Effect of composition and aging on the porous structure of metakaolin-based geopolymers

International audienceA combination of intrusive and small-angle scattering techniques is used to characterize the porous structure of metakaolin-based geopolymers. The influence of the geopolymer paste composition and the aging time in a 100percent relative humidity environment at 20DC are studied. The effect of the alkali activator, the water amount and the silica amount were investigated. The results show a strong "ink-bottle" effect indicating a two-level pore structure: a meso-and macroporous network. Both alkali activator and water amount have a significant impact on porosity and microstructure in the studied formulation range. After a period of one month, the pore structure is stable over a period of one year except for a small closure of porosity, revealed by nitrogen sorption and small-angle neutron scattering (SANS). These results highlight the geopolymer stability in the curing conditions. For the first time, SANS combined with the contrast matching technique was used to determine a fraction of closed porosity in geopolymer pastes. It was found that the geopolymer porosity is mainly open, the closed porosity representing less than 5percent of the pore volume after six months of aging