Ettringite group

Next would come the hydrates containing columns of water molecules, such as are found in the ettringite group, Cag[Al(0H)8]a(S04)s-26HaO then sheets, such as those of the clay minerals or of gypsum and finally the cryohydrates with complete networks of linked waters, as in the case of the hydrates of the gases already mentioned. 

The ettringite group, also called AFt phase in cement paste, stands for Al-Fe-tri (tri=three moles of CS) ofthe formula C3A 3CS H32 in which A1 can be replaced by Fe to some extent. The AFt phase forms in the first few hours (Ifom C3A and C4AF) and plays a role in setting. After afew days of hydration only a little amount of it may remain in cement pastes. It appears as stumpy rods in SEM and the length does not normally exceed a few micrometers. The principle substitutions that exist in the AFt phase are Fe and Si" for AP+ and various anions such as OH", C03, and silicates for S04 . 

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).

Spectroscopic evidence for mechanism (2), arsenic adsorption on oxoanion precipitates, has been provided by Myneni (1995) and Myneni et al. (1997) in the arsenate-ettringite system. Arsenate was found to adsorb to different sites as a function of sorption density. At low sorption density, As oxoanions exchange for surface hydroxyl groups bonded to Al atoms and at higher sorption densities As oxoanions exchange for surface hydroxyl groups bonded to Ca atoms, with minor bonding to Al atoms (Table 2). 

A second area of the applicahon of cements for environmental purposes is that of the stabilization/immobilization of Hquid toxic industrial waste, and in parhcu-lar that of hazardous heavy-metal waste streams containing chromium, vanadium, cadmium, and other metals. As noted in Sechon 5.2.4.2, both Cr and V can enter the inter-chain spaces of ettringite to replace groups (Buhlert and Kuzel,... 

Alnmiiiinm group Alunite Alunogen Ettringite Lake white-. Transparent white... 

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