Transmission Electron Microscopy and Nuclear Magnetic Resonance Studies of Geopolymers for Radioactive Waste Immobilization

Transmission electron microscopy of a geopolymer phase, derived from metakaolin and alkaline silicate solutions and having nominal Na/Al and Si/Al molar ratios of 1 and 2, respectively, showed it to be amorphous on the ∼1 nm scale after curing at 40°C. In samples containing 5 wt% Cs or Sr, Cs inhabited the amorphous phase, whereas Sr was incorporated only partly, being preferentially partitioned to crystalline SrCO3. Heating progressively to dewater the material had little effect on the overall structure for temperatures up to 500°C. From solid‐state nuclear magnetic resonance in material cured at near‐ambient temperatures, Cs, like Na, was found to be basically associated with pore water, but with significant linkage to the aluminosilicate framework, more so than for Na. Subsequent heating to 300°C increased the linkage of Cs and Na to the framework.