Quantification of calcium silicate phases in Portland cements by Si-29 MAS NMR spectroscopy

Si-29 magic-angle spinning (MAS) NMR spectroscopy is shown to be a valuable tool for quantifying two calcium silicates, alite (Ca3SiO5) and belite (Ca2SiO4), in Portland cements. Determination of these minerals involves computer deconvolution of the Si-29 MAS NMR spectra combined with elemental analysis of the bulk SiO2 content. The applicability of the method is demonstrated for various types of Portland cements and it is shown that the monoclinic M(I) and M(III) forms of alite can be distinguished by Si-29 MAS NMR. Inversion-recovery Si-29 spin-lattice relaxation MAS NMR allows quantification of the relative content of alite and belite in an independent way and the results of the deconvolution for two of the cements have been tested and confirmed using this technique. Comparison of the Si-29 MAS NMR results for the alite and belite contents with the traditional Bogue calculation demonstrates that this calculation strongly overestimates the belite content at the expense of alite in Portland cements. Improved correlations are observed when the NMR data are compared with the alite and belite quantities determined from a Taylor-modified Bogue calculation.