The Porous Network and its Interface inside Geopolymers as a Function of Alkali Cation and Aging

Abstract

A combination of original, powerful characterization techniques was used to make a thorough description of solid geopolymers and of the associated effect of varying the alkali cation source - NaOH, KOH, or CsOH - and aging for up to several years. More specifically, the local and pore structures were progressively determined from atomic local scale up to several nanometers by pair distribution function analysis (PDF), small-angle X-ray scattering (SAXS), and longer correlation concerning the pore network, and possible diffusion and accumulation phenomena were unraveled by thermoporosimetry and electrochemical impedance spectroscopy (EIS), respectively. These complementary observations resulted in a picture of an interface between the mineral and the porous network that was correlated to the solvated alkali cation present in the porous solution. After a short time of a few months, the Na-based geopolymer was found to exhibit a smooth interface built up from small "elementary"particles. This contrasted with the K- and Cs-based geopolymers, which presented developed interfaces arising from hierarchically organized smooth particles forming aggregates of fractal outer surface. This striking difference unraveled by SAXS and EIS is ascribed for the Na geopolymer to the contact of the solvated Na(H<inf>2</inf>O)<inf>x</inf>+ cations with the amorphous mineral surface. The K(H<inf>2</inf>O)<inf>x</inf>+ and Cs(H<inf>2</inf>O)<inf>x</inf>+ solvated cations were an integral part of the porous solution, without direct contact with the mineral surface, thus leading to apparently rough interfaces. Dewatering occurred with time, mostly impacting the Na series. Overall, we obtained a detailed picture of the geopolymer series and their changes in time. The environment generated around the kosmotropic (order-making) Na+ alkali cation was more prone to change upon aging toward a non-Debye type relaxation than the initially developed interface supplied by the chaotropic Cs+ alkali cation, which was found to be relatively stable after 5 years. © 2015 American Chemical Society.