High strength metakaolin-based geopolymer foams with variable macroporous structure

Metakaolin-based geopolymer foams were synthesized by mechanical frothing, using hydrogen peroxide as additional foaming agent; three vegetable oils were added in order for the saponification reaction to occur. The combined use of the two foaming techniques (mechanical frothing and H2O2 decomposition) led to interconnected porosity: the in-situ formation of the soap molecules generated by the saponification reaction was exploited to generate macro-porosity. This combined route enabled the production of geopolymer foams with a range of total porosity and strength values, with an optimal sample possessing a total porosity of to ∼81 vol%, an open porosity of ∼79 vol%, and a compressive strength of ∼3.1 MPa. Moreover, factors that influence the compressive strength, the porosity, and the pore size distribution were investigated. The results showed that the cell size and cell window size can be controlled by adding different oils and different contents of peroxide.