Oguchi and Yu Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00414-x(2021) 8:32

Progress in Earth andPlanetary Science

REVIEW

Open AccessA review of theoretical salt weatheringstudies for stone heritage
Chiaki T. Oguchi1*

and Swe Yu2

Abstract
Salt weathering can cause substantial deterioration of natural rocks, building stones, masonry materials, monuments,and engineering structures. Nearly two centuries of salt weathering studies, both theoretically and empirically, havemanifested its power as well as its complexity. This paper attempts to unite the kinds of literature assess the various
theories in the light of the combined information. The theoretical approaches concerning the most cited mechanismsof salt weathering such as crystallization, hydration and thermal expansion of crystalline salts are thoroughly reviewed.
It is understood that there is no universally acceptable hard and sound theoretical information on this topic yet. Moreprecise theories should be developed to elucidate the complications of the mechanisms of salt weathering as well as
to interpret the results of empirical studies.
Keywords: Salt weathering theories, Crystal growth pressure, Hydration, Thermal expansion of salt

1 Introduction
Salt weathering has been implicated as a cause of rockdisintegration since the time of Herodotus, and systematic investigations of salt weathering have been performed over at least 200 years (e.g., Thury 1828; Miller
1841; Luquer 1895; Cooke 1979; Goudie and Viles 1997;
Williams and Robinson 2001; Doehne 2002; Tsui et al.
2003; Van Grieken 2003; Coussy 2006; Espinosa-Marzal
and Scherer 2010; Shahidzadeh-Bonn et al. 2010; Flattet al. 2017). In the nineteenth and twentieth centuries,
the great German desert geomorphologists (e.g., Penck
1891; Passarge 1904; Mortensen 1933) and various
French geomorphologists (e.g., Birot 1954; Pedro 1957a,
1957b; Tricart 1960) greatly extended our knowledge ofsalt weathering. Since the middle of the twentieth century, the majority of research work has been done inEurope (e.g., Goudie et al. 1970; Kwaad 1970; Theoulakis& Moropoulou 1997; Williams and Robinson 1981;
Coussy 2006; Espinosa-Marzal and Scherer 2010; Flattet al. 2017). Further interest in the field was sparked by* Correspondence: ogchiaki@mail.saitama-u.ac.jp
1
Graduate School of Science and Engineering, Saitama University,
Shimo-Okubo 255, Sakura-ku, Saitama 338-8570, Japan
Full list of author information is available at the end of the article

the exploration of theories behind salt weathering (Correns 1949; Weyl 1959; Wellman and Wilson 1965).
Many field and laboratory studies over the last few decades have also shown weathering of salt, regardless ofthe mechanisms involved (Evans 1970; Mustoe 1982,
1983; Pye and Sperling 1983; Goudie 1985; Rögner 1986;
Trenhaile 1987; Yatsu 1988; Matsukura and Kanai 1988;
Cooke et al. 1993; Thaulow & Sahu 2004; McGreevy
1996; McGreevy & Smith 1982; Goudie 1986, 1993,
1999; Goudie and Viles 1997; Robinson & Williams
1996, 1982, 2000; Williams & Robinson 1998). It is ahighly destructive process that is particularly importantin rock decay in a wide range of environments includingMars (Malin 1974). Types of decay due to salt weathering are shown as Fig. 1. These are supposed to bereflected by salt weathering processes. Furthermore, salt
weathering plays a role in the development of many geomorphologic features, such as alveoles and honeycombs(Mustoe 1982), tafoni and cavernous weathering (Bradly
et al. 1978), and pedestal rocks (Chapman 1980) (Fig. 2).
This type of weathering is also an important contributor
to the production of rock meals and debris, especially inarid regions (Beaumont 1968; Goudie 1977, 1983; Goudie et al. 1979, 1984, 1997; Goudie and Day 1980; Goudie & Cooke 1984; Goudie & Watson 1984; Punuru© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate ifchanges were made. The images or other third party material in this article are included in the article's Creative Commonslicence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commonslicence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain
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Oguchi and Yu Progress in Earth and Planetary Science(2021) 8:32

Page 2 of 23

Fig. 1 Patterns of disintegration due to weathering. a Cracking and disintegration, b crumbling, c scaling, d surface pitting, e thick scaling, fdelamination, g cracking, and h expansive dilatation patterns. Scale bar shows 10 mm

et al. 1990; Goudie & Viles 1995), coastal areas (Mottershead 1989; Cardelle et al. 2003), and Antarctica (Prebble1967). Salt weathering may be a common planetaryphenomenon and could be responsible for some aspectsof the Martian landscape (Malin 1974; Clark and Van
Hart 1981; Clark 1998).
Salt weathering of building stones is a primarycause of the loss of historic architecture (Evans 1970;
Winkler 1973; Amoroso and Fassina 1984; Smith and

McGreevy 1988; Winkler 1994; Cooke 1994; Price
1996; Cooke and Gibbs 1993; Gomez-Heras & Fort
2007) and many structures in archeological sites havebeen damaged by this process (Lucas 1925; Winkler
1975; Goudie 1977). Salt also has a major impact on
civil engineering because it damages roads, highways, runways, dams, and building foundations (Netterberg andLoudon 1980; Sayward 1984; Obika et al. 1989; Doornkamp and Ibrahim 1990; Goudie and Viles 1997). In hot,Oguchi and Yu Progress in Earth and Planetary Science(2021) 8:32

Fig. 2 Example of a honeycomb structure at Giza, Egypt

arid areas, such as the Middle East, the full significance ofsalt weathering as an engineering hazard became evident
after the construction boom of the 1970s and 1980s(Doornkamp and Ibrahim 1990).
Despite nearly two centuries of research and observation, there is still no definitive way to overcome salt
weathering. As a result of intensive reviews and textbooks around every decades such by Evans (1970),
Winkler (1994), Goudie and Viles (1997), Doehne andPrice (2010), and Siegesmund and Snethlage (2014),
these studies have been able to be introduced, categorized or organized as focusing to theory, mechanisms,
experimental methods, research tools, and so on. The
present paper also reviews many studies even though
not enough, to summarize the research history of salt
weathering, including the subsequent research results. In
orderthe construction boom of the 1970s to find a
breakthrough to conquer the deterioration of stone andrelated materials for future issues, this review is expected
to help a little in solving future problems.

2 Substances involved in salt weathering
2.1 Moisture

A crucial factor affecting the effectiveness of salt weathering is the presence of moisture and its movement.
Moisture can either provide a source of salt or mobilize
and deposit salt in pores and cracks because only salts

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dissolved in water can enter and move through porous
bodies. Although moisture itself may also substantiallydiminish the mechanical resistance of some rocks andstones, its distribution in the porous material determines
the area where deterioration occurs. This area corresponds to the zone of maximum moisture content(Snethlage and Wendler 1997) and refers not only to thedamage at the base of a wall with rising damp, as described by Arnold (1982), but also to the formation of
gypsum veils at the top of the rising damp area, as discussed by Zehnder (1993, 1996).
The main moisture sources that merit consideration
are dew, fog, rain, and groundwater. In some desert
areas, dewfall enhances the salt weathering processes
and fog water is of special importance in coastal deserts.
Of all these sources, groundwater may be the most important. Ground moisture comes from rain splashingagainst the base of a building and from groundwater rising by capillary suction. Both types of moisture may
contain more ions than (Meredith & Donald 1995) surface water because they can contain salts from streetsand sidewalks or from the slow circulation of undersaturated groundwater between the mineral particles ofsoils. As the groundwater moves through the stone, thesalts may crystallize and effloresce near or on the stone
surface. If those salts are soluble, they are dissolved
again and infiltrate the ground. Therefore, groundwateris also a major source of salts in suitable topographic situations (e.g., sabkhas zones of the Middle East), and thepresence of high groundwater levels is a prime determinant of the efficacy of salt weathering of engineering
structures. Cooke et al. (1982) reported that aggressive
soil conditions constituting an engineering hazard were
produced when saline groundwater was drawn upward
through the soil to produce a capillary fringe that either
reached the ground surface or was sufficiently close to it
to affect foundations.
Water can enter a porous material either as a vapor or
liquid. In the vapor state, the main mechanisms bywhich water can penetrate a porous material are condensation and hygroscopicity. Condensation is categorized assurface condensation and capillary condensation (microcondensation) in pores, which are well-understood, distinct processes (Cammuffo 1988). Adsorption of moistureor surface condensation occurs in warm humid air whenmoisture meets a cooler surface or by cooling of the air.
Moisture also adsorbs readily to capillary walls in anoriented manner owing to the electrostatic attraction incapillaries 0.1 μm or smaller, which can cause stone decay
by moisture expansion.
Salts can also absorb moisture, especially when theambient relative humidity becomes higher than theirequilibrium relative humidity (ERH). Under highhumidity conditions, salts may be hygroscopic or(2021) 8:32

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deliquescent. Hygroscopic salts absorb moisture from theair, but not enough to form a solution, whereas deliquescent salts absorb sufficient moisture to form a solution.
Because concentrated salt solutions have a lower vapor
pressure than does pure water, they have a higher tendency to condense water vapor from the environment toequalize the activity of water in the solution with that in
the vapor phase to reach equilibrium. This process has
also been called classical osmosis, particularly in German
texts (Weber 1984).
In the liquid state, water movement can occur via capillarity and infiltration mechanisms. The capillarity mechanism arises from the attraction of the water and thecapillary material, as well as the surface tension of the liquid, whereas the infiltration mechanism requires hydrostatic pressure and depends on the permeability of thematerial (Vos 1976; Amoroso and Fassina 1984; Weber
1984; Massari and Massari 1993). The rise of groundmoisture by the capillarity mechanism can lead to slow,
progressive salt impregnation (Fig. 3). This upward movement of capillary water through surface and near-surface
materials is sometimes called evaporative pumping and is
the product of high surface temperatures.
The height of the capillary rise, that is, the thickness ofthe capillary fringe, depends on the soil temperature gradient and the nature of the soil, and it is normally less than ameter in clean gravel, although it is up to 10 m in clay.
Cooke et al. (1982) suggested that the capillary rise
under desert conditions is normally no more than 2–
3 m, and rarely, if ever, exceeds 4 m. In some buildings, the thickness of the capillary zone may be exceptionally and unexpectedly high, sometimes
exceeding 6 m (Cooke 1994). The maximum height ofthe capillary rise is often marked by a white rim ofefflorescence with a dark wet outer rim that remains
wet owing to the effect of hygroscopic salts. The
moisture can be retained, especially in fine-grained
masonry, stone, concrete, and mortar. Other complications may arise, with the surface concealment of capillaries by lichens and mosses, which retain moistureand do not permit evaporation from the open ends ofthe channels, keeping the wall wet.
Alves et al. (2011, 2013) investigated the pore networkof several types of stone in laboratory experiments andfound that durability is controlled by porosity. The orderof porosity values from high to low, of limestones, marbles, and granites matched that of durability. These experiments also showed that in addition to porosity,
further analysis of pore size distribution and specific surface area is important for understanding the pore network of building stones. Benavente (2011) summarizes
the importance of measuring the pore size distribution
in building stone deterioration. The main processes are
categorized in the order of pore size (Fig. 3). De Kocket al. (2017) investigated sandstone in Belgian heritageand highlighted the importance of pore size based on
their measurement of rock properties. Capillary poresadsorb and desorb water according to the relative humidity (RH). This water adsorption interferes with water
vapor diffusion and affects critical water content. Stones
with a high number of capillary pores are more prone toadsorbing moisture and are difficult to dry out completely. Finally, the connectivity between all the different
pores and pore sizes defines the overall effect of water.
These studies demonstrate that it is important to understand how water moves inside a porous material. If it moves
as a liquid, it can transport salts, whereas if it moves as avapor, it may be retained through hygroscopicity. Liquidwater is governed by a capillarity mechanism and water
vapor is governed by a diffusion mechanism. The transition
point between these two mechanisms defines the critical
moisture content, Ψc, of a porous material. This parameteris constant for each material and depends mainly on porosity and pore size distribution (Snethlage and Wendler
1997). The actual distribution of moisture in stone depends
on porosity, pore size distribution, and environmental conditions. As Snethlage and Wendler (1997) discuss, the maximum of moisture content resulting from wet-dry cycling iscloser to the surface in denser stones, and deeper andbroader in coarse, porous materials.

Oguchi and Yu Progress in Earth and Planetary Science

2.2 Sources of salts

Salts can originate from various sources (Price 1996).
Solutions of ions, from which two or more destructive
salts may crystallize on or in rock surfaces, may originate
from the following sources: from the rock and be drawn
to the surface by evaporation, from overlying beds andrun down the face of the rock, and from the atmosphereas airborne materials, such as sea spray, sea splash, aerosol, or the reworking of other deposits, and be deposited
directly on the rock surface. Several salts may crystallizedepending on the composition of the solution and on
the environmental conditions.
Important sources of salts include atmospheric pollution, groundwater, seawater, rainwater, rock salts anddeicing salts, building materials, organic/biological metabolism, and inappropriate treatments with incompatible conservation products. The salts that form dependon the ions present, their relative abundance, and environmental conditions that favor evapotranspiration, suchas temperature and RH.
Gaseous pollutants, such as sulfur dioxide and nitrogenoxides, and acids, such as sulfuric, nitric, and hydrochloric
acid, all contribute to salt weathering. Particulates or aerosols, such as carbonaceous spheres, and oxidants, such asozone produced by photochemical reactions, may act asimportant catalysts. For example, sulfur dioxide reacts withdust and other particles and with reactive materials,

Oguchi and Yu Progress in Earth and Planetary Science(2021) 8:32

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Fig. 3 Model of the disruption process by crystallizing salts(modified after Zehnder and Arnold 1989). a Saline solution
evaporates first from large pores, and then smaller pores, whichmakes salts growing in large pores form aggregates or granularcrusts. b Cohesion or strength of the host material is reduced bydisruption along cracks caused by the crystallization pressure ofclustered aggregates growing on pore walls. c Crystallization
concentrated in cracks continues isometrically when the crystals arecovered by solution film, which produces columnar crystals whenevaporation dominates. d Columnar crystal growth thins out as thecracks open, forming whisker-like crystals when their growing tips
are in contact with solution, and the crystal lift debris or fragments
produced by disruption of the host material

including many building stones, to form gypsum (Cammuffo et al. 1983). An important part of this process is theoxidation of sulfur dioxide, either in the gas phase or inmoisture films on building stones. This oxidation is assisted
by atmospheric oxidants, such as ozone and hydrogen peroxide, as well as by catalysts, such as soot or black carbon
and smoke. Sulfates can attack carbonate rocks throughdissolution by sulfuric or sulfurous acid and by converting
calcium and magnesium carbonates to more soluble sulfates or sulfites. The sulfate attack on silicate rocks, in contrast, is not easily measured. Leaching of alkali metals andiron may occur, which often forms black crusts on thestone surface, causing discoloration. Török et al. (2011) investigated the effect of the environment on the formationof these crusts in urban and rural areas and Farkas et al.
(2018) studied the crusts in different countries. Gibeaux
et al. (2018) determined the pollution rates from the speedof color change measured by fixed-point observations.
Efficient automotive combustion is an important contributor of nitrogen oxides, which are readily converted to corrosive nitric acid in the presence of oxidants. Although it isalso a strong acid, nitric acid is less damaging to carbonate
rock than sulfuric acid and sulfates due to the greater reactivity of sulfates with stone. Hydrochloric acid, which readily
dissolves carbonate rock, is an important pollutant source ofchloride ions, in addition to sea spray and desert dust in
many areas. Coal combustion is the major non-natural
source of hydrochloric acid. All these atmospheric pollutants,
after undergoing emission (from natural and anthropogenic
sources), transmission (over short or long distances), immission (reaching certain local environments), and deposition(on dry and wet surfaces with variable reactivities), can cause
devastating salt weathering hazards.
Groundwater is an ample source of salts because thewaters in soil are dilute salt solutions. The solutions
contain chloride, sulfate, nitrate, carbonate, magnesium,
calcium, sodium, potassium, ammonium, and occasionally rare earth elements. Organic pollutants can produce
phosphates and nitrates. Compared with normal soil solutions, those near housing and other sites used byhumans are enriched in nitrate and chloride. Nitrate is(2021) 8:32

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produced by microorganisms from organic waste and chlorides come from the use of deicing salts (NaCl and CaCl2)(Gomez-Heras et al. 2004). The salts rising from thegroundwater readily join salts leached from lime mortar, cement, concrete, plaster, and deicing salts from the streets.
Sea salt in the atmosphere is the greatest contributor ofsalts. Sea spray from ocean waves is a salt spray that contains a high concentration of mineral salts, particularly
chloride ions, and is supplied to rock, masonry, and materials up to 300 miles from coastal waters (Wellman andWilson 1965). Bursting bubbles of seawater form water
droplets of aerosol size, and a further reduction in size iscaused by the evaporation of the water droplets. NaCl, themost common salt in the oceans, is carried as fine sprayand settles on the ground or on the top of buildings, from
where it moves upward and downward. NaCl, as well assome Mg and Ca salts, can form white rims. A high saltcontent can accumulate over time through the depositionof salt spray. High winds blowing over salt lakes can also
supply large quantities of NaCl to nearby areas.
Rainfall is the source of the high chloride content ofgroundwater and salt lakes in coastal regions. Rainfall
chemistry varies with distance from the sea and in inland situations the importance of chloride is reduced
relative to sulfates, nitrates, and carbonates. However,near coasts, sea salt is a major factor controlling rainwater chemistry. Fog water can also account for the deposition of a substantial amount of some ions, which
provide a source of salts on rock surfaces.
Most rocks have a primary content of salt or of chemicalelements that can form salts after they have been released
from the crystal lattice of rock-forming minerals by chemical weathering. Rocks contain salts as their constituents or
as entrapped materials in their pores. Chlorides, sulfates, nitrates, carbonates, and oxides of iron are the most common
constituents. Sulfates are common after natural weatheringof pyrite. Calcium is dissolved from carbonate rocks andtends to form calcium sulfate (gypsum) in polluted air. Sulfates of calcium, sodium, and magnesium are most common among soluble salts in rock (Linnow et al. 2006;
Mamillian 1993). Some bedrock contains old evaporitic
beds as connate salts trapped in the sediment or rock at thetime of deposition. Leaching may transfer these evaporites
to modern evaporite basins in which they may reproduce
soluble salts. Some shale also contains appreciable quantities of salts; for instance, colliery shale waste is a source ofsulfates and may contain 1–5% soluble salt, whereas pyretic
shales are especially prone to producing gypsum.
Salt attack from deicing salts scattered on drives andsidewalks during winter to remove ice has caused moredamage to adjacent stone than any other source by adding salts to the environment (Winkler and Singer 1972).
Deicing salts are introduced to walls by water and also
by wind in very cold weather. The saline solutions of

NaCl, and more recently CaCl2, disaggregate and flake
porous and semiporous stone and concrete on walls by
crystallization pressure, leading to crumbling and spalling. The presence of these salts also increases chemicaldissolution attack. CaCl2 is now more frequently used as
it is more environmentally friendly, replacing persistent
Na ions with Ca ions that are readily absorbed by plants.
Salts may come from Portland cement (Fassina 1994),
which may contain up to 1% soluble alkalis. The ionsleached out may form efflorescences of alkali carbonate
salts, including trona (Charola and Lewin 1979).
Under some conditions, salts and acids may be produced
biologically by a range of microorganisms through biochemical processes. Soils around areas of human activity
are enriched in chlorides and nitrates because humans andanimals deposit excrement and urine, which contain appreciable quantities of chlorides and nitrates, and microorganisms produce nitrates from excrement and waste (e.g.,
Hosono et al. 2006; La Russa et al. 2017; Přikryl et al. 2017;and Lubelli et al. 2018). Although nitrates originate from
biological metabolism, there is no known biogenic source
for the chlorides being concentrated in similar quantities inthe same rising moisture zone. In addition, bacteria canoxidize ammonia to produce nitrate salts, whereas lichensliving on calcareous rocks may secrete oxalic acid, which
reacts with calcite to form calcium oxalate salts.
The use of incompatible materials for restoration andpreservation can produce salts. The materials that cause salt
weathering include acids, alkaline solutions, ionogenic tensides, Portland cement, waterglass products, and siliconates.
Acids neutralized with alkaline solutions and vice versa produce salts. However, the most important deterioration iscaused by salts from waterglass products and Portland cement (Zehender and Arnold 1989). Some stone chemicaltreatment, such as consolidants and sealants, release alkali
metals during curing, especially sodium and potassium
waterglasses. Sodium and potassium cations combine withsulfate, carbonate, and other anions to join the soluble salts
traveling through the masonry. Because the salt types andmoisture sources that aid salt weathering differ greatly, considering the diversity of salts and moisture sources can suggest under which environmental conditions salts are
particularly effective weathering agents.

Oguchi and Yu Progress in Earth and Planetary Science

2.3 Mechanisms of salt weathering

The presence of soluble salts in porous materials is only
harmful when there are changes in environmental conditions leading to crystallization-dissolution cycles. The problem is in explaining why and how salt damage occurs. The
earliest hypothesis (Correns 1926; Correns and Steinborn
1939; Correns 1949) suggested that crystallization pressure
was responsible for weathering via hydrostatic crystallizationpressure and linear crystal growth pressure. Ten years later,
their theory was completed by Weyl (1959) and Wellman

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and Wilson (1965) developed a thermodynamic model tocalculate the crystallization pressure. The hydration pressure
was first suggested by Mortensen (1933) and verified more
recently (e.g., Kwaad 1970). There are other proposedmechanisms, and the interim model contributes tothe understanding of the degradation of porous materials by salt. However, most models have not yet beenverified by the laws of physics, chemistry, or physicalchemistry (thermodynamics). Several equations of thetheories are summarized in Table 1.

from the top surface and the entire system eventually solidifies, thus creating pressure in the closed gap.
The crystallization of the salt by cooling can likewise
apply pressure by the change in volume, but because thesystem is necessarily open, when salt crystallization byevaporation occurs, the pressure cannot be applied by the
total volume change. That is, the holes must be large
enough to allow easy communication with the atmosphere,
and there must be a net loss of water content by evaporation. Furthermore, once the gap widens, it is almost impossible to cause rock disaggregation, as the entire volume
change cannot exert any further destructive force.
Thompson (1862) pointed out that rock collapse is notcaused by an expansion of the total volume, but by thegrowth of crystals in contact with a liquid that tends to deposit the same crystalline material on the solid, until thevoid space in which they are contained is filled. Cobb et al.
(1990) also argued that the damage caused by the
crystallization of sodium sulfate must be due to unknown
crystallization forces, not volume expansion. Furthermore,
Detsch (1914) showed that porous tiles and bricks can be
destroyed as Na2SO3·5H2O crystallizes with decreasing
volume. Thus, the crystallization of the salt is not related
to the volume expansion of the salt-solvent system but isrelated to crystal growth. So far, there have been few studies measuring crystal growth pressure.
Andree (1912) reviewed thoroughly on the subject andfavored the views of Becker and Day (1916). Nonetheless,some researchers (e.g., Schbnikov 1935; Arakelyan 1987)
failed to take into account the tendency of crystals growing against pressure to have concave undersurfaces, withthe weight supported by a narrow rim and thus underestimated the crystallization pressure drastically by overestimating the area of contact between crystal and pore wall.
Becker and Day (1916) observed that if the solution was
sufficiently saturated, the crystal could grow at points whichwere under pressure though stressed crystal growth was
slower than the growth of unstressed crystals. The crystals
not only grew on the top and sides but also grew a rim
around the edges (but not below the central portion of theoriginal crystal) of the bottom which lifted the growing crystals from its support. This lifting took place even if a substantial weight (0.7 g) was placed on the crystal but the rate oflifting was decreased by a factor of 25 over that of a crystal
stressed only by its own weight. Becker and Day (1916) also
remarked that the lifting force cannot be due to, as Bruhns
and Mecklenburg (1913) suggested, capillarity, which would
invoke surface tension pulling the crystal downward.
Taber (1916) confirmed that crystallizing alum, CuSO4
and K2SO4 could be able to lift glass plates several mm
and the crystals exert a linear force regardless of which
crystal face they rested on. The new growth was outward
and downward, the original crystal sat at the top of aterraced hollow the depth of which equaled the height ofOguchi and Yu Progress in Earth and Planetary Science

2.4 Crystal growth pressure

Crystallization of salt from supersaturated solutions candestroy rocks and other porous materials. This salt crystal
growth is the most dangerous of the salt weathering processes, as it generally exerts a wedging force as the salt solution in rock pores and cracks crystallize (Steiger 2005a,
2005b; Genkinger & Putnis 2007; Ruiz-Agudo et al. 2012;
Scherer 2004, 1999, 2006). Because the tensile strength ofrocks is in the range of one third to one tenth of their compressive strength, the pressure caused by crystallization andhydration of salts exceeds that of most rocks and concrete(Knacke and Erdberg 1975). This pressure is called
crystallization pressure or crystal growth pressure andcauses salt weathering in rocks, monuments, buildings, andother structures. The pressure is caused by the volume ofthe salt increasing against the confining force of the rock.
Early knowledge of salt weathering suggested that themechanism of rock collapse was the total volume expansionof the salt-solvent system due to the crystallization of thesupersaturated solution. This hydrostatic crystallizationpressure occurs when the supersaturated solution occupies
a smaller volume than the precipitated crystals and the residual saturated solution (Correns 1926). This pressure isdirectly proportional to the supersaturation ratio. However,as the pressure increases, according to Le Chatelier’s
principle, the solubility of the salt also increases,
thereby reducing the supersaturation ratio. Therefore,
this type of mechanism cannot account for the observed deterioration of porous stones by salts (Kwaad
1970; Duttlinger and Knöfel 1993).
Taber (1929, 1930; cited in Evans 1970) argued thatwater freezing or supercooling is similar to the
crystallization of solutes in unsaturated materials. Freezing depends on intermolecular attraction rather thanvolume expansion, and molecules in solution reach thepoint of attachment to the growing crystal by osmoticpressure. However, this similarity is limited to salt
crystallization involving salt and solvent. This is primarily because only salts, which are a relatively small proportion of the system, are converted to the solid phase,and secondly, crystallization of the salts involves asmaller volume expansion (Correns and Steinborn
1939). In contrast, as the water cools, freezing may begin

Only one method (porosimetry) is enough to determine
this index.

Ipc: Index of total connected porosity
Ipm0.1: Index of microporosity of pores
smaller than 0.1 μm in radius
Pms: Total connected porosity
Pc: Microporosity of pores smaller than
5 μm in radius

Salt Susceptibility Index, SSI, SSI = (Ipc + Ipm0.1)(Pms/Pc)

Yu and Oguchi (2010a, 2010b)

This parameter reflects three kinds of conditions (poresize, evaporation, and rock strength) of salt weathering.

AI: Capillary coefficient (C), evaporation
coefficient (E), and tensile strength (σT)
AV: P-wave velocity (vp)

Alteration index (AI) estimator,
pﬃﬃﬃﬃÞ
AIestim ¼ lnðE100C
σT
Alteration velocity (AV) estimator, P-wave velocity

Angeli et al. (2007)

It is easy to consider because of simple equation.

Petrophysical durability estimator, PDE ¼ XσBenavente et al. (2004a)

Only one method (porosimetry) is enough to determine
this index.

No chemical analyses for saline solution but strength testof solids (rocks) are necessary. The parameter can consider
variable tensile strength of rocks.

Fairly complex methods (determination of pore size
distribution and chemical analysis of saline solution)
are necessary.

For determination of this index, only easy-measurement
parameter (porosity) of solid (rocks) is enough, althoughchemical analysis of saline solution is necessary.

Comments (merits and demerits for determinationof index)

Estimators based on pore structure (X)and various rock strengths (σ)

Durability dimension estimator, DDE ¼

Ordóñez et al. (1997)
Pore size distribution, pore radius (r), andconnected porosity (Pconc)

Calculated crystallization pressure based onpore size distribution and pore radius (Pc)
and tensile strength (St)

Weathering susceptibility index, WSI ¼ PSct

Matsukura and Matsuoka (1996)

Pconc

Total porosity (P), microporosity (Pmicro), andsaturation coefficient (Cs)

Durability estimator, x = [(P. Pmircro)Cs]0.5

Moh'd et al. (1996)r

Saturation coefficient (Cs) and total porosity (P)Durability factor, D ¼ C 2s P

Richardson (1991)

P vðrÞ

Parameters used in durability
estimation

Durability estimator

Table 1 Various durability estimators reported in the literature

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the lifting observed. He also came to similar conclusion
with Becker and Day that growth of loaded crystals was
naturally slower than that of unloaded crystals. However,
the growth of the former could be increased by raising
the level of supersaturation.
Mosebach (1951) measured pressures in excess of 47
atmospheres by growing copper sulfate (CuSO4.5H20)
crystals. The author also showed that different crystal
faces have different abilities to grow against pressure;the (010) face of potassium chromate (K2Cr2O7) could
grow against a pressure more than twice the limit forthe (001). Correns (1949) also found that certain faces of
the alum crystal did not show any growth while others
grew against pressures up to 42 atmospheres. An unsolved paradox is that surfaces parallel to the directionof pressure, with free access to the solution, ceased to
grow at pressures much lower than for surfaces normal
to the pressure.

exert lower crystallization pressures than anhydrous salts(Sperling and Cooke 1985).
Winkler and Singer (1972) and Winkler (1975) used
the Correns equation to calculate the crystallizationpressure of common salts at temperatures of 0 and 50 °C
as the supersaturation ratio increased to 2, 10 and 50. At
C/Cs = 10, rock salt can exert maximum crystallizationpressure of 2190 atm. Rock salt crystals are mainlypresent in areas where high concentration ratios arecaused by continuous evaporation, and transport of salt
upwards can concentrate an almost unlimited amount ofsalt near the edge. A comprehensive review and discussion of crystal growth under these conditions waspresented.
Duttlinger and Knöfel (1993) pointed out that the Correns equation is applicable only at low supersaturation
ratios. That is, if the oversaturation value exceeds 1.3, it
deviates substantially from the theoretical value. They
also showed that the crystallization pressure, frequently
cited by Winkler (1975), was calculated from a highly
unlikely supersaturation ratio in stone pore systems, andAmoroso and Fassina (1984) also discussed this topic.
Cooke and Gibbs 1993demonstrated that Pc in the Correns equation is not necessarily the pressure exerted by
crystal growth. They explain it as follows: “If supersaturation occurs in an unconstrained situation,
crystallization will occur but no pressure will be applied.
When the crystal begins to grow in a confined space in a
porous solid, the volume change will make the solid
elastically elastic. Stress builds up as it begins to deform,
which increases as the deformation increases until it isrelieved by plastic deformation (of ductile solids), cracking (of brittle solids such as stones), or the value of Pc.
The issue with the Correns equation is that in the caseof supersaturation, which can easily occur, Pc is verylarge and is probably not achieved before reaching thecritical stress of internal cracks in porous stones. The internal stresses developed at the onset of cracking willthen be directly proportional to the amount ofcrystallization that has taken place, the crystal molar volume Vc and the elastic moduli of the stone. It follows
that the theoretical crystallization pressures Pc for various salts do not necessarily rank them in order of propensity for causing damage. So long as they all exceed
the stresses required to cause internal cracking, the crystal characteristic that should be an indicator of damage
propensity is the crystallization volume, Vc. This accords
with empirical observation.”Price (1978) also remarked that “How realistic is it tothink that one can get significant degrees of supersaturation in a dirty piece of stone, with any number of possible nucleation sites? For example, won’t the receding
front of solution leave behind a supply of unstressed
crystals that can act as nuclei?”Oguchi and Yu Progress in Earth and Planetary Science

2.5 Influential theories, criticisms, and arguments

The following three theoretical analyses are most influential and cited in the history of salt weathering research. The first is an early theoretical analysis of linear
crystal growth pressure developed by Correns and Steinborn (1939). Correns (1949) analyzed the salt-solutionrock three-phase system based on the experimental observations of Taber (1916). For a thin supersaturated solution membrane, it is assumed that salt crystals in theconfined pore space can grow while the film is maintained at the pore wall interface according to
Pc ¼ ðRT =V c Þ ln ðC=C s Þð1Þ

where Pc is the pressure applied by crystal growth (atm),
R is the gas constant of the ideal gas law (8.3145 J mol−1K−1), T is the temperature (K), Vc is the molecularweight of the solid salt (cm3 mol−1), C is the actual concentration of the solute during crystallization, and Cs is
the saturation concentration of the solute. Much less research has been done on this theory after this formula
was proposed (Wellman and Wilson 1965). Buckley(1951) also contributed a useful review of crystal growthpressure and recognized the existence of crystal growthpotential.
The magnitude of the salt crystallization pressure is
proportional to the degree of supersaturation of the salt
solution. That is, it depends on the supersaturation ratio,
C/Cs; the higher the degree of supersaturation, thehigher the pressure of the salt crystals, with limited
space, to a lesser extent, and able to withstand temperatures inversely proportional to the molar volume of thesalt present. This pressure depends on the type of salt
and its hydrate. Due to the inverse relationship between
crystal growth pressure and molar volume, salt hydrates

(2021) 8:32

Page 10 of 23

Weyl (1959) is more sophisticated in including Correns’model for binary systems (mineral-solution-mineral) byconsidering that crystals can be grown by displacing solid
constraints. In the proposed model, crystal growth exerts
crystallization force against the restraint. Weyl’s (1959) external stress causes crystals to dissolve. Pressure dissolution phenomena and crystallization forces are the resultof the removal or deposition of mineral matter in the contact zone between mineral particles.
In the theory, under pressure dissolution andcrystallization pressures, adjacent grains are separated bya film of solvent in which diffusion takes place up to aneffective normal stress (σ = total normal stress − hydrostatic pressure). It is assumed that the pressures between
grains will be very high. Weyl (1959) and Tada et al.
(1987) found that the thickness of this solvent film must
be of the order of a few molecules or a few nanometers
for the diffusion of the solute to the deposition site togrow the stressed crystals. These solvent films are characterized by a diffusion constant, D, and thickness, h,which may depend on the effective normal stress, σ,across the film. h decreases exponentially with σ,
whereas D does not change substantially throughout thefilm and the solute concentration is a linear function of
the effective normal stress across the film.
As the normal stress exceeds the hydrostatic pressure,the solution film must be able to bear shear stress, andtherefore cannot act mechanically as a liquid. To be active in solution or in precipitation, the solute ions must
be able to diffuse into the film. The rate of diffusion
through this film does not have to be as fast as in freesolution, but it must be much faster than solid diffusion
through adjacent mineral particles. Not all researchers
define the solution film between crystals and pore walls
supported by capillary forces as required for crystal
growth and the resulting crystal growth pressure. Some
researchers believe that close contact with the crystal
wall is necessary (e.g., Weyl 1959).
Weyl (1959) provides different expressions to elucidate
the processes of pressure solution and crystallization
force. The contact surface across which the stress istransmitted is assumed to be a flat, circular plane andthe change in effective normal stress σ with distancex (dσ/dx) is calculated from its center. If dσ/dx is
negative, that is, the effective normal stress increases
inward toward the center of the area of contact, solute diffusion is outward to the less stressed areas andsolute is removed from the zone of contact and hencepressure solution occurs. If dσ/dx is positive, that is,
the effective normal stress increases outward, inward
diffusion may lead to continued deposition of solutefrom the solution film on the growing crystal surface
at the contact area, and thus crystallization force isexerted against the stress.

If the average effective normal stress, σavg, at the margin of the zone of contact is positive (inward diffusion)and at the center is negative (outward diffusion), crystal
growth takes place around the periphery and the crystal
forms a hollow center, as in the experiments of Becker
and Day (1916) and Taber (1916). As the average effective normal stress is increased, the rate of crystal growthdecreases inversely to the stress and the radius of thehollow center shrinks until it disappears.
As the average effective normal stress increases further, the rate of crystal growth continues to decrease
until the stress across the film is constant; hence, there
are no more concentration gradients, and thus no diffusive transfer. Further increases in average effective normal stress changes the direction of solute diffusion, andpressure solution occurs at an increasing rate with theaverage effective normal stress until the film disappears
and solution ceases. Therefore, Weyl’s phenomenological theory could qualitatively confirm and elucidate
the experimental results of Becker and Day (1916), who
observed that alum crystals grew faster under low pressure than under high pressure.
Weyl’s model is directly applicable to a threecomponent system (crystal-solution-rock) in which therelatively insoluble rock mineral differs from the solute,
which crystallizes out. Weyl demonstrates that if the solution is supersaturated, precipitation in the area of contact between the crystal and the pore wall will occur aslong as the ratio of the supersaturation, Δc, to the stresscoefficient of solubility, b, is greater than the average effective normal stress, σavg, between the crystal and thepore wall. Hence, the force of crystallization occurs via
crystal growth. A transition point will occur when the
average effective normal stress becomes equal to the ratio of the supersaturation to the stress coefficient ofsolubility, and no more diffusion will occur at this point.
After this transition, Weyl’s mechanism indicates thatcrystallization will stop and pressure solution will takeover if the average effective normal stress across the solution film exceeds the ratio of the supersaturation to
the stress coefficient of solubility.
In deriving the theory, Weyl assumes that the minerals
have a positive stress coefficient of solubility, that is, theminerals are more soluble under stress than without
stress. If solubility increases with stress, crystallization
can only occur at points of increased stress if the external solution is supersaturated. Therefore, minerals withsmaller but positive fractional pressure coefficients ofsolubility can grow against higher stresses, or with lower
supersaturation. Weyl gives the following practical example. Calcite has a fractional change of solubility withhydrostatic pressure of approximately 10−3 per atm(Owen and Brinkley 1941); therefore, at 1% supersaturation, calcite should crystallize against an effective

Oguchi and Yu Progress in Earth and Planetary Science(2021) 8:32

Page 11 of 23

normal stress of 10 atm. This is of the same order as thetensile strength of rock. If its supersaturation is higher, it
will crystallize against a proportionately higher stress.
Although this assumption is valid for calcite andquartz, if a mineral has a negative pressure coefficient ofsolubility, the force of crystallization increases at pointsof stress and decreases solubility. Under this unstablesystem, needles would grow in the direction of maximum principal stress. However, all common naturalsalts crystallize with an increase in system volume andthus have positive pressure coefficients of solubility.
Weyl’s model even shows that if supersaturation is sufficient, crystals can exert great pressure, without necessarily filling pores. This has also been experimentally
verified by Becker and Day (1916) and Taber (1916).
Even if crystals grow down capillaries, they can continue
to grow in the pores that they fill. The pressure theyexert is controlled by the local supersaturation, rather
than by the size of capillaries.
In fact, Weyl considers Correns’ model as an exceptional case. Although the surface tension, which indicates only whether the interstitial film exists, isnecessary, it is only one of the conditions necessary forcrystallization. It neither accounts for the continuance oflifting by crystallization from the film nor considerspressure solution by the film.
According to Evans (1970), Weyl’s model is the only
acceptable explanation for the observed force of
crystallization (salt damage) and has been accepted as
being more realistic and preferable to other approaches.
Nevertheless, Yatsu (1988) felt Weyl’s model was not entirely convincing because it assumes that the concentration of solute in the interstitial film is a linear functionof the effective normal stress across the film, and thevalidity of this assumption has not been verified. In
addition, Yatsu criticized the facts that no values were
proposed for the stress coefficient of solubility, and thatWeyl’s model did not consider the potential energy during the crystallization from solution. However, data from
Tada et al. (1987), Maliva and Siever (1988), and Dewers
and Ortoleva (1990) validated Weyl’s model and its ability to explain various natural phenomena.
The presence of a clay film in the solution film bordering the two crystal grains increases the force of
crystallization. The clay film consists of a series of platelets with associated water films. These water films provide substantially increased rates of solute diffusion,
which increase the rate of crystallization (Weyl 1959).
Wellman and Wilson (1965), who were inspired by theworks of Everett (1961) and Lewis and Randall (1961),
developed a thermodynamic model for calculating the
crystallization pressure in a porous solid with large poresand small pores in contact with each other. Thermodynamically, there is chemical free energy associatedwith the surface of a crystal and because this free energy
increases with its surface area, the area of its interfaces
should be minimized to minimize the free energy of thecrystal. Hence, in a system containing crystals in equilibrium with a saturated solution, larger crystals will growat the expense of smaller crystals because smaller crystals have a higher free energy per unit mass than larger
crystals owing to the higher surface-volume ratio forsmaller crystals.
The work that must be done by crystal growth on one
face of a crystal is equal to the pressure difference (atm)
between the solid and liquid (Pl − Ps) multiplied by theincrease in crystal volume, dV (cm3). This work must
equal the work required to extend the surface, σdA,
where σ is the interfacial tension between the solution
and the growing crystal (dyne cm−2), and dA is the increase in crystal surface area (cm2). Because σ is independent of V, the equation developed by Wellman andWilson (1965) to calculate the pressure built up in thisway isOguchi and Yu Progress in Earth and Planetary Science

Pl − Ps ¼ σdA=dV

ð2ÞThey inferred that crystallization induced by evaporation or a temperature change occurs in the larger pores
initially, as supported by observations of Zehnder andArnold (1989). The larger crystals in these large pores
will grow at the expense of small crystals in the small
pores, which supply solution to the large pores (Fig. 2).
When the larger pores are completely filled, the crystals
in these pores do not grow out into the smaller poresbecause this (Pl − Ps = σ dA/dV for the crystals to growdown the capillary pores) requires a large increase ininterfacial surface area relative to the small increase involume, which would increase the chemical potential(free energy per mole or per gram of material) of thecrystal extensions hugely.
Thus, the crystals continue to grow until they exert apressure against the pore walls, which raises their chemical potential to that of the crystal extensions in unfilled
pores, and this eventually disrupts the rock. Even when
crystals form suddenly in all the pores, that is, there arelarger crystals in larger pores and smaller crystals in tiny
pores, the smaller crystals dissolve to decrease the freeenergy of the system and the ions are transferred to thelarger crystals. Thus, the larger crystals continue growing until they exert the crystallization pressure on thepore wall.
Wellman and Wilson (1965) state that whether disintegration occurs depends on the smallness of the small
pores (the smaller the pores, the greater the pressure thecrystals can exert) and the value of σ compared with themechanical strength of the porous material. If the surface tension, σ, of the salt multiplied by dA/dV of the(2021) 8:32

Page 12 of 23

micropores is greater than the mechanical strength of
the rock, the large pores will be enlarged.
Wellman and Wilson (1965) proposed that the excesspressure caused by crystallization in a large pore, ΔP(dyne cm−2), derived from the modified Laplace equation, is equal to

They refer to Tiller’s (1991) observation that heterogeneous nucleation of a crystal occurs in the smallest
pores, where the solution is withdrawn and eventuallyreaches high supersaturation ratios, thereby generatinghigh crystallization pressures, according to Correns(1949).
However, the Wellman and Wilson model appears to
be well accepted by many other researchers, such as
Fitzner and Snethlage (1982), Zehnder and Arnold(1989), Rossi-Manaresi and Tucci (1991). Fitzner andSnethlage (1982) developed an equation that considers
the crystallization pressures more explicitly in relation toporous materials. Following the work of Everett (1961),
they approach the theoretical problem of crystallizationpressures in porous media in terms of an analogy withthe thermodynamics of freezing of solutions in porous
materials.
Fitzner and Snethlage (1982) expressed the crystallizationpressure as

Oguchi and Yu Progress in Earth and Planetary Science

ΔP ¼ 2σ ð1=r − 1=RÞ

ð3Þwhere σ is the surface tension (dyne/cm) and R is the radius of the large pore (cm) connected by a capillary ofradius r (cm). Thus, when the excess pressure exceeds
the tensile strength of the rock, the material will fracture. The model predicts that crystallization will originate in larger pores, and for porous materials of equal
mechanical strength, materials with large pores separated by areas of micropores will be most susceptible todisruption by salt crystallization (cf. Honeyborne andHarris 1958).
The Wellman and Wilson model is based on the theoretical freezing model developed by Everett (1961), whoestablished that freezing results in the generation of capillary pressure by a solid growing at the expense of a liquid in a capillary. The thermodynamic background forthis theory, according to Evans (1970), is valid only for
fluid, such as gas-liquid and liquid-liquid interactions,and its extrapolation to solid-liquid interactions, thoughwidely accepted, has not yet been validated. Evans(1970) criticized that Pl − Ps may be very large and yet
causes pressure solution of the crystal so that the rockcannot be forced apart. Tiller (1991) also stated that thecapillary pressure, which is valid for liquids, is a fictitious
quantity when applied to solids.
Rodriguez-Navarro and Doehne (1999) argued that
their experimental observations conflict with Wellman
and Wilson’s (1965) model but that they agreed with theoriginal model for the generation of crystallization pressure proposed by Correns (1949) and completed by
Weyl (1959). They reasoned that the Wellman andWilson model does not include the capillary (suction)pressure P (atm), which is, according to the Laplace andWashburn equation, inversely proportional to the poreradius,P ¼ 2σ cosθ=r

ð4Þ

where σ is the interfacial tension and θ is the contact
angle of the meniscus at the pore wall of radius r (cm).
This means that as evaporation progresses, the saturated
solution is suctioned from the larger pores toward thesmaller pores, where it is concentrated. Thus, once critical supersaturation is reached, crystallization occurs inthe smaller pores, not in the larger ones. The larger
pores act as reservoirs supplying solution to the smaller
ones where high supersaturation ratios can be reached.

P ¼ 2σ ð1=rRÞð5Þwhere P is the crystallization pressure, σ is the ionicinterfacial tension of the salt solution, r is the radius of
the small pores, and R is the radius of the large pores.
According to this theoretical approach, crystallization
occurs first in large pores and only moves into small
pores once the larger ones are full. Until that point thesmall pores act as supply reservoirs for salt solutions.
Thus, stones with a high volume of large pores and ahigher volume of smaller pores are particularly sensitive
to salt weathering and frost weathering.
Fitzner and Snethlage (1982) used their equation to
calculate crystallization pressures for four German sandstones whose pore size distribution had been measured.
The results showed that the pressure should increase
uniformly with decreasing pore size, in contrast to differences in the actual behavior of the stones. Using poregeometry models, they developed multiplying factors V1
and V2 to correct the calculated crystallization pressures.
The factor that gave the best results compared with theexperimental crystallization results was
V 2 ¼ V r=ΣVR

ð6Þ

where Vr is the volume of pores of radius r and VR is
the volume of pores of radius R. This factor does nothave any thermodynamic significance, but it increases
the importance of smaller pores. This approach could be
useful in estimating the durability of a stone if the poresize distribution is known.
A similar approach was used by Rossi-Manaresi and
Tucci (1991) to calculate the resistance to salt deterioration of three biocalcarenites, tuff, marble, and(2021) 8:32

Page 13 of 23

sandstone. Interestingly, they used the first correction
factor, V1, which is expressed as

et al. (2012) investigated the orientation of crystal propagation in the crystallization of MgSO4 and discussed the
crystal growth that occurs inside the gap in detail. With
these recent advances, a revised version of Winkler’s theory has been published (Winkler 1994; Steiger et al. 2014).

Oguchi and Yu Progress in Earth and Planetary ScienceV 1 ¼ V r=VR

ð7ÞIn the case of the German sandstones, V1 did notmatch the experimental results. Rossi-Manaresi and
Tucci (1991) did not compare the calculated values withthe experimental results. Their calculations used changesof pore size distribution in different samples of the samematerial, such as the surface vs. interior. For example,
the surface crust of sandstone had a higher number ofsmaller pores, and hence the higher crystallization pressures should lead to its flaking. The heterogeneity inpore size distribution of the calcarenites was predicted
to produce an alveolar deterioration pattern. Finally,
marble, which had few small pores, was predicted to beresistant to salt crystallization.
Aside from some controversy, crystallization generally
continues even to considerable limiting forces. The crystal growth pressure in the pores causes local tension,
which tends to crack the pores and propagate them bytensile failure. To allow the salt to crystallize, the solution film must be maintained at the salt-rock interface.
The surface tension between the salt and the rock must
be exceeded. Crystallization can continue from thismembrane only if the solute diffuses in, which requiresthat the effective normal stress be reduced inward toward the center of the contact area. Because the solubility of the salt increases with pressure, crystallization canoccur up to the pressure at the pressure rise point only
if the solution retains its degree of supersaturation.
Small supersaturations easily achieved by rapid evaporation or cooling can result in a wedging force of
crystallization that exceeds the tensile strength of rocks,
stones, and other porous building materials. However,for complete crystallization from a rock penetrating solution, several days of rainfall or strong evaporation
without spray are required. Because evaporation generally occurs during heating, crystallization can increase
thermal stress by exacerbating space problems.
Salt weathering research also focuses on pore structure
and porous networks from various perspectives (Fitzner
1988; Angeli et al. 2008; Benavente et al. 2001, 2004b,
2007; Gomez-Heras & Fort 2007; Yu & Oguchi 2013;
Kozlowski et al. 1990). Cardenes et al. (2013),
Thomachot-Schneider et al. (2018), and Török andSzemerey-Kiss (2019) examined the relationship between
freeze-thaw and salt crystallization. Raneri et al. (2015)
used micro X-ray computed tomography (μXCT) to
understand the pore structure. Owing to advances in computing, μXCT is an important technique for rock weathering studies, although in the 1990s, it was impossible todistinguish pores in rocks due to poor resolution. Schiro

2.6 Hydration

Some salt hydrates and anhydrates change easily at atmospheric temperature and RH. Hydration can occur
when the temperature is below the transition point of aparticular salt. At the transition point, a reversible
change occurs from anhydrous salt to hydrate, or lower
hydrate to higher hydrate. The salt can be repeatedly
hydrated and dehydrated in the pores as the amount ofhydration water increases or decreases.
The famous German geomorphologist Mortensen(1933) first recognized the importance of salt hydration
as a factor in desert weathering. In his pioneering dissertation, Mortensen challenged the then accepted view thatthe damage observed in desert rocks was caused by frozen
water, a process similar to saltwater crystallization. Instead, he asserted that crystallization pressure does notoccur because the crystals deposited from saturated solution by evaporation occupy less volume than the original
solution. As the salt hydrates, the pressure decreases as
the volume increases. He proposed that rising solutions
caused by capillary action containing salts, such as
Na2SO4, Na2CO3, MgSO4, and CaSO4, have no or low
crystallization water during the daytime when the soil surface temperature is high, causing surface formation of
crystals. When nighttime cooling to ambient environmental conditions occurs, these anhydrous salts or lower hydrates become more stable hydrates or achieve higher
hydration by absorbing water vapor from the atmosphereto the crystal lattice. The volume expansion of the hydrated salt causes the hydration pressure on the pore wall.
To illustrate the point, Mortensen (1933) calculated
the hydration pressures of salts typically found in deserts
by the theoretically derived equation
P ¼ RT =V ln ðp1 =p2 Þð8Þwhere P is the hydration pressure (MPa or atm), T is theabsolute temperature (K), V is the molar volume ofwater of crystallization (cm3 mol−1 or mL), R is the gas
constant, p1 is the vapor pressure of water at T (mmHg),
and p2 is the dissociation vapor pressure of the hydrate
at T (mmHg). According to this equation, salt hydrationpressure depends directly on the number of moles ofwater gained by a salt during hydration, on temperature,and on the ratio of the atmospheric vapor pressure to
the dissociation vapor pressure of the salt-hydrate. Furthermore, the salt hydration pressure is inversely related
to the volumetric expansion of the salt on hydration.

(2021) 8:32

Page 14 of 23

As with crystallization, the ambient environmental
conditions necessary for a salt to hydrate are complex.
Even at low temperatures, the atmospheric aqueous
vapor pressure must exceed the dissociation vapor pressure of the salt-hydrate system for hydration to happen.
On the other hand, for a given atmospheric vapor pressure, the temperature, which controls the dissociation
vapor pressure, should fall below a certain value for thetransition to occur. Therefore, high RH accompanied bya fall in temperature are merely side effects, which do
not account the formation of hydrates. In short, in
addition to transition vapor pressures, some salts also
have a transition temperature above which only the anhydrous form or a lower hydrate is stable. The effect ofincreasing external pressure on the dissociation vapor
pressure has been considered. Depending on the pressure of the hydrate (Moore 1896), the formation of thehydrate against the confinement force can only proceed
until the dissociation vapor pressure, which increases
with the generation of the expansion pressure, is equal
to the saturation vapor pressure of the free water.
Mortensen (1933) also observed that the transition
temperature of sodium sulfate is 32.4 °C in pure solution
and drops to 17.9 °C in a sodium chloride saturated environment. Below 32.4 °C, the stable form of the anhydrous salt, Na2SO4, or the hydrate, Na2SO4·10H2O,
depends on the vapor pressure. Furthermore, for somesalts, the transfer may be rapid. At 39 °C, the transition
from thenardite to mirabilite is complete within 20 min.
Hydration of Na2SO4 to its heptahydrate form at 31 °C
and of Na2CO3 to its decahydrate form at 32.4 °C shows
the presence of excess water (> 60%) Sodium sulfate
underwent a volume change of more than 300% during
hydration; however, at very low humidity, such as 30%,
hydration does not usually occur.
Mortensen's work is supported by Bonnell and Nottage(1939). They developed simple, robust molds under compression, changed the temperature, and measured volume
changes for different mixtures of hydrated salts (sodium sulfate and magnesium sulfate) and sand. They
revealed that anhydrous salts (or lower hydrates) canbe hydrated (or further hydrated) to moderately highstress. Because these stresses far exceed the tensile
strength of conventional porous building materials,
when hydration occurs in the pores of such building
materials, the salt would probably exert sufficient
force during the hydration to causes the material tocollapse. Bonnell and Nottage (1939) did not adapt
unexpanded pores to the swelling expansion, and thesalt crystals expanded to moderately high stresses regardless of whether they were completely constrainedin one direction. Some salt crystals continue to groweven though the large pores in which the crystals cangrow without stress are not filled.

Cooke and Smalley (1968) suggest that in addition to
the crystal growth pressure, destructive stress can also
be caused by the dehydrated salt at high temperatures indeserts. The presence of both anhydrite and gypsum indeserts suggests that some interconversion may occur,and the presence of other salts tends to promote the dehydration reaction.
Although Cooke and Smalley (1968) cite Detsch (1914),
when they partially filled test tubes with plaster of Parisand water stucco, the test tube often broke due to apparent volume expansion. When bassanite turns into plaster,
the volume increases by about 7%. They theoretically predicted a volume reduction of 7% during the curing ofpastes of CaSO4·1/2H2O and H2O, but in fact there was a
0.5% expansion. Thus, even partially dewatered salts are
expected to exert forces on the walls of pores when they
are hydrated. This effect damages construction materials(Schaffer 1932).
Various forms of hydrated and anhydrous CaSO4 arecommon in deserts, and the weathering mechanism of
hydration-dehydration proposed by Cooke and Smalley(1968) is as follows: (1) Hydrated salts are deposited inrock cracks. (2) Dewatering aids salts under high temperatures in the desert, then subsequently rapid wetting
and hydration occurs, and consequently, the wall of thefracture is stressed. However, according to Goudie andViles (1997), this mechanism is not fully understood. In
many cases, there should be room to expand again, as
the anhydrous salt may be formed initially from the salt
solution in the pore or from the hydrated salt.
Kwaad (1970) presents several conditions that must be
met to allow the rock to collapse due to salt hydration.
First, the process of hydration should be fast enough tobe achieved largely within about 12 h; otherwise, thehydration cannot be caused by the daily change oftemperature. Second, to exert hydration pressure, thehydrated salt should not be able to escape from the holewhere it exists. Third, the hydration pressure must exceed the tensile strength of the rock at the hydration
point.
Winker and Wilhelm (1970) suggested that crystallizationpressure against the pore walls is a one-off process, whereas
the hydration pressure affects the crystalline salts and maychange several times in a season or diurnal cycle. To calculate the pressures theoretically produced by hydration,
Winkler and Wilhelm modified Mortensen’s equation as

Oguchi and Yu Progress in Earth and Planetary Science

P ¼ ½nRT =ðV h − V a Þ  2:3 log ðpw =p0 w Þð9Þ

where P is the hydration pressure (MPa or atm), n is thenumber of moles of water gained during hydration to
the next highest hydrate, R is the gas constant, T is theabsolute temperature (K), Vh is the volume of hydrate(cm3/gm mol of hydrated salt), Va is the volume of the(2021) 8:32

Page 15 of 23

original salt before hydration (cm3/gm mol), pw is thevapor pressure of water at a given temperature (mmHg),
and p′w is the vapor pressure of the hydrated salt
(mmHg).
They calculated the hydration pressures of some important common salts (bassanite-gypsum, thenarditemirabilite, kieserite-hexahydrite, hexahydrite-epsomite,thermonatrite-heptahydrite, and heptahydrite-natron) at
different temperatures and relative humidities by using
the data on the dissociation vapor pressures of the hydrates from Mortensen (1933) and other chemical constants from Weast (1967). They observed that thegreatest hydration pressures (maximum of 2190 atm at
0 °C and 100% RH) occur when bassanite is converted togypsum. This is in excess of the crystallization pressureof ice at − 22 °C and is in excess of the pressure required
to exceed the tensile strength of rocks.
Calcium sulfate hemihydrate (plaster of Paris), which
occurs in nature as bassanite, is a key raw material in
the building industry (Lager et al. 1984). Heating to373–383 K produces a metastable anhydrite, which rehydrates rapidly under normal atmospheric conditions.
The hydration pressure of anhydrite to gypsum causes a
maximum pressure of 2800 atm (283.71 MPa) at 0 °C
and 100% RH (Winkler 1975). The hydration of various
magnesium sulfates is less common (Foster and Hoover1963) and exerts only low pressures. The hydration of
thenardite to mirabilite is fast and effective in salt weathering. In general, low temperatures and high RHs produce
high hydration pressures, whereas high temperatures andlow RHs produce low pressures.
The hydration pressures calculated by Winkler andWilhelm (1970) have been quoted extensively in literature, although Evans (1970) pointed out that Winkler
and Wilhelm (1970) do not discuss the extent to which
the hydration pressures they have calculated can be realized in nature. He also argues that although their data
show that the theoretical pressures increase with RH
and with decreasing temperature, these pressures (2190
atm for CaSO4) have not yet been observed experimentally. Moreover, their calculated hydration pressures forthe bassanite-gypsum system (hemihydrate to dihydrate)
were shown to be inaccurate by Kirchner (1996), who
also calculated the hydration pressures associated withthe halite (NaCl) and hydrohalite (NaCl·2H2O) system.
In fact, Mortensen’s hypothesis of hydration pressure appears logical and was not strongly questioned for morethan 60 years because, as Duttlinger and Knöfel (1993)highlight, there have been few attempts to validate thehydration pressure theory experimentally under controlled conditions and results have been inconclusive.
Duttlinger and Knöfel (1993), in their thorough analysis of Mortensen's hypothesis, derived Eq. (9) above
considering hydration as an osmotic process. When

Duttlinger and Knöfel (1993) rederived Eq. (9) from thesources mentioned by Mortensen, they obtained aslightly different equation that resulted in far smaller
calculated pressures. Because osmotic processes require
the presence of liquid water, Duttlinger and Knöfel concluded that the formation rate of the hydrate, via dissolution of the anhydrous salt and reprecipitation of thehydrate (Snethlage and Wendler 1997), must be a key
factor. They suggested that the hydration rates of the individual salts could explain the observed differences inthe effectiveness of the various salts in salt weathering.
Recrystallization does not always occur from one crystal lattice to another and may pass through the liquidstate. For instance, anhydrite completely dissolves before
it crystallizes as gypsum. The process is restricted to
crystallization. Thus, there is no hydration pressure forthe conversion of anhydrite to gypsum. Further studies
on the hydration-dehydration mechanism of sodium sulfate also showed that while the dehydration of mirabilite
proceeds via multiple dissolution steps ending in a polycrystalline mass of submicron particles, the hydration
rate of anhydrous thenardite is slowed due to the formation of a hydrated film on the crystal surface (Charolaand Weber 1992; Doehne 1994). That is, the hydrationof thenardite does not proceed via absorption of moisture by a solid crystal but only by dissolution of thatcrystal and reprecipitation from solution.
Although the processes of crystallization and hydration
have been recognized for many years, it is still difficult
to differentiate the mechanisms despite a sound basis oftheoretical understanding. However, it has long been
accepted that the combination of crystallization and hydration appears to play an important role in salt weathering. For instance, McIntyre & McTaggart (1979)
concluded that hydration was important because thecrystallization experiments showed that sodium sulfate
was more effective in salt weathering than nonhydrating potassium sulfate. Winkler and Singer (1972)
reported that the rate and role of hydration of rapidly
hydrating MgSO4 inflicted 35% more damage than
Na2SO4 during crystallization cycles.
Nevertheless, in considering the individual mechanisms, there have been many arguments relating to theeffectiveness and importance of crystallization and hydration processes. Wilhelmy (1958) argued that although
the force of crystal growth is real, it is rarely importantin deserts because it involves crystallization from anoversaturated solution. He preferred Mortensen’s mechanism of salt damage because repeated hydration alternating with dehydration is more realistic as the rock isalternately wetted and dried. However, Baker et al.
(1968) noted that granulation by salts also occurs in regions of temperature and humidity where a transition
point cannot be reached and suggested that these

Oguchi and Yu Progress in Earth and Planetary Science(2021) 8:32

Page 16 of 23

phenomena are better explained by the pressure of crystal growth.
Evans (1970) suggested that rocks may be shattered
not only by the hydration of the salts in the rocks, butalso by the initial process of crystallization. He contended that the crystallization process is more firmly
based upon experiment and theory than the hydration
process. He also argued that hydration is probably important for carbonates and sulfates because rock disintegration is often associated with gypsum. However, he
did not explain the larger number of cases where sodium
chloride, a non-hydratable salt, is the main salt associated with disintegrating rock. Moreover, Lewin (1982)
proved that nonhydratable sodium chloride can disintegrate sandstone samples by crystallization pressurealone.
However, we note that sodium chloride can hydrate
under very cool conditions as hydrohalite (NaCl·1/
2H2O) and is stable at and below 0 °C (Johnston 1973).
In addition, halite forms a dihydrate (NaCl·2H20) at temperatures below − 0.1 °C, and it has been suggested thatthe effectiveness of sodium chloride as a weathering
agent at subzero temperatures may, in part at least, beassociated with the formation of this dihydrate (Williams
and Robinson 1981, 2001).
Although it is not easy to separate the relative powers ofhydration and crystallization (Sperling and Cooke 1980a),many laboratory experiments have been performed to try
to determine whether the effectiveness of hydratable salts
as a deterioration agent arises from hydration or
crystallization pressure (Kwaad 1970; Chatterji et al. 1989;
Sperling and Cooke 1985, 1980a). It has been shown thatrock disintegration is possible by salt crystallization weathering alone under nonhydratable conditions or without hydration pressure. For example, Kwaad (1970) observed thatMgSO4 and Na2CO3, which can hydrate during the diurnal
temperature cycle, were less effective than Na2SO4, which
hydrated too slowly and did not absorb any water ofcrystallization during the 12 h period of observation. He inferred from his experimental observations that NaCl stayed
within the terms of the theory of crystal growth because
the amounts of debris produced in the hydration tests andcrystallization tests were similar. Kwaad (1970) also accepted the theory of crystal growth as valid for Na2SO4 because Na2SO4 exceeded the limits of Mortensen’s hydration
theory by being active even when no hydrate could occur
and because its hydration velocity was low. On the other
hand, other experimental work has established that hydration alone can be effective in rock disruption (Sperling and
Cooke 1980b). Therefore, the arguments continue and thismatter still needs to be scrutinized in further investigations.
Scherer (1995) claimed that the pressure exerted bysalt hydration is doubtful and the statements about thehydration of crystals in most of the literature may be

misleading. Scherer argued that crystals cannot absorb
water and swell like a sponge because the crystal structures of the anhydrate and hydrated forms are entirely
different, and therefore the crystal must dissolve andreprecipitate as the new phase. This is true for all crystals; hence, it is unlikely that rock disruption is causedby the hydration pressure of salt crystals. Rather, it isprobably caused by the crystallization pressure during
the growth of the new crystal phase.
Despite numerous experiments and observations of
the process of salt weathering due to the hydration ofvarious salts (Kwaad 1970; Steiger et al. 2008; Goudie
1977; Cooke 1979; Williams and Robinson 1981;
Hamilton & Hall 2008; Steiger & Asmussen 2008; Stergonetch 2008), the nature of weathering processes by hydration require further study. Experiments must be
conducted with greater precision to verify the theoreticalvalues of hydration pressure to evaluate accurately theeffectiveness of crystal growth pressures and hydrationpressures of various salts (Yatsu 1988).

Oguchi and Yu Progress in Earth and Planetary Science

2.7 Differential thermal expansion

A further mechanism whereby salt attack may disrupt
rock was proposed by Cooke and Smalley (1968).
Though they accept the effectiveness of crystallizationpressures, they argue that rock may also be disrupted bystresses caused by hydration of certain salts in confined
spaces and stresses exerted by the thermal expansion ofmany salts in confined spaces. Therefore, the principal
processes of salt weathering include crystal growth fromsolution, volume expansion accompanying hydration,and volumetric expansion of crystalline salts caused bytemperature change (Cooke and Smalley 1968).
If strong surface heating-cooling occurs, the expansion ofcertain salts in the pores may be much greater than that of
the rock, leading to thermal disruption, because the salts
have higher coefficients of volumetric expansion than theminerals in the rock, thereby creating pressures sufficient tocause rock disintegration (Cooke and Smalley 1968). The extent to which salts expand when they are heated depends ontheir thermal characteristics and the temperature ranges towhich they may be subjected. Hanus & Luis (1968) also proposed the thermal expansion of crystalline salts.
Field and laboratory experiments have suggested thatthe expansion and contraction of rock minerals as a result of daily temperature changes in deserts are inadequate by themselves to cause disintegration (Griggs
1936; Parkus 1959; Möller 1990). Griggs (1936) provedthat differential thermal expansion of rock minerals
alone cannot account for rock burst in deserts. For acommon desert salt like NaCl, a typical temperaturechange of 54 °C can produce a volumetric expansion of
approximately 1%, which is considerably greater thanthe expansion that would be expected in the(2021) 8:32

Page 17 of 23

surrounding rock. Halite, gypsum potassium chloride,barium sulfate, and sodium nitrate are common desert
salts that have a greater expansion potential than rockminerals. The volumetric expansions of sodium nitrate,
halite, and potassium chloride are more than three times
that of granite, and sodium nitrate is likely to be themost effective of these salts (Cooke and Smalley 1968).
Many salts obscure this process of thermal expansion
with crystallization and hydration, except for nonhydrating salts, such as halite, which do not usually exert
hydration pressure. The thermal expansion withtemperature increase from freezing to 60 °C is 0.5% forhalite, whereas it is less than 0.2% for granite. This difference appears small, yet the disruptive expansion ofsalts can participate in rock decay. Therefore, if the rockis granite or another rock with similar properties, the effect of temperature changes on salts precipitated fromsolution in crevices and pores in these rocks near thesurface may be considerable. First, large internal stresses
are produced by crystallization, and then the combination of these internal stresses with the stresses causedby salt expansion may cause fracturing. The cracking
produced by salt expansion in turn provides avenues forthe further penetration of saline solutions, and theprocess may continue until the rock disintegrates (Cookeand Smalley 1968).
Cooke et al. (1993) argue that empirical studies of cliff
weathering by Johannessen et al. (1982) support the effectiveness of the differential thermal expansion process.
They showed that in the mid-latitude coastal environment of Oregon, the rate of cliff weathering was at least
10 times faster on sunny south-facing cliffs than on
shaded north-facing cliffs. This contrast is attributed tothe thermal expansion of NaCl on heating in the sprayzone on the south-facing cliffs. The most important factor is probably the absolute range of temperature
differences.
Cooke (1979) deduced the relative effectiveness of thethree most cited salt weathering mechanisms as follows:“Crystal growth seems to be most pronounced whererelatively saline solutions are rapidly evaporated, e.g.,
humid coastal deserts; hydration is probably most significant where diurnal changes of temperature and relative humidity combine to cause hydration thresholds tobe crossed frequently; and thermal expansion of crystalline
salts should be greatest where diurnal temperaturechanges are highest”. Evans (1970) remarked that it is necessary to test experimentally whether the greater differential expansions introduced by salts are adequate. He
argued that any such effect would be additive to that of
crystal growth, which produces its greatest pressures as
the rock dries because when all the solution has evaporated, the dry rock surface heats rapidly. Goudie (1974)
maintained that the thermal expansion mechanism ispossible only theoretically and proved experimentally that
thermal expansion of salts is ineffective in rock disintegration along with insolation and wetting and drying processes. In fact, the importance of pressures due to thethermal expansion mechanism has not been investigatedin any detail, and much more work is required to verify
the existence and degree of effectiveness of this mechanism (Goudie 1974; Obika et al. 1989).

Oguchi and Yu Progress in Earth and Planetary Science

2.8 Development of new theory from current research

Lubelli et al. (2018) stated that new analytical methods
and the categorization of degradation advanced. Certainly, as in Kamh et al. (2019), the categorization of thedegree of deterioration is shown as an example by using
formula previously proposed so far. In addition, researchon the subdivision of the weathered layer has been reported due to advances in microanalysis technology(Přikryl et al. 2017). Furthermore, although it is a special
environment like cave, there are also studies that investigated the relationship with environmental changes using
isotopes (Gázquez et al. 2018).
Espinosa-Marzal and Scherer (2010) pointed out rather in detail the relationships between pore size andcrystallization stress generation. They elucidated that thepresence of small pores, equilibrium thermodynamics indicates that crystallization pressure can result from thecurvature dependence of the solubility of a salt crystal,
whereas non-equilibrium conditions, high transientstresses can occur even in larger pores.
Flatt 2002, 2003, Flatt & Scherer 2002, Flatt et al. 2006
carried out extensive works on crystallization mechanisms. Flatt et al. (2017) summarized the formula for
crystallization pressure as a supplementary technicalpaper. The idea of thermodynamics is introduced; firstly,
the phenomenon of dissolution was explained and thenit was replaced with the ion activity product. If stress is
not generated from the crystallization pressure, the material will not break. There is a continuing movement toseek as much rock properties as possible to determine
the stress generated.
There is also a study comparing the physical properties from accelerated experiments using salt solutions(e.g., Qiang and Zhang 2019; Dassow et al. 2020). Suchproperties of rocks are also examined by using sampled
rocks in fields as well (Germinario and Ákos 2019).
Rock property change is one of the comparing measures of rock durability. Although not from a viewpointof salt weathering but general weathering, Oguchi et al.
(1999) indicated that the mechanical strength declined
first among various properties which means that mechanical property is the most sensitive to weathering. Considering that mechanical strength is the most importantparameter for construction planning, it may be the mostimportant for targeting to prevent strength reduction. In(2021) 8:32

Page 18 of 23

other words, the structure will not collapse, if thestrength can be prevented from decreasing, even thoughweathering or deterioration occur severely. From thispoint of view, the ideas and applications of surface protective agents are increasing (e.g., Manohara et al. 2019).
Research on micro- to meso-scale landforms such as
honeycomb structure or taphoni has also derived fromsalt weathering. These kinds of studies are increased not
only in geomorphological journals but also in materialscience journals (e.g., Chen et al. 2019; Li et al. 2019).
It can be said that this kind of study was originally dealt
in geomorphology field but needs the sense of geochemistry or crystallography fields. Besides, review papers dealing
with influence of risks of recent disasters and environmental problems have been appeared as well (McCabe et al.
2013; Gomez-Heras and McCabe 2015; Menéndez 2018;
Bertolin 2019; Fusade et al. 2019). These multidisciplinary
studies are welcome, although it might be difficult for
many people to understand thoroughly. These multidisciplinary studies are welcome, although it might be difficult for many people to understand thoroughly. This isbecause it is necessary to deal with global warming andthe intensification of disasters, which are becoming more
serious year by year. Fluctuations in temperature and humidity will affect the type of precipitated salt.
Doehne and Price (2010) implied that new challenges in
the future require materials for applied research, technology transfer, and long-term maintenance concerning morethan 800 documents. Applied research and technology
transfer are good at engineering and civil engineering, andtheir theory depends largely on the results of physics andchemistry. The present paper agrees with their statement,
and the following viewpoints are dared to be added.
The change and alteration of a material can be considered
as water-rock reaction, in detail, a solid-solution or a solidgas reaction, in which the material is placed as a deciding
factor. Weathering and alteration of materials can be considered as water-rock reactions (WRI), in detail, solid-solution
reactions or solid-gas reactions. In these cases, material location is one of controlling factors. If this concept applies tothe weathering of a stone building, material is stone, and thelocation of the building represents environmental factors:
temperature, RH, etc. Thus, it is necessary to consider how
to suppress the reaction caused by the intrusion of water.
Regarding long-term weathering of durability, there isan example concept from geomorphology proposed by
Suzuki (2002), called geomorphological equation: Q = f (S,A, R, t), here, Q is geomorphic quantity; S, geomorphic
setting or initial landforms; A, geomorphic agent; R, resistance of landform materials; and t, duration of the acting
agents. This concept simply means that landform changes
can be estimated from above parameters in which particularly important parameters of material properties, timeand environment of the location.

Applying this equation to stone weathering, the equation, Qsw = f (Ssw, Asw, Rsw, tsw), will be expressed. Here,Qsw regards as weathering degree such as the amount removed or destroyed by weathering; Ssw is environmental
condition, i.e., temperature, humidity, atmospheric, andpressure; Asw is salt crystallization pressure or hydrationpressure; Rsw is the resistance of rocks, not only rockstrength but also pore characteristics; and tsw is experimental duration or observing time.
In the viewpoint of microscale, the reaction processes
may be divided by several steps. First, the surface reaction process such as dissolution will occur in a very thinlayer. The dissolved chemical species will react with ion
in solution. Then, solution concentration reaches to thesolubility and excess saturation point of the compounds,
precipitation or crystallization will occur. The pressurecaused by the crystallization becomes higher than therock strength, especially tensile strength, the rock will
destroy. Thus, the parameters of Asw and Rsw in theequation of Qsw = f (Ssw, Asw, Rsw, tsw) is considered tohave the own another equation. Furthermore, the thermodynamics express higher-level concept by itself of the
Asw parameter. By composing all such equations and parameters, a practical equation for salt weathering can beproposed in the future.
Measures for estimating time tsw are also being considered, especially from an archeological or geoscientificperspective to address long-term weathering and deterioration problems, for example, estimating the rate of development of a very thin weathered layer (altered layer)on the surface of a rock (Oguchi 2004). The estimatedvalues are not high accuracy; however, it may be possible
to estimate a certain amount of time as the time that thestone material was exposed to the environment for along time. Collecting such information is an important
approach to long-term problems. The recent studies
from geochemistry and geomorphology can provide abreakthrough for this difficult estimation by using LAICP-MS or cosmogenic exposure ages (e.g., Yokoyama
et al. 2018; Matsuoka et al. 2013).
By obtaining the rough theory relating to long-termchanges are from geomorphology and archeologicalfields and by comparing to the fundamental theory frommaterial science and thermodynamics, it may apply tomore practical problems as a future perspective. Good
cooperation in all these research fields will lead to meaningful conservation and utilization, even though it mighttake a long time. In the next stage of salt weathering research and reviews, it is somehow necessary to organize
and consider these kinds of concepts.

Oguchi and Yu Progress in Earth and Planetary Science

3 Concluding remarks—as a perspective
The concept of crystallization including crystallizationpressure as an important mechanism was first proposed

Oguchi and Yu Progress in Earth and Planetary Science(2021) 8:32

in the early twentieth century. Many equations havebeen proposed and to apply them correctly, it is necessary to consider what type of salt weathering is observed.
Thus, it is important to select and to apply the models
proposed depending on the situation although it is necessary to consider the weathering environment as wellas the types and properties of salts and rocks in salt
weathering studies. A procedure to collect and rebuild
information on these peripheral fields will also be necessary. Appropriate cooperation in all these research fields
will lead to meaningful conservation and utilization will
apply to more practical problems as a future perspective.
Abbreviations
Pc: The pressure applied by crystal growth (atm); R: Gas constant of the ideal
gas law (8.3145 J mol−1 K−1); T: Temperature (K); Vc: Molecular weight of thesolid salt (cm3 mol−1); C: Actual concentration of the solute during
crystallization; Cs: Saturation concentration of the solute; Pl: Pressure of liquid;
Ps: Pressure of solid; V: Crystal volume (cm3); σ: Interfacial tension between
the solution and the growing crystal (dyne cm−2); A: Surface area (cm2);
ERH: Equilibrium relative humidity; NaCl: Sodium chloride (halite);
NaCO3: Sodium carbonate (natron); Na2SO4: Sodium sulfate (thenardite);
RH: Relative humidity; WRI: Water-rock interaction; Qsw: Weathering degree;
Ssw: Environmental condition; Asw: Salt crystallization pressure or hydrationpressure; Rsw: Resistance of rocks; tsw: Experimental duration or observing
time
Acknowledgements
The authors are grateful to research funds provided by JSPS KAKENHI (grant
numbers 26350399 and 22000321).
Authors’ contributions
CTO proposed the idea for this review and contributed some of theliterature. Swe Yu collected the rest of the literature and drafted themanuscript. CTO approved the final manuscript. The authors read andapproved the final manuscript.
Authors’ information
CTO is an associate professor at Saitama University, and a member of the
Geosphere System Research Group, which works on chemical and physicalproperty analyses of various rocks for elucidating weathering processes that
occur in both natural landscapes and cultural properties, especially buildingstones. Swe Yu was a PhD student who worked on salt weatheringexperiments, but he left his research activities and became a civil engineer
to develop his home country. This review is based on his early career review
works.
Funding
Not applicable.
Availability of data and materials
This article is distributed under the terms of the Creative Commons
Attribution 4.0 International License (http://creativecommons.org/licenses/
by/4.0/), which permits unrestricted use, distribution, and reproduction inany medium, provided you give appropriate credit to the original author(s)and the source, provide a link to the Creative Commons license, and indicate
if changes were made.

Declaration
Competing interests
The authors declare no competing interests.
Author details
1
Graduate School of Science and Engineering, Saitama University,
Shimo-Okubo 255, Sakura-ku, Saitama 338-8570, Japan. 2Present address:
ST&T Architect, Yangon, Myanmar.

Page 19 of 23

Received: 1 November 2019 Accepted: 11 February 2021

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