Aluminum Incorporation in the C-S-H Phase of White Portland Cement-Metakaolin Blends Studied by 27Al and 29Si MAS NMR Spectroscopy

The composition and structure of the calcium-silicate-hydrate (C-S-H) phases formed by hydration of white portland cement-metakaolin (MK) blends have been investigated using Al-27 and Si-29 MAS NMR. This includes blends with 0, 5, 10, 15, 20, 25, 30 wt% MK, following their hydration from 1 d to 1 yr. (29) Si MAS NMR reveals that the average Al/Si ratio for the C-S-H phases, formed by hydration of the portland cement-MK blends, increases almost linearly with the MK content but is invariant with the hydration time for a given MK content. Correspondingly, the average aluminosilicate chain lengths of the C-S-H increase with increasing MK content, reflecting the formation of a C-S-H with a lower Ca/Si ratio. The increase in Al/Si ratio with increasing MK content is supported by Al-27 MAS NMR which also allows detection of stratlingite and fivefold coordinated aluminum, assigned to AlO5 sites in the interlayer of the C-S-H structure. Stratlingite is observed after prolonged hydration for MK substitution levels above 10 wt% MK. This is at a somewhat lower replacement level than expected from thermodynamic considerations which predict the formation of stratlingite for MK contents above 15 wt% after prolonged hydration for the actual portland cement-MK blends. The increase in fivefold coordinated Al with increasing MK content suggests that these sites may contribute to the charge balance of the charge deficit associated with the incorporation of Al3+ ions in the silicate chains of the C-S-H structure.