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The construction of nuclear power plants necessitates careful consideration of the discharge and fixation of nuclear waste. Geopolymers are new cement-based materials (CBMs) with three-dimensional cage-like structures that enable effective nuclear waste fixation. In this work, the adsorption of radioactive caesium and strontium ions by sodium aluminosilicate hydrate (NASH) gel, the main component of geopolymers, was investigated using molecular dynamics simulations to obtain nanoscale insights into the ions' interactions with the gel. The formation of strong ion–oxygen bonds allowed both ions to be effectively adsorbed on the NASH surface, but the adsorption ratio of strontium ions (17.2%) was slightly lower than that of caesium ions (21.0%). Because strontium ions are divalent, they can form stronger electrostatic interactions with water molecules and chloride ions, which hinders their approach to the interface. For the same reason, the diffusion coefficient of strontium ions in solution is lower than that of caesium ions. These results provide new insights into the nuclear waste fixation capacity of NASH gel and guidance for the design of new CBMs for radioactive waste disposal. [ABSTRACT FROM AUTHOR]

In this paper, molecular dynamics simulations are used to study the effects of temperature on the transport of chloride and sulfate in the nanopores of aluminum-doped cement-based materials (i.e., CASH gels) exposed to aqueous solutions of NaCl and Na2SO4 at 283, 293, 303, 333, and 363 K. It is shown that high temperatures increase the initial transport rates of water molecules and ions while weakening the hydration layer around ions. This increases the probability of ion–ion and ion–substrate contact and thus makes ions more likely to cluster in solution and be captured by the substrate. Both cluster formation and substrate capture can significantly restrict the free movement of ions in solution and thus gradually reduce the ion transport rate. In addition, since sulfate ions have four oxygen atoms that can capture other ions, large ion clusters form more readily in Na2SO4 solution than in NaCl solution. The capture of these large ion clusters at the interface can cause a "necking" phenomenon that hinders the subsequent transport of water molecules and ions into the nanopore. These results provide a nanoscale basis for designing aluminum-doped cement-based materials with enhanced durability at high temperatures. [ABSTRACT FROM AUTHOR]

Geopolymer concrete offers superior mechanical properties and microstructure, yet micro-level compressive properties and structural evolutions remain insufficiently understood. This study employed molecular dynamics to simulate the uniaxial compressions of the sodium aluminosilicate hydrate (N-A-S-H) under UCZ, BCZ, and TCZ (z-axial compressions with zero, one, and two dimensions restrictions, respectively) conditions at 263 K, 300 K, and 800 K. The results provided valuable insights linking mechanical behavior with structural properties. Stress fluctuations in the yield stage were attributed to the continuous formation and fracture of Al-O-H bonds during micro-molecule processes. In the later compression stages, the rapid increase in Si-O-H groups suggested that water molecules equally attacked Al and Si tetrahedra due to limited voids. Under UCZ and BCZ conditions, slight bond contraction occurred, with the main structural resistance arising from bond angle bending within the skeleton. In contrast, TCZ experienced notable changes in both bond lengths and bond angles due to bilateral displacement constraints. The evolutionary molecular processes exhibited insensitive response to the temperature, especially under TCZ conditions. Additionally, varying trends were observed in different bond-angle styles (e.g., within or inside tetrahedra), providing a crucial insight for the design of N-A-S-H to determine optimal components. • Reactive molecular dynamics was used to simulate uniaxial compressions of sodium aluminosilicate hydrate gels. • Different lateral displacement constraints and temperatures effects was considered in the simulations. • The evolutions of the gel's molecular structure under uniaxial compression were characterized. [ABSTRACT FROM AUTHOR]

Chloride anion attack is a major factor limiting the durability of concrete structures. To clarify the mechanisms by which chloride salts degrade concrete, nanoscale molecular dynamics (MD) simulations were used to study chloride attack on calcium silicate hydrate (CSH), the main component of cement. In MD simulations, a relaxation process is generally required to allow the system to reach equilibrium. However, relaxation is computationally expensive when performing MD simulations of large structural systems. This expense could potentially be avoided by using deep learning techniques. This paper describes the creation of a multi-fidelity physics-informed neural network model of a CSH gel pore containing an aqueous NaCl solution. The neural network's input variables are the ambient temperature and the NaCl concentration and its output variables are the system's energy, the Na-O radial distribution function, and the Na+ and Cl- ion density distributions. After training the model using the results of low-fidelity MD simulations without relaxation and a smaller number of high-fidelity simulations with relaxation, highly accurate outputs were obtained with prediction errors below 3%. Deep learning can thus greatly reduce the computational cost of MD studies of large and complex systems with no appreciable loss of accuracy. [Display omitted] • MPINN models can accurately predict the complex MD simulations (error < 3%). • The MPINN can reduce the computational costs by at least 50% and up to 90%. • The CSH surface adsorbs Na ions strongly, thus leading to adsorption of Cl ions. • Raising the temperature increases the rate at which ions reach the CSH surface. [ABSTRACT FROM AUTHOR]

As a low-carbon, environment-friendly and economical resource for nuclear power generation, radionuclide emission and storage has received worldwide attention. Geopolymer concrete is a green and sustainable building material that can be used to immobilize radionuclides. In the present study, molecular dynamics simulations were conducted to investigate the structural and mechanical properties of sodium aluminosilicate hydrate (NASH) gel, the main component of geopolymer concrete, with/without immobilized radioactive Cs and Sr ions. The three-dimensional structure of NASH gel enabled good immobilization of both radioactive Cs and Sr ions owing to the large radius of Cs ions and high charge density of Sr ions. Addition of Cs ions reduced the strength of the gel and increased the fracture strain, whereas addition of Sr ions increased the strength and significantly increased the ductility. Addition of Sr ions increased the number of penta-coordinated Al in the structure. Consequently, breakage of these bonds required more energy to be absorbed from outside. The nanoscale molecular dynamics simulations provided a theoretical support at atomic level for understanding the structural and mechanical characteristics of geopolymers pertinent to the immobilization of nuclear waste. [Display omitted] • The molecular dynamics approach provided a nanoscopic basis for radioactive waste disposal. • NASH gel has a good immobilization effect on Cs and Sr ions at nanoscopic scale. • Addition of radioactive ions alters the structural and mechanical properties of NASH. • Addition of Sr ions improves the ductility of the structure. [ABSTRACT FROM AUTHOR]

• Reactive molecular dynamics is utilized to simulate shock compression in sodium aluminosilicate hydrate gels. • Distinctive shock-wave induced mechanical response stages are determined. • The relationships between shock-induced wave speed and impact velocity are demonstrated. • Structural deformation mechanisms are proposed in disparate mechanical response stages. Sodium aluminosilicate hydrate (N-A-S-H) gels have gained attention due to their potential use as components of geopolymers to improve structural and mechanical properties. In this study, we investigated the propagation of shock waves in N-A-S-H gels subjected to impact velocities (U p) ranging from 0.1 to 3.0 km/s, as well as the resulting mechanical responses and structural deformations. Our results showed that when U p <0.4 km/s, only one elastic wave existed, and the Hugoniot elastic limit was estimated to be 4.1 GPa. Above this limit, a two-wave structure formed. The elastic and elastoplastic deformation mechanisms involved initial compaction and densification of the N-A-S-H gel structure, followed by bond angle bending. The Hugoniot U s - U p relationship was found to be linear in the elastoplastic region, with a linear parameter λ of approximately 2.75. These new atomistic insights into the shock compression of N-A-S-H gels will provide valuable guidance for future studies. [ABSTRACT FROM AUTHOR]