Mercury 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...

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. 

The earlier literature contains several references to missing porosity, meaning the difference between mercury porosities and total water porosities. The difference clearly arises mainly from the failure of mercury to enter vacated interlayer space but has also been attributed to the encapsulation of gel by CH (D33) or to the presence of large (> 15 pm), closed pores that are not entered (A 15). 

Fig. 9.6 compares curves obtained on first intrusion for pastes of Portland and composite cements. At early ages, pfa or slag cement pastes are more porous than comparable Portland cement pastes because of the relatively slow reactions of the mineral additions, but the pore size distribution curves indicated by MIP are essentially similar in shape. For mature pastes, the apparent distributions for the composite cements indicate that there is a greater proportion of fine porosity. For the pfa cement paste, the mercury porosity obtained at maximum pressure are greater than that of the Portland cement paste. Day and Marsh (D32) agreed with Feldman that discontinuity... 

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). 

Hg-injection curves give us the pore structures measurements such as mercury porosity, distribution of Hg-saturation versus pore-throat size specific surface area deduced from... 

The information on pore size cannot always be directly correlated to the macroscopic measurements obtained by mercury porosity because the definition of a pore is different. In porosimetry, pore entrance sizes are measured, the volume of the cavity to which it gives access is attributed to the diameter of the entrance whilst, in a cross-section or fracture of the same carrier, all the cavities generated by the assembly of particles are viewed. 

Mercury porosity measurements are carried out in the pressure range 14 Pa (ambient) to 415 Pa, which corresponds to pore radii from 2 nm to 200 pm. 

Scanning electron microscopy (SEM) carried out on these crystals reveals different behaviors according to the drying conditions tested. The crystals obtained from dioxane are similar to blocks of sintered particles since they consist of microcrystallites botmd to each other, as if partially welded. In fact, the measurement of mercury porosity indicated a very high porosity. This sintered aspect concords with the non-transmission of hght through the whole crystal. 

The composition of the final support was determined by X-ray fluorescence on the calcined samples. Mercury porosity has b n measured with a Carlo Erba PORO 2000 device and surface area by the usual BET method. 

Figure 3. and pore volume distribution by mercury porosity. Samples calcined at 700 C,... 

Mercury porosity Porous system Little sampling damage Physical decay, structural damage... 

In Unger and Fischer s study of the effect of mercury intrusion on structure, three samples of porous silica were specially prepared from spherical particles 100-200 pm in diameter so as to provide a wide range of porosity (Table 3.16). The initial pore volume n (EtOH) was determined by ethanol titration (see next paragraph). The pore volume u (Hg, i) obtained from the first penetration of mercury agreed moderately well with u fEtOH),... 

Porosity and pore-size distribution usually are measured by mercury porosimetry, which also can provide a good estimate of the surface area (17). In this technique, the sample is placed under vacuum and mercury is forced into the pore stmcture by the appHcation of external pressure. By recording the extent of mercury intmsion as a function of the pressure appHed, it is possible to calculate the total pore volume and obtain the population of the various pore sizes in the range 2 nm to 10 nm. 

Surface Area and Permeability or Porosity. Gas or solute adsorption is typicaUy used to evaluate surface area (74,75), and mercury porosimetry is used, ia coajuactioa with at least oae other particle-size analysis, eg, electron microscopy, to assess permeabUity (76). Experimental techniques and theoretical models have been developed to elucidate the nature and quantity of pores (74,77). These iaclude the kinetic approach to gas adsorptioa of Bmaauer, Emmett, and TeUer (78), known as the BET method and which is based on Langmuir s adsorption model (79), the potential theory of Polanyi (25,80) for gas adsorption, the experimental aspects of solute adsorption (25,81), and the principles of mercury porosimetry, based on the Young-Duprn expression (24,25). 

The phases and their proportions present ia hardened amalgam are controlled by many factors. The composition of the alloy the size, shape, and size distribution of the particles the thermal history of the cast ingot and the comminuted alloy and the surface treatment of the particles are some of the factors for which the manufacturer is responsible. The tooth cavity preparation and the mixing, compacting, and finishing techniques of the dentist can make the difference between satisfactory and unsatisfactory restorations, even with the best of alloys. A minimal amount of residual mercury and porosity are needed to obtain the most serviceable restorations (138). 

Porosity and surface area are routinely measured by nitrogen absorption-desorption, mercury intrusion, and low-angle X ray. The electron microscope (EM) provides direct visual evidence of pore size and pore-size distribution. Thus, a combination of EM and conventional methods of pore-size measurement should provide reliable information on the pore structure of polymers. 

Figure 1.5 shows the cumulative pore volume curve for 5-/rm monosized porous PS-DVB particles with 50, 60, and 70% porosity. The curves were drawn by overlapping the measurements from nitrogen adsorption-desorption and mercury intrusion. A scanning electron micrograph of 5-/rm monosized particles with 50% porosity is shown in Fig. 1.6 (87). 

The H-type cell devised by Lingane and Laitinen and shown in Fig. 16.9 will be found satisfactory for many purposes a particular feature is the built-in reference electrode. Usually a saturated calomel electrode is employed, but if the presence of chloride ion is harmful a mercury(I) sulphate electrode (Hg/Hg2 S04 in potassium sulphate solution potential ca + 0.40 volts vs S.C.E.) may be used. It is usually designed to contain 10-50 mL of the sample solution in the left-hand compartment, but it can be constructed to accommodate a smaller volume down to 1 -2 mL. To avoid polarisation of the reference electrode the latter should be made of tubing at least 20 mm in diameter, but the dimensions of the solution compartment can be varied over wide limits. The compartments are separated by a cross-member filled with a 4 per cent agar-saturated potassium chloride gel, which is held in position by a medium-porosity sintered Pyrex glass disc (diameter at least 10 mm) placed as near the solution compartment as possible in order to facilitate de-aeration of the test solution. By clamping the cell so that the cross-member is vertical, the molten... 

Anode electrode Sintering temp. ("C) Initial porosity (%) FHH equation (Nitrogen adsorption) Mercury porosimetry Average Ds- Contact Angle with electrolyte 0C)... 

Determination of the porosity of a tablet presents the classic problem of defining the appropriate volume to be measured. The displacement medium may be able to penetrate the most minute crevices, as is the case for helium. Other displacement media, such as mercury, are unable to enter the smallest tablet crevices and thus produce different porosity values. Standardization of displacement media is therefore necessary for comparative evaluations. 

Flow Tests. One foot long sand packs using Wilmington oil field unconsolidated sand were prepared for each of the flow tests. Porosity and permeability of all the sand packs were within 30-35% and 100-300 md, respectively. All core packs were evacuated to about 1 mm of mercury (Hg) before saturating them under gravity to assure complete water saturation. Table III gives the core and fluid properties for the flow tests. The properties of the cores were chosen so that they are close to the field conditions reported by Krebs(15). 

The hydrophobic gas layer of the air electrode [4] possesses high porosity (ca. 0,9 cm2/g), such that an effective oxygen supply through this layer is obtained. From the experimental porogrames measured by both mercury and 7 N KOH-porometiy the contact angle 0en of the hydrophobic material with water electrolytes is obtained (0eff =116° 118°). Because of... 

The porosity of the materials were characterized by nitrogen (N2) adsorption-desorption at 77K, mercury (Hg) intrusion and electron microscopy (TEM) (table 1). 

Brenner, A.M., Adkins, B.D., Spooner, S., and Davis, B.H. 1995. Porosity by small-angle x-ray scattering (SAXS) Comparison with results from mercury penetration and nitrogen adsorption. J. Non-crystal. Solids 185 73-77. 

Measurements of particle porosity are a valuable supplement to studies of specific surface area, and such data are particularly useful in the evaluation of materials used in direct compression processes. For example, both micromeritic properties were measured for several different types of cellulosic-type excipients [53]. Surface areas by the B.E.T. method were used to evaluate all types of pore structures, while the method of mercury intrusion porosimetry used could not detect pores smaller than 10 nm. The data permitted a ready differentiation between the intraparticle pore structure of microcrystalline and agglomerated cellulose powders. 

Mercury porosimetry is based on the fact that mercury behaves as a nonwetting liquid toward most substances and will not penetrate the solid unless pressure is applied. To measure the porosity, the sample is sealed in a sample holder that is tapered to a calibrated stem. The sample holder and stem are then filled with mercury and subjected to increasing pressures to force the mercury into the pores of the material. The amount of mercury in the calibrated stem decreases during this step, and the change in volume is recorded. A curve of volume versus pressure represents the volume penetrated into the sample at a given pressure. The intrusion pressure is then related to the pore size using the Washburn equation... 

---