Matériaux argileux de l’Ombella M’Poko (Centrafrique) : Caractérisation et étude des propriétés physiques, mécaniques et microstructurales des produits de cuisson.

Abstract

The work presented in this thesis is a contribution for the valorization of two clay deposits (Bimbo (BIM) and ASECNA (ASEC)) of clay materials in the Ombélla M’Poko Region in Central Afriacn Republic (CAR). The characterization, the evaluation of fired products properties of clay and clay-carbonate mixture for fired clay bricks making is the aim of this study. To this end, four sample representatives of these deposits and named ASEC-1, ASEC-2, BIM-1 and BIM2, was used in this study. At first physico-chemical and mineralogical characterization of the raw samples was done using several techniques. The mineralogical composition of the four samples was similar and made of kaolinite, illite, quartz and goethite and mains minerals. The fired product of these materials has shown that BIM-1, because of its high content in clay minerals, in comparison to the other, exhibits sufficient mechanical and physicochemical properties that make it usable for fired bricks and tiles making. ASEC-1 and ASEC-2 are only usable for fired brick production while BIM-2, because of its high quartz content, can be used as degreaser in the formulation of ceramic pastes from fraction richer in clay minerals for structural control. In the view of improving ceramic products properties, dolomite was mixed to BIM-1 at various dosages and subject to thermal treatment under variable temperature and heating rates to access the influence of these parameters on the technological properties of the resulting products. It was found that to obtained mechanical and physical properties of interest, the dolomite addition should not exceed 10 % (mass basis) and the heating rate should be maximum 10 °C/min. A mixture of BIM-1 with 8 % dolomite (mass basis) was further analyzed in other to evaluate the influence of Dolomite-Clay mixture chemical composition on the mineralogical evolution during firing. The phase transformation was found to be influenced by the raw material composition. In these case, the presence of illite and a considerable amount of iron oxide (goethite), led to the formation of large amount of glassy phase during firing. This liquid phase was favorable to mullite and cristobalite formation at temperature between 1050 and 1150 °C, which induce improved mechanical response. Anortite, gehlenite and spinell are dissolved in the liquid phase as the temperature increases. Beyond 1150 °C, the large amount of glassy phase together with increase heterogeneity of the medium induces a brittle behavior that reduces the mechanical performance of the product.