The gypsum samples with a high content of P2O5 in crystal lattice were prepared with analytical reagents under simulated conditions of P2O5 in crystal lattice formation. Then the samples were calcined to hemihydrates and added to the natural building gypsum to progress the physical and mechanical properties testing of their hardened pastes. The influence of P2O5 in crystal lattice on the performance of gypsum and its mechanisms were studied by scanning electron microscopy, atomic absorption spectrometry, infrared spectroscopy combined with chemical analysis methods. The results showed that P2O5 in crystal lattice significantly reduces the hydration rate and the crystallization supersaturation of dihydrate gypsum, makes crystals thick and structures loose, and the strength of hardened paste reduced. The P2O5 in crystal lattice does not change in the process of calcination, still exists in the gypsum crystal lattice. In the hydration process, the P2O5 in crystal lattice continuously dissolves from the crystal lattice, changes into water soluble HPO4^2-, which ionizes to H^+ and PO4^3-,PO4^3-quickly binds with Ca^2+in the solution, changes to Ca3(PO4)2 which covers on the crystal surface, hinders the gypsum further hydration, results in the strength reduction of hardened paste, and the surplus of H^+ will lead to a decrease in pH value of the slurry.
