C3A-GYPSUM SYSTEM IN ALKALI SULPHATE SOLUTIONS

The hydration mechanism of C3A in the presence of gypsum in excess solutions of 0.1 and 1 molar alkali metals sulphates, for one week at room temperature, was investigated. The effect of lime and alite on the progress of the hydration was studied. In sulphate-rich solutions, the mechanism of C3A hydration was interpreted in terms of the change in the calcium ion concentration of the solution with time. The dormant period was indicated by a plateau of the dissolved calcium and the acceleration period by its depletion. The hydration of C3A in the presence of gypsum was strongly retarded in 0.1 molar alkali sulphate solutions saturated with lime, the dormant period was extended, and the mechanism became diffusion controlled. The hydration was, however, accelerated in 1 molar sodium sulphate solution of the same mixture because of the precipitation of hydroglauberite (Na10(CaSO4)8.6H2O) which was accompanied by the depletion of the dissolved calcium. The hydroglauberite transformed into ettringite (C3A.3CaSO4.30-32H2O) with time.