Predicting rare earth elements concentration in coal ashes with multi-task neural networks

The increasing demand for rare earth elements (REEs) makes them a scarce strategic resource for technical developments. In that regard, harvesting REEs from coal ashes-a waste byproduct from coal power plants-offers an alternative solution to conventional ore-based extraction. However, this approach is bottlenecked by our ability to screen coal ashes bearing large concentrations of REEs from feedstocks-since measuring the REE content in ashes is a time-consuming and costly task requiring advanced analytical tools. Here, we propose a machine learning approach to predict the REE contents based on the bulk composition of coal ashes, easily measurable under the routine testing protocol. We introduce a multi-task neural network that simultaneously predicts the contents of different REEs. Compared to the single-task model, this model exhibits notably improved accuracy and reduced sensitivity to noise. Further model analyses reveal key data patterns for screening coal ashes with high REE concentrations. Additionally, we showcase the utilization of transfer learning to improve the adaptability of our model to coal ashes from a distinct source.

Electrosteric Control of the Aggregation and Yielding Behavior of Concentrated Portlandite Suspensions

Portlandite (calcium hydroxide: CH: Ca(OH)2) suspensions aggregate spontaneously and form percolated fractal aggregate networks when dispersed in water. Consequently, the viscosity and yield stress of portlandite suspensions diverge at low particle loadings, adversely affecting their processability. Even though polycarboxylate ether (PCE)-based comb polyelectrolytes are routinely used to alter the particle dispersion state, water demand, and rheology of similar suspensions (e.g., ordinary portland cement suspensions) that feature a high pH and high ionic strength, their use to control portlandite suspension rheology has not been elucidated. This study combines adsorption isotherms and rheological measurements to elucidate the role of PCE composition (i.e., charge density, side chain length, and grafting density) in controlling the extent of PCE adsorption, particle flocculation, suspension yield stress, and thermal response of portlandite suspensions. We show that longer side-chain PCEs are more effective in affecting suspension viscosity and yield stress, in spite of their lower adsorption saturation limit and fractional adsorption. The superior steric hindrance induced by the longer side chain PCEs results in better efficacy in mitigating particle aggregation even at low dosages. However, when dosed at optimal dosages (i.e., a dosage that induces a dynamically equilibrated dispersion state of particle aggregates), different PCE-dosed portlandite suspensions exhibit identical fractal structuring and rheological behavior regardless of the side chain length. Furthermore, it is shown that the unusual evolution of the rheological response of portlandite suspensions with temperature can be tailored by adjusting the PCE dosage. The ability of PCEs to modulate the rheology of aggregating charged particle suspensions can be generally extended to any colloidal suspension with a strong screening of repulsive electrostatic interactions.

Metal cations as inorganic structure-directing agents during the synthesis of phillipsite and tobermorite

Metal cation identity determines the zeolite topology. Framework topology determines the total zeolite cationic content. Potassium predominantly counterbalances Al anions; sodium and calcium are predominantly structure-directing agents.

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.

Experimental evidence of auxeticity in ion implanted single crystal calcite

We report initial experimental evidence of auxeticity in calcite by ion implanting (1010) oriented single crystalline calcite with Ar⁺ at room temperature using an ion energy of 400 keV and a dose of 1 × 10¹⁴ cm⁻². Lattice compression normal to the substrate surface was observed, which is an atypical result for ion implanted materials. The auxetic behavior is consistent with predictions that indicate auxeticity had been predicted along two crystallographic directions including [1010]. Materials with a positive Poisson’s ratio experience lattice expansion normal to the substrate surface when ion implanted, whereas lattice contraction normal to the surface is evidence of auxetic behavior. Triple-axis X-ray diffraction measurements confirmed the auxetic strain state of the implanted calcite substrates. Reciprocal space maps for the symmetric 3030 and asymmetric 1450 reflections revealed that the implanted region was fully strained (pseudomorphic) to the bulk of the substrate, as is typical with implanted single crystals. A symmetric (3030) ω:2θ line scan was used with X-ray dynamical diffraction simulations to model the strain profile and extract the variation of compressive strain as a function of depth normal to the substrate surface. SRIM calculations were performed to obtain a displacement-per-atom profile and implanted Ar⁺ concentration profile. It was found that the strain profile matches the displacement-per-atom profile. This study demonstrated the use of ion implantation and X-ray diffraction methods to probe mechanical properties of materials and to test predictions such as the auxeticity.

Removal of As(III) by Electrically Conducting Ultrafiltration Membranes

As(III) species are the predominant form of arsenic found in groundwater. However, nanofiltration (NF) and reverse osmosis (RO) membranes are often unable to effectively reject As(III). In this study, we fabricate highly conducting ultrafiltration (UF) membranes for effective As(III) rejection. These membranes consist of a hydrophilic nickel-carbon nanotubes layer deposited on a UF support, and used as cathodes. Applying cathodic potentials significantly increased As(III) rejection in synthetic/real tap water, a result of locally elevated pH that is brought upon through water electrolysis at the membrane/water interface. The elevated pH conditions convert H₃ASO₃ to H₂AsO₃^-/HAsO₃^2- that are rejected by the negatively charged membranes. In addition, it was found that Mg(OH)₂ that precipitates on the membrane can further trap arsenic. Importantly, almost all As(III) passing through the membranes is oxidized to As(V) by hydrogen peroxide produced on the cathode, which significantly decreased its overall toxicity and mobility. Although the high pH along the membrane surface led to mineral scaling, this scale could be partially removed by backwashing the membrane. To the best of our knowledge, this is the first report of effective As(III) removal using low-pressure membranes, with As(III) rejection higher than that achieved by NF and RO, and high water permeance.

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.

Selective sulfur removal from semi-dry flue gas desulfurization coal fly ash for concrete and carbon dioxide capture applications

High-sulfur mixed fly ash residues from semi-dry flue gas desulfurization units in coal-fired power plants are unsuitable for use as supplementary cementitious material (SCM) for concrete production or carbon dioxide utilization. In this work, we explore the potential for upcycling a representative spray dry absorber ash (10.44 wt% SO₃) into concrete-SCM by selective sulfur removal via weak acid dissolution while simultaneously exploring the possibility for CO₂ capture. Towards this effort, parametric studies varying liquid-to-solid ratio, acidity, and CO₂ pressure were conducted in a batch reactor to establish the sulfur removal characteristics in de-ionized water, nitric acid, and carbonic acid, respectively. The dissolution studies show that the leaching of sulfur from calcium sulfite hemihydrate, which is the predominant S phase, is rapid and achieves a concentration plateau within 5 min, and subsequently, appears to be controlled by the primary mineral solubility. Preferential S removal was sufficient to meet SCM standards (e.g., 5.0 wt% as per ASTM C618) using all three washing solutions with 0.62-0.72 selectivity (S^), defined as the molar ratio of S to Ca in the leachate, for a raw fly ash with bulk S^ = 0.3. Acid dissolution with 1.43 meq/g of ash or under 5 atm CO₂ retained > 18 wt% CaO and other Si-, Al-rich phases in the fly ash. Based on the experimental findings, two sulfur removal schemes were suggested for either integration with CO₂ capture and utilization processes using flue gas or to produce fly ash for use as a SCM.

Calcination-free production of calcium hydroxide at sub-boiling temperatures

Calcium hydroxide (Ca(OH)₂), a commodity chemical, finds use in diverse industries ranging from food, to environmental remediation and construction. However, the current thermal process of Ca(OH)₂ production via limestone calcination is energy- and CO₂-intensive. Herein, we demonstrate a novel aqueous-phase calcination-free process to precipitate Ca(OH)₂ from saturated solutions at sub-boiling temperatures in three steps. First, calcium was extracted from an archetypal alkaline industrial waste, a steel slag, to produce an alkaline leachate. Second, the leachate was concentrated using reverse osmosis (RO) processing. This elevated the Ca-abundance in the leachate to a level approaching Ca(OH)₂ saturation at ambient temperature. Thereafter, Ca(OH)₂ was precipitated from the concentrated leachate by forcing a temperature excursion in excess of 65 °C while exploiting the retrograde solubility of Ca(OH)₂. This nature of temperature swing can be forced using low-grade waste heat (≤100 °C) as is often available at power generation, and industrial facilities, or using solar thermal heat. Based on a detailed accounting of the mass and energy balances, this new process offers at least ≈65% lower CO₂ emissions than incumbent methods of Ca(OH)₂, and potentially, cement production.

A calcination-free route to produce calcium hydroxide from alkaline industrial wastes including leaching, concentration, and temperature-swing precipitation.

The role of gas flow distributions on CO2 mineralization within monolithic cemented composites: coupled CFD-factorial design approach

Optimizing the spatial distribution of contacting gas and the gas processing conditions enhances CO₂ mineralization reactions and material properties of carbonate-cementitious monoliths.

Atomic Dislocations and Bond Rupture Govern Dissolution Enhancement under Acoustic Stimulation

By focusing the power of sound, acoustic stimulation (i.e., often referred to as sonication) enables numerous "green chemistry" pathways to enhance chemical reaction rates, for instance, of mineral dissolution in aqueous environments. However, a clear understanding of the atomistic mechanism(s) by which acoustic stimulation promotes mineral dissolution remains unclear. Herein, by combining nanoscale observations of dissolving surface topographies using vertical scanning interferometry, quantifications of mineral dissolution rates via analysis of solution compositions using inductively coupled plasma optical emission spectrometry, and classical molecular dynamics simulations, we reveal how acoustic stimulation induces dissolution enhancement. Across a wide range of minerals (Mohs hardness ranging from 3 to 7, surface energy ranging from 0.3 to 7.3 J/m², and stacking fault energy ranging from 0.8 to 10.0 J/m²), we show that acoustic fields enhance mineral dissolution rates (reactivity) by inducing atomic dislocations and/or atomic bond rupture. The relative contributions of these mechanisms depend on the mineral's underlying mechanical properties. Based on this new understanding, we create a unifying model that comprehensively describes how cavitation and acoustic stimulation processes affect mineral dissolution rates.

Enhancing Polyvalent Cation Rejection Using Perfluorophenylazide-Grafted-Copolymer Membrane Coatings

Surface modification offers a straightforward means to alter and enhance the properties and performance of materials, such as nanofiltration membranes for water softening. Herein, we demonstrate how a membrane's surface charge can be altered by grafting different electrostatically varying copolymers onto commercial membrane surfaces using perfluorophenylazide (PFPA) photochemistry for enhanced ion separation performance. The native membrane's performance-i.e., in terms of divalent cation separation-with copolymer coatings containing a positively charged quaternary ammonium (-N(Me)₃⁺), a negatively charged sulfonate (-SO₃^-), and an essentially neutral zwitterion (sulfobetaine, -N(Me)₂R₂⁺, and -SO₃^-), respectively, indicates that: (a) the sulfonated polymer induces robust Coulombic exclusion of divalent anions as compared to the negatively charged native membrane surface on account of its higher negative charge; (b) the positively charged ammonium coating induces exclusion of cations more effectively than the native membrane; and significantly, (c) the zwitterion polymer coating, which reduces the surface roughness and improves wettability, in spite of its near-neutral charge enhances exclusion of both divalent cations and anions on account of aperture sieving by the compact zwitterion polymer that arises from its ability to limit the size of ions that transport through the polymer along with dielectric exclusion. The outcomes thereby inform new pathways to achieve size- and charge-based exclusion of ionic, molecular, and other species contained in liquid streams.

Temperature-Induced Aggregation in Portlandite Suspensions

Temperature is well known to affect the aggregation behavior of colloidal suspensions. This paper elucidates the temperature dependence of the rheology of portlandite (calcium hydroxide: Ca(OH)₂) suspensions that feature a high ionic strength and a pH close to the particle's isoelectric point. In contrast to the viscosity of the suspending medium (saturated solution of Ca(OH)₂ in water), the viscosity of Ca(OH)₂ suspensions is found to increase with elevating temperature. This behavior is shown to arise from the temperature-induced aggregation of polydisperse Ca(OH)₂ particulates because of the diminution of electrostatic repulsive forces with increasing temperature. The temperature dependence of the suspension viscosity is further shown to diminish with increasing particle volume fraction as a result of volumetric crowding and the formation of denser fractal structures in the suspension. Significantly, the temperature-dependent rheological response of suspensions is shown to be strongly affected by the suspending medium's properties, including ionic strength and ion valence, which affect aggregation kinetics. These outcomes provide new insights into aggregation processes that affect the temperature-dependent rheology of portlandite-based and similar suspensions that feature strong charge screening behavior.

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

Concrete gains its strength from the precipitation of a calcium-alumino-silicate-hydrate (C-A-S-H) colloidal gel, which acts as its binding phase. However, despite concrete's ubiquity in the building environment, the atomic-scale mechanism of C-A-S-H precipitation is still unclear. Here, we use reactive molecular dynamics simulations to model the early-age precipitation of a C-A-S-H gel. We find that, upon gelation, silicate and aluminate precursors condensate and polymerize to form an aluminosilicate gel network. Notably, we demonstrate that the gelation reaction is driven by the existence of a mismatch of atomic-level internal stress between Si and Al polytopes, which are initially experiencing some local tension and compression, respectively. The polymerization of Si and Al polytopes enables the release of these competitive stresses.

How clay particulates affect flow cessation and the coiling stability of yield stress-matched cementing suspensions

The remarkable increase in the flow resistance of dense suspensions can hinder 3D-printing processes on account of flow cessation in the extruder, and filament fragility/rupture following deposition. Understanding the nature of rheological changes that occur is critical to manipulate flow conditions or to dose flow modifiers for 3D-printing. Therefore, this paper elucidates the influences of clay particulates on controlling flow cessation and the shape stability of dense cementing suspensions that typically feature poor printability. A rope coiling method was implemented with varying stand-off distances to probe the buckling stability and tendency to fracture of dense suspensions that undergo stretching and bending during deposition. The contributions of flocculation and short-term percolation due to the kinetics of structure formation to deformation rate were deconvoluted using a stepped isostress method. It is shown that the shear stress indicates a divergence with a power-law scaling when the particle volume fraction approaches the jamming limit; φ → φj ≈ φmax. Such a power-law divergence of the shear stress decreases by a factor of 10 with increasing clay dosage. Such behavior in clay-containing suspensions arises from a decrease in the relative packing fraction (φ/φmax) and the formation of fractally-architected aggregates with stronger interparticle interactions, whose uniform arrangement controls flow cessation in the extruder and suspension homogeneity, thereby imparting greater buckling stability. The outcomes offer new insights for assessing/improving the extrudability and printability behavior during slurry-based 3D-printing process.

Dispersing nano- and micro-sized portlandite particulates via electrosteric exclusion at short screening lengths

In spite of their high surface charge (zeta potential ζ = +34 mV), aqueous suspensions of portlandite (calcium hydroxide: Ca(OH)2) exhibit a strong tendency to aggregate, and thereby present unstable suspensions. While a variety of commercial dispersants seek to modify the suspension stability and rheology (e.g., yield stress, viscosity), it remains unclear how the performance of electrostatically and/or electrosterically based additives is affected in aqueous environments having either a high ionic strength and/or a pH close to the particle's isoelectric point (IEP). We show that the high native ionic strength (pH ≈ 12.6, IEP: pH ≈ 13) of saturated portlandite suspensions strongly screens electrostatic forces (Debye length: κ-1 = 1.2 nm). As a result, coulombic repulsion alone is insufficient to mitigate particle aggregation and affect rheology. However, a longer-range geometrical particle-particle exclusion that arises from electrosteric hindrance caused by the introduction of comb polyelectrolyte dispersants is very effective at altering the rheological properties and fractal structuring of suspensions. As a result, comb-like dispersants that stretch into the solvent reduce the suspension's yield stress by 5× at similar levels of adsorption as compared to linear dispersants, thus enhancing the critical solid loading (i.e., at which jamming occurs) by 1.4×. Significantly, the behavior of diverse dispersants is found to be inherently related to the thickness of the adsorbed polymer layer on particle surfaces. These outcomes inform the design of dispersants for concentrated suspensions that present strong charge screening behavior.

Electrochemically Enhanced Dissolution of Silica and Alumina in Alkaline Environments

Dissolution of mineral surfaces at asymmetric solid-liquid-solid interfaces in aqueous solutions occurs in technologically relevant processes, such as chemical/mechanical polishing (CMP) for semiconductor fabrication, formation and corrosion of structural materials, and crystallization of materials relevant to heterogeneous catalysis or drug delivery. In some such processes, materials at confined interfaces exhibit dissolution rates that are orders of magnitude larger than dissolution rates of isolated surfaces. Here, the dissolution of silica and alumina in close proximity to a charged gold surface or mica in alkaline solutions of pH 10-11 is shown to depend on the difference in electrostatic potentials of the surfaces, as determined from measurements conducted using a custom-built electrochemical pressure cell and a surface forces apparatus (SFA). The enhanced dissolution is proposed to result from overlap of the electrostatic double layers between the dissimilar charged surfaces at small intersurface separation distances (<1 Debye length). A semiquantitative model shows that overlap of the electric double layers can change the magnitude and direction of the electric field at the surface with the less negative potential, which results in an increase in the rate of dissolution of that surface. When the surface electrochemical properties were changed, the dissolution rates of silica and alumina were increased by up to 2 orders of magnitude over the dissolution rates of isolated compositionally similar surfaces under otherwise identical conditions. The results provide new insights on dissolution processes that occur at solid-liquid-solid interfaces and yield design criteria for controlling dissolution through electrochemical modification, with relevance to diverse technologies.

A Nitrogen- and Self-Doped Titania Coating Enables the On-Demand Release of Free Radical Species

For potential applications such as suppressing the onset of peri-implant infections, a doped titania coating was developed to induce free radical release because of its ability for microbial elimination. The coatability of the sol-gel precursor is robust since the suspension's rheology can be modified to attain uniform and complete surface coverage. The coating is composed of a mixture of anatase and rutile polymorphs doped with nitrogen (N^3-), and it contains substoichiometric Ti²⁺ and Ti³⁺ species. Nitrogen doping results in a 0.4 eV band gap shift, while the defects induce photocurrent generation under visible light excitation up to 650 nm. Greater currents were observed in the nitrogen-doped titania at wavelengths above 450 nm vis-à-vis its (singularly) self-doped counterparts. The (photo)electrochemical behavior and photoactivity of the coating were evaluated by assessing redox species formation in a background aqueous solution. In the absence of any illumination, the coating behaved as an insulator and inhibited the activities of both oxidative and reductive species. On the other hand, under illumination, the coating enhances oxidation processes and inhibits reduction reactions within a near-field region wherein release of free radicals occurs and is constrained (delimited).

Next-Generation Asymmetric Membranes Using Thin-Film Liftoff

For the past 30 years, thin-film membrane composites have been the state-of-the-art technology for reverse osmosis, nanofiltration, ultrafiltration, and gas separation. However, traditional membrane casting techniques, such as phase inversion and interfacial polymerization, limit the types of material that are used for the membrane separation layer. Here, we describe a novel thin-film liftoff (T-FLO) technique that enables the fabrication of thin-film composite membranes with new materials for desalination, organic solvent nanofiltration, and gas separation. The active layer is cast separately from the porous support layer, allowing for the tuning of the thickness and chemistry of the active layer. A fiber-reinforced, epoxy-based resin is then cured on top of the active layer to form a covalently bound support layer. Upon submersion in water, the cured membrane lifts off from the substrate to produce a robust, freestanding, asymmetric membrane composite. We demonstrate the fabrication of three novel T-FLO membranes for chlorine-tolerant reverse osmosis, organic solvent nanofiltration, and gas separation. The isolable nature of support and active-layer formation paves the way for the discovery of the transport and selectivity properties of new polymeric materials. This work introduces the foundation for T-FLO membranes and enables exciting new materials to be implemented as the active layers of thin-film membranes, including high-performance polymers, two-dimensional materials, and metal-organic frameworks.

Method for experimentally determining N-A-S-(H) solubility

Inorganic polymer binders, also sometimes called geopolymers or alkali-activated cements, can serve as an alternative to ordinary portland cement (OPC) in concrete.  The development of thermodynamic models to predict phase development and, ultimately, engineering properties, of inorganic polymer binders is an important step toward enabling their widespread use. However, such models require self-consistent solubility data of the primary phase in inorganic polymer binders, sodium aluminosilicate hydrate(s). To date, there is very little solubility information available for this phase. Here, a rigorous method for synthesizing sodium aluminosilicate hydrate(s) of controlled composition, and for measuring its solubility is presented. This approach allows complete stoichiometric control over the (initial) solution composition to elucidate directly the development of N-A-S-H composition as it relates to a given solution composition. A review of previous literature related to the solubility of other cementitious materials is presented, and the need for thermodynamic data is discussed. Finally, a sample calculation is presented for determining the solubility product (Ksp) of a laboratory synthesized sodium aluminosilicate hydrate.

Revealing How Alkali Cations Affect the Surface Reactivity of Stainless Steel in Alkaline Aqueous Environments

Stainless steel is a ubiquitous structural material and one that finds extensive use in core-internal components in nuclear power plants. Stainless steel features superior corrosion resistance (e.g., as compared to ordinary steel) due to the formation of passivating iron and/or chromium oxides on its surfaces. However, the breakdown of such passivating oxide films, e.g., due to localized deformation and slip line formation following exposure to radiation, or aggressive ions renders stainless steel susceptible to corrosion-related degradation. Herein, the effects of alkali cations (i.e., K⁺, Li⁺) and the interactions between the passivated steel surface and the solution are examined using 304L stainless steel. Scanning electrochemical microscopy and atomic force microscopy are used to examine the inert-to-reactive transition of the steel surface both in the native state and in the presence of applied potentials. Careful analysis of interaction forces, in solution, within ≤10 nm of the steel surface, reveals that the interaction between the hydrated alkali cations and the substrate affects the structure of the electrical double layer (EDL). As a result, a higher surface reactivity is indicated in the presence of Li⁺ relative to K⁺ due to the effects of the former species in disrupting the EDL. These findings provide new insights into the role of the water chemistry not only on affecting metallic corrosion but also in other applications, such as batteries and electrochemical devices.

Revealing How Alkali Cations Affect the Surface Reactivity of Stainless Steel in Alkaline Aqueous Environments

Stainless steel is a ubiquitous structural material and one that finds extensive use in core-internal components in nuclear power plants. Stainless steel features superior corrosion resistance (e.g., as compared to ordinary steel) due to the formation of passivating iron and/or chromium oxides on its surfaces. However, the breakdown of such passivating oxide films, e.g., due to localized deformation and slip line formation following exposure to radiation, or aggressive ions renders stainless steel susceptible to corrosion-related degradation. Herein, the effects of alkali cations (i.e., K⁺, Li⁺) and the interactions between the passivated steel surface and the solution are examined using 304L stainless steel. Scanning electrochemical microscopy and atomic force microscopy are used to examine the inert-to-reactive transition of the steel surface both in the native state and in the presence of applied potentials. Careful analysis of interaction forces, in solution, within ≤10 nm of the steel surface, reveals that the interaction between the hydrated alkali cations and the substrate affects the structure of the electrical double layer (EDL). As a result, a higher surface reactivity is indicated in the presence of Li⁺ relative to K⁺ due to the effects of the former species in disrupting the EDL. These findings provide new insights into the role of the water chemistry not only on affecting metallic corrosion but also in other applications, such as batteries and electrochemical devices.

New insights into the atomic structure of amorphous TiO2 using tight-binding molecular dynamics

Amorphous TiO₂ (a-TiO₂) could offer an attractive alternative to conventional crystalline TiO₂ phases for photocatalytic applications. However, the atomic structure of a-TiO₂ remains poorly understood with respect to that of its crystalline counterparts. Here, we conduct some classical molecular dynamics simulations of a-TiO₂ based on a selection of empirical potentials. We show that, on account of its ability to dynamically assign the charge of each atom based on its local environment, the second-moment tight-binding charge equilibration potential yields an unprecedented agreement with available experimental data. Based on these simulations, we investigate the degree of order and disorder in a-TiO₂. Overall, the results suggest that a-TiO₂ features a large flexibility in its local topology, which may explain the high sensitivity of its structure to the synthesis method being used.

Anomalous variations in the viscous activation energy of suspensions induced by fractal structuring

HYPOTHESIS

In suspensions, the activation energy of viscous flow is an important property that controls the temperature dependence of the viscosity. However, the differentiated roles of the properties of the liquid phase and the structure of the solid particles in controlling the activation energy remain unclear. We propose here that particle fractal structuring yields an anomalous behavior in the activation energy of viscous flow.

EXPERIMENTS

The rheology of two series of suspensions consisting of glass beads suspended in poly(1-decene) was investigated over a wide range of solid volume fractions (0.00 ≤ φ ≤ 0.55). These suspensions were characterized by their viscosity (η, Pa∙s) via shear rate sweeps and by their yield stress (Pa) via oscillatory amplitude sweeps.

FINDINGS

Interestingly, for suspensions consisting of nominally smaller particles (d₅₀ ≈ 5 µm), we observe an anomalous decrease in the activation energy (E_a, kJ/mol) of viscous flow with increasing solid fraction. Based on oscillatory rheology analyses, it is suggested that such anomalous behavior arises due to entropic effects that result from the formation of fractally-architected cooperatively rearranging regions (i.e., agglomerates) in the suspension.

A thermodynamics-based approach for examining the suitability of cementitious formulations for solidifying and stabilizing coal-combustion wastes

Cementitious binders are often used to immobilize industrial wastes such as residues of coal combustion. Such immobilization stabilizes wastes that contain contaminants by chemical containment, i.e., by uptake of contaminants into the cementitious reaction products. Expectedly, the release ("leachability") of contaminants is linked to: (i) the stability of the matrix (i.e., its resistance to decomposition on exposure to water), and, (ii) its porosity, which offers a pathway for the intrusion of water and egress of contaminant species. To examine the effects of the matrix chemistry on its suitability for immobilization, an equilibrium thermodynamics-based approach is demonstrated for cementitious formulations based on: ordinary portland cement (OPC), calcium aluminate cement (CAC) and alkali activated fly ash (AFA) binding agents. First, special focus is placed on computing the equilibrium phase assemblages using the bulk reactant compositions as an input. Second, the matrix's stability is assessed by simulating leaching that is controlled by progressive dissolution and precipitation of solids across a range of liquid (leachant)-to-(reaction product) solid (l/s) ratios and leachant pH's; e.g., following the LEAF 1313 and 1316 protocols. The performance of each binding formulation is evaluated based on the: (i) relative ability of the reaction products to chemically bind the contaminant(s), (ii) porosity of the matrix which correlates to its hydraulic conductivity, and, (iii) the extent of matrix degradation that follows leaching and which impact the rate and extent of release of potential contaminants. In this manner, the approach enables rapid, parametric assessment of a wide-range of stabilization solutions with due consideration of the matrix's mineralogy, porosity, and the leaching (exposure) conditions.

New insights into the sol-gel condensation of silica by reactive molecular dynamics simulations

The sol-gel method is an attractive technique to synthesize homogeneous silicate glasses with high purity while relying on a lower synthesis temperature than in the melt-quench method. However, the mechanism and kinetics of the condensation of the silicate network in aqueous solution remain unclear. Here, based on reactive molecular dynamics simulations (ReaxFF), we investigate the sol-gel condensation kinetics of a silica glass. The influence of the potential parametrization and system size is assessed. Our simulation methodology is found to offer good agreement with experiments. We show that the aqueous concentration of the Si(OH)₄ precursors and the local degree of polymerization of the Si atoms play a crucial role in controlling the condensation activation energy. Based on our simulations, we demonstrate that the gelation reaction is driven by the existence of some local atomic stress that gets released upon condensation.

Direct observation of pitting corrosion evolutions on carbon steel surfaces at the nano-to-micro- scales

The Cl⁻-induced corrosion of metals and alloys is of relevance to a wide range of engineered materials, structures, and systems. Because of the challenges in studying pitting corrosion in a quantitative and statistically significant manner, its kinetics remain poorly understood. Herein, by direct, nano- to micro-scale observations using vertical scanning interferometry (VSI), we examine the temporal evolution of pitting corrosion on AISI 1045 carbon steel over large surface areas in Cl⁻-free, and Cl⁻-enriched solutions. Special focus is paid to examine the nucleation and growth of pits, and the associated formation of roughened regions on steel surfaces. By statistical analysis of hundreds of individual pits, three stages of pitting corrosion, namely, induction, propagation, and saturation, are quantitatively distinguished. By quantifying the kinetics of these processes, we contextualize our current understanding of electrochemical corrosion within a framework that considers spatial dynamics and morphology evolutions. In the presence of Cl⁻ ions, corrosion is highly accelerated due to multiple autocatalytic factors including destabilization of protective surface oxide films and preservation of aggressive microenvironments within the pits, both of which promote continued pit nucleation and growth. These findings offer new insights into predicting and modeling steel corrosion processes in mid-pH aqueous environments.

Anion Capture and Exchange by Functional Coatings: New Routes to Mitigate Steel Corrosion in Concrete Infrastructure

Chloride-induced corrosion is a major cause of degradation of reinforced concrete infrastructure. While the binding of chloride ions (Cl^-) by cementitious phases is known to delay corrosion, this approach has not been systematically exploited as a mechanism to increase structural service life. Recently, Falzone et al. [Cement and Concrete Research72, 54-68 (2015)] proposed calcium aluminate cement (CAC) formulations containing NO₃-AFm to serve as anion exchange coatings that are capable of binding large quantities of Cl^- ions, while simultaneously releasing corrosion-inhibiting NO₃^- species. To examine the viability of this concept, Cl^- binding isotherms and ion-diffusion coefficients of a series of hydrated CAC formulations containing admixed Ca(NO₃)₂ (CN) are quantified. This data is input into a multi-species Nernst-Planck (NP) formulation, which is solved for a typical bridge-deck geometry using the finite element method (FEM). For exposure conditions corresponding to seawater, the results indicate that Cl^- scavenging CAC coatings (i.e., top-layers) can significantly delay the time to corrosion (e.g., 5 ≤ d_f ≤ 10, where d_f is the steel corrosion initiation delay factor [unitless]) as compared to traditional OPC-based systems for the same cover thickness; as identified by thresholds of Cl^-/OH^- or Cl^-/NO₃^- (molar) ratios in solution. The roles of hindered ionic diffusion, and the passivation of the reinforcing steel rendered by NO₃^- are also discussed.

Revealing the Effect of Irradiation on Cement Hydrates: Evidence of a Topological Self-Organization

Despite the crucial role of concrete in the construction of nuclear power plants, the effects of radiation exposure (i.e., in the form of neutrons) on the calcium-silicate-hydrate (C-S-H, i.e., the glue of concrete) remain largely unknown. Using molecular dynamics simulations, we systematically investigate the effects of irradiation on the structure of C-S-H across a range of compositions. Expectedly, although C-S-H is more resistant to irradiation than typical crystalline silicates, such as quartz, we observe that radiation exposure affects C-S-H's structural order, silicate mean chain length, and the amount of molecular water that is present in the atomic network. By topological analysis, we show that these "structural effects" arise from a self-organization of the atomic network of C-S-H upon irradiation. This topological self-organization is driven by the (initial) presence of atomic eigenstress in the C-S-H network and is facilitated by the presence of water in the network. Overall, we show that C-S-H exhibits an optimal resistance to radiation damage when its atomic network is isostatic (at Ca/Si = 1.5). Such an improved understanding of the response of C-S-H to irradiation can pave the way to the design of durable concrete for radiation applications.

Thermometer Effect: Origin of the Mixed Alkali Effect in Glass Relaxation

Despite the dramatic increase of viscosity as temperature decreases, some glasses are known to feature room-temperature relaxation. However, the structural origin of this phenomenon-known as the "thermometer effect"-remains unclear. Here, based on accelerated molecular dynamics simulations of alkali silicate glasses, we show that both enthalpy and volume follow stretched exponential decay functions upon relaxation. However, we observe a bifurcation of their stretching exponents, with β=3/5 and 3/7 for enthalpy and volume relaxation, respectively, in agreement with Phillips's topological diffusion-trap model. Based on these results, we demonstrate that the thermometer effect is a manifestation of the mixed alkali effect. We show that relaxation is driven by the existence of stressed local structural instabilities in mixed alkali glasses. This driving force is found to be at a maximum when the concentrations of each alkali atom equal each other, which arises from a balance between the concentration of each alkali atom and the magnitude of the local stress that they experience.

Cements in the 21st Century: Challenges, Perspectives, and Opportunities

In a book published in 1906, Richard Meade outlined the history of portland cement up to that point1. Since then there has been great progress in portland cement-based construction materials technologies brought about by advances in the materials science of composites and the development of chemical additives (admixtures) for applications. The resulting functionalities, together with its economy and the sheer abundance of its raw materials, have elevated ordinary portland cement (OPC) concrete to the status of most used synthetic material on Earth. While the 20th century was characterized by the emergence of computer technology, computational science and engineering, and instrumental analysis, the fundamental composition of portland cement has remained surprisingly constant. And, although our understanding of ordinary portland cement (OPC) chemistry has grown tremendously, the intermediate steps in hydration and the nature of calcium silicate hydrate (C-S-H), the major product of OPC hydration, remain clouded in uncertainty. Nonetheless, the century also witnessed great advances in the materials technology of cement despite the uncertain understanding of its most fundamental components. Unfortunately, OPC also has a tremendous consumption-based environmental impact, and concrete made from OPC has a poor strength-to-weight ratio. If these challenges are not addressed, the dominance of OPC could wane over the next 100 years. With this in mind, this paper envisions what the 21st century holds in store for OPC in terms of the driving forces that will shape our continued use of this material. Will a new material replace OPC, and concrete as we know it today, as the preeminent infrastructure construction material?

Confined Water in Layered Silicates: The Origin of Anomalous Thermal Expansion Behavior in Calcium-Silicate-Hydrates

Water, under conditions of nanoscale confinement, exhibits anomalous dynamics, and enhanced thermal deformations, which may be further enhanced when such water is in contact with hydrophilic surfaces. Such heightened thermal deformations of water could control the volume stability of hydrated materials containing nanoconfined structural water. Understanding and predicting the thermal deformation coefficient (TDC, often referred to as the CTE, coefficient of thermal expansion), which represents volume changes induced in materials under conditions of changing temperature, is of critical importance for hydrated solids including: hydrogels, biological tissues, and calcium silicate hydrates, as changes in their volume can result in stress development, and cracking. By pioneering atomistic simulations, we examine the physical origin of thermal expansion in calcium-silicate-hydrates (C-S-H), the binding agent in concrete that is formed by the reaction of cement with water. We report that the TDC of C-S-H shows a sudden increase when the CaO/SiO₂ (molar ratio; abbreviated as Ca/Si) exceeds 1.5. This anomalous behavior arises from a notable increase in the confinement of water contained in the C-S-H's nanostructure. We identify that confinement is dictated by the topology of the C-S-H's atomic network. Taken together, the results suggest that thermal deformations of hydrated silicates can be altered by inducing compositional changes, which in turn alter the atomic topology and the resultant volume stability of the solids.

Topological Control on Silicates' Dissolution Kinetics

Like many others, silicate solids dissolve when placed in contact with water. In a given aqueous environment, the dissolution rate depends on the composition and the structure of the solid and can span several orders of magnitude. Although the kinetics of dissolution depends on the complexities of both the dissolving solid and the solvent, a clear understanding of which structural descriptors of the solid control its dissolution rate is lacking. By pioneering dissolution experiments and atomistic simulations, we correlate the dissolution rates-ranging over 4 orders of magnitude-of a selection of silicate glasses and crystals to the number of chemical topological constraints acting between the atoms of the dissolving solid. The number of such constraints serves as an indicator of the effective activation energy, which arises from steric effects, and prevents the network from reorganizing locally to accommodate intermediate units forming over the course of the dissolution.

Nature of radiation-induced defects in quartz

Although quartz (α-form) is a mineral used in numerous applications wherein radiation exposure is an issue, the nature of the atomistic defects formed during radiation-induced damage has not been fully clarified. Especially, the extent of oxygen vacancy formation is still debated, which is an issue of primary importance as optical techniques based on charged oxygen vacancies have been utilized to assess the level of radiation damage in quartz. In this paper, molecular dynamics simulations are applied to study the effects of ballistic impacts on the atomic network of quartz. We show that the defects that are formed mainly consist of over-coordinated Si and O, as well as Si-O connectivity defects, e.g., small Si-O rings and edge-sharing Si tetrahedra. Oxygen vacancies, on the contrary, are found in relatively low abundance, suggesting that characterizations based on E' centers do not adequately capture radiation-induced structural damage in quartz. Finally, we evaluate the dependence on the incident energy, of the amount of each type of the point defects formed, and quantify unambiguously the threshold displacement energies for both O and Si atoms. These results provide a comprehensive basis to assess the nature and extent of radiation damage in quartz.

Stretched Exponential Relaxation of Glasses at Low Temperature

The question of whether glass continues to relax at low temperature is of fundamental and practical interest. Here, we report a novel atomistic simulation method allowing us to directly access the long-term dynamics of glass relaxation at room temperature. We find that the potential energy relaxation follows a stretched exponential decay, with a stretching exponent β=3/5, as predicted by Phillips's diffusion-trap model. Interestingly, volume relaxation is also found. However, it is not correlated to the energy relaxation, but it is rather a manifestation of the mixed alkali effect.

Expression of intercellular adhesion molecules in the bladder of patients with interstitial cystitis

OBJECTIVES

To study the presence of intercellular adhesion molecule-1 (ICAM-1), P-selectin, and E-selectin, as well as the cytoskeletal components talin and vinculin that bind to cellular adhesion molecules (CAMs), in bladder biopsies from patients with interstitial cystitis (IC) and controls. IC is a sterile, bladder disorder characterized by urinary frequency and pelvic floor pain. The pathologic bladder findings include defective urothelium, activated mast cells, and variable inflammation. Mast cells can induce the expression of CAMs necessary for initiation of inflammation.

METHODS

Fresh frozen biopsies were analyzed immunocytochemically from 2 female normal bladders, 10 female IC bladders, 1 clear margin of transitional cell carcinoma of female bladder, 1 normal foreskin, 1 transitional cell carcinoma of foreskin, and 1 inflamed male finger.

RESULTS

Of the 10 IC samples, 9 were positive for ICAM-1, 6 for P-selectin, 6 for vinculin, 5 for talin, and 4 for E-selectin, all exclusively perivascular. Both normal bladders were negative for ICAM-1 and P-selectin and faintly positive for E-selectin, and one was weakly positive for talin and vinculin; the normal foreskin was negative. The "control" samples from the transitional cell carcinoma of the bladder and foreskin, as well as the inflamed finger skin, were positive only for ICAM-1. An increased number of activated mast cells associated with ICAM-1 was noted in IC.

CONCLUSIONS

These results showed that ICAM-1 is expressed in IC, with variable expression of the other markers studied, supporting the different degrees of bladder inflammation noted in patients with IC.