Development of geopolymer-zeolite composites

Geopolymer-zeolite composites were produced mixing different geopolymer matrices with a synthetic commercial zeolite. Metakaolin was selected as the main precursor for the geopolymer matrix, because it is the ideal material for the production of geopolymers due to the high reactivity and purity, compared to other starting raw materials as fly ashes and natural clays. A potassium or sodium silicate activating solution was used for the geopolymerization process and the commercial zeolite Na13X was used as filler. The microstructure of a metakaolin-based geopolymer consists of nano-particulates separated by micro- and mesopores [1,2]. Moreover, geopolymers can be regarded as the amorphous counterpart or precursor of crystalline zeolites. Indeed, the final geopolymer structure consists of an amorphous network of SiO4 and AlO4- tetrahedral units connected by oxygens and charge-balanced by hydrated alkali cations, while zeolites are crystalline hydrated aluminosilicates with 3-dimensional structures. The substitution of Al3+ for Si4+ results in negatively charged aluminosilicate lattice for both the materials, that needs to be balanced by extra-framework cations (generally Na+, K+ and Ca2+), resulting movable and exchangeable by other metal ions. Since zeolite is partially reactive in alkaline medium [3], the production process and the synthesis parameters were investigated to limit the reaction of the zeolite. In fact, the main goal of the study was to combine the peculiar and defined microporosity of zeolite with the meso- and macroporosity of the geopolymer matrix, together with the possibility to consolidate the zeolite powder. Actually, the requirement of supporting or shaping powdery zeolites is important for industrial applications and often the manufacturing and assembling of supports and zeolites add complexity and cost to the final product. Because of the high accessibility of pores, these porous composites with 3-dimensionally interconnected and distributed open pores, may be used as catalyst, filters and sorbents, furthermore, zeolites, and in particular zeolite X are largely used for the CO2 gas adsorption [4]. The selected composites were deeply characterized in term of microstructure, mineralogical composition, porosity and specific surface area, together with the ability to adsorb CO2, in order to highlight one of the possible functional properties of the new material.