Materials Research. 2010; 13(3): 395-399 © 2010 *e-mail: hotza@enq.ufsc.br Measuring and Modeling the Plasticity of Clays Fernando Augusto de Andrade, Hazim Ali Al-Qureshi, Dachamir Hotza * Núcleo de Pesquisa em Materiais Cerâmicos – CERMAT, Departamentos de Engenharia Mecânica – EMC e Engenharia Química – ENQ, Universidade Federal de Santa Catarina – UFSC, CEP 88040-900, Florianópolis, SC, Brazil Received: April 1, 2010; Revised: June 29, 2010 The measurement of plasticity in clay bodies is crucial in order to get products free of defects and with less processing time. However, tests which simulate the behavior of the clay during processing and the mathematical modeling of some of its characteristics, particularly the plasticity, become difficult because many variables are involved and there is no consensus on the choice of method to be used. This study aimed to develop a mathematical model based on compression test to evaluate the plasticity of clays. Three types of clays were studied with different levels of moisture and their indices of plasticity were also characterized by the Atterberg’s and Pfefferkorn’s methods. The experimental data were well fitted by the theoretical curves for a wide range of clay plasticity. Moreover, it was possible to observe a correlation between effective stress of compression and paste moisture within each group of clay. Keywords: clays, extrusion, plasticity, modeling 1. Introduction The plasticity in the processing of ceramic materials is a fundamental property since it defines the necessary technical parameters to convert a particulate ceramic body to a component with a given shape by application of pressure. The plasticity, in this case, and particularly in clay mineral systems, is defined as a property that shows shape changes without rupture when a clay body with added water is submitted to an external force. Furthermore, when the force is removed or reduced below to a value corresponding to the yield stress the shape is maintained 1 . The main factors that affect the clay plasticity, according to Barba et al. 2 and Händle 3 , are related to physical characteristics of the solid, particularly the particle size distribution and its specific surface area, the water characteristics (viscosity, surface tension, etc.), the solid mineralogical composition (clay mineral type, proportion of non- plastic minerals, etc.), the dispersion state of the particles that depends on the ionic change capacity and nature and proportion of additives, as well as on the ceramic body temperature. Relevant process-related factors affecting clay plasticity are application of pressure, body temperature and characteristics of additives used 4 . However, the plasticity determination is not always an easy task since it cannot be immediately applied and interpreted. In fact, there are several methods for measurement and characterization of the plasticity of a clay body, although its experimental determination, in some cases, is operator-dependent, causing difficulties in interpreting the results 5 . Among the methods, the Atterberg’s plasticity index, the Pfefferkorn’s plasticity index, stress/strain curves, indentation and rheological measurements are the most applied. The Atterberg’s plastic limit is the lowest water content at which the body can be rolled into threads without breaking. The Atterberg’s liquid limit is the water content at which the body begins to flow, using a specific apparatus. The difference between both values is called the Atterberg’s plasticity index 6 . Alternatively, the Pfefferkorn method determines the amount of water required to achieve a 30% contraction in relation to the initial height of a test body under the action of a standard weight 7 . As with other types of materials, a compression test can be used to evaluate the plasticity of clays. Baran et al. 8 formulated their workability concept for clays using compression tests in cylindrical samples, allowing to determine the optimum amount of moisture for each clay studied. Ribeiro et al. 5 evaluated the plasticity of extrudable clays by compression tests and found that the measured samples were ruptured at 50-55% deformation. In a typical test curve, a great deal of information is obtained: modulus of elasticity, yield strength, maximum deformation and rupture strength. Those parameters are strongly influenced by the moisture of the clay and its chemical or phase composition. Clays may present a wide range of plasticity values 9 . Typical values of Atterberg’s plasticity index for Kaolinitic clays range from 5 to 22; for ilittic clays, from 39 to 51; and for montmorillonitic clays, up to 600. 2 Usually measurements of clay plasticity are undertaken without considering a formal description of this physical behavior through a modeling approach. A model would not only describe the process in a broader and deeper way, but it also might be used for predicting a system’s behavior with a lower experimental effort. In this paper, a mathematical model for evaluation of the plasticity of clay bodies was developed from applied concepts of the plasticity theory by using the stress/strain diagram under compression. 2. Mathematical Modeling of Compression Test The mathematical knowledge applied to metallic porous materials was used as a basic tool for plasticity modeling of clays 10 taking into account few experimental parameters. To define the processing parameters, it was assumed that the clay compact, which has a cylindrical shape, deforms axially and symmetrically. When the compressive force is applied, the height