Silica gel is an inorganic and amorphous polymer that is formed by the condensation of silicate tetrahedrons using oxygen as the binding site, giving rise to the siloxane bond (Si-O-Si) and, ultimately, nanometer-sized particles.11 The siloxane groups are in the interior of the particles, and the surface is composed of silanol groups (Si-OH). Based on previous studies,6-9,11 it is known that amorphous silica has increased solubility in solutions with pH values higher than 10. Therefore, the amorphous silica contained in the rice husk ash can be solubilized when treated with a sodium hydroxide solution that has a pH value greater than 10. Thus, amorphous silica is dissolved and forms soluble silicate, which has a solubility of 876 mg/L in water at 25 ºC.12 When solubilized, the silica is in the monomeric form of silicic acid - Si(OH)4, as shown in Equation 1.13