Design of lightweight metakaolin based geopolymer foamed with hydrogen peroxide

Monolithic metakaolin based geopolymer foams were synthetized by direct foaming using hydrogen peroxide (H2O2) as a blowing agent and surfactants to stabilize the resulting gas bubbles. The influence of the H2O2 content and the nature of the surfactant (cationic vs air entraining) have been investigated in the liquid and the solid state. The reaction rate constants for the oxygen produced by the catalytic decomposition of hydrogen peroxide (k = 5.5 x 10(-4) and 2.1 x 10(-4) s(-1) for the air entraining and cationic based geopolymer foams, respectively) are reported for the first time, as calculated from volume expansion data in the liquid state. These constants were found to be independent of the initial H2O2 content. In the solid state, although the physical, mechanical, and microstructural properties of the resulting foams are known to be influenced by the amount of H2O2 in the geopolymer paste, the chemical composition of the surfactant had a startling effect on the morphology and topology of the macroporous network. Indeed, while faceted closed pores form in the cationic based geopolymer, a 3D network of connected pores forms in the air entraining one. As expected therefore, the cationic samples were found to have a much higher compressive strength than the air entrained ones (0.57-5.9 MPa compared with 0.26-4.3 MPa, respectively) for equivalent apparent densities (varying from 0.23 to 1.1 g cm(-3)).