Density Functional Theory Study of the Crystal Structure and Infrared Spectrum of a Synthetized Ettringite Mineral

Abstract

Abstract: One of the most important hydration phases of Portland cement is ettringite, a calcium

sulfo-aluminate mineral (Ca6Al2(OH)12(SO4)3·26H2O) showing a great capacity of adsorbing radionuclides

and other contaminant cationic and anionic species, or incorporating them into its crystal

structure. In this work, the X-ray diffraction pattern and infrared spectra of a synthetized ettringite

sample are recorded and simulated, employing theoretical methods based on Density Functional

Theory. Despite the complexity of this phase, the calculated structure, X-ray diffraction pattern

and infrared spectrum are in excellent agreement with their experimental counterparts. Since the

calculated and experimental spectra are consistent, the main infrared bands are assigned using a

normal coordinate analysis, some of them being completely reassigned with respect to other experimental

works. The good agreement found provides strong support for the computational methods

employed towards their use for studying the surface adsorption properties and the incorporation

of contaminations in its structure. The density of reactive groups at the surfaces of ettringite is

reported, and the surface adsorption of water molecules is studied. These surfaces appear to be

highly hydrophilic, in agreement with the experimental finding that the ettringite structure may

include more water molecules, at least up to 27, one more than in its standard formula.