The interlayers in LDHs are also occupied by water molecules accompanying the anions counterbalancing the excess of positive charges created by the cation isomorphic substitution in the hydroxides sheets.
[2][3][4] The thickness of the interlayer is sufficient to host a variety of relatively large anions often present as impurities: B(OH)−4, SeO2−4, SeO2−3...[5][6] As other LDHs, AFm can incorporate in their structure toxic elements such as boron[5] and selenium.
[6] Some AFm phases are presented in the table here below as a function of the nature of the anion counterbalancing the excess of positive charges in the Ca(OH)2 hydroxide sheets.
To convert the oxide notation in LDH formula, the mass balance in the system has to respect the principle of the conservation of matter.
The anions X– counterbalancing the positive charge imbalance born by the sheet are located in the interlayer whose spacing is much larger than in the layered structure of brucite or portlandite.
When the carbonate concentration increases in the hardened cement paste (HCP), hydroxy-AFm are progressively replaced, first by hemicarboaluminate and then by monocarboaluminate.
The stability of AFm phases increases with their carbonate content as shown by Damidot and Glasser (1995) by means of their thermodynamic calculations of the CaO-Al2O3-SiO2-H2O system at 25 °C.
[1] As stressed by Matschei et al. (2007), the impact of small amounts of carbonate on the nature and stability of the AFm phases is noteworthy.