GEOPOLYMER MATERIALS ON THE FLYASH BASIS - LONG-TERM PROPERTIES

The influence of the external environment on the properties of fly ash-based geopolymers exposed to various conditions (different humidity levels, autoclave, dry conditions at 80 C, immersed in water) was investigated for two years. The strength values of geopolymers kept under the various conditions did not fall over this time. The maximum values could be achieved if the geopolymers were kept under dry conditions at 80 C and in air at a relative humidity ranging from 40 to 50%. The strength values do not change if the geopolymers are kept in water, in an autoclave or in an environment with a relative humidity of 95%. Long-term exposure does not result in any change in microstructure and few 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 efflorescence. The NMR 23Na, 27Al and 29Si spectra revealed a practically unchanged occurrence of structures in samples after long-term exposure to various conditions as compared with the initial structures. XRD and IR showed little evidence of the formation of new phases after various exposure conditions. No significant morphological changes (formation of crystalline phases) in samples could be seen 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 crystalline zeolites. Thus, the fly ash-based geopolymers can be regarded as stable hydraulic binders.