Non-evaporable and bound water

As with calcium silicate pastes, the gelatinous nature of the principal reaction product renders any definition of chemically bound water somewhat arbitrary. The three definitions of water content described in Section 

2 for calcium silicate pastes are relevant to cement pastes, viz. non-evaporable water, chemically bound water and water seemingly essential to the formation of the hydration products in a saturated paste. 

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 above 550°C is due partly to CO2 and partly to the final stages of dehydration of C-S-H and the hydrated aluminate phases. It is not practicable to distinguish the contributions from TG evidence alone, and, unless evolved gas analysis is used, a separate determination of COj should be made. As with calcium silicate pastes, serious errors arise if TG determinations are carried out on material that has been treated with an organic liquid, e.g. to stop hydration. Losses above 550°C of more than about 3%, referred to the ignited weight, indicate serious carbonation either from this or other causes. 

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