Long-term thermal loading study on the dehydration behavior of Ca-bentonites of Ünye (Ordu, NE Turkey)

The aim of this study is to monitor the changes in physicochemical and mineralogical properties of three Ca-bentonite samples from Unye (Ordu, NE Turkey). The effect of long-term (20, 30 and 40 days) thermal loading (100, 150 and 200 °C) treatments on the dehydration behavior of Ca-bentonites (CaB) was characterized in terms of the variations in the cation exchange capacity (CEC), specific surface area (SSA), pH, swelling index (SI), crystal structure and molecular fingerprints. CEC of raw CaB samples (52.3–62.7 meq/100 g) shows maximum decrease at 40 days to 29.9–37.3 meq/100 g at 100 °C, 19.4–25.4 meq/100 g at 150 °C and 20.9–23.9 meq/100 g at 200 °C. Long-term thermal loading also lowers the SSA of CaB samples from 42.8–51.4 m2g−1 to 19.6–30.6 m2g−1 at 100 °C, 15.9–20.8 m2g−1 at 150 °C and 17.1–19.6 m2g−1 at 200 °C. The strong alkalinity of the CaB samples (pH 9.3–9.7) changes from slight to moderate (7.7–8.6) at 200 °C. The swelling index of CaB samples shows narrow-range variations from 4.4–7.5 mL/2 g (raw) to 4.2–5.3 mL/2 g at 100 °C, 4.2–5.8 mL/2 g at 150 °C and 3.7–5.8 mL/2 g at 200 °C at 40 days. Thermal loading has also the imprints on the molecular scale in the loss of capillary condensed and adsorbed water vibrations (~ 3388 cm−1). The dehydration effect at 100 and 150 °C thermal loadings is revealed by the broadening and asymmetry of the basal reflection with a decrease in the intensity; on the other hand, 200 °C treatment at 40 days results in the appearance of ~ 1 nm reflection which is suggested to be the beginning of illitization.