In situ synchrotron X-ray pair distribution function analysis of the early stages of gel formation in metakaolin-based geopolymers
Abstract not provided
White, Claire E↗
Engineering topics
White, Claire E
Publications and source records attributed to White, Claire E.
X-ray microtomography shows pore structure and tortuosity in alkali-activated binders
Durability of alkali-activated binders is of vital importance in their commercial application, and depends strongly on microstructure and pore network characteristics. X-ray microtomography ({mu}CT) offers, for the first time, direct insight into microstructural and pore structure characteristics in three dimensions. Here, {mu}CT is performed on a set of sodium metasilicate-activated fly ash/slag blends, using a synchrotron beamline instrument. Segmentation of the samples into pore and solid regions is then conducted, and pore tortuosity is calculated by a random walker method. Segmented porosity and diffusion tortuosity are correlated, and vary as a function of slag content (slag addition reduces porosity and increases tortuosity), and sample age (extended curing gives lower porosity and higher tortuosity). This is particularly notable for samples with {>=} 50% slag content, where a space-filling calcium (alumino)silicate hydrate gel provides porosity reductions which are not observed for the sodium aluminosilicate ('geopolymer') gels which do not chemically bind water of hydration.
36 MATERIALS SCIENCE↗
Effect of Temperature on the Local Structure of Kaolinite Intercalated with Potassium Acetate
Kaolinite intercalated with potassium acetate is of great interest in the areas of environmental remediation and industrial application; however, its exact atomic structure and the changes which occur when heated have remained largely elusive. Here, neutron pair distribution function analysis is used to investigate the local structural characteristics of this complex material, revealing that hydrated potassium acetate exists as a single layer in the interlamellar spacing of kaolinite. Furthermore, the potassium ions within the intercalated complex are most likely associated with the resonance structure of the acetate molecules, and upon heating (and decomposition of the carbon containing molecules), these ions become strongly associated with the negative charge located on the oxygen atoms in the alumina layers of dehydroxylated kaolinite. Several possible orientations of hydrated potassium acetate within the interlamellar spacing of kaolinite have been proposed and investigated using density functional modeling, revealing the complex nature of this material. Nevertheless, this investigation has shown that the dehydroxylated form of the intercalated compound contains highly strained alumina and available alkali (potassium), making it a viable alternative to traditional aluminosilicates.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
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.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
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.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗