Rapid increases in the production of gypsum waste from the Flue Gas Desulphurization process necessitates that proper impoundments be built with an adequate understanding of the strength behavior of the sedimented gypsum slurry. This article presents laboratory experimental results that describe its physical characteristics and strength behavior. The physical characterization includes a particle size analysis and specific gravity of solids, which conform to ASTM standards. The strength behavior is described from a series of consolidated isotropically undrained triaxial compression tests that is performed at six levels of effective confining stress with two tests for each effective confining stress. The results and parameters are presented via critical state analyses in q-p' and e-p' spaces as well as a Mohr-Coulomb model. The analysis reveals two distinguishable regimes during shearing, which are classified as structured and destructured regimes, at which the sediment undergoes a change in strength behavior from being an intact structure to becoming a broken structure. A unique critical state line is found to be unparalleled with the isotropic compression line.