Chemical aspects on chromate in cement

Abstract

A certain amount of chromate can be added to calcium aluminate and calcium aluminate ferrite, 2 compounds present in clinker, without being extractable with water. Solutions of sulfate and the chemically related selenate and molybdate can extract about 2/3 of the added chromate. Also a certain amount of molybdate can be added to calcium aluminate without being extractable with water. About 2/3 of the added molybdate can be extracted with solutions of sulfate, selenate and chromate. Chromate present in clinker can partly be extracted with solutions of sulfate, selenate and molybdate. The present results indicate that chromate is partly present in crystals of calcium aluminate and calcium aluminate ferrite and that the correlation between chromate and sulfate in cement is due to an equilibrium between these two compounds inside and outside the crystals. The results also indicate that chromate, sulfate, selenate and molybdate can substitute each other. As cement always contains gypsum, which is partly water soluble, the possibility to make chromatefree cement is limited.