Fly ash and kaolinite-based geopolymers: processing and assessment of some geotechnical properties

Abstract

Fly ash is an industrial waste material of the thermal power plants. At present, India produces about 100 million tons of fly ash per annum and most of them are currently dumped in landfills, thus creating a threat to the environment. In recent years, attempts are made to increase the effective utilization of fly ash in the country. This study reveals the morphological, mineralogical, and geotechnical properties of fly ash and fly ash–kaolinite-based geopolymer materials prepared with low-calcium fly ash (ASTM Class F) and kaloinite with activating solution of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) at different curing conditions. Both the fly ash and the fly ash–kaolinite-based geopolymers are prepared in molds, and the unconfined compressive strength (UCS) for each combination is obtained. The UCS of geopolymers at different concentration of alkali, curing temperature, and time is studied. The microstructural and geotechnical characterization of fly ash and geopolymers are obtained using the X-ray diffraction, X-ray fluorescence, and scanning electron microscope (SEM). The low-calcium fly ash and kaolinite-based geopolymers are found to exhibit excellent UCS as compared to that of other geotechnical materials such as clay. It is also observed that fly ash–kaolinite-based geopolymer exhibits more compressive strength than that prepared only with the fly ash. The compressive strength of the geopolymers increases with the concentration of NaOH up to 12 molar beyond which the strength decreases. The compressive strength of geopolymers increases with the improvement of curing condition. Finally, water absorption test is performed to know the water absorption characteristics of the geopolymers.