© Faculty of Mechanical Engineering, Belgrade. All rights reserved FME Transactions (2019) 47, 42-47 42 Received: Septeber 2018, Accepted: November 2018 Correspondence to: Stefano Vitali Keser Italia S.r.l. Via Marche 5, 41049, Sassuolo, Italy E-mail: stefano.vitali84@gmail.com doi: 10.5937/fmet1901042V Stefano Vitali Keser Italia S.r.l. Italy Loris Giorgini Professor University of Bologna Italy Overview of the Rheological Behaviour of Ceramic Slurries Given that the majority of raw materials used in the ceramic industry have sedimentary origins, their formation is conducted in aqueous medium, reason for which most ceramic production processes involve water for manufacturing diverse semi-finished materials. However, the rheological behaviour inherent of ceramics may infer significant impact on their processing, defining crucial parameters such as solid-to-water ratio, deflocculation level, and tpye and amount of chemical additives to be used. Thus, the present work explores the intricacy of essential aspects of rheology in ceramic slurries, intending to provide an overview of the chemical, physical and practical influence on the manufacturing of tiles from both industrial and scientific points of view. Keywords: Thixotropy, Clay, Rheology, Zeta potential, Suspension, Dispersion. 1. INTRODUCTION The industry constantly evolves toward the usage of more efficient materials aiming to achieve optimal cost- benefit relations and enhanced performance. Within this context, it is imperative to acknowledge the importance held by composite materials [1-5], where ceramics play a major role due to their elevated toughness and envi– ronmental resistance. For this reason and to allow a bet– ter comprehension of their properties, the analysis of manufacturing steps become essential. Ceramic slurries formed by mineral grinded powder and water are extensively used in industrial manufac– turing techniques such as spray-drying, slip-casting and tape-casting to manufacture diverse products and vari– eties of tiles. The way of dispersion of such powder on aqueous medium is imperative to understand the expec– ted rheological behavior of the mixture, which basically stands for the flow of matter and how it deforms. For instance, according to Tsetsekou et al. [3], the particles must have an appropriate settlement rate and be able to remain in suspension to avoid segregation and assure a homogeneous density gradient over the manu– factured part. Simultaneously, the slurries should be insensitive to solid content and chemical composition variations as well as storage time. The importance of having a high enough solid ratio is highlighted by Lange et al. [4], depicting that it is critical for the attainment of adequate casting rates and to minimize the energy consumption demanded by the following step of drying, once the less moisture there is to be removed, the less energy is spent. Lee et al. [5] state that the rheology conditions have utmost influence on the performance quality of the slip for sanitaryware casting, sticking the piece in the mold, causing an improper cast rate, an improper trim quality, soda spots and even a precocious mold release. Consequently, the general resistance properties can be severely affected producing brittle casts [5], which demean important mechanical thresholds for a structural part such as to compression and impact [6]. Tsetsekou et al. [3] analyzed the stabilization of alumina slurries of high solids content by using different types of dispersants, and found out that the slurry solid content increased the density in both green and fired states of the ceramics, decreased the post-firing shrin– kage and the products presented enhanced mechanical properties. Souza et al. [7] used humic acid extracted from subbituminous coal [8] as a dispersant in an alumina suspension, finding optimal values of 0.016 wt% of humic acid (which is near to the viscosity when using sodium polyacrylate), showing prominent results for slip casting by achieving remarkable colloidal stability and attaining high density parts, attaining high density parts, existance of negative surface charges, absence of metallic complexes in the molecules, and pointing out the alkaline pH 11 as ideal once it allows the humic acid to have an enhanced zeta potential and decreases its turbidity. The most important parameters to be taken into account for the preparation of an adequate slurry can be summarized as: • Solid-to-water ratio: also known as specific gravity, it has a prevailing influence on the thixotropy, i.e. the long-term viscosity, defining casting properties of the mix such as casting rate and plasticity of the cast part. • Types and amount of chemical additives: used to control deffloculation and thixotropy, this resource is able to modify the slip in an economic way, for example diminishing the dispersion when using ball clayis and kaolin in slurry form, and to prevent the concentration of fines in the mold/cast interface. • Deflocculation level (determined by organic disper– sants): influences the plastic characteristics of the slip, being crucial to obtain a homogeneous and