Effects of alkali and alkaline-earth cations on the high-pressure sound velocities of aluminosilicate glasses

Physics and Chemistry of Minerals - A black tourmaline sample from Seagull batholith (Yukon Territory, Canada) was established to be a schorl with concentrations of Fe2+ among the highest currently...     

The reaction of montmorillonite to mixed-layer clay: the effect of interlayer alkali and alkaline earth cations

The Wyoming bentonite was saturated with alkali and alkaline earth cations and allowed to react hydrothermally for one month at 300°C, and one week and one month at 400°C. Generally, the rate of formation of collapsed layers for the alkali clays was inversely related to interlayer hydration energy. This pattern of reaction is expected if layer dehydration results from an attraction between negatively charged 2:1 layers and the positive interlayer cation. In this mechanism, the greater the hydration energy of the interlayer region, the greater the charge that must develop on the 2:1 layers to cause dehydration, and, therefore, the slower the reaction rate. Reaction rate for the alkaline earth clays was directly related to interlayer hydration energy. Clays saturated with cations of hydration energy equal to or greater than Na⁺ did not react at 300°C, and ceased to react at 50% expandable layers at 400°C. Those saturated with cations of lower hydration energy continued to form mica-like layers with increasing run time at both 300° and 400°C. Clays saturated with monovalent cations reacted by a process of gradual transformation of smectite layers into collapsed layers, whereas those with divalent cations reacted directly from the smectite structure to form a rectorite-like phase.     

线性加载下考虑土体非线性变形的一维固结理论

深厚软黏土一维固结计算中,初始应力和附加应力分布形式对固结度存在较大影响。假设土体初始应力和附加应力均沿深度线性变化,推导了线性加载时的双曲线一维固结控制方程,并用差分法加以解答。同时,在不同的外荷载qa、压缩层厚度H、初始模量E0、附加应力分布系数β及边界条件下,分析了初始应力和附加应力沿深度线性变化对平均固结度Us、Up的影响。结果表明,当顶面和底面均透水时,除了E0≤388 k Pa且H≥25 m情况外,初始应力和附加应力沿深度线性变化对Us、Up的影响不大。但当顶面透水而底面不透水时,在土体初始应力和附加应力的两种不同分布形式下计算的Us、Up相差较大,特别是对于深厚软黏土,这种差别更加明显。另外,还可以看出,考虑初始应力和附加应力随深度变化时,Tv不再适合作为描述时间的无量纲因子。     

Effects of dry density and exchangeable cations on the diffusion process of sodium ions in compacted montmorillonite

As a part of the safety assessment of the geological disposal of high-level radioactive waste, the effects of dry density and exchangeable cations on the diffusion process of Na⁺ ions in compacted bentonite were studied from the viewpoint of the activation energy for diffusion. The apparent self-diffusion coefficients of Na⁺ ions in compacted Na-montmorillonite and in a Na- and Ca-montmorillonite mixture were determined by one-dimensional, non-steady diffusion experiments at different temperatures and dry densities. A unique change in activation energy as a function of dry density was found for the Na⁺ ions in compacted Na-montmorillonite. The activation energy suddenly decreased from 18.1 to 14.1 kJ mol^− 1 as the dry density increased from 0.9 to 1.0 Mg m^− 3, whereas it increased to 24.7 kJ mol^− 1 as the dry density increased to 1.8 Mg m^− 3. Examination of the effect of exchangeable cations on the activation energies determined that the activation energies were almost constant, approximately 25 kJ mol^− 1, for the montmorillonite specimens at a dry density of 1.8 Mg m^− 3. However, three different activation energy values were obtained at a dry density of 1.0 Mg m^− 3. These findings cannot be explained by the conventional diffusion model (the pore water diffusion model), which suggests that the predominant diffusion process alternates among pore water diffusion, interlayer diffusion, and external surface diffusion.     

考虑饱和黏土初始各向异性的循环边界面塑性模型

天然土体的初始各向异性通常可对其后继循环特性产生显著影响。现有考虑循环载荷作用的土体弹塑性模型,往往采用类似修正剑桥模型的椭圆形屈服面,已有研究表明,该椭圆形屈服面因其拉伸弹性区域偏大,针对天然K0固结状态的土体,其计算精度较差。基于新近提出的广义各向同性硬化准则,在边界面方程中引入初始各向异性张量,并采用空间滑动面破坏准则（SMP）的变换应力法,建立了能考虑饱和黏土初始各向异性的循环边界面塑性模型。分别针对等压和偏压固结的饱和黏土静、动三轴试验进行模拟,结果表明,该模型能合理反映土体的初始各向异性及其后继循环动力特性。     

Kinetics and mechanisms of iron redox reactions in silicate melts: The effects of temperature and alkali cations

The kinetics and the mechanisms of iron redox reactions in molten Fe-bearing pyroxene compositions have been investigated by Raman spectroscopy and X-ray absorption Near Edge Structure (XANES) experiments at the iron K-edge. The former experiments have been made only near the glass transition whereas the latter have also been performed from about 1300 to 2100 K. The same kinetics are observed with both techniques. They are described by characteristic times that depend primarily on temperature and not on the initial redox state. At high temperatures, where both kinds of reactions could be investigated, these times are similar for oxidation and reduction. From these characteristic times we have calculated as a function of temperature and composition a parameter termed effective redox diffusivity. For a given melt, the diffusivities follow two distinct Arrhenius laws, which indicate that the mechanisms of the redox reaction are not the same near the glass transition and at high temperatures. As is now well established, diffusion of divalent cations is the dominant mechanism at low temperatures but the enhanced kinetics observed for alkali-bearing melts indicate that Li⁺ and Na⁺ also participate in ionic transport. At superliquidus temperatures, in contrast, diffusion of oxygen represents the dominant mechanism.     

A two-surface plasticity model for cyclic behavior of saturated clay

This paper presents a two-surface plasticity model for describing some important features of saturated clay under cyclic loading conditions, such as closed hysteresis loops, cyclic shakedown and degradation, and different stress–strain relations for two-way loading. The model, namely ACC-2-C, is based on the elastoplastic model ACC-2 (an adapted Modified Cam Clay model with two yield surfaces) developed by Hong et al. (Acta Geotech 11(4):871–885, 2015). The small-strain nonlinearity concept is adopted to achieve the nonlinear characteristics of clay during unloading–loading stage. The new hardening law related to accumulated deviatoric plastic strain is proposed for the inner surface to describe the cyclic shakedown and degradation. Following the advantages of the ACC-2 model, the constitutive equations are simply formulated based on the consistency condition for the inner yield surface. The model is conveniently implemented in a finite element code using a stress integration scheme similar to the Modified Cam Clay model. The simulation results are highly consistent with experimental data from drained and undrained isotropic cyclic triaxial tests in normally consolidated saturated clay under both one-way and two-way loadings.

     

A low-dimensional subsurface model for saturated and unsaturated flow processes: ability to address heterogeneity

A low-dimensional model that describes both saturated and unsaturated flow processes in a single equation is presented. Subsurface flow processes in the groundwater, the vadose zone, and the capillary fringe are accounted for through the computation of aggregated hydrodynamic parameters that result from the integration of the governing flow equations from the bedrock to the land surface. The three-dimensional subsurface flow dynamics are thus described by a two-dimensional equation, allowing for a drastic reduction of model unknowns and simplification of the model parameterizations. This approach is compared with a full resolution of the Richards equation in different synthetic test cases. Because the model reduction stems from the vertical integration of the flow equations, the test cases all use different configurations of heterogeneity for vertical cross-sections of a soil-aquifer system. The low-dimensional flow model shows strong consistency with results from a complete resolution of the Richards equation for both the water table and fluxes. The proposed approach is therefore well suited to the accurate reproduction of complex subsurface flow processes.     

Tracer diffusion of some alkali,alkaline-earth and transition element ions in a basaltic and an andesitic melt,and the implications concerning melt structure

The diffusion properties of Na, Cs, Sr, Ba, Co, Mn, Fe and Sc ions in a basaltic and an andesitic melt have been determined experimentally using the radiotracer residual-activity method, and narrow platinum capillaries, over the temperature range 1,300–1,400° C. Diffusion of all cations follows an Arrhenius relationship; the values of the activation energies range from 24 kcal mol^–1 for Na to 67 kcal mol^–1 for Co in the andesitic melt, and from 39 kcal mol^–1 for Na to 65 kcal mol^–1 for Cs in the basaltic melt. Relative diffusivities in the basaltic melt, but not in the andesitic melt, correlate with assumed ionic radii values. Each cation, except Na⁺, diffuses faster in the basaltic melt than in the andesitic melt over the studied temperature range. Sodium shows similar diffusivity in the two melts.Compensation diagrams incorporating new and some previously-published data indicate that Cs probably diffuses by different mechanisms in different silicate glass and melt systems. Iron has a relatively high activation energy which is consistent with its part occupancy of tetrahedral co-ordination polyhedra.     

A smoothed cap model for variably saturated soils and its robust numerical implementation

This paper deals with the numerical implementation of a cap model for unsaturated soils. It provides a brief review of existing cap model approaches, based on which an improved model formulated in terms of generalised effective stress and matric suction is derived and described in detail. Although the proposed model is a multisurface plasticity model, it can efficiently be implemented using only single‐surface projections because of the smoothness of the model, which is obtained by construction. Numerical algorithms are provided for these single‐surface stress projections, using a single‐equation approach whenever possible. The robustness of the utilised single‐equation approaches is enhanced by proposing problem‐fitted start‐up procedures based on investigations of the nonlinear projection equations. A comparison of the model response with extensive material test data is used to validate the model and to demonstrate the robust application of the approach to silty sands and low to medium plasticity clays. Copyright © 2015 John Wiley & Sons, Ltd.     

Generalized plasticity state parameter‐based model for saturated and unsaturated soils. Part 1: Saturated state

The behavior of granular materials is known to depend on its loose or dense nature, which in turns depends both on density and confining pressure. Many models developed in the past require the use of different sets of constitutive parameters for the same material under different confining pressures. The purpose of this paper is to extend a basic generalized plasticity model for sands proposed by Pastor, Zienkiewicz and Chan by modifying the main ingredients of the model flow—rule, loading–unloading discriminating direction and plastic modulus—to include a dependency on the state parameter. The proposed model is tested against the available experimental data on three different sands, using for each of them a single set of material parameters, finding a reasonably good agreement between experiments and predictions. Copyright © 2010 John Wiley & Sons, Ltd.     

饱和盐渍土分数阶蠕变模型及试验研究

采用等温条件下饱和盐渍土的固结蠕变试验研究了其蠕变特性。在元件模型的基础上,通过引入Abel阻尼器和渗透吸力后,建立了饱和盐渍土的分数阶蠕变模型,从表观现象学方面展开了含盐量与饱和盐渍土蠕变特性的相关性讨论。结合试验结果和分数阶模型,拟合分析了渗透吸力同次固结系数与压缩指数的比值C_a/C_c和初始剪切模量以及初始剪切应变的相互作用关系。在蠕变试验的基础上,通过不同应力水平、含盐量的试验结果对分数阶模型进行了拟合分析,并结合给出的黏度系数公式对模型的有效性进行了验证。分析结果表明:次固结系数与压缩指数的比值C_a/C_c随着渗透吸力的增大而增大且呈指数关系,初始剪切模量随着渗透吸力的增大而减小,初始剪切应变与渗透吸力呈现出线性关系。相比与整数阶元件模型,所提出的分数阶蠕变模型更适合于饱和盐渍土蠕变行为的预测。结合试验和模型分析结果,发现含盐量的增大促进了盐渍土的蠕变行为。     

白垩系饱和冻结砂岩蠕变试验及本构模型研究

冻结法作为穿越富水软岩地层的重要施工方法,冻结壁的长期稳定性对于工程安全有着至关重要的作用。蠕变破坏是诱发冻结壁变形的显著特点之一,对研究冻结岩石蠕变的特性有重要的理论和工程意义。以白垩系饱和冻结砂岩为研究对象,开展–10℃低温冻结条件下,不同围压(0、2、4、6 MPa)的三轴蠕变力学试验。分析了饱和冻结砂岩蠕变变形,根据现有黏弹塑性模型开展了参数辨识并探究蠕变参数的变化规律,基于此提出考虑温度及损伤效应的蠕变本构模型。研究结果表明:低温冻结削弱蠕变过程中颗粒间的相互胶结力,使其蠕变特征明显;而围压却在一定程度上抑制饱和冻结砂岩内部损伤的发展,导致稳态蠕变速率随围压的升高出现明显的下降趋势。随围压的增加饱和冻结砂岩的蠕变破坏形态呈现出从剪切破坏到张拉破坏再到局部塑形硬化破坏的变化过程。在黏弹塑性模型的基础上,总结蠕变参数E₁、E₂和η₂随荷载的增加呈现先增后减的趋势,拐点为屈服应力;而参数η₀在大于屈服应力后出现并呈现先增后减的趋势。结合冻结岩石蠕变数据对定义的应力-低温耦合蠕变本构模型进行了参数...     

A model of water allocation in alkali feldspar,derived from infrared-spectroscopic investigations

Polarizedinfrared (IR) spectra of sanidine crystals from Volkesfeld, Eifel show the existence of two broad pleochroic absorption bands at 3,400 and 3,050 cm^?1. Because overtones near 5,150 cm^?1 were observed, the former bands are assigned to OH stretching frequencies of H₂O molecules. On the basis of the pleochroic scheme of the bands it is proposed that H₂O molecules occur as structural constituents entering theM site of the sanidine structure; the plane of the H₂O molecules lies parallel to the symmetry plane.     

分形渗透模型在饱和冻土中的应用

冻土中的渗透系数对于评估冻土工程中的水,热和溶质迁移至关重要。以往研究表明,渗透系数主要依赖孔隙结构,经常被描述为孔径大小和孔隙率,但是这两个参数并不能充分地表征孔隙结构。为加强对孔隙结构的描述,引用分形理论研究了冻土中的渗透系数。基于非均匀毛细管束模型和分形理论,提出了饱和冻土中渗透系数的分形模型,并提出通过土体冻结特征曲线获取冻土中孔径分布的理论方法。为了验证分形模型的有效性,对已有实验数据进行分析。分析表明,分形渗透系数模型是毛细管分维、最大孔径、黏度和迂曲度的函数,孔径分布变化是导致冻土渗透系数变化的根本原因。通过对比,计算值与实测值吻合较好。结果表明分形模型可以较好的预测冻土中的渗透系数,研究结果可为冻土渗透机理研究提供参考。     

Anisotropic bounding-surface plasticity model for the cyclic shakedown and degradation of saturated clay

The cyclic behaviours of embedded offshore structures under different cyclic loading levels are related to the cyclic shakedown and degradation of the surrounding soils. In the present study, a damage-dependent bounding-surface model based on a newly proposed hardening rule was developed to predict the cyclic shakedown and degradation of saturated clay and the effect of the initial anisotropic stress state. By extending the Masing’s rule to the bounding-surface plasticity theory, the stress reversal point is taken as the generalised homological centre of the bounding surface. With movement of the generalised homological centre, at lower stress amplitudes, the cyclic process ends at a steady state, and cyclic shakedown is reached. At higher stress amplitudes, a damage parameter related to the accumulated deviatoric plastic strain is incorporated into the form of the bounding surface, which is hence able to contract to model degradations in stiffness and strength. To take into account the effects of initial anisotropic conditions on the cyclic behaviour of soils, an initial anisotropic tensor is introduced in the bounding surface. The developed model is validated through undrained isotropic and anisotropic cyclic triaxial tests in normally consolidated and overconsolidated saturated clay under both one-way and two-way loadings. Both cyclic shakedown and degradation are well reproduced by the model, as is the anisotropy effect induced by the initial anisotropic consolidation process.     

Configurational thermodynamics of aluminous pyroxenes: A generalized pair approximation

A pair approximation is used to estimate the effects of short-range order on the thermodynamic properties of aluminous clinopyroxenes on the joins diopside (CaMg-Si₂O₆)-jadeite (NaAlSi₂O₆) and diopside-CaTs (CaAl₂SiO₆). The generalized pair approximation is the simplest model for concentrated solutions which includes short-range order. Short-range order is expected to be especially significant in coupled solid solutions, such as aluminous pyroxenes, since atoms of different valence substitute for each other. The calculations show that the random model, in which the configurational entropy is calculated as if atoms on each crystallographic site mix randomly, is appropriate as a first approximation. The excess entropy relative to the random model behaves regularly, is always negative, and becomes more negative as temperature decreases or the ordering energies increase. The excess entropy relative to the random model can be modeled reasonably well with a simple power series, or Margules-type, formulation. In contrast, the excess entropy relative to a molecular model, in which the ideal activity is assumed to be equal to some mole fraction, is irregular, can be positive or negative, and even changes in sign with variations in temperature and composition. The configurational enthalpy is positive at high temperatures, and becomes negative with decreasing temperature or increasing ordering energy. The mixing enthalpy can have non-configurational contributions, in addition to the effective short-range configurational contributions considered explicitly. The pair approximation predicts an ordering transition from C2/c to P2₁/n for CaTs and diopside-CaTs solutions at moderate to low temperatures, respectively. A field where C2/c orders to C2 is also found. A higher order approximation, different relative ordering energies, or quantitative consideration of strain contributions is required to account for the C2/c to P2/n transition in omphacites. There is no justification for molecular models, in which the configurational entropy is calculated as if endmember “molecules” were mixing in the crystal, in either concentrated or dilute solutions. Molecular models do not represent limiting ordered states for coupled solid solutions.     

Immobilization of Ni by Al-modified montmorillonite: A novel uptake mechanism

The sorption capacity of montmorillonite clay minerals for small cations, such as Ni²⁺, can be greatly enhanced by modifying the clay mineral with Al(III). In this study, the mechanisms of Ni uptake by Al-modified montmorillonite were studied using extended x-ray absorption fine structure (EXAFS) spectroscopy of powders and polarized EXAFS spectroscopy of self-supporting clay films to delineate the binding structure of Ni formed as a function of the reaction conditions. Analysis of powder EXAFS spectra of wet pastes, collected from Ni-treated Al-modified montmorillonites reacted at pH 5-8, 25°C or 80°C (to accelerate the reaction process), and reaction times ranging from 1 month to 9 yrs, showed that Ni was surrounded on average by 6 O atoms at a distance of 2.05 Å and 6 Al atoms at 3.01 Å, suggesting the incorporation of Ni into a gibbsite-like structure. Only at pH 8, Ni-containing precipitates were congruently formed. Polarized EXAFS spectroscopy of self-supporting Ni-reacted Al-modified montmorillonite clay films showed a pronounced angular dependency of the spectra of the Ni-doped gibbsite, indicating that the orientation of this Ni-doped gibbsite coincided with the layering of the montmorillonite. Data analysis suggested that Ni is included slightly above and below the vacant octahedral sites of the postulated interstitial gibbsite monolayer. This newly identified mechanism of metal uptake by Al-modified montmorillonite provides a large metal sorption capacity and, because the metal is included in a monolayer gibbsite or gibbsite “islands” formed in the interstitial space of the clay mineral, it potentially leads to a permanent sequestration of the metal from the environment.