HYDRATION OF C3A IN THE PRESENCE OF CA(OH)2, CASO4.2H2O AND CACO3

The hydration of C3A in the presence of CaSO4.2H2O, CaCO3 and always Ca(OH)2 in excess has been studied with X-ray diffraction. Using a special technique the diagrams were taken directly from the pastes. From the data obtained conclusions regarding the sequence of reactions and the possible phase assemblages in the system C3A.Ca(OH)2.18H2O - C3A.3CaSO4.32H2O - C3A.3CaCO3.32H2O at 22-degrees-C were drawn. It is shown that the presence of CO2 in the pastes prevents the reaction of ettringite formed in the course of the early hydration stages with C3A to monosulfate at the end of the induction period. Instead the concentration of ettringite remains almost unchanged and the crystallisation of hemicarbonate, monocarbonate, or both is observed.