Compositional and structural principles

AFm phases have layer structures derived from that of CH by the ordered replacement of one Ca ion in three by AF or Fe (Fig. 6.1) (A7-A9). The principal layers thus defined alternate with interlayers containing the X anions, which balance the charge, and H2O molecules. The replacement of Ca by the smaller AF or Fe ions distorts the structure of the principal layer, alternate Ca ions moving in opposite directions from its central plane. This allows each to coordinate the oxygen atom of an interlayer HjO molecule in addition to its six OFI ions. The principal layer, together with the H2O molecules thus bonded to the Ca ions, has the composition [Ca2(Al,Fe)(0Fl)g-2Fl20]. In the simpler AFm structures, these units are stacked in such a way as to produce octahedral cavities surrounded by three FljO molecules from each of the adjacent layers. These cavities may contain 

X anions, HjO molecules, or both. In CjA CaClj lOHjO (or [CajAKOHlg- IHjOjCl), each cavity contains a Cl ion. In C3A CaS04-12H20, one half contain SO anions and the remainder contain two HjO molecules. In C4AH13, all contain one OH anion and one HjO molecule. 

The unit cells of all AFm phases are based on hexagonal structural elements with a = 0.57-0.59 nm. These values are somewhat less than 3/3 times that of CH. The layer thickness, c, depends on the nature of the X anion and the amount of interlayer water, which can be varied by stepwise dehydration from the highest hydration state. AFm phases exist in which a complete, additional layer of H O molecules is present between the principal layers. There is no restriction on the separation between adjacent layers this allows large anions, and also intercalated neutral molecules (D19,D20), to be present in interlayer sites. Many AFm phases readily undergo changes in 3 water content and anion exchange (F24,D20), the latter including that of CO3 for OH . Due care must therefore often be taken to exclude CO, and where necessary also to control the humidity. 

Complex sequences of stacking the layers, or ordered patterns of filling off the cavities where their contents are not all the same, lead to unit cells that 1 

The layer thicknesses indicate that the CO3 ions are oriented perpendicular to the layers in C4AC0.5H12 and parallel to them in the other phases containing COf . In all these phases, some octahedral cavities contain a COj ion, and others varying combinations of H2O molecules and OH ions. In the early literature, C4ACHH (or C3A CC H,) was incorrectly identified as a C3A hydrate, and C4AC0 5H,2 as a polymorph of C4AH13. Both phases readily form on carbonation of the C4A hydrates. 

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