Abstract
The hydration of tri-calcium aluminate in the presence of gypsum has been examined by electron optical methods as a function of pH and hydration time. The location of the alumino-sulphate phase has been related to the solubility of aluminate ions from the precursive platelet phase. At pH ∼ 11.5 the alumino-sulphate precipitate is crystalline and precipitates from solution. When first formed it is deficient in calcium and sulphate with respect to ettringite. If the deficiency is too great, transformation to calcium aluminate monosulphate hydrate may occur as an intermediate step before the equilibrium phases are obtained. At a higher pH (∼ 12.5) the fibres of alumino-sulphate are smaller and more rounded. Long tubular fibres similar to those seen in portland cement were not observed. This has been explained as a consequence of using pure tricalcium aluminate, rather than alkali-containing material. The information obtained has been used to predict the effect of some inorganic additives on the phases produced during the setting of portland cement.
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