Karfa Traoré

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.

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.

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.

Numerical study of cooling water via an evaporatively cooled immersed porous tube

SYNOPSIS We present a numerical study of the cooling of a volume of water through the wall of a cylindrical terracotta tube, the pores of which are saturated with water. The internal surface of the wall carries a water film which evaporates by forced convection of hot and dry air. Taking into account the physical properties of the material and that the aspect ratio of the tube used has a maximum value (D/L = 0.32), variations of the thermophysical properties of the air between the entry and the exit of the tube are ignored. The mathematical model describing the physical processes is based on the Darcy equation of fluid flow through a saturated porous material, the equation of Laplace related to pressure and the equation of heat in polar coordinates. One can then describe the space-time evolution of the field of temperature inside the wall as well as the temporal evolution of the average temperature of water.