Characterisation and ceramic application of clays from North Africa

Abstract

ABSTRACT The mineralogical, chemical, physical and thermal analyses of the representative clays from North Africa (Algeria, Morocco and Tunisia) have been studied for their potential use in traditional ceramic industry. The clay fractions of the Moroccan and Algerian clays are essentially composed of illite (38 and 21%, respectively) and kaolinite (17 and 12%, respectively) as predominant minerals, with subordinate I/Sm mixed-layer (10 and 3%, respectively). The non-clay minerals are quartz, calcite, dolomite and occasionally plagioclase and haematite. Tunisian clays are composed of similar proportions of kaolinite (15%), smectite (15%), illite (12%) and palygorskite (9%), whereas their associated minerals are quartz (30%), calcite (15%) and rarely plagioclase (4%). The chemical data show agreement with estimated mineralogical compositions. All the samples contain large amounts of iron (>5.6%) and earth-alkaline oxides (>6.9%), and high values of LOI (>12%). Algerian clays show high plasticity (PI = 40%), requiring particular attention and careful temperature control during drying to avoid the deformation and the formation of cracks in the ceramic bodies, whereas the Tunisian and Moroccan clays (PI = 18% and 16%, respectively) show acceptable behaviour in shaping and drying. The average grain-size distribution demonstrates a substantial amount of the silt and clay fractions in raw materials which are therefore suitable for easy shaping of paste without any special need for further adjustments. Indeed, the amount of fraction upper 63 µm is lower less than 2%. The main transformations during firing are influenced by the abundance of components such as Fe2O3, CaO, MgO, K2O and Na2O and observed above 1000°C with the appearance of new crystalline phases, especially mullite, spinel, plagioclase, diopside and haematite. The technical parameters of fired pieces (firing shrinkage, water absorption and flexural strength) fall within the ceramic international standards (ISO).