Reactions controlling the formation, composition and disintegration of ettringite are critical in determining the overall stability and strength of cements and concretes derived from dry-flue gas desulfurization wastes. These wastes typically consist primarily of fly ash, along with, CaSO{sub 3}, and CaSO{sub 4} and unreacted Ca(OH){sub 2}. The sulfites and sulfates in these materials react with the portiandite (Ca(OH){sub 2}) along with the glassy aluminosilicate Portion of the fly ash to form calcium sulfo-aluminate minerals. Ettringite, the most important of these, is the main contributor to the compressive strength development of the FGD waste mixtures. Excessive ettringite formation causes first lateral and horizontal swelling of the concrete which often leads to destructive crack formation. It has been shown that the quantity of ettringite formed and compressive strength of the FGD waste mixtures reach a maximum until ettringite begins to disintegrate. Because the formation mechanisms of ettringite are not entirely understood, swelling in FGD derived products is difficult to predict. This study focused on the formation and disintegration mechanism of ettringite in FGD waste mixtures with varying contents of aluminum (hydr)oxides, and calcium (hydr)oxides of the starting materials. Mineralogical transformations were documented in X-ray diffraction and scanning electron microscope (SEM) observations of the materials.