Fe³⁺-substituted tobermorites: Synthesis and characterization by Mössbauer spectroscopy and other techniques

The isomorphous substitution of Fe³⁺ for Si⁴⁺ in the tobermorite (Ca₅Si₆H₂O₁₈·4H₂O) structure and its effects on the cation exchange properties of tobermorites were investigated. Fe³⁺- substituted 11Å tobermorites were synthesized under hydrothermal conditions. The cation exchange capacities of these Fe³⁺-substituted tobermorites ranged between 82-122 meq/100g while specific Cs sorption ranged from 420-800 K_d. ⁵⁷Fe Mossbauer Spectroscopy, in addition to other techniques, was used to investigate the Fe-substitution in the tobermorite structure. None of the Mossbauer spectra revealed any absorption line due to Fe²⁺. The observed spectra were fitted to one or more closely overlapping quadrupole doublets. The assignment of Fe³⁺ among the available tetrahedral and octahedral sites was made based on the distinct differences in the observed isomer shifts. The extent of Fe³⁺_IV vs. Fe³⁺_VI site occupancies was determined from the corrected intensities of the respective quadrupole doublets.