The structure of the C-S-H gel in hydrated cement samples prepared with and without the addition of silica nanoparticles is studied in this work through the deconvolution of 29 Si MAS-NMR spectra. X-ray diffraction and energy dispersive X-ray microanalysis are also considered for the analysis. A method with improved sensitivity and reliability is proposed for the deconvolution of the spectra into elementary components involving a good fitting of both the spectral curve and its second derivative. The increased sensitivity of the deconvolution proves to be especially interesting for the characterization of low concentration components in the NMR spectra, and the high reliability allows properly defining the effect of nanosilica addition on the C-S-H gel nanostructure. In fact, the addition of nanosilica is found to produce a pozzolanic reaction that results in an increase of the mean silicate chain length and an enhancement of the aluminum incorporation into the C-S-H gel phase.