Studies of Early Stages of Paste Hydration of Cement Compounds, I

The paste hydration of C₄AF with and without the addition of lime and/or gypsum was studied by means of electron-optical and X-ray diffraction techniques. It was found that in the paste hydration of C₄AF iron did not separate out but remained in the structure of the hydrated compounds, probably substituting for aluminum. In a neat paste of C₄AF and H₂O, at first di- and tetracalcium aluminate hydrate crystals formed, but later cubic C₃AH₆ crystals appeared. In the presence of Ca(OH)₂, C₂AH_x crystals did not form; otherwise the course of the reaction was the same as without lime. Any CO₂ present formed monocarboaluminate. When gypsum alone was present, ettringite crystals first appeared which later changed to monosulfate hydrate. In the presence of both lime and gypsum, the reactivity of C₄AF was much reduced. Ettringite crystals were somewhat stabilized by lime. The formation of monocarboaluminate was hindered in the presence of gypsum.