A New Investigation of the Cl − –CO 3 2− Substitution in AFm Phases

Chloride to carbonate substitution in AFm phase of general formulae [Ca 2 Al(OH) 6 ]·[Cl 1− x ·(CO 3 ) x /2 ·(2+ x /2)H 2 O] (or 3CaO·Al 2 O 3 ·(CaCl 2 ) 1− x ·(CaCO 3 ) x ·(10+ x )H 2 O 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 [Ca 2 Al(OH) 6 ]·[Cl 1− x ·(CO 3 ) x /2 ·(2+ x /2)H 2 O] solid solution with 0.25≤ x ≤0.95, isotypic with the recently described chloro‐carboaluminate of [Ca 2 Al(OH) 6 ]·[Cl 0.5 ·(CO 3 ) 0.25 · 2.25H 2 O] composition. A second metastable monoclinic [Ca 2 Al(OH) 6 ]·[Cl 1− x ·(CO 3 ) x /2 ·(2+ x /2)H 2 O] 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 [Ca 2 Al(OH) 6 ]· [Cl 1− x ·(CO 3 ) x /2 ·(2+ x /2)H 2 O] 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 [CO 3 ] 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.