Ceramics-Silikáty 58, (1) 12 - 20 (2014)

The influence of the external environment on the properties of geopolymers on the basis of fly ashes exposed to various conditions (environments of different humidity levels, autoclave, dry conditions at 80^oC, immersed in water) has been investigated for two years. The strength values of geopolymers kept under various conditions do not drop during the period of two years. The maximum values could be achieved if the geopolymers were kept under dry conditions at 80^oC and in the air at a relative humidity ranging from 40 to 50 percent. The strength values do not change practically if the geopolymers are kept in water, in an autoclave or in an environment with the relative humidity of 95 percent. The long-term exposure of fly ash-based geopolymers does not result in any change in their microstructure and practically no new phases arise. The exposure to water is accompanied by a gradual leaching of sodium until its almost complete transition to water. The exposure of geopolymers to alternating humid and dry conditions gives rise to efflorescences. The NMR ²³Na, ²⁷Al and ²⁹Si spectra of fly ash-based geopolymers revealed a practically unchanged occurrence of structures in samples after their long-term exposure to various conditions as compared with the initial structures. Practically no evidence (X-ray diffraction and IR analyses) of any formation of new phases after various exposure conditions. No significant morphological changes (the formation of crystalline phases) in samples kept under above mentioned conditions could be identified after two years as compared to the initial state; hence, the original amorphous character of the geopolymers is preserved. The structure of man-made geopolymers can probably be deduced rather from the amorphous mineral allophane than from crystaline zeolites. Therefore, thanks to a comprehensive long-term investigation, the fly ash-based geopolymers can be regarded as stable hydraulic binders.