Kinetic study of the drying process of clay bricks

This work deals with the drying kinetics of three red clays with different mineralogical composition and grain size distribution used for the production of clay bricks. The kinetic study was performed using thermo-gravimetry (TG) in both non-isothermal and isothermal mode in the 50-200 °C range. To the knowledge of the authors, this is the first time that the drying process of clay bricks is studied using the TG method to present a general model at molecular scale of the phenomenon. The observed drying mechanism is composed of a fast initial step and a slow final step. The former is characterized by an Avrami-like mechanism with an Avrami coefficient n 1 comprised between 1 and 2. This kinetic parameter indicates a diffusive control in three dimensions with instantaneous or deceleratory nucleation and refers to desorption of adsorbed water from the surface of all mineral phases. The second step is also characterized by an Avrami-like mechanism with n 2 = 1 and points to a diffusion-controlled reaction in two dimensions, with instantaneous nucleation. This step involves dehydration of plastic clay minerals such as illite, smectite and IS through diffusion of water molecules within the two-dimensional interlayer. The apparent activation energies related to the first fast mechanism have similar values ( < 32 kJ mol -1 ) for all samples. The apparent activation energy E a for the second slower step is dependent upon the mineralogical composition and is > 35 kJ mol -1 . The dependency of the apparent activation energies on grain size distribution and sample mass was also assessed.