The dissolution of fly ashes in undersaturated alkaline solutions

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 degrees 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 longterm reactivity was related to reactive Al and Si content. Dissolution in the dilute solution was consistent with reaction extent in R3 mixtures at 25 degrees C, and but greater for reactions at 40 and 60 degrees C. These results help us understand fly ash dissolution and predict reactivity under conditions relevant to those used to evaluate performance in cementitious mixtures.