Insight on Tricalcium Silicate Hydration and Dissolution Mechanism from Molecular Simulations

Atomistic Insights into Silicate Dissolution of Metakaolinite under Alkaline Conditions: Ab Initio Quantum Mechanical Investigation

This study employs computational chemistry to investigate the detailed mechanisms behind the dissolution of thermally activated clays, which are emerging as promising supplementary cementitious materials (SCM) for enhancing concrete properties and reducing carbon footprint. Specifically, the study employs a first-principles methodology for obtaining activation energies (ΔEa) involved in the dissolution of metakaolinite (MK) silicate units using NaOH and KOH activators. The investigation includes considerations of hydrolyzing oxo-bridging covalent bonds, van der Waals (vdW) interactions, and the influence of water molecules surrounding alkali cations. The study employs the enhanced dimer method within density functional theory (DFT) to propose four models for determining the activation energies required to break oxo-bridging bonds. The results demonstrate that KOH generally requires lower activation energies than NaOH, particularly when considering vdW interactions. They also highlight the lower activation energy required for commencing the dissolution of silicate units and emphasize the significance of the hydration shell around cations. The proposed methodology contributes to establishing a systematic database of atomistic activation energies, essential for atomistic kinetic Monte Carlo upscaling and mesoscopic forward dissolution rate calculations in clays. This holds relevance in understanding their reactivity within cementitious materials.

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.

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.

First-principles study on the mechanical properties of cement mortar modified with functionalized graphene oxide

CONTEXT

In this paper, first-principle calculations reveal that the shear strength of the graphene-cementitious interface (G/C-S-H) (12 MPa) is lower than that of the epoxy, hydroxyl and carboxyl graphene-cementitious interfaces (G-O/C-S-H, G-OH/C-S-H and G-COOH/C-S-H) (21 MPa, 29 MPa and 14 MPa). This indicates that the introduction of functional groups helps to improve the mechanical properties of the graphene-cementitious contact interface. Electrical analysis of the interface reveals that functional groups adsorbed on graphene change the electron distribution on the graphene surface. The formation of a contact interface between graphene and cementitious not only promotes the interaction between the two, but also serves as a bridge connecting the graphene and the cementitious, exacerbating the charge transfer between the two and promoting the generation of solid chemical bonds.

METHOD

All calculations were performed by the CASTEP module in Materials Studio software, using the GGA-PBE functional for structural optimization. The convergence criteria for the geometry optimization are set to a self-consistent field iteration convergence criterion of 2.0 × 10-6 eV and a structural optimization convergence criterion of 0.02 eV/Å.

Dissolution of Forsterite Surface in Brine at CO2 Geo-storage Conditions: Insights from Molecular Dynamic Simulations

Evaluating the long-term security of geological deep saline aquifers to store CO₂ requires a comprehensive understanding of mineral dissolution properties. Molecular dynamics simulations are performed to study the dissolution of forsterite in deep saline aquifers. The forsterite surface is found to be covered by three H₂O molecular layers, hindering CO₂ from directly contacting the surface. The dissolution rates at 350 K are increased by more than 10¹² with the presence of Mg defects or salt ions in solutions. The more disordered surface in pure water caused by Mg defects accounts for the acceleration of dissolution, while absorbed Cl^- ions on the surface in NaCl and KCl solutions accelerate the dissolution through electrostatic interactions. Comparatively, the frequent attacks from alkaline earth cations in MgCl₂ and CaCl₂ solutions to the surface contribute to the enhanced dissolution. In the acidic H₃OCl solution, the electrostatic interactions between O atoms in H₃O⁺ and the surface facilitate the dissolution. Interestingly, the ionic clusters of CO₃^2-/HCO₃^- and Na⁺ in Na₂CO₃/NaHCO₃ solution promote the dissolution process. This work provides molecular insights into forsterite dissolution in deep saline aquifers and guidance toward the optimization of CO₂ geo-storage conditions.

Dissolution of β-C2S Cement Clinker: Part 1 Molecular Dynamics (MD) Approach for Different Crystal Facets

A major concern in the modern cement industry is considering how to minimize the CO2 footprint. Thus, cements based on belite, an impure clinker mineral (CaO)2SiO2 (C2S in cement chemistry notation), which forms at lower temperatures, is a promising solution to develop eco-efficient and sustainable cement-based materials, used in enormous quantities. The slow reactivity of belite plays a critical role, but the dissolution mechanisms and kinetic rates at the atomistic scale are not known completely yet. This work aims to understand the dissolution behavior of different facets of β-C2S providing missing input data and an upscaling modeling approach to connect the atomistic scale to the sub-micro scale. First, a combined ReaxFF and metadynamics-based molecular dynamic approach are applied to compute the atomistic forward reaction rates (RD) of calcium (Ca) and silicate species of (100) facet of β-C2S considering the influence of crystal facets and crystal defects. To minimize the huge number of atomistic events possibilities, a generalized approach is proposed, based on the systematic removal of nearest neighbors' crystal sites. This enables us to tabulate data on the forward reaction rates of most important atomistic scenarios, which are needed as input parameters to implement the Kinetic Monte Carlo (KMC) computational upscaling approach. The reason for the higher reactivity of the (100) facet compared to the (010) is explained.

Ab initio mechanism revealing for tricalcium silicate dissolution

Dissolution of minerals in water is ubiquitous in nature and industry, especially for the calcium silicate species. However, the behavior of such a complex chemical reaction is still unclear at atomic level. Here, we show that the ab initio molecular dynamics and metadynamics simulations enable quantitative analyses of reaction pathways, thermodynamics and kinetics of the calcium ion dissolution from the tricalcium silicate (Ca₃SiO₅) surface. The calcium sites with different coordination environments lead to different reaction pathways and free energy barriers. The low free energy barriers result in that the detachment of the calcium ion is a ligand exchange and auto-catalytic process. Moreover, the water adsorption, proton exchange and diffusion of water into the surface layer accelerate the leaching of the calcium ion from the surface step by step. The discovery in this work thus would be a landmark for revealing the mechanism of tricalcium silicate hydration.

Dissolution of Portlandite in Pure Water: Part 1 Molecular Dynamics (MD) Approach

The current contribution proposes a multi-scale bridging modeling approach for the dissolution of crystals to connect the atomistic scale to the (sub-) micro-scale. This is demonstrated in the example of dissolution of portlandite, as a relatively simple benchmarking example for cementitious materials. Moreover, dissolution kinetics is also important for other industrial processes, e.g., acid gas absorption and pH control. In this work, the biased molecular dynamics (metadynamics) coupled with reactive force field is employed to calculate the reaction path as a free energy surface of calcium dissolution at 298 K in water from the different crystal facets of portlandite. It is also explained why the reactivity of the (010), (100), and (11¯0) crystal facet is higher compared to the (001) facet. In addition, the influence of neighboring Ca crystal sites arrangements on the atomistic dissolution rates is explained as necessary scenarios for the upscaling. The calculated rate constants of all atomistic reaction scenarios provided an input catalog ready to be used in an upscaling kinetic Monte Carlo (KMC) approach.

Exchange Reactions at Mineral Interfaces

Exchange reactions are a family of chemical reactions that appear when mineral surfaces come into contact with protic solvents. Exchange reactions can also be understood as a unique interaction at mineral interfaces. Particularly significant interactions occurring at mineral surfaces are those with water and CO₂. The rather complex process occurring when minerals such as calcium silicate hydrate (C-S-H) phases come into contact with aqueous environments is referred to as a metal-proton exchange reaction (MPER). This process leads to the leaching of calcium ions from the near-surface region, the first step in the corrosion of cement-bound materials. Among the various corrosion reactions of C-S-H phases, the MPER appears to be the most important one. A promising approach to bridging certain problems caused by MPER and carbonation is the passivation of C-S-H surfaces. Today, such passivation is reached, for instance, by the functionalization of C-S-H surfaces with water-repelling organic films. Unfortunately, these organic films are weak against temperature and especially weak against abrasion. Exchange reactions at mineral interfaces allow the preparation of intrinsic, hydrophobic surfaces of C-S-H phases just at room temperature via a metal-metal exchange reaction.

Role of Mg Impurity in the Water Adsorption over Low-Index Surfaces of Calcium Silicates: A DFT-D Study

Calcium silicates are the most predominant phases in ordinary Portland cement, inside which magnesium is one of the momentous impurities. In this work, using the first-principles density functional theory (DFT), the impurity formation energy (Efor) of Mg substituting Ca was calculated. The adsorption energy (Ead) and configuration of the single water molecule over Mg-doped β-dicalcium silicate (β-C2S) and M3-tricalcium silicate (M3-C3S) surfaces were investigated. The obtained Mg-doped results were compared with the pristine results to reveal the impact of Mg doping. The results show that the Efor was positive for all but one of the calcium silicates surfaces (ranged from −0.02 eV to 1.58 eV), indicating the Mg substituting for Ca was not energetically favorable. The Ead of a water molecule on Mg-doped β-C2S surfaces ranged from –0.598 eV to −1.249 eV with the molecular adsorption being the energetically favorable form. In contrast, the Ead on M3-C3S surfaces ranged from −0.699 eV to −4.008 eV and the more energetically favorable adsorption on M3-C3S surfaces was dissociative adsorption. The influence of Mg doping was important since it affected the reactivity of surface Ca/Mg sites, the Ead of the single water adsorption, as well as the adsorption configuration compared with the water adsorption on pristine surfaces.

Reactivity of Different Crystalline Surfaces of C3S During Early Hydration by the Atomistic Approach

Early hydration of tricalcium silicate (C₃S) has received great attention over the years due to the increased use of composite cement with a reduced number of clinker phases, especially the addition of what should be very reactive C₃S to guarantee early strength. Although many mechanisms have been proposed, the dissolution of polygonal C₃S at the material interface is not yet fully understood. Over the last decade, computational methods have been developed to describe the reaction in the cementitious system. This paper proposes an atomistic insight into the early hydration and the dissolution mechanism of calcium from different crystalline planes of C₃S using reactive force field (ReaxFF) combined with metadynamics (metaD). The reactivity and thermodynamic stability of different crystal planes were calculated from the dissolution profile of calcium during hydration at 298 K. The simulation results, clearly describe the higher reactivity of (01¯1¯), (011), (100), and (1¯00) surfaces of C₃S due to the strong interaction with the water, whereas, the dissolution profile explains the lower reactivity of (1¯1¯0), (110), (01¯0) and the effect of water tessellation on the (001), (010) planes.

Structure, orientation, and dynamics of water-soluble ions adsorbed to basal surfaces of calcium monosulfoaluminate hydrates

Transport of water molecules and chloride ions in nanopores of hydrated cement paste (HCP) is proven to adversely affect the long-term durability of reinforced concrete structures exposed to seawater or deicing salts. The resistance against chloride attack is primarily associated with the chloride binding capacity of the main HCP constituents. Experimental tests revealed that AFm phases of HCP play a central role in binding the chloride ions. However, many aspects of AFm-solution interactions were largely unknown, especially at their interfaces. This was the motivation of the current study, in which the atomistic processes underlying the transport of water-soluble ions are investigated in detail using the classical molecular dynamics (MD) method. To this end, an aqueous layer containing various concentrations of sodium chloride solution is sandwiched between two basal surfaces of calcium monosulfoaluminate hydrate, which is the most abundant phase of AFm. The adsorption mechanisms of water molecules and diffusing ions are then characterized for inner- and outer-sphere distance ranges from the basal surfaces of monosulfoaluminate. It is found that the self-diffusion coefficient of the chloride and sodium ions present in the outer-sphere range are 83% and 47% larger than those residing in the inner-sphere range. With increasing the distance from the solid surface, an increase in the self-diffusion coefficient is captured. This increase in mobility is larger for chloride ions than sodium ions. This can be understood based on the observation that the inner- and outer-sphere complex formation are the governing adsorption mechanisms for the chloride and sodium ions, respectively.

Water Adsorption on the β-Dicalcium Silicate Surface from DFT Simulations

β-dicalcium silicate (β-Ca2SiO4 or β-C2S in cement chemistry notation) is one of the most important minerals in cement. An improvement of its hydration rate would be the key point for developing environmentally-friendly cements with lower energy consumption and CO2 emissions. However, there is a lack of fundamental understanding on the water/β-C2S surface interactions. In this work, we aim to evaluate the water adsorption on three β-C2S surfaces at the atomic scale using density functional theory (DFT) calculations. Our results indicate that thermodynamically favorable water adsorption takes place in several surface sites with a broad range of adsorption energies (−0.78 to −1.48 eV) depending on the particular mineral surface and adsorption site. To clarify the key factor governing the adsorption of the electronic properties of water at the surface were analyzed. The partial density of states (DOS), charge analysis, and electron density difference analyses suggest a dual interaction of water with a β-C2S (100) surface including a nucleophilic interaction of the water oxygen lone pair with surface calcium atoms and an electrophilic interaction (hydrogen bond) of one water hydrogen with surface oxygen atoms. Despite the elucidation of the adsorption mechanism, no correlation was found between the electronic structure and the adsorption energies.

Atomic-scale investigation of physical adsorption of water molecules and aggressive ions to ettringite's surfaces

The strength and durability of cementitious composite materials are adversely affected by the ingress of water molecules and aggressive ions into their intrinsic meso- and nano-pore spaces. Among various phases of hydrated cement paste (HCP), aluminum-rich phases play an important role in controlling the diffusivity of aqueous solutions, which can contain aggressive ions. To this date, however, there has been no systematic study to understand the adsorption mechanisms and chloride binding capacity of the aluminum-rich phases of HCP. This research gap has been the motivation of the current study to investigate the physical adsorption characteristics of ettringite as the main aluminum-rich phase of HCP and the primary hydrated product of calcium sulfoaluminate cement. Through a set of Molecular Dynamics simulations supported by macro-scale experimental tests, a fundamental insight into the molecular origins of the diffusion of water molecules, as well as sodium and chloride ions, in contact with ettringite is provided. As the primary objective of this study is to evaluate the transport properties at and near solution/solid interfaces, the molecular adsorption mechanisms are characterized for inner- and outer-sphere distances from the solid substrate. With an in-depth understanding of the structure and dynamics of water molecules and aggressive ions in contact with ettringite's surfaces, the outcome of this study provides reliable measures of physical adsorption, binding capacity, and self-diffusion coefficient, which can be further employed to introduce strategies to avoid the degradation of a wide variety of cementitious materials exposed to harsh environmental conditions.

Surface dissimilarity affects critical distance of influence for confined water

In this study, the properties of nano-confined water, such as density, orientation etc., are monitored across varying confinement spacing to determine the critical distance of influence between dissimilar surfaces.