Substituting cement with pozzolans is important for various applications, like radioactive
waste management. In this study, 10–30 % metakaolin and fly ash were added to Portland
cement to gain insight into their effects on the structure formation, pore structure,
and surface properties of cement. Early hydration of these composites was monitored
using NMR relaxometry, showing that metakaolin exhibited pozzolanic activity after
8 h, while filler effect was observed for fly ash. FFC NMR and T1-T2 correlation relaxometry
revealed stronger water-solid interaction for the composites compared to pure cement.
The dominance of the CSH gel pores increased with the additives in dry and wet state.
Water diffusion in the capillary pores, followed by H2O-D2O exchange, was slower in
metakaolin composites than in fly ash containing samples. The novel combination of
NMR, SEM and N2 porosimetry showed an advantage of metakaolin additive over fly ash
for the long-term safe disposal of radioactive waste.
