The middle period of hydration

Studies using ion-thinned sections, wet cells and backscattered electron images of polished sections show that a space develops between the shell and the anhydrous material (S40,S41,S68) (Fig. 7.6c). In this respect, the hydration of cement differs from that of C3S, in which the C-S-H grows directly over the C3S surfaces, without any detectable separation (S41). By 12 h, the spaces are up to 0.5 pm wide. They are likely to be filled with a highly concentrated or colloidal solution, and the shells are evidently sufficiently porous at this stage that ions can readily migrate through them (S68). The existence of spaces shows that reaction proceeds by dissolution and precipitation further evidence for this is provided by the fact that the C-S-H also deposits on the surfaces of pfa particles, if these are mixed with the cement (D28). Some other relatively unreactive or inert admixtures behave in the same way. 

Towards the end of the middle period, a renewed growth of AFt crystals takes place (D27,P3I,S4I) (Fig. 7.6d). They are markedly more acicular than those formed earlier their lengths are typically 1-2 pm, but sometimes up to 10 pm. Their formation is associated with a shoulder on the heat evolution curve (Section 7.5.1). Their formation implies an increase in the rate of reaction of the aluminate, or less probably the ferrite phase, which is probably related to the reaction of the alite (S68). 

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