Molecular Dynamics Simulations of $γ$-Belite(010)-Water Interfaces with High-Dimensional Neural Network Potentials

Belite – dicalcium silicate Ca$_2$SiO$_4$ – is a main constituent of low-carbon cement. In this work, we study several terminations of the (010) surface of $γ$-belite, its most stable polymorph, by molecular dynamics simulations. The energies and forces are provided by a high-dimensional neural network potential trained to density functional theory data. Water can interact in molecular form as well as dissociatively with the investigated interfaces, and the degree of dissociation is determined primarily by the protonation of SiO$_4$ groups accessible at the surface. A major part of the simultaneously formed hydroxide ions is adsorbed at surface calcium atoms, whose octahedral coordination spheres are completed by additional water molecules. The T3 termination, which is most stable in vacuum, shows only little reactivity in water. For the only slightly less stable T2 termination, however, two distinct types of surface defects are observed. The type I defect is even stable in vacuum and leads to a reconstruction of the entire surface, while the type II defect is only found in the presence of water. Overall, our results suggest that a variety of structures may be formed at the Ca$_2$SiO$_4$(010) surface, which are stabilized in the presence of water.

💡 Research Summary

This research presents a sophisticated molecular-level investigation into the interaction between $\gamma$-belite ($Ca_2SiO_4$), a critical component in the production of low-carbon cement, and water. Utilizing advanced computational techniques, the study employs Molecular Dynamics (MD) simulations powered by High-Dimensional Neural Network Potentials (HDNNP). These potentials, trained on high-fidelity Density Functional Theory (DFT) data, allow the researchers to bridge the gap between quantum mechanical accuracy and the large-scale temporal and spatial requirements of molecular dynamics, enabling the observation of complex interfacial phenomena that were previously computationally inaccessible.

The study focuses on the (010) surface of the most stable $\gamma$-belite polymorph, examining various surface terminations. A primary discovery is the mechanism of water dissociation at the interface. The researchers found that the degree of dissociation is primarily driven by the protonation of $SiO_4$ groups that are accessible at the surface. As water molecules dissociate, the resulting hydroxide ions ($OH^-$) adsorb onto surface calcium atoms, effectively completing their octahedral coordination spheres alongside additional water molecules. This provides a fundamental atomistic understanding of how the chemical environment of the surface changes during the initial stages of hydration.

A detailed comparison of different surface terminations, specifically T2 and T3, revealed distinct chemical and structural behaviors. The T3 termination, which is the most stable in a vacuum, exhibits minimal reactivity when in contact with water, maintaining its structural integrity. In contrast, the slightly less stable T2 termination demonstrates significant structural complexity and sensitivity to the environment. The study identifies two specific types of surface defects in the T2 termination: Type I defects, which are stable even in vacuum and trigger a large-scale reconstruction of the entire surface, and Type II defects, which are uniquely induced by the presence of water.

Ultimately, the findings demonstrate that the presence of water is a fundamental driver in stabilizing a variety of complex and diverse structures at the $Ca_2SiO_4(010)$ surface. By elucidating the atomistic details of how water interacts with and alters the belite surface through dissociation and adsorption, this work provides essential insights into the fundamental processes of cement hydration. This knowledge is crucial for the future development of sustainable, low-carbon construction materials, as it enables more precise control over the chemical and structural evolution of cementitious-water interfaces, paving the way for next-generation cement chemistry.