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 3 5 wt. % opal-CT and 65 wt. % Ca-rich montmorillonite (CaM). Samples were he ated at various temperatures between 100-1300 degreesC for 2 h. Thermal gra vimetric (TG), derivative thermal gravimetric (DTG), and differential therm al analysis (DTA) curves were determined. Adsorption and desorption of N-2 at liquid N-2 temperature for each heat-treated sample was determined. X-ra y 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 th e clay by dehydration, dehydroxylation, recrystallization, shrinkage, fract ure, etc. The activation energies related to the dehydration and dehydroxyl ation of CaB calculated from the thermogravimetric data are 33 and 59 kJ mo l(-1), respectively. The average deformation enthalpies, in the respective temperature intervals between 200-700 degreesC and 700-900 degreesC, were e stimated to be 25 and 205 kJ mol(-1) using CEC data and an approach develop ed in this study. The specific surface area (S) and the specific micropore- mesopore volume (V) calculated from the adsorption and desorption data, res pectively, show a "zig zag" variation with increasing temperature to 700 de greesC, but decrease rapidly above this temperature. The S and V values wer e 43 m(2) g(-1) and 0.107 cm(3) g(-1), respectively, for untreated bentonit e. 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, wh ere the irreversible dehydration is completed without any change in the cry stal structure, the porosity of CaM reaches its maximum.