ABSTRACT
The hydration of the tricalcium aluminate in presence of different amounts of gypsum in excess water has been studied from 2 min to one week at 30 degree by means of X-ray diffraction as well as the chemical methods of analysis. The sulfate ion concentration in the bulk solution increases with increasing gypsum content. It remains constant during the first 15 min then decreases with the formation of ettringite which maximum is reached around three hours at an almost total drop of the sulfate ion concentration. The concentration of aluminium is depressed by the presence of sulfate. It passes through a minimum after 15 min then increases up to one week. At early hydration stage the monophases appear beside the ettringite. They become the predominant phases after the ettringite maximum with the simultaneous consumption of the unreacted tricalcium aluminate. The complete conversion of ettringite to the low sulfate form is retarded in mixtures of originally high sulfate content