Free water porosity

Fig. 8.5 Relations between porosities (volume percentages) and water/ccmcnt ratio for mature Portland cement pastes. The experimental data are for pastes at least 8 months old, and the calculated curves relate to a typical cement aged 18 months. Open symbols total water porosities. Filled or half-filled symbols mercury porosities. Curve A total water porosity. Curve B free water porosity. Curve C capillary porosity. References to data O (P20) O (S77) A (F33) V (M68) (S78) (F34) 9 (019) (M68) (D3I) 3 (H4I). In the last two cases, porosities by volume were estimated from data referred in the original sources to masses of dried paste, assuming the tatter to have contained 0.23 kg of water per kg of cement having a specific volume of 3.17 x 10 m kg h...

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. 

Typical results (Fig. 8.7) show that the distribution moves to smaller values as hydration proceeds. The observed porosity is mainly in the 3 nm to 1 pm range for young pastes, and in the 3-100 nm range for mature pastes. For mature pastes of low w/c ratio, which according to the Powers-Brownyard theory consist entirely of hydration product, nearly all the porosity is below 50 nm (S77). We shall refer to the porosities obtained using mercury at the maximum pressures employed as mercury porosities. Typical values for mature pastes (Fig. 8.5) are somewhat lower than the calculated free water porosities. 

These results suggest that, in attempts to relate strength to porosity, the total water porosity should not be used, the capillary or free water porosity or the volume of pores above a certain size being more appropriate. Parrott and Killoh (P30), in relation to their modelling of properties, similarly considered the volume, size and continuity of the larger pores to be the relevant quantities. 

Calculations based on reaction stoichiometry and densities of phases support the conclusions from experimental observations that mature pastes of composite cements are more porous than comparable pastes of Portland cements. This is indicated by the results in Table 7.3, 9.4 and 9.6. Similar calculations for 180-day-old pastes of w/s 0.45 indicate free water porosities of about 24% for a typical Portland cement, 35% for a cement with 40% slag, 35% for one with 40% pfa and 32% for one with 30% microsilica. The calculated values are in all cases somewhat higher than observed mercury porosities (F34,F41). 

As previously mentioned, a minimum level of soil moisture is necessary for successful biodegradation. The continuous circulation of air during bioventing results in the evaporation of soil moisture. For this reason, the design of these systems must include an appropriate installation for adding water to the contaminated zone. Care must be taken to avoid the addition of excess water. If soil moisture is significantly increased, e.g., above the limit of 85%, air circulation is no longer effective due to the decrease in free soil porosity. 

In order to increase the flow rate without too much pressure, Experiment 4 was performed with a Fann filter press which has a wider cross sectional area. A constant air pressure of 100 psi was applied, the flow rate was 26 times that of Experiment 1 while the NaCl concentration was only slightly higher than that of Experiment 1. Although the flow rate was much increased in Experiment 4, the result was similar to Experiment 1. The water retained in the clay (Column 8) determined by drying was found to be close to the amount of anion-free water. The porosity of the sediment was 0.4 and the average pore diameter was 4466 X. It was concluded from this experiment, that the anion-free water was immobile even at 100 psi and 7.4 ft/day. The pore size distributionQof the sample showed 90% of the pores to have a diameter above 350 A and less than 3% of the pores to have a diameter below 100 X (Figure 4). 

Anion-free water determined by using a chloride ion electrode agrees well with data given in the literature. (2) A new equation has been proposed for the bound water calculation. (3) The mobility of the anion-free water was found to be affected by pressure, porosity and electrolyte concentration. (4) Compaction experiments indicated that the anion-free water will not move until all the bulk water has been removed. (5) It is possible to increase the ratio of bound water to bulk water in a sample through compaction experiment. 

In addition to the above suite of well logs, the newest type of log was obtained via NMR (here called CMR) as shown in Figure 7.37. In this new method, the capillary, clay-bound, and free water (on the right) as determined by the NMR log, are subtracted from the total porosity as determined by the density tool (not shown) to obtain the hydrate saturation in the middle column. 

Figure 7.37 (See color insert following page 390.) 5L-38 CMR logs showing hydrate extent at depths between 900 and 930 m. Note that hydrates are obtained by the difference (middle column) between the total porosity as determined by density (not shown), and the capillary, clay-bound and free water determined by NMR.

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. 

In an aquifer, the total Fickian transport coefficient of a chemical is the sum of the dispersion coefficient and the effective molecular diffusion coefficient. For use in the groundwater regime, the molecular diffusion coefficient of a chemical in free water must be corrected to account for tortuosity and porosity. Commonly, the free-water molecular diffusion coefficient is divided by an estimate of tortuosity (sometimes taken as the square root of two) and multiplied by porosity to estimate an effective molecular diffusion coefficient in groundwater. Millington (1959) and Millington and Quirk (1961) provide a review of several approaches to the estimation of effective molecular diffusion coefficients in porous media. Note that mixing by molecular diffusion of chemicals dissolved in pore waters always occurs, even if mechanical dispersion becomes zero as a consequence of no seepage velocity. 

The ion-salt complex also includes free water in pore dead ends of open porosity, which does not participate in the flow (Figure 2.8, a). For this reason, the total amoxmt of immobile water within open porosity may be noticeably greater than physically-bonded. This amount depends on properties of the ground and rates of flow, and may sometimes be assumed equal to the difference between open and effective (active) porosity. In loams and clays the immobile water can take the entire volume of open porosity. 

The role of fiber porosity on dyeing behavior as discussed by Zollinger and coworkers also included the influence of dye size [349] and the relative amounts of bound and free water... 

Equations T3.5 and T3.6 in Table 4.3 are obtained by considering the activation energy for diffusion as a function of material moisture content. Equations T3.7 through T3.10 are not based on the Arrhenius form. They are empirical and they use complicated functions concerning the discrimination of the moisture and temperature effects (except, of course. Equation T3.7). Equation T3.11 is more sophisticated as it considers different diffusivities of bound and free water and introduces the functional dependence of material moisture content on the binding energy of desorption. Equation T3.12 introduces the effect of porosity on moisture diffusivity. 

Solution. Volume porosity may be determined by completely immersing a dry sample of known weight in water and boiling to eliminate air. After boiling, the water is adjusted to a measured volume. The free water is then poured off and measured. The difference between the total volume and the volume of the free water is the total external volume of the solid. The internal pore space is equivalent to the difference between the wet and dry weight of the sample (assume a specific gravity of unity for water). This difference in weight divided by the total external volume of the solid (as determined above) is the volume porosity ratio. This value is approximate and does not include porosities of less than 10 A diameter. ... 

A solute diffuses through free water in a gel. Due to the screening effect and the presence of bound water, the diffusion rate will slow. It is necessary to understand the diffusion coefficient D in the gel and the release rate of the solute when any DDS is to be designed [2]. If the volume fraction of the free water, namely the porosity fraction, is , ) can be given by the Mackie and Meares equation [6]. 

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