This thesis investigates the potential use of geopolymers to encapsulate intermediate level waste (ILW), particularly Magnox swarf contaminated with Al metal and Cs/Sr-loaded clinoptilolite. Both wastes have different interactions with the encapsulation matrix. For Magnox swarf waste containing trace Al metal, the pH of the encapsulating matrix is a key factor that controls corrosion and release of hydrogen. Cs and Sr can leach from contaminated clinoptilolite into the encapsulating geopolymer and therefore the chemical interactions of these ions with the matrix have been investigated.
A fundamental understanding of the geopolymer system used for encapsulation was developed. This involved investigating the influence of different precursor on the mechanical properties. It was shown showed that metakaolin based geopolymers are unstable at room temperature when in contact with an atmosphere with a low relative humidity and excessive drying shrinkage occurs. This shrinkage can be reduced by adding inert fillers which have low impact on the mortar viscosity and mechanical strength.
Magnox waste and Al-metal have been encapsulated in metakaolin based geopolymers and surface interactions studied using SEM-EDX and XRD. In addition the corrosion rates were determined. Magnox swarf does not react with the geopolymers matrix, while Al-metal rapidly corrodes. However, by using a metakaolin with a low molar Si:Al ratio and controlling the molar Al:Na ratio in geopolymers the corrosion can be significantly reduced and allows encapsulation of this difficult waste stream.
Surface reactions of Cs/Sr-contaminated clinoptilolite and geopolymers were also studied. Simulated wastes containing Cs+ and Sr2+ salts were mixed with geopolymers and the influence of the cations on the geopolymer microstructure and leaching were investigated. Mixing Cs/Sr-contaminated clinoptilolite with activation solution causes surface dissolution of clinoptilolite with release of Cs and Sr ions into the matrix. Leaching of Cs contaminated geopolymers showed that Cs+ ions can be immobilised at concentrations up to 10 wt%. Sr2+ reacts with the activating solution and dissolved metakaolin and is build chemically into the structure.
The research has resulted in a number of key conclusions related to the stability of metakaolin derived geopolymers and their interactions with the selected wastes.