Effect of Phosphorus Impurity on Tricalcium Silicate T1: From Synthesis to Structural Characterization

Alite is the major compound of anhydrous Portland cement: it is composed of tricalcium silicate Ca3SiO5 (C3S) modified in composition and crystal structure by ionic substitutions. Alite is also the main hydraulic phase of cement and the most important for subsequent strength development. Using raw meals (rich in Ca3P2O8) as alternative fuels in cement plants raises the question about the effect of phosphorus on C3S and its consequences on reactivity with water. This paper deals with a systematic study of C3S triclinic T1 polymorph doped with P2O5 in the range 0–0.9 wt%. All the samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electron-microprobe analysis. The appearance of a phase rich in phosphorus is shown. It displays a structure derivative of the α′H–Ca2SiO4 polymorph, noted α′H–C2S(P). As phosphorus content increases, C3S is more and more decomposed into free lime and α′H–C2S(P). The α′H phase was detected from 0.1 wt% P2O5 and located at the interfaces of C3S grains. Two identification keys are proposed in order to highlight the α′H–C2S(P) phase: the XRD angular window at 2θCu=32.8°–33.2° and a smooth aspect on SEM micrographs.