<p>Low-carbon alkali-activated slag (AAS) is among the most common alkali-activated materials (AAMs). To further lower CO₂ emissions and optimize the material system, we proposed a scheme of using phosphorous slag (PS) to substitute ground granulated blast-furnace slag (GGBS) in sodium carbonate (NC) activated slag system. we conducted a systematic study on the mechanical properties of the NC-activated slag/PS blends at normal temperature and examined the influences of different substitution amounts of phosphorus slag and NC equivalents on the performance of the material system. The hydration process was analyzed using hydration flow and chemical shrinkage. The hydration products were characterized via XRD and TGA. Moreover, the pore structure and pH value were also analyzed. When the substitution dosage of PS was not greater than 30%, the 3 d compressive strength of the systems was improved to a certain degree. However, in the medium and later periods, the compressive strength of the systems was slightly lower than that of the control group. The 90 d compressive strength of the control group 4SC-0% was 47.6 MPa, which was 4.0 MPa lower than the 28 d one of itself, presenting a strength retrogression phenomenon, while all the test groups demonstrated a continuous growth law. When the substitution dosage of PS was not more than 30%, the hydration reaction of the AAS system was facilitated, whereas when the substitution amount was 50%, the hydration of the system was conspicuously slowed down. The incorporation of phosphorous slag was capable of enhancing the volume stability of the material system. The hydration products of this system were likely to be manasseite, calcite, and C-S-(A)-H. When the incorporation amount of phosphorous slag increased, the quantity of the hydration products reduced, which might result in the generation of C-N-S-A-H. The study proposed the methodology for designing weak base-activated slag/PS.</p>