Ceramics-Silikáty 58 (3) 188-197 (2014)

Preparation and properties of fly ash-based geopolymer foams

An extremely porous geopolymer material – an incombustible alumino-silicate foam – can be prepared by incorporating gas-forming agents into mixtures consisting of an alkaline activating agent (NaOH + sodium water glass) and fly ash. Aluminium powder or materials containing elemental Si (silica fume, FeSi or SiC) may be used as gas-forming agents. Macroscopic pores are given rise thanks to the release of gaseous hydrogen generated during the reaction of aluminium, respectively Si, in a strong alkaline environment. The formation of the foam becomes more intense if aluminium powder is used instead of silica fume. The effect of silica fume can only be observed at concentrations ranging from 5 to 10 percent by weight and at temperatures above 60°C because of a low content of elemental Si in silica fume. The foam formation takes place immediately after the addition of aluminium powder to the mixture but the foam collapsing must be prevented by incorporation of portland cement or lime. The Si reaction in the strong alkaline environment is boosted by the presence of an alumino-silicate material, ie the fly ash. Geopolymer foams prepared by addition of gas-forming aluminium powder or silica fume are characterized by similar pore sizes; the pore size is comparable with that observed in lime-silicate materials produced in autoclave. The volume weight of geopolymer foams is similar to that of other inorganic foams. The strength of geopolymer foams is dependent on their volume weigth. Mechanical properties of geopolymer foams were stable over the investigated period of a year. Geopolymer foams are incombustible and can resist temperatures of up to 1000°C without any sign of decomposition. Their shape does not change after exposure to temperatures ranging from 400 to 800°C. In contrast to lime-silicate foams and cement-based foams the firing of geopolymer foams is not accompanied by the formation of any cracks or by any additional changes (rehydration).

Keywords: Geopolymer, Foam, Macro-porosity, Fly ash