Sato and Hattanji Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-018-0241-2(2018) 5:84

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open AccessA laboratory experiment on salt weathering
by humidity change: salt damage induced
by deliquescence and hydration
Masato Sato1* and Tsuyoshi Hattanji2

Abstract
A laboratory experiment on salt weathering was carried out under changing humidity conditions. Three types of rock
(tuff and dense and porous sandstone) with sodium chloride, sodium sulfate, or magnesium sulfate were subjected tohumidity oscillations from 20%RH to 98%RH during 6 h periods at 20 °C. Prior to the weathering experiment, the rockspecimens were oven-dried, and the water supply was restricted to only that from the air during the experiment. Salt
on the rock specimens was deliquesced/hydrated at almost 100%RH during the high-humidity period and crystallized/
dehydrated during the low-humidity period. Sodium chloride, which has high deliquescence ability, caused the mostintensive weathering. Porous sandstone with sodium chloride was completely broken down after 100 cycles ofhumidity changes. The other two types of rock with sodium chloride showed flaking or swelling on their surfaces.
For dense sandstone with sodium chloride, the Equotip rebound value decreased as the humidity cycles increased.
Sodium chloride has a high susceptibility of deliquescence at humidities of more than 80%RH. Weathering by sodium
chloride would occur extensively on the surfaces of stone heritages and geoheritages in humid coastal spray zones.
Magnesium sulfate induced a weight loss in porous sandstone, flaking from the surface of tuff, and cracks on thesurface of dense sandstone. Sodium sulfate had almost no effect in the humidity-change experiment, although salt
efflorescence was obvious. The results for magnesium sulfate, salt deliquescence, hydration, and crystallization in thehumidity-change experiment were easily repeated. Magnesium sulfate might have more influence than sodium sulfate
when air humidity fluctuates in a short period.
Keywords: Rock breakdown, Weight loss, Equotip hardness value, Longitudinal wave velocity, Sodium chloride,
Sodium sulfate, Magnesium sulfate

Introduction
Salt weathering is one of the major processes of stone decaythat causes serious damage to brick walls (Goudie 1977;
Kuchitsu et al. 2000), structural foundations (Mottershead
1994; Takahashi et al. 1994), and historical buildings (Gauri
1990; Kamh 2005; Thomachot-Schneider et al. 2011). In
natural environments, salt weathering occurs in coastal
areas (Matsukura and Matsuoka 1991, 1996; Mottershead1982), deserts (Cook 1981; Goudie and Cook 1984; Viles
and Goudie 2013), and the Antarctic inland (Fahey
1985; Marchant and Head 2007; Miotke 1982; Selby
and Wilson 1971). Field observations undertaken globally* Correspondence: msato@geoenv.tsukuba.ac.jp
1
Graduate School of Life and Environmental Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan
Full list of author information is available at the end of the article

in a range of environments have suggested the importanceof microclimatic conditions in stone decay, including
temperature (Hœrlé 2006), insolation (Gunzburger and
Merrien-Soukatchoff 2011; McFadden et al. 2005), moisture(Mol 2014), and humidity (Viles 2005).
Many laboratory experiments have simulated salt
weathering under the conditions of “cyclic total immersion”
or “continuous partial immersion”. In cyclic total immersion
experiments, rock specimens are repeatedly subjected tototal immersion in a saturated salt solution followed bydrying (Goudie et al. 1970; Robinson and Williams
1982; Williams and Robinson 1998). In continuous partial
immersion experiments, specimens partly immersed insaturated solutions are subjected to environmental conditions (Goudie 1986; Oguchi and Yuasa 2010; Robinson
and Williams 2000). However, these conditions, in which© The Author(s). 2018 Open Access 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, andreproduction in any 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.

Sato and Hattanji Progress in Earth and Planetary Science(2018) 5:84

abundant salt water is supplied to the rock specimens,
are not likely to occur in the field, with the exceptionof intertidal zones and places with nearby groundwater.
In addition, the drying temperature affects the formation of salt crystals (Aly et al. 2015). The disjuncture
between field environments and laboratory experiments
has often been pointed out (McGreevy and Smith 1982;
Smith et al. 2005).
Air humidity is a water source affecting stone heritages.
Arnold and Zehnder (1989) pointed out the importance ofmonitoring air humidity and temperature for theirpreservation, as these environmental conditions control
salt crystallization and hydration. Goudie and Viles (1997)
assumed that air humidity is an important factor in salt
crystallization from solution, change of phase (hydration/
dehydration), and crystallization pressure. Seasonal salt
deliquescence and precipitation have been shown to occur
on stone heritages in the presence of air humidity changes(Goudie 1977; Matsukura et al. 2004). In dry areas such as
Antarctica and Mars, air humidity is also a major sourceof water, and salt weathering may be induced by cyclicchanges in it (Campbell and Claridge 1987; Gough et
al. 2016, 2017; Heinz et al. 2016; Wellman and Wilson
1965). The influence of humidity on hydration/dehydration has been thoroughly examined (Benavente et al.
2004; Rodriguez-Navarro and Doehne 1999; Steiger andAsmussen 2008).
Salt deliquescence occurs when the vapor pressure ofa saturated salt solution on salt crystals is less than thevapor pressure of water in the air. Fahey (1985) and
Goudie (1993) subjected rock specimens to six different
climatic conditions. Both studies suggested that rockbreakdown is most intensive under high humidity conditions close to 100%RH. These experimental studies
suggested that only salt deliquescence and subsequentsalt crystallization could cause the breakdown of rock.
However, they partly immersed the rock specimens inwater or a salt solution. They also subjected the specimens
to humidity changes along with temperature fluctuations.
This study entailed a laboratory experiment on salt
damage to rock specimens occurring when humidity
fluctuates at a constant temperature. The water source
was restricted to only that from air humidity.

Methods/Experimental
Rock properties

Three types of rock (Oya tuff, Aoshima sandstone, andUbara sandstone) were used. Oya tuff is dacitic or rhyolitic
marine pumice tuff of the Miocene age; it was quarried in
Utsunomiya, central Japan. In Japan, a variety of buildings
and stone statues are made from Oya tuff, which is often
damaged by salt weathering (Oguchi and Yuasa 2010).
Aoshima sandstone is a very fine-grained, well-cemented
sandstone of the Pliocene age; it was collected in the

Page 2 of 10

Nichinan Coast, Miyazaki Prefecture, southwest Japan.
Ubara sandstone is a fine-grained, weakly cemented,
porous sandstone of the Miocene age. The sample wascollected on the southeast coast of Chiba Prefecture,
central Japan. Tafoni and honeycomb weathering features
are well developed on type B shore platforms (Takahashi
et al. 1994; Tsujimoto 1987) in these coastal areas and areoften associated with salt weathering (Bradley, Hutton,
and Twidale 1978; Mustoe 1982; Mottershead 1994;
Mottershead and Pye 1994; Strini et al. 2008; Young 1987).
Table 1 shows the physical properties of these rocks.
Rock surface hardness was evaluated with the reboundvalue (L-value) of a type D Equotip hardness tester (Proceq). We used the single impact method, which is one oftwo methodologies for hardness tests with Equotip (Aokiand Matsukura 2007); this method measures the mean
L-values of single impacts at ten different points. The longitudinal wave velocity (P-wave velocity) was measured with
an ultrasonic tester (Pundit Plus, CNS Farnell).
Humidity-change weathering experiment

Eight cubic specimens with a side of 5 cm were preparedfrom the Oya tuff and Aoshima sandstone. Only fourcylindrical specimens with a diameter and a height of
3.5 cm were prepared from the Ubara sandstone becauseof the difficulties in shaping the material into cubes. The
initial values of the weight, L-value, and P-wave velocity
were measured after oven drying these specimens at
110 °C for 48 h.
Saturated solutions of sodium chloride (NaCl), sodium
sulfate (Na2SO4), or magnesium sulfate (MgSO4) wereprepared at 20 °C. The rock specimens were immersedwith one of the three salt solutions or distilled water for
72 h. After immersion, the specimens were oven dried
again at 110 °C for 48 h to provide them with salt prior
to the humidity-change experiment.
Each specimen was placed on a sieve with 2 mm openings and was subjected to a total of 200 cycles of humidity
changes in an ambient chamber, in which the humidity
was oscillated from 20 to 98%RH every 6 h (Figs. 1 and 2).
The temperature of the air in the chamber was kept at
20 ± 2 °C during the experiment. Air temperature andrelative humidity in the chamber were recorded every
5 min with a data logger (TR-72wf, T&D) with accuracies
of ± 5% for RH and ± 0.5% for temperature. The sensor
required 7 min to reach a 90% response. Weight, L-value,
and P-wave velocity of the specimens were measured
every 20 cycles. For Oya tuff and Aoshima sandstone, four
specimens were used for measuring the weight; the otherfour specimens were used for measuring the L-value andP-wave velocity. The L-value and P-wave velocity were
not measured in the Ubara sandstone because the specimens were small. All fragments remaining on the sieve
were weighed. In addition, the weights of the specimens

(2018) 5:84

Sato and Hattanji Progress in Earth and Planetary Science

Page 3 of 10

Table 1 Properties of the rocks used in experiments
Bulk density,ρbulk (g/cm3)

True density,
ρtrue (g/cm3)

Effective porosityne (%)

Tensile strength,
St (MPa)*

Equotip reboundvalue, L

Longitudinal wave
velocity, Vp (km/s)

Ubara sandstone

1.56

2.67

41.8

1.67

392

0.84

Oya tuff

1.39

2.39

42.3

1.37

452

2.30

Aoshima sandstone

2.40

2.66

5.3

5.75

593

3.32

*The tensile strengths were taken from Tsujimoto (1987) for Ubara sandstone and Yamada et al. (2005) for Oya tuff and Aoshima sandstone, respectively

were measured every hour only for the firsthumidity-change cycle in order to estimate the amount ofwater that absorbed from the air on the specimens in thehigh-humidity period.
Continuous wetting/drying weathering experiment

Another weathering experiment involving repeated wetting/drying with a salt solution was carried out in order to compare its results with those of the humidity-change experiment. Eight cubic specimens each of Oya tuff and Aoshima
sandstone were prepared in the same way as in thehumidity-change experiment. Four cubes were used formeasuring the weight; the other four cubes were used formeasuring the L-value and P-wave velocity. After measuring

the initial weight or initial L-value and P-wave velocity, thespecimens were subjected to a total of 20 cycles of wetting
and drying, each of which was composed of the following
treatment steps: the specimens were (1) immersed in one of
the three salt solutions or distilled water for 24 h at 20 °C,(2) oven dried at 110 °C for 48 h, and (3) immersed in
distilled water for 24 h to remove the salt that had crystallized in the specimens. After cooling, they were (4)oven dried again at 110 °C for 48 h and (5) weighed or
their L-value and P-wave velocity measured. As in thehumidity-change experiment, all fragments remaining on
the sieve were weighed.

Results and discussion
Amount of salt in the humidity-change experiment

Table 2 shows the amount of salt contained in the rockspecimens for each combination of rock and salt. The
porous Oya tuff and Ubara sandstone contained moresalt than the Aoshima sandstone did. The amounts of
Na2SO4 were smaller than those of the other two salts,
which reflected the lower solubility of Na2SO4 (19.0 wt%)
than that of NaCl (26.4 wt%) and of MgSO4 (25.2 wt%)
at 20 °C.
Salt pathway in the humidity-change cycle

The weights of the rock specimens increased in thehigh-humidity period and decreased in the low-humidity
period. Figure 3 shows that there were significant increases

Fig. 1 Experimental design. a Humidity-change experiment. The
weathering cycle was repeated 200 times. Weight, Equotip reboundvalue, and longitudinal wave velocity were measured every 20
cycles. b Wetting/drying experiment. The weathering cycle wasrepeated 20 times. Weight and Equotip rebound value were
measured at the end of each weathering cycle

Fig. 2 Details of the humidity-change cycle. A cycle consisted ofhigh- and low-humidity periods lasting 6 h in total. The solid line
represents the relative humidity measured at 5 min intervals. The
dashed line represents the setting of the ambient chamber(2018) 5:84

Sato and Hattanji Progress in Earth and Planetary Science

Table 2 Salt content of rock specimens used in the humiditychange experiment
Sodiumchloride

SodiumsulfateMagnesium
sulfateUbara sandstone

5.3

2.9

6.1

Oya tuff

7.7

4.2

8.6

Aoshima sandstone

0.5

0.2

1.0

The values represent gram weights of salt in a 100 g salt-free rock specimen

in the weights of the specimens with NaCl during thehigh-humidity period. The specimens with MgSO4 also
showed larger increases than the specimens with Na2SO4
or no salt. The weight increases of the specimens with
Na2SO4 were comparable to the values of the specimens
with no salt. The weight oscillation means that the rockspecimens absorbed water from the air in the high-humidity period, and they dried out in the low-humidity period.
At the beginning of the high-humidity period, all threesalts appeared to form a solution by dissolving into thewater absorbed from the air, indicating that salt deliquescence occurred. The practical deliquescence relative humidity (DRH) at 20 °C is 75.6%RH for NaCl (halite), 83.3–
95.1%RH for Na2SO4 (mirabilite), and 92.7%RH for
MgSO4 (epsomite) (Apelblat and Korin 1998, 2002;
Apelblat and Manzurola 2003). The relative humidity inthe ambient chamber rapidly increased and reached almost
100%RH within a few minutes. The high salt content of
the rock specimen and low DRH of the NaCl were thecauses of the largest weight gain in the high-humidity
period.
Hydration also occurred in the high-humidity period.
Flatt (2002) and Steiger and Asmussen (2008) described
the phase change of Na2SO4. Under the experimental
conditions (20 °C), Na2SO4 exists in two stable phases:

Fig. 3 Weight gains in the first high-humidity period. The value
represents the amount of water absorbed into specimens during thehigh-humidity conditions. The specimens with NaCl showed thelargest absorption. The specimens with MgSO4 showed greater
gains in weight than the specimens with Na2SO4 or without salt
(distilled water)

Page 4 of 10

anhydrate (thenardite) and decahydrate (mirabilite). At
the beginning of the experiment, thenardite should
crystallize on the rock surface because it directly crystallizes
from a saturated solution above 32.4 °C. At the beginningof the high-humidity period, salt deliquescence shouldcause a saturated solution to form. Because the saturated
solution is supersaturated to mirabilite, mirabilite
crystallizes from a salt solution (Flatt 2002). In fact,
mirabilite crystallized in our experiment 30 min after
the first high-humidity period started. In the low-humidity
period, mirabilite should crystallize from the solution at
95.6%RH and dehydrate to thenardite below 76.4%RH(Steiger and Asmussen 2008). The crystallization of
thenardite is dominant below the equilibrium humidity of
86.6%RH, although both mirabilite and thenardite crystallize
directly from solution (Steiger and Asmussen 2008).
The sequence of deliquescence and hydration of
thenardite should occur at the beginning of every highhumidity period. However, the deliquescence and phasechanges of Na2SO4 were most distinct in the first highhumidity period. These changes became unclear as thenumber of humidity cycles increased, and the rock surface
became covered with the efflorescence of Na2SO4. Kwaad(1970) suggested that the hydration rate of Na2SO4 is muchslower than that of MgSO4. Dehydration of mirabilite
might hardly have occurred under our experimental
conditions, in which the air humidity oscillated over ashort interval. Deliquescence of Na2SO4 might havebeen hindered by the high DRH of mirabilite (95.6% at
20 °C), and efflorescence of Na2SO4 remained on therock surface.
MgSO4 has three stable hydrated forms: monohydrate(kieserite), hexahydrate, and heptahydrate (epsomite) (Chouand Seal 2003; Vaniman et al. 2004). The hexahydrate ismetastable. Though a large number of metastable forms of
MgSO4 have been identified besides hexahydrate, thebehavior of these metastable forms is still unclear (Chipera
and Vaniman 2007). At the beginning of the experiment,
kieserite should have crystallized on the rock surface.
Steiger et al. (2008) suggested that hydration of kieseriteand formation of hexahydrate occur rapidly when therelative humidity exceeds the DRH of kieserite (60%RH
at 25 °C), although the reaction proceeds above theequilibrium humidity of kieserite and hexahydrate (42%RH
at 25 °C). The rapid reaction occurs as a two-step processof deliquescence and subsequent crystallization. They also
reported that epsomite did not form above the equilibrium
humidity of hexahydrate and epsomite (51%RH at 25 °C).
However, they did not confirm which hydrated crystals
form at relative humidities above the DRH of hexahydrate(81%RH at 25 °C). Epsomite would form when the relative
humidity exceeds the DRH of hexahydrate. Although we
could not distinguish the hydration pathways in our experiment, deliquescence of kieserite and direct crystallization

Sato and Hattanji Progress in Earth and Planetary Science(2018) 5:84

of epsomite would have occurred within a few minutes of
exposure to humidity reaching almost 100%RH in theambient chamber. The epsomite and hexahydrate would
have crystallized in the low-humidity period. The kieserite
might have occasionally crystallized under the humidity
conditions of our experiment (Balboni et al. 2011). Dehydration of epsomite and formation of hexahydrate
would also have occurred, whereas the kieserite formation
might have been limited by the low equilibrium humidityof hexahydrate and kieserite.

Page 5 of 10aWeight and L-value changes in the humidity-change
experimentAmong the three kinds of salt, NaCl caused the mostdamage to all types of rock in the humidity-change
experiment. Ubara sandstone with NaCl was completely
broken down after 100 cycles (Figs. 4a and 5a). The cycle
0 data in Fig. 5 refers to the ratio of the weight immediately before the first cycle to the initial salt-free weight;
the specimens became heavier from salt precipitation. Oya
tuff and Aoshima sandstone did not show any obvious
weight losses (Fig. 5a), although a few granular fragments
were deposited under the sieve in the cases of the specimens
with salt. The samples of Oya tuff with NaCl showed flaking
on their surfaces (Figs. 6a), while Aoshima sandstone with
NaCl showed swelling on the surface (Fig. 6c). The
L-values of the Oya tuff and Aoshima sandstone specimens with NaCl decreased as the number of cycles increased (Fig. 5b). The L-value of these specimens could
not be measured after their surfaces became rough. The
final L-value was 43.3% of the initial hardness after 120
cycles for Oya tuff and 32.7% after 160 cycles for Aoshima
sandstone.
MgSO4 caused the second most amount of damage tothe specimens. The weight of Ubara sandstone with MgSO4
decreased to 38.1% of the initial weight after 200 cycles.
The samples of Oya tuff with MgSO4 showed flaking on
their surfaces (Fig. 6b) and a reduction in their L-value,abcFig. 5 Weight (a), Equotip rebound value (b), and longitudinal wave
velocity (c) measured during the humidity-change experiment. All
the measured values are divided by the initial value measured
before the specimens were immersed in the salt solutions. Recordsof longitudinal wave velocity from 80 to 120 cycles went missing
because of mechanical trouble. The solid line, dotted line, anddashed line represent the results for Oya tuff, Aoshima sandstone,
and Ubara sandstone, respectively. The symbols correspond to thefollowing combinations: filled circles, NaCl; triangles, Na2SO4; squares,
MgSO4; open circles, no salt (distilled water)

although the decrease over the first 20 cycles (from 140.3 to70.9%) was greater than the change in the subsequent
180 cycles (from 70.9% to 45.8%). The decrease in the first
20 cycles was likely induced by salt deliquescence andhydration. The hydration of kieserite and crystallization of
the hydrated crystals led to minor rock breakdown andminor changes to the surface micromorphology. ThesebcFig. 4 Ubara sandstone samples subjected to 100 humidity-change cycles. The initial shape of each sample was cylindrical, 3.5 cm in diameterand height. The internal diameter of the sieve was 7.5 cm, and the opening was 2 mm. a NaCl caused the most intensive breakdown. The
sandstone completely disintegrated into sand particles. b The specimen with Na2SO4 kept its initial shape, although the sides of the cylinder
showed irregular morphology due to flaking. c The specimen with MgSO4 showed moderate weathering, although it obviously disintegrated

Sato and Hattanji Progress in Earth and Planetary Science(2018) 5:84acPage 6 of 10bdFig. 6 Oya tuff and Aoshima sandstone with NaCl or MgSO4 subjected to 200 humidity-change cycles. The scale bars are 1 cm long. Oya tuff
with NaCl (a) and MgSO4 (b) showed irregular morphology on their surface. Millimeter-scale fragments flaked from the surface of the Oya tuff
with MgSO4. Aoshima sandstone with NaCl (c) showed swelling on the surface. Cracks parallel to the rock surface were observed on the edge of
the Aoshima sandstone with MgSO4 (d)changes caused the L-value to decrease. Aoshima sandstone with MgSO4 had cracks parallel to the rock surface(Fig. 6d). Cracks occurred on the six faces of the cube.
Na2SO4 was relatively inactive as a weathering agent in
the humidity-change experiment, with the exception of theweight loss of Ubara sandstone (14.2%) after 200 cycles.
The L-value of the Oya tuff with Na2SO4 fluctuated withina range from 84.5 to 101.5%. The L-value of Aoshima
sandstone showed indistinct changes, except in the first
20 cycles. The L-value for Aoshima sandstone with
Na2SO4 had a similar trend to that with MgSO4. Tabular
fragments flaked from the surface of the Oya tuff with
Na2SO4 when the specimen was immersed in distilledwater after the weathering experiment.
The P-wave velocities of all the specimens were constantor increased slightly during the experiment (Fig. 5c). Only
Oya tuff with NaCl recorded a significant decrease fromthe start to the first 20 cycles.
None of the rock specimens showed any obvious cracks,
flaking, or swelling at the beginning of thehumidity-change experiment. Therefore, we concluded

that the damage caused by the initial immersion in a salt
solution and oven drying was relatively weak in comparison with the damage induced by the subsequent humidity
changes. The results of the wetting/drying experiment described later supported this conclusion.
The degree of damage induced by each salt was in proportion to the weight gain during the high-humidity
period. The lower DRH of NaCl resulted in greater water
absorption from the air, and the greater absorption, in turn,induced greater weight losses and L-value reductions. The
degree of damage would also have been affected by the factthat NaCl does not hydrate substantially. The hydration of
two sulfates involves volume expansion of salt crystals. The
hydrated crystals might have filled pores in the rockand hindered infiltration of water (Steiger and Asmussen
2008). The specimens with NaCl could absorb greater
amounts of water because NaCl only dissolved and madea solution with deliquescence.
The specimens with Na2SO4 were less damaged than
the specimens with MgSO4, despite that the DRH of
Na2SO4 was lower than or comparable to the DRH of

Sato and Hattanji Progress in Earth and Planetary Science(2018) 5:84

MgSO4. Ruiz-Agudo et al. (2007) carried out a laboratory
experiment on the solutions of Na2SO4 and MgSO4. They
reported that Na2SO4 and MgSO4 were both extremely
damaging. Balboni et al. (2011) explained that the different
behaviors of the two sulfates are due to the different
viscosities of their solutions. They assumed that thehigh viscosity of MgSO4 retards the solution transportand crystal nucleation. High viscosity also causes pores toclog, which retards hydration and deliquescence. In our
experiment, as previously mentioned, the slow hydration/
dehydration rates of Na2SO4 and the high DRH of
mirabilite would have restricted their activity. Na2SO4
is generally regarded as one of the most aggressive salts
in both the field and the laboratory (Goudie and Viles 1995,
1997; Smith and McGreevy 1983; Thomachot-Schneider et
al. 2011). However, the relative importance of MgSO4 to
Na2SO4 might increase in environments in which available
water is limited and the humidity is extremely high.
The difference in the water supply method affected
how the sulfates induced breakdown of the rocks.
Ruiz-Agudo et al. (2007) reported that Na2SO4 caused
localized damage and detachment, whereas MgSO4
induced cracks and fractures. Their experiment, inwhich the lower part of the specimen was immersed in asalt solution, induced salt crystallization inside the rockinterior. In contrast, in our experiment, both sulfatesconcentrated in the surface layer because of the limited
water supply from the surface of the rock. Cracking
parallel to the rock surface, which occurred in the
Aoshima sandstone with MgSO4 and in the Oya tuff
with Na2SO4, suggests that salt weathering occurred only
within several millimeters of the surface. The interiors
were not weathered much because the P-wave velocitiesof the specimens did not decrease. López-Arce et al.
(2011) suggested that porous dolostone is sensitive todiurnal cycles of air humidity at depths up to 3 cm. In our
experiment conducted in a short time interval, the depth
at which air humidity could have had an effect would havebeen less than a centimeter from the rock surface.
The specimens with NaCl did not show any cracks.
Because NaCl tends to crystallize on the surface, thiscrystallization pattern is referred to as efflorescence. In
contrast, Na2SO4 and MgSO4 tended to crystallize withinrock pores, referred to as subflorescence. Salt subflorescence induces greater crystallization pressure than salt
efflorescence does (Doehne 2002; Moteershead 2013;Rodriguez-Navarro and Doehne 1999). The differencein the crystallization patterns may be reflected in theabsence or presence of cracks on the surface.
Comparison with continuous wetting/drying experiment

The results of the wetting/drying experiment contrasted
with those of the humidity-change experiment (Fig. 7).
Oya tuff immersed in MgSO4 disintegrated rapidly, andPage 7 of 10

Fig. 7 Weight (a) and Equotip rebound value (b) measured during
the continuous wetting/drying experiment. The solid line represents
the results for Oya tuff, and the dotted line represents those for
Aoshima sandstone. The symbols correspond to the following
combinations: filled circles, NaCl; triangles, Na2SO4; squares, MgSO4;
open circles, no salt (distilled water)

no fragments remained on the sieve after 16 cycles. The
L-value of this specimen could not be measured after 2cycles. Among the Aoshima sandstone specimens, MgSO4
showed the second greatest weight loss and L-value reduction. Although the effect of NaCl was comparable to thatof Na2SO4 for Oya tuff, NaCl was obviously inactive for
Aoshima sandstone. Na2SO4 substantially damaged the
Oya tuff and caused the most damage to the Aoshima
sandstone.
The weights of some of the specimens could not bemeasured after the sieve openings became clogged withrock fragments. In addition, the L-values could not bemeasured after the surfaces of the specimens becamerough.
For Aoshima sandstone specimens with Na2SO4, thefinal weight was 15.4% of the initial weight after 13 cycles,
and the final L-value was 24.1% of the initial hardness after
9 cycles. Oya tuff specimen with Na2SO4 disintegrated rapidly; the final weight was 43.8% of the initial weight after
13 cycles. The L-value of another Oya tuff specimen rapidly decreased; the final value was 44.8% of the initial hardness after 9 cycles. Aoshima sandstone immersed in
NaCl did not show any distinct changes in either
weight or L-value.
The less hydrated forms of each salt, i.e., halite, thenardite,and kieserite, would crystallize by oven drying at 110 °C.
Breakdown and disintegration mainly occurred when thespecimens were immersed in distilled water. Hydration of
thenardite and kieserite would have occurred during theimmersion. We used distilled water in the wetting sequence,
whereas the previous experiments often used salt solutions

Sato and Hattanji Progress in Earth and Planetary Science(2018) 5:84

in various concentrations. Thenardite and kieserite in rockspecimens dissolve in distilled water, and their solution
would have reached saturation as the water penetrated therock. There was no sign of rock breakdown during thesubsequent oven drying and immersion in salt solution,although crystallization and hydration would have beenpossible if salt remained in the specimens.
As we described above, the dominant weathering processes in the wetting/drying experiment would be salt
crystallization of halite and hydration of thenardite or
kieserite. These processes are almost the same as those
expected in the humidity-change experiment, i.e., deliquescence/recrystallization of halite and hydration of
thenardite and hexahydrate of MgSO4. The difference indamage between the two experiments thus did not result
from the salt pathways, despite that the crystallization/
hydration pressures vary with temperature and humidity(Flatt 2002). The difference between these two experiments
was mainly due to the difference in the water supply
methods.
Comparison of rock properties

Pore size distribution, micropore volume, and specificsurface area generally affect susceptibility to physicalweathering (Benavente et al. 2004; Matsukura and Matsuoka
1996; Matsuoka 1990; Ordóñez et al. 1997; Yu and Oguchi
2010; Arnold and Zehnder 1989). The specific surface
area probably affects the salt weathering susceptibility
by humidity changes because a larger specific surface
area causes greater absorption.
Ubara sandstone with higher porosity and lower tensile
strength (Table 1) showed the greatest weight losses(Fig. 4). Although Oya tuff has comparable properties
to Ubara sandstone, Oya tuff has large pores resulting
from the alternation of pumice, while Ubara sandstone iscomposed of uniform fine sand. These facts imply that the
micropore volume and the specific surface area of Ubara
sandstone are greater than those of Oya tuff. The differencesin micropore volume might have affected the weathering
rates in the humidity-change experiment.

Conclusions
This study highlights that salt weathering obviously occurs
under conditions in which the water supply is limited toair humidity. The humidity oscillation induced a cycle ofrepeated salt deliquescence, hydration, and crystallization.
The degrees of damage were strongly correlated with thevolume of water absorbed in the rock specimens.
NaCl caused the most damage to the specimens. Porous
and weak sandstones with NaCl were completely broken
down after 100 cycles of humidity change. The other two
types of rock also showed concentrated damage on theirsurfaces. The lower DRH of NaCl resulted in more water
being absorbed from the air, which in turn induced greater

Page 8 of 10

weight losses and L-value reductions. The absence of hydrated forms would also have facilitated water absorption
from the air.
The salt pathways of Na2SO4 would be deliquescenceof thenardite and subsequent recrystallization of mirabilite. For MgSO4, deliquescence of hexahydrate andrecrystallization of epsomite would also occur. MgSO4
induced damage in all of the rock specimens, whereas
Na2SO4 had almost no effect in our experiment. Na2SO4
may be ineffective as an agent of salt weathering in highhumidity conditions. The volume expansion with hydrationof Na2SO4 might hinder infiltration of water into therock. The high DRH of mirabilite would also prevent
salt deliquescence.
Aoshima sandstone with MgSO4 developed cracks parallel to its surface that reflected the different crystallization
patterns (efflorescence or subflorescence). MgSO4 tends
to form subflorescence, unlike NaCl and Na2SO4. The
tendency was observed under the condition in which alimited volume of water from air humidity was supplied to
the rock surface.
Tafoni, which is often associated with salt weathering,occasionally appears hundreds of meters away fromcoastlines. In supratidal zones, salts are provided by sea
spray on their surface. In such conditions, NaCl is supplied
to and precipitates on the rock surface in stormy weather;subsequent humidity fluctuations would induce salt
weathering.
Sulfates are often found on building materials, andsodium sulfate is thought to be one of the most aggressive
salts. However, the restricted activity of Na2SO4 in our
experiment suggests that deliquesce and hydration of
Na2SO4 are hindered by the clogging of rock pores withthe hydrated salts. MgSO4 did not show such behavior,although deliquesce and hydration of MgSO4 would
have occurred. MgSO4 would be more effective than
Na2SO4 under conditions in which the water supply islimited to air humidity.
Abbreviations
DRH: Deliquescence relative humidity; L-value: Equotip rebound value;
MgSO4: Magnesium sulfate; Na2SO4: Sodium sulfate; NaCl: Sodium chloride;
P-wave velocity: Longitudinal wave velocity
Acknowledgements
We are grateful to the anonymous reviewers for their helpful andconstructive comments that contributed to improving the manuscript.
Funding
This study was financially supported by a JSPS Grant-in-Aid for Scientific
Research (25560143; principal investigator, N. Matsuoka).
Availability of data and materials
Requests for data should be sent to the contact author.
Authors’ contributions
Both authors designed the experiments, wrote the manuscript, and read andapproved the final manuscript.

Sato and Hattanji Progress in Earth and Planetary Science(2018) 5:84

Competing interests
Both authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Graduate School of Life and Environmental Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan. 2Faculty of Life andEnvironmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba,
Ibaraki 305-8572, Japan.
Received: 30 April 2018 Accepted: 13 November 2018

References
Aly N, Gomez-Heras M, Hamed A, Álvarez De Buergo M, Soliman F (2015) The
influence of temperature in a capillary imbibition salt weathering simulation
test on Mokattam limestone. Mater Constr 65:e044
Aoki H, Matsukura Y (2007) A new technique for non-destructive fieldmeasurement of rock-surface strength: an application of an Equotip hardness
tester to weathering studies. Earth Surf Proc Land 32:1759–1769
Apelblat A, Korin E (1998) The vapour pressures of saturated aqueous solutions of
sodium chloride, sodium bromide, sodium nitrate, sodium nitrite, potassium
iodate, and rubidium chloride at temperatures from 227 K to 323 K. J Chem
Thermodyn 30:59–71
Apelblat A, Korin E (2002) The vapour pressure of water over saturated solutionsof sodium sulfate, calcium bromide, ferric chloride, zinc nitrate, calcium
nitrate, and lithium nitrate at temperatures from 278.15 K to 323.15 K. J
Chem Thermodyn 34:1621–1637
Apelblat A, Manzurola E (2003) Solubilities and vapour pressures of saturated
aqueous solutions of sodium tetraborate, sodium carbonate, and magnesium
sulfate and freezing-temperature lowerings of sodium tetraborate andsodium carbonate solutions. J Chem Thermodyn 35:221–238
Arnold A, Zehnder K (1989) Salt weathering on monuments. In: 1st International
Symposium on the Conservation of Monuments in the Mediterranean Basin,
Bari, 7–10 June 1989
Balboni E, Espinosa-Marzal RM, Doehne E, Scherer GW (2011) Can drying andre-wetting of magnesium sulfate salts lead to damage of stone? Environ
Earth Sci 63:1463–1473
Benavente D, Garcı́a del Cura MA, Garcı́a-Guinea J, Sánchez-Moral S, Ordóñez S(2004) Role of pore structure in salt crystallisation in unsaturated porous
stone. J Cryst Growth 260:532–544
Bradley WC, Hutton JT, Twidale CR (1978) Role of salts in development of granitic
tafoni, South Australia. J Geol 86:647–656
Campbell IB, Claridge GGC (1987) Antarctica: soils, weathering processes andenvironment. Elsevier, Amsterdam
Chipera SJ, Vaniman DT (2007) Experimental stability of magnesium sulfate
hydrates that may be present on Mars. Geochim Cosmochim Ac 71:241–250
Chou I-M, Seal RR (2003) Determination of epsomite–hexahydrite equilibria by
the humidity–buffer technique at 0.1 MPa with implications for phaseequilibria in the system MgSO4–H2O. Astrobiology 3:619–630
Cooke RU (1981) Salt weathering in deserts. P Geologist Assoc 92:1–16
Doehne E (2002) Salt weathering: a selective review. In: Siegfried S, Thomas W,
Axel V (eds) Natural stone, weathering phenomena, conservation strategies
and case studies: introduction, Geological Society of London special
publication vol 205. Geological Society of London, London, pp 51–64
Fahey BD (1985) Salt weathering as a mechanism of rock breakup in cold
climates: an experimental approach. Z Geomorphol 29:99–111
Flatt RJ (2002) Salt damage in porous materials: how high supersaturations are
generated. J Cryst Growth 242:435–454
Gauri KL (1990) Decay and preservation of stone in modern environments.
Environ Geol Water S 15:45–54
Goudie AS (1977) Sodium sulphate weathering and the disintegration of
Mohenjo-Daro, Pakistan. Earth Surf Proc Land 2:75–86
Goudie AS (1986) Laboratory simulation of “the wick effect” in salt weathering ofrock. Earth Surf Proc Land 11:275–285
Goudie AS (1993) Salt weathering simulation using a single-immersion
technique. Earth Surf Proc Land 18:369–376

Page 9 of 10

Goudie AS, Cooke R, Evans I (1970) Experimental investigation of rock weatheringby salts. Area 2:42–47
Goudie AS, Cooke RU (1984) Salt efflorescences and saline lakes; a distributional
analysis. Geoforum 15:563–582
Goudie AS, Viles HA (1995) The nature and pattern of debris liberation by salt
weathering: a laboratory study. Earth Surf Proc Land 20:437–449
Goudie AS, Viles HA (1997) Salt weathering hazard. John Wiley & Sons, Chichester
Gough RV, Chevrier VF, Tolbert MA (2016) Formation of liquid water at low
temperatures via the deliquescence of calcium chloride: implications for
Antarctica and Mars. Planet Space Sci 131:79–87
Gough RV, Wong J, Dickson JL, Levy JS, Head JW, Marchant DR, Tolbert MA(2017) Brine formation via deliquescence by salts found near Don Juan Pond,
Antarctica: laboratory experiments and field observational results. Earth
Planet Sc Lett 476:189–198
Gunzburger Y, Merrien-Soukatchoff V (2011) Near-surface temperatures and heatbalance of bare outcrops exposed to solar radiation. Earth Surf Proc Land 36:
1577–1589
Heinz J, Schulze-Makuch D, Kounaves SP (2016) Deliquescence-induced wetting
and RSL-like darkening of a Mars analogue soil containing various
perchlorate and chloride salts. Geophys Res Lett 43:4880–4884
Hœrlé S (2006) Rock temperatures as an indicator of weathering processes
affecting rock art. Earth Surf Proc Land 31:383–389
Kamh GME (2005) The impact of landslides and salt weathering on Roman
structures at high latitudes—Conway Castle, Great Britain: a case study.
Environ Geol 48:238–254
Kushitsu N, Ishizaki T, Nishiura T (2000) Salt weathering of the brick monuments
in Ayutthaya, Thailand. Dev Geotech Eng 84:319–327
Kwaad FJPM (1970) Experiments on the granular disintegration of granite by saltaction. University Amsterdam Fysisch Geografi sch en Bodemkundig
Laboratorium Publicatie 16:67–80
López-Arce P, Fort R, Gómez-Heras M, Pérez-Monserrat E, Varas-Muriel MJ (2011)
Preservation strategies for avoidance of salt crystallisation in El Paular
Monastery cloister, Madrid, Spain. Environ Earth Sci 63:1487–1509
Marchant DR, Head JW (2007) Antarctic dry valleys: microclimate zonation,
variable geomorphic processes, and implications for assessing climatechange on Mars. Icarus 192:187–222
Matsukura Y, Matsuoka N (1991) Rates of tafoni weathering on uplifted shore
platforms in Nojima-Zaki, Boso Peninsula, Japan. Earth Surf Proc Land 16:51–56
Matsukura Y, Matsuoka N (1996) The effect of rock properties on rates of tafoni
growth in coastal environments. Z Geomorphol Supp 106:57–72
Matsukura Y, Oguchi CT, Kuchitsu N (2004) Salt damage to brick kiln walls inJapan: spatial and seasonal distribution of efflorescence and moisture
content. B Eng Geol Environ 63:167–176
Matsuoka N (1990) Mechanisms of rock breakdown by frost action: an
experimental approach. Cold Reg Sci Technol 17:253–270
McFadden LD, Eppes MC, Gillespie AR, Hallet B (2005) Physical weathering in arid
landscapes due to diurnal variation in the direction of solar heating. Geol
Soc Am Bull 117:161–173
McGreevy JP, Smith BJ (1982) Salt weathering in hot desert: observation on thedesign of simulation experiments. Geogr Ann 64A:161–170
Miotke F (1982) Physical weathering in Taylor Valley, Victoria Land, Antartica.
Polar Geogr Geol 6:71–98
Mol L (2014) Investigations into the relationship between changes in internalmoisture regimes and rock surface deterioration in cavernous sandstonefeatures. Earth Surf Proc Land 39:914–927
Mottershead DN (1982) Coastal spray weathering of bedrock in the supratidal
zone at East Prawle. South Devon Field Studies 5:663–684
Mottershead DN (1994) Spatial variations in intensity of alveolar weathering of a
dated sandstone structure in a coastal environment, Weston-super-Mare, UK.
In: Robinson DA, Williams RBG (eds) Rock weathering and landform
evolution. Wiley, New York, pp 151–174
Mottershead DN (2013) Coastal weathering. Treatise on Geomorphology 4:228–244
Mottershead DN, Pye K (1994) Tafoni on coastal slopes, South Devon, U.K. Earth
Surf Proc Land 19:543–563
Mustoe G (1982) The origin of honeycomb weathering. Geol Soc Am Bull 93:108–115
Oguchi CT, Yuasa H (2010) Simultaneous wetting/drying, freeze/thaw and salt
crystallization experiments of three types of Oya tuff. In: Přikryl R, Török Á (eds)
Natural stone resources for historical monuments, Geological Society of London
special publication vol 333. Geological Society of London, London, pp 59–62
Ordóñez S, Fort R, García del Cura MA (1997) Pore size distribution and the
durability of a porous limestone. Q J Eng Geol 30:221–230

Sato and Hattanji Progress in Earth and Planetary Science(2018) 5:84

Robinson DA, Williams RBG (1982) Salt weathering of rock specimens of varying
shape. Area 14:293–299
Robinson DA, Williams RBG (2000) Experimental weathering of sandstone bycombinations of salts. Earth Surf Proc Land 25:1309–1315
Rodriguez-Navarro C, Doehne E (1999) Salt weathering: influence of evaporation
rate, supersaturation and crystallization pattern. Earth Surf Proc Land 24:191–209
Ruiz-Agudo E, Mees F, Jacobs P, Rodriguez-Navarro C (2007) The role of saline
solution properties on porous limestone salt weathering by magnesium andsodium sulfates. Environ Geol 52:269–281
Selby M, Wilson A (1971) The origin of the labyrinth, Wright Valley, Antarctica.
Geol Soc Am Bull 82:471–476
Smith BJ, McGreevy JP (1983) A simulation study of salt weathering in hot
deserts. Geogr Ann 65A:127–133
Smith BJ, Warke PA, McGreevy JP, Kane HL (2005) Salt-weathering simulations
under hot desert conditions: agents of enlightenment or perpetuators of
preconceptions? Geomorphology 67:211–227
Steiger M, Asmussen S (2008) Crystallization of sodium sulfate phases in porous
materials: the phase diagram Na2SO4–H2O and the generation of stress.
Geochim Cosmochim Ac 72:4291–4306
Steiger M, Linnow K, Juling H, Gülker G, AEl J, Brüggerhoff S, Kirchner D (2008)
Hydration of MgSO4•H2O and generation of stress in porous materials. Cryst
Growth Des 8:336–343
Strini A, Guglielmin M, Hall K (2008) Tafoni development in a cryotic
environment: an example from Northern Victoria Land, Antarctica. Earth Surf
Proc Land 33:1502–1519
Takahashi K, Suzuki T, Matsukura Y (1994) Erosion rates of sandstone used for amasonry bridge pier in the coastal spray zone. In: Robinson DA, Williams RBG(eds) Rock weathering and landform evolution. John Wiley & Sons,
Chichester, pp 175–192
Thomachot-Schneider C, Gommeaux M, Fronteau G, Oguchi CT, Eyssautier S,
Kartheuser B (2011) A comparison of the properties and salt weathering
susceptibility of natural and reconstituted stones of the Orval Abbey(Belgium). Environ Earth Sci 63:1447–1461
Tsujimoto H (1987) Dynamic conditions for shore platform initiation. Science
Report of the Institute of Geoscience, University of Tsukuba, Sec A 8:45–93
Vaniman DT, Bish DL, Chipera SJ, Fialips CI, Carey JW, Feldman WG (2004)
Magnesium sulphate salts and the history of water on Mars. Nature 431:663–665
Viles HA (2005) Microclimate and weathering in the central Namib Desert,
Namibia. Geomorphology 67:189–209
Viles HA, Goudie AS (2013) Weathering in the central Namib Desert, Namibia:
controls, processes and implications. J Arid Environ 93:20–29
Wellman HW, Wilson AT (1965) Salt weathering, a neglected geological erosive
agent in coastal and arid environments. Nature 205:1097–1098
Williams RBG, Robinson DA (1998) Weathering of sandstone by alunogen andalum salts. Q J Eng Geol Hydroge 31:369–373
Yamada T, Aoki H, Takahashi M, Matsukura Y (2005) Effect of rock properties on
rates of salts weathering: a laboratory experiment. J Japan Soc Eng Geol 46:
72–78 (in Japanese with English abstract)
Young ARM (1987) Salt as an agent in the development of cavernous
weathering. Geology 15:962–966
Yu S, Oguchi CT (2010) Role of pore size distribution in salt uptake, damage, andpredicting salt susceptibility of eight types of Japanese building stones. Eng
Geol 115:226–236

Page 10 of 10