Structural changes in smectites subjected to mechanochemical activation: The effect of the occupancy of the octahedral sites

Abstract The effect of the configuration and composition of the octahedral sheet on the mechanochemical stability of different smectites, both trioctahedral (natural and synthetic hectorites, Hc, and saponite, Sp) and dioctahedral (montmorillonites, Mt) subjected to a short period of high energy grinding (HIG) was evaluated. The higher extent of amorphization for dioctahedral than for trioctahedral smectites was confirmed from the results obtained from XRD analysis, infrared and MAS NMR spectroscopy. After extensive grinding, 29Si MAS NMR spectra indicated transformation of the Si-tetrahedra from a layered structure (Q3 sites) to Si-tetrahedra in a three-dimensional network (Q4 sites). However, the Q3 signal still dominated the spectra of trioctahedral samples, rather than the Q4 signal that indicates formation of an amorphous phase. 27Al MAS NMR spectra confirmed the transformation of the octahedral Aloct and tetrahedral Altet present in the layered structure of Mt into an amorphous phase containing both Altet and Aloct coordination. FTIR spectra of the unground samples showed the presence of absorption bands associated with OH groups attached to octahedra with Mg atoms in the central positions, while none of the bands attributed to structural OH groups in Mt could be seen in the spectra after grinding. The 1H NMR spectra exhibited strong signals of structural OH after 6 min of grinding for Hc and Sp. Similar trends were seen in the DTG and H2O mass profiles for the dehydroxylation step: the peaks attributed to structural OH groups were not observed in Mt after treatment, while they were still detected for both hectorites and saponite.