Symmetry induced stability in alkali doped calcium-silicate-hydrate

TL;DR

This study uses quantum chemistry calculations to show that symmetrical distribution of alkali atoms in calcium-silicate-hydrate enhances its stability, informing the design of more durable, low-CO2 construction materials.

Contribution

It reveals the relationship between alkali atom symmetry and stability in C-S-H, providing insights into optimizing alkali doping for durable, low-emission cement materials.

Findings

Symmetrical alkali distribution increases stability.

Moderate alkali concentrations are optimal for stability.

Structural mechanisms involve migration barriers and charge distribution.

Abstract

CO$_2$ emissions originating from the construction industry have a significant impact on global warming where the production of ordinary Portland cement clinker is responsible for approximately 8\% of all human-made CO$_2$. Alkali doped calcium-silicate-hydrate (C-S-H) is a critical silicate material since the use of blended cements and alkali-activated materials in construction industry can substantially reduce human-made CO$_2$ emissions. However, the effect of alkali doping (Na and K) on the long-term stability and associated durability of C-S-H remains an open question. Here, using first principles quantum chemistry calculations on the model crystalline phase clinotobermorite, we show that there is a strong interplay between the thermodynamic stability of alkali doped C-S-H and the symmetry of the alkali atoms in the structure. Our results reveal that a symmetrical distribution of alkali atoms leads to a higher stability value such that stable structures with moderate alkali concentrations can be obtained provided that the alkali atoms are allowed to settle into a symmetrical distribution. We investigate the associated structural mechanisms by calculating the migration barriers of alkali atoms within the material, the electronic charge distribution in the material and the variation of basal spacing by using both computational methods and X-ray diffraction analysis.