Thermal transformation kinetics of a kaolinitic clay A. D. Pekdemir 1 • Y. Sarıkaya 2 • M. O ¨ nal 2 Received: 6 March 2015 / Accepted: 13 June 2015 / Published online: 11 July 2015 Ó Akade ´miai Kiado ´, Budapest, Hungary 2015 Abstract The objective of the present study was to use the zero-order reaction rate (ZORR) assumption and transition state theory (TST) to examine the kinetics of the thermal changes in solids. The method was firstly applied to the thermal transformation of spinel–mullite in a kaolinitic clay. The selected clay contains kaolinite (K), illite (I), and quartz (Q) minerals. The cylindrical compacts of the clay powder with a diameter of 40.22 mm were prepared under 150 MPa by uniaxial pressing using water (7 % by mass) as binder. The compacts after dried at 100 °C for 4 h were fired at various temperatures in the interval of 900–1050 °C by changing the time between 20 and 120 min. Besides the bulk density measurements, phase transformation in the compacts depending on the firing temperature was examined by X-ray diffraction and scanning electron microscopy techniques. Time derivative of the bulk densities for each firing tem- perature was taken as the transformation rate constant (k) according to the ZORR assumption. Arrhenius equation for the transformation was found as k = A exp(-E # / RT) = (136 kg m -3 s -1 ) exp(-12,022 J mol -1 /RT) where A is the proexponential factor, E # is the activation energy, R is the universal gas constant and T is the absolute tem- perature of firing. The most general thermodynamic relation of the activation for the transformation was obtained from the TST with the SI units as DG # =-RT ln K # = DH # - TDS # = 89,367 ? 224T where DG # , K # , DH # , and DS # are the Gibbs energy, equilibrium constant, enthalpy, and entropy for the activation of the thermal transformation, respectively. The positive value of the DG # between 900 and 1050 °C indicated that the activation complex for the transformation does not form spontaneously. Keywords Activation thermodynamics  Arrhenius equation  Density  Kaolinitic clay  Transformation kinetics  Transition state theory Introduction Thermal transformation in a compact is important for the ceramic production. Transformation kinetics is based on the measurement of any physical parameter depending on the time at different temperatures [1, 2]. Some of these parameters were arranged in rows as porosity, surface area, hardness, shrinkage, and density [3–8]. Several methods were reported in the literature for the determination of the kinetics parameters such as rate constant, activation energy, and proexponential factor as well as equilibrium constant, enthalpy, entropy, and Gibbs energy of the acti- vation [9–14]. Since the kaolinitic clays (kaolins) are used as raw material in conventional porcelain production, their thermal behaviors have been extensively investigated [13, 15–17]. Major clay mineral of kaolins is called as kaolinite. Kaolinite is a hydroxylated aluminum silicate with an ideal structural formula Al 2 Si 2 O 5 (OH) 4 . Chemical equation of thermal transformations for kaolinite mineral in the successive temperature intervals is reported as follows: Al 2 Si 2 O 5 OH ð Þ 4 ! Al 2 Si 2 O 7 þ 2H 2 O 450700  C ð Þ 2Al 2 Si 2 O 7 ! Al 4 Si 3 O 12 þ SiO 2 925950  C ð Þ 3Al 4 Si 3 O 12 ! 2Al 6 Si 2 O 13 þ 5SiO 2 9501050  C ð Þ & M. O ¨ nal onal@science.ankara.edu.tr 1 Institute of Mineral Research and Exploration, C ¸ ankaya, 06800 Ankara, Turkey 2 Department of Chemistry, Faculty of Science, University of Ankara, Tandog ˘an, 06100 Ankara, Turkey 123 J Therm Anal Calorim (2016) 123:767–772 DOI 10.1007/s10973-015-4866-8