Bound water composite cement pastes

Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of non-evaporable water, referred to the ignited weight. Copeland et al. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression ... 

The content of chemically bound water is approximately that retained on equilibration at 11% RH of a sample not previously dried below saturation. For fully hydrated pastes of typical cements, it is about 32%, referred to thel ignited weight (F13,T35). There are no systematic data relating this quantityi to cement composition. The total content of water essential for completei hydration in a saturated paste is defined as that present in such a pastel having the minimum w/c ratio at which complete hydration is possiblel... 

The loss below the CH step is due to decomposition of C-S-H and the hydrated aluminate phases. Although the TG curves of pure AFm phases are markedly stepped in this region (Fig. 6.2), those of cement pastes normally show only slight indications of steps. Weak peaks can, however, sometimes be seen on DTG curves. The absence of steps is probably due to a combination of low crystallinity, the presence of other phases and the presence of AFm phases of different compositions in mixture or solid solution or both. For typical experimental conditions with a 50 mg sample, heating rate of 10 deg C min and Nj flow rate of 15 ml min , the volatiles retained at about 150°C, after correction for COj, correspond to the non-evaporable water, and those retained at about 100"C to the bound or 11% RH water, but this last temperature, in particular, is very dependent on experimental conditions (T5). 

Fig. 4.62 The bound water content in Portland cement pastes measured at difTerent time and curing temperature (according to [194]). Cement composition C3S—58%, CjS—22%, C,A—8%, C,AF—5.4%, gypsum—3.4%, w/c = 0.25...

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