The effective volumes of waters of crystallization & the thermodynamics of cementitious materials

Hydrates are significant components of cements and concrete. We examine the effective volumes of waters of crystallization for these materials, where the “effective volumes” are the difference per water molecule between the formula volume of the hydrate and of its parent anhydrate. These effective volumes cover a small range around 15 cm³ mol⁻¹ (≅ 23 ų per water molecule), unlike the wider range for general inorganic materials.

We also examine the thermodynamic properties of the cementitious phase, which follow the generally observed correlation of relating to their molar volumes. We establish “effective” additive oxide parameters for enthalpy and for molar volume, which are useful in confirming experimental values and in predicting as-yet undetermined values. Their Debye temperatures approximate to 600 K; this Debye temperature is well above ambient temperature and suggests that the vibrational modes of these cementitious phases are only partially excited and that the materials are hard. Ferrate-containing materials generally have a lower Debye temperature (∼273 K) implying that they may be softer than other cementitious materials.

These observations may be useful in checking for errors in data and anomalies in behavior among related cementitious materials.