A commercial blast furnace slag was activated using either sodium hydroxide or hydrous sodium metasilicate, and the degree of hydration was determined by 29Si magic angle spinning nuclear magnetic resonance (NMR). The results are compared with measurements made using scanning electron microscopy image analysis (SEM-IA). The results from both 29Si NMR and the SEM-IA measurements indicated a fast initial reaction with the alkali, and similar degrees of hydration throughout the reaction. The 29Si NMR results were analyzed using two different methods for fitting the residual slag in the decomposition of the 29Si NMR spectra: the first method used the spectrum of the anhydrous slag, whereas the second method used the spectrum of the dissolution residue of the hydrated sample. Only the first method provided a satisfactory simulation. The degree of hydration and the Al/Si atomic ratio within the CSH, deduced by 29Si NMR were in agreement with SEM-IA and EDX analyses.