Calcium silicate coordination

Ft is doubtful whether XRD can distinguish a mixture of tobermorite and C-S-H from a uniform material of intermediate crystallinity the situation may lie between these extremes (A30). This question is discussed further in Section 11.7.4. Crystallization is probably favoured by low bulk density its extent is apparently minimal in calcium silicate bricks (P49), but considerable in aerated concretes (A30). In cement-silica materials, substantially all the AljOj appears to enter the C-S-H, which as its Ca/Si ratio decreases can accommodate increasing amounts of tetrahedrally coordinated aluminium (S70). NMR results (K34) support an early conclusion (K62) that 1.1-nm tobermorite, too, can accommodate aluminium in tetrahedral sites. Small amounts of hydrogarnet have sometimes been detected, especially in products made from raw materials high in AljOj, such as pfa or slag. Minor amounts of tricalcium silicate hydrate (jaffeite C, S2H,) have sometimes been detected (A29,K61). 

Portland cement and high-alumina cements contain, in addition to calcium silicate phases, calcium monoaluminate, CaAl204 (or CA in cement chemist s shorthand, where C = CaO and A = AI2O3). The Al NMR spectra of this compound, in which the Al is exclusively in tetrahedral coordination, and a number of other calcium alu-minates have been determined (Muller et al. 1986), and more recently, using satellite transition spectroscopy (SATRAS) which has allowed the multiple tetrahedral sites in the various calcium aluminates to be distinguished (Skibsted et al. 1993). The NMR parameters for the synthetic aluminates and a number of their hydration products are shown in Table 5.4. 

On the other hand, it is likely that most reactions of silica and silicates in aqueous solutions do involve a temporary expansion of the coordination number from 4 to 5 or 6. Also in certain crystalline silicates, the silicon atom is coordinated with six oxygen atoms, for example, in thaumasite (82). This was also confirmed by Edge and Taylor (83). who worked out the structure of this calcium silicate sulfate carbonate crystal and demonstrated the presence of Si(OH), " ions. 

As mentioned previously, because of the hydrophilic character of the clay minerals, water molecules can intrude into the interlayer spaces. The swelling of the layer leads to extremely low water permeability of layer silicates. Water permeability, however, strongly depends on the cations in the interlayer spaces. The different cations coordinate the different number of water molecules (Berend et al. 1995 Cases et al. 1997). Monovalent ions (e.g., sodium ions) decrease water permeability greater than bivalent ions (e.g., calcium ions). It can well be seen in saline soils where water permeability is unfavorable. 

Figure 8.29. Relationship between the shifts of some inorganic calcium compounds and the mean Ca-O distance of the coordination polyhedron. Full circles denote oxides, full squares denote silicates and full diamonds denote carbonates. The open symbols denote sulphates and phosphate.

Although as has been stated above, most biominerals are based on what is readily available to organisms for forming structures, calcium and silicate-based systems, there are some very important lessons to be learned by coordination chemists aiming towards new materials . We need only think of the strength of rather light bones, which are some ten times stronger than ordinary concrete. When we consider that it is necessary to reinforce concrete with iron wires to achieve an... 

A third group of sodium rare earth silicates is formed by the smallest lanthanides from erbium to lutetium and yttrium. The original description of the stmctures as Pbn2j (Maksimov et al., 1967) was later changed to Pcn2i, for NaYSi04 (Merinov et al., 1979) after a different choice of axes. This structure is isostmctural with Ca2Si04, but with the octahedrally coordinated calcium atoms replaced by sodium and rare earth atoms. 

Figure 3.14 The coordination of water molecules about the Cc atoms situated above the ditrigonal hole in the silicate layer. The coordination is larger than for the mx and calcium atoms and very distorted. 

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