The recycling of basic oxygen furnace slag in the steelmaking process is currently limited by its phosphorus content. Phosphorus is known to induce remarkable phase segregation in the slag microstructure and is only present in phases belonging to the C2S–C3P solid solution, the iron-containing phases being left free of phosphorus. The mechanism of phosphorus insertion into calcium silicate structures was studied by a combination of TEM, XRD and 29Si and 31P solid-state NMR. Upon P2O5 addition, [PO4]3 − units are incorporated into the dicalcium silicate structure by substituting [SiO4]4 − groups, charge balance being maintained by creation of calcium vacancies. This substitution leads to a stabilization of the β- and α-Ca2SiO4 phases at room temperature and results in the formation of more polymerized calcium silicate secondary phases. The obtained P-substituted α-Ca2SiO4 compounds belong to the Ca2 − x/2Si1 − xPxO4 solid solution and exhibit lattice parameters similar to those of the calcium phosphosilicate Ca15(PO4)2(SiO4)6 phase.