Thermodynamic stability of waste glasses compared to leaching behaviour

The thermodynamic stability of products obtained from the high-temperature treatment of municipal solid wastes and their associated residues (bottom ash, fly ash, filter cake, optional additives) can be estimated by calculation of their free energy of hydration ΔG_hydr by a polyhedral approach. This approach has been applied on a series of 23 samples originating from high-temperature treatment processes operated under a range of conditions, and 3 thoroughly characterised standards. For vitreous or vitrocrystalline samples, it is demonstrated that Si and Ca contents clearly control their thermodynamic stability, and that the type of incineration process plays only a minor role. Silicon directly influences the durability of the samples, while Ca governs the pH during corrosion, which in turn affects the thermodynamic stability. It is also shown that there is a tight inverse relationship between the calculated thermodynamic stability of the samples and their rates of dissolution under aggressive conditions of corrosion. Attempts to compare the results to the large literature database of results obtained from nuclear high-level waste glasses, their proxies and other analogs (ancient and commercial glasses) are limited by sample preparation constraints. It is however concluded that the calculated thermodynamic stability of these “waste glasses” offers a valid estimate for their relative quality and, in turn, for their durability.