Geopolymerization is an innovative technology that can transform several solid aluminosilicate materials into useful products called geopolymers or inorganic polymers. Although the geopolymerization mechanism is not well understood, the most proposed mechanism includes four parallel stages: (a) dissolution of solid aluminosilicate materials in alkaline sodium silicate solution, (b) oligomerization of Si and/or Si-Al in aqueous phase, (c) polymerization of the oligomeric species, and (d) bonding of undissolved solid particles in the polymer. It is obvious that polymerization in sodium silicate solutions comprises a fundamental process in geopolymerization technology. Therefore, this article aims at studying experimentally the polymerization stage in synthetic pure sodium silicate solutions. The structure of sodium silicate gels as a function of the SiO2/Na2O molar ratio is examined and their hardness as well as hydrolytic stability are determined. In addition, the effect of aluminum incorporation in the hydrolytic stability of these gels is also examined. Finally, the structure of sodium silicate and aluminosilicate gels is correlated to the measured properties drawing very useful conclusions that could be applied on geopolymerization technology.