Synchrotron pair distribution function analyses of ye'elimite-based pastes

The study of nanocrystalline phase atomic ordering is intrinsically complicated. The presence of additional crystalline phase(s) exacerbates this challenge. Here, a synchrotron pair distribution function (PDF) is used to characterise the local atomic order of the nanocrystalline phases in ye'elimite-containing pastes. A multi r-range analysis approach has been used, where the 3-5 nm r-range allows the crystalline phase contents to be determined and the 0.16-3.5 nm is used to characterise the atomic ordering in the nanocrystalline components. Quantitative phase contents have been also derived from these analyses. Specifically, five stoichiometric ye'elimite pastes with variable anhydrite and water contents have been prepared. For the pastes without anhydrite addition, ettringite was not present and two calcium aluminium monosulfate phases were required for good PDF fits. In addition to crystalline calcium aluminium monosulfate (AFm) (c approximate to 2.86 nm), a nanocrystalline AFm phase with a significantly smaller c-unit cell parameter value, c approximate to 2.68 nm, was required, having about a 6 nm particle size. For a paste obtained with 31 wt% of anhydrite addition, the main crystalline phase was ettringite with the aluminium hydroxide having a nano-gibbsite local structure with quite a small average particle size, approximate to 3.5 nm.