311

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.

313

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

315

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).

318

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

319

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

321

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.

322

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.

323

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).

324

Photo 1. Scanning electron micrograph of magnesium sulphate (limestone efflorescence)

Photo 2. Scanning electron micrograph of porous spherical particle from crust of Malaga

Cathedral.