No abstract

Hardening of lime-based binders is a process where carbonation of portlandite plays an important role. Additionally, carbonation of hydrated lime is of general technical interest, e.g., for carbon dioxide capture. The mechanisms responsible for this reaction are not well-known. In this work, we study the carbonation of portlandite without water and with one water molecule at atomistic scale. Density functional theory (DFT) is used to simulate the reaction path and to accurately calculate minima and transition states on the potential-energy surface. We find that water significantly lowers reaction barriers and stabilizes the partially carbonated reaction product, which gives atomistic proof to experimental results. From our atomistic point of view, we explain the reasons for this effect of water on the carbonation of portlandite.

Rapid CO 2 mineralization in natural and synthetic metal silicates provides a potentially scalable solution to address the untethered global carbon emissions. When metal silicates react with humidified CO 2 -rich fluids, a nanometer-thick water film adsorbs on mineral surfaces. Experiments show that such nanoscale reactive environments demonstrate an enhanced level of carbonic acid formation, metal-(bi)carbonate surface complexation, and fast carbon mineralization. Hindered by the spatiotemporal complexities of in situ measurements at the solid−liquid interface, the mechanistic picture of carbon mineralization mechanisms in water films remain obscure. Here, we leverage reactive and nonreactive molecular simulations to probe the elementary reaction steps involved in the interaction of bicarbonate with metal silicate surfaces. We observe that a reverse proton transport between the bicarbonate and surface hydroxides drives carbonate production and surface metal carbonate complexation in agreement with in situ spectroscopy measurements. The resultant carbonate can also contribute to the ligand-enhanced dissolution that appears to be slightly favorable over carbonate-unassisted dissolution. We also discuss the potential implications of metal carbonate complex formation and dissolution on lowering the growth's configurational entropy penalty and the rise of interfacial carbon mineralization pathways.