Effect of kaolinite, illite and talc on the processing properties and mullite content of porcelain stoneware tiles Daniel Magagnin a,c , Carla Margarete Ferreira dos Santos b , Augusto Wanderlind c , Jeanini Jiusti c , Agenor De Noni Jr. c,n a Eliane Revestimentos Cerâmicos, Rua Maximiliano Gaidzinski, 245, 88845-000 Cocal do Sul, Brazil b Instituto Federal Catarinense – Campus Avançado Sombrio, Rua Francisco Caetano Lumertz, 818 – Bairro Januária, 88960-000 Sombrio, Brazil c Reactors and Industrial Processes Research Group, Materials Science and Engineering Post-graduate Program, Universidade do Extremo Sul Catarinense, Avenida Universitária, 1105, 88806-000 Criciúma, Brazil article info Article history: Received 16 April 2014 Received in revised form 9 September 2014 Accepted 10 September 2014 Available online 18 September 2014 Keywords: X-ray diffraction Ceramics Sintering Fracture abstract In this study, the effect of talc, illite and kaolinite on some properties of porcelain stoneware tiles was investigated. Raw materials available locally in a Brazilian ceramics production plant were used. The mixtures were defined through the use of a 2 3 factorial design of experiments. The starting point was a composition reported in the literature as follows: 35% kaolinite, 25% quartz and 40% albite. The mixtures were prepared by wet grinding, followed by spray drying. The conformation was performed by uniaxial pressing. The firing was carried out in an electric furnace, with the temperature varying between 1160 and 1260 1C, according to the maximum densification of each composition. The residence time was 6 min. Two cooling conditions were applied: 0.8 and 4.0 1C/s. The dry bulk density, fired porosity, pyroplastic deformation and three-point flexural strength were measured. The contents of mullite, quartz and vitreous phase were quantified using the Rietveld method with fluorite as the internal standard. The results showed that there is an optimum content of mullite which maximizes the increase in the flexural strength of the pieces submitted to rapid cooling. It was possible to distinguish the effect of the mineralogical phases on the formation of the mullite and the processing properties. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Porcelain stoneware is a class of ceramic tiles (BIa; ISO 10.5454). In comparison with the other classes it has a high content of the vitreous phase (50–70%), low apparent porosity ( o1%) and high technical performance [1]. It is composed mainly of a mixture of clay minerals, quartz and feldspar. The clay mineral fraction provides plasticity and flexural strength and forms mullite and the vitreous phase during firing. Feldspar is the main component responsible for the formation of the vitreous phase, accelerating the sintering. Quartz is the mineral with the highest melting point and it aids the thermal and dimensional stability [2–5]. Considering their composition, porce- lain tiles can be compared with triaxial ceramics [6]. Thus, the main mechanisms generally accepted as being responsible for the mechan- ical properties are related to: (a) the interconnection of the mullite crystals; (b) the dispersion of second-phase particles which limit the flaw size and deflect the crack path; and (c) the strengthening of the matrix associated with the difference in the thermal expansion coefficients of the matrix and the dispersed crystalline particles [7]. The porcelain manufacturing process is comprised of three main stages: (1) wet grinding and homogenization of the raw materials (the particle size distribution being  100% o74 μm), followed by drying of the suspension in a spray-dryer, residual moisture from 5 to 7%; (2) uniaxial pressing at 35–45 MPa; and (3) fast firing for 30– 60 min (cold to cold) with a maximum temperature in the range of 1180–1230 1C to obtain the maximum densification [2,5]. The lower firing time and temperature, in comparison to triaxial porcelain, determines two important effects: (1) no significant formation of secondary mullite [4]; and (2) thermal stress during the cooling, due to the fast cooling rate and allotropic transformation of quartz [8]. Two events which occur during the fast cooling of porcelain tiles are described in the literature: (1) the formation of macroscopic residual stresses; and (2) an increase in the Griffith flaw size [9]. These events affect the development of the mechanical properties in opposite ways, simultaneously increasing and decreasing the flexural strength, respectively. Thus, one way to maximize the mechanical properties would be to generate residual stresses without increasing significantly the Griffith flaw size. The authors [9] verified that one of their compositions, referred to as C4 (25% quartz, 35% kaolinite and 40% feldspar), resulted in the maximization of the mechanical Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2014.09.049 0921-5093/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail addresses: daniel.magagnin@eliane.com (D. Magagnin), acw@unesc.net (A. Wanderlind), jeanini@unesc.net (J. Jiusti), agenordenoni@unesc.net (A. De Noni Jr.). Materials Science & Engineering A 618 (2014) 533–539