Bauxite `red mud' in the ceramic industry. Part 2: production of clay-based ceramics Vincenzo M. Sglavo a, *, Stefano Maurina a , Alexia Conci a , Antonio Salviati a , Giovanni Carturan a , Giorgio Cocco b a Dipartimento di Ingegneria dei Materiali, Universita Á di Trento, Via Mesiano 77, 1-38050 Trento, Italy b Dipartimento di Chimica Fisica, Universita Á di Sassari, Via Vienna 2, 1-07100 Sassari, Italy Received 4 February 1999; received in revised form 1 April 1999; accepted 11 May 1999 Abstract Some potential uses of red mud as a raw component in clay mixtures for ceramic bodies production are presented. The in¯uence of increasing amounts of red mud on the forming procedure, sintering and ®nal properties was analyzed. Samples were produced by uniaxial pressing and slip casting. Two dierent clays are used as basic materials, the former being currently employed for the production of bricks by extrusion, the second Ð almost pure Kaolin Ð for high quality ceramic manufacturing. In both cases the addition of red mud led to more de¯occulated solid±water systems and an increase of the critical moisture content. Mixtures pre- pared with the ®rst clay and red mud loads up to 50% were ®red at 850  C. The red mud content did not in¯uence the sample porosity while determining a strength decrease attributed to the inertness of red mud at the working temperature. Samples pro- duced using the second clay and red mud (0 ± 20%) were ®red at 950 and 1050  C. The addition of red mud determined increases of density and ¯exural strength which can be accounted for by the formation of a larger amount of glassy phase at higher red mud contents. The results of this work indicate excellent perspectives for using `red mud' as raw material in mixtures with clay for the production of ceramic bodies. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Clays; Red mud; Sintering; Mechanical properties; Porosity; Bauxite 1. Introduction The enormous quantity of `red mud' discharged by industries producing alumina from bauxite represents an environmental and economical problem. Numerous reports proposing re-uses of `red mud' have been advanced, especially for the production of ceramic bodies or cements. 1±11 Along this line of research, a deep characterization of the red mud has been presented in a companion paper, hereafter Part 1. 12 Special atten- tion has been paid to the structural transformations and cross-reactions induced by heating representative batches of red mud up to 1400  C. A full knowledge of the thermal behaviour seemed to us of prior concern for possible applications requiring mixing with other raw materials and ®ring at dierent temperatures. Dried red mud was substantially inert up to 900  C, the loss of H 2 O from aluminium hydroxides and of CO 2 from silico-alumino-carbonates being the only detect- able eects. Between 900 and 1100  C complementary reactions occurred, yielding Ca 3 Al 2 O 6 , NaAlSiO 4 and Na 2 SiO 5 . Concurrent with the melting of nepheline-like compounds, the colour turned from orange-red to dark red. At higher temperatures major components such as Fe 2 O 3 and TiO 2 reacted to give Fe 2 TiO 4 with O 2 evolu- tion. This reaction was responsible for the intensi®ca- tion of the brown colour above 1200  C. 12 These results were used as guidelines to test the red mud as a raw component in clay mixtures for the pro- duction of ceramic bodies. In this work we report on the in¯uence of the red mud on the forming procedure, strength, porosity and colour of conventional clay-based products obtained with dierent processing techniques. Two dierent clays were considered. The ®rst one (A) is industrially used for the production of bricks and it is commonly mixed with a less valuable clay producing a plastic paste upon addition of 25% of water. The 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(99)00156-9 Journal of the European Ceramic Society 20 (2000) 245±252 * Corresponding author. Tel.: +39-0461-882468; fax: +39-0461-88 1977. E-mail address: sglavo@ing.unitn.it (V.M. Sglavo).