Physical-chemistry of sodium silicate gelation in an alkaline medium

The behavior of sodium silicate solutions in an alkaline medium has been studied in the 11.56-9 pH range by adding different amount of hydrochloric acid into a concentrated commercial solution ([Si] = 7 mol/L, Si/Na = 1.71, pH = 11.56). The formed products and their evolution during long ripening (up to 150 days) have been characterized by cryo-SEM, elementary analysis (ICP-AES), X-ray diffraction and surface area and relative density measurements. In the studied narrow ranges of pH (11.56-9) and silicon concentration (7-0.2 mol/L) four different situations have been observed: (i) a stable and clear solution, (ii) a reversible and transparent physical gel; (iii) a soluble white gel characterized by a significant contraction during ripening and (iv) an irreversible gel which presents a slow syneresis leading to a consolidate solid. The characterizations of the different solids, liquids and gels have shown that the observed behaviors were the results of the formation of nanometric soluble NaSi1.87O4.24 particles and/or insoluble silica-like (NaSi12.66O25.82) grains and of the contribution of a dissolution/precipitation mechanism.