Abstract Four crystal forms of solid solutions of Na 2O in 3CaO.Al 2O 3 have been synthesized. Chemical, crystal-optical, X-ray diffraction and IR-spectroscopic investigations have been made. The hydration activity at the early and later stages has been studied. It is shown that the hydration process can be divided into three stages. The analysis of the kinetic curves permitted the conclusion that the retardation of the hydration process is controlled by diffusion of the liquid phase through the product layer.