Ettringite structure

Pollmann, H., Kuzel, H. J. Wenda, R. 1989. Compounds with ettringite structure. Neues Jahrbuch fur Mineralogie, Abhandlungen, 160, 133-158. 

Hassett, D. J., McCarthy, G. J., Kumarathasan, P. Pflughoeft-Hassett, D. 1990. Synthesis and characterization of selenate and sulphate-selenate ettringite structure phases. Materials Research Bulletin, 25, 1347-1354. 

The type and concentration range of the AFt solid solutions are given in many reports by Pollmaim et al. [91-94]. The solid solutions in the ettringite structure can be formed within the following range of compositions [92] ... 

Leachability of trace elements from CCP is a function of their type and hydration behaviour. The leachability of metals from lignite and sub-bituminous CCP is lower than from bituminous CCP due to the formation of hydrated minerals. Leachability of Se and B from Fort Union and PRB CCP is probably controlled by metal substitution within the ettringite crystal structure (Hassett Hassett 1988 Hassett et al. 1989 Kumatharasan et al. 1990 Solem-Tishmack... 

Incorporation in cement minerals will lead to a similar relationship, which may be described by a distribution ratio with the exception that uptake may be much greater than that of surface sorption ( ). Such mechanisms may apply to C-S-H, AFt, and AFm phases. The mechanism of incorporation may be by isomorphic substitution of a particular species within a crystal lattice. A good example here is the exchange of SO in ettringite for another anion. Another possibility is the adsorption to sites within a crystal structure, as may occur at silicate sites within C-S-H. 

Moore, A. W. Taylor, H. F. W. 1970. Crystal structure of ettringite. Acta Crystallographica Section B Structural Science, 26, 386-393. 

Fig. 6.4 Crystal structure of ettringite. (A) Part of a single column in (1120) projection A = Al, C = Ca, H = O of an OH group, W = O of an HjO molecule. Hydrogen atoms are omitted, as are the H2O molecules attached to those calcium atoms lying in the central vertical line of the figure. (B) Projection on the aft-plane, showing columns (large circles) and channels (small circles) the unit cell, with d = 1.123 nm, is outlined. Modified from Struble (S59).

The ettringite crystals on the surface of the cement particles are so fine that they cannot bridge the gap between the cement particles and therefore do not form a solid structure. This is the basis of the delay in solidification upon adding gypsum to cement. Without gypsum the tricalcium aluminate immediately reacts with water to calcium aluminate hydrate, which fills the space between the cement particles with its large crystals and leads to very rapid solidification of the cement slurry. 

None of these hypotheses were verified experimentally [87]. The specific smface area of ettringite crystals is much lower than the surface of C-S-H and the latter one does not shows swelling [87]. Finally, there is no room for more water in the crystal structure of ettringite (there are 26 molecules in the stracture) [87]. 

The structure of monoclinic ettringite was determined by Taylor and Moore [95], This is formed of colurtms of empirical formula Ca3[Al(0H)g]12H20 +, parallel to the c axis. There are the charmels between the columns with SO ions and other... 

Fig. 9.21 Crystal structure of ettringite (according to [95]) a projection along the c axis columns (H) and tetrahedra SO (S). b details of column structure p—ions at z=0 and 1/2 (superposition), q and r—Ca for z= 1/4 and % respectively. Empty circles—OH" ions coordinated around AP+ ions or Ca + ions or water molecules coordinated only with Ca + inside the column. Ca—O bond at z= 1/2 plotted with continuous lines, at z=-1/2 with dashed lines. All the coordinates z are given as a fraction of the pseudo-cell height (4,072 pm)...

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