The main component of the cement hydration, are both, the calcium silicate hydrate (C–S–H) and calcium silicate hydrate with Al (C–S(A)–H), whose composition is characterized by its calcium to silicon ratio (Ca/Si), which normally varies from 0.6 to 1.6. The theoretical Ca/Si ratios of the synthesized gels were compared with those of the experimental gels, which were determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES). In addition, the microstructure of the gels was studied by spectroscopic techniques: infrared and Raman spectroscopy and nuclear magnetic resonance. By the double-decomposition method used in this work (1 day at 25 °C, inert atmosphere and pH = 12.3), only C–S–H and C–S(A)–H gels with a maximum Ca/Si ratio ranging from 0.8 to 1.0 were synthesized. However, the structures of the gels are slightly different as the Ca/Si ratio increases. Calcium silicate hydrate, with and without Al has been synthetized following double decomposition method, with different Ca/Si ratio. Analyzing Ca/Si ratio of the formed compounds, indicate that C–S(A)-H with Ca/Si ratios from 0.6 to 1.1 can be formed. Highlights Calcium silicate hydrate, with and without aluminum, was synthesized by the double-decomposition method with different nominal Ca/Si ratio. Ca/Si ratio ranging from 0.8 to 1.0 in the C–S–H synthesized by the double-decomposition method. C–S(A)–H gel with Ca/Si ratio <0.6, cannot be synthesizes by the double-decomposition method. As nominal Ca/Si ratio increases, Q 1 units increases in the C–S–H gel structure.