The phosphorus slag (PS) can be used as a supplementary cementitious material due to its potential hydrating activity. However, its usage has been limited by its adverse effects, including prolonged setting and lowered early-stage strength. In this study, we achieved ultrafine granulation of PS using wet-milling (reducing d50 to as low as 2.02 μm) in order to increase its activity, and examined the physico-chemical properties of the resulting materials, including particle-size distribution, slurry pH, zeta potential, and activity index, as well as how their replacement level and granularity affect the setting time and mechanical performance of PS-cement mixture systems. The results suggested that as the granularity increases, there are significant boosts in the uniformity of particle sizes, slurry pH, and activity index, and the effects on cement paste, including setting times, and early- and late-stage strengths, are significantly mitigated. When d50=2.02 μm, the slurry becomes strongly alkaline (pH=12.16) compared to the initial d50=20.75 μm (pH=9.49), and the activity is increased by 73％; when used at 40％ replacement, the PS-cement mixture system can reach a 28 d compressive strength of 93.2 MPa, 36％ higher than that of the pure cement control group.