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The Mechanism of Masonry Decay Through Crystallization
Pages 120-144

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From page 120...
... Liquid water deposits dissolved matter wherever evaporation occurs. The site of this crystallization is determined by the dynamic balance between the rate of escape of water from the surface and the rate of resupply of solution to that site.
From page 121...
... Each repeated imbibition of salt solution can be expected to reduce the remaining free volume of the pore by one quarter of that value, until the pore is filled with deposited solute. But there is no analogy in this process to the expansive force that develops when water filling a pore transforms into ice or when certain types of solid phases filling a pore recrystallize into higher hydrates (as, for example, when sodium sulfate (Na2SO4)
From page 122...
... It is proposed herein that the necessary condition for surface decay is the establishment of a steady state in which the rate of diffusion of water through a thin layer of the porous solid at the surface is balanced by the rate of replenishment of water to that site from.the source freservoir) of the solution.
From page 123...
... Solute is deposited a distance ~ inside the stone at the height h above the reservoir. The radius of the pore opening at the site of deposition is r; the average radius of the channel through which solution migrates is R; the length of the migration path is L
From page 124...
... where ~ is the contact angle of wetting of the meniscus at the walls of the pore, and r is the radius of the pore at the liquid surface.8 If there is a distribution of pore sizes in the solid, an effective radius can be adopted that represents the weighted average of the contributions of the various pores to the resultant surface Strivings force. This driving force draws solution to the surface to replace that which departs via evaporation.
From page 125...
... When evaporation is occurring, a steady state tends to be established in which Apnea has that value which produces a Poiseuille flow just sufficient to balance the rate of escape of liquid at the evaporation site. The driving force in the Poiseuille equation involves the effective radius r at the height h.
From page 126...
... Crystallization One of the common types of salt decay is that caused by deposition of sodium chloride in stone and brick.3 The source of the salt may be seawater or groundwater, deicing practices, or aerosol particles. In this case the solution just below the exposed surface tends to be a saturated sodium chloride solution; the temperature is the ambient temperature, and the solution migrating within the solid is dilute.
From page 127...
... This served to confine all liquid migration to the internal capillary network of the stone. Salt solutions of known concentrations were fed into the glass vessel at the rate necessary to maintain the liquid there at constant level.
From page 128...
... Under these conditions a steady state was established within six to eight hours in which the solution migrated upward through the interior of the stone column to the exposed surfaces where the water evaporated, depositing the sodium chloride. The lower part of the
From page 129...
... Before each run, the stone column was removed from the apparatus, washed free of deposited salt, soaked for two weeks in daily changes of distilled water to remove any remnants of the previously imbibed salt solution, and dried. After each run, the site and depth of the surface decay were measured.
From page 131...
... 131 N a, Cat cn ,S: _ EM ~ O 0'~ C.)
From page 133...
... As has already been shown, only the larger pores in the distribution of pore sizes contribute significantly to the Poiseuille flow that brings solution to the exposed surface. These pores are responsible for the water imbibition that occurs during the first hour or several hours after immersion.
From page 134...
... Immersion of the same test block in boiling water for five hours, or in room-temperature water after exhaustive evacuation, yields the weight increase shown as "maximum water absorption." The fully water-saturated test block, allowed to air-dry, loses water much more rapidly and completely than it had imbibed-the water, and by a different mechanism, showing that evaporation occurs from most of the pores at the surface, whereas liquid migration occurs mainly through the larger capillaries. The test block was 7.2 x 5.5 x 15.4 cm; its dry weight was 1338.8 g; its dry density was 2.20 g/cm3.
From page 135...
... This does not seriously affect the validity of the resulting average, since, as has been shown, such small pores do not contribute significantly to the observed flow rate. An alternative method of estimating the effective radius for Poiseuille flow would be to measure the rate of effusion of liquid through a plug of the stone of known dimensions under a controlled driving force and divide by the number of capillaries contributing to the total flow.
From page 136...
... 136 CONSERVATION OF HISTORIC STONE BUlEDINGS FIGURE 7 Scanning electron micrographs showing the internal pore character of t New Hampshire sandstone. Magnification employed to estimate frequencies of occurrences of pores of average radius between: 7a 0.05 and 0.005 mm; 7b 10.0 and 1.0 ,um; 7c 5.0 and 0.5 ~m; 7d 1.0 and 0.1 ,um.
From page 137...
... Mechanism of Masonry Decay Through Crystallization 137 d FIGURE 7 Continued
From page 138...
... _ - Ruler Holder 1 FIGURE 8 Experimental arrangement for determining the Laplace radius effective in generating the surface pressure in a masonry specimen. The height, x, to which liquid has risen in the stone column is observed visually by means of the darkening effect of wetting as a function of time, t, of contact with the bulk liquid.
From page 139...
... The slope of the straight portion of the curve yields the effective Laplace radius, r = 0.28 ,um Contact angle taken to be zerol.
From page 140...
... That is, the tortuosity factor is taken as equal to unity for this rather porous solid.~4 The contact angle for the aqueous solutions against the polar x-quartz surfaces of this stone's pores is taken as zero degrees, and the interfacial tension is taken as that of a saturated NaCl-glass interface. The remaining relevant data are collected in Table 1, which also compares the values predicted by equation 8 with those observed experimentally.
From page 141...
... The necessary condition for the occurrence of this type of decay is the development of a steady state at the exposed surface, wherein FIGURE 10 Granite surface stone in the lower course of a New York City landmark building has been subjected to the action of salt used in de-icing the adjacent street. The surface has lifted up In numerous places, forming blisters with a layer thickness that ranges from 0.5 to 1.5 mm.
From page 142...
... The quantitative relationship between the thickness of surface deterioration and the characteristics of the liquid and solid media are derivable from classical physical chemistry via the Fick and Poiseuille laws. The parameters needed to describe the porous nature of the
From page 143...
... These considerations, and the related experimental observations, establish beyond reasonable doubt that the deposition from solution of a simple, nonhydrated salt, such as sodium chloride, in the pores at the surface of a stone generates pressures sufficient to break down the induration. We are convinced of the reality of the phenomenon and can now account for it in detail and predict where and under what conditions it will occur.
From page 144...
... is defined empirically as the correction factor needed to make the calculations for certain theoretical models of pore structure agree with experimental data.


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