Incorporation of aluminum in the calcium silicate hydrate (C-S-H) of hydrated Portland cements: A high-field Al-27 and Si-29 MAS NMR Investigation

摘要

The calcium silicate hydrate (C-S-H) phase resulting from hydration of a white Portland cement (wPc) in water and in a 0.3 M NaAIO(2) solution has been investigated at 14 and 11 hydration times, respectively, ranging from 6 h to 1 year by (27)AI and Si-29 MAS NMR spectroscopy. (27)AI MAS NMR spectra recorded at 7.05, 9.39, 14.09, and 21.15 T have allowed a determination of the (27)AI isotropic chemical shift (delta(iso),) and quadrupolar product parameter (P-Q = C(Q)root1 + eta(Q)(2)/3) for tetrahedrally coordinated AI incorporated in the C-S-H phase and for a pentacoordinated AI site. The latter site may originate from AI(3+) substituting for Ca2+ ions situated in the interlayers of the C-S-H structure. The spectral region for octahedrally coordinated AI displays resonances from ettringite, monosulfate, and a third aluminate hydrate phase (delta(iso) = 5.0 ppm and P-Q = 1.20 MHz). The latter phase is tentatively ascribed to a less-crystalline aluminate gel or calcium aluminate hydrate. The tetrahedral AI incorporated in the C-S-H phase has been quantitatively determined from (27)AI MAS spectra at 14.09 T and indirectly observed quantitatively in Si-29 MAS NMR spectra by the Q(2)(1AI) resonance at -81.0 ppm. A linear correlation is observed between the Si-29 MAS NMR intensity for the Q(2)(1AI) resonance and the quantity of AI incorporated in the C-S-H phase from 27AI MAS NMR for the different samples of hydrated wPc. This correlation supports the assignment of the resonance at delta(iso)(Si-29) = -81.0 ppm to a Q(2)(1AI) site in the C-S-H phase and the assignment of the (21)AI resonance at delta(iso)((21)AI) = 74.6 ppm, characterized by P-Q((27)AI) = 4.5 MHz, to tetrahedrally coordinated AI in the C-S-H. Finally, it is shown that hydration of wPc in a NaAIO(2) solution results in a C-S-H phase with a longer mean chain length of SiO4 tetrahedra and an increased quantity of AI incorporated in the chain structure as compared to the C-S-H phase resulting from hydration of wPc in water. [References: 38]