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In order to transform the crystalline form of Ca₂SiO4 (C₂S) in phosphorus-containing slag from monoclinic β-polycrystalline to square γ-polycrystalline, a volume expansion of about 11% was generated, which caused the phosphorus-containing slag to undergo self-powdering. The CaO-SiO₂-Al₂O₃-MgO-MnO-P₂O₅-FeO slag system was analyzed using FactSage7.1 thermodynamic software, and the effects of different P₂O₅, FeO and basicity on the mineral phase composition of slag system were analyzed in the range of 1300~1700 °C. It was shown that P₂O₅, FeO and basicity all have an effect on the composition of the mineral phases. When the mass fraction of P₂O₅ in the slag was lower than 0.25%, it had less effect on the transformation of C₂S crystalline structure. When the P₂O₅ content was higher than 0.25%, it was favorable to the generation of low-melting-point substances, but the P₂O₅ in the slag reacted with C₂S in the silicate phase, making P⁵⁺ solidly soluble in C₂S, inhibiting the transformation of β-C₂S to γ-C₂S and hindering the self-powdering of the slag. The FeO content in the slag system ranged from 20% to 28%, and as the FeO content increased, the C₂S content in the silicate phase decreased from 33.3% to 25.9%, while the temperature at which the silicate was completely dissolved into the liquid phase decreased from 1600 °C to 1500 °C and the complete melting temperature of the slag decreased. The low FeO content facilitates the self-powdering of slag. In the high-phosphorus slag, at temperatures below 1450 °C, with the increase of basicity, the proportion of C₂S in the silicate phase first increased and then decreased. With basicity at 1.8; the highest content of silicate phase, accounting for 33.7%; and the temperature exceeding 1450 °C, the silicate phase dissolved into the liquid phase, which is conducive to the removal of phosphorus from the slag, achieving the self-powdering of high-phosphorus slag.