Finally na17a15016 has discrete chains composed of

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Unformatted text preview: s to form a non-planar 12-membered ring which is then joined by pairs of oxygen atom bridges to adjacent rings, thus generating an infinite chain of alternating 12- and 8membered rings with A1-0, 175-179pm and Al-Ot 173-4pm. Finally, Na17A15016 has discrete chains composed of five A104 tetrahedra sharing comers with almost linear angles (160" and 173") at the bridging 0 atoms and with '*D. F. SHRIVER G. C. FARRINGTON, and Chem. and Eng. News, May 20, 42-57 (1985), and references cited therein. 5 9M. G . BARKER,. G . GADDand M. J. BEGLEY,. Chem. P J Soc., Chem. Cammun., 379-81 (1981). M . G. BARKER, P. G . GADD and S. C. WALLWORK, hem. Soc., Chem. J. C C~mmun.,16-7 (1982). 5 Ternary and more complex oxide phases 97.3.4 251 the various A1-0 distances again falling in the range 170-180pm. Note that the unusual formula Na17A15016 (i.e. Na3.4A103.2) is nearly the same as Na3A103 which would have been the stoichiometry if the chains of A104 tetrahedra had been infinite. Tricalcium aluminate, Ca&06 Tricalcium aluminate is an important component of Portland cement yet, despite numerous attempts dating back over the preceding 50 y, its structure remained unsolved until 1975.@') The basic unit is now known to be a 12-membered ring of 6 fused {A104} tetrahedra [A16018]~~as shown in Fig. 7.16; there are 8 such rings per unit cell surrounding holes of radius 147 pm, and the rings are held together by Ca ions in distorted sixfold coordination to give the struttural formula Ca9A16018. The rather short C a - 0 distance (226 Pm> and the observed compression of the {cao,} octahedra may indicate some strain and this, together with the large holes in the lattice, facilitate the rapid reaction with water. The products of hydration depend sensitively on the temperature. Above 21" Ca3A1206 gives the hexahydrate Ca3&06.6H20, but below this temperature hydrated di- and tetra-calcium aluminates are formed of empirical composition 2CaO.Al203.5-9H20 and 4CaO.Al203.12- 14H20. This is of great importance in cement technology (see Panel) since, in the absence of a retarder, cement reacts rapidly with water giving a sharp rise in temperature and a "flash set" during which the various calcium aluminate hydrates precipitate and congeal into an unmanageable mass. This can be avoided by grinding in 2-5% of gypsum (CaS04.2H20) with the cement clinker; this reacts rapidly with dissolved aluminates in the presence of Ca(OH)2 to give the calcium sulfatoaluminate, 3CaO.Al203.3CaS04.31H20, hich is much less w soluble than the hydrated calcium aluminates and 6 op.MONDAL nd J . W. a (1975). JEFFREY, Acta Cryst. B31, 689-97 Figure 7.16 Structure of the [A16018]'*+ unit in Ca3A1206 (i.e. Ca9A16018). The AI-0 distances are all i n the range 175 f 2 pm. therefore preferentially precipitates and prevents the premature congealing. Another important calcium aluminate system occurs in high-alumina cement (cirnent fun&). This is not a Portland cement but is made by f using limestone and bauxite with small amounts of Si02 and Ti02 in an open-hearth furnace at 1425-1500"; rotary kilns with ta...
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