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

Papp, Vanda and Ardelean, Ioan and Bulátkó, Anna and László, Krisztina and Csik, Attila and Janovics, Róbert and Kéri, Mónika (2025) Effect of metakaolin and fly ash on the early hydration and pore structure of Portland cement. CEMENT AND CONCRETE RESEARCH, 196. No.-107928. ISSN 0008-8846

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Abstract

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.

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