NEW INVESTIGATION OF THE Cl(-)-CO3(2-) SUBSTITUTION IN AFm PHASES

Chloride to carbonate substitution in the AFm phase of general formula [Ca2Al(OH)6].[Cl1-x.(CO3)x/2.(2+x/2)H2O] (or 3CaO.Al2O3.(CaCl2)1-x.(CaCO3)x.(10+x)H2O written in cement notation) was characterised by studying 12 powdered samples synthesised at 25 and at 85 C. The existence of a solid solution was noted. It formed one stable rhombohedral [Ca2Al(OH)6].[Cl1-x.(CO3)x/2.(2+x/2)H2O] solid solution with x = 0.25-0.95, isotopic with the recently described R3c chloro-carboaluminate of [Ca2Al(OH)6].[Cl0.5.(CO3)0.25.2.25H2O] composition. A second metastable monoclinic [Ca2Al(OH)6].[Cl1-x.(CO3)x/2.(2+x/2)H2O] solid solution has also been identified in a less extended x domain. The structural relationship between these two stable and metastable chloro-carboaluminate polymorphs showed similarities with the structural transition of the Friedel's salt (from rhombohedral high-temperature-structure to monoclinic low-temperature-structure at about 35 C). The two [Ca2Al(OH)6].[Cl1-x.(CO3)x/2.(2+x/2)H2O] solid solutions were simultaneously present, in equivalent amounts, in samples synthesised at room temperature, whereas only the stable rhombohedral polymorph was observed for samples synthesised at 85 C. Raman spectroscopy was used to locate carbonate in the structure. The [CO3] symmetric stretching mode was observed at 1068 cm-1 for monocarboaluminate (x = 1) for carbonate directly bonded to main layer, while it was observed at 1086 cm-1 for both chloro-carboaluminate polymorphs, which is characteristic of an interlayer carbonate.