Three landfill test cells were constructed using AFBC by-products and Class F coal fly ash at a site in central Illinois and were monitored over a 5 year period as part of a DOE METC effort to characterize by-products from clean coal technologies. The field programme documented rapid loss of structural integrity in cell 1, which contained no fly ash, whereas cells 2 and 3, constructed with an FBC-fly ash mix, showed improved strength and permeability over time. These differences in material behaviour are related to loss of soluble minerals from cell I and enhanced silicate reactivity in cells 2 and 3. Ettringite and thaumasite, minerals associated with failure of Portland cement concrete, do not appear to be the primary causes of cell 1 deterioration; ettringite remained stable, while thaumasite formed after significant deterioration had already taken place. Ettringite remained a primary cement-forming mineral in cells 2 and 3, but increased availability of reactive silica led to formation of calcium silicate hydrate rather than thaumasite, increasing long-term durability.