Ceramics-Silikáty 49 (4) 230-236 (2005)

FORMATION AND STABILITY OF CRYSTALLOHYDRATES IN THE NON-EQUILIBRIUM SYSTEM DURING HYDRATION OF SAB CEMENTS

M. Palou, J. Majling, M. Dováľ, J. Kozanková, S. Chandra Mojumdar

The present experimental work characterizes different kinetic regimes and crystallohydrate products of the hydration reaction in C₂AS–C₄A₃S–CS–C₂S–C system, relevant to Sulfoaluminate Belite Cement (SAB). In the given system, two associations of phases (I and II) were identified, each is composed of 4 minerals, as well as two singular points with 3 minerals (S1, S2) and a single point of gehlenite (P1). The computational method established that the hydration products are gehlenite hydrate and ettringite. The ratio of these two crystallohydrates depends on the mineralogical composition in the given phase associations. The hydration reaction of this system is a heterogeneous and non-equilibrium solid phase transformation forming from an anhydrous solid phases to crystallohydrate phases in the presence of water. The advantage and complexity of this heterogeneous process is that during hydration heat is evolved and the paste sets and hardens limiting thus the dissolution of primary anhydrous phases and ion transport. Then the reaction is at great extent retarded. Some primary hydration products (C–S–H, AH₃, CH) convert through secondary reactions to the gehlenite hydrate, thermodynamically more stable. Eight samples were prepared along gehlenite hydrate - ettringite composition. Apart from the single gehlenite point, ettringite is formed at the beginning of reaction in all samples, but over the time and under the non-equilibrium conditions it was decomposed to monosulphate. After 28 - day curing at high temperature and under saturated stream pressure, gehlenite hydrate was identified by SEM. The intensity of hydration reaction from calorimetric measurements depends on the content of C₄A₃S in the mixtures.

Keywords: Hydration rate, Gehlenite hydrate, Ettringite, Monosulphate, Stability