BOTTOM-UP DESIGN OF A CEMENT FOR NUCLEAR WASTE ENCAPSULATION

The chemical interactions between waste and the cement, in which it is encapsulated, are dominated by the chemistry of the pore water in the cement. Hence tailoring the interaction between waste and cement requires designing the chemistry of the cement matrix. In this paper, the chemical design of a new cement compatible with aluminium metal will be discussed. The starting point was the realization that hydration of magnesia to form brucite should yield a pH around 10 and that this is a favourable pH for aluminium passivation, certainly when compared with the much more alkaline environment induced by Portland cement. A range of fillers was studied and metakaolin and fused silica found to be compatible with the desired pH. Further work with fused silica showed that the brucite reacts with the fused silica to give magnesium-silicate-hydrate (M-S-H) gel and that addition of magnesium carbonate allows buffering the early pH. Encapsulation trials with aluminium showed that little or no corrosion occurred in the time-frame studied.