Microstructure et propriétés technologiques des grès céramiques élaborés à partir de l’argile alluviale d’Ebebda (Centre-Cameroun) : effet de l’ajout des cendres de cortex de cacao

Abstract

The aim of this study was to use cocoa cortex ashes (CCA) as an alternative fluxing agent for the manufacture of low temperature ceramic stoneware from Ebebda alluvial clay (EB). The raw materials (EB, CCA and SY) have been subjected to chemical, mineralogical and microstructural characterizations. The chemical composition of EB showed SiO2 and Al2O3 as predominant oxides and the mass ratio SiO2 / Al2O3 > 2 as well as the loss on ignition value of 12.46 %; CCA is marked by the presence of the fluxing oxide K2O (45.83 %), the syenite has shown a simultaneous existence of the two fluxing oxides: K2O (5.97 %) and Na2O (4.18 %). Mineralogical analysis showed kaolinite as the main mineral (55.8%) of EB, associated with quartz (27.1 %), hematite (7.8 %) and rutile (2.5 %); in CCA, augite (39.2 %), potassium oxide (38.7 %), potassium phosphate (10.5 %) and arcanite (5.4 %) were observed as crystalline phases; syenite showed albite (33.1 %), microcline (28.7 %), hornblende (18.5 %), quartz (10.1 %) and biotite (9.3 %) as minerals. The EB microstructure showed poorly crystallized kaolinite platelets while CCA showed in addition to white clusters, a black tone indicating the residual carbon. Samples of mixtures comprising EB and CCA (0, 10, 20 and 30 % by mass) were produced and heat treated at 1050 and 1100 °C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed. γ-alumina, mullite, akermanite, anorthite and grossular are the new crystalline phases formed. In addition, the technological properties of the specimens have been determined. The results showed that at 1100 °C., the shrinkage during sintering increases up to 8.5 % for the samples containing 10% of CCA (C10), then decreases up to 2% for the samples containing 30 % of CCA. The C10 specimen has a compact appearance with a water absorption of 2 %, a density of 2.41 g / cm3 and a flexural strength of 32 MPa; while C20 and C30 have a fragile structure. The rest of the work reported the comparison of the effect of CCA to that of syenite (SY) on the physico-mechanical properties and the microstructure of ceramic stoneware produced from EB + CCA and EB + SY mixtures containing 0, 10, 20 and 30 % of each additive. At 1100 °C, mullite and ɣ-alumina are identified as new common mineral phases resulting from the cooking of all the specimens. Akermanite, anorthite and grossular are present in C20 and C30. Specimen C10 has a more compact structure compared to S10, S20 and S30, while the latter two formulations are more compact than C20 and C30 respectively. For the EB + SY mixture, the formulation which has appreciable technological properties is S30 (water absorption 3.87 %; apparent density 2.37 g / cm3 and flexural strength 29.6 MPa); while for the EB + CCA mixture, C10 has interesting properties. According to ISO 13006, the products obtained with 10 % CCA are vitrified sandstones and those obtained from EB + SY mixtures are semi-vitrified stoneware. In order to optimize the technological properties of ceramic stoneware, a partial substitution of CCA by SY was carried out. The sample composed of 10 % CCA + 10 % SY fired at 1100 °C showed more interesting properties (water absorption 1 %; apparent density 2.45 g / cm3 and flexural strength 41, 43 MPa).