Nanoscale Charge Balancing Mechanism in Alkali Substituted Calcium-Silicate-Hydrate Gels

TL;DR

This study uncovers a nanoscale charge balancing mechanism in alkali-substituted calcium-silicate-hydrate gels, revealing how alkali incorporation stabilizes structures and enhances mechanical properties, with implications for sustainable cementitious materials.

Contribution

It introduces a molecular-level charge balancing mechanism in alkali-modified C-S-H gels, supported by first-principles simulations, advancing understanding of their stability and properties.

Findings

Charge balancing stabilizes alkali-containing C-S-H structures.

Alkali incorporation improves mechanical properties.

Charge balanced structures have lower formation energies.

Abstract

Alkali-activated materials and related alternative cementitious systems are sustainable material technologies that have the potential to substantially lower CO$_2$ emissions associated with the construction industry. However, the impact of augmenting the chemical composition of the material on the main binder phase, calcium-silicate-hydrate gel, is far from understood, particularly since this binder phase is disordered at the nanoscale. Here, we reveal the presence of a charge balancing mechanism at the molecular level, which leads to stable structures when alkalis (i.e., Na or K) are incorporated into a calcium-silicate-hydrate gel, as modeled using crystalline 14{\AA} tobermorite. These alkali containing charge balanced structures possess superior mechanical properties compared to their charge unbalanced counterparts. Our results, which are based on first-principles simulations using density functional theory, include the impact of charge balancing on the optimized geometries of the new model phases, formation energies, local bonding environments, bulk moduli and diffusion barriers of the alkali atoms within the crystals.