Characterisation and hydration of ye'elimite containing cements

Calcium sulphoaluminate cements (CSA) with the main phase ye’elimite are a promising class of non-Portland cements which are gaining increasing interest from the cement industry. However, the hydration kinetics of CSA cements varies strongly, even for cements with similar composition and fineness. Understanding the origin of this variation was the main motivation for this thesis. Earlier studies indicated that the type and composition of ye’elimite was related to the different hydration kinetics. Iron-rich CSA cements often show faster hydration kinetics compared to those with low iron contents. The addition of easily soluble calcium sulphate such as gypsum mitigates this effect, resulting in a harmonization of the hydration sequences and kinetics. The literature reports that stoichiometric ye’elimite has an orthorhombic symmetry at ambient conditions, whereas a cubic symmetry is stabilised by the presence of minor elements such as iron. Thus, previous to this work, it was often assumed that there was a link between the presence of iron, the polymorphism of ye’elimite and the hydraulic reactivity. This thesis provides new insights into the effect of iron on the formation and hydration of different ye’elimite types, including the effect of gypsum and mayenite additions. We found that ye’elimite is formed from the reaction of the intermediate calcium aluminates such as krotite with anhydrite. Iron primarily accelerates the formation of krotite and as a result also of ye’elimite. However, this leads to the faster decomposition of ye’elimite and as a result, mayenite starts to form. The investigation of the ye’elimite hydration revealed five hydration periods. The onset and length of these periods is largely controlled by the solution composition. We could further show that two main hydration reactions occur. The first hydration reaction is the formation of ettringite, monocalcium aluminate decahydrate and amorphous aluminium hydroxide and occurs primarily during the initial and dormant period. The second hydration reaction is the formation of monosulphate and gibbsite-like aluminium hydroxide, occurring primarily during the acceleration and main hydration period. Increasing the water to binder ratio generally favours the first reaction. Gypsum accelerates the hydration and modifies the hydrates assemblage by favouring the formation of ettringite rather than that of monosulphate, independent of the type of ye'elimite used. We could simulate the hydration kinetics of iron-rich ye’elimite by blending stoichiometric ye’elimite with synthetic mayenite. Hence, it appears that the presence of mayenite explains the different kinetics. We investigated the chemical shrinkage of neat ye’elimite and the effect of gypsum. The experiments revealed a transitory chemical expansion, during which bound water is released. This could be linked to the transformation and crystallization of previously formed metastable amorphous phases such as aluminium hydroxide. The transitory chemical expansion was followed by a recovery period when the shrinkage and the bound water contents returned to their original values. One possible explanation for this phenomenon is the swelling of the amorphous aluminium hydroxide.