Topological origin of phase separation in hydrated gels

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.

Beyond the Average: Spatial and Temporal Fluctuations in Oxide Glass-Forming Systems

Atomic structure dictates the performance of all materials systems; the characteristic of disordered materials is the significance of spatial and temporal fluctuations on composition-structure-property-performance relationships. Glass has a disordered atomic arrangement, which induces localized distributions in physical properties that are conventionally defined by average values. Quantifying these statistical distributions (including variances, fluctuations, and heterogeneities) is necessary to describe the complexity of glass-forming systems. Only recently have rigorous theories been developed to predict heterogeneities to manipulate and optimize glass properties. This article provides a comprehensive review of experimental, computational, and theoretical approaches to characterize and demonstrate the effects of short-, medium-, and long-range statistical fluctuations on physical properties (e.g., thermodynamic, kinetic, mechanical, and optical) and processes (e.g., relaxation, crystallization, and phase separation), focusing primarily on commercially relevant oxide glasses. Rigorous investigations of fluctuations enable researchers to improve the fundamental understanding of the chemistry and physics governing glass-forming systems and optimize structure-property-performance relationships for next-generation technological applications of glass, including damage-resistant electronic displays, safer pharmaceutical vials to store and transport vaccines, and lower-attenuation fiber optics. We invite the reader to join us in exploring what can be discovered by going beyond the average.