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Cement stabilization of metallurgical residues is a frequently implemented option to reduce the associated environmental risks and thus simplify residue disposal procedures. As chemical stabilization is the first and main purpose of the process, physical testing of these stabilized residues is usually kept simple with unconfined compressive strength, permeability, and durability as the most frequently employed control tests. These tests are, however, unable to deliver a true image of the potentially complex mechanical behavior after stabilization of some important residues. One example is treated sludge. Stabilizing agents and fine residues are usually mixed at high water contents, resulting in a very porous microstructure, with fragile bonds. Under loading, rapid destructuration and compaction is possible and this process may be prone to material instabilities. These phenomena are experimentally and theoretically illustrated herein. The experimental illustrations are brought from a microstructural study and undrained triaxial campaign on a stabilized jarosite. The theoretical explanations are obtained from an elasto-plastic model previously employed for soft rocks. CE Database subject headings: Stabilization; Sludge; Elastoplasticity; Microstructure; Cement; Waste management; Waste disposal.