The effect of thermal treatment on some of the physicochemical properties of a bentonite

CT and 65 wt. % Ca-rich montmorillonite (CaM). Samples were heated at various temperatures between 100–13008C for 2 h. Thermal gravimetric (TG), derivative thermal gravimetric (DTG), and differential thermal analysis (DTA) curves were determined. Adsorption and desorption of N2 at liquid N2 temperature for each heat-treated sample was determined. X-ray diffraction (XRD) and cation-exchange capacity (CEC) data were obtained. The change in the d(001) value and the deformation of the crystal structure of CaM depend on temperature. Deformation is defined here as changes of the clay by dehydration, dehydroxylation, recrystallization, shrinkage, fracture, etc. The activation energies related to the dehydra-tion and dehydroxylation of CaB calculated from the thermogravimetric data are 33 and 59 kJ mol21, respectively. The average deformation enthalpies, in the respective temperature intervals between 200– 7008C and 700–9008C, were estimated to be 25 and 205 kJ mol21 using CEC data and an approach developed in this study. The specific surface area (S) and the specific micropore-mesopore volume (V) calculated from the adsorption and desorption data, respectively, show a ‘‘zig zag’ ’ variation with in-creasing temperature to 7008C, but decrease rapidly above this temperature. The S and V values were 43 m2 g21 and 0.107 cm3 g21, respectively, for untreated bentonite. They reach a maximum at 5008C and are 89 m2 g21 and 0.149 cm3 g21, respectively. The XRD data clearly show that, at 5008C, where the irre-versible dehydration is completed without any change in the crystal structure, the porosity of CaM reaches its maximum