Winnefeld – Thermodynamic Modelling of the Hydration of Calcium Sulfoaluminate Cements
Frank Winnefeld, Barbara Lothenbach
Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Concrete and Construction Chemistry, Überlandstrasse 129, 8600 Dübendorf, Switzerland
Calcium sulfoaluminate cements (C$A) are a promising low-CO2 alternative to ordinary Portland cements. In this study, the hydration two C$A cements has been investigated experimentally by means of conduction calorimetry, TGA, XRD, pore solution analysis and SEM between 1 hour and 28 days at w/c ratios of 0.72 and 0.80, respectively. Besides the major phase ye’elimite, C$A-1 contained calcium monoaluminate, C$A-2 belite as minor hydraulic phase. As calcium sulfate, gypsum was added in the case of C$A-1, whereas anhydrite was used in C$A-2.
Based on the experimental data, a thermodynamic hydration model for C$A cements based on cement composition, hydration kinetics of clinker phase and calculations of thermodynamic equilibria by geochemical speciation has been established.
During the first hours of hydration, ettringite and gel-like Al(OH)3 form from the hydration of ye’elimite, (4CaO∙3Al2O3∙SO3). After the depletion of the calcium sulfate, monosulfate starts to form. The depletion of calcium sulfate is visible in the conduction calorimetry as a shoulder or maximum in the heat flow curve. In the case of C$A-2, strätlingite (2CaO∙Al2O3∙SiO2∙8H2O) occurs as further hydration product, as the belite present starts to hydrate after several days. From the pore solution analysis it is evident, that strätlingite acts as a sink for the potassium released from the hydrating linker phases.
The pore solution chemistry is dominated at early age (first hours of hydration) by Na, K, Ca, Al and sulfate, the pH is between 10 and 11. After that, the main part of the calcium sulfate addition is consumed in hydration reaction, causing a strong decrease of Ca and sulfate concentration and an increase of pH to 12.5-12.8. The saturation indices calculated from the pore solution data reveal that the pore solution is undersaturated with respect to C-S-H phases. The silica dissolved is instead incorporated into strätlingite.
The two types of calcium sulfate in C$A-1 and C$A-2 behave differently. The analysis of the pore solution shows in the case of C$A-1 that it is saturated with respect to gypsum until the gypsum is depleted after 16 hours of hydration. The anhydrite in C$A-2 is undersaturated beyond 1 h of hydration due to its slower dissolution kinetics compared to the gypsum.
The microstructure of both cements is already quite dense after a hydration time of 16 hours. After 28 days of hydration, only a small volume of pore space is available. C$A-1 exhibits a homogeneous microstructure, whereas C$A-2 appears more inhomogeneously with large amounts of big strätlingite crystals present after 28 days.
The thermodynamic model developed in this study can be used to predict the hydration of C$A cements, allowing an easy and fast parameter variation like clinker composition, amount of calcium sulfate or water/cement ratio.