Calcium-mono-aluminate (CA or CaAI2O4) and calcium-di-aluminate (CA2 or CaAl4O7) are the main phases in iron free high alumina cements (CAC) which are used in demanding refractory applications. It is well known that the hydraulic reactivity of calcium aluminate phases increases with the calcium content of the phase, and therefore CA shows higher reactivity than CA2. Some literature even claims that CA2 would be an almost inert phase with regard to hydraulic reactivity. This study investigates the hydration of pure CA and blends of CA and CA2 using heat flow calorimetry and quantitative in-situ X-ray diffraction (QXRD) during hydration. Heat flow was calculated by combining the QXRD data obtained during hydration of the mixes together with the standard enthalpies of formation of the participating phases. Comparing the observed heat flow from calorimetry with the heat flow calculated from QXRD data, good conformance could be obtained. The results show a clear pronounced influence of CA2 on hydration of CAC and its heat of hydration during the first 22 hours. After more than 10 hours the hydration of CA2 gives the major contribution to the heat flow.
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