Hydration process and rheological properties of cement pastes modified by orthophosphate addition

The use of cement by nuclear industry for confining low radioactive waste knows a constant increase. The interest bore to this material is due to its mechanical and chemical properties. However, these properties may be modified by the nature and the amount of wastes introduced with the mixing solution especially with large amount of phosphate. Precipitation of hydroxylapatite in the cement paste at the very beginning after mixing was emphasized from a particular concentration of orthophosphate. It was also found that this phenomenon leads to an early rigidification of the cement paste. From a kinetics point of view, the evolution of hydration was both delayed and slowed down according to a pessimum of phosphate concentration. These results indicate a competition between a adsorption process of phosphate on the cement phases which delay the hydration of the cement pastes and a process of precipitation of hydroxylapatite which does not.