Utilisation of fly ash and brine in a geopolymeric material

Abstract

An increasing demand for electricity in modem society has resulted in the burning of large quantities of coal and ultimately the production of large quantities of fly ash. The petrochemical industry, if based on coal in a country such as South Africa, also produce large quantities of fly ash. In a semi-arid country like South Africa, there is a need to recover water. Processes currently in use for the recovery of wastewater produce large quantities of brines. These brines are stored in waste dams, which are not only expensive to maintain, but also pose a potential threat to the environment. The process of geosynthesis led to the development of a new type of material, namely geopolymers. Geopolymers can best be viewed as a polymeric silicon-oxygen¬aluminium framework with alternating silicon and aluminium tetrahedra joined together in three directions by sharing all the oxygen atoms. Cations such as Na+, K+, Ca2+ and H3O+ must be present in the framework cavities to balance the negative charge generated by the Al3+ in IV-fold co-ordination. It was attempted in this study to manufacture a geopolymeric binder, supplying most of the ingredients through waste materials. In the first set of experiments, matrices containing different amounts of fly ash, kaolinite, sodium hydroxide, sodium silicate and brine or water were synthesised by mixing and heating at 50°C for 24 hours. Compressive strength measurements showed a maximum strength of 4.05 MPa after 28 days. Leaching tests indicated that sodium was the best stabilised showing a stabilisation of between 30 and 40% (70 to 60% of the sodium initially added leached out again). The anions were stabilised to a lesser extent. Infrared spectra obtained confirmed an aluminosilicate structure. The second set of experiments was done to obtain the optimum curing conditions. Matrices containing the same amounts of fly ash, kaolinite, sodium hydroxide, sodium silicate and brine or water were cured at different temperatures and for different time periods. The matrices containing water showed a maximum compressive strength of 7.25 MPa after 28 days when cured at 60°C for 48 hours, while their brine-containing counterparts showed a maximum compressive strength of 7.76 MPa after 28 days when cured at 70°C for 72 hours. Infrared spectra obtained confirmed an aluminosilicate structure while X-ray diffraction patterns obtained indicated a largely amorphous product. In the third set of experiments matrices containing different amounts of fly ash, metakaolinite, sodium hydroxide, sodium silicate and brine or water were synthesised by mixing and heating at the optimum conditions determined previously. Compressive strength measurements indicated a maximum strength of 1.45 MPa after 28 days. Leaching tests indicated a higher stabilisation of the cations than in the first set of experiments. Potassium was the best stabilised, showing a stabilisation of above 80%. The anions were again stabilised to a lesser extent. Infrared spectra obtained confirmed an aluminosilicate structure while X-ray diffraction patterns obtained indicated a largely amorphous material.