Precipitation of calcium-alumino-silicate-hydrate gels: The role of the internal stress

Investigation of Carbonation Kinetics in Carbonated Cementitious Materials by Reactive Molecular Dynamics Simulations

Calcium carbonate (CaCO3) precipitation plays a significant role during the carbon capture process; however, the mechanism is still only partially understood. Understanding the atomic-level carbonation mechanism of cementitious materials can promote the mineralization capture, immobilization, and utilization of carbon dioxide, as well as the improvement of carbonated cementitious materials' performance. Therefore, based on molecular dynamics simulations, this paper investigates the effect of Si/Al concentrations in cementitious materials on carbonation kinetics. We first verify the force field used in this paper. Then, we analyze the network connectivity evolution, the number and size of the carbonate cluster during gelation, the polymerization rate, and the activation energy. Finally, in order to reveal the reasons that caused the evolution of polymerization rate and activation energy, we analyze the local stress and charge of atoms. Results show that the Ca-Oc bond number and carbonate cluster size increase with the decrease of the Si/Al concentration and the increase of temperature, leading to the higher amorphous calcium carbonate gel polymerization degree. The local stress of each atom in the system is the driving force of the gelation transition. The presence of Si and Al components increases the atom's local stress and average charge, thus causing the increase of the energy barrier of CaCO3 polymerization and the activation energy of carbonation.

Effects of temperature and CO2 concentration on the early stage nucleation of calcium carbonate by reactive molecular dynamics simulations

It is significant to investigate the calcium carbonate (CaCO3) precipitation mechanism during the carbon capture process; nevertheless, CaCO3 precipitation is not clearly understood yet. Understanding the carbonation mechanism at the atomic level can contribute to the mineralization capture and utilization of carbon dioxide, as well as the development of new cementitious materials with high-performance. There are many factors, such as temperature and CO2 concentration, that can influence the carbonation reaction. In order to achieve better carbonation efficiency, the reaction conditions of carbonation should be fully verified. Therefore, based on molecular dynamics simulations, this paper investigates the atomic-scale mechanism of carbonation. We investigate the effect of carbonation factors, including temperature and concentration, on the kinetics of carbonation (polymerization rate and activation energy), the early nucleation of calcium carbonate, etc. Then, we analyze the local stresses of atoms to reveal the driving force of early stage carbonate nucleation and the reasons for the evolution of polymerization rate and activation energy. Results show that the higher the calcium concentration or temperature, the higher the polymerization rate of calcium carbonate. In addition, the activation energies of the carbonation reaction increase with the decrease in calcium concentrations.

Atomistic origin of kinetics in hydrated aluminosilicate gels upon precipitation

Calcium-alumino-silicate-hydrate (CaO-Al2O3-SiO2-H2O, or C-A-S-H) gel, which is the binding phase of cement-based materials, greatly influences concrete mechanical properties and durability. However, the atomic-scale kinetics of the aluminosilicate network condensation remains puzzling. Here, based on reactive molecular dynamics simulations of C-A-S-H systems formation with varying Al/Ca molar ratios, we study the kinetic mechanism of the hydrated aluminosilicate gels upon precipitation. We show that the condensation activation energy decreases with the Al/Ca molar ratio, which suggests that the concentration of the Al polytopes has a great effect on controlling the kinetics of the gelation reaction. Significantly, we demonstrate that 5-fold Al atoms are mainly forming at high Al/Ca molar ratios since there are insufficient hydrogen cations or extra calcium cations to compensate the negatively charged Al polytopes at high Al/Ca molar ratios during accelerated aging.

Nanoscale Insight into the Effect of Calcium on Early-Age Polymerization of CNASH Gels

Sodium-containing calcium-alumino-silicate-hydrate (CNASH) gels, the primary binder phase of alkali-activated materials (AAMs), significantly impact the performance of the AAM. Although the effect of the calcium content on the AAM has been extensively studied in the past, few studies focus on the effect of calcium on the structure and performance of gels at a molecular scale. As an important element in gels, the effect of calcium in gels on its atomic-scale properties remains unclear. This study establishes a molecular model of the CNASH gel via reactive molecular dynamics (MD) simulation and verifies the feasibility of the gel model. By employing the reactive MD, the effect of calcium on the physicochemical properties of gels in the AAM is investigated. The simulation highlights that the condensation process of the system containing Ca is accelerated dramatically. This phenomenon is explained from the perspective of thermodynamics and kinetics. The increased calcium content enhances the thermodynamic stability and reduces the energy barrier of the reaction. Then, the phenomenon is further analyzed through the nanosegregation in the structure. It is proved that this behavior is driven by the weaker affinity of calcium for aluminosilicate chains than the particles in the aqueous environment. The difference in affinity leads to nanosegregation in the structure, making Si(OH)₄ and Al(OH)₃ monomers and oligomers closer for better polymerization.

New insights into the early stage nucleation of calcium carbonate gels by reactive molecular dynamics simulations

The precipitation of calcium carbonate (CaCO₃) is a key mechanism in carbon capture applications relying on mineralization. In that regard, Ca-rich cementitious binders offer a unique opportunity to act as a large-scale carbon sink by immobilizing CO₂ as calcium carbonate by mineralization. However, the atomistic mechanism of calcium carbonate formation is still not fully understood. Here, we study the atomic scale nucleation mechanism of an early stage amorphous CaCO₃ gel based on reactive molecular dynamics (MD) simulations. We observe that reactive MD offers a notably improved description of this reaction as compared to classical MD, which allows us to reveal new insights into the structure of amorphous calcium carbonate gels and formation kinetics thereof.

Predicting Fracture Propensity in Amorphous Alumina from Its Static Structure Using Machine Learning

Thin films of amorphous alumina (a-Al₂O₃) have recently been found to deform permanently up to 100% elongation without fracture at room temperature. If the underlying ductile deformation mechanism can be understood at the nanoscale and exploited in bulk samples, it could help to facilitate the design of damage-tolerant glassy materials, the holy grail within glass science. Here, based on atomistic simulations and classification-based machine learning, we reveal that the propensity of a-Al₂O₃ to exhibit nanoscale ductility is encoded in its static (nonstrained) structure. By considering the fracture response of a series of a-Al₂O₃ systems quenched under varying pressure, we demonstrate that the degree of nanoductility is correlated with the number of bond switching events, specifically the fraction of 5- and 6-fold coordinated Al atoms, which are able to decrease their coordination numbers under stress. In turn, we find that the tendency for bond switching can be predicted based on a nonintuitive structural descriptor calculated based on the static structure, namely, the recently developed "softness" metric as determined from machine learning. Importantly, the softness metric is here trained from the spontaneous dynamics of the system (i.e., under zero strain) but, interestingly, is able to readily predict the fracture behavior of the glass (i.e., under strain). That is, lower softness facilitates Al bond switching and the local accumulation of high-softness regions leads to rapid crack propagation. These results are helpful for designing glass formulations with improved resistance to fracture.

Topological origin of phase separation in hydrated gels

HYPOTHESIS

Depending on their composition, hydrated gels can be homogeneous or phase-separated, which, in turn, affects their dynamical and mechanical properties. However, the nature of the structural features, if any, that govern the propensity for a given gel to phase-separate remains largely unknown. Here, we argue that the propensity for hydrated gels to phase-separate is topological in nature.

SIMULATIONS

We employ reactive molecular dynamics simulations to model the early-age precipitation of calcium-alumino-silicate-hydrate (CASH) gels with varying compositions, i.e., (CaO)_1.7(Al₂O₃)_x(SiO₂)_{1 -x}(H₂O)_{3.7 +x}. By adopting topological constraint theory, we investigate the structural origin of phase separation in hydrated gels.

FINDINGS

We report the existence of a homogeneous-to-phase-separated transition, wherein Si-rich (x ≤ 0.10) CASH gels are homogeneous, whereas Al-rich (x > 0.10) CASH gels tend to phase-separate. Furthermore, we demonstrate that this transition is correlated to a topological flexible-to-rigid transition within the atomic network. We reveal that the propensity for topologically-overconstrained gels to phase-separate arises from the existence of some internal stress within their atomic network, which acts as an energy penalty that drives phase separation.