Geopolymer is the inorganic polymer commonly made from kaolin, clay, fly ash, or slag. Obsidian mineral is the new candidate material for geopolymer formation. Obsidian is a material that used in ancient eras, but nowadays, this function has been replaced by various metals. Obsidian consists of cristobalite and sodium aluminum silicate. Obsidian was reacted with disodium metasilicate (Na2SiO3) to form a mineral-based geopolymer. An analysis of the formation mechanism through X-ray diffraction and Fourier transform infrared spectroscopy showed that the geopolymer formation from obsidian takes place in two stages. The first stage is the formation of a geopolymer from a reaction among cristobalite, disodium metasilicate, and amorphous aluminum silicates, whereas the second stage is the incorporation of crystalline sodium aluminum silicate into the former geopolymer structure. Geopolymer is usually utilized to form brick or concrete for an infrastructure purpose, but in this research, the geopolymer is proposed to control the release rate of elements from fertilizer. The result of potassium release test shows that the geopolymer has a slow-release property.
Keywords: Mineral; geopolymer; obsidian; slow release fertilizer.