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

Abstract

²⁹ Si 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 ²⁹Si 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 ²⁹Si MAS NMR. Inversion–recovery ²⁹Si 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 ²⁹Si 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.