Current study explores the estimation of reactivity and dissolution mechanism in low glassy content siliceous fly ashes. Two different fly ashes directly collected from thermal power stations were used in this study. Dissolution experiments were carried out in different sodium hydroxide-based alkaline environments and at different curing temperatures. The dissolution of elements from fly ash is directly related to the glassy portion. During the dissolution process, the crystalline phases are not affected, and it remains the same in various alkaline environments. Higher molarity and temperature enhance the dissolution process. © 2020, Springer Nature Singapore Pte Ltd.