The influence of CaSO4.2H2O upon the hydration character of 3CaO.Al2O3

Abstract

Summary This paper presents a quantitative interpretation of results of differential thermal analysis of hydration reactions at 2, 12,23 and 52°C of 3CaO.Al2O3 mixed with 0, 0·25, 2·5, 10 or 20% gypsum, and compares these with length changes obtained from the hydration of compacts of these mixtures. Estimates of the relative reaction rates from d.t.a. peak areas and measurement of length change during these reactions showed marked agreement, although interpretation had to be made with caution. It was concluded that the formation of ettringite (high sulpho-aluminate) had no direct effect upon reaction rate, but that the reactivitv of 3CaO.Al2O3 was reduced by the sorption of (SO4)2− ions on active sites of its surface. Gypsum reduces the rate of conversion of the hexagonal hydro-aluminates to the cubic hexahydrate, probably also by sorption of (SO4)2− ions, and thus is a second mechanism by which they can retard the normal hydration of 3CaO.Al2O3. The degree to which a given addition of gypsum retards the hydration of 3CaO.Al2O3 at a particular temperature is affected by a balance between (1) the concentration of (SO4)2− ions on and around the surface of the 3CaO.Al2O3; (2) the rate of reaction of (SO4)2− ions with the hexagonal hydro-aluminates; (3) the thickness of the hexagonal hydro-aluminate layer around the 3CaO.Al2O3 grain. Reactions at the various temperatures are discllssed in terms of the above parameters.