Comparing the short-term leaching behaviour of cements containing supplementary cementitious materials in different leachants for low-level waste encapsulation

Pulverized fly ash (PFA) is commonly used as a supplementary cementitious material (SCM) in cement and is permitted for use in cements and grouts used for the encapsulation of low-level nuclear waste. However, with a global decrease in availability of PFA predicted in the near future, alternative SCMs are required. In nuclear encapsulation, a key parameter for a new SCM is that it does not change the leaching behaviour of the grout. We present promising initial leaching data from two SCMs: metakaolin and waste paper fly ash. These were tested for 24 h in two leachants, deionized water and an artificial groundwater, tracking the pH and leachability of a range of elements as the samples equilibrated with the leachants. The study suggests that for many elements, both metakaolin and waste paper fly ash show a very similar leaching behaviour to PFA grout formulations despite differences in the pH equilibration. In general, the tests suggest that the composition of the elute is dominated by leaching from the cementitious phases, and not the SCM. This suggests that further longer-term leaching studies are required as part of testing for SCM suitability for nuclear grouts in order to understand the connections between leaching and grout microstructure.