The effect of iron and phosphorus addition on the formation and the hydration of dicalcium silicate Ca2SiO4 has been investigated. Synthesized solid solutions were analysed by X-ray diffraction, infrared spectroscopy and scanning electron microscopy. The hydraulic reactivity of synthesized powders was followed by means of isothermal conduction microcalorimetry. The results showed that the beta, alpha' and alpha-Ca2SiO4 formation with Fe and P additions less than 9.50 wt.% Fe2O3 and 8.45 wt.% P2O5. The solid solution of a' polymorph exhibited an early hydration higher than that in P modification. (c) 2005 Elsevier B.V. All rights reserved.