Ab initio mechanism revealing for tricalcium silicate dissolution

Carbonation under Varying Humidity Conditions: A 3D Micro-Scale Kinetic Monte Carlo Approach

This paper investigates the impact of varying humidity conditions on the carbonation depth in hardened cement paste using a 3-dimensional microscale kinetic Monte Carlo (kMC) approach. The kMC algorithm effectively simulates the carbonation process by capturing the interplay between CO2 diffusion and relative humidity at the microscale, providing insights into macro trends that align with historical models. The study reveals that the maximum carbonation depth is achieved at relative humidity levels between 55 and 65%, where the balance between water and CO2 diffusion is optimized. At lower relative humidity levels (<55%), a lower carbonation depth is observed. Conversely, at higher relative humidity levels (>65%), increased water content impedes CO2 diffusion, resulting in reduced carbonation depth for cement paste. The kMC model demonstrates a parabolic relationship between relative humidity and carbonation depth. Time series analysis shows that Fick's law is consistently followed, with carbonation depth following the relationship x = k√t at constant relative humidity. The kMC also breaks down the event cycle which shows that after an equilibrium (in terms of rate of events) is achieved between CO2 and H2O at a relative humidity of 75%, a shift occurs in the dominance from reactive to transport processes at a relative humidity of 85%. These findings highlight the importance of humidity in influencing carbonation rates on the one hand and demonstrate the effectiveness of the kMC approach in simulating these complex interactions at the microscale on the other hand.

Crumbling crystals: on the dissolution mechanism of NaCl in water

Dissolution of ionic salts in water is ubiquitous, particularly for NaCl. However, an atomistic scale understanding of the process remains elusive. Simulations lend themselves conveniently to studying dissolution since they provide the spatio-temporal resolution that can be difficult to obtain experimentally. Nevertheless, the complexity of various inter- and intra-molecular interactions require careful treatment and long time scale simulations, both of which are typically hindered by computational expense. Here, we use advances in machine learning potential methodology to resolve at an ab initio level of theory the dissolution mechanism of NaCl in water. The picture that emerges is that of a steady ion-wise unwrapping of the crystal preceding its rapid disintegration, reminiscent of crumbling. The onset of crumbling can be explained by a strong increase in the ratio of the surface area to volume of the crystal. Overall, dissolution comprises a series of highly dynamical microscopic sub-processes, resulting in an inherently stochastic mechanism. These atomistic level insights contribute to the general understanding of dissolution mechanisms in other crystals, and the methodology is primed for more complex systems of recent interest such as water/salt interfaces under flow and salt crystals under confinement.

Theory-driven design of cadmium mineralizing layered double hydroxides for environmental remediation

The environmental concern posed by toxic heavy metal pollution in soil and water has grown. Ca-based layered double hydroxides (LDHs) have shown exceptional efficacy in eliminating heavy metal cations through the formation of super-stable mineralization structures (SSMS). Nevertheless, it is still unclear how the intricate coordination environment of Ca2+ in Ca-based LDH materials affects the mineralization performance, which hinders the development and application of Ca-based LDH materials as efficient mineralizers. Herein, we discover that, in comparison to a standard LDH, the mineralization efficiency for Cd2+ ions may be significantly enhanced in the pentacoordinated structure of defect-containing Ca-5-LDH utilizing both density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Furthermore, the calcination-reconstruction technique can be utilized to successfully produce pentacoordinated Ca-5-LDH. Subsequent investigations verified that Ca-5-LDH exhibited double the mineralization performance (421.5 mg g-1) in comparison to the corresponding pristine seven coordinated Ca-7OH/H2O-LDH (191.2 mg g-1). The coordination-relative mineralization mechanism of Ca-based LDH was confirmed by both theoretical calculations and experimental results. The understanding of LDH materials and their possible use in environmental remediation are advanced by this research.

Sediment resuspension as a driving force for organic carbon transference and rebalance in marginal seas

The transfer of particulate organic carbon (POC) to dissolved organic carbon (DOC; OC transferP-D) is crucial for the marine carbon cycle. Sediment resuspension driven by hydrodynamic forcing can affect the burial of sedimentary POC and benthic biological processes in marginal sea. However, the role of sediment grain size fraction on OC transferP-D and the subsequent impact on OC cycling remain unknown. Here, we conduct sediment resuspension simulations by resuspending grain-size fractionated sediments (< 20, 20-63, and > 63 μm) into filtered seawater, combined with analyses of OC content, optical characteristics, 13C and 14C isotope compositions, and molecular dynamics simulations to investigate OC transferP-D and its regulations on OC bioavailability under sediment resuspension. Our results show that the relative intensities of terrestrial humic-like OC (refractory DOC) increase in resuspension experiments of < 20, 20-63, and > 63 μm sediments by 0.14, 0.01, and 0.03, respectively, likely suggesting that sediment resuspension drives refractory DOC transfer into seawater. The variations in the relative intensities of microbial protein-like DOC are linked to the change of terrestrial humic-like OC, accompanied by higher DOC content and reactivity in seawater, particularly in finer sediments resuspension experiments. This implies that transferred DOC likely fuels microbial growth, contributing to the subsequent enhancement of DOC bioavailability in seawater. Our results also show that the POC contents increase by 0.35 %, 0.66 %, and 0.93 % in < 20, 20-63, and > 63 μm resuspension experiments at the end of incubation, respectively. This suggests that the re-absorption of OC on particles may be a significant process, but previously unrecognized during sediment resuspension. Overall, our findings suggest that sediment resuspension promotes the OC transferP-D, and the magnitudes of OC transferP-D further influence the DOC and POC properties by inducing microbial production and respiration. These processes significantly affect the dynamics and recycling of biological carbon pump in shallow marginal seas.

Bioactive ceramic-based materials: beneficial properties and potential applications in dental repair and regeneration

Bioactive ceramics, primarily consisting of bioactive glasses, glass-ceramics, calcium orthophosphate ceramics, calcium silicate ceramics and calcium carbonate ceramics, have received great attention in the past decades given their biocompatible nature and excellent bioactivity in stimulating cell proliferation, differentiation and tissue regeneration. Recent studies have tried to combine bioactive ceramics with bioactive ions, polymers, bioactive proteins and other chemicals to improve their mechanical and biological properties, thus rendering them more valid in tissue engineering scaffolds. This review presents the beneficial properties and potential applications of bioactive ceramic-based materials in dentistry, particularly in the repair and regeneration of dental hard tissue, pulp-dentin complex, periodontal tissue and bone tissue. Moreover, greater insights into the mechanisms of bioactive ceramics and the development of ceramic-based materials are provided.

Molecular insight into the initial hydration of tricalcium aluminate

Portland cement (PC) is ubiquitously used in construction for centuries, yet the elucidation of its early-age hydration remains a challenge. Understanding the initial hydration progress of tricalcium aluminate (C3A) at molecular scale is thus crucial for tackling this challenge as it exhibits a proclivity for early-stage hydration and plays a pivotal role in structural build-up of cement colloids. Herein, we implement a series of ab-initio calculations to probe the intricate molecular interactions of C3A during its initial hydration process. The C3A surface exhibits remarkable chemical activity in promoting water dissociation, which in turn facilitates the gradual desorption of Ca ions through a metal-proton exchange reaction. The dissolution pathways and free energies of these Ca ions follow the ligand-exchange mechanism with multiple sequential reactions to form the ultimate products where Ca ions adopt fivefold or sixfold coordination. Finally, these Ca complexes reprecipitate on the remaining Al-rich layer through the interface-coupled dissolution-reprecipitation mechanism, demonstrating dynamically stable inner-sphere adsorption states. The above results are helpful in unmasking the early-age hydration of PC and advancing the rational design of cement-based materials through the bottom-up approach.

Unveiling the heavy-metal ion critical role in γ-dicalcium silicate: from solidification to early hydration

The heavy-metal ion critical role in γ-dicalcium silicate (γ-C2S) both in terms of solidification mechanism and hydration is still unclear. In this work, the solidification mechanism and the effect on initiating hydration of these three heavy-metal ions (Ba, Cd, and Cr) in γ-C2S is systemically studied by well-defined ab initio calculations. The calculated results show that the solid solution tendency of ions originates from the charge contribution, and the charge localization caused by the doping of Cr ions weakens the surface water adsorption. These insights will provide theoretical guidance for the low-carbon cement development by γ-C2S.

The initial stages of cement hydration at the molecular level

Cement hydration is crucial for the strength development of cement-based materials; however, the mechanism that underlies this complex reaction remains poorly understood at the molecular level. An in-depth understanding of cement hydration is required for the development of environmentally friendly cement and consequently the reduction of carbon emissions in the cement industry. Here, we use molecular dynamics simulations with a reactive force field to investigate the initial hydration processes of tricalcium silicate (C3S) and dicalcium silicate (C2S) up to 40 ns. Our simulations provide theoretical support for the rapid initial hydration of C3S compared to C2S at the molecular level. The dissolution pathways of calcium ions in C3S and C2S are revealed, showing that, two dissolution processes are required for the complete dissolution of calcium ions in C3S. Our findings promote the understanding of the calcium dissolution stage and serve as a valuable reference for the investigation of the initial cement hydration.

Dynamic dissolution of Cm3+ ions incorporated at the calcite-water interface: an ab initio molecular dynamics simulation study

The stability of actinide-mineral solid solution in a water environment is critical for assessing the safety of nuclear-waste geological repositories and studying actinide migration in natural systems. However, the dissolution behavior of actinide ions incorporated at the mineral-water interface is still unclear at the atomic level. Herein, we present metadynamics simulations of the reaction pathways, thermodynamics and kinetics of trivalent curium ions (Cm3+) dissolving from calcite surfaces. Cm3+ ions incorporated in different calcite surfaces (i.e., terrace and stepped surfaces) with distinct coordination environments have different reaction pathways, free energy barriers and free energy changes. We found that Cm dissolution from a stepped surface is more favorable than that from a terrace surface, both thermodynamically and kinetically. In addition, water molecules seem to promote the detachment of curium ions from the surface by exerting a pulling force via water coordination with Cm3+ and a pushing force via proton migration to the surface layer and water diffusion in the vacant Cm site. Thus, the findings from this work prove to be a milestone in revealing the dynamic dissolution mechanism of trivalent actinides from minerals and would also help predict the dissolution behaviors of other metal ions at the solid-water interface in chemical and environmental sciences.