Construction and Building Materials 25 (2011) 813–822

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Construction and Building Materials

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Deposition of particles on gypsum-rich coatings of historic buildings in urbanand rural environments

J. Sanjurjo Sánchez a,*, J.R. Vidal Romaní a, C. Alves b,1

a Instituto Universitario de Geología ‘‘Isidro Parga Pondal”, Campus de Elviña Universidade da Coruña, 15071 A Coruña, Spainb Centro de Investigação Geológica, Ordenamento e Valorização de Recursos, Campus de Gualtar Universidade do Minho, 4710-057 Braga, Portugal

a r t i c l e i n f o

Article history:Received 26 June 2009Received in revised form 14 July 2010Accepted 15 July 2010Available online 11 August 2010

Keywords:Atmospheric particlesAir pollutionGypsum coatingsGypsum crustsDust coatingsHistoric mortarsDecay

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.07.001

* Corresponding author. Tel.: +34 981167000; fax:E-mail address: jsanjurjo@udc.es (J.S. Sánchez).

1 Financial support from FEDER.

a b s t r a c t

Deposition of gaseous and particulate atmospheric pollutants causes decay of historic mortars to givegypsum-rich coatings by sulphation of lime mortars and blackening of gypsum mortar, resulting in gyp-sum coatings. Particulate pollution emitted by industrial sources and vehicular traffic is responsible forthe deterioration. XRF and SEM analyses of these coatings and their comparison with both the composi-tion of dust coatings formed by the deposition of gaseous and particulate matter in an urban and a rurallocality allows assessing which pollution sources are the most damaging for these materials, knowing theelemental composition of these emissions.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Atmospheric chemical constituents can precipitate by wet anddry deposition. The deposition involves a close interaction betweenthe atmosphere and the surface. Measuring deposition by analyz-ing the deposits formed on rock surfaces allows knowing the realeffect of the pollutants and the deposition rate on that rocks. Draw-ing on the surface of building façades, it has been reported that drydeposition plays a major role in the deposition of acid substanceson buildings. The characteristics of individual underlying surfacesoften allows determining the mass-transfer rates [1,2]. Dependingon the characteristics of the surface, interaction of pollutants fromthe air may vary causing varied dry deposition rates [3].

The industrial atmospheric emissions have been increased inthe last decades causing important changes in the conditions ofconservation of stone buildings. Fossil fuel combustion by indus-trial facilities and vehicular engines is a major source of anthropo-genic particulate emissions into the atmosphere. However, fewresearch works on dry deposition of particles have been performedin comparison to gaseous compounds [4]. Studies on buildingmaterials have been limited to deposition of SO2, NO2, HNO3 andorganic pollutants [5–9]. Of all the constituents produced by this

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combustion, SO2 is considered as the most important for stone de-cay processes. They react with calcite (CaCO3) from marble, calcar-eous rocks and lime mortars to form calcium sulphate(CaSO4�2H2O). Also, NOx gases increase the acidity and thus theoxidation capability of the SO2 to react with CaCO3 and to give cal-cium sulphate [10]. Because the solubility of calcium sulphate ishigher than that of CaCO3, they are dissolved in rainwater andcan penetrate into inner pores of the building materials. Afterevaporation, these salts give rise to stress in the building materialscausing deterioration [11].

1.1. Atmospheric particles

The term ‘atmospheric particles’ is used to refer fine solid or li-quid particles suspended in air. They may be originated by eithernatural or anthropogenic sources. Natural sources greatly exceedanthropogenic emissions, but anthropogenic particle emissionsare frequently concentrated in urban areas. These are emitted di-rectly into the atmosphere and are frequently mechanically trans-ported. Their diameter ranges between 0.002 and 0.1 mm.Standards for particle concentrations have been focused on differ-ent classes of particle, based on particle diameter, but PM10 parti-cles (particles of <10 lm in diameter) and PM2.5 (particles ofdiameters <2.5 lm) are considered the most active portion in hu-man health impacts [12]. PM10 contribute to coarse mass in theform of soil dust (mostly silicate minerals, aluminium, iron and

814 J.S. Sánchez et al. / Construction and Building Materials 25 (2011) 813–822

dust), sea spray (sodium and chloride), and plant particles. Othersources of coarse aerosols include products of reaction of gaseousnitric acid (HNO3) on soils [13].

Anthropogenic sources of atmospheric gases and particles arevery important in urban areas. NOx, CO and lead can be generatedfrom gasoline combustion in vehicle engines, although no lead-en-riched gasoline exist at present in the EU. Estimated emissions oftraffic exhaust are responsible of 10% of the CO2 emissions globally.The combustion of diesel fuel is not a significant source of SO2 but amajor source of NOx. These last engines produce carbonaceous par-ticles (carbon soot or fly ash by incomplete combustion) and met-als (Al, Si, Zn, Cd, Pb) [14]. Simão et al. [15] refers a study ofColebeck published in 1995 that indicates that, in the UK, on-roaddiesel combustion contribution to airborne elemental carbon levelit is though to be between 80% and 95%. Industrial emissions are asignificant source of most of these gases (SO2, NOx, CO2) and parti-cles. Among pollutant industrial facilities, thermal power stationsand chemical industries produce most of these gas and particulateemissions from coal and oil combustion. For that reason, the ques-tion of which pollution sources are the most important is a keyquestion to protect the health of the habitants and the historicalbuildings of urban areas from decay.

1.2. Gypsum coatings on granitic rock buildings and air pollution

Rock coatings are accretions on rock surfaces whose constitu-ents have been transported to these from a few microns or thou-sands of kilometres [16]. The different types of existing coatingson rock surfaces have been classified according to their composi-tion and attributed to different sources. However, their occurrenceis not the same either on natural rock surfaces or rocks ashlars ofHeritage building façades. In particular, gypsum coatings are veryfrequent on building stones. They have been found and studiedin diverse Heritage buildings, on different rock types and havebeen identified from rural to urban areas in all climatic conditions.

In urban areas, there are important differences between build-ings constructed with siliceous and calcareous rocks. In both casesgypsum coatings are very common. Because of the elevated airconcentration of sulphates, Ca from calcium carbonate reacts withatmospheric sulphate to give crusts that deteriorate the rock sur-face. The catalytic effect of particles from motor vehicles exhaustemissions in the formation of gypsum has been observed in labora-tory experiments with carbonate rocks [17] and silicate rocks [16],with particulate from diesel engine exhausts having a much moremarked effect than particulate from gasoline ones. Therefore, gyp-sum has been linked to air pollution but there is the problem of thesource of Ca in siliceous rocks. Some authors have specifically pos-tulated that gypsum coatings are formed by combination of sul-phates from air pollution and Ca from feldspars [18]. Sulphateorigin has been attributed to the oxidation of rock pyrites [19].For others authors, they are produced by nucleation and sulphationfrom clay and soil airborne particles [1] or by microorganisms [20].However, these processes cannot explain the formation of thickand compact coatings, due to both the low proportion of Ca of gra-nitic rocks and the low crystallization of gypsum by nucleation,even in experiments under polluted atmospheres with high sul-phate concentrations [21].

Other authors have proposed alternative sources of calcium sul-phate, as [22,23] the sulphation of calcium from mortars (veryprone to sulphation because they are a huge source of easily solu-ble calcium and salts) or [24,25] the use of gypsum plaster on thegranite ashlars of the buildings. A study of long-time exposure oflimestones on different urban environments have shown that soil-ing rate, evaluated by chromatic changes, after three years washigh in a busy road with high diesel traffic, low in a urban back-ground site (around 200 m from roads) and intermediate in sam-

ples exposed in roads with low frequency of diesel poweredtraffic [26]. In a study of limestone façades, Schiavon et al. [27]concluded for a link between air pollutants related to vehiculartraffic (such as particles from diesel motors) and sulphate crustcomposition.

Hélène Cachier, coordinator of the European project CARAMEL(Carbon content and origin of damage layers in European monu-ments) has been quoted as saying ‘‘unlike in past eras, transportis now the overwhelming contributor of atmospheric gaseousand particulate pollutants in most European urban zones.”(http://ec.europa.eu/research/environment/newsanddoc/article_2387_en.htm). Grossi and Brimblecombe [28], referring data fromthe same CARAMEL project, indicate asymptotic lightness values(lower values correspond to darker surfaces) that in areas withhigh traffic intensity and in road tunnels are around half of thosefound in urban background sites (the authors refer elemental car-bon values that in areas with high traffic intensity and in road tun-nels are above three times higher than in urban background sites).

The deterioration and blackening of gypsum plasters by deposi-tion of atmospheric particles could explain its appearance as blackgypsum-rich coatings, similar to the gypsum crusts developed onlimestones. A recent study has allowed identifying six differenttypes of gypsum-rich coatings (types 1–6) on granite rock build-ings [29], most of them related with previous gypsum or calciumcarbonate mortars. These mortars are very prone to depositionand accumulation of atmospheric particles and, therefore, veryuseful to determine the mass-transfer rates of particles and gas-eous substances. Thus, types referred as 1, 4 and 5 coatings havebeen related to previous gypsum plasters and have been formedby deterioration due to the interaction of atmospheric factors sur-rounding the façades of the buildings. As that study reported, type1 coatings are remnant gypsum plasters. Since these, type 4 coat-ings are formed by the deposition of particles on the surface andtype 5 are developed by re-precipitation of calcium sulphate dis-solved on the surface from gypsum plasters or Ca-rich mortars ofupper areas of the façades. On the contrary, the type 6 coatingsare formed by precipitated calcium sulphate, washed from upperareas of the façades on the ashlars surface. Moreover, the type 2coatings correspond to original lime mortar coatings that haveundergone in situ sulphation processes (not necessarily connectedto runoff). This fact allows developing a calcium sulphate or gyp-sum surface layer of variable thickness. Also, the type 3 coatingscome from mixed gypsum and calcium carbonate mortars. Themain characteristics of these coatings are exposed in Table 1.

After the study of composition and origin of these gypsum coat-ings, some questions have remained unsolved. The first one is theeffect of the air pollution from both urban (from traffic exhaust)and industrial sources in the formation of gypsum and the deposi-tion of atmospheric particles. The second one is which particles areoriginal components of the plasters and mortars of the buildings,and which are originated from air pollution sources. Althoughsome particles are undoubtedly originated by deposition of atmo-spheric particles, some others could be related with both origins.This discussion is very important to know which is the originalcomposition of the historic mortars and which are the most impor-tant sources of air pollution.

1.3. Aim of this work

The aim of this paper is to present the diverse composition oftypical particle content found in the gypsum coatings, formed, mod-ified or deteriorated, at least in part, by deposition of atmosphericparticles and gases in urban and rural environments. The compari-son of these particles with the particle content of a dust coatingfound on one of the studied building façades, originated due to thestrong air pollution could contribute to identify components that

Table 1Characterization of the different types of gypsum-rich coatings on granitic rock historic buildings. From Sanjurjo Sánchez et al. [29].

Coating Description Layers Thickness Composition Particles

Type 1 Gypsum plaster Inner <1 cm Ca, S, C Gypsum, soil airborne particles, Ba-rich particlesThick black–grey Outer 10–20 lm Si SiO2

Type 2 White–grey layer Inner Ca, C CaCO3, sandOuter 10–400 lm Ca,S, C, P Gypsum, soil airborne particles, halite, others

Type 3 Dark-brown layer Inner 200–600 lm C, Ca, S, Gypsum, sandOuter 1–30 lm Ca, S, C Gypsum, carbon particles, sand

Type 4 Black crust Inner 0.5–3 cm Ca, S, C SandOuter 100–200lm Ca, S, C, P Gypsum, soil airborne particles, others

Type 5 Nodulated black crust Inner 3–4 mm Ca, S Gypsum, soil airborne particles, othersOuter <5 mm Ca, S, P, C Gypsum, sand

Type 6 Grey-black coating 0.2–2 mm Ca, S Gypsum, soil airborne particles, halite, others

Table 2Listo of studied buildings from A Coruña and Betanzos, with indication of façadesorientation, age and applied rock types. Rock type key: SPL, San Pedro leucogranite;BG, Bregua Granodiorite; PG, Parga granite; PL: Parga leucogranite.

Building Locality Façade Rocktype

Year

Capuchinas Church A Coruña S SPL 1715Houses of Paredes A Coruña S BG 1778Colegiata de Sta. María

del CampoA Coruña W SPL and

BG1899

N SPL andBG

1302–1899

E SPL andBG

1302–1795

S SPL andBG

1302–1899

Church of Santiago A Coruña W SPL andBG

1502

N SPL andBG

12thCentury–1502

Church of San Francisco Betanzos W PG andPL

15th Century

S PG andPL

15th Century

E PGChurch of Sta. María de

AzogueBetanzos W PG and

PL15th Century

S PG andPL

15th Century

E PG 15th CenturyN PG and

PL15th Century

Church of Santiago Betanzos W PG andPL

15th Century

N PG andPL

15th Century

E PG 1900

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can be used as pollution markers, and air pollution sources thatcauses damage to historical building rock ashlars. Also, a comparisonof the composition of a the dust coating composition with both airpollution data from local pollution control stations and data fromtraffic intensity are considered to discuss the origin of the compo-nents of these dust coatings.

2. Materials and methods

2.1. Study area

A Coruña is a city located on the NW coast of Spain, with approximately 250 000inhabitants. Heritage buildings of the city have been constructed with two types ofgranitic rocks from local quarries: San Pedro leucogranite and Bregua granodiorite.The San Pedro leucogranite is white fine-grained granite, sometimes with feldsparsslightly deformed and oriented. The Bregua granodiorite is a milky grey coarse-grain rock with biotite and muscovite. The climate of the city is sub-humid Medi-terranean with Atlantic trend. The average annual temperature is 13.9 �C and theaverage annual rainfall 1000 mm [30].

Next to the city of A Coruña (20 km east–southeast) is located the village of Bet-anzos. It has around 10 000 inhabitants, at a non-industrialized area. The climaticconditions are very similar to A Coruña, with an average annual temperature of12.3 �C and an average annual precipitation of 900 mm [30]. Betanzos was animportant town in the Middle Ages. It has an important medieval old town. Heri-tage buildings have been constructed with granitic rocks of similar characteristicsto those of A Coruña: the Parga leucogranite and granite. The former is white med-ium-fine grain granite, equigranular with biotite and muscovite. The latter is greyporphidic granite with megacrystals of K-feldspar, biotite and muscovite. In thiscase, the leucogranite was only used in the construction of arcades of the façades,so that almost all ashlars are granite.

A compilation of recent data [31] of wind measurements indicates that in bothplaces, the prevailing wind direction is S–N/SSW–NNE although the resultant meanforce of the winds is S–N direction. In winter, strong winds are recorded. Evenwinds of 14–19 km/h can be recorded an average of 3–4 days per month at this timeof year, most of them NE–SW sense.

2.2. Features of the studied buildings

Most of the façade buildings of A Coruña, built with one or both rock types (leu-cogranite and granodiorite) showed large covering of gypsum-rich coatings, most ofthem black crusts. Although all the studied buildings are located in the old townarea, and therefore, they are similarly exposed to industrial air pollution sources,the buildings are exposed to different air pollution emissions from vehicular trafficby their situation in the city. This fact supports the suitability of the sampled build-ings to study the effect of traffic exhaust in gypsum coating formation. The studiedbuildings (Table 2) are Las Capuchinas Church (IC), Church of Santiago (SC), Coleg-iata de Sta. Maria del Campo (CSM) and the Houses of Paredes (CP). The first onewas built using only San Pedro leucogranite ashlars, while the last one was builtonly with Bregua granodiorite. The other two buildings (the Church of Santiagoand the Colegiata de Sta. Maria del Campo) were constructed using both rock types.The buildings studied in Betanzos are the Church of San Francisco (SF), Church ofSta. Maria de Azogue (SMA) and the Church of Santiago (SB). All of them are mostlyconstructed with the Parga granite, although arcades were constructed using theParga leucogranite.

2.3. Atmospheric pollution data

Data from air pollution originated from vehicular traffic in the area are providedby traffic mean daily intensity studies by the council of A Coruña and direct obser-vation in Betanzos. In A Coruña, the estimated average daily traffic is about

100 000 vehicles. In the surroundings of the studied buildings, vehicular trafficintensities are very different from one to others and even among different façadesof the same building. For that reason, mean daily traffic intensities are a useful toolto know the intensity of pollution that could affect to each façade. From these inten-sities, gaseous and particulate emissions from vehicular engines can be approxi-mated considering studies of the composition of traffic exhaust [32].

Regarding to industrial sources of air pollution, in the outskirts of A Coruñathere are a thermal power station (Sabón), an oil refinery and an aluminium pro-duction factory (situated 10 km W), which emit high amounts of pollutant gasesand particles (see Table 3). Another thermal power stations (Meirama) is locatedapproximately 20 km S from this city. Also, quantitative NOx emissions form a cera-mic industry 20 km W from A Coruña are known. Emissions from these power sta-tions and industrial sources of atmospheric gases and particles have been compiledfrom environmental annual reports published by the government of Spain from2000 to 2007 (Table 4). Data reports have been built from emissions inventories(EPER [33]).

Control pollution stations near to the industrial sources and in the outskirts ofthe city of A Coruña allow us to know the air pollutants composition in the area.These stations are shown in Fig. 1. Data of PM10, SOx, NO2, and NOx, are available

Table 3Annual mean industrial emissions of gases and particulate matter in the study area (years 2001–2006; source: EPER, Ministry of Environment of Spain).

Point Emission industry CO (tn/yr) CO2 (tn/yr) NOx (tn/yr) SO2 (tn/yr) Cd (kg/y) Cr (kg/y) Pb (kg/y)

A Sabon – 614 000 870 5084 167 417.3 258.5B Oil refinery 1220 1.565 000 2766.7 12 433.3 23.75 – –

Alcoa 6123.3 161 500 – 741 – – –C Meirama 602 4450 000 10 310 66 280 48.2 203 –D Campo ceramic Ind. – – 229 000 – – – –

Table 4Gaseous SO2, NO2, NOx and PM10 air content recorded in the control stations of the studied area (Xunta de Galicia, 2000–2007).

Point Station Council SO2 (lg/m3) NO2 (lg/m3) NOx (lg/m3) PM10 (lg/m3)

1 A Coruña A Coruña 12.3 31.8 103 17. 72 Arteixo Arteixo 5.9 7.0 11 16.03 Grela A Coruña – – – 17.04 Lañas Arteixo 20.1 22.3 – 22.06 Pastoriza Arteixo 22.4 14.8 21.7 17.05 Paiosaco Arteixo 22.3 19.1 – 19.07 Sorrizo Arteixo 22.7 – 9.0 20.78 San Vicente Cambre 10.8 16.6 28.5 –9 Mesón Carral 12.7 16.9 27.3 17

10 MonteXalo Carral 12.7 16.3 7.0 16.011 Xalo Carral 13.3 13.0 26.5 19.2512 Cerceda Cerceda 11.0 20.0 27.0 19.613 Rodís Cerceda 12.0 15.0 23.0 –14 Paraxón Cesuras 10.3 18.6 25.0 18.915 Benantes Miño 10.3 7.9 8.7 20.916 Cendón Laracha 10 16. 7 – –

816 J.S. Sánchez et al. / Construction and Building Materials 25 (2011) 813–822

between 2000 and 2007, but not all the stations have registered all this data everyyear. These atmospheric data permit comparing surface deposition on the coatingsof particles to the atmospheric content in the area, and thus the effect of the air pol-lution in the historic building.

Hourly concentration levels of SO2, NO2/NOx and PM10 were continuously reg-istered by the automatic monitor system of the pollution control stations of thegovernment of Galicia. These data were obtained by the Public Health Departmentof the Government of Galicia [34] and the National Institute of Meteorology. In thestation grid, measurements of SO2 and NO2/NOx are carried out by UV fluorescenceand chemiluminescence, respectively. For PM10 measurements are performed bydifferent methods (beta attenuation, oscillating micro-balance) depending on thestation.

2.4. Analysis of coatings and particles

A field study was performed, surveying the location and frequency of the gyp-sum-rich coatings on all the studied façades. The aim of this field survey was to ob-serve the distribution patterns of these coatings, selecting the most appropriate andrepresentative samples. Carefully sampling was carried out to study the particulatecomposition of the coatings, to cause the least possible damage to the façades. Also,a dust coating have been found and sampled to analyze its composition. This couldbe useful to distinguish the particles that come from either atmospheric pollutionor the original particulate composition of mortars.

About 94 coating samples were taken from the façades. Little amount of everysample was taken carefully, to limit the impact on the ashlars, scraping off the ash-lars surface, taking off the inner surface of the coatings. For a detailed study of coat-ings by Scanning Electron Microscopy (SEM), fragments of samples were dried andcoated with gold. Both surface and polished cross-sections were prepared and ob-served in a JEOL JSM 6400 Scanning Electron Microscope at 16 keV. Also, ElectronDispersive Spectroscopy Analyses (EDS) were performed with an Oxford 200 IncaEnergy EDS equipment. From SEM and EDS results, the chemical qualitative compo-sition of the coatings was compared to air pollution data. Also, semiquantitativemeasurements of soil airborne particles were performed from the samples. In caseswhere sufficient sample amounts were available X-ray Diffraction (XRD) analyseswere carried out to research the mineralogical composition of the samples. About5 g of each sample was powdered and analyzed in a D5000 SIEMENS X-rayDiffractometer.

An X-ray Fluorescence (XRF) from a dust coating of the Casas de Paredes Façadewas carried out to determine its chemical composition. 30 g of sample was takenand two 10 g aliquots for every sample were powdered and analyzed in a Fluores-cence Spectrometer S4 Pioneer of wavelength dispersion Bruker-Nonius. Two other5 g aliquots were calcined to 975 �C to obtain loss on ignition (LOI).

3. Results

3.1. Air pollution data results

In the outskirts of A Coruña, two thermal power station andother industrial sources emit high amounts of atmospheric pollu-tants (Table 3). Pollution control stations are located leeward andwindward from the industrial sources of A Coruña area. Thus, pol-lution measured in these stations reflects the effect of the indus-trial emissions from the sources of A Coruña area.

As the prevailing winds come from S and SSW (Fig. 1), it can beexpected that dry deposition of gases and atmospheric particlesoriginated from industrial sources on the building rock surfaceswill be stronger on the S or SSW façades, although this wind com-pound the 30% of winds of the area. Also, the prevailing winds indi-cate that industrial pollution sources located at the W of the cityshould not affect too much both the city and other nearbylocations.

Gaseous and particulate emissions from factories allowed us toknow the content of main industrial emissions (CO, CO2, SOx, NOx,NO2 and PM10) in the area (Table 3). Recorded particle content(PM10) is very similar in all the pollution control stations (Table 4).As can be observed, NO2 and NOx mean annual concentrations arealso similar in most stations and in the outskirts of A Coruña,excepting data from the A Coruña control station where very highmean values has been registered. Both NO2 and NOx gases areunstable gases produced by industrial pollution but above all byvehicle engines. Thus, these higher measurements in A Coruñacan be related to vehicular traffic air pollution.

Maps of the annual mean concentration of pollutant gases andPM10 in the studied area have been built from the pollution datarecorded by the air quality monitoring stations (Fig. 1). Isolineshave been drawn by interpolation of mean data of neighbour sta-tions. These maps can be used to establish the background levelof each pollutant in the two studied places and to compare thedeposition of particles on the coatings taken from the façades. This

Fig. 1. Map of the study area with the studied locations and the situation of the pollution control stations and industrial emission focuses, with a wind data graphic. Smallmaps of annual mean concentration of pollutant gases and particles measured in the pollution control stations are shown below.

J.S. Sánchez et al. / Construction and Building Materials 25 (2011) 813–822 817

background estimation can be considered as resultant of bothindustrial and vehicular traffic pollution, and independent of theeffect of the contribution of local sources as traffic exhaust in thesurroundings of the façade of each building. The resulting map sug-gests that a certain pattern related to the prevailing winds is ob-served for all the measured pollutant gases. Comparing thebackground of the gases and particles in A Coruña and Betanzosit can be seen that the mean annual PM10 is similar in the studiedlocations. However, SO2 mean annual concentration is slightlyhigher in A Coruña, and both NO2 and NOx are substantially moreconcentrated in this location. These gases (NO2 and NOx) are re-lated to traffic of vehicles. Thus, it is expected that the contributionof gaseous and atmospheric particles to the coatings studied in ACoruña will be higher not only due to industrial pollution sourcesbut also vehicular traffic. Strong episodic contributions of gaseousand particulate matter from the industrial sources cannot be esti-mated from these data.

Based on the previous considerations the relation between thedeposition of atmospheric pollutants from industrial sources andthe composition of the coatings should include the orientation ofthe façade. This will determine its exposure to prevailing winds(SN sense) that transport the industrial pollutants in the atmo-sphere. However, all the S façades studied in A Coruña are locatedin the areas more exposed to traffic of vehicles. Thus, this compar-

ison must be dismissed. A similar pattern is found in Betanzos. Inthis location the deposition of particles is lower than in A Coruña.Furthermore, industrial pollution and traffic intensities are muchlower. South and west façades, theoretically most exposed toindustrial pollution sources are also not comparable in this casebecause the west and north façade of San Francisco Church, thenorth façades of both Santiago Church and Santa Maria de AzogueChurhc are exposed to little traffic intensities, while west façadesof Santa Maria de Azogue and Santiago Church are located in areasclosed to traffic. The low traffic intensities in this locality could beoverlapped by the background pollution.

Exposure to traffic exhaust, varies from building to building andfaçade to façade in A Coruña. Las Capuchinas Church and Houses ofParedes are very exposed. They are located in areas of heavy trafficand there are bus stops beside both façades. The other two build-ings considered in this city are located in areas of low traffic. Spe-cifically, traffic is very low in the W side of the Church of Santiagoand a little bit higher in the N one. In the N and W front of theColegiata de Sta. Maria del Campo there is no traffic, although thereis low traffic intensity in the N and E façades.

The three buildings studied in Betanzos are located in the vil-lage centre, exposed to low road traffic, as this is an area restrictedto traffic. Therefore, there is low air pollution from vehicular emis-sions. Only the W and N (in its E corner) façades of the Church of

818 J.S. Sánchez et al. / Construction and Building Materials 25 (2011) 813–822

San Francisco, the W façade of the Church of Sta. Mara de Azogueand N one of the Church of Santiago, are located on streets of verylow intensity traffic.

Regarding traffic exhaust, two factors are important to evaluateits effect: the traffic intensity and the elemental composition of theparticles of the traffic exhaust. The traffic intensity is an importantissue that can be approximated by direct observation studies. Thisis an important contributor to the dry deposition of gases and par-ticles on the building façades. Analysing the differences of intensi-ties among building façades can allow us to asses the contributionof road traffic to the formation of gypsum coatings. Tables 7 and 8shows data of traffic intensities taken from the studies carried outby the A Coruña city council and direct observations in Betanzos.

Various references provide data to solve the second question.Data can be approximated form Pierson and Brachaczek [32]. Theyanalyzed the composition of traffic exhaust from engines by gaso-line and diesel combustion. Summarised data of the elementalcomposition of the exhaust are shown in Table 5. Although threedifferent types of atmospheric particles have been traditionallyconsidered as pollution markers (fly ash, metallic rich particlesand silicon particles), as they are typically originated by combus-tion of fossil fuel from both industrial focuses and engines, trafficexhaust data allow indicate some other possible elements relatedto pollution, such as Br, K, Mg and Ca.

3.2. Particles on dust coatings

SEM observations of the dust coating sample revealed a hetero-geneous composition. These are detritic coatings 0.5–1 cm thickand discontinuous, containing fly ash and carbon soot, microorgan-isms, airborne soil particles, and other particles. EDS analyses showC and Si as main components, Al and Fe as minor components and

Table 5Summary of composition and emisión rates (mg/km) of airborne mater form on–road veh

Element Gasoline (mg/km) Diesels (mg/km) Overall (mg/km

C 34 ± 21 725 ± 117 141N 1.1 ± 0.8 16 ± 2 3.4Na 0.09 ± 0.37 6.6 ± 1 1Mg 0.7 ± 0.3 8 ± 1 1.7Al 0.2 ± 0.5 8.5 ± 1 1.4Si 0.5 ± 0.7 14 ± 2 2.5P 0.07 ± 0.06 1.3 ± 0.2 0.24S(SO4

�2) 0.4 (3.4) ± 0.9 23 (42)±5 3.9 (9.4)Cl 0.8 ± 0.4 �3.1 ± 2.5 0.4K 0.17 ± 0.08 1.5 ± 0.2 0.3Ca 1.3 ± 0.3 5.8 ± 1.4 2Ti 0.006 ± 0.01 0.12 ± 0.03 0.23Cr 0.001 ± 0.003 0.02 ± 0.01 0.003Mn 0.08 ± 0.01 0.35 ± 0.04 0.115Fe 0.32 ± 0.32 5 ± 0.9 0.96Cu 0.04 ± 0.02 0.22 ± 0.09 0.07Zn 0.04 ± 0.04 1.4 ± 0.1 0.22Br 5.75 ± 0.45 �0.8± 4.3Ba 0.03 ± 0.01 0.66 ± 0.03 0.8Pb 12.4 ± 1.6 11.5 ± 3 12.3Other elements �5.9 ± 2.1 39.9 ± 22.2 4.7Gross mass 51 ± 30 865 ± 161 177Sum 66 ± 22 890 ± 118 193

Table 6XRF results of dust coating taken from the Houses of Paredes façade.

Oxide SiO2 CaO Al2O3 K2O Na2O

(%) 39.7 3.2 14.5 2.70 0.98

ZnO SrO BaO MnO Rb2O CuO

0.789 0.023 0.260 0.132 0.026 0.047

traces of other elements, mainly K, Mg, S and Ca, although some-times Pb, Ti, Ba and other elements are detected.

Results from XRF analyses of the coating reveal Table 6 that SiO2

(39.7%) and Al2O3 (14.5%) are the mayor components, probably dueto the deposition of soil airborne particles on the building façades.Although a bigger concentration could be expected for some typi-cal pollution particles, these are not detected or only detected asminor components. In this sense, mayor components detected onexhaust analyses (Pb, Br, S, Ca, C, Mg and Fe) or emitted by indus-trial focuses (S, C, Pb and Cr) are either major (Fe, Ca, Mg, S, C, K, Na,P), minor (Ba, Mn, Cl, Pb) or traze (Cr, Cu) components of the dustcoatings. To compare, previous data from dust coatings from A Cor-uña (urban dust, local suburban and local urban samples) haveshown very low S and N content (3–3%, 0.08% and 0.18% of S and2.8%, 0.25%, 0.1% of N, respectively) and moderate C content (18,5.14 and 2.56, respectively) [35]. This S content is similar to thatfound in this XRF analysis, supporting the atmospheric origin ofthis dust coating.

Fe2O, K2O and MgO are important components of the dust coat-ing (above 1%), and others as TiO2, ZnO or Pb are over 0.5%. Also,important loss on ignition (LOI) has been measured, reaching25.2%. This could be due to a high water content of this coatingand to carbonaceous particles or components. A TDA–TGA analysis(Fig. 2) reveals losses of 3.62% at 200 �C, and a continuous loss thatreaches 12.23% at 600 �C and 8.008% at 1050 �C with peaks at 350–430 �C and 820–900 �C probably due to dehydration of silicatephases and breakdown of silicate clays, respectively [36].

3.3. Particles on gypsum coatings

SEM observations and EDS measurements allowed detectingdifferent elements on the gypsum coatings. They could be attributedto either the deposition of atmospheric particles or the original

icles, from Pierson and Brachaczek [32].

) Gasoline (% of total) Diesels (% of total) Overall (% of total)

67 ± 42 84 ± 14 79.52 ± 2 1.9 ± 0.3 1.90.2 ± 0.7 0.8 ± 0.1 0.61.3 ± 0.6 0.9 ± 0.15 10.3 ± 0.9 1 ± 0.2 0.81 ± 1.3 1.6 ± 0.2 1.40.13 ± 0.11 0.15 ± 0.02 0.140.9 (7) ± 3 2.7 (4.9) ± 0.9 2.2 (5.3)1.6 ± 0.8 0 0.20.3 ± 0.2 0.17 ± 0.03 0.22.5 ± 0.7 0.7 ± 0.2 1.10.01 ± 0.02 0.014 ± 0.004 0.0130.001 ± 0.006 0.002 ± 0.001 0.0020.16 ± 0.025 0.039 ± 0.004 0.0650.60 ± 0.6 0.6 ± 0.1 0.540.07 ± 0.03 0.025 ± 0.01 0.040.08 ± 0.08 0.16 ± 0.01 0.12511.2±0.9 <0.05 2.40.07 ± 0.02 0.077 ± 0.003 0.0724 1.3 ± 0.3 6.96�13.42 3.86 0.74100 100 100129 ± 43 103 ± 14 109

SO3 Fe2O Cl MgO P2O5 TiO2

1.7 5.29 0.26 1.45 2.58 0.752

PbO ZrO2 Cr2O3 NiO Y2O3 LOI

0.429 0.029 0.022 0.010 0.003 25.2

Fig. 2. GTA diagram of the dust coating sample.

J.S. Sánchez et al. / Construction and Building Materials 25 (2011) 813–822 819

composition of the mortar, considering that high Ca and S val-ues are due to the gypsum of the mortars and coatings, and some-what lower Si and Al values to the mortar sand. Some particles thatcould be correlated to exposure to air pollution do not seem topresent a pattern that allows us to relate them. Fe-rich particles(Fig. 3a) have been detected by SEM observations and EDS analysesin almost all samples. Although it can be related to soil airborne

Fig. 3. Particles in surface layers of the gypsum-rich coatings: (a) Fe-rich particles in andgypsum coating from Capuchinas Church, A Coruña, (c) soil airborne particles deposited oa gypsum layer of Capuchinas Church.

particles, a pattern between these particles and Fe detection hasnot been found. Also, other metalic particles such as Cr or Ti andBa, cannot be only related to traffic exhaust. In particular, Fe, Cror Ti could be part of the existing mortars in the buildings, as theyhave been used in the past to give colour [37]. The detection of Ba(Fig. 3b) could be also attributed to hydroxide Ba treatments ap-plied to mortars in the past, to avoid their decay [38]. Other ele-ments such as K, Na or Mg can be detected as part of the mortarsand.

The content of soil airborne particles (Fig. 3c) on the surface ofthe coatings has been observed and semi-quantified and results ofEDS analyses have also been considered. Despite that importantamounts of soil airborne particles are found on the samples takenfrom the façade of Casas de Paredes, the most exposed to traffic ex-haust, the occurrence of K, Na and Mg in these samples is not nec-essarily higher. Furthermore, K has been detected in most samplesof not deteriorated or fairly deteriorated mortars (types 1 and 2).For that reason, it does not seem indicative of exposure to pollu-tion. Also, an association between Na and Cl has been found, asthe two study areas are located in a coastal area, so the existenceof sodium chloride in many of the coatings is expected (as indeedis detected) (Fig. 3d).

A certain association between the façades more exposed to traf-fic exhaust and the detection of Pb-rich particles on the coatingsamples has been found. Pb-rich particles appear more often onthe samples taken from the façades of Capuchinas Church and Ca-sas de Paredes in A Coruña, more exposed to pollution by vehiculartraffic (up to 50 and 100 times the traffic intensity of the other

on a coating from Santiago Church (SB-2) from Betanzos, (b) Ba-rich particles on an a gypsum coating of Casas de Paredes, A Coruña and (d) halite particles included in

Table 7Particles observed by SEM and detected by EDS on the surface of coating samples from A Coruña.

Building Façade Aprox. trafficintensities(vehicles/day)

Coatingtype

Numberofsamples

Heightrange(m)

Colour P Fe Pb Ba Cr K Mg Ti Na Cl Flyash

Soilairborneparticles

Otherelements

IC S 12 000 1 1 7–9 Yellow – – – – – + – – – + – – –4 7 0–9 Black + – + + + + + + + + + ++ –5 2 6–10 Black + + + – – + + – + + + ++ –6 1 8–10 Grey – + + – – + + – + + – ++ –

CS W 0 (closed to traffic) 3 1 0–3 Black + + + + – + + + + + + ++ –N 0 (closed to traffic) 2 8 0–10 Grey–

black+ + + + – + + + + + + ++ Zn, Mn,

3 2 0–6 Black – + + – – + + + + – + + F4 5 0–4 Black – + – – – + + + + + – ++ –

E 300 2 3 0–10 Orange–brown

+ + – – – + + – + – – ++ –

3 3 0–5 Black + – – + + + + + + + – + –S 300 2 3 0–1.5 Black + + – – – + + + + – – ++ –

3 2 0–10 Grey–black

– + – – – + – – + + + + –

4 3 0–5 Black – + – – – + + + + + + +++ FSC W 300 1 1 2.5–3 Red – + – – – – – + – + – + –

2 1 0–1.5 Ocre–brown

– + – – – + + – + + – + –

4 1 0–7 Black – + – – – – – – + + + + –6 1 0–4 Grey – + – – – – – – + + – ++ –

N 300 3 1 0–8 Grey–black

– + – – – + + + – + +++ –

4 3 0–7 Black – + – – – – – – + + + + –6 3 0–4 Grey – + – – – – – – + + – +++ –

CP S 24 000 1 1 7–8 Grey–black

+ + – – – + – – + + – +++ –

2 2 0–4 Grey–black

– – + – – – – + + + ++ –

5 2 4–11 Black + + – – – + – + + – + +++ –3 2 4–4.5 Grey–

black– + + – – + – – + + + +++ –

6 2 4–4.5 Black – + – – – – – – + – + –

Table 8Particles observed by SEM and detected by EDS on the surface of coating samples from Betanzos.

Building Façade Aprox. traffitintensities(vehicles/day)

Coatingtype

Numberofsamples

Heightrange(m)

Colour P Fe Pb Ba Cr K Mg Ti Na Cl Flyash

Soilairborneparticles

Otherelements

SF W 200 2 1 0–5 Brown + + + + – – – – – – – ++ –S 300 2 6 0–2 Red–

brown+ + – – – + + – – – – ++ –

E 0 (closed to traffic) 2 4 0–3 Brown + + – – – + + – – + – + –2 3 0–6 Black–

grey– + – + – – + – + – – + –

3 1 2–3 Grey–black

– + – – – – – – – – – ++ –

SM W 0 (closed to traffic) – – – – – – – – – – – – – –S 300 2 4 4–8 Black – + + + – + – – – + – ++ –E 0 (closed to traffic) 2 1 1–10 Grey–

black– + + + – – – – – – – ++ –

N 200 2 2 4–8 Black – + – – – + – – – + – ++ –SB W 0 (closed to traffic) – – – – – – – – – – – – – –

N 300 2 7 0–10 Brown + + – – + + + + + + – ++ –E 0 (closed to traffic) 2 4 0–4 Brown–

orange+ + – – – + + – + + – ++ –

820 J.S. Sánchez et al. / Construction and Building Materials 25 (2011) 813–822

buildings, respectively). There are also indications of associationbetween the exposure of the façades to traffic exhaust and thedetection of fly ash (up to twice more frequent fly ash in exposedfaçades), which is a clear indicator of pollution since they are typ-ically produced by the combustion of diesel [14]. In the town of ACoruña, this observation agrees with the relative exposure of thefaçades at different intensities of vehicular traffic.

The close coincidence between exposure to traffic pollution, Pb-rich particles, fly ash and a high proportion of soil airborne parti-

cles indicates the façade of the Casas de Paredes is the most ex-posed to pollution. Also, the façade of Capuchinas Church, thesouth façade of Colegiata de Santa María and the north façade ofSantiago Church in A Coruña show variable but important fly ashand soil airborne particles content. These façades are the most ex-posed to vehicular traffic exhaust, and they show thicker gypsumlayers developed on the surface of the mortars as previous studieshave shown in [29]. This fact could be related to the effect of thehighest concentration of NO2/NOx on the coatings (as related by

J.S. Sánchez et al. / Construction and Building Materials 25 (2011) 813–822 821

Pio et al. [10]) regarding that the SO2 background concentration isnot very different in A Coruña and Betanzos. Therefore, it can bealso concluded that local air pollution point sources (exposure totraffic exhaust) are a more determinant factor in the decay of themortars than the background due to industrial pollution, at leastin A Coruña.

Regarding the influence of industrial emissions, as two thermalpower stations in the studied area emit high amounts of pollutantgases and particles, trace element levels in coal ash reported in theliterature do not agree with our results. In their studies, Singh andKumar [39] detected important amounts of Cr, Hg, Mo, Ni, Se, Znand Cu. In our case, only Pb has been observed and Cr and Zn aredetected in two buildings from A Coruña (IC and CP) and one fromBetanzos (SB). The buildings of A Coruña are the most exposed totraffic exhaust because traffic intensities are the highest besidesome of their façades. In the case of the buildings of Betanzos,the Cr is probably related to the mortar, as Cr-rich particles canbe used to give colour to the mortars. From these elements, someauthors [37] have demonstrated the ubicuous presence of Zn, Cuand Pb in patinas from monuments, always in higher content thanin the substrate rock, suggesting an atmospheric origin.

To check this observation, data obtained from A Coruña werecompared with those from Betanzos. Fly ash has not been detectedand Pb-rich particles have only been detected in two coatings fromBetanzos. However, no correlation with pollution by vehicular traf-fic is observed for the Pb content in this place. Even this elementcan be used to give colour to mortars, such as Fe or Cr as has beenreported in the literature [40]. Also, metalic particles except Fe arescarce in this location. Moreover, a lower soil airborne particle con-tent is generally observed.

In Table 9 the results of the XRD analyses are shown. The mostabundant minerals correspond to those expected in coatingsformed from gypsum mortars (gypsum and calcite due to theaggregate, and quartz, albite and microcline due to the mortarsand). Quartz, albite and microcline as other minerals such as mi-cas or clinochlore can also be related to soil airborne particles.Thus, they may be indicative of the presence of a significant pro-portion of these particles in these samples, at least above thedetection limit of the method. Also, halite is found in some sam-ples. This salt is more frequent in A Coruña, as this is a coastal area

Table 9XRD of some samples from A Coruña and key: gypsum (G), calcite (Cc), quartz (Q), albite (clinochlore (Ch), calcium phosphate (CP), hydrated calcium silicate (HCS), aliminic phosph

Building Façade Coating type Num. of samples G Cc Q A

IC S 1 1 +++ – + +4 3 +++ – + +5 2 ++ – + +

CS S 2 2 +++ – + +S 4 1 +++ – + +E 2 1 – + + +N 1 1 ++ – + +

2 1 ++ – + +SC W 1 1 – – + +

6 1 ++ – + +N 6 1 ++ – + +

6 1 +++ – + +CP S 2 1 ++ +++ ++ +

3 1 +++ – + +5 1 ++ – ++ +6 1 +++ – + +DCa 1 – – ++ +

SF W 2 1 ++ ++ + +S 2 2 ++ ++ + +E 2 1 ++ ++ + +

SB E 2 1 ++ ++ + +N 2 1 ++ ++ + +

a DC (dust coating).

of greater influence has a clear atmospheric source. The XRD detec-tion of certain mineral species such dolomite, hematite and mag-netite (Fe-rich minerals) and some phosphates (calciumphosphate, sodium phosphate or potassium–aluminium phos-phate) may be related to either atmospheric pollutants or mortarcomponents as has been related above. Fe-rich and phosphate min-erals are not frequently detected.

4. Conclusions

The particle content of gypsum coatings formed by decay ofmortars (types 1, 2 and 4 coatings defined in Sanjurjo Sánchezet al. [29]) has been studied and compared to atmospheric pollu-tion data and dust coatings formed by deposition of atmosphericparticles. This comparison has allowed us to obtain twoconclusions:

– The sulphation of the calcium carbonate mortars by atmo-spheric SO2 to give calcium sulphate is not necessarily due tolocal pollution but background air pollution. This conclusion issupported by two facts. The first one is that gypsum coatingsare generally observed in Betanzos despite local pollution totraffic exhaust is not important. The second one is that meanannual SO2 concentrations measured in both localities (back-ground) are not very different.

– The deposition of some particles is a good indicator of local pol-lution due to traffic exhaust. The results of this work suggestthat, fly ash, soil airborne particles and Pb-rich particles arethe best indicators of pollution compared to other elementssuch as Fe, Cr or Ti, which have occasionally been consideredin pollution studies and can also be found in mortars due totheir use to give colour to them [39]. This has been previouslyobserved in other studies [29].

It has been shown that the background of air pollution mea-sured at an urban and a rural locality in the same geographic area(due to both industrial sources and traffic exhaust) influences theparticle deposition at the regional level. However, local pollutionseems to be more important to cause strong decay of mortars. As

A), microcline (M), mica (Mi), halite (H), dolomite (D), hematite (He), magnetite (Mg),ate (AP), sodium phosphate (SP), potassium calcium phosphate (PCP).

M Mi H D He Mg Ch CP HCS AP SP PCP

– – + – – – – – – – – –– – – – – – – – + – – –– – – – – – – – + – – –+ + – – – – – – – – – –+ – + – – – – + – – – ++ – – – – – – – – – – –+ + – – – – – – – – – –+ + – – – – – – – – – –– + – + + + + – – – –+ + – – – – – – – – – –+ + – – – – – – – – – –+ – + – – – – + – – – –+ – – – – – – – – – – –+ – – – – – – – – – – –+ + – – – – – – – – – –+ – – – – – – – – – – –+ + – – – – + – – – – –+ + – – – – – – – – – –+ – – – – – – – – + – –+ + – – – – – – – + + –+ + – – – – + – – – – –+ + – – – – – – – – – –

822 J.S. Sánchez et al. / Construction and Building Materials 25 (2011) 813–822

results have showed, the differences in the particle content ob-served on the studied façades can be related with the local pollu-tion. Vehicular traffic determines and increases the soiling,formation and development of gypsum coatings by mortar decay,probably due to the increase of sulphation with deposition of par-ticles and the highest concentration of NO2/NOx in the atmosphere.Thus, exposure to local vehicular traffic seems to be more decisivethat exposure to industrial pollution as suggests the higher particlecontent in the facades exposed to high traffic intensities (and asevidenced the dust coating found in Casas de Paredes in A Coruña).

Acknowledgments

The Centro de Investigação Geológica, Ordenamento e Valor-ização de Recursos is supported by the Pluriannual program ofthe Fundação para a Cência e a Tecnologia, funded by the EuropeanUnion (FEDER program) and the national budget of the PortugueseRepublic.

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