Synchrotron radiation pair distribution function analysis of gels in cements

Abstract

The analysis of atomic ordering in a nanocrystalline phase with small particle sizes, below ≈5 nm, is intrinsically complicated becausef the lack of long-rangerder. Furthermore, the presence of additional crystalline phase(s) mayxacerbate the problem, as is the case in cement pastes. Here, we use theynchrotron pair distributionunction (PDF) chiefly to characterize the local atomicrder of the nanocrystalline phases, gels, in cement pastes. We have used a multi-range analysis approach, where the ~4-7 nm-range allows determining the crystalline phase contents, the ~1-2.5 nm-range is used to characterize the atomic ordering in the nanocrystalline component, and the ~0.2-1.0 nm-range gives insights about additional amorphous components. Specifically, we have prepared our alite pastes with variable water contents, and the analyses showed that a defective tobermorite, Ca11Si9O28(OH)2·8.5H2O, gave the best fit. Furthermore, the PDF analyse suggest that the calciumilicate hydrate gel is composed of this tobermorite and amorphous calcium hydroxide. Finally, this approach has been used to study alternative cements. The hydration of monocalcium aluminate and ye’elimite pastes yield aluminum hydroxide gels. PDF analyse show that these gels are constituted of nanocrystalline gibbsite, and the particle size can be a small as 2.5 nm.