A comprehensive study on the gelation process of silica gels from sodium silicate

The sol-gel route via sodium silicate precursor is known as a practical synthesis method to prepare silica gels in a cost effective way. In the current research, rheological properties of the synthesized silica gels were investigated at different pH values. The gelation time (tgel) was determined from the rheological measurements at different frequencies of 0.1, 0.3 and 0.5 Hz. The results were analyzed using rheological equations of state and the concept of the gelation point was discussed. The results revealed that tgel increases by increasing the pH value. Moreover, it was proved that the fractal value (df) is dependent on the pH value. The relaxation exponent (Δ) value was measured as 0 < Δ < 1/2, which shows that the gel system follows the non-stoichiometrically balanced reactions. Finally, it was shown that the gelation process is divided into three stages, namely, induction, gelation and over curing. The results revealed that by increasing the pH value, stage I is extended while stage II is diminished.