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

Abstract

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 27Al and 29Si MAS NMR. This includes blends with 0, 5, 10, 15, 20, 25, 30 wt% MK, following their hydration from 1 d to 1 yr. 29Si 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 27Al MAS NMR which also allows detection of strätlingite and fivefold coordinated aluminum, assigned to AlO5 sites in the interlayer of the C–S–H structure. Strätlingite 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 strätlingite 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.