Speaker
Description
Ettringite (AFt) is a key phase influencing the early-stage hydration and rheology of calcium aluminate, calcium sulfoaluminate and blended cements. However, a quantitative and mechanistic understanding of early-stage hydration - specifically nucleation and growth- remains unclear, which is fundamental for optimizing the material performance.
In this study, we have developed the synthesis of AFt in a highly controlled, dilute system with titration-precipitation, enabling in-situ monitoring of physiochemical process using Ca ISE (ion selective electrode), pH, optical and conductivity electrodes every 5 seconds. The time-resolved data from this process is further implemented into a thermodynamic-kinetic model framework based on classical nucleation theory and population balance method, evaluating physically significant parameters such as solubility product, interfacial energies, alongside nucleation kinetics.
The prime focus of this study is to understand the influence of superplasticizers, essential for rheology control, yet reported to be detrimental for cement hydration. Various polycarboxylate ethers (PCEs) were studied in the controlled titration precipitation method, revealing that PCEs not only alter the nucleation, causing delay, but also stabilize in a highly sulfated system the thermodynamically less stable phase, monosulfate (AFm), by altering the nucleation sites. This kinetic inhibition is observed to have a concentration dependence, leading to a shift in the distribution of AFt and AFm phases.
TEM evolution over the solid formation revealed the simultaneous formation of both phases, without intermediate transition, influenced by the local supersaturation. Thermodynamic-kinetic modelling accurately described the early formation stages - nucleation and growth, resulting in reasonable thermodynamic parameters. These results offer crucial insights into early-stage physicochemical process, which could optimize the usage of various superplasticizers and workability in blended cements.
Affiliations
Lab of Sustainable Energy Carriers and Processes, PSI Center for Energy and Environment Sciences, 5232 Villigen PSI, Switzerland, hari.ravindran@psi.ch
| Keywords | Calcium sulfoaluminates, PCE, early-stage hydration, kinetic-thermodynamic model, in-situ monitoring |
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