Many of the by-products from coal combustion and flue gas desulfurization are cementitious, which suggests utilization in some settings replacing conventional cements. Information on the long-term stability of these materials in Nature can be obtained from studies of landfilled by-products. Five coal combustion by-products (CCBs) landfilled at four sites in the USA were characterized by X-ray diffraction. Where moisture was available during or after disposal, the materials were changed chemically, physically and mineralogically over time, in processes analogous to diagenesis of buried sediments. At three of the sites, initially formed crystalline-phase assemblages were transformed into an assemblage dominated by ettringite and thaumasite. At one site where a high-sodium by-product was landfilled, an assemblage of Na-rich phases, including a zeolite and a zeolite-related nosean-hauyne phase, as well as tobermorite, was observed. This assemblage is unique and previously unobserved in CCBs. Diagenesis associated with many of these CCBs reduced strength and increased permeability after only a few years in the natural environment. The characteristics of the altered by-products resemble those of soils more than concrete. Initially promising 7 or 28 day laboratory tests of strength and permeability may not be characteristic of these materials on exposure to the environment. Blending of CCBs with fly ash to increase the proportion of cementitious C-S-H, and controlling subsequent moisture additions, could minimize deleterious by-product diagenesis.