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 NaSi_1.87O_4.24 particles and/or insoluble silica-like (NaSi_12.66O_25.82) grains and of the contribution of a dissolution/precipitation mechanism.