Ceramics-Silikáty 54 (1) 53-59 (2010)

THE C₃A - GYPSUM SYSTEM IN ALKALI SULFATE SOLUTIONS

H. Y. Ghorab, A. Antar, M. Ahmed N. Taha

An investigation series is carried out on the mechanism of hydration of C₃A in the presence of gypsum in excess solutions of 0.1 and 1molar alkali metals sulphates, for one week at room temperature. The effect of lime and alite on the progress of hydration is studied. In sulphate-rich solutions, the mechanism of C₃A hydration is interpreted in terms of the change in the calcium ions concentration of the solution with time. The dormant period is indicated by a plateau of the dissolved calcium and the acceleration period by its depletion. The hydration of C₃A in the presence of gypsum is strongly retarded in 0.1molar alkali sulphate solutions saturated with lime, the dormant period is extended, and the mechanism becomes diffusion controlled. The hydration is, however, accelerated in 1molar sodium sulphate solution of the same mixture because of the precipitation of hydroglauberite (Na₁₀(CaSO₄)₈·6H₂O) which is accompanied by the depletion of the dissolved calcium. The hydroglauberite transforms to ettringite (C₃A·3CaSO₄·30-32H₂O) with time.

Keywords: Hydration, C₃A, Gypsum, Alkali sulphate solutions