Consolidation of cementitious and geopolymeric materials involves silicate-based gel formation. This in situ mechanism is difficult to identify because it occurs in a complex and developing system representing only a minority phase. A study based on the behaviour of acidified sodium silicate solutions in alkaline medium, was therefore initiated in order to define the conditions of irreversible setting. A concentrated sodium silicate solution ([Si]=7 mol/l, pH=11.56, Si/Na=1.71) was used as starting solution. 29Si NMR spectroscopy, SAXS and elementary chemical analyses (ICP-AES) were used to characterize the various solutions. Acidification of initial solution, leads in a range of relatively low pH and silicon concentration to various gels formation: (i) reversible transparent gels made up of aggregates of particles (Si7O18H4Na4) and which do not change over time, (ii) soluble white gels that lead to gradual formation of a soluble solid consisting of colloid composition of NaSi1.87O4.24, (iii) “irreversible” gels which provide a syneresis phenomenon leading to formation of a strongly consolidated solid made up of soluble phase rich in sodium similar to white gels (NaSi1.87O4.24) and an insoluble phase type silica of composition NaSi12.66O25.82.