Mineralogical and mechanical transformations in the bulk of a red mud tailing dam Based on optical, SEM, X-ray and chemical analyses of dam samples, we intended to identify the natural and man-made components of this construction. The dam was built up mainly by slag, cinder and fly ash of coal based power plants, transported and deposited by hydraulic means, with or without cement slurry supply. Mineralogical, structural and textural changes were depicted in some components during sedimentation, dehydration and crystallization. The amorphous and highly hydrated material passes into (micro) crystalline bulk of calcite and sulphates, with sensible volumetric variations. Due to inhomogeneity of the materials, tension fields may be generated in the dam body, resulting consequences in mechanical stability.