Influence of raw materials and potassium and silicon concentrations on the formation of a zeolite phase in a geopolymer network during thermal treatment

The development of porous materials based on geopolymers allows the benefits of eco-consumption, good insulating properties and good mechanical properties to be combined. For geopolymers to be useful, the development of an understanding of their properties under various conditions is important. Attention was given to the structural evolution of porous materials with heating. The structural evolutions were investigated using thermal analysis and infrared spectroscopy. The formation of a crystalline phase was observed and identified as a zeolite. To elucidate the roles played by the raw materials, silicon concentration and potassium concentration on the formation of the crystalline phase, three parameters in the foam synthesis process were modified. The influence of silica fume and metakaolin on the appearance of the crystalline phase was examined through the reactivity of the raw materials in contact with a potassium solution. Different behaviors directly linked to the network composition were observed. Silica-fume-based samples showed phase crystallization and dehydroxylation due to the large concentration of silicon, which likely led to the formation of hydrated silica species. The formation of the zeolite-phase network upon the application of heat depended only on the silica dissolution and was linked to the composition of silica in terms of the silicon species and grain size. Studying the crystalline phase formation by variation of the amount of potassium allowed the determination of a maximum silicon concentration and a minimum potassium concentration. The formation of the zeolite phase occurred at a potassium concentration greater than 3.5 mol·L− 1 and a silicon concentration lower than 16.5 mol·L− 1, which confirmed the presence of F-zeolites.

► Potassium geopolymer foam presents the formation of zeolite phase at 350 °C. ► Samples synthesized only with metakaolin have a stable network with heating. ► The use of silica with high grain size stops the zeolite formation. ► The zeolite formation depends on the silicon and potassium concentration. ► The zeolite specie was identify as a F-zeolite.