Multimineral reactions controlling secondary phase evolution in a hyperalkaline plume

This paper presents a modelling study of the influence of a hyperalkaline leachate on a host rock in a geological disposal facility for low- to intermediate-level nuclear waste disposal, when compared with a 15-year experiment designed to simulate the geochemical and mineralogical evolution of the system. The hyperalkaline leachate may be produced by the interaction of groundwater with the cementitious backfill surrounding the waste form. The rock sample is from the Borrowdale Volcanic Group, which comprises orthoclase, quartz, dolomite, muscovite, haematite and calcite as the major mineral phases, with various kinetic dissolution/precipitation rates. Multimineral reactions with the multiple chemicals in the leachate are responsible for the variation in chemical concentration and temporal evolution of different primary and secondary phases observed in the experimental study. The temporal variation in the concentrations of calcium (Ca²⁺), silicate (SiO₃ ²⁻), magnesium (Mg²⁺) and carbonate (CO₃ ²⁻) ions and pH has been modelled, which agrees well with the experimental results. The mineral evolution process has confirmed that dolomite and calcium silicate hydrate (CSH) gel (CSH phases) will dissolve and generate new secondary phases, such as calcite and saponite-K over the 15-year experiment.