Quantitative and qualitative assessment of the amorphous phase of a Class F fly ash dissolved during alkali activation reactions – Effect of mechanical activation, solution concentration and temperature

A detailed analysis of the effects of mechanical activation and temperature on the dissolution process of a Portuguese Class F fly ash was developed. The evolution of the silica and alumina dissolved and transferred to the aluminosilicate gel was characterised. Quantification of bulk vitreous mass was performed using X-ray diffraction, together with the identification of the most relevant amorphous phases dissolved. The breakage process is responsible for further modifications of the original particle than just the exposure of additional surface area and increased structural disorder. X-ray diffraction and Rietveld amorphous bulk quantification proved to be an inadequate methodology to quantify the effects of mechanical activation. Increasing dissolution temperature proved to be very effective in terms of bulk mass dissolved, with higher dissolution values after 16 h, at 80 °C, than after 90 days, at 20 °C. The semi empirical Avrami model was found to be satisfactorily fitted to the experimental data regarding the dissolution percentage as a function of time. Compressive strength evolution was found to be related to the release of amorphous bulk phases, and in particular to the transfer of aluminium into the aluminosilicate gel, throughout the entire curing period of 90 days.