Hydration of calcium aluminate cement blended with anhydrite

Calcium aluminate cements (CACs) are used in combination with calcium sulfates as binders in self-levelling mortars. The influence of commercial CAC/anhydrite ratio on the solid phase assemblage and pore solution at water to binder (w/b) ratios of 0.40 and 0.72 was investigated between 1.5 h and 91 d of hydration using a multi-method approach including x-ray diffraction, thermogravimetric analysis and thermodynamic calculations. The reduction of the extent of hydration at the lower w/b ratio can be explained by a lack of water for complete hydration and lack of space. In the system with a CAC/anhydrite weight ratio of 1, ettringite and AH(3) were the main hydration products, whereas in the case of a CAC/anhydrite ratio of 2.3, in addition to ettringite and AH(3), monosulfate was present along with stratlingite and at a later age. In contrast to the thermodynamic modelling results, anhydrite persisted even at low calcium sulfate contents as the precipitation of AH(3) around the anhydrite grains seemed to lead to a chemical blocking.