Development of novel low pH Magnesium Silicate Hydrate (M-S-H) cement systems for encapsulation of problematic nuclear wastes

There are more than 100,000 tonnes of nuclear waste currently stored in the UK, waiting for final disposal. Composite cements consisting of Portland cement (PC) and blast furnace slag (BFS) have a good track record in encapsulation of a range of nuclear wastes. However, the pH of this system is relatively high (~13.3) for encapsulating wastes that containing trace levels of aluminium, as this can react under high pH conditions. The aim of this research was to develop novel cement systems with lower pH (~10) for encapsulating wastes containing aluminium metal and Magnox swarf. The hydration of magnesium oxide to form brucite produces a pH around 10 and this is a favourable pH for aluminium passivation. A range of reactive fillers were investigated and silica fume (SF) found to be the most suitable to achieve the desired pH. Identification of the hydrated phases in MgO/SF samples showed that magnesium silicate hydrate (M-S-H) gel is the main hydration product. Brucite (Mg(OH)2) also forms in the early stages of hydration but then reacts with SF to produce additional M-S-H gel. The system has been improved by addition of sodium hexameta-phosphate (Na-HMP) as a dispersant, magnesium carbonate to control the early pH and sand to minimise the drying shrinkage. The physical, chemical and mechanical properties of the improved MgO/ SF system have been investigated and compared with the control PC/BFS system. Magnox swarf and aluminium 1050 (Al) supplied by National Nuclear Laboratory (NNL) were used as metal wastes. These metals were encapsulated in the control PC/ BFS system and the optimised MgO/ SF system. The interaction between the metals and the two cement systems has been investigated by monitoring H2 generation, studying the microstructure by SEM and the crystalline phases by XRD. Al strips were firmly bound into the optimised MgO/ SF system and no H2 gas was detected during the test period. The corrosion of Al is very limited in the optimised MgO/SF system compared to the control system. Magnox swarf was found to show similar corrosion behaviour when encapsulated in both the control system and the optimised MgO/SF system. The M-S-H gel forming cement system developed in this research is novel and may have potential for encapsulating certain types of problematic legacy wastes generated from the nuclear industry