A Raman Study of the Sulfated Cement Hydrates: Ettringite and Monosulfoaluminate

Ettringite (the AFt Ca6Al2(OH)12·(SO4)3·26H2O compound) and monosulfoaluminate (the AFm Ca4Al2(OH)12·SO4·6H2O compound) phases were synthesised by the coprecipitation method and studied by micro-Raman spectroscopy. Hydrogen bond network, involving sulfate anions, ensures the cohesion of the structure of these two compounds; the columnar struture of ettringite as well as the lamellar structure of monosulfoaluminate. Raman spectroscopy has been used as a probe to investigate mainly the inter-columnar and the inter-lamellar regions of the structures. Raman spectra allowed the characterization of the local environment of sulfate anions and the hydrogen bond networks. A significant asset brought by Raman spectroscopy is the ability to work on hydrated cement products without specific sample preparation; i.e. without a risk to dammage these hydrated compounds. Several examples of the possibilities brought by Raman spectroscopy in cement chemistry are given in this paper. A space group redetermination of the ettringite structure was performed on the basis of the point symmetry of sulfate tetrahedra. The inter-columnar region of ettringite was compared to the inter-lamellar region of monosulfoaluminate as these two structural parts contain the same species. The thermal behaviors at the beginning of the dehydration (up to 390 K) and the iron substitution in ettringite were also investigated.