Quantification of the C–S–H hydrates and anhydrous material in plain and blended cement systems can be performed by deconvolution of 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. NMR data are reliable for simple cement systems, but with the incorporation of supplementary cementitious materials, quantification is often uncertain. For example, the overlap of peaks from slag cement and C–S–H in 29Si MAS NMR spectra causes problems with deconvolution. A novel method was developed to address these difficulties. 29Si MAS NMR was combined with a selective dissolution method. The hydrate peaks in slag blends can now be quantified without interference from the slag peak. This new method enables the silica remaining in unreacted slag to be estimated, thus allowing the degree of slag hydration to be quantified. Hydrate Al/Si ratios correlate well with data from analytical transmission electron microscopy (TEM). Analysis of the dissolution residues by TEM and 29Si MAS NMR indicates that they consist of a mixture of unreacted slag, a hydrotalcite-type phase, and small amounts of aluminosilicate gel. The origin of the aluminosilicate gel needs further investigation.