Mechanisms of reaction and equilibria

Quartz reacts relatively slowly, and substantial proportions of C S-H having a Ca/Si ratio of about 1.75 are formed as intermediate products, which gradually react with it to give C-S-H of lower Ca/Si ratio. This process competes with the formation of a-CjS hydrate. The conditions should be such as to minimize the latter process, since subsequent reaction of u-CjS hydrate with quartz is slow and may not be completed in the time available. Fine grinding of the quartz and adequate mixing with the cement are therefore essential. 

The structure of the individual layers in tobermorites was described in Section 5,4.2. In l.l-nm tobermorites, these layers arc stacked in such a way that oxygen atoms of the bridging tetrahedra (Fig. 5.7) not attached to two silicon atoms can closely approach those of the adjacent layer. A suggestion (T56.M5I) that interlayer Si-O-Si links are present in anomalous tobermorites was confirmed by chemical (W32) and Si NMR (W21,KI6) studies of silicate anion type. The compositions and densities of naturally occurring tobermorites show that the contents of the pseudocell (Table 11.1) always deviate markedly from the idealized formula CsSj.Hs (M5I). There are at least three effects  

1-nm tobermorite is readily synthesized using CH and finely ground quartz at 180 C. It is much less readily formed if amorphous silica is used. Synthetic studies show that normal tobermorite is an intermediate in the transition from C--S--H(l) to anomalous tobermorite (E9.H61). Its formation in preference to anomalous tobermorite is favoured by short time, low temperature, high Ca/Si ratio, and presence of Al in the absence of alkali. The presence of Af together with alkali favours the formation of anomalous tobermorite (E9). The presence of alkali without Al has been variously found to impede crystallization (E9) or to favour formation of anomalous tobermorite (H61). Most of these results are readily explainable in terms of the structural differences described above. Al is also reported to accelerate the formation of tobermorite and to retard its replacement by. onotlite at temperatures around 175 C (K62). Mitsuda and Chan (Ml 12) found the tobermorite in some aerated concretes to be anomalous. Aluminium-substituted tobermorites have cation exchange properties (K63). 

Hara and Inoue (H61) studied the lattice parameters of tobermorites. They confirmed an early observation (K62) that c increases with aluminium substitution, but found that several other variables affect the parameters to smaller extents. The a-axial lengths of highly crystalline, natural tobermorites appear to be lower (0.560-0.562 nm for the pseudocell) than those of the synthetic materials (M5I). 

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