Effect of metakaolin and fly ash on the early hydration and pore structure of Portland cement

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 T 1 -T 2 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 H 2 O-D 2 O exchange, was slower in metakaolin composites than in fly ash containing samples. The novel combination of NMR, SEM and N 2 porosimetry showed an advantage of metakaolin additive over fly ash for the long-term safe disposal of radioactive waste. • Metakaolin and fly ash as additives in cement composites and their effect on the pore structure were compared. • During the early hydration of cement, metakaolin exhibited pozzolanic activity significantly earlier than fly ash. • Stronger solid- water interaction in case of pozzolan containing composites was observed compared to Portland cement. • LF-NMR, SEM and N 2 porosimetry revealed the micro- to macroscale changes of the cement structure caused by the pozzolans. • Diffusion of water in the capillary pores was an order of magnitude slower in the metakaolin composites.