In the present paper a theoretical study is presented on the dissolution (reaction) of pulverised powder coal fly ash. A shrinking core model is derived for hollow spheres that contain two regions (outer hull and inner region). The resulting analytical equations are applied to the dissolution experiments by Pietersen (Mat.Res.Soc.Symp.Proc., Vol. 178, Materials Research Society, 1990, p. 139; Ph.D. Thesis, Delft University of Technology, The Netherlands, 1993), yielding reaction rates at various temperatures and pH for two class F fly ashes. It is revealed that the available amount of reactive fly ash is proportional to the glass content of the fly ash, and that the reaction rate is proportional to this glass content as well. Moreover, it is concluded that the outer region is less reactive than the inner region, and that these reactivities are proportional to a power of the hydroxyl concentration. Subsequently, experimental data and model are used to assess the magnitude of inner and outer region. It seems that the outer hull of solid spheres and cenospheres are having the same thickness, about 2 mgrm. Based on the observed trends a reaction mechanism is proposed which accounts for the glass content and composition of the fly ash (and that is applicable to slags as well). Finally, using the reaction product, thermodynamic properties of the studied fly ashes are derived: the free energy, enthalpy and entropy of reaction.