Caractérisations structurale et mécanique de céramiques texturées : mise en forme par le procédé de coulage en bande-congélation (freeze tape casting)

National audienceLes argiles sont majoritairement constituées de minéraux argileux, dont la plupart appartiennent à la famille des phyllosilicates. Ces matières premières argileuses sont largement utilisées pour la mise en forme de céramiques dites « conventionnelles ou silicatées». Les propriétés finales obtenues après frittage dépendent de la nature chimique des constituants de ces minéraux argileux et de leurs proportions relatives, ainsi que de la présence de phases secondaires et des procédés mis en oeuvre pour élaborer ces produits céramiques. Le procédé de coulage en bande congélation (noté FTC) est une technique de mise en forme par voie liquide découle du coulage en bande conventionnel (noté TC) et du coulage congélation [1-3]. Le procédé FTC consiste à couler une suspension sur un plancher réfrigéré. D'après la littérature, une congélation rapide permet la croissance de petits cristaux de glace et la préservation de la structure du matériau [1]. A l'inverse, plus la congélation est lente, plus les cristaux sont gros. Pour cette étude, les produits sont congelés à des températures inférieures à-20 °C ce qui entraîne la formation de cristaux de glace d'épaisseur de l'ordre de quelques microns [2,3]. La morphologie particulière de ces argiles peut permettre d'obtenir des produits texturés, présentant des propriétés d'usage intéressantes pour des applications techniques, et ce à partir du procédé de coulage en bande-congélation [4,5]. Dans cette étude, trois matières premières argileuses ont été utilisées, notées KCR (fournie par Imerys), HCR (provenant du Cameroun) et KORS (provenant du Burkina Faso) respectivement. L'objectif est de mettre en évidence l'influence de la morphologie initiale des particules argileuses et/ou du procédé de mise en forme sur les propriétés structurales, mécaniques et thermiques des produits frittés à 1200°C. les argiles KCR et KORS sont essentiellement constitués de kaolinite (plaquettes hexagonales) tandis que HCR est essentiellement composé d'halloysite à morphologie sphéroïdale et tubulaire. De plus KORS et HCR contiennent des teneurs intéressantes en oxydes métalliques. Les valeurs de porosité après mise en forme FTC et TC, et avant frittage, sont comprises entre 70 et 80% (FTC) et entre 30 et 40% (TC). Le frittage à 1200°C entraîne une diminution significative de ces valeurs pour les échantillons issus du TC en dessous de 10%. Cependant les valeurs de porosité d'échantillons issus du FTC ont tendance à augmenter légèrement (de 75 à 80%). Les valeurs de résistances mécanique en flexion biaxiale montrent que les échantillons frittés, issus aussi bien du TC que du FTC, évoluent dans le sens croissant : KORS-HCR < KHCR < KCR. Cette tendance est partiellement corrélée aux valeurs de taux de porosité. Les phases cristallines et la génération d'une phase amorphe impactent également cette évolution des valeurs de résistance mécanique. La déstructuration de l'agencement des plaquettes apportée à travers le FTC comparativement au TC conduit à une forte diminution des propriétés mécaniques ainsi que des valeurs de conductivité thermique. Référence

TECHNOLOGICAL PROPERTIES OF RAW CLAY BASED CERAMIC TILES: INFLUENCE OF TALC

Local clay from Burkina Faso has been used as a basic raw material in the formulation of ceramic tile with a natural talc (0 - 4% wt) as a flux. The used sintering temperatures are between 950 and 1100 °C with one hour as hold at heating rate of 5 °C∙min-1. The different technological properties (shrinkage, water absorption and mechanical strength) are improved when the sintering temperature exceeds 1000 °C. The mixture with 4% wt of talc provides better properties than the other grades. The tiles obtained at 1050 °C with 4% wt of talc have similar characteristics to those obtained at 1100 °C without talc. An energy gain with a difference of temperature of 50 °C could be made with the use of talc as the adjuvant

Caractérisations morphologique et comportementale d’argiles riches en kaolinite et en halloysite pour une application éventuelle au procédé de coulage en bande-congélation

National audienc

Microstructural, physical, and mechanical characteristics of adobes amended with cement-metakaolin mixtures

International audienc

PREPARATION OF POZZOLAN WITH A RAW CLAY MATERIAL FROM BURKINA FASO / PREPARATION D’UNE POUZZOLANE A BASE D’UNE MATIERE PREMIERE ARGILEUSE DU BURKINA FASO

A raw clay material from Burkina Faso has been characterized for pozzolan elaboration. The analysis showed that SiO2 (45.91 wt. %), Al2O3 (28.29 wt. %) and Fe2O3 (9.75 wt. %) are the main oxides and kaolinite (71 wt. %) is the principle mineral. The sample has been activated at 680 °C during 2 or 5 hours. The pozzolanic activity has been accessed by coupling several methods. The chemical methods showed the good lime fixing ability by the activated sample. At the first 24 hours, more than 50 wt. % of the lime is fixed and at 28 days around 90 wt. % of lime are fixed. Mineralogical analyses showed the disappearing of portlandite (CH) and the formation of new C-S-H hydrates. The pozzolanic index (81 %) calculated from mechanical strength of mortars made with 25 wt. % substitution of cement by the pozzolan confirm it suitability for portland cement replacement in the field of building materials

Study of thermal shock and chemical durability of a porcelain formulated from local raw materials from Burkina Faso

This work evaluates the resistance to chemical attack and thermal shock of porcelain formulated from local raw materials from Burkina Faso for their validation in real use. A kaolinitic clay, a pegmatite and sand were used for the formulation of porcelain tiles. Some samples were shaped by casting into porous molds and sintered at a temperature of 1240 °C. The average heating rate is 10 °C/min up to the final given temperature. These porcelains were immersed to a depth of 25 mm in test solutions and kept closed at 20 °C for 12 days. Six concentrations of test solutions were prepared to perform these tests: ammonium chloride (100 g/L); hydrochloric acid (3% and 18% by volume); lactic acid (5% by volume); citric acid (100 g/L) and alkali KOH (30 g/L and 100 g/L). The test showed that these porcelains are resistant to chemical attacks with insignificant mass variations ranging from −0.030 to 0.053 wt%. The results of the thermal shock test show that formulated porcelains are resistant to the brutal variation of temperature. The results obtained qualify the porcelains for industrial and real use

Comparative Properties of Porous Phyllosilicate-Based Ceramics Shaped by Freeze-Tape Casting

Porous phyllosilicate-based ceramics were manufactured by freeze-tape casting from clays of different particle size and morphology in order to characterize their microstructure and stress to rupture changes before and after firing. Three raw clays were selected: HCR (77% Halloysite–10Å), KORS (29% kaolinite), and KCR kaolin (98% kaolinite). These clays exhibited a monomodal distribution and were used to prepare four slurries, three with each clay material and one consisting of a mixture of KCR and HCR labeled KHCR. After shaping by freeze-tape casting, the porosity and stress to rupture obtained by a biaxial flexural test were collected for disk-like samples after drying and sintering at 1200 °C. Results showed that KCR ceramic materials had the highest biaxial bending strength (70 ± 1.1 MPa) and those from KORS had the highest porosity value (80 ± 1%). SEM observation revealed a difference in microstructure and texture for the manufactured porous ceramic materials. In the KCR ceramic disks, the flattened pores appeared more textured, and the primary mullite crystallites formed a rigid skeleton within the amorphous phase. KORS ceramic materials showed a small quantity of secondary mullite needles which were randomly dispersed in a vitreous phase. The relatively important vitreous phase in the porous materials of HCR led to the bulk formation of small mullite particles. The biaxial flexural strength values were related to the presence of the mullite, as well as to the microstructure (volume, morphology, and size distribution of pores) after sintering

Physical, thermal and mechanical properties of adobes stabilized with fonio (Digitaria exilis) straw

International audienceThis study deals with the influence of fonio straw on the physical, thermal and mechanical properties of adobes. The raw materials (soil and fonio straw from Burkina Faso) were characterized from the chemical (ICP-AES), mineralogical (XRD, DSC-TGA, IR), geotechnical (particle size distribution, Atterberg limits, methylene blue value) and microstructural (SEM-EDS) points of view. The physical (thermal conductivity, capillarity water absorption, porosity, erodibility) and mechanical (compressive and flexural strength) properties of the adobes were studied with specific attention paid to their damage and fracture behavior. Because of the biochemical composition of fonio straw (presence of quasi-crystalline cellulose and hemicelluloses, which are hydrophilic compounds), its association with the clay matrix increased water absorption and was accompanied by a significant porosity due to the air trapped during mixing. The insulating character of the cellulose and the low density resulting from the high porosity contributed to an appreciable reduction of the thermal conductivity of these adobes. The use of small amounts of fonio straw improved the mechanical properties of the adobes and made them less brittle. This improvement was linked to the good adhesion between fonio straw and the clay matrix, greatly reduced propagation of fissures in the composites and the high tensile strength of fonio straw because of its cellulose content. Thus, fonio-straw-reinforced adobes have interesting properties for use as cheap construction materials in the Sahelian zones and could contribute significantly to the thermal comfort of the inhabitants in this hot climat

Physical, thermal and mechanical properties of adobes stabilized with fonio (Digitaria exilis) straw

International audienceThis study deals with the influence of fonio straw on the physical, thermal and mechanical properties of adobes. The raw materials (soil and fonio straw from Burkina Faso) were characterized from the chemical (ICP-AES), mineralogical (XRD, DSC-TGA, IR), geotechnical (particle size distribution, Atterberg limits, methylene blue value) and microstructural (SEM-EDS) points of view. The physical (thermal conductivity, capillarity water absorption, porosity, erodibility) and mechanical (compressive and flexural strength) properties of the adobes were studied with specific attention paid to their damage and fracture behavior. Because of the biochemical composition of fonio straw (presence of quasi-crystalline cellulose and hemicelluloses, which are hydrophilic compounds), its association with the clay matrix increased water absorption and was accompanied by a significant porosity due to the air trapped during mixing. The insulating character of the cellulose and the low density resulting from the high porosity contributed to an appreciable reduction of the thermal conductivity of these adobes. The use of small amounts of fonio straw improved the mechanical properties of the adobes and made them less brittle. This improvement was linked to the good adhesion between fonio straw and the clay matrix, greatly reduced propagation of fissures in the composites and the high tensile strength of fonio straw because of its cellulose content. Thus, fonio-straw-reinforced adobes have interesting properties for use as cheap construction materials in the Sahelian zones and could contribute significantly to the thermal comfort of the inhabitants in this hot climat

[Ceramic clay from Burkina Faso used in building construction] : Céramiques d'argile du Burkina Faso utilisées en construction immobilière

http://www.soachim.org/index.php?option=com_remository&Itemid=53&func=fileinfo&id=109International audience2 raw clays samples from Burkina Faso were analyzed in order to improve the ceramic process of roof tiles and small bricks manifacture. The major phases etected by X-ray diffraction in the 2 samples are kaolinite, illite, orthose and quartz. In addition, feeble quantities of albite and montmorillonite are also detected. The thermal behavior between 25°C and 1100°C studied by DTA and DTG is mainly explained by the individual role of clay minerals in studied samples. The sintering of clay ceramics at 1100°C was characterized by dilatometry. In general, dilatometric curves shapes are characterized by successive stages of strain and/or densification in specific temperature ranges. The extend of the observed phenomena is mainly related to the clay mineral fraction in samples. It evidences that clay mineral have a critical role in the sintering of ceramic. A useful application of the terra cotta is found with roof tiles manufacture which imbibition rate and permeability during rain are connected with microstructure and porosity. The microstructure of sintered clays is studied by SEM and the open porosity and permeability are determined. In general, the distribution of size and open porosity appears to be similar in both materials and composite microstructures are evidenced with large grains embedded in a sintered clay matrix. But local heterogeneities are observed in one of the ceramics at interfaces between large grains and matrix phase. They are supposed to have an effective role in pore connectivity through the material. It confirms the importance of pore morphology and connectivity in ceramics, induced mostly by clay type and the sintering process