Magdalena Balonis

Cooling rate effects in sodium silicate glasses: Bridging the gap between molecular dynamics simulations and experiments

Although molecular dynamics (MD) simulations are commonly used to predict the structure and properties of glasses, they are intrinsically limited to short time scales, necessitating the use of fast cooling rates. It is therefore challenging to compare results from MD simulations to experimental results for glasses cooled on typical laboratory time scales. Based on MD simulations of a sodium silicate glass with varying cooling rate (from 0.01 to 100 K/ps), here we show that thermal history primarily affects the medium-range order structure, while the short-range order is largely unaffected over the range of cooling rates simulated. This results in a decoupling between the enthalpy and volume relaxation functions, where the enthalpy quickly plateaus as the cooling rate decreases, whereas density exhibits a slower relaxation. Finally, we show that, using the proper extrapolation method, the outcomes of MD simulations can be meaningfully compared to experimental values when extrapolated to slower cooling rates.

The influence of slightly and highly soluble carbonate salts on phase relations in hydrated calcium aluminate cements

The addition of slightly (CaCO3) and highly soluble (Na2CO3) carbonate salts is expected to favor the formation of carboaluminate phases in hydrated calcium aluminate cements (CACs). A multi-method approach including X-ray diffraction, thermogravimetric analysis, and thermodynamic calculations is applied to highlight that the “conversion phenomena” in CACs cannot be mitigated by the formation of carboaluminate phases (monocarboaluminate: Mc and hemicarboaluminate: Hc) which are anticipated to form following the addition of carbonate salts. Here, carboaluminate phase formation is shown to depend on three factors: (1) water availability, (2) carbonate content of the salts, and their ability to mobilize CO32− species in solution, and (3) lime content associated with the carbonate salt. The latter two factors are linked to the composition and solubility of the carbonate agent. It is concluded that limestone (CaCO3), despite being a source of calcium and carbonate species, contributes only slightly to carboaluminate phase formation due to its low solubility and slow dissolution rate. Soluble carbonate salts (Na2CO3) fail to boost carboaluminate phase formation as the availability of Ca2+ ions and water are limiting. Detailed thermodynamic calculations are used to elucidate conditions that affect the formation of carboaluminate phases.

Vertical scanning interferometry: A new method to quantify re-/de-mineralization dynamics of dental enamel

OBJECTIVE Remineralization and demineralization are processes that compete in the oral environment. At this time, numerous therapeutic agents are being developed to promote remineralization (precipitation) or suppress demineralization (dissolution). To evaluate the relative efficacy of such treatments, there is a need for non-invasive, real-time, high-resolution quantifications of topographical changes occurring during demineralization and remineralization. METHODS Vertical scanning interferometry (VSI) is demonstrated to be a quantitative method to assess reactions, and topographical changes occurring on enamel surfaces following exposure to demineralizing, and remineralizing liquids. RESULTS First, the dissolution rate of enamel was compared to that of synthetic hydroxyapatite (HAP) under acidic conditions (pH=4). Second, VSI was used to compare the remineralization effects of F(-)-based and CCP-ACP agents. The former produced a remineralization rate of ≈349nm/h, similar to simulated body fluid (SBF; concentration 4.6×) while the latter produced a remineralization rate of ≈55nm/h, corresponding to 1.7× SBF. However, the precipitates formed by the CCP-ACP agent are found to demineralize 2.7× slower than that produced by its F(-)-counterpart. SIGNIFICANCE Based on this new VSI-based data, a remineralization factor (RF) and demineralization (DF) factor benchmarked, respectively, to 1× SBF and the demineralization rate of human enamel are suggested as figures of merit of therapeutic performance of dental treatments. Taken together, the outcomes offer new insights that can inform clinicians and researchers on the selection of remineralization strategies.

The influence of metakaolin on limestone reactivity in cementitious materials

Recent studies have demonstrated that in the presence of limestone (CaCO3), carbonate-AFm phases (i.e., hemi- and/or mono-carboaluminate) may be stabilized at the expense of sulfate-AFm, which is more commonly found in cement systems. This suggests that enhancing AFm phase formation may be a novel way of incorporating increased quantities of limestone as a reactive component in cement-based systems. Often, in an ordinary portland cement (OPC), the quantity of the AFm hydrates formed is limited by the availability of aluminum. Therefore, as means of enhancing AFm phase formation, this paper evaluates metakaolin addition to determine how it affects limestone reactions and carbonate-AFm formation in the OPC systems. The results of a multi-method study including: X-ray diffraction with Rietveld refinement (QXRD), strength measurements, thermogravimetric analysis, and thermodynamic calculations are used to quantify solid phase constitutions, and the extent of limestone that has been consumed in reaction. Obtained results suggest that pozzolanic reactions which occur when metakaolin is used as an aluminous source are observed to be beneficial in offsetting the dilutive effects of OPC replacement noted in blended cement formulations.

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 ≤ df ≤ 10, where df 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.

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.

On the Application of Inertial Microfluidics for the Size-Based Separation of Polydisperse Cementitious Particulates

The early‐age performance of concrete is determined by the properties of the cementitious binder and the evolution of its chemical reactions. The chemical reactivity, and to some extent, the composition of cementitious particles can depend on particle size. Therefore, it is valuable to physically separate cementing minerals into well‐defined size classes so that the influences of both particle size and composition on reaction progress can be studied without the confounding effects of a broad particle size distribution. However, conventional particle separation methods (e.g., density fractionation, wet sieving, field-flow extraction, ultrasonification-sedimentation) are time-consuming and cumbersome and result in poor particle yields and size-selectivity, thus, making them unsuitable for processing larger volumes of cementitious powders (on the order of grams). This study applies a novel inertial microfluidics (IMF) based procedure to separate cementitious powders on the basis of their size. Special attention is paid to optimizing operating variables to ensure that particles in a fluid streamline achieve unique equilibrium positions within the device. From such positions, particles can be retrieved as per their size using symmetrical outlet configurations with tuned fluidic resistances. The approach is critically assessed in terms of: (1) its ability to separate cementitious powders into narrow size bins, and therefore its feasibility as a fractionation procedure, and (2) quantitatively relating the operating parameters to the particle yield and size selectivity. The study establishes metrics for assessing the ability of IMF methods to classify minerals and other polydisperse particles on the basis of their size.

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 (Ea, 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.

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.