Effects of short-term high-temperature calcinations on the physico-chemical and mineralogical properties of Ca-bentonites from Ünye (Ordu, NE Turkey)

Three Ca-bentonite samples from Ünye (NE Turkey) were calcined up to 1000 °C with 200 °C increments at 25 min. The physico-chemical and mineralogical effects of high-temperature loadings were evaluated by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) and loss on ignition (LOI) analysis. The results of the experiments show that the dehydration up to 400 °C does not alter the properties of Ca-bentonites significantly; however, the endothermic reactions due to dehydroxylation between 400 and 800 °C cause the deformation in the Ca-smectite crystal structure revealed by the collapse of the basal reflection from 1.4 to 1.0 nm and the gradual disappearance of OH stretching of structural hydroxyl groups and decrease in specific surface area and the closure of micropores. By calcination at 1000 °C, the specific surface area reaches the minima of 1.33–2.05 m 2 g −1 for all the samples, and the structure of Ca-smectite is almost completely decomposed for CaB 2 and CaB 3 , however, partially decomposed for CaB 1 due to incomplete dehydroxylation stage. The crystallinity of opal-CT intensifies, and the formation of amorphous silica phases is promoted for all samples at 1000 °C; however, the formation of mullite is enhanced only for CaB 2 and CaB 3 . The sintering effect is revealed as much larger aggregates with more rounded morphologies. It is also determined that the calcination up to 1000 °C does not affect the mesoporous characteristics of Ca-bentonite samples.