Capillary suction, 5.28

When water comes into contact with a porous material such as concrete, it is absorbed rapidly by the underpressure in the pores caused by what is called capillary action. This action depends on the surface tension, viscosity, and density of the liquid, on the angle of contact between the liquid and the pore walls and on the radius of the pore. In concrete, the contact angle is small due to the presence of molecular attraction between the liquid and the substrate (that is, between water and cement paste). Under these conditions, a drop will spread on a flat surface, while the meniscus of a capillary pore will rise above the level of the surrounding liquid and be concave towards the dry side in Chapter 14 we will see how this aspect can be changed by hydrophobic treatment. 

In theory, capillary action is stronger as the pore dimensions decrease. On the other hand, the smaller the pores become, the slower the transport will be due 

A sorptivity test is normally carried out to measure capillary suction in concrete. 

The bottom surface of a previously dried sample is placed in contact with water at atmospheric pressure. In general, capillary absorption is measured as mass (or volume) of liquid absorbed per unit of surface (i, g/m for mass or m /m for volume) in time t (a test normally lasts from four to 24 h). The development of i as a function of time is of the type  

This relationship, empirically derived from the observation of experimental data, is commonly used to define the parameter S. This is correct only for very porous materials or in the early stage of capillary action. In fact, in concrete with a low w/c ratio, the square root law (Eq. (6)) is changed to a power law with exponent lower than 0.5. 

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Slurry or slip casting provides a relatively inexpensive way to fabricate unifonn-thickness, thin-wall, or large cross section shapes [4o, 44, 45, 46, 42 aiid 48]. For slip casting, a slurry is first poured into a porous mould. Capillary suction then draws the liquid from the slurry to fonn a higher solids content, close-packed, leather-hard cast on the inner surface of the mould. In a fixed time, a given wall thickness is fonned, after which the excess slurry is drained. 

Capillary Suction Processes. The force needed to remove water from capillaries increases proportionately with a decrease in capillary radius, exceeding 1400 kPa (200 psi) in a 1-p.m-diameter capillary. Some attempts have been made to use this force as a way to dewater sludges and cakes by providing smaller dry capillaries to suck up the water (27). Sectors of a vacuum filter have been made of microporous ceramic, which conducts the moisture from the cake into the sector and removes the water on the inside by vacuum. Pore size is sufficiently small that the difference in pressure during vacuum is insufficient to displace water from the sector material, thus allowing a smaller vacuum pump to be effective (126). 

Funicular state is that condition in diying a porous body when capillary suction results in air being sucked into the pores. 

For transporting foam, the critical capillary pressure is reduced as lamellae thin under the influence of both capillary suction and stretching by the pore walls. For a given gas superficial velocity, foam cannot exist if the capillary pressure and the pore-body to pore-throat radii ratio exceed a critical value. The dynamic foam stability theory introduced here proves to be in good agreement with direct measurements of the critical capillary pressure in high permeability sandpacks. 

Experiments to determine specific resistance, based on Equation 7, have usually been carried out by some form of vacuum filtration. These methods are time-consuming and subject to error. More rapid techniques such as the measurement of capillary suction time (CST) can be used (8), although these do not give absolute values of specific resistance. Nevertheless, the CST method is very useful for rapidly obtaining comparative data on the flocculation of fairly concentrated suspensions by polymers (9). In the present work, specific resistance has been determined by an automated technique, which will be described below. 

The questions of interest to an engineer in this case are How do the initial concentration, the particle size, and the nature of the interparticle potential affect the structure of the dispersion, the structure of the final specimen, and the processing time How long does the process take What kinds of chemical additives are suitable The permeability and the capillary suction in the mold determine the rate of production of the specimens. How does one adjust the two to optimize production These questions require a basic understanding of colloid and surface science and phenomena. 

Low capillary suction - decreases the rate of film thinning. 

Capillary suction draws water into these structures, and moves their cell walls from a glassy to rubbery state as they hydrate. 

The second approach to characterize wetting considers the ability of the fluid to penetrate a powder bed. It involves the measurement of the extent and rate of fluid rise by capillary suction into a column of powder, better known as the Washburn test. Considering the powder to consist of capillaries of radius R, the equilibrium height of rise he is determined by equating capillary and gravimetric pressures, or... 

On top of these intrinsic contributions to 17 there may be additional ones, like capillary suction (exerted on the ends of the film) and external fields such as gravity. 

So far we have described in some detail the development of wetting layers by accretion. Obviously, the other direction is also possible a thick, macroscopic liquid layer on a solid can thin (by gravity, evaporation or capillary suction) until the layers become of colloidal thickness. Eventually, the same final situation should be attained, unless the dynamics of dewetting leads to hysteresis. 

Although the maximum pore radius distribution in conventional gypsum microstructures usually lies within the range of capillary suction action (100 nm to 1 mm) [44, 52], the roughly 50% water absorption that occurs in untreated gypsum is often reduced to less than 5 wt% [44, 45] at standard market application rates of just 0.3 - 0.5 wt% H-siloxane. The optimum amount to use depends upon the gypsum raw material, the fineness of the grain in the plaster of Paris, its specific sur ce area, and the reaction temperature. 

Formation of the layer in a plaster mould can be compared to filtration of a suspension through a partially dewatered layer at the boundary by the effect of capillary suction of the mould pores. As has been proved by many authors, the layer thickness x... 

The rate of body formation can be controlled by the amount and size of pores in the plaster mould, since capillary suction is given by tbe relationship Ap = 2y cos0/r, where y is surface tension of suspension, 0 is the angle of contact, r is the radius of the capillary pore. At a certain Ap, the rate of body formation reaches a maximum (cf. Satava and Pribylova, 1969). This is why the problems of plaster mould porosity and its control are very significant. 

A major difference between the two methods is the process by which the liquid is removed from the slurry. In slip casting, the liquid is removed by capillary suction provided by a porous mold. In contrast, the liquid is removed by evaporation in tape casting. In most tape casting processes, the liquid is nonaqueous because the higher volatility allows faster evaporation. 

Surface tension—the capillary suction created by a receding liquid meniscus can be extremely high. 

Surface Tension These effects are very common and worth a few more comments. Capillary suction created by a receding liquid meniscus can create very high pressures for collapse. The quantitative... 

Mobile liquid binding produces cohesion through interfacial forces and capillary suction. Three states can be distinguished in an assembly of particles held together by a mobile liquid (Fig. 21-92). 

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