Optimization of mechanical activation of fly ash

Abstract

More than 5.5 million tonnes of fly ash (FA), a by-product originating from coal combustion in thermal power plants (TPP), is produced in Serbia annually. Only small part of the FA is reused, mostly by cement industry, while the rest of it is disposed on landfills. Given its chemical composition and particle size distribution, FA has huge potential for reuse, especially in construction industry. Some of the most promising ways for FA utilization in building materials industry are synthesis of alkali-activated materials (AAM) and binders in which different quantities of Portland cement (PC) are substituted with FA (up to 80%). However, potential use of FA strongly depends on its reactivity. Among factors that affect FA reactivity, particle size distribution and amorphous phase content are the most important. A way to improve the reactivity of FA is mechanical activation, a process where mechanical energy is used to increase chemical reactivity of material. The main results of the mechanical activation process are particle size reduction, increase in specific surface area, generation of structural defects, increase in amorphous phase content, which all lead to better reactivity of material. In this paper, FA from TPP “Kolubara”, Serbia, was mechanically activated in planetary ball mill using different ball-to-ash ratios, from 1:3 to 1:20. Characterization of raw and activated FA, such as particle size distribution, mineral and chemical composition, morphology and specific surface area, was carried out. The effects of mechanical activation of FA on the properties of resulting binders, both AAM and binder with PC, were also investigated. It was found that mechanical activation, even with the lowest ball-to-ash ratio (1:3), caused drastic changes in particle size distribution and specific surface area of FA. Significant increase in compressive strength of mortars based on mechanically activated FA, relative to mortars based on raw FA, indicated that reactivity of FA was considerably improved by mechanical activation. Further increase in the ball-to ash ratio led to less significant changes in the properties of the activated FA and therefore, less considerable increase in compressive strength of the synthesized mortars. It was concluded that, in terms of the balance between time and energy consumption, on the one hand, and improvement of the FA properties on the other, further increase of ball-to-ash ratios was not justified.