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The Mechanism of Masonry Decay Through Crystallization SEYMOUR Z. LEWIN One of the most common and extensive sources of deterioration of stone, brick, mortar, plaster, and concrete is the consequence of crystallization phe- nomena that take place in pores, channels, and cracks at and near exposed surfaces. 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. The former is a function of temperature, air humidity, and local air currents. The latter is controlled by surface tension, pore radii, viscosity, and the path length from the source of the solution to the site of the evapo- ration. The detailed nature of this balance determines the form that the decay will take. If the rate of resupply of solution to the surface is sufficient to keep pace with the rate of evaporation, the solute deposits on the external surface and is characterized as an efflorescence. If the rate of migration of solution through the pores of the masonry does not bring fresh liquid to the surface as rapidly as the vapor departs, a dry zone develops just beneath the surface. Solute is then deposited within the stone at the boundary between the wet and dry regions, generating spells, flakes, or blisters. The site of crystal deposition can be predicted by applying the physical- chemical laws governing capillarity, viscous flow, and diffusion. These con- siderations disclose the quantitative relationship between the porosity of the masonry and the dimensions of the flakes, blisters, or spells that develop, as well as the manner in which the decay progresses. Data from controlled experiments in which salt decay is induced in labo- ratory specimens, together with measurements on examples of salt decay in buildings and monuments in a variety of environments, confirm the validity of these insights. Seymour Z. Lewis Professor, Department of Chemistry, New York University. 120 .

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Mechanism of Masonry Decay Through Crystallization 121 Exposed stone and other masonry materials are subject to a number of deteriorating influences, chief among which are the effects of crys- tallization, freezing, acidic attack, and mechanical erosion. The reality and ubiquity of the phenomenon termed "salt decay" are recognized by many of those concerned with the conservation of buildings and monurnents,~-3 but the detailed- mechanism by which the crystalli- zation of waterborne substances can break up the surface of a some- what porous solid has not hitherto been objectively demonstrated. When water at 0 C changes into ice, there is a volume increase of 9 percent. If liquid water is confined in a pore or crack, and this phase transformation takes place, it is evident that the resulting expansive force can damage the host solid. It is also clear that susceptible materials can be dissolved by acidic substances generated from air pollutants {e.g., fossil-fuel combustion products!, microorganisms, associated minerals (e.g., sulfides that undergo oxidation), or vegetation. Such attack can destroy surface modeling and sculpted details and weaken internal induration that binds the grains of the solid together. Similarly, the manner in which mechanical abrasion {e.g., the sandblasting effect of wind-driven dust) erodes a surface is readily visualized. However, it is not irnrnediately evident why the deposition of a solute from a solution into a pore or crack at the surface of a solid should damage the latter. Consider, for example, the evaporation of a sodium chloride solution at-a stone surface. When, as a consequence of the escape of water vapor, the solution reaches saturation, it contains at ordinary temperatures about 26 percent solid matter by weight. Hence, a pore filled with such a solution con have only about one- quarter of its volume taken up by the residue left when evaporation is complete. 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) transforms at -high humidity into Na2SO4 10 H2O). Nevertheless it is a fact that the deposition of a simple, nonhydrat- able solid, such as sodium chloride, during the evaporation of its so- lution in the pores of stone and masonry, can disrupt the solid. The external manifestations of this disruption are similar to those produced by the freezing of water in the pores of the surface of the solid scaling, flaking, and blistering and/or crumbling of the surface. ..

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22 CONSERVATION OF HISTORIC STONE BUILDINGS It is the..purpose of this paper toiinvestigate whether the so-called salt decay is due solely to the deposition of solute at a stone surface, to determine the conditions under which salt decay occurs, and to establish the quantitative relationships between the type of decay ob- served and the physical properties of the liquid and solid phases in- volved. THEORY The experimental section of this paper demonstrates that salt decay . . . . . . .. . . . . . . . ... occurs only when solute.is deposited within the pores ot the solid that is, a certain distance beneath the external surface (usually a frac- tion of a millimeter to a few millimeters). This can occur when- the rate at which~water departs from the surface of the solid via evaporation is equal to the rate at which fresh solution is brought to the surface via. migration through the internal capillary system of the solid. If migration of solution to the surface is faster than the rate of cIrying, then liquid oozes out onto the exposed surface, and solute is deposited on top of that external surface. This corresponds to the formation of visible efflorescences.4 5 Although they may be unsightly, . and usually indicate that subsurface crystallization is occurring elsewhere, they -are not, per se, damaging to the stone. If the migration of solution toward the exposed surface is very slow, then very little deposition of solutes takes place. Whatever.deposition does occur is deep within the stone and does not manifest itself in the form of surface decay. 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. The principle of this mechanism is depicted schematically in Figure 1. Evaporation by Diffusion The ~ying-out of solution within a pore opening at the surface occurs by diffusion of water vapor through a layer of thickness ~ centimeter of the porous solid. The rate of diffusion, ~ grams per square centimeter per second, is expressed by Fick's first law: [ = D(dC/~, { 1 ) where D is the diffusion coefficient in cm2 see-i, and dC/dX is. the concentration gradient across the diffusion layer.6

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Mechanism of Masonry Decay Through Crystallization r I' is' FIGURE 1 Parameters involved in the proposed mechanism for masonry deterioration due to deposition of solute from solution. The masonry, M, is in contact with a reservoir of solution, S. 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. 123

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24 CONSERVATION OF HISTORIC STONE BUILDINGS Because the air at the surface of exposed masonry is generally in motion, the aqueous tension at the surface of the solid tends to be constant, and the concentration gradient can be expressed in terms of the difference in vapor pressures of the water at the solution surface, Ps' and in the ambient air, Pa' divided by the diffusion-layer thickness, a: ~ = D(PS Pa)(M W./Nk~/~. (2) In the steady state the rate of escape of water from 1 cm2 of exposed surface of the porous solid is equal to the diffusion rate, I, times the fraction of open area at the solid surface, Fs The latter is related to the porosity of the solid and is typically between 0.05 and 0.40 for natural stone and other masonry materials.7 Replenishment by Capillary Migration Solution is drawn to the surface of the porous solid by capilIarity. The interfacial tension, By, at the free surface of the liquid provides the driving force that draws the liquid to the surface through the capillary network from the source. The equilibrium pressure difference at the liquid surface in a circular pore as a result of the interfacial tension is given by the Laplace equation: Ups = 2 ~ cos sir, (3) 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. The flow of liquid through the capillary net- work under this driving force is governed by Poiseuflle's law: AV ~ R4 AP _ = . _ At 8~ L' (4) where R is the effective radius averaged over the total length L of the capillary network through which the flow is occurring, V is the volume of solution passing through 1 cm2 of pores in the time t, and ~ is the viscosity in poise {g see-i cm-.9 The term AP/L is the total gradient of pressure from the solution reservoir to the evaporation site.

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Mechanism of Masonry Decay Through Crystallization The driving force, UP, in an empty, uniform capillary of radius r is given by equation 3 above. As the liquid rises in the capillary, the driving force diminishes, since part of the surface pressure must provide the hydrostatic pressure to support the column of liquid of height h: 125 ~ ~ cos ~ Apnea = r hPg. 1,5) If there were no evaporation occurring, the liquid would rise in the capillary until the surface pressure and hydrostatic pressure became equal, i.e., apnea would be zero. 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. The frictional force limiting the rate of flow involves a different parameter, R. which describes an effective radius for the entire length of capillaries through which the liquid moves to get from the source to the evaporation site. It is shown in the experimental section that, operationally for ma- sonry, these two parameters generally will have quite different values. The reason is as follows. Whereas the surface (driving) force involves the inverse first power of the pore radius, the viscous "opposing) force involves the capillary radius raised to the fourth power. Thus, for small capillaries The condition for laminar flow is R ~ 1), the rate of Poi- seuille flow falls very rapidly as the radius decreases. This has the important practical consequence that in a porous solid containing a range of pore sizes, only the upper part of the pore-size-distnbution curve contributes significantly to the rate of flow of liquid to the evaporation site during the times involved in the wet-to-dry cycling of masonry in buildings and monuments. On the other hand, the sur- face force increases inversely as the radius decreases. Therefore, pores too small to participate significantly in the viscous flow do neverthe- less make an important contribution to the net driving force drawing the liquid through the capillary network. The Steady State In the steady state the rate of escape of water is equal to Fs times J. The rate of replenishment is equal to the Poiseuille flow rate, ~V/At, times the fractional area of the solid that consists of contributing

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126 CONSERVATION OF HISTORIC STONE BUILDINGS capillaries, Fp, times the liquid density, p, and weight fraction, Fw, of water in the solution. Thus: FsJ=FppFw(/\V//~15. Substituting equations 2, 4, and 5 into equation 6 yields: FSD(Ps Pa)(M.W./Nk~ Fp pFw=R4 (25/cosU _ h pa) \ r and (6) 17) 8 ~ L 8FsD~L(Ps Pa) (M.W./Nk 1) =- Fp p Fw ~ R4 { ~ cos ~ - h p g} r /l8) If, because of evaporation of water, solute crystallizes a distance ~ beneath the exposed surface of the porous solid, and if this is the source of the deterioration of the surface, then equation 8 permits the quan- titative prediction of the extent of the surface decay (i.e., the thickness of the flake, blister, or powder layer). Such prediction can be made directly and rigorously on the basis of the properties of the solid (po- rosity, pore-size distribution), the solution (concentration, vapor pres- sure, interfacial tension, viscosity, density), the solvent (diffusion coef- ficient, molecular weight), and the environment (temperature, relative humidity). Detenoration from NaC! 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. The following values will be taken as fairly representatively of this system: D = 0.22 cm2 see-i (for water vapor in air at 1 atm and 20 Call Ps = 19 tort = 0.025 atm (for 5.3 M NaCI)

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Mechanism of Masonry Decay Through Crystallization Pa = 10 tort = 0.0125 atm (for air at 60% relative humidity) M.W.=l~gmol-i Nk = 82.06 cm3 atm mol- ~ deg-i T p cos ~ = 1.00 = 293K 1.00 g cm-3 = 82.0 dyne cm-1 (for 5.3 M NaCl) 127 g = 980 dyne g- 1 h p g, the hydrostatic pressure term, will generally be negligible relative to the surface pressure term (2 my cos Or. Substituting these values into equation 8 yields the following result: = 3.17 x 10 L F {Fp R4) {91 which permits the prediction of the thickness of surface decay that will result from crystallization of sodium chloride. The prediction employs no arbitrary, empirical parameters; it is based on data on the location of the decay zone {L), the concentration of the salt in the reservoir of solution (~ /Fw), and the pore characteristics of the solid (Fs r/Fp R4~. EXPERIMENTAL TEST OF THEORY Design of the Experiment The validity of equation 8 has been tested by a series of laboratory experiments in which the conditions during the deposition of sodium chloride at an exposed stone surface were controlled and measured. The experimental arrangement is shown schematically in Figure 2. A rectangular sandstone column, 60 cm x 2.5 cm x 5 cm, was mounted in a glass vessel inside a Plexiglas box with its lower 5 cm immersed in a sodium chloride solution. The neck of the glass vessel was sealed with a plug of paraffin wax 1 cm thick so that liquid could not migrate up the external surface of the stone column. 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. A constant, uniform flow of air at 60 percent relative humidity

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128 . CONSERVATION OF HISTORIC STONE BUILDINGS ; ~ 1 NaCI Sol 'a Glass Beads for Support 1 1 _. .. it'= ~wc ~ ~ ~ ~ _~ _ J I Stone Column Paraffin Wax Seal Air at 20 C and 60% R.H. 1 ~ FIGURE 2 Experimental arrangement for producing salt decay. Lower end of masonry column is immersed in a salt solution, which is able to reach the exposed surface only by capillary rise through the interior of the stone. A uniform flow of air at controlled tem- perature and relative humidity is maintained over the exposed sur- face of the stone. and 20 C was maintained over the exposed surface of the stone col- umn. 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

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Mechanism of Masonry Decay Through Crystallization column's surface received solution faster than it dried out, and a heavy deposit of salt formed on the stone. With increasing distance from the reservoir of solution, the thickness of the salt deposit di- minished until at a certain height the surface of the stone appeared to be darker than the part above it. This indicated that the pores in the darker surface contained some liquid, but~very little salt was visible on the outer surface. The typical appearance of the stone column after a two-week run with a saturated salt solution is shown in Figure 3a and with a half-saturated solution in Figure 3b. The stone was damaged only in the region where the external deposition of salt had diminished to minute amounts that is, where the rate of arrival of solution at the exposed surface was approxi- mately equal to the rate of evaporation of the water, so that salt deposited within the surface rather than on ton of it. The damaged 129 ~ . . , ~ ~ ~ ~ . ~ ~ ~ . .' ~ .1 1 r - . 1 surface layer ot stone was nelo togetner ny tne sunsurtace crystals of sodium chloride. However, when the salt deposit was washed away, the area of decay became readily apparent as can be seen in Figure 4. This type of experiment was conducted on the same sandstone column employing solutions of sodium chloride at three different concentrations. 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. The location and thickness of the decayed zone were different for each of the different salt concentrations, as can be seen in Figure 5. Experimental Data Effective PoiseuiRe Rachus The data recorded in these controlled salt-deposition experiments rel- ative to the surface decay are summanzed in Table 1. To use these results to test the theory, it is necessary to evaluate the porosity of the stone. A type of measurement proposed here as particularly useful in this respect is the record of the rate of water imbibition as a function of the time of immersion of a test block. The data for the Longmeadow, New Hampshire, ferruginous sandstone employed in this work are shown in Figure 6. The total porosity is estimated from (a) the volume of water imbibed after extremely long room-temperature immersion, or, equivalently, (b) that which is taken up during five hours of immersion in boiling

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~ - - - - ~ l ~ - - O a' . ~ ~ ~ - - e~ it ~ ~ ~ ~ o o ~ ; _ ~ y m- ~ o ~ o ~4 _ ~~ -3 o,= o ~~.~ +, I ~ ~: )? ~ ~~ ~ ~ . c - 8 ~ ~ ~ Am. 3 ~-

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134 8 7 6 5 hi: m m CONSERVATION OF HISTORIC STONE BUILDINGS 4 Maximum Water Absorption _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - n 1 a I I I '` ~ I I o o 5 10 15 20 35 40 [TIME, hours] 1/2 , o 5 10 15 FIGURE 6 The rate of imbibition of water by a test block of New Hampshire sandstone yields information about the effective porosity for liquid flow through the internal capillary network. There is an initial rapid gain in weight due to the filling of the larger capillaries, then a very slow, diffusion-controlled process that requires marry months (in fact, more than three years) to fill all the interior spaces. 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. imbibition per 100 g of stone. Ibis value yields an effective fraction of intemal cross-sectional area participating in Poiseuflle transport of: Fp= {~2.20~0.042~2/3 = 0.20 cm2 capillary area per 1 cm2 of {11) stone cross section. From the comparison of total pore area with the area participating in Poiseuille flow (0.20/0.32), it follows that the upper 0.63 of the pore-

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Mechanism of Masonry Decay Through Crystallization 135 size-distribution curve should be considered in evaluating the effective radius, R. to be employed in equation 8. For the present study, this part of the curve has been estimated from the scanning electron mi- crographs, representative examples of which are reproduced in Fi-gure 7. The frequencies of occurrence in 3-,um-wide pore-size intervals per 1 cm2 of cross section in fracture surfaces have been estimated; these frequencies, multiplied by the average radius in the interval, have been raised to the fourth power; and the weighted average has been com- puted. The result is that the effective pore radius for Poiseuille flow in this stone is estimated as: R = {1.6 + 0.21 x 10-3 cm. (12) In this averaging technique the smallest pores those whose di- mensions were less than 0.1 Amwere not included. 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 Poi- seuille 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. This approach would also involve microscopic detection and counting of pores in cross sections of the stone and does not appear to offer any advantages of precision or convenience over the technique adopted in the present work. Effective Laplace Radius It remains now to estimate the effective pore radius at the stone surface that determines the surface (driving) force. This value is most reliably derived from the rate of advance of liquid through the capillary network of the stone when no evaporation is taking place, and when the hy- drostatic (retarding) force is negligible. Under these conditions the Washburn equational applies: ~ ~ ' 1,13) where x is the distance that the interfacial tension, A, draws the liquid of viscosity, A, through a capillary of radius, r, in time, t. Figure 8

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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 occur- rences 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.

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Mechanism of Masonry Decay Through Crystallization 137 d FIGURE 7 Continued

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38 CONSERVATION OF HISTORIC STONE BUILDINGS Sat'd. Vapor Enclosure - Ru ler Column Support T-- -1 _ \ _ I \ ~ , Liquid ~ n~ umn Constant Flow Device :] . _ _ Constant ~ , _ _ Level ~ _~ ~ <, ___ ,..... _ - 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. The inner bent tube in the constant flow device ensures that the position of contact of the liquid source with the stone column remains constant. shows a convenient arrangement for carrying out this type of mea- surement,~3 and Figure 9 shows the data obtained for the sandstone under study. The slope, m, of the graph of x versus tile is given by: ~ i/2 m= 2Y ~ (rcos0~/2, (14)

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J 10 8 6 4 f , ~ ~~ m = A/ yr/271 O At. I 0 2 - 4 6 8 10 12 14 16 18 20 a, FIGURE 9 Graph of height of capillary rise, x, versus tl'2 of contact of a New Hampshire sandstone column with a reservoir of 5.3 M NaC1 solution. The slope of the straight portion of the curve yields the effective Laplace radius, r = 0.28 ,um Contact angle taken to be zerol.

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140 CONSERVATION OF HISTORIC STONE BUILDINGS chloride, since these properties refer to the solution that is crystallizing at the stone surface and not to the solution in the interior, which may be more dilute. The internal path length, L, has been taken as equal to the vertical distance from the source of the solution to the decay zone (i.e., equal to hi. 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 ex- perimentally. The largest source of uncertainty in this test of the theory is the estimation of the effective Poiseuflle radius, R. It will be noted that the experimental results agree very satisfactorily with the pre- dicted values, within the precision of the data. In these experiments the order of magnitude of the thickness of the deteriorated surface layer resulting from these salt solutions in this particular sandstone proves to be between a fraction of a millimeter and one or two millimeters. Our observations, and those of others, in studies of the decay of exposed stone and masonry in buildings and monuments have disclosed that the natural decay of many other sand- stones and other types of natural stone and masonry results in surface losses of the same order of magnitude. That is, when initial salt decay is indicated i.e., when a single layer of stone has been lifted up in the form of a blister, spell, or flakethe thickness of the deterioration is in the vicinity of a millimeter. Thus, it appears that the parameters characteristic of the present experimental setup are similar to those commonly encountered in practical instances of crystallization-in- duced deterioration of masonry. CONCLUSIONS The present work demonstrates that the mechanism of the so-called salt decay of exposed stone and masonry consists in the deposition of solutes from solution within the pores of the solid close to the surface. This is characteristically manifested in the form of a thin layer of the surface that lifts up in the form of a blister, peels outward as a spell, flakes off, or powders away. The initial thickness of this surface decay is of the order of a millimeter. When this thickness of surface has separated, the decay process may be initiated again in the underlying, still sound stone, resulting in a second such decay layer under the first. The processes can then proceed again beneath these layers, and so on.

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Mechanism of Masonry Decay Through Crystallization 141 In some cases, many successive layers of decay can be recognized, all of them similar in character, with thicknesses from a fraction of a millimeter to 1 or 2mm. An example of the occurrence of blisters 1 mm thick on exposed granite is shown in Figure 10; examples of the multiplication of decay layers, progressing from the outer surface of the exposed stone toward the interior, are shown in Figure 11. 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 sub- jected 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.

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142 _ FIGURE lla Cross sec- tion of the surface layers of a salt-decayed sand- stone. CONSERVATION OF HISTORIC STONE BUILDINGS _> _ _ FIGURE llb A sandstone sculpture in the sculpture garden of the Brooklyn Museum, New York City, showing the development of multiple layers of surface decay, resulting from successive salt-decay processes proceeding from the outside inward.: the rate of evaporation of water via diffusion through a layer of the porous solid is balanced by the viscous flow of solution from the reservoir to that site through the internal capillary network. The quantitative relationship between the thickness of surface deterio- ration and the characteristics of the liquid and solid media are de- rivable from classical physical chemistry via the Fick and Poiseuille laws. The parameters needed to describe the porous nature of the

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Mechanism of Masonry Decay Through Crystallization 143 solid are obtainable from water imbibition, capillary rise, and pore- size-distribution measurements. Laboratory experiments conducted under controlled conditions yield results that are in good agreement with the predictions of this theory. 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. We do not yet understand how the requisite disruptive pressures can be developed in the pores of the solid. The fundamental question that remains to be addressed is: How do crystals that have grown from a solution until they fill the volume of a pore continue to grow at the areas of direct contact between crystal and pore wall? REFERENCES AND NOTES 1. S.Z. Lewin and A.E. Charola, Scanning Electron Microscopy in the Diagnosis of "Diseased" Stone, Scarming Electron Microscopy, 1978, vol. I, pp. 695-703, SEM, AMP O'Hare, Ill. 2. A.E. Charola and S.Z. Lewin, Examples of Stone Decay Due to Salt Efflorescence, Third International Congress on the Deterioration and Preservation of Stones, Venice, 24 October 1979, in press. 3. S.Z. Lewin and A.E. Charola, The Physical Chemistry of Deteriorated Brick and Its Impregnation Technique, Congress for the Brick of Venice, 22 October 1979, Venice, Proceed~ngs, pp. 189-214, University of Venice, Italy. 4. S.Z. Lewin and A.E. Charola, Aspects of Crystal Growth and Recrystallization Mechanisms as Revealed by Scanning Electron Microscopy, Scanning Electron M~cros- copy, 1980, vol. I, pp. 551-558, SEM, AMP O'Hare, Ill. 5. A.E. Charola and S.Z. Lewin, Efflorescences on Building Stones; SEM in the Characterization and Elucidation of the Mechanisms of Fonnation, Scarming Electron Microscopy, 1979, vol. I, pp. 379~87, SEM, AMP O'Hare, Ill. 6. W. lost, Diffusion in Solids, Laquids, Gases, Academic Press, N.Y., 1952, 8ff. 7. A.E. Scheidegger, The Physics of Flow Through Porous Media, Univ. of Toronto Press, 1960, p. 13. 8. F.A.L. Dullien and V.K. Batra, Determination of the Structure of Porous Media, In Flow Through Porous Media, American Chemical Society, Washington, D.C., 1970, 17ff. 9. T.L. Poiseuille, Gompt. Rend., 11, 961 jl8401; 12, 112 {1841~; 15, 1167 {18421. 10. There is some uncertainty concerning the appropriate value of the contact angle, 0, for this system. The data of D.D. Eley and D.C. Pepper, Trans. Faraday Soc., 42, 697- 702 {19461, suggest that ~ = 0 degrees for water In plugs of powdered Pyrex glass. The data of B.V. Deryagin, M.K. Melr~ikova, and V.I. Krylova, Colloid I. USSR, 14, 459 (19521, suggest ~ = 60 to 70 degrees for water spreading through a packing of quartz sand.

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144 CONSERVATION OF HISTORIC STONE BUILDINGS However, advancing contact angles tend to be different from receding angles, which tend to be zero; of. W. Rose and R.W. Heins, I. Colloid Sci., 17, 39 {19621. 11. A. Winkelmann, Wied. Ann., 22, 1, 152 ~18841; 23, 203 {1884i; 26, 105 {18851; 33, 445 ~18881; 36, 92 {18891. 12. E.W. Washburn, Phys. Rev., 17, 27~83 jl9211; see also V.G. Levieh, Physico- chemical Hydrodynamics, Prentiee-Hall, Englewood Cliffs, NJ., 1962, pp. 382-383. 13. J.N. Chan, Gypsum Plaster as a Prototype in the Study of the Physical Chemistry of Solid Porous Media, Ph.D. thesis, New York University, October 1980. 14. The tortuosity factor, T. is defined empirically as the correction factor needed to make the calculations for certain theoretical models of pore structure agree with ex- perimental data. F.A.L. Dullien calculates, based on his particular model of porosity, that the tortuosity for sandstone would be in the range of 1.5 to 1.7; see AIChE [. 21, 299 {1975i. However, he points out that "using different models of pore structure, widely different values may be obtained for T. some of which completely lack any physical meaning." {See Porous Media Fluid Transport and Pore Structure, Academic Press, N.Y., 1979, p. 22,71. It may be noted that if L is significantly greater than h (i.e., T > 1l, the calculated value of ~ will be too large by that tortuosity factor. This effect is in the opposite direction and would tend to offset any overestimation of ~ due to the contact- angle factor Cf. ref. 101.