Determination of porosities by pyknometry

Total water porosities are obtained experimentally from the loss in weight when a saturated paste is D-dried or subjected to some procedure regarded as equivalent, such as heating to constant weight at 105 C. Reabsorption of water by materials thus dried gives identical results (D32). The procedures should be carried out under COj-free conditions. A value for the specific volume of the evaporable water must be assumed this has usually been 1.00 X lO m kg . Fig. 8.5 includes typical values thus obtained. 

Powers and Brownyard (P20) found that porosities of D-dried pastes obtained using helium as the pyknometric fluid were lower than those obtained using water. Feldman (F33) confirmed this and found that a lower value was also obtained using methanol if a D-dried sample was exposed to 

Feldman (F33) explained his results in terms of the Feldman-Sereda model as follows. If the paste is D-dried, water is lost from the interlayer spaces. If the sample is immersed in water, this water is reabsorbed, and the total water porosity therefore includes the volurrie of the interlayer space. Methanol does not penetrate into this space, and helium does so only slowly, so that lower porosities are obtained. If the sample is equilibrated at 11% RFI, the interlayer spaces are largely filled, and the lower value is obtained irrespective of the fluid used. The helium porosities reported by Feldman were near to the calculated free water porosities. 

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