The co-preparation of phosphogypsum (PG) and silica fume (SF) to form PG-based composite cementitious material (PGCM) is an effective way to promote the resource utilization of PG, with enormous environmental benefits. This paper mainly focuses on high-dosage β hemihydrate phosphogypsum (β-HPG), formulates the \({L}_{9}({3}^{4})\) type orthogonal test scheme with SF, quicklime, polycarboxylate superplasticizer (PCS) and protein retarder (PR) as variables, and conducted physical and mechanical test on PGCM. The test results for PGCM cured for 14 days under natural, absolute drying, and saturated conditions showed that its flexural strength (FS) and unconfined compressive strength (UCS) fell within the ranges of 1.1-5.3 MPa and 3.3-22 MPa, respectively. The water absorption rate ranges from 16 ~ 26%, and the flexural and compressive softening coefficients are 0.38 ~ 0.64 and 0.27 ~ 0.59, respectively. Range analysis pointed out that the PR and SF have the greatest influence on the mechanical properties, and the water resistance performance is mainly affected by SF and PR, and the optimal ratios corresponding to different performance indicators were obtained by range analysis. Scanning electron microscope (SEM), x-ray diffractometer (XRD), Fourier-transform infrared (FTIR) and thermogravimetric (TG) test were also carried out on the specimen with the highest (PGCM1) and lowest (PGCM7) UCS under absolute dry state. The SEM analysis revealed the mechanism by which the 0.3% protein retarder weakens the strength: the dihydrate gypsum crystal structure of PGCM7 with 0.3% protein retarder is shorter, and has fewer connection points, a looser microstructure, more pores and a decrease in density, and leads to the poor mechanical performance. The hydration products of PGCM1 and PGCM7 were characterized by XRD and FTIR, and both samples contain the dihydrate gypsum (DG), quartz (SiO2), and calcium silicate hydrate (C-S-H) gel. The TG and DSC test showed the PGCMs have good thermal stability.
