THE POTENTIAL USE OF FLY ASH WITH A HIGH CONTENT OF UNBURNED CARBON IN GEOPOLYMERS

The electric and thermal energy produced by combusting of fossil fuels - coal appears to be still dominant in a near future. Therefore, high quantities of fly ashes produced worldwide by heating and thermal power plants will even increase. In the Slovak Republic, nine tenths of fly ash is hydraulically transported to landfill areas or mines, causing ecological risks. Only a marginal part is used as a secondary raw material, mainly in the building industry. However, according to the European legislation only fly ashes with a low content of unburned coal residues, expressed by the loss on ignition (LOI), are allowed to be utilized. With the aim to alternatively use fly ashes, including those already deposited, various factors have been studied as to their influence on the compressive strength evolution of geopolymers containing untreated black coal fly ashes with a high LOI. Geopolymers with a mechanical strength of up to 55 MPa were prepared after 90 days of hardening by manipulating some crucial parameters of the activation solution and the geopolymerization mixture. An influence of the replacement of up to 20 % of fly ash by various alumosilicate materials such as slag, kaoline, metakaoline, bolus and calcined bolus on the hardened geopolymers' mechanical properties was also analysed.