A New Investigation of the Cl−-CO32− Substitution in AFm Phases
Résumé
Chloride to carbonate substitution in AFm phase of general formulae [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) has been characterized by studying 12 powdered samples synthesized at 25° and at 85°C. The existence of a solid solution has been highlighted. It forms one stable rhombohedral [Ca2Al(OH)6]·[Cl1−x·(CO3)x/2·(2+x/2)H2O] solid solution with 0.25≤x≤0.95, isotypic with the recently described inline image 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 shows 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 are simultaneously present, in quite equivalent weight amounts, in samples synthesized at room temperature, whereas only the stable rhombohedral polymorph is observed for samples synthesized at 85°C. Raman spectroscopy has been successfully used to locate carbonate in the structure. The [CO3] symmetric stretching mode is observed at 1068 cm−1 for monocarboaluminate (x=1) for carbonate directly bonded to main layer, while it is observed at 1086 cm−1 for both chloro‐carboaluminate polymorphs, which is characteristic of interlayer carbonate.