# Effects of Relative Humidity on Dissolution and Carbonation Potential of Portlandite

## TL;DR

This study explores how humidity affects the dissolution of portlandite and its carbonation potential, using simulations and experiments to understand the process better.

## Contribution

The study introduces a novel simulation approach combining GCMC-MD and metadynamics to analyze portlandite dissolution and carbonation under varying humidity.

## Key findings

- Calcium dissolution from portlandite is likely to be trapped as inner-sphere adsorbed atoms.
- Activation energies for surface detachment and diffusion were calculated.
- Humidity affects the thermodynamic stability of surface sites, with a threshold at 40% RH.

## Abstract

The carbonation process
of cementitious materials is gaining attention
as a way to mitigate anthropogenic CO2 emissions. Because
the reactions occur as interfacial dissolution and precipitation within
thin adsorbed water film on the substrate, understanding water adsorption
and its influence on carbonation reactivity is essential. In this
study, the water adsorption isotherms on portlandite surface were
simulated by hybrid Grand Canonical Monte Carlo (GCMC)–Molecular
Dynamics (MD) simulations and compared with the experiment. With the
simulated water film under various relative humidity (RH), biased
MD simulations were performed using a well-tempered metadynamics scheme
to investigate how calcium dissolved from the portlandite surface.
The results revealed that the dissolved Ca was likely to be trapped
parallel to the substrate surface as inner-sphere adsorbed (adatom).
The activation energies for detachment from the surface sites and
for surface diffusion were calculated. In addition, the thermodynamical
stability of the surface sites at various RH was evaluated by calculating
the Gibbs energy for reaction (Δ
r

G), showing a bilinear relationship with a decreasing
trend until 40% RH and constant values above that, implying the threshold
RH for Ca dissolution. Our simulations also revealed that the perpendicular
movement of the dissolved Ca was restricted within where the H2O layer was present, which may spatially limit subsequent
nucleation and crystal growth of calcium carbonate and inhibit complete
carbonation of the substrate. For comparison with these simulations,
experiments were also performed to study the degree of carbonation
(DoC) of portlandite at various RH. The experimental trend showed
good consistency with simulations with respect to the reactivity threshold
and the reaction-saturated RH.