In this work we present results on the structutural properties of clay deposits localized in northern Rio de Janeiro sate in Brazil, where this kind of soil is abundant and used for production of ceramics, maily brics and roof tiles. The soil samples were dried, grounded and passed through a sieve with nominal aperture of 840 μm. The resulting homogeneus powder was very representative of the natural soil, which presents 95% particles with grain sizes below 50 μm. This powder was extruded in order to obtain bricks with average dimensions of 100 X 20 X 10 mm. The bricks were dried in air at room temperature during one week and subsequently at 110 ^oC during 24 hours. Thereafter the bricks were submitted to a slow firing process in a furnace of high heat capacity. The temperature was slowly raised up to 600 ^oC and kept constant during 60 minutes in order to avoid cracks owing to a quartz phase transition that occurs at 575 ^oC. Then the temperature was increased to a firing temperature T_f, and kept at this value during 180 min. Subsequently the sample was cooled at a controlled rate of 1.5 ^oC/min. Similar procedures were applied for preparing bricks with T_f = 300, 400, 500, 600, 700, 800, 900, 950, 1000, 1050, 1100 and 1200 ^oC. Powder difraction data were collectes from the treated brick with a conventional diffractometer Seifert URD65. The experimental conditions were 3^o ≤ 2θ ≤ 75^o,  Δ2θ = 0,02^o and counting time 3 s. A granulometric study of untreated samples was performed and alowed a quantitative analyse of three different fractions by the Rietveld method using the refinement program GSAS. Samples from differents soil depths treated at 1200 ^oC were also quantitatively analyzed and showed small differents in their compositions of cristobalite, mullite and hematite. The effect of the thermal treatment in the clay structural properties was also analyzed by FT-IR spectroscopy. In the IR region of 780 cm^-1 to 1010 cm^-1 were detected different peaks of silicates in dependence of the temperature T_f.

References:

1. Toledo R., Dos Santos D.R., Faria Júnior, R.T., Carrio J.G., Auler L.T., Vargas H.. Gas release during clay firing and evolution of the ceramic poperties. Appl. Clay Sci., Amsterdam, 27, 3-4, 151-157, 2004. 

2. Dos Santos D.R., Toledo R., Faria Júnior, R.T., Carrio J.G., Silva M.C., Auler L.T., Vargas H., Evolved gas analysis of clay materials. Rev. Sci. Instrum., Melville, 74, 1, 663-666, 2003.

3. A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory. Los Alamos, EUA. Copyright, 1985–2000, The Regents of the University of California, 2001.

Acknowledgments: Mackpesquisa, CAPES, FAPERJ

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