CHARACTERIZATION OF CALCIUM ALUMINATE HYDRATES AND RELATED HYDRATES OF CEMENT PASTES BY AL-27 MQ-MAS NMR

Al-27 multi quantum (MQ) MAS NMR spectroscopy was used for the first t ime to characterize calcium aluminate hydrates, which are of importanc e 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 Ca3Al(OH)(12), [CaAl(OH)(4)][OH(H2O)(1.5)], [Ca 2Al(OH)(6)](OH). 3H(2)O, [Ca2Al(OH)(6)](2)(CO3). 5H(2)O, [Mg2Al(OH)(6) ](CO3)(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 spectr a allow one, more easily than the MAS-only NMR spectra, to obtain the chemical shift, delta(iso), and the quadrupolar parameters nu(Q) and e ta, which label each site and bring information on its symmetry and en vironment. The distributions of the aluminum environments were observe d for each site. In [Ca2Al(OH)](6)(OH)3H(2)O, Al-27 MAS spectrum demon strates the presence of two octahedral aluminum sites. In the C-S-H su bstituted with Al, tetrahedral aluminum is observed, in bridging and n onbridging sites of the silicate chains, mostly in the bridging sites for the sample investigated.