salt weathering: a selective review eric doehne

Salt Weathering: A Selective Review

ERIC DOEHNE

The Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, CA, USA90049-1684 (e-mail: [email protected])

Abstract: The past decade has seen a growing scientific interest in thestill poorly understood subject of salt weathering, a phenomenon withsignificant cultural and economic consequences. This interest has led toan increase in research results and growing clarification of the rolessalts play in weathering and decay. There are now over 1,800 researcharticles on the topic of salt weathering originating from severaldisciplines, as well as over 6,000 references on the general problems ofbuilding material decay. In order to navigate such a vast collection ofdata and knowledge, this article describes the multidisciplinary natureof the study of salt damage to porous building materials, provides aframework for considering the complexity of salt damage, and serves asa selective literature survey largely focused on recent work and thosearticles with relevance for conservation.

"In time, and with water, everything changes"

—Leonardo da Vinci

Over the past ten years, there has been increasingscientific interest in building material decayphenomena, especially the incompletelyunderstood subject of salt weathering. Thereaction of salts with moisture in rock outcropsproduces an intriguing array of weathering formssuch as tafoni, honeycombs and pedestal rocks,as well as abundant rock debris (Goudie & Day1980; Mustoe 1982; Smith 1994). Saltweathering of building materials can be equallydestructive (Haynes, O'Neill et al. 1996). Saltweathering is widely recognized as one of theprimary agents in the deterioration of historicalarchitecture, structures in archaeological sites,and archaeological objects (Schaffer 1932;Lewin 1982). Understanding and finding ways toreduce damage caused by salts holds greatsignificance for the conservation of materialcultural heritage (Torraca 1982; Amoroso &Fassina 1983; Goudie & Viles 1997). Moreover,damage to modern concrete building foundationscaused by salts has recently been the subject oflitigation leading to multi-million dollar

settlements and judgments (Haynes 2002;Kasdan, Simonds et al. 2002). Salt weathering isindisputably a process with profound culturaland economic consequences.

Salts have long been known to damage porousmaterials (Herodotus 420 B.C.; Luquer 1895)through the production of physical stressresulting from the crystallization of salts in pores(Taber 1916; Jutson 1918). Salts can damagestone and other building materials through arange of other mechanisms as well, such asdifferential thermal expansion, osmotic swellingof clays, hydration pressure, and enhancedwet/dry cycling caused by deliquescent salts(Smith 1994; Nash 2000). A 1997 book byGoudie and Viles summarizing salt weatheringhazards raised awareness of the problem andbroadened the field of investigation in recentyears. Despite this, many unresolved issuesrelated to salt weathering remain, ranging fromthe details of the damage mechanisms (Flatt2002) to the interactions between substrate,environment and salt type (Charola 2000;Blaeuer et al. 2001), and the expression of thisprocess in the landscape (Smith, Warke et al.

Geological Society Special Publication Natural Stone, Weathering Phenomena, Conservation Strategies and Case Studies. Vol 205, 51-64.

SALT WEATHERING ERIC DOEHNE

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2000) and in building stone (Kuchitsu, Ishizakiet al. 2000; Schwarz, Gervais et al. 2000).

This article is intended to describe themultidisciplinary nature of the study of saltdamage to porous building materials;communicate the complexity of salt weatheringand discuss open questions; and serve as alimited literature survey largely focused onrecent work and those articles with relevance forconservation.

Background

The majority of the published literature on saltweathering originates from a collection ofdistinct disciplines, each responsible for specificcontributions: geomorphology, environmentalscience, geotechnical and materials science, geo-chemistry, and, lastly, conservation, perhaps themost multi-disciplinary of the fields. There is arich literature in geomorphology on saltweathering phenomena, particularly thatpublished by scientists from the United Kingdom(Smith 1994; Goudie & Viles 1997). Environ-mental scientists have studied the important roleof salts in atmospheric aerosols and their fallout(Pósfai, Anderson et al. 1995; Torfs & VanGrieken 1997). The geotechnical field has dealtwith expansive sulfate soils, or salt heave,(Xiaozu, Jiacheng et al. 1999), concrete decayfrom salt crystallization (Haynes, O'Neill et al.1996; Mehta 2000; Haynes 2002), and testing ofbuilding materials using standardized saltcrystallization tests such as ASTM aggregatesound test C88-90 (ASTM 1997), and RILEMPEM/25 (RILEM 1980). Materials scientistshave provided useful insights on the fundamentalmechanisms of salt attack (McMahon, Sandberget al. 1992; Scherer 2000). In the geochemicalliterature, the concepts of pressure solution, forceof crystallization, and displacive growth areimportant to understanding that salt attack is apart of a larger set of interrelated behaviours(Carstens 1986; Maliva & Siever 1988; Minguez& Elorza 1994). Finally, the conservationliterature is vast and especially rich in casestudies and documentation of methods ofmitigating salt damage (Schwarz & Roesch1993; Young 1995; Siedel 1996; Charola,Henriques et al. 1998; Price 2000; Unruh 2001).German speaking conservation scientists,conservators and architects have published anumber of important books on the topic of salts

and cultural heritage (Hammer and Tinzl 1996;Arendt and Seele 2000; Kraus 2002).

Nevertheless, much of the conservation literatureon salts can be difficult to survey, since manyarticles are published in conference proceedings(Price 1996). Two recent summaries of the lit-erature illustrate the breadth of applicableresearch: an overview of the role of salts in thedeterioration of porous materials by Charola(2000) and a discussion of salts and crusts bySteiger (2002).

Following these recent efforts, this paperpresents a current survey of the multidisciplinarycontributions to the field of salts and buildingdeterioration. A large bibliography of researcharticles on salts and building deterioration phe-nomena was assembled in order to facilitate theintegration of these related researches. Someobservations based on this survey are discussedbelow.

A recitation of the sometimes significant,sometimes subtle, differences in terminology anddefinitions employed in the literature of saltweathering reveals the multidisciplinary natureof the scientific literature. The same basicphenomena are variously described in differentdisciplines and countries as salt attack, saltdamage, salt crystallization, salt crystallisation(UK), haloclastisme (French), and salzsprengung(German). Less common terms include salt burst,salt exudation, haloclasty, salt decay, saltcrystallization pressure, crystal wedging, saltfretting, and salt heave in soils.

In the geomorphology literature, the terms"honeycomb weathering", "alveolar weathering",and the less common descriptors "stone lattice","stone lace", and "fretting" (Mustoe, 1982),reference a characteristic form of salt weatheringin stone defined by irregular cavities in otherwiseregular rock surfaces, although in recentpublications those terms are increasinglydiscarded in favor of the term "tafoni" forcavities large and small. Substantial research hasbeen performed by geomorphologists on tafoni(singular: tafone), a term that normally refers topits and caverns in rock faces resulting fromextensive salt weathering of stone in salt-rich anddesert environments (Kirchner 1996; Turkington1998). Tafoni also occur in humid temperateenvironments; however, it is not yet clear thatsalt weathering plays a role in the formation oftafoni in this type of climate (Mikulas 2001).


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In the conservation literature, the term "risingdamp" is widely used to characterize saltweathering, but in Australia this process ofmoisture and salts mobilized by capillarity anddeliquescence is regionally referred to as "saltdamp" (Young 1995). Many buildings and sitesare also affected by salt-rich aerosols, which mayaccumulate and be mobilized by "falling damp"or "penetrating damp."

In the concrete literature, the terms "salt attack"and "sulfate attack" are sometimes incorrectlyused interchangeably (Mehta 2000), and recentauthors have proposed the terms physical saltattack or salt hydration distress as distinct fromthe chemical reaction of salts with concrete(Haynes, O'Neill et al. 1996; Hime, Martinek etal. 2001; Haynes 2002). Stone conservatorstypically use the terms desalting, salt extraction,or desalination, referring to salt reductionmethods using immersion, intermittent washing,poulticing or compress treatments.

Given the variety of terms used and the diversityof disciplines researching the topic of saltweathering, it seems clear that at least some ofthe apparent contradiction in the literature ismerely the result of differences in terminology(Hime, Martinek et al. 2001).

Salt Damage: Dealing with Complexity

Two major difficulties encountered by thoseinvestigating salt weathering are the multiplevariables involved and the fact that thephysiochemical reactions of greatest importanceoccur along thin, nanometer-scale films inside aporous solid. Such interactions are inherentlydifficult to study in situ and in tempo, furtherinhibiting understanding of the degree ofinteraction between possible variables. To aidunderstanding of the variables, it is useful toclassify them as properties of the substrate,solution, salt type, or environment (Fig. 1). Theproperties interact in specific ways determinedby thermodynamic equilibria and kinetic factorsto produce a range of salt behaviours.

It is important to note that not all salt behavioursresult in deterioration. For example, theproduction of surface efflorescence is oftenimpressive and highly visible, but generallyresults in little damage. Likewise, salt creep can

move salts over the surface of materials but, asthe name implies, is a surficial process. Saltssuch as magnesium sulfate can actually bindtogether (i.e., cement) previously fracturedmaterial.

Understanding the properties that contribute to aparticular weathering effect, and differentiatingbetween benign and damaging salt behaviors,allows the investigator to focus on the latter.Typical damage behaviors or processes for saltscan include surface scaling, deep cracking,expansion, granular disintegration, surfacepowdering and microcracking. For particulardamage mechanisms, such as crystallizationpressure, the degree of damage (sometimesmeasured as the damage factor, damage functionor dose-response function) is often attributable tospecific properties and can be the single mostimportant metric (Viles 1997). For example, inthe case of sodium sulfate crystallization, thedegree of supersaturation and the location ofcrystallization appear to be the keys tounderstanding the degree of damage (Rodriguez-Navarro & Doehne 1999a). In turn, there are afew important properties and kinetic factors,such as the evaporation rate, that largely controlhow much damage results from this mechanism(Fig. 1).

By focusing on the nature (i.e., moment, locationand type) and, especially, the degree of saltdamage, the field of variables, or properties,directly responsible can be narrowedconsiderably and investigators can begin toanswer the questions most relevant to specificdamage types. Why are certain types of stonemuch more vulnerable than other types to saltdamage (Goudie 1999b)? Why are certain saltsmuch more damaging than other salts (Hoffmann& Grassegger 1995; Aires-Barros & Mauricio1997; Rodriguez-Navarro & Doehne 1999a)? Isdamage caused mostly by relatively rare en-vironmental events (rapid cooling) or cumulativeeveryday stresses (humidity cycling)? What arethe long-term effects of various conservationtreatments, such as desalination or consolidation,on salt damage? How can desalination andpreventive conservation efforts be enhanced?Can general agreement be achieved regarding thefundamental mechanisms of salt weathering?What are the appropriate equations to use incalculating crystallization pressure (Duttlinger &Knöfel 1993; La Iglesia, Gonzalez et al. 1997;


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Substrate PropertiesSurface Area, Porosity, Pore Size Distribution, Permeability, Internal Surface Roughness, Adsorption, Surface Energy, Crystal Defects - type and density, Pore Shape, Tensile Strength, Wet Strength, Substrate surface relation to evaporative front, Colloidal effects (Clay or Fe)

Solution PropertiesSurface TensionViscosityActivity Coefficient of SoluteMolar ConcentrationThickness of supersaturated zoneThickness of diffusion layerHydraulic pressureVapor PressureEquilibrium Relative HumiditySolubilitySustainable degree of supersaturation VolumeCapillary CondensationSolubility changes with P or THygroscopicity

Salt PropertiesCrystal Type and MorphologyHydration States, Stability RangeDensityThermal Expansion Coefficient

Environmental PropertiesWind velocity and directionTurbulenceBoundary layerEvaporative CoolingTemperatureHumidity (macro and micro)Support moisture content

Figure 1. Diagram of properties, factors and behaviours in the salt crystallization process.

Kinetic FactorsSolution Evaporation RatesCrystal Nucleation RatesCrystal Growth RatesCrystal Expansion RatesCrystal Dissolution RatesSolution Diffusion RatesSolution Transport RatesSolution Vaporization RatesHeat Transfer Rates

Thermodynamics

System Equilibrium (salts, solution, environment, substrate)

Salt Behaviours (Processes)EfflorescenceSubflorescenceSalt CreepSalt Crust developmentWhiskers - Basal Growth FormWhiskers - Tip Growth FormDissolution of existing saltsHydration/DehydrationThermal expansion stressCyclical ProcessesEffect on crack propagationCrystallization Pressure Pressure SolutionHydration Pressure

Important Properties and FactorsFor Supersaturation:

Evaporation Rate (surface area, temperature, humidity, wind) Cooling Rate (temperature, wind) Pre-existing salts (thenardite dissolves, then mirabilite ppts) Nucleation/Growth rates

For Location: Evaporation Rate/Solution Transport Rate Solution properties (surface tension, viscosity) Pore size distribution (more small pores = more damage)

Key to DamageDegree of SupersaturationLocation of crystal growth

Salt DamageScalingDeep CrackingUniform ExpansionMicrocrackingGranular DisintegrationDelamination


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Benavente, Garcia del Cura et al. 1999; Scherer1999; Charola 2000)? Can the large number oflaboratory simulation studies of salt weatheringbe reconciled with field data and explained usingexisting theories? Can salt weathering formssuch as tafoni be accurately modeled usingexisting knowledge? How do the behaviors ofwater, salts, and substrate compare at theimportant mesopore scale (2-50 nanometers)versus the macro scale (Brown 2001; Hochella2002)? These complex questions, together withsuch basic questions as how the hydration ofsalts progresses and how crystallizationpressures are sustained in situ, are still in need ofstudy (Charola & Weber 1992; Doehne 1994;Steiger, Beyer et al. 2000). Nevertheless, theresearch performed in recent years and surveyedherein has significantly advanced overall under-standing of the damage caused by saltweathering processes.

Deterioration due to salts: Theory andLaboratory Observations

Several important advances in salt damagearising from theoretical and laboratory researchhave application for the conservation field anddeserve highlighting. The research summarizedhere addresses the precise nature of therelationship between salt stress and material poresize; how supersaturated solutions develop andare maintained; the contradictory behavior of thecommon salts halite and gypsum in the field asopposed to the laboratory; and the important roleof humidity cycling in salt crystallization.

George Scherer (1999; 2000) has recentlyadvanced the theoretical framework for un-derstanding stress caused by the crystallizationof salts in pores. This research helps explain whysalt crystallization may not damage certainmaterials: in some cases, the local stress field isnot large enough to propagate critical flaws inthe material. While stones with large pores tendto fare better than those with abundant smallpores, Scherer's work points out that large poreseffectively behave like small pores when filledwith salt. Scherer (1999) finds that the maximumpressure salt crystallization can achieve is highlydependent on pore size, predicting that most ofthe damage occurs when salt growth migratesfrom larger to smaller pores in the size range of 4nm to 50 nm. Scherer's work clarifies the visualmodel of how salts growing on the nanometer-

scale thin film between salt and stone can resultin stresses that exceed the strength of mostporous building materials. Putnis & Mauthe(2001) have noted that halite cementation inporous sandstone reinforces the observation thatlarger pores fill before smaller pores, confirmingthat fluids in small pores can better maintainhigher supersaturation.

It has long been known that the solution must besupersaturated, in order for salt crystallization todamage porous materials (Goudie & Viles 1997).What has been unclear until recently is how, in amaterial with abundant nucleation sites, such asupersaturated solution was created andmaintained.

Dei et al. (1999) investigated the experimentalcrystallization of potassium nitrate salt in Italianmarble, sandstone and travertine samples. Theyfound that the pore size distribution was shiftedtowards larger pores and the porosity wasreduced after salt crystallization. This appears tobe the result of blocking of micropores due tocrystallization of salts in the zone between largeand small pores. Thermal analysis of the samplesto identify the salts showed that the shape andtiming of the exothermic peak indicative ofcrystallization varied depending on the poresystem and stone type.

The important relationships between the rate atwhich supersaturation increases, nucleation, andpore size distribution was studied by Putnis et al.(1995). They found that nucleation wassuppressed in fine-grained substrates inexperiments with gypsum and other salts. Thiswork provides provides another explanation forempirical observations that stones with abundantmicropores are often have a higher decay rate instandard salt crystallization tests. Also, it helpsexplain how high supersaturations may be ob-tained in materials with a high internal surfacearea.

Kashchiev and Van Rosmalen (1995) present away to calculate the supersaturation a solutioncan maintain in a pore of a given radius beforecrystallisation occurs. Due to the Laplace effectof curvature, in small pores (less than 1 micron)a higher supersaturation can be reached beforecrystallization begins. Pores also alter thecrystallization of ice, dramatically reducing thenormal freezing point (Baker et al. 1997).


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Some authors have taken the idea of crystal-lization pressure and applied it to less solubleminerals such as calcite (McBride and Picard2000), suggesting that crystallization of calcite ina calcite-rich rock may result in the productionof tafoni. However, conventional saltcrystallization theory requires the pore wall andthe salt to be incompatible in order to generatecrystallization pressure (Scherer 2000), so someother mechanism may be at work.

Crystallization can be characterized as needing adriving force, such as a supersaturated solution,to create and sustain it. The generally acceptedkinetic pathways to produce such a solution ormode of supersaturation generation are rapidcooling, for salts with a strong solubilitydependence on temperature, and rapidevaporation for the remainder of salts(Rodriguez-Navarro, Doehne et al. 1999). Athird reaction pathway was first noted byChatterji & Jensen (1989), more recently byDoehne, Selwitz et al. (2001), and in detail byFlatt (2002). This pathway is the presence offine-grained pre-existing salts that dissolverapidly to produce a supersaturated solution,such as is caused by the wetting of thenardite,leading to the rapid precipitation of mirabilitefrom solution, as observed in the environmentalSEM (Doehne 1994; Rodriguez-Navarro &Doehne 1999a).Another unresolved conundrum is the contrastbetween severity of damage in the field causedby halite and gypsum (Kirchner 1996), two ofthe most common salts, as compared to theirrelatively benign behavior in the laboratory(Smith & McGreevy 1988; Goudie & Viles1997; Robinson & Williams 2000).Understanding gypsum's behaviour is especiallyimportant to stone conservation in view of thecommon occurrence in the field of "black crust"containing abundant gypsum, a product of thereaction of regional sulfate air pollution withbuilding stone (Verges-Belmin 1994; Wittenburg& Dannecker 1994). Despite the ongoingreduction of sulfate air pollution over the past 20years, the rate of loss of carbonate stone surfacehas not slowed proportionally at sites such as St.Paul's Cathedral in London. This discrepancy hasbeen attributed to salt crystallization damagefrom pre-existing gypsum (Trudgill, Viles et al.1991).

Gypsum also plays a significant role in thetypical problems of masonry buildings near thesea. Salt weathering related to sodium chloride

and gypsum from sea spray salts and soilcapillary waters is addressed in a recent casestudy documenting the decay history of the siteof Civil Palaces in Alicante, Spain (Louis, DelCura et al. 2001). The desalination of porousbuilding materials typically results in the non-proportional removal of salts (Bromblet &Verges-Belmin 1996). If gypsum is present, theremoval of hygroscopic salts may decrease itsmobility since the solubility of gypsum issignificantly increased by the presence of sodiumchloride and other salts (Robinson & Williams2000).

Other open questions pertaining to salt damageof porous materials are the role of the range ofhumidity cycling, the potential presence ofdamage thresholds, and the degree to whichdamage over the long term can be reduced bydecreasing humidity fluctuations. Experimentson ceramic tiles loaded with NaCl, Na2SO4 andCaSO4.2H2O and simulating the humidity cycles(43%-55% RH and 0%-11% RH) of museumdisplay cases (Nunberg & Charola 2001) overthe course of six months found that measurabledeterioration of the tiles had taken place. Thisran contrary to expectations that no damagewould occur in a closed environment withlimited humidity cycling well below thedeliquescence point of the salt (equilibriumrelative humidity or RHeq). The discovery ofdamage suggests that while environmentalcontrol significantly reduces the rate of damage,additional research is needed to understand thelimits of environmental control and how thedamage was produced. In this example, the modeof solution supersaturation is not clear. However,the filling of larger pores with salt and thecreation of smaller pores may play a role, asmight capillary condensation of water in finepores combined with limited humidity cyclingexerted over time (Rucker, Holm et al. 2000).The results of Nunberg and Charola (2001)suggests that environmental control may provideless protection than previously believed (Price2000).

The standard salt crystallization tests (ASTMand RILEM) for building materials, wheresample blocks are repeatedly immersed insaturated solutions of sodium sulfate and thendried, are widely used to estimate resistance tofrost damage and general durability (Price 1978;Marschner 1979). Nevertheless, there has beensome controversy over the applicability of thetests, given the severity of the test and the some-


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times inconsistent results (Sheftick 1989; Hunt1994). Recent research has addressed thisproblem from two directions: examining theway the test actually operates (Rodriguez-Navarro, Doehne et al. 2000a) and proposingnew standard test methods which more closelysimulate typical conditions (Benavente, Ordóñezet al. 2001). The assumption of many researchershas been that the damage from this test occurreddue to the hydration of sodium sulfate. More re-cent work suggests that direct crystallization ofthenardite (Na2SO4) may occur (Rodriguez-Navarro, Doehne et al. 2000a) in addition torapid mirabilite (Na2SO4.10H2O) crystallizationduring immersion (Doehne, Selwitz et al. 2001;Flatt 2002). Goudie (1999a) found that salt crys-tallization tests of limestone were poor predictorsof frost resistance. Efforts to discover parametersthat correlate with building stone durabilitycontinue, with emphasis on those that can bemeasured more easily than salt crystallizationresistance (Moh'd, Howarth et al. 1996;Ordonez, Fort et al. 1997; Nicholson 2001).

New Methods for Studying Salt Weath-ering

An important turning point in the advancementof salt weathering studies has been thedevelopment of methods that provide data onmaterial behavior in situ and in tempo, such asultrasonic field testing (Goudie 1999b; Simon &Lind 1999), acoustic emission (Storch & Tur1991; Grossi, Esbert et al. 1997), time-lapse andEnvironmental Scanning Electron Microscopy(ESEM) (Doehne 1997; Rodriguez-Navarro &Doehne 1999b), and three-dimensional imagingmethods such as NMR (Rucker, Holm et al.2000) and micro-CT scanning (Degryse, Geet etal. 2001). Such methods add the third and fourth(time) dimensions to previous studies and aid inseparating the effects of salts from other decayphenomena. For example, Pel (2000) found thatthe absorption of a concentrated NaCl solution incalcium-silicate brick, as seen when using NMR,revealed a sharp wetting front. Unexpectedly, thesodium was clearly seen to lag behind themoisture profile. Little sodium, if any, wasobserved near the wetting front, an apparentresult of the interaction of sodium ions with thepore surface. In tests of 21 limestones using non-destructive methods, Goudie (1999b) noted thateven durable stones which showed no visibledecay after salt crystallization tests were found to

have suffered significant decreases in theirmodulus of elasticity values, indicating loss ofstrength. Limestone durability was generallyfound to correlate with high values of modulusof elasticity, lower water absorption capacities,high densities and low salt uptakes.

Deterioration due to Salts: Field Observa-tions

A wide range of important field observations ofdecay to porous building materials over severaldecades has been summarized by Arnold andcolleagues (Arnold & Küng 1985; Arnold &Zehnder 1985; Arnold & Zehnder 1988; Arnold& Zehnder 1991). These observations clearlyestablish the strong relationship between the rateof salt decay and the environmental conditions.Arnold discusses the morphology of the saltcrystals and its relationship to the supportmoisture content. With a relatively dry substratewith a slow evaporation rate, the crystals take onan acicular form; when more moisture is present,a salt crust may form (Arnold & Küng 1985;Arnold & Zehnder 1985). Subsequentobservations of climate and salt behavior in sixchurches over the course of five years showedthat… "hygroscopic salts crystallize periodicallyaccording to the variations of relative humidityand to a much lesser extent of temperature.Within continuously heated rooms crystallizationand decay are related to variations in relativehumidity caused primarily by the heating andsubordinately by natural variations of the outsideclimate. In non heated rooms seasonal variationsof temperature also induce periodic crystalliza-tion." (Arnold & Zehnder 1988). Arnold alsodocumented the fractionated pattern of salts inwalls with rising damp, dividing the wall intofour zones in vertical succession differentiatedby degrees of damage and salt content. Thelowest zone was perpetually damp with saltsremaining in solution, succeeded by the zone ofgreatest damage and, finally, the uppermost zonemarking the limit of rising moisture caused bydeliquescent salts. The highest salt content doesnot always correspond to the greatest damage,since ion mixtures that are strongly hygroscopicwill not crystallize under normal conditions. Thesources of the salts are attributed by Arnold tothe extensive use of cement-based mortars andconcrete in large restoration programs over thepast 50 years, as well as salts from air pollutionand rising damp. Soluble salts from cement-


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based mortars and concrete are still today an im-portant cause of decay to historic buildingmaterials, despite efforts to improve cementquality and reduce the overall use of thesematerials (Moropoulou 2000).

A brief review of other more recent fieldobservations on salt weathering is considerednext. Some useful insights into rates of saltweathering have recently been gained throughthe measurement of tafoni depth using rocksurfaces with a range of known ages (Matsukura& Matsuoka 1991; Sunamura 1996; Norwick &Dexter 2002). Tafoni are cavities or pits inotherwise homogeneous rock surfaces. Thecurrent consensus appears to confirm the saltweathering origin of most tafoni (Pye & Motter-shead 1995; Mikulas 2001; Norwick & Dexter2002). For tafoni that has developed over the lastcentury, linear recession rates are estimated at0.6 to 5.2 mm/year (Sunamura 1996). Tafonidepth was also found to develop twice as fast asthe width of the pit, providing additionalconfirmation for the salt weathering origins oftafoni. More recent analysis has shown thatwhile there is significant scatter in the depthmeasurements, it appears that the weathering rateis actually non-linear, with a time lag followedby rapid recession and then a much slower rateof loss as the tafoni age. This pattern generates asigmoidal or “S” curve, a typical example ofwhich is given by the following equation:

D = b1 + e(b2+(b3/t)) (1)

where D is the depth of the tafoni, t is the age ofthe surface and b1, b2, and b3 are constantssolved for mathematically (Norwick & Dexter2002).

A speculative explanation for the non-linearitysuggests that the rate of loss is first limited bylow salt concentrations, then becomes dominatedby a positive feedback loop with an increasingrate of damage related to increasing saltconcentration, followed by a negative feedbackloop as the scale of the tafoni reaches a pointwhere it apparently no longer allows efficientsalt weathering (Norwick & Dexter 2002). Thisnon-linear pattern is typical for many othernatural weathering processes (Goudie 1995).Such non-linearity may help explain why saltweathering rates measured in laboratory settingshave been much more rapid than those typicallyfound in field studies. While erosion rate datafrom the study of stone structures for

conservation is surprising uncommon, suchpatterns of low rates of decay, followed by arapid increase have been noted by Charola(2000) and Snethlage, Wendler et al. (1996). Thefact that field salt weathering rates are typicallynon-linear and may show a low initial rate ofloss suggests that stone conservators shouldinterpret short-term observations of overallstability of vulnerable structures, such as wallpaintings, with caution.

One of the advantages marble possesses forresisting salt damage is its minimal porosity andtendency not to have a strong capillary suction(which can lead to rising damp). However, recentstudies of the marble monuments at thearchaeological site of Delos, Greece have shownthat salts deposited from atmospheric aerosolsare responsible for damage of marble sculpture(Jeannette 2000; Chabas & Jeannette 2001).Additionally, recent work on soil chemistry inHawaii using strontium isotope data has foundthat about 50% of the calcium in soil samplesfrom a coastal location originated from saltspray, not soil mineral weathering as expected(Whipkey, Stewart et al. 2000). This suggeststhat salt spray has a strong effect on the localenvironment, perhaps dependent on overall windand storm patterns.

An extensive survey and analysis of bedrock inthe Valley of the Kings, Luxor, Egypt recentlyresulted in warnings concerning the hazards toancient tombs from swelling clays and saltsactivated by moisture from the exhalations oftourists as well as flash floods (Wüst & McLane2000; Wüst & Schlüchter 2000). Expansion fromhydration of anhydrite and crystallization cyclingof abundant sodium chloride where cited asimportant decay mechanisms in important Royaltomb sites, such as Seti I, which have seen arapid increase in stone loss since their openingby archaeologists.

Warke and Smith (2000) suggest that "Saltweathering is a threshold phenomenon wheredecay is manifest only when a stress/strengththreshold is crossed." This conclusion is basedon the analysis of two similar sandstone blocks,one sound and one damaged, with similar saltcontent and distribution. They found that NaClwas deeper and more mobile in the stone (6 cm)than expected from surface damage (2 cm). Astrong correlation of decay rate with envi-ronmental variation has recently beendocumented by Cantón, Pini et al. (2001). In a


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range of experiments that the weathering rate ofa calcareous mudstone was found to most closelyfollow the number of wetting/drying cycles.Some useful cautionary notes on the real worlddistribution of salts in sandstone can be found in(Turkington & Smith 2000). The researchersconcluded that "The widely held perception thaturban environments are 'dry' with shallowsurface wetting of building stone does not appearto hold true for certain building stone."

Salt weathering is traditionally associated withcoastal and desert environments (Theoulakis &Moropoulou 1999), however regions with atropical monsoon climate may also be effectedby salt weathering due to the strong contrastbetween the rainy and dry season. Two recentexamples are the famous temples at Angkor inCambodia (Uchida, Nakagawa et al. 2000) andthe brick monuments in Ayutthaya, Thailand(Kuchitsu, Ishizaki et al. 2000). At Angkor, oneof the main causes of deterioration was identifiedas crystallization pressure from gypsum andphosphate minerals growing between exfoliatedlayers of sandstone. The sulfur and phosphate arederived from rain water leaching of bat guano,which subsequently moves into the stone throughrising damp (Uchida, Nakagawa et al. 2000). AtAyutthaya, gypsum efflorescence is found in therainy season, with more soluble salts such asthenardite appearing in the dry season. Mostdamage occurs at the beginning of the dry seasonto brick surfaces, and Kuchitsu Ishizaki et al.(2000) propose methods to reduce waterimpregnation of the bricks in the rainy season asa conservation measure.

Interrelationship between Salt Weather-ing and Other Decay Mechanisms

With an understanding of the properties nec-essary for a particular salt damage mechanism tobe activated, it is possible to contemplatemodifying those properties to better understandthe damage mechanism and to developpreventive conservation measures. Changing aparameter such as surface tension using surface-active agents was recently found to have a strongeffect on damage from sodium sulfatecrystallization since it directly affects where saltscrystallize (Rodriguez-Navarro, Doehne et al.2000b). The authors speculated that biologicallyproduced surfactants might have a similar effect.Two articles have subsequently found that the

presence of bacteria, associated biofilms, andbiopolymers greatly increases the damage causedby salts (May, Papida et al. 2000; Papida,Murphy et al. 2001).

The relationship between salts and frostweathering was explored recently by Williams &Robinson (2001). The authors extended therange of salts known to intensify frostweathering (potassium and ammonium alums)and confirmed the significance of halite in theprocess. Williams & Robinson (2001) show thatthe degree of damage to stone varied greatlydepending on the combinations of salts involved.While most salts contributed to damageproportionally when used in combination, somesalt mixtures caused intensified damage out ofproportion to the component salts. The topic ofsalt mixtures and their effect on salt damage andother decay mechanisms is complex since thebehaviour of the mixture cannot be predictedfrom single salt data (Steiger & Dannecker 1995;Steiger 1996; Steiger & Zeunert 1996).However, the use of computer models such asthe semi-empirical ion interaction model ofPitzer provides an important predictive tool forstudying most field situations where saltmixtures are the rule (Clegg & Brimblecombe2000; Steiger, Beyer et al. 2000).

Dragovich (1997) studied the weathering ofmarble tombstones near Sydney, Australia andwas surprised to measure a low weathering rate(540 microns/ 100 years). The seaward facingsides of the marble blocks, which were exposedto greater salt input, unexpectedly weatheredmore slowly than the landward side. Thissuggests that even in coastal environments saltweathering is only one aspect of the fullweathering process.

Prevention, Mitigation, and TreatmentOptions

Avoiding damp is a fundamental principle formaintaining any structure where porous materialsare used (Ashurst & Ashurst 1988). The mostimportant mitigation method is prevention,through the physical separation of buildingmaterials from soil moisture and salts with thetraditional "damp-proof course." This is typicallyan impermeable barrier such as plastic (currentstructures), glazed brick, bitumen, or othermaterial. The service life of the plastic sheeting


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used in modern concrete slab construction is notwell known. In existing salt-laden structures,salts can be removed through poulticing and insome cases the affected masonry may bereplaced. However, in the past few decades, thecost of such replacement has led to increasinguse of chemical "damp-proof courses" in theform of injected siloxane. Below, a range oftreatment options and research into the reductionof salt damage is discussed.

Salts tend to cause damage in building materialswhen activated by a change in humidity,temperature or the presence of liquid water. Ifthe composition of the salts is known, thencalculating the stability range (RH andtemperature) for the salts allows establishment ofthe appropriate conditions for minimizingdamage due to a change of state. Establishingsuch controlled conditions should substantiallyextend the life of the material. An important newreport by Price and colleagues (2000) documentsthe development of an expert system to providesuch information and is an extremely usefulrepository of detailed thermodynamic and kineticinformation concerning individual salts, saltmixtures, and their calculated and actualbehavior. Other researchers have also exploredthis approach (Aires-Barros & Mauricio 1997).Unfortunately, controlled environments for stoneobjects and structures tend to be the exceptionrather than the rule, and field experiencesuggests that damage tends to occur wheneversignificant concentrations of salts are present inporous masonry (Andreas Arnold, personalcommunication).

The practice of poulticing salts using highlyabsorbent materials such as clays or wood pulphas long been employed by stone conservators toremove salts from important wall paintings andcarved stone. Study of the effectiveness of thesemethods has shown that, while chloride andnitrate are readily removed, sulfate removal ismore difficult. Artificial anionic clays(hydrotalcites) are recommended for enhancedextraction of the less soluble gypsum (Vicente &Vicente-Tavera 2001). Vicente & Vicente-Tavera (2001) also noted the loss of extractionefficiency under conditions of high humidity.Simon, Herm et al. (1996) found that a new highporosity compress render was generally moreeffective at removing salts than a traditionalsacrificial lime plaster, although the introductionof water to the surface during the treatmentincreased nitrate concentrations higher up on the

wall. Siedel also noted the difficulty ofcontrolling desalination (Siedel 1996), achievingresults that were "contradictory and sometimeseven undesirable on large monuments."

Two desalination methods (total immersion andintermittent washings) were studied recently fortheir potential for use in the removal of saltsfrom ceramic tiles by Freedland & Charola(2001). The authors found that for more solublesalts, such as NaCl, repeated washings weremore effective, while long-term immersion wasmore efficient for less soluble salts such as gyp-sum as well as for sodium sulfate in ceramic tileswith fine pores. Based on a substantial body ofexperimental work, Charola, Freedland et al.(2001) have developed a reproducible method tocalculate the salt remaining in an object afterdesalination.

An interesting effort to move sodium chloridefrom a brick vault into an adsorbing mortarabove the vault used differential humidity tomobilize the salts (Larsen & Bøllingtoft 1999;Larsen 2001). With a low relative humidityabove the vault and a moist environment beneath(set below the deliquescence point of the salts), itwas found that the kinetics for NaCl movementsolely attributable to humidity are slow, and thatNaCl most readily moves into the mortar (fromdamaged bricks) when liquid water is present.

A newly proposed approach mitigating saltweathering is the reduction of the interfacialenergy between the salt and the stone using aspecially selected non-swelling polymer with theappropriate surface properties (Scherer, Flatt etal. 2001). By placing such a barrier between thepore walls and salts, it is anticipated that greatersurface energy compatibility may be achievedand the salts should stop growing when theycome in contact with the polymer.

Long experience with salt weathering of buildingfoundations in Adelaide, Australia, a region witha Mediterranean climate and salt-rich soil, hasresulted in a host of practical methods formitigating this damage (Blackburn & Hutton1980; Young 1995). Young found that decay ismost commonly found at the base of walls,where solutions migrating from local salt-richsoils damage masonry, mostly in cases where adamp-proof course is either lacking, has beendamaged or was bridged by later masonry. Themost effective treatment is replacement of thesalt-laden stone and reinstallation of a damp-


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proof course. The high cost of this treatment islimits its use to severe cases or situations wheresufficient funding is readily available. The mostcommon treatment is injection of waterproofingagents (siloxane) to provide a chemical damp-proof course and desalination of the salt-ladenmasonry. The longevity of these treatmentsremains uncertain, with some failures attributedto problems with desalination or incompleteinjection. Saline soils are an increasing regionalproblem in south and southeast Australia, andsalt weathering of heritage structures in thisregion has begun to mount (Spennemann 2001).

Planetary Salt Weathering

Salt weathering is a global phenomenon, presentin Antarctica as well as Death Valley. Thisdiversity has been appreciated by geologists andplanetary scientists, who have suggested thatsalt-weathering mechanisms may help explainunusual features on Mars (Malin 1974).Photographs from Mars taken at the Pathfinderlanding site reveal features remarkable similar tohoneycomb weathering structures on Earth(Rodriguez-Navarro 1998). McCord (2001)found that under conditions typically found onthe Jovian moon Europa, magnesium sulfatecould remain hydrated on a geologic time scale.

Summary and Future Work

As of this writing, there are over 1,800 ref-erences in the scientific literature on the topic ofsalt weathering of porous materials and over6,000 references addressing general problems ofbuilding material decay. Many of these articlesdescribe unique case studies that may be difficultto interpret or apply to other situations. In orderto navigate this vast collection of data andknowledge borne of disparate disciplines, thisarticle serves as an organizing framework to con-sider the complexity of salt damage, presents anoverview of multidisciplinary research and openquestions on this topic, and provides a limitedliterature survey largely focused on recent workand those articles with relevance forconservation. The research surveyed here hasclearly advanced our understanding of thedamage caused by salt weathering processes, andraises important questions for the conservation ofmaterial heritage. Future investigation into saltweathering should include more quantitative

measurements of field weathering rates andlaboratory studies using methods such as three-dimensional imaging to further illuminate therelationships between salt weatheringmechanisms and the properties of solution,substrate and environment, and to contribute to abetter understanding of the action and kinetics ofsalt mixtures in weathering stone. Finally,improving methods to prevent and mitigate saltweathering of building materials clearly deservesmore effort and attention.

The author is indebted to Dr. Michael Steigerand Dr. Andrew Putnis for their comments andcritical reviews, which greatly improved themanuscript. This paper was supported by theGetty Conservation Institute under the SaltResearch project. Also, the support of StefanSimon, William Ginell, and Jeanne MarieTeutonico and Alberto de Tagle isacknowledged. Konrad Zehnder generouslycontributed many useful references. The fullbuilding materials decay bibliography isavailable from the author.

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