m.i. carretero and e. galan departamento de cristalografla...
TRANSCRIPT
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MARINE SPRAY AND URBAN POLLUTION AS THE MAIN FACTORS OF STONE
DAMAGE IN THE CATHEDRAL OF MALAGA (SPAIN)
M.I. CARRETERO and E. GALAN
Departamento de Cristalografla y Mineralogfa, Facultad de Qufmica, Universidad de Sevilla,
Apdo. 553, E-41071 Sevilla, Spain
Abstract
The Cathedral of Malaga is located in a highly polluted district, near the shoreline of the
Mediterranean sea. The Cathedral is made of sandstone and limestone and was built between
1528 and 1782. The weathering forms usually observed outside the monument were: Crusts,
efflorescences, alveolar weathering, crater formations, grain disgregation, cracking, fissuring,
swelling, contour scaling and loss of material. Alveolar weathering and crater formations are
chiefly exhibited by limestone, whereas grain disgregation with loss of relief is observed
mainly in sandstone. Efflorescences consist essentially of magnesium sulphate, and crusts of
gypsum and microspherules originating from urban pollution. Halite is also present in
efflorescences and crusts formed on the fa~ades exposed to the direct action of marine spray.
Stones obtained from the same quarries as those used to build the monument were exposed
to accelerated ageing tests (wetting-drying and salt crystallization using magnesium sulphate)
and found to develop similar weathering forms as the Cathedral stones. Based on this study,
marine spray and urban pollution can be considerer as the two main agents for stone damage
in the monument. Less significant altering agents include pigeon activity, anthropogenic
degradation and iron grappling, which can result in serious local damage.
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1. Introduction
The Cathedral of Malaga, like others in Spain, was built on the foundation of the
former Main Mosque of the city. Its construction spanned two and a half centuries (from 1528
to 1782, when the works were eventually stopped for the lack of funding and the South tower
and other decorative parts were left unfinished).
The Cathedral has been the subject of additional works (repair of the domes,
restoration of the stained-glass windows, emergency repairs in the North tower, etc.) during
the XX century; however, the current condition of structural stones in the Cathedral is
unsatisfactory on the whole. The main purpose of this work was to investigate the causes and
mechanisms by which the Cathedral's stones have been deteriorated.
2. Location and environmental conditions of the Cathedral
The Cathedral of Malaga, which faces the West-East, lies in the southern part of the
city, very close to the Mediterranean shoreline; in fact, as can be seen in some lithographs
of the XIX century, the building was separated by only a few houses from the beach at the
time.
The Cathedral is located in a highly polluted district. Only one of the surrounding
streets (off the East fa9ade) is a pedestrian street. Heavy traffic in the streets off the South
and West fa9ades virtually throughout the day results in high atmospheric and acoustic
pollution around the building. Matters worsened with the opening of a massive garage
opposite the South fa9ade a few years ago.
The city of Malaga has a typically Mediterranean climate, with warm summers and
not very cold winters. Based on data from the Weather Service of the city, the average
temperature for the past 32 years ranged from 12.1°C in January to 25.3°C in August. Rainfall
is scant (annual average 574.6 mm); however, rains are usually erratic and torrential. The
annual number of sunshine hours is 2856 on average and daily thermal oscillations are only
moderate (about l0°C). Finally, the relative humidity varies little during the year (from 60%
in June to 73% in November and January, with 68% as the average).
The direction of prevailing winds throughout the year, SE-NW, favours the transport
of marine spray to the building's stones, particularly in the south and west areas, and in the
higher ones (North tower), which are not sheltered by the surrounding buildings.
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3. Structural stone
The structural stones of the Cathedral consist essentially of limestone and sandstone.
The materials were characterized in situ in various respects by using non-destructive
techniques based on ultrasound transmission velocity (with a Pundit instrument), water
absorption (by use of a Karsten tube "pipe method") and mechanical resistance (with a
Schmidt hammer). In addition, the real and apparent density, porosity and porometry of the
stones were determined by applying a combination of Hg-injection, N2-adsorption (BET) and
image analysis by optical microscopy, to small fragments obtained from various ashlars that
were also characterized by X-ray diffraction (by use of the powder method, CuKa radiation
and an Ni filter on a Philips PW 1130/90 diffractometer), polarizing microscopy, atomic
absorption spectrometry and standard methods of wet chemical analysis.
Sandstone is located in the first and second bodies of the main fa~ade and the towers.
There are three different types of sandstones in the building, viz. orthoquartzites with siliceous
cement (Group A), and protoquartzites and subarcoses with dolomitic cement (Group B).
The physical properties of Groups A and B are quite similar (Table 1); interestingly,
they possess many bottleneck pores. The samples in Group B exhibit higher microporosity
than those in Group A. The openings of bottleneck pores are less than 4 µm in radius for
Group B and more than 4 µm for Group A.
Limestones are present in three varieties, all of which are sandy and bioclastic. Lime
stones in Group A are located mainly in the North tower and the West portion of the terrace.
These limestones exhibit a low ultrasound transmission velocity and a high porosity and water
absorption capacity (Table l); they have large pores (60-400 µm) and bottleneck pores with
openings less than 4 µmin radius. The limestones in Group B are located in the North tower
and on the sides of the building. They are characterized by the presence of dolomite (10%)
and by a much higher ultrasound transmission velocity and lower porosity and pore size
(40-200 µm) than those for Group A (Table 1). They also have bottleneck pores, which,
however, are larger than 4 µm in radius at the mouth. Finally, the third limestone variety
(Group C) is scarcely present in the monument and in a heavily degraded condition (basically
in the form of contour scaling). They are dolomitic limestones (30% of dolomite) and differ
from the others in their increased concentrations of some trace elements such as Mn, Ni, Sr
and Cr, as well as in the presence of a channel pore network and in lower microporosity.
Group A limestones come from the Mio-Pliocene Almayate quarries, about 25 km
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East of the city. Those from which the limestones of Groups B and C were obtained are no
longer accessible as they lie beneath highly populated urban nuclei. Pennotriassic sandstones
were brought from the vicinity of Malaga; however, only the quarries for those in Group B,
located in Cerro Coronado (about 2 km NW of the city centre) are currently accessible
(Carretero, 1993; Galan and Carretero, 1994a, 1994b). Samples from these quarries were also
obtained in order to study fresh, unweathered stone.
4. Weathering forms
The weathering forms most commonly seen in the building include crusts, efflor
escences, fissuring, cracking, grain disgregation with loss of relief, alveolar weathering, crater
formations, swelling, contour scaling, loss of material, biogenic crusts and plants.
Crusts and efflorescences are essentially concentrated in cornices and sheltered zones.
Material losses are specially outstanding in the sandstone cornices of the South tower, where
large pieces of stones are about to become detached. Alveolar weathering and crater forma
tions are typical of limestone, while grain disgregation with loss of relief is more common
place in sandstone. Finally, contour scaling is observed in both sandstones and group C
limestones.
Alternative weathering agents can lead to significant damage. Such is the case with
the detrimental -and anaesthetic- effect of pigeon's excrements that cover most of the
fa9ades, with anthropogenic action, and with iron grappling, which results in breakage and
material losses in some areas.
5. Study of crusts, efflorescences and contour scaling
Crusts and efflorescences from the different limestone and sandstone groups were
sampled in various orientations at different levels. Samples were studied by X-ray diffraction,
chemical analysis and scanning electron microscopy (using a Jeol JSM-5400 microscope) with
energy dispersive X-ray analysis (EDX).
Efflorescences
Efflorescences were found to consist predominantly of crystalline magnesium sulphate
with a variable number of water molecules, in the form of kieserite (MgS04
-tt20), starkeyite
(MgS04 ·4H20), hexahydrite (MgS04 -6H20) and epsomite (MgS04 ·7H20). They also
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contain smaller amounts of calcium sulphate as basanite (CaS04 ~H20) and gypsum
(CaS04 ·2H20), as well as potassium and sodium bicarbonate as kalicinite (KHC03) and
nahcolite (NaHC03), respectively. All these salts frequently occur in monument efflorescences
(Arnold and Zehnder, 1990).
The chemical analysis of the efflorescences was consistent with their X-ray diffraction
results. All samples were found to contain so4- 2 and Mg2+, in addition to lower proportions
of HC03-, Ca2+, Na+ and K+. No nitrates, and only traces of chlorides, were detected. The
ion concentrations in the efflorescences were similar for the limestone and sandstone in the
different groups studied, which suggests that the weathering products are similar for both
lithotypes.
SEM observations confirmed the previous results; limestone and sandstone
efflorescences were found to be very similar and to consist chiefly of magnesium sulphate
(Photo 1, Fig. 1) and occasional gypsum.
The ions that form the efflorescences may come from the stone itself or from atmo
spheric pollution, marine spray, mortar or residual paint in some zones of the building. In
order to check the actual origin of the ions, we carried out a study of mortar and paint
samples collected from the same zone as the efflorescences. The study revealed that both the
mortar and the paint consist of quartz, calcite and gypsum but no magnesium mineral;
therefore, the mortar may be a source of sulphur but not of magnesium -the former may also
have originated from the heavy urban pollution in the Cathedral's surroundings.
Magnesium can essentially be supplied by the stone itself since several groups of
limestones and sandstones, which are scattered randomly throughout the building, contain
dolomite. To a lesser extent, Mg may also have come from marine spray owing to the
nearness of the Cathedral to the sea.
Crusts
The study of crusts showed them to contain gypsum as the sole mineral; exceptionally
some crusts on limestone facing the South also contained halite. Generally crusts contain
higher concentrations of S, Na, Zn and Pb than the unaltered underlying stones; to a smaller
extent, the Mn, Cu, Sr, Ni and Ba contents -the last element occurs in crusts on sandstones
are also higher (Fig. 2).
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The chemical analysis for soluble salts in the crusts of both types of stone revealed
the presence of so4- 2 and Ca2+, and, in smaller amounts, HCo3-, ci-, Na+ and Mg2+, as well
as K+ traces. Based on these chemical results and on the mineralogical composition of the
efflorescences, crusts can also be assumed to contain the following salts in lower proportions:
nahcolite, hydrated magnesium sulphate, kalicinite (traces) and halite (traces in sandstone
crusts only). Limestone samples exhibited greater amounts of c1- and Na+. The presence of
halite in some samples was confirmed by XRD.
The scanning electron microscopy study of the crust samples revealed the
predominance of gypsum, accompanied by smaller amounts of recrystallized calcite and halite.
This last mineral, highly abundant in the samples on sandstones facing South and East, was
occasionally covered with gypsum.
Crusts were occasionally observed to include porous spherical particles (Photo 2) from
automobile emissions (Ross et al., 1989; Ausset et al., 1992), consisting essentially of S, Ca,
Al, and Si, as well as smaller amounts of Fe and V (Fig. 3). These particles play a prominent
role in the formation of gypsum crusts as they contain V, a catalyst for the oxidation of S02 to S03 that precedes the formation of sulphuric acid, which will attack the stone and form
gypsum (Ausset et al., 1992).
The presence of halite in limestone crusts and its complete or virtual absence from
sandstone crusts is the likely result of the sample location rather than the nature of the
underlying material (the sea is the source of ci- and Na+). Thus, the samples with the largest
amounts of halite were those facing South (the sea) at a height not sheltered by neighbouring
buildings. Sandstone samples, which contain less halite than limestones, are located below
these, so they are more effectively sheltered from marine spray by the surrounding buildings
(NW-W-SW orientation). It should be noted that the prevailing wind direction, SE-NW,
facilitates the transport of marine spray to the SW fa9ade of the building.
Gypsum is the main component of limestones and sandstones crusts because it is the
least soluble of all the salts present in the efflorescences. Likewise, crusts contain more
nahcolite than kalicinite, the two of which occur in similar proportions in efflorescences.
Contour scaling
Contour scaling was studied by determining the concentration of weathering products
between the inner part, the detached scale and the zone between scales in sandstones and in
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Group C limestones (the lithotypes where this type of alteration was observed). The sole
weathering product found was gypsum, particularly in the intermediate zone (5% versus traces
in the detached zone). No gypsum was found in the inner zone, however.
6. Accelerated ageing tests
6.1. Methodology
We carried out wetting-drying and salt crystallization accelerated ageing tests. The
latter involved complete immersion of the samples in 10% magnesium sulphate. The tests
were chosen based on the environmental conditions of the building and the weathering
products found. Stones were cut into prismatic specimens of 5 x 5 x 10 cm. A longer than
normal drying time (42 h rather than 21 h) was used in the tests, with an overall 40 cycles
of 48 h each. After the salt crystallization test was finished -and before the physico-chemical
properties of the materials were determined--, salts were removed from the inside of the
stones by immersion in a vessel through which a water stream was continuously passed for
48 h. Three specimens were used in each test. The results are given as the means for the
three. Changes during the tests were followed from macroscopic observations of alteration and
weight losses from the specimens; also, after the tests were finished, the following physical
properties were determined: ultrasound transmission velocity, porosity, real and apparent
density, water absorption capacity and mechanical resistance. Also, the specimens that were
subjected to the tests were examined under the scanning electron microscope.
6.2. Wetting-drying test
The wetting-drying test revealed no change in the sandstone specimens and only small
losses in the limestone specimens that never reached 0.6% after 40 cycles (Fig. 4).
Neither type of stone exhibited macroscopic weathering forms after 40 cycles;
however, the scanning electron microscope revealed limestones to be more porous after the
test.
Regarding changes in the physical properties, sandstones underwent none; however,
limestone specimens exhibited slightly decreased ultrasound transmission velocity, apparent
density and mechanical resistance, as well as substantially increased porosity and capillarity
coefficient (Table 2).
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This change in the physical properties of the limestone after the wetting-drying test
was a result of the stone composition, with over 70% of calcite, a highly anisotropic mineral.
The two main expansion coefficients had opposite signs. Thus, a thermal gradient of 30°C
produces a theoretical expansion of 0.075% along the c-axis and a contraction of 0.015%
along the x-axis, which combined theoretical result is a volume expansion of 0.045% (Galan,
1991). With successive wetting and heating cycles, calcite expands and contracts alternately,
which creates internal forces that lead to grain disintegration and increased porosity; these in
turn increase the water absorption capacity and decrease stone compactness, consistent with
the changes observed in the physical properties of the building's stones as weathering
progressed (Table 1).
6.3 Salt crystallization test
This test had a drastic effect on limestones. After 6 cycles, the weight loss reached
35.6% and the specimens fractured (Fig. 4). However, in the first 3 cycles -before the stone
disintegrated-, a weight gain due to the presence of salts in the pores of limestone was
observed; in the subsequent crystallization-<lissolution cycles, the salts caused stones to break.
Weathering of limestone begins with the formation of efflorescences, grain disgregation and
edge rounding -the weathering forms observed in the building's limestone. Subsequently,
cracks appear along weak lines by effect of -essentially compositional- texture differences
in the material that eventually disintegrate the specimen.
Sandstone subjected to salt crystallization by complete immersion undergoes extensive
weathering. In the first 5 cycles, sandstone gained weight -as did limestone- due to the
presence of salts in the specimen; however, it gradually lost weight afterwards up to 33.2%
at 40 cycles (Fig. 4).
After 10 cycles, efflorescences and grain disgregation were observed on the specimen
sides; also, after 15 cycles, edges were rounded. At 25 cycles, fissuring and cracking started
to occur; the resulting fissures and cracks became deeper at 30 cycles. At 40 cycles, there was
contour scaling and material loss by effect of salts emerging from the fractures. The
weathering forms observed were thus grain disgregation with loss of relief, efflorescences,
fissuring and fracturing, in addition to contour scaling at a high number of cycles. All these
weathering forms were observed in the building's sandstone.
The scanning electron microscope revealed marked weathering of limestone and
sandstone after the salt crystallization test. Limestone was disgregated and sandstone exhibited
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fractured quartz by effect of the crystallization pressure prior to grain disgregation.
This test considerably decreased the ultrasound transmission velocity for sandstone
(Table 2), consistent with the macroscopic weathering signs observed in the stones and the
weight loses obtained in the test. The velocity for limestone could not be measured because
the specimens broke after 6 cycles. Porosity increased after 40 cycles and the apparent density
decreased relative to the levels before the test The water capillary absorption capacity and
the resistance to uniaxial compression of limestone could not be measured because the
specimens broke after 6 cycles. Sandstone exhibited an increased capillarity coefficient and
markedly decreased mechanical resistance after the test (Table 2). These changes in physical
properties with weathering were also observed in the building's stones (Table 1).
7. Discussion
From the results obtained in this work it follows that the location and climatological
conditions of the Cathedral of Malaga appear to be the main weathering agents for the stones.
In fact, the building is located very near the sea and the prevailing wind direction throughout
the year, SE-NW, facilitates the transport of marine spray to the building, particularly in the
South and East areas and in the higher zones (North tower), which are unsheltered by the
surrounding buildings - this was the zone were the highest halite concentrations were detected.
Marine spray contains dissolved salts that subsequently reached the inside of stones owing
to the high relative humidity of the environment. In addition, magnesium sulphate, the major
salt in the efflorescences, has a very high crystallization pressure. The environmental
conditions around the Cathedral give rise to crystallization-dissolution cycles that damage the
stone.
On the other hand, the Cathedral is subject to the effects of heavy traffic virtually
throughout the day, especially off its West and South fa9ades. The action of car exhaust on
stones, and spherical particles contained in these emissions, which act as catalysts in the
formation of sulphuric acid, can be highly detrimental. The acid attack dissolves the grain
cement, leading to disgregation and the formation of preferential ways for access of water
containing substances that damage stones. In addition, some reaction products such as gypsum
differ in composition from natural stone and hence also in some physical properties such as
the expansion coefficient and solubility, so they eventually detach themselves from stones by
effect of temperature oscillations.
The alteration forms found in the stones subjected to the accelerated ageing tests were
320
similar to those observed in the building itself, as were the changes in the experimental
specimens and those in stones of the building in variously altered conditions. This confinns
that temperature changes and successive crystallization-dissolution cycles for salts (mainly
magnesium sulphate) resulting from the environmental conditions of the city cause stone
weathering in the Cathedral of Malaga.
8. Conclusions
Marine spray and urban pollution are the two main agents for stone damage in the
monument. Less significant altering agents include pigeon activity, anthropogenic degradation
and iron grappling, which can result in serious local damage.
9. References
ARNOLD, A. & ZEHNDER, K. 1990. Salt weathering on monuments. 1st International Symposiwn on the
Conservation of Monwnents in the Mediterranean Basin. Ed. Zezza, F., Grafo Edizioni, Brescia (Italy). 31-58.
AUS SET, P.; LEFEVRE, R.; PHILIPPON, J. & VENET, C. 1992. Large scale distribution of fly-ash particles inside
weathering crusts on calcium carbonate substrates. Some examples on french monuments. Ilnd International
Symposiwn on the Conservation of Monwnents in the Mediterranean Basin. Ed. Decrouez, D.; Chamay, J. & Zezza,
F., Geneve (Switzerland). 121-139.
CARRETERO, M.I. 1993. La piedra de la Catedral de Malaga. Estado de a/teraci6n y tratamientos de conservaci6n.
Ph. Thesis. Seville University. 590 pp.
GALAN, E. 1991. The influence of temperature changes on stone decay. Weathering and air pollution. 1st Course
of University School ofMonwnent Conservation. Community of Mediterranean Universities. Lago di Garda (Portese),
Venezia, Milano. Ed. Mario Adda, Bari (Italy). 119-129.
GALAN, E. & CARRETERO, M.I. 1994a. Estimation of the efficacy of conservation treatments applied to a
permotriassic sandstone. IIIrd International Symposiwn on the Conservation of Monwnents in the Mediterranean
Basin. Ed. Fassina V., Ott H. and Zezza F. Venice (Italy), 947-954.
GALAN, E. & CARRETERO, M.I. 1994b. Metodologfa para valorar la eficacia de los tratamientos de conservaci6n
de la piedra. Aplicaci6n a la caliza de la torre de la Catedral de Malaga. Bo!. Soc. Esp. Min., 17, 179-191.
ROSS, M.; McGEE, E.S. & ROSS, D.R. 1989. Chemical and mineralogical effects of acid deposition on Shelburne
Marble and Salem Limestone test samples placed at four NAPAP weather-monitoring sites. Am. Mineral., 74, 367-
383.
! f-
I t-
limestone
sandstone
Group A
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Water mechanical speed of
absorption resistence porosity ultrasounds (mis) (Kg/cm2) dl!nsil]'.
(indirect method) (ml/min) (Schmidt (% Vol.) (g/cm )
(pipe method) hammer)
weathered 1550 2.56 100 37. I 2.67 ·········-----·····-···-······-· ·····-···-···--··--··-·····-···"-······ ...................................................................................... .......... ·························· ........................ .
highly weathered 1100 7.55 <100 40.9 2.63
hulk
dt:nsitr, (g/cnr )
1.64
1.56
Group B ···-1~~ ... ~~.~~-=~~·- ·-···-·· ·····-·-~~ .................................... ?.:.?.~ ................................. ~.~?. ................................ 1 .~:.3. ...................... ~:.?.3 2.20
2.12
Group C
Groups A and B
weathered 3850 0.33 278 21.4 2.70 highly
835 37.0 weathered 150 16.5 2.72 2.27
low weathered 3300 0.03 650 14.3 2.69 2.30 ···--· .. ········--·-······---···· ·····- ·····--·-·--····-··-······ .. ········ .............. -............................ ····•····································· ···································· . ······················· . ·································
·-··-::V.~~t-~-=~~--··- -·········---~:7.~9. .... --··········· ······· ·····-···?.·.~·~······ ·· · ···· ·· ................. ~?.?. ............... ······-·· ... ~.~.:~ .................... 2.:.~8......... 2.23 highly
weathered 1600 1.82 380 18.5 2.64 2. 15
Table 1. Physical properties of sandstones and limestones from Malaga Cathedral
speed of ultrasounds
porosity densi7, bulk density capillarity mechanical
(mis) coefficient resistence (direct
(%Vol.) (g/cm ) (g/cm3) (Kg.m2.min°5) (Kg/cm2)
method)
quany 2037 35.0 2.69 1.76 267 50
limestone wetting-drying 1912 37.7 2.66 1.62 382 30
salt 39.5 2.68 1.57 ----- -----
crystallization ----·-
quany 3472 13.0 2.68 2.35 17 470
sandstone wetting-drying 3438 12.9 2.66 2.37 17 461
salt 2326 16.8
crystallization 2.68 2.21 78 372
Table 2. Physical properties of sandstones and limestones from quarries before and after
accelerated ageing tests.
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s
Mg
0 . [II)(:) tCl . 11 0 -+
Figure 1. Chemical analysis by EDX of limestone efflorescence (photo 1).
200..-~~~~~~~~~~----.
ppm
o~....._ ....... ~...._....._ ....... ~...._.....___.__, Mo Ni Co Cu Zn Pb Sr Ba Cr Cd
- Limestone, group B -+- Crust
SOOr-~~~~~~~~~~----.
400 .......... . . ...... .... .... . .. .. .... ...... . . .. ... . . .
300 ...... .... .. .... . .......... ............ .... ..... . .
-+-Sandstone, group B -- Crust
Figure 2. Chemical analysis of trace elements from crusts and inalterated stones.
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Ca Si
Figure 3. Chemical analysis by EDX of porous spherical particle from crust of Malaga
Cathedral (photo 2).
increase of weight (%)
-10 .. .. --- limestone (W-D)
-+- limestone (S C)
-20 .... . - sandstone (W-D)
"*" sandstone (S C)
-30 ... .. .
-40 __ __.__----1. __ ...J._ _ ___i. __ ...J._ _ ___.._ __ J__ _ _J
0 5 10 15 20 25 30 35 40
number of cycles
Figure 4. Increase of weight of limestones and sandstones during accelerated ageing tests (W
O: wetting-drying, S C: Salt crystallization).
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Photo 1. Scanning electron micrograph of magnesium sulphate (limestone efflorescence)
Photo 2. Scanning electron micrograph of porous spherical particle from crust of Malaga
Cathedral.