Mass transport mechanisms of caesium- and strontium-loaded geopolymer cement wasteforms under repository conditions

Geopolymers have been highlighted as an alternative to Portland cement blends for
the immobilisation of low- and intermediate-level radioactive waste in the UK. Caesium-
137 and strontium-90, abundant radioisotopes in these waste streams, could be effectively
disposed of using these materials. This thesis aims to contribute to the safety case for
geopolymer cements by understanding the mass transport processes and radionuclide retention
mechanisms of caesium and strontium in geopolymers under conditions relevant to
UK geological disposal.
Geopolymers loaded with caesium and strontium were subjected to monolithic leach
tests to determine how the mass transport processes and radionuclide retention mechanisms,
are affected by temperature, irradiation, and the presence of ion exchange material.
Results indicate that caesium transport occurs through surface exchange and diffusion,
while strontium displays more complex behaviour, with surface exchange and diffusion occurring
alongside an additional third mechanism involving crystallisation of the amorphous
geopolymer gel. Differences in caesium and strontium release rates are attributed to differences
in incorporation mechanisms: caesium binds into the alkali aluminosilicate hydrate
(geopolymer) gel in charge-balancing extra-framework sites, but competes with potassium
ions for sites, whereas strontium is present in both charge-balancing extra-framework sites
and in stable additional phases such as strontium carbonate or strontium hydroxide.
Leach tests revealed that temperature strongly influences mass transport processes,
with diffusion becoming more dominant at higher temperatures for both caesium and
strontium systems. In contrast, irradiation of caesium- or strontium-loaded geopolymers,
and inclusion of caesium- or strontium-loaded ion exchange material (clinoptilolite) had a
relatively minimal effect.
Overall, geopolymers demonstrated low leachability and minimal structural changes
across all tested conditions, demonstrating good performance as wasteform materials under
conditions relevant to UK geological disposal. These findings support the potential of
geopolymers as an effective alternative to Portland cement blends, strengthening the UK
safety case for their application in radioactive waste disposal.