Using Particle Characterization to Study Fly Ash Dissolution and Leaching in Water and KOH Solution

This paper presents the results of the physical and chemical changes of Class F fly ashes exposed to either deionized water or to a 0.4 mol/L KOH solution for 15 or 45 minutes. The changes in physical and chemical makeup of thousands of individual particles before and after being exposed to solutions are analyzed using an autonomous scanning electron microscope. The results show that individual fly ash particles with CaO > 70% by mass completely dissolved. Particles with a CaO < 30% by mass leached sulfur and the remaining undissolved particles showed no measurable change in diameter or physical appearance. Similar observations were made from three different fly ash sources despite showing differences in their total oxide content. These results suggest that only certain particles with a narrow chemistry (CaO < 30% by mass) are responsible for the early age leaching of sulfur. This means that different fly ash particles could be responsible for modifying the sulfur balance in concrete observed in fly ash mixtures at different ages. These observations also give insights into the initial interactions between fly ash and alkali activators in alkali-activated materials. Keywords: automated scanning electron microscope (ASEM); characterization; dissolution; energy-dispersive spectrometer (EDS); fly ash; leaching.