Exploring Mechanisms of Hydration and Carbonation of MgO and Mg(OH)2 in Reactive Magnesium Oxide-based Cements

TL;DR

This study uses density functional theory simulations to investigate the reaction mechanisms of hydration and carbonation in MgO-based cements, revealing how water and CO2 interactions influence reaction pathways and rates.

Contribution

It provides detailed atomic-level insights into hydration and carbonation mechanisms in MgO and Mg(OH)2, advancing understanding for improved magnesium-based cement development.

Findings

Carbonation of MgO is hindered by water molecules.

Carbonation of Mg(OH)2 is hindered by carbonate/hydrate layers and excess water.

Reaction pathways and energy barriers differ between MgO and Mg(OH)2 surfaces.

Abstract

Reactive magnesium oxide (MgO)-based cement (RMC) can play a key role in carbon capture processes. However, knowledge on the driving forces that control the degree of carbonation and hydration and rate of reactions in this system remains limited. In this work, density functional theory-based simulations are used to investigate the physical nature of the reactions taking place during the fabrication of RMCs under ambient conditions. Parametric indicators such as adsorption energies, charge transfer, electron localization function, adsorption/dissociation energy barriers and the mechanisms of interaction of H2O and CO2 molecules with MgO and brucite (Mg(OH)2) clusters are considered. The following hydration and carbonation interactions relevant to RMCs are evaluated i) carbonation of MgO, ii) hydration of MgO, carbonation of hydrated MgO, iii) carbonation of Mg(OH)2, iv) hydration of Mg(OH)2 and v) hydration of carbonated Mg(OH)2. A comparison of the energy barriers and reaction pathways of these mechanisms shows that the carbonation of MgO is hindered by presence of H2O molecules, while the carbonation of Mg(OH)2 is hindered by the formation of initial carbonate and hydrate layers as well as presence of excessed H2O molecules. To compare these finding to bulk mineral surfaces, the interactions of the CO2 and H2O molecules with the MgO(001) and Mg(OH)2 (001) surfaces are studied. Therefore, this work presents deep insights into the physical nature of the reactions and the mechanisms involved in hydrated magnesium carbonates production that can be beneficial for its development.