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.

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