Substituted Tobermorites: ²⁷Al and ²⁹Si MASNMR, Cation Exchange, and Water Sorption Studies

²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy revealed that the maximum amount of Al that can be substituted for Si in the tobermorite structure is 15 to 20 mol%. Powder X‐ray diffraction and cation exchange studies corroborate the above finding. Anomalous tobermorite structures resulted in all cases, and hydrogarnet appeared beyond the 15 to 20 mol% Al substitution limit for Si. The sorption of water molecules by synthetic [Al³⁺+ Na⁺]‐substituted tobermorites and their cation‐exchanged forms heated at 200°C under vacuum (≥10⁻³ torr) was measured at 25°C in order to probe the nature of the ion‐exchange cavity. Samples with ≤30 mol% of Al substitution for Si showed isotherms of Brunauer, Demming, Demming, Teller (BDDT) type I and gave better linearity with the Langmuir plot than with the BET plot. Samples with ≥35 mol% of Al substitution for Si showed BDDT type II isotherms and gave better linearity with the BET than with the Langmuir equation. The Langmuir monolayer capacity was found to depend on the Al content and on the cationic form. The Li⁺‐ and Na⁺‐exchanged tobermorites with about 20 mol% of Al substitution for Si showed the highest Langmuir monolayer water sorption capacity. The Cs⁺‐exchanged tobermorites showed a smaller capacity than the Li⁺‐ or Na⁺‐exchanged samples, which can be ascribed to clogging of the ion‐exchange cavity by the large Cs⁺ ions.