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

A combination of intrusive and small-angle scattering techniques (USAXS/SAXS and SANS) was used to characterize the porous structure of metakaolin-based geopolymers. The influence of the geopolymer paste composition and the aging time in a 100% relative humidity environment at 293 K were 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 the alkali activator and the 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 slight closure of pores, revealed by nitro­gen sorption and small-angle neutron scattering (SANS). These results highlight the geopolymer stability in these curing conditions. For the first time, SANS combined with the contrast matching technique was used to determine the fraction of closed pores in metakaolin-based geopolymers. It was found that the geopolymer porous network is mainly open and that the fraction of closed pores represents less than 5% of the total pore volume after six months of aging. These conclusions are supported by transmission and scanning electron microscopy observations and by the determination of the chemical composition of the interstitial solution over time.