Regulating the chemical foaming reaction to control the porosity of geopolymer foams

In lightweight geopolymers foamed with aluminium powder, it is desirable to increase the porosity without increasing the amount of aluminium usage. In this study, the kinetics of aluminium reaction is manipulated to increase the porosity of geopolymers without adding extra foaming agent, and the impact on porosity development and the characteristics of binding skeleton is investigated. It is shown that adjusting the ratio of alkali activators regulates the oxidation rate of aluminium powder and impacts the extent of foaming. Samples with higher aluminium oxidation rate developed higher porosities and reached lower densities without adding more foaming agent. Size distribution of the voids becomes wider, and their circularity is reduced in higher chemical foaming reaction rates. Also, by increasing the foaming rate, lower silica to alumina ratios contribute in geopolymer gel network, and geopolymer binder loses its compactness.