Polycondensation in alkali silicate solutions comprises a fundamental process of the geopolymerization technology. Previous works had shown that the hydrolytic stability of sodium silicate gels depends on the SiO2/Na2O ratio. Sodium silicate gels totally insoluble in water can be produced at SiO2/Na2O molar ratios higher than 4.4. This article aims at elucidating the effect of tetra-coordinated aluminum addition on the hydrolytic stability of sodium silicate gels. According to the results, the aluminum addition stabilizes the sodium silicate gels in an aqueous environment. A sodium silicate gel with SiO2/Na2O molar ratio 3.48, which is totally soluble in deionized water at ambient temperature, can be transformed to insoluble sodium hydroaluminosilicates with the addition of tetrahedral aluminum at Al/Si molar ratios higher than 0.08. In addition, this article studies the structure of prepared sodium hydroaluminosilicates and draws very useful conclusions for the geopolymerization technology.