The dissolution of fly ashes in undersaturated alkaline solutions

Abstract

This study investigated the dissolution of 8 class C and 8 class F fly ashes in undersaturated conditions at pH 12.4–13.8 and 25–60 °C. Class C fly ashes showed preferential dissolution of Al over Si, while congruent dissolution was observed for low Ca fly ashes. Si in solution inhibited the dissolution of class C ashes, while Ca and Al inhibited the dissolution of class F ashes. NaOH dissolved class F fly ashes faster than KOH at the same concentrations. Non-bridging oxygen per tetrahedra correlated with dissolution rates in early hours while long-term reactivity was related to reactive Al and Si content. Dissolution in the dilute solution was consistent with reaction extent in R³ mixtures at 25 °C, and but greater for reactions at 40 and 60 °C. These results help us understand fly ash dissolution and predict reactivity under conditions relevant to those used to evaluate performance in cementitious mixtures.