rad singenit

Journal of Cultural Heritage 13 (2012) 175186

Available online at

www.sciencedirect.com

Original article

The origin of syngenite in black crusts on the lim(Belgra tur

Vesna M hilDanica Sa University of lograpb LADIR, UMR 7c The Highway

a r t i c l

Article history:Received 25 February 2011Accepted 1st September 2011Available online 12 October 2011

Keywords:LimestoneBlack crustSyngeniteGypsumFertiliser

Black crusts formed on limestone built into the Kings Gate represent the most important process in stonedeterioration that is occurring in this part of the monumental complex of the Belgrade Fortress (Serbia).Of special importance is the association of salts (namely gypsum and syngenite). Syngenite is a commonsecondary deposit on granite monuments and on medieval stained glass (i.e., on K-containing materials).However, its appearance over calcareous substrates is not apparent, particularly in cases where cementmortar was not used for bounding. The origins of the potassium and sulphate ions required for syngenite

1. Researc

Syngenitbuildings coite, cementmaterials li

The blacbuilt into thnant and mbeside comblocks alon

CorresponE-mail add

(V. Matovic), s(A. Kremenovi(D. Sreckovic-B

1296-2074/$ doi:10.1016/j.formation are related to meteoritic water, which penetrates the soil above the arch of Kings Gate. Waterdissolves some soil components and becomes enriched with various ions before coming into contact withthe limestone blocks. Enriched water contains two times more K+ and SO42 ions than pristine meteoriticwater does. The source of the required ions is potassium-sulphate that is present in agricultural fertilisersthat are used above the monument. The proposed mechanism for syngenite formation was additionallysupported with laboratory experiment. The results of X-ray diffractometry and SEM-EDS analyses oflimestone treated with potassium-sulphate solution and sulphuric acid suggest the possibility that thesyngenite was formed over calcite:

CaCO3 + K2SO4 + H2SO4 K2Ca(SO4)2 H2O + CO2However, the complex mechanisms of gypsum and syngenite formation under natural conditions (vari-

able concentration of potassium and sulphate ions, intermediates phases, temperature changes, humidity,the amount of disposable water etc.) do not exclude the possibility of syngenite formation over gypsum.

2011 Elsevier Masson SAS. All rights reserved.

h aims

e is the common mineral in black crusts derived onnstructed of potassium rich building materials (gran-

mortar, etc.). However its presence on potassium poorke limestone is extremely strange.k crusts that have developed on the limestone blockse Kings Gate (Belgrade Fortress, Serbia) are the domi-ost destructive decay forms. The presence of syngenitemon gypsum and calcite in black crusts on limestoneg with the variable amounts of syngenite motivated the

ding author. Tel.: +38 11 13 33 67 02.resses: [email protected], [email protected]@yahoo.com (S. Eric), [email protected]), [email protected] (P. Colomban), [email protected]).

aims of this study. Therefore, in this study were taken into consid-eration:

variable composition of the black crusts depending on the posi-tion of the limestone blocks;

determination of the source of the ions responsible for the pres-ence of different phases in the black crusts;

the explanation of the mechanism of black-crust formation, par-ticularly with regard to syngenite-bearing crusts.

2. Introduction

Cultural monuments all over the world have been traditionallymade of different stone types. Monuments in Serbia, especially inBelgrade were built most frequently of limestone and sandstonedue to their pronounced abundances, decorative appearance andproperties conducive to ease of manipulation.

see front matter 2011 Elsevier Masson SAS. All rights reserved.culher.2011.09.003de Fortress, Serbia) the role of agricul

atovic a,, Suzana Eric a, Aleksandar Kremenovic a, Preckovic-Batocanina, Nenad Matovic c

Belgrade, Faculty of Mining and Geology, Department of Mineralogy, Petrology, Crystal075 CNRS, and Universit-Pierre-and-Marie-Curie, 94230 Thiais, FranceInstitute, Kumodraska 257, Belgrade, Serbia

e i n f o a b s t r a c testone monument Kings Gatee fertiliser

ippe Colombanb,

hy and Geochemistry, Djusina 7, Belgrade, Serbia

176 V. Matovic et al. / Journal of Cultural Heritage 13 (2012) 175186

However, upon exposure to various physical and chemical fac-tors over many years, limestone is subject to a variety of aggressivestone decay processes that result in varying degrees of degradation.Deterioration processes of limestone depend on its own propertiesand on theBlack cruststone surfarun-off.

Three mon marble:and black enThey usuallit to becomtors that gothe importathe type of derived on loaded (SO2els, the SO2atmosphereposition, limcrusts, whicinuence oferation of bmost cases,a polluted amatter founble effect: acolour) [8] in the substInitially, crufacade elemthus are nowith moistutually becocoal (whichbuildings) aare also prethe formatition to the pquartz, glassignicant through thecompoundsthe stone tphate and cthe surfaceintegrationcrusts.

The stonumental fopure limestBecause it himpacts formain damaso-called bof black crumechanismcrusts formHowever, sother sulphpresence ofof joints as tsyngenite mcence on co[21].

3. Site characteristics and historical background on theKings Gate

The Kings Gate is located in the central and oldest part of theenvirue

44estorid bys nu

mpre Kithonss, t

andy) anriansetritjtova

posuse otylesof cod Beas bu

and rthes ovg phffensfortrte is er paf thith ag tec

quesuth fd sto

pilas abov

closepreshe Ba

imat

ate the wwwn thent

e anity isated g plar pols as

vehiing nt p

Thert ofm3. Tal vand w exterior factors to which it has been exposed [13].s are a common form of deterioration that affect onces sheltered from the direct inuence of the water

ain types of black crust were dened in observation black crust with decayed substrate, compact depositcrustration showing a sound substrate underneath [4].

y spontaneously separate from the basement and causee soiled and disintegrated. Among debated several fac-vern the locations and morphologies of black crusts,nt are local atmospheric pollution, microclimate andstone [1,59]. Similarly, syngenite-rich black crusts areK-containing stained glass that are exposed to heavily> 40 g/m3) atmospheres [1012]. At lower SO2 lev-concentration and the relative humidity in the ambient

play only minor roles [11]. Due to their mineral com-estones are very susceptible to the deposition of blackh regularly occur on surfaces sheltered from the direct

atmospheric ows. The moisture required for the gen-lack crusts is supplied by fog and dew [1] and [13]. In

black crusts form on limestone surfaces as a result oftmosphere (i.e., as a result of the airborne particulated in urban areas) [5,14,15]. Their presence has dou-esthetic (as they completely cover the stones originaland physicochemical (as they induce physical changesrate that eventually leads to the splitting of the stone).sts form through deposition of pollutants and dust onents that are not directly exposed to rainwater (andt naturally washed). Supercial deposits then interactre from the atmosphere adhere to the surface and even-

me harder and solidied. Particles from the burning of are particularly abundant in deposits within very oldlong with particles from the burning of other fossil fuelssent. The inuence of S-containing dust particulates onon of sulphate crusts has been reported [12]. In addi-roducts of fuel combustion, black crusts contain calcite,sy globules and some air-borne particles [1]. The mostcomponent of black crust is gypsum, which is formed

interaction of CaCO3 within the substrate with sulphur. The transformation of calcium carbonate minerals ino the gypsum causes some dramatic changes [9]. Sul-alcite are generated through precipitation on and under

[16]. Therefore, the main cause of the limestone dis- below the crust is gypsum that originates from black

e built into the Kings Gate, which is part of the mon-rtress complex at Kalemegdan in Belgrade, is almostone (with a CaO/MgO/K2O ratio of 0.989/0.01/0.001).as been exposed to various natural and anthropogenic

almost 300 years, it exhibits several forms of decay. Thege has been caused by various salts contained in thelack crusts on the monument. The major compoundssts are calcite, gypsum and syngenite. The formation

of gypsum, which is a common compound of blacked on limestone and marble, is well known [1719].yngenite is rare mineral and is usually associated withates on granite monuments and buildings due to the

K-feldspar. As a rule, syngenite develops on the insidehe consequence of cement utilisation [20]. Additionally,ay occur either as a compound of white efores-ncrete blocks or along cement joints between bricks

urban the con2027and hiroundecontainmost i

Theautochof accetowerscenturHungaalgal-dthe la

Thethe catural sphase invadegate wnichesThe nowreathbuildintrian oof the the gayoungation oends wbuildintechniThe soprolelateralbridgepletelyGate rfrom t

3.1. Cl

Climfrom http://ence omonumaveraghumidis situheatinThe aidecadeted byincreaspollutawind).tral pa70 g/seasondays aonment of Belgrade (the capital of Serbia; Fig. 1a) abovence of the Sava and the Danube Rivers (444914north,ast). The monument is situated in a famous culturalcal complex called the Belgrade Fortress, which is sur-

the Kalemegdan Park (Fig. 1b). The Belgrade Fortressmerous gates and towers, but the Kings Gate is thessive.ngs Gate is constructed of white lajtovacki,ous limestone that is Miocene in age. Due to easehese reef limestones were widely used for bastions,

gates since the time of the despot Stevan (11thd later during the occupations of the Turks and Austro-

(17th19th centuries). Pure algal, algal-bryozoan andal varieties of limestone can be distinguished withincki limestone beds below the fortress.ition of the Kings Gate inside of the fortication wasf its complex constructive fabric in various architec-

along with its long-lasting construction [22]. The rstnstruction occurred during the time when the Turkslgrade (16931696). Subsequently, the older part of theilt in the form of a vault passage with blind arcadinglateral chambers through the fortress bastion (Fig. 1c).rn portal on the gate is simple, with two proled stonyerarched with a cordon wreath (Fig. 1d). The secondase (from 1718 to 1721) took place during the Aus-ive in Belgrade and included extensive reconstructioness. The passage from the older to the younger part ofclearly marked with an arch and an ironclad door. Thert, with a lower arch, spatially represents a continu-e arched passage with several lateral passageways. It

remarkable portal designed in the Baroque style. Thehnique used was signicantly different from previous

and the portal contains stony blocks with brick rows.acade is composed of regular, richly ornamented andny blocks, whereas the arch passage is marked with twoters and overarched with a cordon wreath (Fig. 1e). Ae the trench in front of the southern facade is com-ed with a chevron-shaped ravelin structure. The Kingsents the best-preserved part of the Belgrade fortressroque period.

ic conditions

data were collected for the period 19612010 (dataRepublic Hydro-Meteorological Service of Serbia -.hidmet.gov.rs) and analysed to determine their inu-

e degradation processes occurring at the investigated. The average annual temperature is 11.7 C, and thenual rainfall is 669.5 mm/year. The average relative

69.5%. As Kalemegdan, including the monument itself,in the centre of the city and is close to busy roads,nts and factories, the air pollution is pronounced.lution problem has increased rapidly in the last few

a consequence of the high quantities of gasses emit-cles and fuel combustion; these also contribute to thecontent of sulphur (IV)-oxide in the atmosphere. Thiseriodically decreases during the kosava (southeastern

average annual content of black smoke in the cen- Belgrade (near the Kings Gate) is between 50 andhe contents of black smoke and SO2 display daily andriations. Variations are also observed during workingeekends. Occasionally, the black smoke content in this

V. Matovic et al. / Journal of Cultural Heritage 13 (2012) 175186 177

Fig. 1. (a, b)

part of the of SO2 in th60 g/m3 (tand 150 gHydro-met

4. Materia

4.1. Sample

Seven c50 mm diameast arcadesections wewhile one saFive samplestone includtance (two s(30 cm in lemeasuring 20 cm fromsample 3, fr Location of the Kings Gate in Belgrade, Serbia; (c) schematic section through the Kings

city exceeds 100 g/m3. Although the highest valuesis part of the city are noted, the value rarely exceedshe limit values of black smoke and SO2 are 50 g/m3

/m3, respectively, based on data from the Republiceorological Service of Serbia).

ls and methodology

s

ore samples were selected (water-drilled, nominaleter, 240300 mm depth) from the arch and from the

. Samples were used to characterise the stone (thin-re prepared from each core for petrographic analysis,mple with black crust from arch was analyzed by SEM).s were used to determine physical properties of theing porosity, water absorption, density and frost resis-amples were not tested because core was broken). Corength) from the arch were cut into three samples for

pore size distribution by Hg porosimetry (sample 1 at the surface; sample 2 at 5 cm from the surface; andom the surface).

The blac(that is buistone particmicroscopelows: two two samplesouth entra(T5 and T6the north e the arch black crustspectrometSEM/EDS.

Three so50 m left ofions.

Quantitawere perfosurface. TheH2SO4.

One samfrom a degate).Gate [22]; (d) north facade of the gate; (e) south facade of the gate.

k crusts were separated by hand from the host rocklt into the gate) and afterwards any remaining lime-les were picked out with special tools under a binocular. Ten samples of black crusts were collected as fol-samples from the wall on the left (T1 and T2) ands from the right side (T3 and T4) of the gate near thence; two samples from the left side in the central part); two samples from the wall on the right side nearntrance (T7 and T8) and two from the ceiling wallaxis (T10 and S2). The mineral phases present in alls were determined by XRPD, IR absorption and Ramanry, while the sample T10 was additionally analysed by

il samples were taken at 7 m above, 10 m below and the entrance to determine the variation of K and SO4

tive chemical analyses of the embedded limestonermed for a sample from a depth of 20 cm below the

same sample was chemically treated with K2SO4 and

ple of mortar was taken between the limestone blockspth of 2 cm (inner left side of central part of the


4.2. Petrophysical properties of limestone

To examine the rate and nature of the stone damage, a determi-nation of petrophysical properties (e.g., bulk and apparent density,porosity, wusing laborStandards aStandards funit of appstatic weighatmospheripowdered sing uid. Tofor real andincludes clomechanicaltance, as ituids will pof limestonintrusion pthe Milestointrusion pand enablesval.

Water amass of waat atmosphages of theto constantcycles of freinvolved resamples arethat, sampl(15C) for 2

4.3. Chemis

The chembuilt into twere determtrated HCl, 0.1 M HCl (Cwhile conte300 Spectro

4.4. X-ray a

Phase coand chemicder diffractPW-1710 dfrom 5 to 7and 0.5 s of

4.5. SEM-ED

Morphopresent in identied uthat was coter. The samSputter coavacuum con

4.6. IR absorption

IR spectra were recorded on KBr pellets with a Perkin-Elmer 983double-beam spectrometer. The powdered black crust was mixed

ry KB

man

sam, hi

Horibnfocicrosw

he di), thcro)p

seco

emic

conns. F

conconteer. SBaCl2

pH

celer

iece ent

M sted aDS an

ults

trop

rograthat

all ands alg

h, paundsamisiicriteesh

lentded

are csil reallisents nifer) wearryith s

limeere

fromes ofater absorption, and frost resistance) was performedatory tests according to national standards (Serbianre concerning methods of investigation close to Britishor stone). The bulk densities are given as mass perarent volume. Volumes were determined by hydro-ing of specimens soaked and suspended in water underc pressure [23]. Real densities were estimated fromamples using a pycnometer with water as the divid-tal porosity values were calculated according to values

bulk densities [23]. Although the total porosity (whichsed pores) is signicant and has a great inuence on the

properties of stone, open porosity is of greater impor- has a direct impact on the likelihood that undesiredenetrate the stone. To estimate the porous structuree, open porosity was determined using a mercury-orosimeter (Carlo Erba Porosimeter 2000) and usingne 100 Software System. This high-pressure mercury-orosimeter operates within the 0.1200 MPa interval

the estimation of pores within the 7.5 nm15 m inter-

bsorption values were determined by measuring theter absorbed by the samples after immersion for 48 heric pressure; the values were expressed as percent-

initial mass of the samples previously dried at 105 C weight [24]. Resistance to frost was determined by 25ezing to 25 C and thawing [25]. The test procedurepeating of cycles where one cycle is: the water soaked

placed in the freezing chamber for 4 h (20C); afteres are totally immersed in the water in the thawing tank

h.

try of limestone and mortar

ical composition of the limestone and mortar that arehe Kings Gate was determined. Cations in limestoneined from solution of the sample dissolved in concen-

and in mortar from solution of the sample dissolved ina and Mg contents were obtained complexometrically,nts of K and Na were determined using an AAS Analystmeter).

nalyses

mpositions of powdered black crust and of powderedally treated limestone were determined by X-ray pow-ion (XRPD). The XRPD was performed on a Philipsiffractometer. The diffraction patterns were obtained0 2 using CuK radiation with a step scan of 0.02 2 counting time at each step.

S

logies and chemical compositions of mineral phasesblack crust and in chemically treated limestone weresing a JEOL JSM-6610LV Scanning Electron Microscopennected to an X-Max Energy Dispersive Spectrome-ples were covered with gold using a BALTEC-SCD-005

ting device, and the results were recorded under high-ditions.

with dcency.

4.7. Ra

ThelteredYvonThe copus mto a fewith t488 nmfor (mi5 100

4.8. Ch

ThesolutioThe ionand K trometa 10% using a

4.9. Ac

A pmonumand 0.1extracSEM-E

5. Res

5.1. Pe

Petimply ture ismicro-tify it awhitiscomponium rare mnent mEmptyas rouncles),

FosrecrystFragmforamiFig. 2bwith sprock w

Thecrust win sizethe sizr powder and was sintered under pressure to translu-

spectrometry

ples were analysed with a multichannel notch-gh-sensitivity LaBram HR microspectrometer (Jobin-a, France) equipped with a Peltier-cooled CCD matrix.

ality of the instrument can be adapted with an Olym-cope and x50 objective to reduce the analysed volumem3. The same spot can be illuminated alternativelyfferent laser lines (air-cooled Ar+ ion laser at 532 ande more energetic blue excitation being better suitedorous samples. Counting times ranged from 3 100 tonds.

al analysis of soil solutions

tents of Na+, K+ and SO42 ions were determined in soilifty grams of soil was dissolved in 1 L of distilled water.tent was analysed after 3 days in a ltered solution. Nants were ascertained using an AAS AAnalyst 300 Spec-O42 contents were determined gravimetrically withsolution, while pH values of solutions were recorded

meter (CyberScan pH11/110Eutech instruments).

ated weathering test with K2SO4 and H2SO4 solution

of limestone (approximately 2 cm3 in size) from the was immersed in a solution of 0.1 M potassium sulphateulphuric acid solution. After two days, the sample wasnd air-dried. Such a prepared sample was analysed byd XRPD.

hysical properties of limestone

phic analyses of stones from the Kings Gate arch the most dominant material built into the struc-ochemical, sparite limestone. Textural features and

macrofaunal characteristics of the limestone iden-al rudstone (algal biosparrudite). The limestones are

le yellow or beige in colour. The prevailing allochemical are fossil-skeletal fragments of red algae (Lithotam-ssimum; family Corallinaceae) (Fig. 2a). The remains

in composition with a well-developed and promi-texture consisting of micron-sized rectangle chambers.icular cavities (from 0.3 to 0.8 mm in diameter), as well

cavities lled with spores or sparry calcite (concepta-ommon inside the algal remains.mains of Bryozoans (up to 300 m in size) that areed and lled with sparry calcite were noted also.of some other species (e.g., gastropods and different

species such as Nummulites of about 1 mm in size;re also recognised and have chambers partially lled

calcite. The algal biosparrudite is a weakly consolidatedparite bounds.stone sample from the gates ceiling wall with the blackanalysed by SEM-EDS (Fig. 3a). Calcite crystals ranging

2 to 8 m are clearly visible in eld 1 (Fig. 3b), whereas calcite crystals in eld 2 (commonly in pores) range


Fig. 2. (a) Theforaminifera o

widely fromrecrystalliseVesicles, mefrom 20 to 8and the hos30 to 150

Accordinheavy rock

absorption and frost susceptibility. The results show that averagevalue of bulk density is 1.84 g/cm3 and real density is 2.73 g/cm3.Absolute porosity varies from 13.7 to 34.3%. Values for water

tion are in agreement with absolute/open porosity, whereasragesigniults

metrt a de avere b

ecreaabsorpthe aveity. A the resporosicores aand thpores aume d microscopic appearance of red algae Lithotamnium ramisissimum; (b)f built limestone; (c) gypsum in pore of limestone XPL.

5 to 30 m (Fig. 3c). The abundant fossil remains ared calcite grains up to 300 m in size (eld 3, Fig. 3d).sopores and channels observable in eld 4 range in size0 m (Fig. 3e). Finally, eld 5 involves both black crustt limestone. The thickness of the black crust varies fromm (Fig. 3f).g to its physical properties, algal rudstone is a fairlythat is extremely porous and shows pronounced water

porosity forbelow the saverage porto 260 nm ideposition oblack crustsincreased frand reecte(Fig. 4). Thiwater insidperiods, enain bulk denthe cohesiotested to frken alreadysamples). T

All of thetals in porethe black crand to grea

5.2. Chemic

Accordinlimestone bcalcium carsium and so

In the mis probablylow and mcalcium is hlime morta

5.3. X-ray pcrusts

X-ray pcrusts formthe presencsum and syquartz, feldtion limit ofthe samplein samples arch, gypsugenite presthe sample

SEM anamineral phaand syngenthrough EDarch of the (i.e., in a forsyngenite a value of 14.94% reects high water-absorption capac-cant quantity of capillary pores is conrmed withof the measurements obtained by mercury-intrusiony (Fig. 4). The data from fresh stones (samples fromepth of 20 cm) presented total porosity values of 15.7%rage pore radius was 46.5 m (Fig. 4b). The majority ofetween 10 and 100 m in radius. The relative pore vol-ses with the decrease in pore diameter. Values of total

samples taken directly from the surface and from 5 cmurface are the same and display signicant decreases ine diameter compared to the pristine stone (from 32 mn surface samples) (Fig. 4b). This nding indicates thef material within pore spaces at the stones surface (e.g.,

and associated salts). The values of specic surface areaom the interior towards the surface of the core sampled a 20% increase in the number of ne/capillary poress increase in ne-pore number is undesirable becausee the pores tends to be absorbed and retained for longbling condensation [26]. In the same way, the decreasesity (from 2.20 to 1.87 g/cm3) resulted in a reduction inn of the limestone microstructure. All stone samplesost are not resistant because ve of them were bro-

after eight (two samples) and 10 cycles (other threehe mass loss varies from 14.1 to 58.5% (average 37.36%).

noted changes, especially the presence of gypsum crys- spaces observed in thin sections (Fig. 2c), indicates thatust leads to changes in microstructural characteristicster susceptibility of limestone to salt decay.

al composition of limestone and mortar

g to results obtained from the chemical analyses, theuilt into the Kings Gate (Table 1) contains about 98.11%bonate. The magnesium content is low, whereas potas-dium are present in negligible amounts.

ortar is present large amount of insoluble residue, which related to the presence of quartz. Potassium content isost likely derived from feldspar alteration. Content ofigh as expected considering that this is a historically

r (Table 1).

owder diffraction and SEM-EDS analyses of black

owder diffraction analyses were conducted on blacked on the Kings Gate at Kalemegdan. They revealede of three main mineral phases, namely calcite, gyp-ngenite. The presence of other mineral phases such asspar and others that occur in amounts below the detec-

X-ray diffraction method is possible. Calcite prevails in taken close to the entrance (on the left wall), whereastaken from the right side of the entrance and from them is the most abundant component. Although is syn-ent to a lesser extent, its greatest amount is evident in

from the arch (Fig. 5).lysis enabled the determination of the morphologies ofses present within the black crusts. The largest gypsumite forms were noted and were additionally conrmedS analysis of the black crust sample from the interiorKings Gate. Gypsum occurs in specic, tabular crystalsm of a micron-sized, so-called desert rose; Fig. 6a), whileppears as combination of (hk0) and (00l) crystals of up to


Fig. 3. (a) Sample of limestone from the gate ceiling wall with the positions of the analysed elds; (b, c) different sizes of calcite crystals; (d) fossil with recrystallised calcite;(e) vesicles, mesopores and channels; (f) black crust and the host limestone.

Fig. 4. Pore size distribution in limestone: (a) cumulative curve of pore size distribution; (b) histogram of pore size distribution; 1: sample of limestone taken at 20 cm fromthe surface; 2: sample of limestone taken at 5 cm from the surface; and 3: sample of limestone taken from the surface.


Table 1Chemical composition of inbuilt limestone and mortar.

Limestone Mortar

Cations mg/dm3 Oxides Weight % Cations mg/dm3 Oxides Weight %

Ca 39.320 CaO 55.02 Ca Mg 0.325 MgO 0.54 Mg Na 0.038 Na2O 0.05 Na K 0.035 K2O 0.04 K

i.r. 1.20

i.r.: insoluble residue.

60 m in size (Fig. 6b). Carbonaceous, porous air-born particles thatare spheroidal in shape and up to 50 m in size were also detected(Fig. 6c, d). Similar (but much smaller) syngenite crystals (between20 and 30 m in size) were observed in the sample from the leftinner wall close to the entrance (Fig. 6e). In black crust formed onthe right side of the wall, syngenite occurs in elongated, prismaticor columnar aggregates (Fig. 6f).

5.4. IR and Raman (micro)spectrometry

The representative Raman spectra obtained by sampling theblack crust and by baseline subtraction were compared (Fig. 7a).The calcite Raman signature consists of a strong narrow peakat 1086 cm1 (symmetric CO3 stretching mode), a weak peak at712 cm1 (deformation) and broader peak (liberation) at 280 cm1

[27] and [28]. A series of spectra characteristic of gypsum (CaSO42H2O), hemihydrate (CaSO4 0.5H2O) and anhydrite (CaSO4) (eitherpure or in combination) are also observed. Some differences regard-ing bandwidth and intensity with regard to literature spectra areobserved [2931] and are likely due to the crystal preferential ori-entation. Syngenite is conrmed (Fig. 7a, T8, S2 & T10) by its strong

Fig. 5. Represeferent positionCc: calcite).

narrow dou1135, 1170are also obs

although narrow foanhydritefor the cm1 peastretchingdisorder. when a st

for the saare obser

Such a vMHSO4 [35are presentthat the wdrate (101(1022 cm1

phase (T6-bby the laser(1610 cm1

doublet, T1combustion

have the rst pun addarrowscop, ander, tenit

stille

ter soy of tact wts of. It sheri

or exenvircannotshowsto almoshow avery nspectrophasesHowevto syng

5.5. Di

Wavicinitin contcontenTable 2atmospions. Furban ntative X-ray powder diffraction diagrams of the black crust from dif-s on the northern part of the Kings Gate (Sy: syngenite; Gy: gypsum;

(sample 4, T

5.6. X-ray d

In the limof potassiumas those id(Fig. 8). Cal32.786 CaO 45.901.512 MgO 2.510.091 Na2O 0.120.254 K2O 0.29

i.r. 15.05

blet at 9851005 cm1 and by lower intensity peaks at, 495, 445, and 425 cm1 [3234]. Unexpected featureserved:

literature spectra show peak signatures that are veryr gypsum and slightly broadened for hemihydrate and

compounds [29,32], a strong broadening is observed495 cm1 and the 1007 (gypsum) or 1022 (anhydrite)k (T6). The latter peaks are those of the asymmetric-

SO4 mode and so the broadening arises from someThe upper Eg wavenumber mode is usually broadenedrong H-bond is formed;mple T8-blue excitation, additional broad componentsved at 465500 and 843 cm1.

ibrational signature is characteristic for acid sulphate,36]. Thus, a variety of sulphates and acid sulphates

in the crust. The formation of acid sulphate provesater may have low pH. The observation of hemihy-5 cm1, T6 sample, green excitation) and anhydrite, T2 sample, blue excitation) and of the intermediatelue excitation) could be due to the local heating induced

beam because of the black colour. However, graphite, T10, T2) and amorphous carbon (13601600 cm1

0) traces are also evident and could have arisen from dust (Fig. 7b) On the other hand, the MHSO4 compound

been caused by a laser-induced transformation. Fig. 7cepresentative IR spectra that correspond (respectively)re gypsum [31] and to a calcite-gypsum mixture. Theseitional CO3 antisymmetric stretching mode at 1380 and

asymmetric bending modes at 872 and 712 cm1. IRy appears to be less efcient for identifying the different

calcite and gypsum are the only identied components.he small shoulders at 1192 and 660 cm1 could be duee [37].

d water-soil solution

lutions of three soil samples collected from the nearbyhe Kings Gate were used to simulate the water that isith the limestone that is built into the monument. The

K+, Na+ and SO42 ions in these solutions are reported inhould be emphasised that in nature soil is dissolved byc water that already contains variable amounts of theseample, rainwater in Belgrade (as well as in some otheronments) contains signicant amounts of sulphate ions

able 2) [38].

iffraction and SEM-EDS analyses of treated limestone

estone sample that was treated with a 0.1 M solution sulphate and sulphuric acid, identical mineral phases

entied in the black crusts were identied by XRPDcite is not completely transformed into other phases.


Fig. 6. SEM-EDcarbonaceous left inner wall

In the centring this treaextent) of gof treated stion occurrcavities andin elongateas combinacavities (Fig

6. Discussi

6.1. Black c

X-ray dience of gypmineral phaof black cruof gypsum calcite is alS analyses of black crust sampled from the ceiling wall: (a) desert rose gypsum; (b) priparticle with gypsum; (d) spheroidal, porous, carbonaceous particle with syngenite; (e) t; (f) long prismatic crystals of syngenite sampled from the right inner wall close to the en

al part of the sample calcite remained unchanged dur-tment. Signicant amounts of syngenite and (to a lesserypsum were formed on the surface. SEM-EDS analysesamples showed that the prevailing syngenite genera-ed in marginal parts of the sample (e.g., mostly inside

ssures; Fig. 9a, b). Syngenite was generally developedd prismatic and needle-shaped forms. Gypsum crystalstions of (0kl), (hk0) and (0k0) were also formed inside. 9c, d).

on

rust formation

ffraction and vibrational analyses conrmed the pres-sum and syngenite, along with calcite, as the mainses. Their variable abundance depends on the positionst formation on the monument. Hence, the amountsand syngenite are notably greater (and the content ofmost negligible) in the black crust that formed on the

central parton the inneof the innerthe prevailiemphasisedlected nearthe amountthe variablethere is a wawith soil. Thuously duriin this areaquence of tare presentsummer). Ling dry seasgates interiAlthough agrade) alreathat circulasmatic and tabular euhedral syngenite crystals; (c) spheroidal porousabular subhedral to euhedral crystals of syngenite sampled from thetrance.

of the ceiling wall (Figs. 5b and 7c). In samples collectedr wall, on the left of the arch (Fig. 5c) and on the right side

wall near the entrance (Fig. 5a), calcite and gypsum areng phases (and is syngenite less important). It should be

that calcite is the prevailing phase in the sample col- the entrance. One possible reason for the dependence of

of each mineral phase on the position of black crusts is amount of water inside the gate. Above the gates archter-resistant layer made of crushed limestone and lledis layer enables water from the arch to drip off contin-

ng rainy season. At the same time, moisture is retained for shorter time periods than elsewhere. As a conse-he high capillary porosity, smaller amounts of water

inside the limestone in the arch itself (even during theimestone blocks near the entrance are usually dry dur-ons. Relative humidity in this place is lower than in theor and is almost equal to atmospheric relative humidity.tmospheric ows in urban environments (such as Bel-dy contain high concentrations of sulphate, the watertes through soil above the gate contains particularly


Fig. 7. (a) Rep th 532showing the c

signicant aing of owconcentratithe soil furcentrationssource of poshould be tahave pH vapH below 2pH values o

The presSEM-EDS anon the gatecrystals or ibonaceous are sphericnumerous p

Carbonaor coal buremitted bycharacteristmight be caand on the

Table 2Concentration

Ions (mg/dm

K+

Na+

SO42

pH of soil so

1: sampled aboto the gate; 3:

te ord [4s up

of gn be

+ H2as alre of resentative baseline subtracted Raman spectra recorded on black crust fragments wiarbon doublet; (c) representative IR spectra. See text for detail.

mounts of sulphate and potassium ions (Table 2). Plant-ers above the gate in spring certainly leads to higherons of sulphate and potassium ions than are found inther from the gate. Most probably, the increased con-

are related to potassium fertiliser (K2SO4), which is thetassium ions and which enables syngenite formation. Itken into consideration that rainwater and soil solutionslues of < 7 (Table 2); acid sulphate commonly forms at, therefore its formation from meteoritic waters with

nuclea[44] andependerationrain ca

CaCO3

It hgate arf 6 requires further study.ence of gypsum, calcite and syngenite was conrmed byalyses. Depending upon the position of the black crust

, syngenite might occur either in large tabular euhedraln smaller, prismatic euhedral-to-subhedral grains. Car-particles identied inside the investigated black crustsal-shaped with external walls that are occupied withores that are variable in size and shape (Fig. 6c, d).ceous particles are generated during incomplete oilning under high temperatures and in urban areas are

vehicles [3942]. These particles are responsible foric black/grey colour of the investigated crusts [8]. Theytalysts during the oxidation of SO2 into SO3 in the air

stones surface [1] and [43]. They are able either to

s of K+, Na+ and SO42 ions in soil solutions.

3) Sample

1 2 3 4

8.94 6.51 3.76 0.141.61 1.63 1.58 0.22

26.32 11.25 10.03 4.53

lutions 6.08 6.10 6.55 6.90

ut 7 m above the gate entrance; 2: sampled 10 m below the entrancesampled 50 m to the left of the entrance to the gate; 4: rainwater [38].

to evolve inleads to bliticularly obBecause dirphate ions mthe presencthat enhancgate. The otacid (i.e., su

6.2. Syngen

Potassiulimestone mof with cemThree possiation. The and mica, saplagioclaseclase and mpotassium i

2KAlSi3O8 +

1 2K+ + CO32 (green) and 488 (blue) nm exciting laser lines; (b) recorded spectrum

(in the presence of moisture) to accumulate CaSO45]. The possibility for hydration/dehydration processeson temperature oscillations and relative humidity. Gen-ypsum through reactions between limestone and acid

presented as (1):

SO4 + 2H2O CaSO42H2O + H2O + CO2 (1)

eady been mentioned that limestone blocks within thehigh capillary porosity (Fig. 4a), which enables gypsum deeper parts. Pressure released during crystallisationstering and aking of limestone. This process is par-vious on the sample from the central part of the arch.ect inuence of ows (i.e., acid rain) is excluded, sul-ight enter from two sources. The rst possibility is that

e of the ions is indirectly related to atmospheric owse sulphate ions percolating through the soil above theher possible source is moisture that produces sulphuriclphate through the oxidation of SO2).

ite formation

m ions, which are required for syngenite generation ononuments (particularly those built with mortar insteadent mortar as the bounding material), are a mystery.

ble sources of potassium might be taken into consider-rst source is sand used for mortar. In addition to quartznd often contains some amounts of alkali feldspars and

. Transformation (weathering) of alkali feldspars (ortho-icrocline) from sand into clay minerals (2) could bringons into solution1:

2H2O + CO2 K2CO3+4SiO2 + Al2Si2O5(OH)4 (2)

.


Fig. 8. X-ray powder diffraction diagram of limestone treated with potassium sulphate (Sy: syngenite; Gy: gypsum; Cc: calcite).

However, according to chemical analysis of mortar, content ofpotassium which derived from feldspar alteration is low (Table 1).On the other hand, it is possible, that its primary chemical compo-sition has been changed over the time under percolation of water.

Another source is apparently the soil above the gate, which(like soil elsewhere) contains some amount of dissolved potassium.Finally, the third, and likely the most important, possible source ispotassium fertiliser.

The possible source of potassium could be also atmosphere, aspotassium can occur either in the state of the ion in precipitation(rainwater, snow) or in the form of various solid compounds inaerosols. Our analysis of rainwater (Table 2) show that contentof potassium is 0.14 mg/dm3. However, according to data fromthe Republic Hydro-meteorological Service of Serbia the averageannual content of potassium in precipitation is 0.45 mg/dm3 (from0.1 to a maximum of 1.5 mg/dm3).

Fig. 9. SEM-EDcrystals of synS analyses of limestone treated with potassium sulphate: (a) needles to long prismatic crgenite in limestone cavities (detail); (c) gypsum and syngenite crystals; (d) shattered cystals of syngenite in limestone cavities; (b) needles to long prismaticrystals of gypsum.


Under laboratory conditions, syngenite was formed easily whena thin gypsum plate was left in potassium sulphate solution atroom temperature. After 2 weeks, the plate was dried naturally inthe air. A soft, thin crust that formed over the plate was identiedas syngenitdevelopme

The expotassium-simulate thcite. Potassfertiliser, wacid. Same bisulphate. analyses (FiIt should beiment react(IV) oxide).of cavities aaggregates central parporosity of reaction en

The reladened but

Growth obe seen in

CaCO3 + K2

Minerals,(Fig. 9c). T

2CaCO3 + K+ C

Syngeniteever, the surface ofsolution lKings gat

Finally, tnatural contory. In addslower proc

7. Conclus

The varipositions obuilt into thof potassiumponents deinteracts wing soil) or

Possiblemortar andpotassium ienriched byamount of Kis twice as hthe soil in t

Through simulation of the mechanism of syngenite formation,it has been conrmed experimentally that syngenite may formdirectly under the inuence of potassium sulphate and sulphuricacid on calcite when concentrations of K+ and SO42 ions are high.

wled

auth mernce h6.

nces

. Amo3.ugrulnbul-. Bell, servatoniolo

Polluronteck crurikryl, Geol. Chariew ofl ston, Geolassinents, atheriical Sorimbllutionrimbnethl346oise

urally by serochiMelchatheriMelchdievalm. Gl

Dolskeldingsamuf

fated cusset

bonataceou. Lefevss, in: g pheiety, Lugini,ultur. rk,ldingsrk,g; the

(2011Begon

Portous ne635rocke

sonry cks, CoPopovS B.Bosity S B.B880).S B.B8Camufordinge [46]. This method reects the possibility of syngenitent due to already formed gypsum.periment in which limestone was treated withsulphate solution and sulphuric acid was performed toe possibility of syngenite formation directly from cal-ium-sulphate solution served in the experiment as ahile another source of potassium SO4 ions was sulphuricresults were obtained through the use of potassiumThe results of X-ray diffraction (Fig. 8) and SEM-EDSg. 9) conrmed the generation of gypsum and syngenite.

mentioned that during the rst few hours of the exper-ion, gas release was noted (most probably of carbon

Syngenite was formed both on the surface and insides euhedral, elongated prismatic crystals and prismatic(Fig. 9a, b). The greatest amount of calcite occurred ints of the broken sample. In spite of the high capillarylimestone, the insufcient time given for the chemicalabled the limestone to remain unchanged.tionship between the derived phases was not clearly

included the following:

f prismatic aggregates of syngenite over calcite that can Fig. 9a, b (3):

SO4 + H2SO4 K2Ca(SO4)2H2O + CO2 (3)

gypsum and syngenite can be seen in the treated samplehere is a possibility of their synchronous formation (4):

2SO4 + 2H2SO4 + H2O K2Ca(SO4)2H2OaSO42H2O + 2CO2 (4)

formation over gypsum could not be excluded; how-presence of syngenite was not observed even on the

partly spent gypsum crystals (Fig. 9d). Gypsum dis-ink to syngenite was not observed in black crusts on thee, too.

he quantities of potassium and sulphate ions seen underditions are not as high as those observed in the labora-ition, the formation of gypsum and syngenite are muchesses and are also much more complex.

ions

able amounts of syngenite resulted from the variablef the black crust formed over the limestone that wase gate, which in turn results in variable concentrations

and sulphate ions. The concentrations of these com-pend on the composition of atmospheric water, whichith limestone on contact either indirectly (by penetrat-directly as moisture.

sources of potassium and sulphate ions are atmosphere, soil, but probably the main source, particularly ofons is soil. Soil above the gates arch is likely probably

potassium fertiliser (i.e., with potassium sulphate). The+ and SO42 ions in the water that penetrates the soiligh as the amounts found in the water that penetrates

he nearest vicinity.

Ackno

Theing theof Scie17601

Refere

[1] G.G198

[2] A. TIsta

[3] F.Gpre

[4] I. TSci.

[5] G. FBlaR. P333

[6] A.Erevura205

[7] V. Fumwelog

[8] P. Bpol

[9] P. BR. S317

[10] G. WnatiedMic

[11] M. we

[12] M. meChe

[13] D. bui

[14] D. Csul

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por[24] SRP

(19[25] SRP[26] D.

accgement

ors would like to thank Jugoslav Krstic for perform-cury porosimetry measurements. The Serbian Ministryas nancially supported this work under contract No.

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The origin of syngenite in black crusts on the limestone monument King's Gate (Belgrade Fortress, Serbia) the role of ag...1 Research aims2 Introduction3 Site characteristics and historical background on the King's Gate3.1 Climatic conditions

4 Materials and methodology4.1 Samples4.2 Petrophysical properties of limestone4.3 Chemistry of limestone and mortar4.4 X-ray analyses4.5 SEM-EDS4.6 IR absorption4.7 Raman spectrometry4.8 Chemical analysis of soil solutions4.9 Accelerated weathering test with K2SO4 and H2SO4 solution

5 Results5.1 Petrophysical properties of limestone5.2 Chemical composition of limestone and mortar5.3 X-ray powder diffraction and SEM-EDS analyses of black crusts5.4 IR and Raman (micro)spectrometry5.5 Distilled water-soil solution5.6 X-ray diffraction and SEM-EDS analyses of treated limestone

6 Discussion6.1 Black crust formation6.2 Syngenite formation

7 ConclusionsAcknowledgementReferences

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