is subject to a preferential solid state diffusion mechanism at face to face of the remaining kaolinite layers at high temperatures. This mechanism is favoured by higher heating rates, mainly in the temperature range corresponding to the structural reorganisation of the metakaolin phase.

Structural transformation of a kaolinite and calcite mixture to gehlenite and anorthite

Kaolinite mixed with calcite was sintered at low temperature (1100 °C; 5 °C/min). The successive phase transformations are metakaolinite to gehlenite and then anorthite, although the available phase diagram indicates a direct anorthite recrystallization. Transmission electron microscopy and electron diffraction studies of nanocrystallites revealed that the transformation path is favored by the structural similarities of phases. In particular, the pseudolayers of gehlenite have a major orientation relationship with the initial metakaolinite layers. The gehlenite axis, [001] G , is parallel to the metakaolinite axis, [001] A . This direct transition is favored by the existence of Si tetrahedral units and 4-fold-coordinated Al in both structures. Ca atoms, initially in the interlayer spacing of metakaolinite, remain in the interlayers of gehlenite. During the second transformation step, anorthite recrystallizes from gehlenite with axis [020] A parallel to [210] G . It is proposed that this orientation relationship is favored by the orientation and shape of Ca-atom channels through both structures, along [001] G and [100] A axes.