Abstract—In this study, we investigated the thixotropic behavior of two clays used in fabrication of ceramic. The structural kinetic model (SKM) was used to characterize the thixotropic behavior of two different kinds of clays used in fabrication of ceramic. The SKM postulates that the change in the rheological behavior is associated with shear-induced breakdown of the internal structure of the clays. This model for the structure decay with time at constant shear rate assumes nth order kinetics for the decay of the material structure with a rate constant. Keywords—Ceramic, clays, structural kinetic model, thixotropy, viscosity. I. INTRODUCTION HE clays are commonly used in many industrial applications, especially in the manufacture of ceramics, drug manufacturing, the treatment of polluted water, such as the adsorption of toxicorganic compounds [1], [2], paper, cement, pharmaceuticals [3] and stabilization of emulsions of oil-water [4]. The study of the rheological properties of clays is important in industrial applications such as in a ceramics factory building in the determined purpose of the good operating conditions of the pumps during the realization of ceramics operation. The rheological properties of clays depend on different factors, including the type and concentration of clays [5], the size of the solid particles, the temperature [6] and pH of the clay [7]. Many clays exhibit the thixotropic behavior, in which, the apparent viscosity of material decreases with time of shearing at constant shear rate. Although many researchers have investigated the time- dependent rheological behavior of materials, in general, the thixotropic characteristics of many types of clay are not so carefully studied. The objective of the present study is to investigate the effect of mass concentration and shear rate on thixotropic behavior of two different clays used in fabrication of ceramic. II. THEORETICAL BACKGROUND The thixotropic behavior of clays can be modeled using the structural kinetic model (SKM), which has been successfully employed for concentrated suspensions of minerals [8], [9]. The kinetic structural approach assumes that the change in the rheological behavior is associated with shear-induced breakdown of the internal structure in the clays. The analogy L. Hammadi, N.Boudjenane, R. Houdjedje, R. Reffis, and M. Belhadri are with the Laboratory of Rheology, Transport and Treatment of the Complex Fluids, University of Science and Technology, Mohamed Boudiaf, B.P. 1505, El M’Naour, Oran 31000, Algeria (e-mail: hammadi7280@yahoo.f). with chemical reactions can be used to express the structural breakdown process in the following form: Structured state → Non െ structured state (1) The rate of breakdown of the clays structure during the shearing process depends on the kinetics of the above reaction. The model assumes that the structure of food products breaks down irreversibly under the effect of shear without significant buildup. The structured state of the thixotropic structure at any time t and under an applied shear rate, ߛሶ , can be represented by a dimensional structural parameter: ߣൌ ߣሺ ߛ ,ݐሶሻ (2) which ߣሺ ߛ,ݐሶሻ is defined as: ߣሺ ߛ ,ݐሶሻ ൌ ఎఎ ಮ ఎ బ ఎ ಮ (3) where ߟ ଴ is the initial apparent viscosity at t=0 (structured state), and ߟ ஶ is the equilibrium apparent viscosity as ∞→ݐ (non-structured state). Note that, both ߟ ଴ and ߟ ஶ are functions of the applied shear rate only. The dimensionless structural parameter, ߣ, is subjected to the following conditions: initially, at the fully structured state, ݐൌ0; ߣൌ ߣ ଴ , and at non-structured state, ∞→ݐ; ߣൌ ߣ ஶ . The rate of structural breakdown can be expressed as: െ ௗఒ ௗ௧ ൌ ሺ ߣെ ߣ ஶ ሻ ௡ (4) where ൌ ሺ ߛሶሻ is the rate constant and  is the order of the structure breakdown reaction. At a constant applied shear rate, integration of (4) from t=0 to t yields: ሺ ߣെ ߣ ஶ ሻ ଵ௡ ൌ ሺ െ 1ሻ ݐ൅ሺ ߣ ଴ െ ߣ ஶ ሻ ଵ௡ (5) Substituting (3) into (5) yields for a constant shear rate: ቀ ఎఎ ಮ ఎ బ ఎ ಮ ቁ ଵ௡ ൌ ሺ െ 1ሻ ݐ൅1 (6) The form of (6) is valid only under the constant shear rate conditions. III. MATERIALS AND METHODS A. Materials A clay (Clay of Nedroma) and a slip of ceramic were recovered from the new ceramic tiles Company Remich, L. Hammadi, N. Boudjenane, R. Houdjedje, R. Reffis, M. Belhadri Modeling the Time-Dependent Rheological Behavior of Clays Used in Fabrication of Ceramic T World Academy of Science, Engineering and Technology International Journal of Physical and Mathematical Sciences Vol:9, No:7, 2015 1400 International Scholarly and Scientific Research & Innovation 9(7) 2015 ISNI:0000000091950263 Open Science Index, Physical and Mathematical Sciences Vol:9, No:7, 2015 publications.waset.org/10003306/pdf