Dissolution mechanism of fly ash to quantify the reactive aluminosilicates in geopolymerisation

Predicting of fly ash (FA) performance in alkali environment is difficult due to its heterogeneous nature. This paper investigates the dissolution process of FA in NaOH-activated geopolymers toward quantification of its reactivity. Dissolution rates were influenced much more by temperature (25–145 °C) and time (2–24 h) than by NaOH molarity (8–16 M). Higher temperatures increased the kinetic energy of the system, thus increasing the effectiveness of solute molecular bond breakage by solvent molecules and intermolecular attraction. Analysis by FTIR, ICP, XRD and SEM-EDS implies that FA consists of reactive materials, partially reactive materials, and inert materials. The reactive materials dissolve rapidly during geopolymerisation. The partially reactive materials are cenospheres, whose outer vitreous Si shell dissolves gradually, while the inert materials remain un-reacted. To avoid overestimating or underestimating the reactive material content in FA, it is proposed to modify the dissolution method to match the intended curing temperature and to set the dissolution test duration fitting with the initial setting time of the geopolymer paste at the curing temperature of interest. It is not only the quantity of reactive material that is important but also the Si/Al ratio of the reactive material. Consequently, it is recommended that quantification of reactive material be analysed by ICP dissolution rather than EDS analyses of the undissolved material.