Basic chemistry of autoclave processes

Understanding of the chemistry of autoclave processes is due primarily to the work of Kalousek and co-workers (K32,K59-K62). Above about I. SO C. for the time scales of a few hours that are used in practice, two features of cement hydration chemistry are added to those relevant at lower temperatures. Firstly, the hydration products tend to crystallize in the absence of reactive silica, C-S-H tends to be replaced by a structurally unrelated, crystalline phase, a-CjS hydrate. Secondly, the range of siliceous materials having effective pozzolanic properties is widened, and includes quartz and various other crystalline minerals, if sufficiently finely ground. 

Menzel s results were thus explained. Addition of a small amount of quartz brings the bulk Ca/Si ratio to 2.0, which is that of u-C,S hydrate. This phase is relatively dense and crystallizes as rectangular tablets the product is porous and weak. With larger amounts of quartz, pozzolanic reaction occurs the CFl is consumed, formation of u-C,S hydrate is avoided, and a C-S-Fl of low Ca/Si ratio is formed. The porosity is reduced, and a strong material results. The optimum addition of quartz is the maximum that can be taken up further additions act only as a diluent, and the strength decreases. Table 11.1 gives crystal data for u-C S hydrate and other phases. 

Name l.l-nm tobermorite l.l-nm tobermorite a-Dicalcium silicate hydrate 

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). 

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