Mechanical Irreversibility and Strength Overcompensation in a Gypsum Dehydration–Rehydration Cycle

Abstract Dehydration–rehydration reactions of hydrous minerals modify rock strength, fluid transport, and deformation in reactive geological environments. Although dehydration‐induced weakening is widely studied, the mechanical consequences of subsequent rehydration remain poorly constrained. We combine triaxial compression tests with microstructural and pore‐network analyses of original gypsum, fully dehydrated anhydrite, and rehydrated gypsum. Dehydration markedly reduces strength and suppresses pronounced serrated stress drops, whereas rehydration produces strength overcompensation and a more localized strain‐softening response. Rehydration does not simply restore the original fabric but produces a reconstructed microstructure containing newly formed gypsum crystals, locally infilled voids, and a reconfigured accessible pore‐throat network. Reaction‐induced stresses may contribute to this restructuring but were not measured directly. These results show that gypsum rehydration can create a mechanically distinct, overcompensated state rather than simply reverse dehydration.