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.

27Al multi quantum (MQ) MAS NMR spectroscopy was used for the first time to characterize calcium aluminate hydrates, which are of importance in the chemistry of high alumina and Portland cements. Substitution sites of silicon by aluminum in the calcium silicate hydrates (C-S-H) which are the main component of Portland cement paste were studied too. Synthetic samples of Ca(3)Al(OH)(12), [CaAl(OH)(4)][OH(H(2)O)(1.5)], [Ca(2)Al(OH)(6)](OH).3H(2)O, [Ca(2)Al(OH)(6)](2)(CO(3)).5H(2)O, [Mg(2)Al(OH)(6)](CO(3))(0.5).3H(2)O, Al(OH)(3), and C-S-H substituted by aluminum were prepared. In most of the samples, the two dimension 3Q-MAS NMR spectra allow one, more easily than the MAS-only NMR spectra, to obtain the chemical shift, delta(iso), and the quadrupolar parameters nu(Q) and eta, which label each site and bring information on its symmetry and environment. The distributions of the aluminum environments were observed for each site. In [Ca(2)Al(OH)](6)(OH).3H(2)O, (27)Al MAS spectrum demonstrates the presence of two octahedral aluminum sites. In the C-S-H substituted with Al, tetrahedral aluminum is observed, in bridging and nonbridging sites of the silicate chains, mostly in the bridging sites for the sample investigated.