From natural clinoptilolite to hierarchical designed porous geopolymer-zeolite monoliths: Synthesis, characterization and formation mechanism

Abstract

This work reports a novel approach to tailored design of clinoptilolite based hierarchically porous geopolymer-zeolite foams (CFs) by concurrent use H2O2 solution and montmorillonite as pore formers. Natural clinoptilolite, calcined clinoptilolite and aluminate activator were used as starting mix to produce the hybrid foams. The resultant products possess multiscale distributed pores – micropores (<2 nm), meso-macropores (50–175 nm) and macropores (0.1–0.2 μm, 100 μm-2 mm). The formation of macropores in micron-millimeter range is due to the decomposition of H2O2. Montmorillonite act as another pore-forming agent and construct macropores in submicron range. Hierarchical porous geopolymer-zeolite monoliths can be produced by varying the composition. The dissolution of clinoptilolite, montmorillonite, illite, metazeolite and anorthite contribute to the formation of geopolymer gel, while quartz and albite remains inert. The characters of CFs are highly depended on the clinoptilolite/metazeolite mass ratio. The porosities, bulk densities and BET surface areas range from 35.2 to 70.3 %, 0.808–1.325 g/cm3 and 2.557–23.330 m2/g, respectively. In addition, mechanisms for the formation of hierarchical porous geopolymer-clinoptilolite monoliths was proposed. These porous materials with tailored porosity are favored applied as building materials (thermal insulation). In contrast to the traditional foaming techniques, our work presents the possibility of directing the tailored synthesis of geopolymer-zeolite foams with at least three level of pores.