Kemija u Industriji (Jan 2011)
Calcium Aluminate Cement Hydration Model
Abstract
Calcium aluminate cement (AC) is a very versatile special cement used for specific applications. As the hydration of AC is highly temperature dependent, yielding structurally different hydration products that continuously alter material properties, a good knowledge of thermal properties at early stages of hydration is essential. The kinetics of AC hydration is a complex process and the use of single mechanisms models cannot describe the rate of hydration during the whole stage.This paper examines the influence of temperature (ϑ=5–20 °C) and water-to-cement mass ratio (mH /mAC = 0.4; 0.5 and 1.0) on hydration of commercial iron-rich AC ISTRA 40 (producer: Istra Cement, Pula, Croatia, which is a part of CALUCEM group), Figs 1–3. The flow rate of heat generation of cement pastes as a result of the hydration reactions was measured with differential microcalorimeter. Chemically bonded water in the hydrated cement samples was determined by thermo-gravimetry.Far less heat is liberated when cement and water come in contact for the first time, Fig. 1, than in the case for portland cement (PC). Higher water-to-cement ratio increases the heat evolved at later ages (Fig. 3) due to higher quantity of water available for hydration. A significant effect of the water-to-cement ratio on the hydration rate and hydration degree showed the importance of water as being the limiting reactant that slows down the reaction early. A simplified stoichiometric model of early age AC hydration (eq. (8)) based on reaction schemes of principal minerals, nominally CA, C12A7 and C4AF (Table 1), was employed. Hydration kinetics after the induction period (ϑ < 20 °C) had been successfully described (Fig. 4 and Table 2) by a proposed model (eq. (23)) which simultaneously comprised three main mechanisms: nucleation and growth, interaction at phase boundary, and mass transfer. In the proposed kinetic model the nucleation and growth is proportional to the amount of reacted minerals (eq. (18)), the interaction at phase boundary was described by a bimolecular consumption of both reactants (eq. (19)), cement and free water, while the mass transfer mechanism was described relative to the limiting reactant (eq. (21)).Increasing temperature from 5 to 20 °C decreases the rate of nucleation and growth (NR) (Fig. 6), increases the rate of interaction (I) according to Arrhenius law (E12 ≈43 kJ mol-1) (Fig. 7), and increases the rate of mass transfer (k) linearly (Fig. 8).
