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McLean, VA, United States

Painter P.,Pennsylvania State University | Veytsman B.,George Mason University | Youtcheff J.,Turner Fairbanks Highway Research Center
Energy and Fuels | Year: 2014

A simple attenuated association model for the aggregation of asphaltenes in solution is described. The model has only two parameters, a single equilibrium constant and the molecular weight of the "monomer" (unassociated asphaltene molecule). Both parameters were estimated from results reported in the literature and account for the variation of the degree of association with the concentration very well. Using Flory's treatment of reversible association and Prigogine's result that the chemical potential of an associating species is equal to the chemical potential of the monomeric species, equations for the free energy of mixing and its derivatives are derived. These equations were then used to model the phase behavior of solutions. Good agreement with observed experimental behavior was obtained using solubility parameters for asphaltenes in a narrow range near 20 MPa0.5, consistent with values reported in the literature. © 2014 American Chemical Society. Source


Painter P.,Pennsylvania State University | Veytsman B.,George Mason University | Youtcheff J.,Turner Fairbanks Highway Research Center
Energy and Fuels | Year: 2015

An attenuated association model describing the aggregation of asphaltenes in solution is extended to derive an equation for the weight-average degree of association and account for phase behavior. The weight-average molecular weight is calculated to be higher than number average, as it must be for a polydisperse material, but not by enough to explain the very large differences in these quantities reported in the literature. Binodals and spinodals are calculated using expressions derived previously, but modified to account for free volume (thermal expansion) differences. The phase behavior of asphaltene solutions is examined in more detail, particularly in the dilute solution regime. It is shown that the formation of nanoaggregates significantly affects the critical value of the χ interaction parameter. The phase diagram is highly asymmetric and the phase boundary approaches the pure solvent composition limit. This has a number of implications in terms of asphaltene solution characterization and the nature of asphaltene solutions. The results indicate that there are toluene insoluble asphaltene components, but these could exist as microphase-separated clusters stabilized against further aggregation by steric and kinetic factors. This would explain the large difference between observed number and weight-average molecular weights. In addition, because of the shape of the binodal curve at low concentrations, experimental data that have previously been interpreted in terms of a critical cluster or micelle concentration are shown to be consistent with a microphase separation. © 2015 American Chemical Society. Source


Painter P.,Pennsylvania State University | Veytsman B.,George Mason University | Youtcheff J.,Turner Fairbanks Highway Research Center
Energy and Fuels | Year: 2015

A guide to the solubility of asphaltenes in a range of solvents is constructed through the use of an association model to account for asphaltene nanoaggregation and its effect on phase behavior and Hildebrand solubility parameters to model interactions between aggregates and solvent. Solvents are classified according to their polarity and ability to self-associate (e.g., through hydrogen bonds). In addition, estimates of the contribution of free volume terms to interaction parameters indicate that a further distinction must be made between solvents with flexible molecules (such as the n-paraffins) and those that are relatively inflexible (such as toluene). A "bare" interaction parameter (π0) is calculated, and it is this parameter that is related to Hildebrand solubility parameters. For nonpolar and weakly polar solvents, a critical value of the solubility parameter difference (Δδc) between an asphaltene or asphaltene component and solvent is calculated to be ±3.5 MPa0.5 at 25 °C for a nonpolar or weakly polar solvents with largely inflexible molecules and a molar volume of 100 cm3/mol. For flexible solvents such as the n-paraffins, free volume effects are larger and Δδc is about ±2.8 MPa0.5. For strongly polar solvents that have limited flexibility, the equivalent critical value of the solubility parameter difference is also calculated to be ±2.8 MPa0.5. Hydrogen bonded solvents like methanol are calculated to be immiscible with asphaltenes, with miscibility being defined as forming a single phase across the composition range (at 25°C). Miscibility maps are constructed in terms of the calculated phase boundary at the critical point, δA ± Δδc, where δA is the asphaltene component solubility parameter, plotted as a function of solvent molar volume. The solubility of asphaltenes and asphaltene components in various solvents is discussed. The solvent that defines asphaltene identity, toluene, is predicted to dissolve only a limited range of asphaltene components. This is consistent with various reported experimental observations. However, solubility is often defined in terms of an absence of a visible precipitate, and on the basis of recent work in the literature, toluene solutions may contain some microphase-separated material stabilized against further aggregation by steric and kinetic factors. © 2015 American Chemical Society. Source


Painter P.,Pennsylvania State University | Veytsman B.,George Mason University | Youtcheff J.,Turner Fairbanks Highway Research Center
Energy and Fuels | Year: 2015

An attenuated association model previously developed to describe the aggregation of asphaltenes in solution is extended to a description of the phase behavior of bitumen and asphalts. Work reported in the literature using various experimental techniques shows that these materials have complex phase structures, but essentially consist of two amorphous phases: one is predominantly asphaltenes and the other is largely maltenes. In the work reported here, these materials were first modeled as pseudo-two-component mixtures, where one component consists of a self-associating component that, for the most part, corresponds to asphaltenes, while the second component is a non-self-associating component that essentially consists of maltenes. It is shown that the critical value of the Flory X interaction parameter is significantly reduced in these mixtures, relative to asphaltene solutions, and calculated binodals (coexistence curves) are very broad and asymmetric, with a phase boundary that approaches the pure maltene composition limit. This indicates that, above a critical value of X, bitumen and asphalts phase-separate into an almost-pure maltene fraction. However, the asphaltene-rich phase consists of an appreciable non-self-associating fraction that varies with temperature and overall bitumen composition. Ternary phase diagrams were also calculated by assuming that bitumen is a pseudo-three-component system that consists of a self-associating component, identified as asphaltenes, saturates, and aromatics plus resins (combined). These ternary mixtures are predicted to phase-separate into two phases: one consists of asphaltenes and aromatics plus resins, but with only small amounts of saturates, while the second phase consists of a mixture of saturates and aromatics plus resins with only trace amounts of asphaltenes. The calculations are consistent with experimental measurements of the glass-transition temperatures of these materials and microscopic observations of phase behavior. © 2015 American Chemical Society. Source


Bentz D.P.,U.S. National Institute of Standards and Technology | Ardani A.,Turner Fairbanks Highway Research Center | Barrett T.,Purdue University | Jones S.Z.,U.S. National Institute of Standards and Technology | And 6 more authors.
Construction and Building Materials | Year: 2014

Limestone (calcium carbonate, CaCO3) has long been a critical component of concrete, whether as the primary raw material for cement production, a fine powder added to the binder component, or a source of fine and/or coarse aggregate. This paper focuses on the latter two of these examples, providing a multi-scale investigation of the influences of both fine limestone powder and conventional limestone aggregates on concrete performance. Fine limestone powder in the form of calcite provides a favorable surface for the nucleation and growth of calcium silicate hydrate gel at early ages, accelerating and amplifying silicate hydration, and a source of carbonate ions to participate in reactions with the aluminate phases present in the cement (and fly ash). Conversely, the aragonite polymorph of CaCO3 exhibits a different crystal (and surface) structure and therefore neither accelerates nor amplifies silicate hydration at a similar particle size/surface area. However, because these two forms of CaCO3 have similar solubilities in water, the aragonite does contribute to an enhancement in the reactivity of the aluminate phases in the investigated systems, chiefly via carboaluminate formation. In 100% ordinary Portland cement (OPC) concretes, 10% of the OPC by volume can be replaced with an equivalent volume of limestone powder, while maintaining acceptable performance. A comparison between limestone and siliceous aggregates indicates that the former often provide higher measured compressive strengths at equivalent levels of hydration, even when the two aggregate types exhibit similar elastic moduli. This suggests that the interfacial transition zone in the limestone-based concretes exhibits a higher degree of bonding, likely due to the favorable physical (texture) and chemical nature of the limestone surfaces. These observations reinforce the value of utilizing limestone to increase the performance and sustainability of 21st century concrete construction. Source

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