The paper deals with aspects concerning the changes in the crystalline structure of alite doped with phosphorus. A number of six samples containing different proportions of P₂O₅ were sintered in the laboratory by iterative thermal treatment at 1450°C. Thus, alite triclinic crystallisation was evinced irrespective of the content in P₂O₅. Also, major shifts of the Bragg reflections have been noticed, which indicates that phosphorus has been incorporated into the alite crystalline structure by its substituting for the silicon in the matrix. Moreover, formation of a new crystalline phase has been evinced when the P₂O₅ content exceeded 0.9% by weight.