The applicability, reliability, and repeatability of Si-29 MAS NMR for determination of the quantities of alite (Ca3SiO5) and belite (Ca2SiO4) in anhydrous Portland cement was investigated in detail for 11 commercial Portland cements and the results compared with phase quantifications based on powder X-ray diffraction combined with Rietveld analysis and with Taylor-Bogue calculations. The effects from paramagnetic ions (Fe3+) on the spinning sideband intensities, originating from dipolar couplings between Si-29 and the spins of the paramagnetic electrons, were considered and analyzed in spectra recorded at four magnetic fields (4.7-14.1 T) and this has led to an improved quantification of alite and belite from Si-29 MAS NMR spectra recorded at "high" spinning speeds of v(R) = 12.0-13.0 kHz using 4 or 5 mm rotors. Furthermore, the impact of Fe3+ ions on the spin-lattice relaxation was studied by inversion-recovery experiments and it was found that the relaxation is overwhelmingly dominated by the Fe3+ ions incorporated as guest-ions in alite and belite rather than the Fe3+ sites present in the intimately mixed ferrite phase (Ca2AlxFe2-xO5). (C) 2009 Elsevier Inc. All rights reserved.