Evolution of zeolite crystals in geopolymer-supported zeolites: effects of composition of starting materials

The role of the characteristics of raw materials and starting mix proportion on geopolymerization and evolution of zeolitic crystals from an amorphous geopolymeric gel was investigated. Different types and compositions of metakaolin, fly ash, and slag were activated using same waterglass and NaOH-based alkali activator. The test results showed that the reactivity of alkali-activated raw materials greatly depends on the Si/Al and Si/Na ratios of the starting mix, which affected the strength development and zeolite formation. It was further found that low-Ca class F fly ash was the most suitable to obtain zeolite Na-P1 crystals. Alternatively, high-Ca class C fly ash promoted the synthesis of hydroxysodalite and calcite. Higher porosity (52%) and surface area (99.5 m²/g) were achieved for the geopolymer-supported zeolites. These materials can be potentially used as membranes/filters for removal of hazardous contaminants (colloidal foulants, macromolecules, heavy metals/radionuclides) from water owing to their porous structure and affinity of zeolites towards cations.