Acta Metallurgica Slovaca, Vol. 17, 2011, No. 3, p. 169-176 169 SURFACE AREA CHANGE OF KAOLIN CAUSING ANNEALING B. Plešingerová 1 , G. Súčik 1 , M. Fabián 2 1 Department of Ceramics, Faculty of Metallurgy, Technical University of Košice, Letná 9, 040 01, Slovakia 2 Institute of Geotechnics - Slovak Academy of Science, Košice, Watsonova 45, Slovakia Received 08.07.2011 Accepted 27.09.2011 Corresponding author: Beatrice Plešingerová, Telephone number: +421055602 2304, Department of Ceramics, Faculty of Metallurgy, Technical University of Košice, Letná 9, 040 01, Slovak Republic, E-mail: Beatrice.Plesingerova@tuke.sk Abstract In case of this study of washed kaolin (WK) from the locality Sedlec (CZ) water begins to release before the temperature attains 400 °C and the dehydroxylation process finishes at 750 °C. The specific surface area of washed kaolin (19 - 19.5 m 2 .g -1 ) starts to decrease at annealing from 200 to 450 °C. In the temperature range of 550 to 750 °C when the dehydroxylation conversion degree achieves the value of 0.4 to 0.9 the specific surface area stagnates at 16 – 17.5 m 2 .g -1 . A marked decrease in the value of the specific surface area was observed at a temperature of 950 °C. Thermal activated “kaolin”, converted from 80 -90 % to metakaolin , shows little decrease of specific area at the temperature of 550 °C for time 60 - 90 min. up to 650 °C but shorter time (30 min.). Specific surface area and reactivity are very difficult for geopolymeric binders. Keywords: minerals, kaolinite, dehydroxylation, metastable phases, surface area 1 Introduction Structure changes of solid substances occur at chemical decomposition and phase transformations. The transformation can be very fast or slow. It is well known, that the reactivity of materials is very sensitive to its structural disordering. If gas is formed at the decomposition sudden release of gas may cause permanent or temporary expansion and increase of specific surface area of material. For example, an important growth of specific surface area occurs at thermal decomposition of carbonates of MgCO 3 , MgCa(CO 3 ) 2 and FeCO 3 . Maximum value of surface area is detected at 0.8 (i.e. 80 %) conversion of MeCO 3 to MeO [1,2]. Similarly, vulcanized hydrated glasses and hydrated minerals with disordered structure expanse at sudden heating-up. If the bond water in the structure releases the surface and/or the porosity increases (e.g. vermiculite. perlite) [3]. Kaolinite is a clay mineral belonging to layered aluminosilicate minerals of the kaolin group. The kaolin mainly consists of high-defected kaolinite with some impurities of feldspar and quartz. The particles of kaolinite are very fine, less than 2 μm. Basic kaolinite particle is a hexagonal platelet formed from a tetrahedral sheet [Si 2 O 5 ] 2- linked through oxygen atoms to an octahedral sheet of alumina [Al 2 (OH) 4 ] 2+ . These 2 sheets form layers. ¾ of the hydroxyl (-OH) groups are bond on top and ¼ in octahedral sheets [4,5]. The kaolinite (Al 2 Si 2 O 5 (OH) 4 ) contains 39.5 wt.% Al 2 O 3 , 46.5 SiO 2 and 13,96 wt.% H 2 O. The kaolin is hydroscopic. As kaolinite is heated to 300 °C the physically adsorbed water desorbs. Heating to higher temperature causes the release of –OH groups from kaolinite and