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

A white calcium bentonite (CaB) from the Kutahya region, Turkey, contains 35 wt. % opal-CT and 65 wt. % Ca-rich montmorillonite (CaM). Samples were heated at various temperatures between 100-1300 degreesC for 2 h. Thermal gravimetric (TG), derivative thermal gravimetric (DTG), and differential thermal analysis (DTA) curves were determined. Adsorption and desorption of N-2 at liquid N-2 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 dehydration and dehydroxylation of CaB calculated from the thermogravimetric data are 33 and 59 kJ mol(-1), respectively. The average deformation enthalpies, in the respective temperature intervals between 200-700 degreesC and 700-900 degreesC, were estimated to be 25 and 205 kJ mol(-1) 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 increasing temperature to 700 degreesC, but decrease rapidly above this temperature. The S and V values were 43 m(2) g(-1) and 0.107 cm(3) g(-1), respectively, for untreated bentonite. They reach a maximum at 500 degreesC and are 89 m(2) g(-1) and 0.149 cm(3) g(-1), respectively. The XRD data clearly show that, at 500 degreesC, where the irreversible dehydration is completed without any change in the crystal structure, the porosity of CaM reaches its maximum.