Methodology for analysis of the reactivity of coal fly ash using selective dissolution by hydrofluoric acid

The use of some additions as cement replacement has played an important role in the amount of clinker reduction. Due to its huge worldwide availability one of the most important and recognised replace agent is the fly ash, but is also estimated that only 30 to 40% of fly ash is used, although there are some challenges to be overcome, such as the reactive potential quantification. Chemical analysis is typically determined by the oxides content and does not take into account the vitreous and crystalline phase ratio. Therefore, this paper describes in detail one selective dissolution method, based in the hydrofluoric acid attack, to the quantification of the vitreous phase (reactive) and crystalline phases (not reactive). Fly ash from Portuguese Pego thermoelectric power plant was submitted to different attacks. To confirm the method and its effects, chemical and physical analysis were performed, such as STA, XRF, XRD with Rietveld refinement, SEM, EDS and laser diffraction granulometry. The results can be used to quantify the potential reactivity of this type of fly ash. The best results were achieved with 1% hydrofluoric acid attack during 6 hours.