Aluminum Incorporation in Calcium Silicate Hydrates (C−S−H) Depending on Their Ca/Si Ratio

27Al magic angle spinning nuclear magnetic resonance (27Al MAS NMR) spectroscopy at different magnetic fields was used to characterize the aluminum incorporation in the tetrahedral−octahedral−tetrahedral (Te−Oc−Te) structure of calcium silicate hydrates (C−S−H), which are the main constituents of the hydrated cement-based materials. C−S−H of different calcium/silicon ratio (0.66 < Ca/Si < 1.7) were synthesized in the presence of aluminum. Two different aluminum/silicon ratios (0.1 and 0.3) were tested. The maximum Al(IV)/[(Al(IV) + Si] ratio in the C−S−H that could be detected in these series of experiments was 0.17. Results show in this case that, when the tetrahedral sheet is formed by linear silicate chains, Al3+ preferentially substitutes a nonbridging Si4+. The rupture of the chains, caused by an increase of the Ca/Si ratio, makes such a position unstable and a redistribution of the aluminum in the tetrahedral sites occurs. Results also indicate that the substitution of Si4+ cannot take place when the tetrahedral sheet is composed of dimers (i.e., for high Ca/Si ratios). In these cases, Al3+ substitutes Ca2+ in the interlayer space (5-fold coordinated) and in the octahedral sheet (6-fold coordinated). However, this kind of substitution remains limited. The amount of aluminum incorporated in the C−S−H structure increases with the length of chains. Results confirm that C2AH8 is not a time-stable phase.