Synchrotron X-ray pair distribution function analysis of tricalcium silicate pastes

Abstract

The analysis of nanocrystalline and amorphous components within cement pastes that contain high amounts of crystalline phases is very challenging. Synchrotron powder diffraction jointly with the pair distribution function (PDF) methodology is very useful to characterize complex cement pastes. PDF data can give information about the local structure and bonding environments of the non-crystalline components such as aluminate hydrate and calcium-(aluminum)-silicate-hydrate gels. The main goal of this work is to characterize amorphous and nanocrystalline gels which are present in cementitious pastes by total scattering PDF analyses in selected real-space ranges. Moreover, the PDF approach also allows quantitatively determining the nanocrystalline and microcrystalline contents. Several sets of tricalcium silicate pastes have been studied and the results are reported: (i) pastes from different tricalcium silicate polymorphs, such as monoclinic and triclinic; and (ii) the influence of selected parameters in triclinic tricalcium silicate pastes. For all the PDF analyses, a multi r-range approach was followed: the higher r-range (40 – 70 Å) is used to determine the microcrystalline phase contents, portlandite and unreactive alite; then, the intermedium r-range (10 – 25 Å) allows characterizing the atomic ordering in the nanocrystalline fraction of the C-S-H gel; and finally, the low r-range, below 10 Å, gives insight about the chemical nature of the additional amorphous component. It was concluded that for all the untreated tricalcium silicate pastes, the crystal structure of tobermorite-11 Å fits the nanocrystalline contribution of the C-S-H gel in the 10 – 25 Å r-region much better than tobermorite-14 Å.