Preparation and characterization of foamed geopolymers from waste glass and red mud

•Foamed geopolymers were obtained by re-using two waste (glass cullet and red mud).•As liquid component NaOH solution or the filtrate obtained by red mud slurry dewatering was used.•The foaming process of sodium silicate (aluminate) hydrates determine important volume increase.•Open pores with a wide range of sizes (1–100μm) were present.•The obtained foamed geopolymers are promising for thermal and sound insulation. New foamed geopolymer materials were successfully synthesized from two waste i.e. red mud and cullet soda-glass, by thermal treatment at 600–800°C for 1h. The geopolymers were obtained by the alkali activation with sodium hydroxide solution or the liquid part of red mud slurry (filtrate) of waste glass (cullet) with/without red mud admixture. Thermal treatment of these geopolymers, at temperatures between 600 and 800°C determined an important volume increase, due to a foaming process specific for sodium silicate (aluminate) hydrates, the main components of these materials. When red mud substitutes 25% of brown glass, softening and partial melting of material occurs at higher temperatures (due to the presence of complex refractory crystalline phases with Al content); for these compositions the increase of volume due to foaming process is noticed at higher temperatures. Open pores with a wide range of sizes (1–100μm) were assessed by scanning electron microscopy; this highly porous microstructure, formed by the bubbling and coalescence of bubbles, is specific for foam materials. The compressive strength values of foamed geopolymers are in the range 2.1–8.6MPa, and as expected, decrease with the increase of open porosity.