Abstract: In allusion to the low strength and poor water resistance of phosphogypsum-based cementing material，alkaline activator was applied in improving its mechanical properties and water-resistance.Hydration products and pore structure were characterized by using scanning electron microscope，X-ray diffraction，and mercury porosimetry.Results showed that hemihydrate calcium sulfate which was obtained by calcining phosphogypsum at 140 ℃ for 4 h was doped with slag and lime according to the mass ratio of 60∶40∶4，the mass ratio of water to binder was 0.6，and 1%（mass fraction） alkali activator was added，and then phosphogypsum-slag-based cementing material was prepared，the compressive strength and flexure strength reached 40.6 MPa and 11.3 MPa，respectively，and the softening coefficient was 0.84；the structure frameworks of calcium sulfate dehydrate and ettringite with the dense network structure of C-S-H gel which enfolded each component had high performances on mechanics and water resistance.

Key words: phosphogypsum, alkaline activator, ettringite, C-S-H gel, water resistance