Fifty-nine coal combustion products were collected from coal-fired power plants using various dry flue gas desulfurization (FGD) processes to remove SO2. X-ray diffraction analyses revealed duct injection and spray dryer processes created products that primarily contained Ca(OH)(2) (portlandite) and CaSO(3)center dot 0.5H(2)O (hannebachite). Most samples from the lime injection multistage burners process contained significant amounts of CaO (Iiine), CaSO4 (anhydrite), and CaCO3 (calcite). Bed ashes from the fluidized bed process were often dominated by CaSO4 but also contained CaCO3, CaO (lime), and MgO (periclase). Cyclone ashes were similar in composition to the bed ashes but contained more unspent sorbent and CaSO4 and less MgO. Fly ash in all samples ranged from 10 to 79 wt%. Samples usually exhibited two distinct swelling episodes. One Occurred immediately after water was applied due to hydration reactions, especially the conversion of CaO to Ca(OH)(2) and CaSO4 to CaSO(4)center dot 2H(2)O (gypsum). The second began between 10 and 50 d later and involved formation of the mineral ettringite (Ca-6[Al(OH)(6)](SO4)(3)center dot 26H(2)O). The final pH after 112 d ranged from 10.0 to 12.1. If samples are incubated under 'closed' (i.e. incomplete recarbonation with atmospheric COD and alkaline weathering conditions, gypsum and portlandite are initially formed followed by the conversion of the gypsum to ettringite. Closed, alkaline conditions typically can occur when FGD products are placed in confined settings such as a road embankment or buried as a discrete layer as occurs in some surface mine reclamation projects. (c) 2005 Elsevier Ltd. All rights reserved.