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The goal of the presented work was to find the most favorable conditions for the synthesis and stabilization of chemically pure ettringite and monosulfate. The reaction was carried out by mixing pure tricalcium aluminate (C[sub.3]A) and gypsum (CS¯H[sub.2]) in an excess amount of water. The impact of hydration time (2–7 days), C[sub.3]A:CS¯ molar ratio (1:1–1:3) and water vapor pressure of the selected drying agents (anhydrite-III and supersaturated CaCl[sub.2] solution) on the phase composition of the products was evaluated. After 7 days of hydration, either ettringite or monosulfate was obtained as the main product, depending on the C[sub.3]A:CS¯ molar ratio. The synthesis carried out at a C[sub.3]A:CS¯ molar ratio of 1:3 produced pure ettringite. In the case of the sample characterized by the ratio of 1:1 (typical of monosulfate), a considerable portion of ettringite (27.9%) was present in the final products along the AFm phase. Therefore, a different synthesis method has to be selected in order to obtain pure monosulfate. The results showed that thermal analysis, X-ray diffractometry and FTIR spectroscopy can be used to distinguish the characteristic features of ettringite and monosulfate.

Methane gas hydrates, crystalline inclusion compounds formed from methane and water, are found in marine continental margin and permafrost sediments worldwide. This article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate‐bearing sediments. Formation phenomena include pore‐scale habit, solubility, spatial variability, and host sediment aggregate properties. Physical properties include thermal properties, permeability, electrical conductivity and permittivity, small‐strain elastic P and S wave velocities, shear strength, and volume changes resulting from hydrate dissociation. The magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate‐bearing sediments to changes in mechanical, thermal, or chemical boundary conditions. These predictions are vital for mitigating borehole, local, and regional slope stability hazards; optimizing recovery techniques for extracting methane from hydrate‐bearing sediments or sequestering carbon dioxide in gas hydrate; and evaluating the role of gas hydrate in the global carbon cycle.

Methane hydrates have important industrial and climate implications, yet their formation via homogeneous nucleation under natural, moderate conditions is poorly understood. Obtaining such understanding could lead to improved control of crystallization, as well as insight into polymorph selection in general, but is hampered by limited experimental resolution. Direct molecular dynamics simulations using atomistic force fields could provide such insight, but are not feasible for moderate undercooling, due to the rare event nature of nucleation. Instead, we harvest ensembles of the rare unbiased nucleation trajectories by employing transition path sampling.We find that with decreasing undercooling the mechanism shifts from amorphous to crystalline polymorph formation. At intermediate temperature the 2 mechanisms compete. Reaction coordinate analysis reveals the amount of a specific methane cage type is crucial for crystallization, while irrelevant for amorphous solids. Polymorph selection is thus governed by kinetic accessibility of the correct cage type and, moreover, occurs at precritical nucleus sizes, apparently against Ostwald’s step rule. We argue that these results are still in line with classical nucleation theory. Our findings illuminate how selection between competing methane hydrate polymorphs occurs and might generalize to other hydrates and molecular crystal formation.

Gas hydrates provide alternative solutions for gas storage & transportation and gas separation. However, slow formation rate of clathrate hydrate has hindered their commercial development. Here we report a form of porous ice containing an unfrozen solution layer of sodium dodecyl sulfate, here named active ice, which can significantly accelerate gas hydrate formation while generating little heat. It can be readily produced via forming gas hydrates with water containing very low dosage (0.06 wt% or 600 ppm) of surfactant like sodium dodecyl sulfate and dissociating it below the ice point, or by simply mixing ice powder or natural snow with the surfactant. We prove that the active ice can rapidly store gas with high storage capacity up to 185 V g V w −1 with heat release of ~18 kJ mol −1 CH 4 and the active ice can be easily regenerated by depressurization below the ice point. The active ice undergoes cyclic ice−hydrate−ice phase changes during gas uptake/release, thus removing most critical drawbacks of hydrate-based technologies. Our work provides a green and economic approach to gas storage and gas separation and paves the way to industrial application of hydrate-based technologies. Gas hydrates have promising energy storage applications, a main bottleneck being their slow formation kinetics. Here, the authors demonstrate that by dispersing kinetic promoters in porous ice as active ice for gas hydrate formation, a minute-level formation process can be achieved for hydrate-based technologies.

The mechanisms involved in the formation/dissociation of methane hydrate confined at the nanometer scale are unraveled using advanced molecular modeling techniques combined with a mesoscale thermodynamic approach. Using atom-scale simulations probing coexistence upon confinement and free energy calculations, phase stability of confined methane hydrate is shown to be restricted to a narrower temperature and pressure domain than its bulk counterpart. The melting point depression at a given pressure, which is consistent with available experimental data, is shown to be quantitatively described using the Gibbs–Thomson formalism if used with accurate estimates for the pore/liquid and pore/hydrate interfacial tensions. The metastability barrier upon hydrate formation and dissociation is found to decrease upon confinement, therefore providing a molecular-scale picture for the faster kinetics observed in experiments on confined gas hydrates. By considering different formation mechanisms—bulk homogeneous nucleation, external surface nucleation, and confined nucleation within the porosity—we identify a cross-over in the nucleation process; the critical nucleus formed in the pore corresponds either to a hemispherical cap or to a bridge nucleus depending on temperature, contact angle, and pore size. Using the classical nucleation theory, for both mechanisms, the typical induction time is shown to scale with the pore volume to surface ratio and hence the pore size. These findings for the critical nucleus and nucleation rate associated with such complex transitions provide a means to rationalize and predict methane hydrate formation in any porous media from simple thermodynamic data.

This work examines how different clay environments influence the formation of methane gas hydrates. The study shows that hydrate growth in clay-rich systems is noticeably slower when the water–sediment ratio is high, mainly because a larger portion of the water becomes tightly bound to the fine clay particles and is not readily available for hydrate formation. The experiments also reveal clear differences in induction time and the overall cooling behaviour of the system when pressure and sediment type are varied. These factors collectively affect both the onset and the rate of hydrate growth. During the repeated thawing and cooling cycles, there was an abrupt increase in the formation of the hydrate once the samples went beyond the sub-cooling range and moved closer to the saturated point. This result clearly highlights the importance of the texture of the clay and the distribution of the water in the process of hydrate formation.

— The conditions for the formation of gas hydrates associated with subsea permafrost in the Kara Sea have been predicted based on numerical modeling. The forecast of the distribution of the relic submarine permafrost and related methane hydrate stability zone is given on the basis of solving the equation of thermal conductivity. According to modeling data, an extensive thermobaric relic submarine permafrost zone is predicted within the Kara Sea shelf. The greatest thickness (up to 600 m) of the permafrost is confined to the Taimyr shelf. Based on the results of the analysis of our model, drilling and seismic data, the southwestern shelf of the Kara Sea has insular or sporadic permafrost. In the northeastern part, the nature of the permafrost is also discontinuous, despite the greater thickness of the frozen strata. For the first time, accumulations of cryogenic gas hydrates on the Taimyr shelf have been characterized. The latest drilling data, seismic data reinterpretation, and numerical modeling have shown that the gas hydrate reservoir is confined to unconformably occurring Silurian–Devonian and underlying Triassic–Jurassic strata. The thickness of the gas hydrate reservoir varies from 800 to 1100 m. Based on the interpretation of CDP data and their comparison with model calculations, frozen deposits, and sub-permafrost traps of stratigraphic, anticline and anticline-stratigraphic types were identified for the first time. These pioneering studies allowed us to characterize the thickness and morphology of the gas hydrate reservoir, give a preliminary seismostratigraphic reference, and identify potentially gas hydrate-bearing structures. Due to the favorable thermobaric and permafrost-geothermal conditions, most of the identified traps may turn out to be sub-permafrost accumulations of gas hydrates. In total, at least five potential accumulations of gas hydrates were discovered, confined to structural depressions; the Uedineniya Trough and its side included the Egiazarov Step and North Mikhailovskaya Depression.

In this study, molecular dynamics (MD) simulations were employed to elucidate the processes and underlying mechanisms that govern the adsorption and accumulation of gas (represented by N[sub.2]) at the hydrophobic solid–liquid interface, using the GROMACS program with an AMBER force field. Our findings indicate that, regardless of surface roughness, the presence of water molecules is a prerequisite for the adsorption and aggregation of N[sub.2] molecules on solid surfaces. N[sub.2] molecules dissolved in water can cluster even without a solid substrate. In the gas–solid–liquid system, the exclusion of water molecules at the hydrophobic solid–liquid interface and the adsorption of N[sub.2] molecules do not occur simultaneously. A loosely arranged layer of water molecules is initially formed on the hydrophobic solid surface. The two-stage process of N[sub.2] molecule adsorption and accumulation at the hydrophobic solid/liquid interface involves initial adsorption to the solid surface, displacing water molecules, followed by N[sub.2] accumulation via self-interaction after saturating the substrate’s surface. The process and underlying mechanisms of gas adsorption and accumulation at hydrophobic solid/liquid interfaces elucidated in this study offer a molecular-level understanding of nano-gas layer formation.

Submarine methane seepage can potentially be promoted by the dissociation of the marine hydrates surrounding the continental margins due to oceanic warming since the Last Glacial Maximum. This seepage could be archived by authigenic carbonates at seeping sites, but the time lag caused by heat transmission through the sediment column leads to an inconsistency between the ages of the carbonate and the period of bottom water warming. Here we present the records of the authigenic carbonate crust from drilling site D1 in the Mid‐Okinawa Trough. Uranium–thorium dating results show that the carbonate crust mainly grew downwards during 14–6 ka. Gas hydrates hosted in the relatively thin stability zone dissociated in a rapid response to bottom water warming and intensified the methane seepage. Our study better supports the hypothesis that a considerable amount of methane can be released from marine hydrates due to global climatic changes. Plain Language Summary Methane hydrates are ice‐like crystalline compounds and often found in deep‐ocean marine sediments. Ocean warming in the past could have destabilized methane hydrates and led to a rapid discharge of free methane gas. Geological information on this methane escape could be preserved by carbonate rocks. To date, the hypothesis of the control of ocean warming on hydrate melting since the Last Glacial Maximum (19,000–26,000 years ago) has not been fully tested. To examine this hypothesis, we used the rock samples of seep‐carbonate retrieved by seafloor drilling in the Mid‐Okinawa Trough. Uranium–thorium dating results show that the seep carbonates formed between 6,000 and 14,000 years ago. It took less time for heat to diffuse downwards from warming seawater to trigger hydrate melting at this location than in most of other oceans at similar water depths. The smaller time lag makes the carbonate rock age close to the period of ocean warming. Our results better support that global ocean warming could potentially release methane from gas hydrates during glacial–interglacial transitions. The consequent methane transport into the oceans and likely also the atmosphere might impact ocean acidification and climatic warming. Key Points Presentation of the chronology of methane seepage by dating authigenic carbonates collected from drilling core D1 in the Okinawa Trough Ocean warming–induced hydrate dissociation led to the downward growth of authigenic carbonate for 14–6 ka after the Last Glacial Maximum This carbonate record better supports the link of hydrate dissociation with ocean warming due to a smaller thermal lag effect

Synergistic effects of the synthetic calcium silicate hydrates (C-S-H)/polycarboxylate (PCE) and Na.sub.2SO.sub.4 on hydration properties and microstructure of cement-lithium slag (LS) composite binder were analyzed. Results showed that C-S-Hs-PCE and Na.sub.2SO.sub.4 exhibited a synergistic effect on hydration acceleration of LS-cement binder. Na.sub.2SO.sub.4 increased alkalinity of interstitial solution and promoted dissolution of LS. Dissolved Al and Si from LS powder reacted with dissolved SO42- from Na.sub.2SO.sub.4 to produce extra hydrates, and C-S-Hs-PCE accelerated pozzolanic reaction as well as hydration reaction via nucleation effect collaborated with dispersing effect. C-S-Hs-PCE accelerated reaction process of Na.sub.2SO.sub.4 via nucleation effect, and activation effect of Na.sub.2SO.sub.4 provided more newly-formed hydrates to act as nucleation seeds or crystal skeleton for the hydration of new phases. Newly formed hydrates promoted exceedingly the appearance of network, leading to a refinement of microstructure.