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• Molar ratio design of PPSC mix ratio. • The composition of PPSC was optimized by simulation method. • The effect of chemical composition on the mechanical properties of PPSC was simulated. In this paper, the mechanical properties of excess-sulfate phosphogypsum slag cement (PPSC) were studied by experimental data, microscopic tests and molecular dynamics (MD) simulations. Specifically, the properties of PPSC in terms of setting time, compressive and flexural strengths were investigated by experiment. The hydration products of PPSC were tested and observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Furtherly, at the atomic scale, the relationship between nanoscale and macroscopic mechanical properties of PPSC was established. The mechanism of the effect of chemical composition on the mechanical properties of PPSC was studied at multiple scales, and the compound synergistic effect between chemical compositions was further studied. The results show that the mechanical properties of PPSC increase with the increase of CaO/SO 3 , and decrease with the increase of SiO 2 /Al 2 O 3. CaO and Al 2 O 3 can improve the mechanical properties of PPSC. The high mobility and structural instability lead to CaO/SO 3 has greater effect on the mechanical properties of PPSC than that of SiO 2 /Al 2 O 3. When CaO/SO 3 is 1.8 ∼ 2.0 and SiO 2 /Al 2 O 3 is 3.5 ∼ 3.7, the mechanical properties of PPSC are better. Moreover, when CaO/SO 3 is 2.0 and SiO 2 /Al 2 O 3 is 3.5, the mechanical properties of PPSC are the best. At this time, the compound synergistic effect of alkaline activator and sulphate activation is the best. This paper provides an important reference for the composition design and application of PPSC. [ABSTRACT FROM AUTHOR]