Claire E. White

Density functional modeling of the local structure of kaolinite subjected to thermal dehydroxylation.

Understanding the atomic-level changes that occur as kaolinite is converted (thermally dehydroxylated) to metakaolin is critical to the optimization of this large-scale industrial process. Metakaolin is X-ray amorphous; therefore, conventional crystallographic techniques do not reveal the changes in local structure during its formation. Local structure-based experimental techniques are useful in understanding the atomic structure but do not provide the thermodynamic information which is necessary to ensure plausibility of refined structures. Here, kaolinite dehydroxylation is modeled using density functional theory, and a stepwise methodology, where several water molecules are removed from the structure, geometry optimization is carried out, and then the process is repeated. Hence, the structure remains in an energetically and thermodynamically feasible state while transitioning from kaolinite to metakaolin. The structures generated during the dehydroxylation process are validated by comparison with X-ray and neutron pair distribution function data. Thus, this study illustrates one possible route by which dehydroxylation of kaolinite can take place, revealing a chemically, energetically, and experimentally plausible structure of metakaolin. This methodology of density functional modeling of the stepwise changes in a material is not limited in application to kaolinite or other aluminosilicates and provides an accurate representation of the local structural changes occurring in materials used in industrially important processes.

What is the structure of kaolinite? Reconciling theory and experiment.

Density functional modeling of the crystalline layered aluminosilicate mineral kaolinite is conducted, first to reconcile discrepancies in the literature regarding the exact geometry of the inner and inner surface hydroxyl groups, and second to investigate the performance of selected exchange-correlation functionals in providing accurate structural information. A detailed evaluation of published experimental and computational structures is given, highlighting disagreements in space groups, hydroxyl bond lengths, and bond angles. A major aim of this paper is to resolve these discrepancies through computations. Computed structures are compared via total energy calculations and validated against experimental structures by comparing computed neutron diffractograms, and a final assessment is performed using vibrational spectra from inelastic neutron scattering. The density functional modeling is carried out at a sufficiently high level of theory to provide accurate structure predictions while keeping computational requirements low enough to enable the use of the structures in large-scale calculations. It is found that the best functional to use for efficient density functional modeling of kaolinite using the DMol3 software package is the BLYP functional. The computed structure for kaolinite at 0 K has C1 symmetry, with the inner hydroxyl group angled slightly above the a,b plane and the inner surface hydroxyls aligned close to perpendicular to that plane.

In situ X-ray pair distribution function analysis of accelerated carbonation of a synthetic calcium–silicate–hydrate gel

Calcium–silicate–hydrate (C–S–H) gel is the main binder component in hydrated ordinary Portland cement (OPC) paste, and is known to play a crucial role in the carbonation of cementitious materials, especially for more sustainable alternatives containing supplementary cementitious materials. However, the exact atomic structural changes that occur during carbonation of C–S–H gel remain unknown. Here, we investigate the local atomic structural changes that occur during carbonation of a synthetic calcium–silicate–hydrate gel exposed to pure CO2 vapour, using in situ X-ray total scattering measurements and subsequent pair distribution function (PDF) analysis. By analysing both the reciprocal and real-space scattering data as the C–S–H carbonation reaction progresses, all phases present during the reaction (crystalline and non-crystalline) have been identified and quantified, with the results revealing the emergence of several polymorphs of crystalline calcium carbonate (vaterite and calcite) in addition to the decalcified C–S–H gel. Furthermore, the results point toward residual calcium being present in the amorphous decalcified gel, potentially in the form of an amorphous calcium carbonate phase. As a result of the quantification process, the reaction kinetics for the evolution of the individual phases have been obtained, revealing new information on the rate of growth/dissolution for each phase associated with C–S–H gel carbonation. Moreover, the investigation reveals that the use of real space diffraction data in the form of PDFs enables more accurate determination of the phases that develop during complex reaction processes such as C–S–H gel carbonation in comparison to the conventional reciprocal space Rietveld analysis approach.

Treatment of hydrogen background in bulk and nanocrystalline neutron total scattering experiments

Nuclear incoherent neutron scattering contributions present a challenge in the structural characterization of many classes of materials. This article introduces methods for the correction of nanoparticle, bulk crystalline and amorphous powder neutron scattering data with significant incoherent contributions from hydrogen, and describes the effects the corrections have on the resulting atomic pair distribution function data sets. The approach is presented in the context of the PDFgetN data-reduction program [Peterson, Gutmann, Proffen & Billinge (2000). J. Appl. Cryst. 33, 1192].

The use of XANES to clarify issues related to bonding environments in metakaolin: a discussion of the paper S. Sperinck et al., “Dehydroxylation of kaolinite to metakaolin-a molecular dynamics study,” J. Mater. Chem., 2011, 21, 2118–2125

We provide a discussion of some issues raised by a recent paper published in Journal of Materials Chemistry regarding the local structure of metakaolin. Furthermore, we show using synchrotron X-ray absorption near-edge spectroscopy (XANES) that tri-coordinated aluminium sites can exist in metakaolin, providing new evidence regarding the coordination environment of aluminium in metakaolin.

Nanoscale Charge-Balancing Mechanism in Alkali-Substituted Calcium–Silicate–Hydrate Gels

Alkali-activated materials and related alternative cementitious systems are sustainable technologies that have the potential to substantially lower the CO2 emissions associated with the construction industry. However, these systems have augmented chemical compositions as compared to ordinary Portland cement (OPC), which may impact the evolution of the hydrate phases. In particular, calcium-silicate-hydrate (C-S-H) gel, the main hydrate phase in OPC, is likely to be altered at the atomic scale due to changes in the bulk chemical composition, specifically via the addition of alkalis (i.e., Na or K) and aluminum. Here, via density functional theory calculations, we reveal the presence of a charge balancing mechanism at the molecular level in C-S-H gel (as modeled using crystalline 14 Å tobermorite) when alkalis and aluminum atoms are introduced into the structure. Different structural representations are obtained depending on the level of substitution and the degree of charge balancing incorporated in the structures. The impact of these substitutional and charge balancing effects on the structures is assessed by analyzing the formation energies, local bonding environments, diffusion barriers and mechanical properties. The results of this computational study provide information on the phase stability of alkali/aluminum containing C-S-H gels, shedding light on the fundamental atomic level mechanisms that play a crucial role in these complex disordered materials.

Pair distribution function analysis of amorphous geopolymer precursors and binders: the importance of complementary molecular simulations

Abstract Geopolymers are a class of alternative cementitious material, synthesised via alkaline activation of aluminosilicate precursors. The atomistic nature of geopolymer precursors and binders remain largely elusive due to their inherent amorphicity and heterogeneity; nevertheless, pair distribution function analysis is one experimental technique capable of elucidating accurate structural representations of these amorphous materials, when combined with advanced molecular simulation methods. Here, it is shown that, when analysed in isolation, some valuable information can be gained from pair distribution functions of geopolymer precursors and binders. However, when used in conjunction with molecular simulations such as density functional theory and coarse-grained Monte Carlo analysis, there is the potential to generate accurate atomistic representations revealing new information regarding local structural environments. A novel methodology combining real-space refinements and density functional simulations in an iterative manner (DFT-PDF) has been used to generate an accurate structural representation of the geopolymer precursor metakaolin. The structural representation obtained from this technique reveals the existence of III-coordinated aluminium, which exemplifies the power of the DFT-PDF iterative methodology in probing uncommon chemical environments in materials. The potential for density functional theory-based coarse-grained Monte Carlo analysis to elucidate the structure of geopolymer binders and other heterogeneous materials is also discussed.

Structure of kaolinite and influence of stacking faults: Reconciling theory and experiment using inelastic neutron scattering analysis

The structure of kaolinite at the atomic level, including the effect of stacking faults, is investigated using inelastic neutron scattering (INS) spectroscopy and density functional theory (DFT) calculations. The vibrational dynamics of the standard crystal structure of kaolinite, calculated using DFT (VASP) with normal mode analysis, gives good agreement with the experimental INS data except for distinct discrepancies, especially for the low frequency modes (200-400 cm(-1)). By generating several types of stacking faults (shifts in the a,b plane for one kaolinite layer relative to the adjacent layer), it is seen that these low frequency modes are affected, specifically through the emergence of longer hydrogen bonds (O-H⋯O) in one of the models corresponding to a stacking fault of -0.3151a - 0.3151b. The small residual disagreement between observed and calculated INS is assigned to quantum effects (which are not taken into account in the DFT calculations), in the form of translational tunneling of the proton in the hydrogen bonds, which lead to a softening of the low frequency modes. DFT-based molecular dynamics simulations show that anharmonicity does not play an important role in the structural dynamics of kaolinite.

Extracting differential pair distribution functions using MIXSCAT

Differently weighted experimental scattering data have been used to extract partial or differential structure factors or pair distribution functions in studying many materials. However, this is not done routinely partly because of the lack of user-friendly software. This paper presents MIXSCAT, a new member of the DISCUS program package. MIXSCAT allows one to combine neutron and X-ray pair distribution functions and extract their respective differential functions.

Mechanism of zinc oxide retardation in alkali-activated materials: an in situ X-ray pair distribution function investigation

Alkali-activated materials are a new class of sustainable materials that can help supplant highly CO2-intensive ordinary Portland cement (OPC). Chemical admixtures and additives that manipulate the hydration and setting of OPC are readily utilized in the construction industry. However, for alkali-activated materials, the impact of these additives on the evolution of the atomic structure of the binder gel is largely unknown. Here, we utilize nano-ZnO (nanoparticles of zinc oxide), a known retarder for OPC hydration, and investigate its influence on the alkali-activation reaction in high and low calcium alkali-activated materials (slag and metakaolin systems, respectively). Using isothermal calorimetry and in situ X-ray pair distribution function analysis, the mechanism of nano-ZnO retardation in alkali-activated materials is uncovered, revealing that calcium plays a pivotal role in dictating whether nano-ZnO has an impact on the alkali-activation reaction. These results also provide important insight on the ability of slag and metakaolin-based alkali-activated materials to effectively immobilize zinc within the binder gel, which is of relevance to the waste solidification/stabilization community.