Powers-Brownyard model

Brunauer and co-workers (B55,BI08) considered that the gel particles of the Powers-Brownyard model consisted of either two or three layers of C S-H, which could roll into fibres. D-drying caused irreversible loss of interlayer water, and the specific surface area could be calculated from water vapour sorption isotherms, which gave values in the region of 200m g for cement paste. Sorption isotherms using N2 give lower values of the specific surface area this was attributed to failure of this sorbate to enter all the pore spaces. 

The Feldman-Sereda model was based on the studies of sorption properties, porosities and relations between water content and physical properties. Alone among the proposed models, it is clearly compatible with the microstructural evidence and with the probable relationships between C-S-H gel and crystalline compounds. It is incompatible with that of Brunauer, but not with the essential features of that of Powers and Brownyard in its original form if the nature of the gel porosity is reinterpreted. Calculations of bound water (Section 7.3.3) indicate that about a third of the gel porosity of the Powers-Brownyard model is interlayer space, the remainder being micro or fine meso porosity of the kind shown in Fig. 8.4. However, as that figure illustrates, the boundary between interlayer space and micropores is ill defined. 

Table 8.1 Calculated porosities, based on the Powers-Brownyard model...

Parrott and co-workers (P30,P32,P35,P33) described a more sophisticated method for modelling the hydration process. The fraction of the total water porosity that was below 4nm was calculated by multiplying the volume fraction of C-S- H by an appropriate factor, which depended on whether the C-S-H was formed from alite or belite, the temperature and the amount of space available. The constants assumed were based on experimental data obtained using a procedure based on methanol sorption (Section 8.3.4). The effect of drying was allowed for (P35) by introducing a factor of 0.7 - -1.2(RH — 0.5) for 0.5 < RH < 1, or of 0.7 for RH 0.5. These refinements allow some deviation from the Powers-Brownyard postulate of a fixed volume ratio of gel porosity to product. Typical results for the volume fractions of pores larger than 4 nm in mature pastes of a cement with an alite content of 56% were approximately 0.26, 0.16 and 0.07 for w/c ratios of 0.65, 0.50 and 0.35, respectively (P32). For the two higher w/c ratios, these results are near the capillary porosities of Powers and Brownyard, but for w/c 0.35 the latter value is zero. 

Water sorption isotherms for hep show marked hysteresis. Powers and Brownyard (P20) found that, while it was difficult to obtain reproducible desorption curves, the low-pressure part of the water vapour resorption curve varied little with w/c ratio, between different Portland cements, or, if allowance was made for the contents of unreacted cement, with the degree of hydration. This was their main direct evidence for the conclusion (Section 8.2.1) that the properties of the hydration product considered in their model were essentially independent of these variables. However, the water sorption iostherms obtained by different investigators have varied considerably (e.g. Refs P20 and S79), and it is not clear to what extent the above conclusion would stand had different desorption conditions been used. 

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