XRD and combined SEM-EDS analysis of long-term hydration products of ye'elimite

Abstract

Ye'elimite (3CaO·3Al2O3·CaSO4) is a material that is of a growing interest particularly in terms of (i) ecological applications for immobilization from waste industrial aqueous solutions the hazardous ionic species and (ii) production of low CO2 emission ye'elimite-containing cements. In this regard, analysis of the hydration reactions and mechanisms of formations of the hydration products in “ye'elimite − H2O” system is of practical importance. The present study reports on results of the morphological, compositional, and microstructural analysis of the hydration products of ye'elimite that were formed in solution at water-to-ye'elimite ratio (by weight) of 3 and a long hydration time of 270 days. The X-ray diffraction, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) measurements showed that the precipitated hydration products comprise three main crystalline phases: ettringite, gibbsite, and calcite as well as the residual (i.e., partially hydrated) ye'elimite. Homogeneously precipitated individual particles of the hydration products/phases exhibit well pronounced morphologies that are typical of particles of these phases. The SEM-EDS measurements also provided a direct evidence of the heterogeneous formation on the surface of ye'elimite particles of a gibbsite coating that, according to a reasonable assumption in some literature reports, may be responsible for a strong retardation of the ye'elimite hydration. The hydration and carbonation reactions as well as the mechanisms of through-solution precipitation of hydration products are in detail analyzed. Strong carbonation of the ettringite particles and precipitation of the calcite with a large relative content provide an evidence of a high CO2 capture capacity of the hydrating ye'elimite. A comparative analysis of obtained results with those reported in literature is also conducted.