The influence of building materials on salt formation in rural environments

ORIGINAL ARTICLE

The influence of building materials on salt formationin rural environments

Vesna Matovic • Suzana Eric • Danica Sreckovic-Batocanin •

Philippe Colomban • Aleksandar Kremenovic

Received: 26 August 2013 / Accepted: 25 January 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Blocks of limestone and sandstone used in

walls of the Manasija Monastery complex showed damage

caused by the efflorescence and subflorescence of different

salts in a low-pollution rural environment. In addition to

common salts such as thenardite, thermonatrite, trona and

gypsum, a small amount of eugsterite and darapskite was

also present. Although these sodium and sodium–calcium

salts formed where cement mortar was used for repairs, the

lithological type and chemical composition of the substrate

also had an influence on the development of distinct salts.

The interaction between limestone and sandstone (sub-

strates) and a solution rich in sodium (without calcium)

was successfully simulated in the laboratory. The presence

of gypsum and sodium–calcium sulphate, as experimental

products, solely on the limestones indicated that, in addi-

tion to the cement-mortar solution, part of the calcium

required for the formation of calcium and double sodium–

calcium salts could occur from this substrate.

Keywords Soluble salts � Na–Ca sulphate � Limestone �Sandstone � Porosity

Introduction

Stone building and monument decay has been a subject of

scientific and official debate in recent decades. The causes

of such decay and scientifically supported hypotheses about

weathering mechanisms, including experimentally docu-

mented degradation phases, are already well known, but

the types of salt and their influence on building materials

are still the focus of numerous research projects. The

successful and appropriate protection of cultural heritage

objects requires good knowledge not only of building

materials, but also of the nature and development mecha-

nisms of various degradation products. Soluble salts are the

most dangerous and significant cause of decay (Winkler

and Singer 1972; Charola 2000; Doehne 2002; Cardell

et al. 2003; Flatt 2002; Scherer 2004).

Various soluble salts occur in cultural-historical monu-

ments built of porous stone (limestone or sandstone),

including sulphates, nitrates, carbonates and chlorides of

Na, K, Ca and Mg (Charola 2000; Arnold and Zehnder

1991; Steiger 2005; Benavente et al. 2004, 2007; Ruiz-

Agudo et al. 2007). The most common and destructive salts

are gypsum (CaSO4�2H2O), thenardite (Na2SO4), trona

Na3(CO3)(HCO3)�2(H2O) and mirabilite (Na2SO4�10H2O).

When these salts’ hydration/crystallisation pressure exceeds

the porous substrate’s resistance, the released stress dam-

ages the stone in the form of scaling, spalling, etc. (Doehne

2002; Lewin 1989; Gaudie 1987). The sources of these salts

vary (Doehne 2002; Cardell et al. 2003; Winkler and Singer

1972; Winkler 1997; Gauri and Bandyopadhyay 1999;

Kloppmann et al. 2011). Ions of a possible salt may occur

V. Matovic (&) � S. Eric � D. Sreckovic-Batocanin �A. Kremenovic

Department of Mineralogy, Crystallography, Petrology

and Geochemistry, Faculty of Mining and Geology,

University of Belgrade, Djusina 7, Belgrade, Serbia

e-mail: [email protected]

S. Eric


D. Sreckovic-Batocanin


A. Kremenovic


P. Colomban

LADIR, UMR 7075 CNRS, Universite Pierre and Marie Curie,

C49, 4 Place Jussieu, 75252 Paris 05, France


123

Environ Earth Sci

DOI 10.1007/s12665-014-3101-4

due to capillary forces brought on during building con-

struction (Rosch and Schwarz 1993), be derived during the

reaction of atmospheric factors with components of the

stone (Camuffo et al. 1983; Cardell et al. 2008; Chabas et al.

2000) or be of anthropogenic origin (Winkler 1978). Mor-

tar, common in modern building construction and an

obligatory component of cultural heritage restoration pro-

jects, is also significant and a common source of salts.

Mortar contains high concentrations of Ca, Na, K and SO4

ions whose permeability enables the easy migration of

water solutions, dissolution and subsequent release and

deposition of salts that will damage a stone (Duffy et al.

1993; Perry and Duffy 1997). For example, these ions can

affect the salt crystallisation resistance of built materials

(Klisinska-Kopacza and Tislova 2013).

Stone damage due to salts activity is a result of very

complex physical–chemical reactions (Charola 2000).

Identifying the interactions between a variety of factors is

commonly performed in laboratory simulations and ageing

tests (Goudie 1986; Goudie and Viles 1997; La Iglesia

et al. 1994; Benavente et al. 2001; Warke and Smith 2007).

The treatment of various stone types with salt solutions

(Goudie and Viles 1997; La Iglesia et al. 1994; Rothert

et al. 2007; Cultrone et al. 2008; Yu and Oguchi 2010),

most commonly with Na2SO4 solution (Rodriguez-Navarro

et al. 2000; Tsui et al. 2003; Espinosa Marzal and Scherer

2008; Steiger and Asmussen 2008), enables the comparison

of destructive activities caused by formed salts. Cardell

et al. (2008) have considered the reaction products and

decay forms of limestone weathering caused by certain

magnesium, calcium, sodium and potassium sulphate

solutions. Based on the mineralogical observation of

derived salts, the authors have concluded that the chemical

weathering of stone leads only to solutions of magnesium

sulphate. According to Ruiz-Agudo et al. (2007), differ-

ences in the damage created by Na-sulphate and Mg-sul-

phate have been due mainly to differences in their

crystallisation patterns, and they have experimentally

demonstrated that the mechanism of salt weathering by Na-

sulphate consists of the detaching of successive stone

layers, while Mg-sulphate induces crack formation and

propagation within the bulk of the stone.

The Manasija Monastery is a lithologically heteroge-

neous building in a non-polluted rural environment. It was

built between 1407 and 1418 and located in the central part

of Serbia, near Despotovac city. Some types of salts in

form of efflorescence and subflorescence were formed on

the surfaces of built blocks. It should be emphasised that

the formation of the above mentioned salts has generally

been linked to the utilisation of cement mortar, during

either the building or reparation of cultural monuments, but

the role of substrate on the type of formatted salts has not

yet been made clear. In the aim to examine possibilities of

Ca and double Na–Ca salts crystallisation in laboratory

conditions, lithotypes built in the Manasija monastery were

treated with sodium-rich solution. The premise for the

chemical treatment was: Is substrate source of the part of

calcium ions required for salt formation? The results

obtained on this way could enable a better understanding of

mechanism of salts formation in natural conditions, espe-

cially the double sodium–calcium salts in porous building

materials.

Site characteristics and historical background

The Manasija Monastery is located in a mountain-valley

region in the central part of Serbia, near Despotovac city

(Fig. 1a). The climate in this area is temperate continental

(average annual temperature is 11.4 �C) with moderately

warm summers (21.2 �C) and rather cold winters (0.7 �C;

data from the Republic Hydro-Meteorological Service of

Serbia). The average annual rainfall is 600–750 mm/year

and the relative humidity (RH) reaches maximum values in

winter (80–85 %), while the minimum values occur in July

and August (65 %). According to data from the Agency for

Environmental Protection, the average annual concentra-

tions of air pollutants in 2011 were SO2 11.7 lg/m3; NO2

26.7 lg/m3; and CO 0.78 lg/m3.

The Manasija Monastery is one of the most significant

monuments of medieval Serbian heritage. It was founded

by Despot Stefan Lazarevic between 1407 and 1418 and

consists of the Holy Trinity church, a refectory and forti-

fication with eleven towers connected by massive walls,

with an exterior guard wall and pedestrian path on the

rampart level (Janicijevic 1998) (Fig. 1a). The church

facades were made of sandstone blocks, while porous

limestone was used for the parapets and profiled elements

on the facades. The eastern and south-eastern church

facades are authentic, excluding the window stone frames,

while the west and narthex facades were rebuilt during

1960–1970 (Fig. 1b). In addition to massive walls built of

compact grey limestone blocks held together by mortar

courses, the fortification includes ten rectangular towers

built mainly of porous limestone and rare sandstone and

compact grey limestone blocks. All frontal parts of the

passages throughout the towers were built of tufa blocks.

Materials and methodology

Samples

Salt samples were taken from stone blocks built in the

Monastery towers and fortification walls in August 2011.

The main criteria for the sampling positions were: (1)

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significant quantities of salt in the form of efflorescence on

the stone surface (the facade of the church had no visible

salt) and (2) the originality of the blocks from the period of

Monastery construction. Salt samples were taken from the

interior walls and vaults of towers where only joints were

repaired 40 years ago. Beside the form of salt efflores-

cence, the subflorescence is evident on some sandstone

blocks (Fig. 2). Their surfaces are subject to scaling and

one salt sample was taken from the new opened surface.

The number of salt samples required for X-ray powder

diffraction (XRPD), SEM/EDS and Raman spectrometry

(*5 g) were mechanically removed from the substrate and

packed into vials. The locations of the collected salt sam-

ples are given in Table 1 and Fig. 1a.

Thirty-three stone samples were collected for charac-

terisation of their mineral compositions. These samples

were taken from labile zones in the stone blocks where

efflorescence was lacking. Samples used for the determi-

nation of pore size distribution were separated after their

petrographic observation.

Analytical methods

Petrographic analyses of the built stone were performed on

thin sections using a Leica DMLSP microscope for polar-

ised light that was connected to a Leica DC 300 digital

camera.

Determination of physical properties of stone (e.g.,

bulk and apparent density, total and open porosity, water

absorption, and hydric properties) was performed using

laboratory tests according to European Standards. Total

and open porosity values were calculated according to

values for real and bulk densities according to test EN

1936 (2006). Hydric behaviour of these stones was

characterised by the water absorption test EN 13755

(2008), and the capillary absorption test EN 1925 (1999).

All the tests performed for each lithological type were

carried out on 5 9 5 9 5 cm samples taken from Ma-

nasija Monastery.

Pore size distribution was determined using a mercury

intrusion porosimeter (Carlo Erba Porosimeter 2000) and

using the Milestone 100 Software System on stone samples

taken from the towers. This high-pressure mercury intru-

sion porosimeter operates within the 0.1–200 MPa inter-

val and enables the estimation of pores within the

0.005–100 lm interval.

X-ray diffraction analyses were used to determine the

phase compositions of powdered salts and powdered and

chemically treated limestone and sandstone. The XRPD

was performed using a Philips PW-1710 diffractometer.

The diffraction patterns were obtained from 5� to 60� 2Husing CuKa1,2 radiation with a step scan of 0.02� 2H and a

0.5 s step time.

Fig. 1 a Panoramic view of the Monastery Manasija with 11 towers (http://pes2011.elfak.ni.ac.rs/?page_id=873); b The Despot tower and

rebuilt facade of Church narthex (photo: V. Matovic)

Fig. 2 Sandstone scaling caused by subflorescence (stone block in

vault of the tower no. 11); salt sample 8 was taken from interior

surface of detachment scale

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http://pes2011.elfak.ni.ac.rs/?page_id=873

SEM–EDS was performed using a JEOL JSM-6610LV

scanning electron microscope connected to an X-Max

energy dispersive spectrometer to identify the morpholo-

gies and chemical compositions of the mineral phases

present in the salts and chemically treated limestone and

sandstone. The samples were covered with gold and carbon

using a BALTEC-SCD-005 sputter coating device, and the

results were recorded under high vacuum conditions.

Raman spectrometry was performed on the samples

using multichannel notch filtered, high-sensitivity Senterra

(Bruker Optics) and HR (Horiba Jobin–Yvon) microspec-

trometers equipped with a Peltier-cooled CCD matrix. An

Olympus microscope (950 objective) was used to produce

an analysed volume of a few lm3. The same spot was

illuminated alternately with different laser lines [532 (both

instruments) and 780 (Senterra) nm]. The step times ranged

from 3 9 100 to 5 9 100 s.

Chemical treatment of built stone

Stone samples including (1) porous limestone, (2) grey

limestone and (3) sandstone were cut into cubes approxi-

mately 1 cm3 in size and put in 50 cm3 glasses with pre-

viously prepared solution of 0.1 M Na2SO4. Each cube was

soaked up to halfway in a solution of 0.1 M Na2SO4. The

samples were kept in the solutions for 15 days and then air-

dried for 15 days (18–25 �C air temperature and 42–55 %

humidity). The treated samples (where salts were formed)

were analysed by SEM/EDS and XRPD with the goal of

determining the formation of salts containing calcium.

Results and discussion

Petrophysical characteristics of built stone

The optical study of the samples and thin sections showed

that the rocks could be characterised as porous limestone

(PL), grey limestone (GL) or sandstone (S). This order

represents the sequence of representation in the towers

and walls. Porous limestones dominated the towers and

sandstone occurred sporadically, while the fortification

walls were built of grey limestone. Tufa was used in the arc

elements of towers. However, this rock was not a subject of

this study because salts did not appear on it. The stone

samples estimated porosimetric parameter values are pre-

sented in Fig. 3.

Porous limestone (PL) was Miocene in age and light

brown in colour (Fig. 3a). The rock consisted of microcrys-

talline calcite and variable amounts of a terrigenous com-

ponent (commonly 5–10 % quartz clasts; Fig. 3a). The

micrite and microsparite groundmass was frequently reddish

in colour due to the presence of iron oxides. The results

of physical properties show that value of bulk density is

2.18 g/cm3 and real density is 2.66 g/cm3. Total porosity is

18.1 and open porosity is 13.6 %. Values for water absorption

6.1 % is in agreement with open porosity and the absorption

coefficient by capillarity (11.22 g/m2 s1/2; Fig. 3d). The pore

size distribution was unimodal, with an average pore radius of

7.9 lm. The percentage of pores with a radius between 10 and

100 lm hardly approaching 10 % and those with a radius

of \10 lm dominating (*85 %; Fig. 3e, f).

Grey limestone (GL) was Jurassic in age and the dom-

inant stone in the fortification wall facade, but rare in the

towers. The rock was dark grey, fine-grained rock cut with

numerous veins of secondary calcite (Fig. 3b) and con-

sisted of carbonates (95–98 % CaCO3) and insignificant

amounts of clayey minerals. It contained rare skeletal

remains of marine organisms bound by a micrite matrix

(Fig. 3b) and was very hard and compact rock (bulk den-

sity is 2.69 g/cm3 and real density is 2.70 g/cm3 with a

total porosity of 0.59 % and open porosity is 0.2 %). The

average pore radius of 22 lm (probably microcracks),

displaying a bimodal capillary pressure saturation curve

(two maxima in the ranges of 0.1–1 lm and 10–100 lm;

Fig. 3f). Water absorption is 0.13 % and the capillary

absorption coefficient (0.064 g/m2 s1/2; Fig. 3d).

Sandstone (S) was Lower Miocene in age and consisted

of quartz, feldspar, rock fragments, rare micas, cement and

a matrix (Fig. 3c). Quartz, as the dominant mineral,

occurred in angular or subangular, rarely isometric grains

that varied in size from 0.25 to 0.5 mm in well-sorted

Table 1 List of salt samples

with their locations and

substrate and extent of their

appearance

Number of samples Location Substrate Surface/thickness

1 Entrance in the tower 3 Porous limestone 1.2 9 0.7 m2/0.5–1 cm

2 Tower 2 Porous limestone 1 9 0.7 m2/0.5–1.2 cm

3 Tower 2 Porous limestone 0.6 9 0.5 m2/0.5–0.8 cm

4 Tower 2 Porous limestone 1.2 9 1 m2/0.5–1 cm

5 Main stairways Grey limestone 0.5 9 0.6 m2/0.1–0.5 cm

6 Fortification walls Porous limestone 0.3 9 0.5 m2/0.3–0.7 cm

7 Entrance in the tower 10 Sandstone 1.4 9 1.2 m2/0.5–1.2 cm

8 Vault of tower 11 Sandstone 0.3 9 0.2 m2/0.3–0.6 cm

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sandstone. The quartz grains were either homogenous or

built mosaic aggregates. Feldspar also occurred in the form

of orthoclase, microcline and plagioclase. Orthoclase and

microcline occurred in subrounded, commonly fresh, very

rare kaolinitised grains measuring 0.4–0.8 mm across,

while plagioclase occurred as lamellar, fresh grains mea-

suring 0.4–0.7 mm in size. Mica occurred as orientated

muscovite flakes, while rock fragments were determined to

be cherts, quartzites and volcanic rocks. Cement (carbona-

ceous to siliceous-calcitic) occurred as a contact-pore

Fig. 3 Images and photomicrographs of stones from the Monastery

Manasija, a Porous limestone; b Grey limestone; c Sandstone.

d Capillary absorption curves of sandstone (S) and limestone (PL,

GL), e, f Distribution of pore size and relative volume of pores in

different rocks (S sandstone, GL grey limestone, PL porous limestone)

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filling. The sandstone (S) was characterised by a relatively

high total porosity of 17.8 % and open porosity is 13.0 % (a

bulk density is 2.34 g/cm3 and real density is 2.84 g/cm3). It

showed a unimodal pore size distribution with an average

pore radius of 13.9 lm and the majority of pores have

radius over 1 lm (* 70 % vol. of rock; Fig. 3e, f). Value

for water absorption is 6.2 % and the absorption coefficient

by capillarity is 12.47 g/m2 s1/2 (Fig. 3d).

XRPD, SEM–EDS and Raman analyses of salts

collected from the monument

The X-ray powder diffraction analyses conducted on the

collected salt samples (samples no. 1–4) revealed the pre-

sence of different salt types in addition to the commonest

calcite (obtained from limestone). Thenardite (Na2SO4)

prevailed in the majority of samples and eugsterite

Fig. 4 XRPD diagrams of the salts (samples 1–8) from walls of the Monastery complex (Da darapskite, Td thenardite, Eu eugsterite, T trona,

Th thermonatrite, Cc calcite, Q quartz)

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(Na4Ca(SO4)3�2H2O) was another identified sulphate salt

(Fig. 4). Nitrate-sulphate salt (darapskite (Na3(SO4)

(NO3)�H2O)) was found in the samples dominated by the-

nardite. Carbonate salts were identified in the fifth salt

sample that developed on the limestone: trona

(Na3(CO3)(HCO3)�2(H2O) and thermonatrite Na2CO3�(H2O) (Fig. 4). Only calcite and quartz were identified in

sample 6. XRPD analyses of the salts that formed on the

sandstone in samples 7 and 8 suggested that the major

phase was thenardite (Fig. 4).

Fig. 5 SEI images with EDS spectrums, a eugsterite and thenar-

dite—sample 1; b thermonatrite—sample 5; c thenardite—sample 8;

Representative Raman spectra, d sulphates: main phase thenardite

plus eventually eugsterite; e–f main phases trona, darapskite and

bassanite (see Table 2; samples 1–5)

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The morphologies of the mineral phases were deter-

mined using SEM and their semi-quantitative chemical

composition determined using EDS analyses. Eugsterite

occurred in elongate, prismatic, pseudorhombic forms

approximately 2–10 lm in size. Thenardite occurred in

bipyramidal shapes approximately 6–10 lm in size (sam-

ple 1, Fig. 5a). The thermonatrite from sample 5 developed

in prismatic or tabular aggregates ranging in size from 5 to

20 lm (Fig. 5b), while only thenardite was observed in salt

sample 8 that occurred in the form of tabular and irregular

grains ranging from 2 to 20 lm in size (Fig. 5c). Due to its

low sensitivity to nitrogen, the SEM–EDS analysis did not

identify darapskite.

The representative Raman spectra collected on salt

samples 1–5 (Table 1) are shown in Fig. 5d–f. The spectra

are grouped as a function of the main fingerprints, sul-

phates (strong 990 cm-1 symmetric stretching mode of a

SO4 tetrahedron) and (hydroxyl) carbonates (strong

ca1090 cm-1 peak of the symmetric stretching mode of a

CO3 unit). Characteristic Raman signatures from the lit-

erature are listed in the Table 2 for the presence of all

potential minerals. It should be noted that the efficiency of

Raman spectroscopy to identify the different sulphate and

carbonate phases is obviously limited in the lack of accu-

rate references covering the low wavenumber region.

However, XRPD and SEM/EDS results are more efficient

in regard to mineral composition of investigated samples.

The sulphate fingerprint was the characteristic signature

of the XO4-based structure, with a strong and narrow

symmetric A1 X–O stretching mode, degenerated F

asymmetric stretching (1060–1150 cm-1) and bending

(617–645 cm-1) triplets with a degenerated E symmetric

bending doublet (450–465 cm-1). The modes at lower

wavenumbers included libration XO4 (280 cm-1). As evi-

denced by the sulphate signatures, the peak relative

intensity varied widely. However, the Raman peak inten-

sity depends heavily on the orientation of the crystal axis

vs. the electric vector of the laser beam. Orientation varied

based on the spot under analysis (see further figures) and

was not characteristic of phase allotropy. The thenardite

and darapskite signatures, as listed in Table 2, agreed with

Fig. 5d spectra. Bands at 1365 and 1455 cm-1 did not

belong to sulphate compounds and may have corresponded

to weddellite (CaC2O4�2H2O), carbonates (see below) or

even nitrates. Traces of gypsum (1008 cm-1) were also

observed, and no other bands were observed at higher

Table 2 Characteristic Raman signatures of formed phases as well as phases that could be in paragenesis or mismatched in Raman spectra

Name Composition Raman signature/cm-1 Observed References

aQuartz SiO2 464 X Tournie et al. (2011)

Calcite CaCO3 1437, 1086, 712 X Culka and Jehlicka (2010)

Dolomite CaMg(CO3)2 1443, 1098, 725 – Kramar et al. (2010)

Anhydrite CaSO4 1018 – Tournie et al. (2011)

Bassanite CaSO4�0.5H2O 1008, 1026 X Prasad (1999)

Darapskite Na3(SO4)(NO3)�H2O 1123, 1059, 993, 640, 456 X Jentzsch et al. (2012)

Eugsterite Na4Ca(SO4)3�H2O 1084, 1125 X This work

Kalicinite (aq. sol.) KHCO3 1364, 1312, 1016, 640 – Rudolph et al. (2008)

Glauberite Na2Ca(SO4)2 1002 X? Clark et al. (2010)

Yusupov et al. (1983)

Gypsum CaSO4�2H2O 1137, 1007, 673, 621, 416 X Maguregui et al. (2010)

Prasad (1999)

Natron Na2CO3�10H2O 1071, 716 ? Sarmiento et al. (2008)

Mirabilite Na2SO4�10H2O 1086, 989.3, 615, 446 ? Hamilton and Menzies (2010)

Metastable-Mirabilite Na2SO4�7H2O 1106, 987.6, 464, 592 ? Hamilton and Menzies (2010)

Pirsonite Na2Ca2(CO3)3�2H2O 1070, 710 ? Zaitsev and Keller (2006)

Shortite Na2Ca(CO3)3 1399, 1088, 1069, 730, 709, 695 – Mernagh et al. (2011)

Thenardite Na2SO4 1258, 1149, 992, 626, 432 ? Agayo et al. (2011)

Maguregui et al. (2012)

Sarmiento et al. (2008)

Morillas et al. (2012)

Thermonatrite Na2CO3�H2O 956, 579 ? Gomez-Laserna et al. (2013)

Trona Na3(CO3)(HCO3)�2H2O 1061, 225, 186 X Gomez-Laserna et al. (2013)

Witherite BaSO4 1424, 1061, 693 – Bucca et al. (2009)

Notably peaks wavenumber in bold

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wavenumbers. A second group of Raman signatures

exhibited a strong narrow peak at 1062 cm-1 and a well-

defined doublet at *1310–1430 cm-1 (Fig. 5e, f), the

latter of which is characteristic of carbonate ion. Strong

differences were observed in the low wavenumber range

(\300 cm-1) according to the presence of different phases.

One spectrum exhibited a 995–1062 narrow doublet, but

other spectra showed an additional broader band at

1028 cm-1. Witherite exhibits a strong 1061 cm-1 peak,

but the rareness of this phase allowed us to rule out this

assignment. Darapskite, a mixed sulphate-nitrate, has a

strong 1059–993 cm-1 doublet (Table 2) that fits well with

the observed spectrum. A broad band before a strong nar-

row peak is characteristic of H–XOn vibration units: the X–

O–H stretching mode is decoupled and its wavenumber is

lower due to the H-bond. Accordingly, features appear in

the O–H stretching region (2500–4000 cm-1), although the

995–1062 cm-1 doublet compound does not exhibit bands

above 2000 cm-1. The low wavenumber signature of the

doublet compound was also very different from that of the

other. A natron Raman spectrum exhibits a strong

1071 cm-1 peak and the stronger peak of the glauberite

spectrum is observed at 1002 cm-1. The observed spec-

trum was thus consistent with a mixture of the later two

phases.

The spectra with medium broad 1028 cm-1 (H–O–

C = O) stretching mode and narrow 3438 cm-1 (O–H

stretching mode) bands could be assigned to trona.

Accordingly, the broad band centred at 3075 cm-1 was the

H2O stretching massif.

Generally, thenardite was the main phase in samples 1–3

and trona the dominant phase in sample 5, while a mixture

of sulphate phases with calcite and darapskite was most

likely in sample 4.

SEM–EDS and XRPD analyses of salts obtained

in the laboratory

The chemical treatment of limestone with a solution of

sodium sulphate caused some chemical reactions between

the solution and substrate. In addition to thenardite, small

amounts of tabular, pseudo-hexagonal gypsum crystals

were formed on the grey limestone. A radial and fan-

shaped aggregate of sodium-calcium sulphate occurred on

the gypsum crystals (Fig. 6a, b).

Double Na–Ca sulphate in the form of a fan-shaped

aggregate was also identified after chemical treatment of a

porous limestone sample using EDS analyses (Fig. 6c, d).

The crystallisation of thenardite was observed on all sub-

strates. XRPD performed on the treated samples identified

only thenardite, while other mineral phases (gypsum and

Na–Ca sulphate) occurred in insignificant amounts below

the applied method’s detection limit. However, it was clear

that the solution of Na2SO4 led to a partial dissolution of

Fig. 6 a SEI images of salts on limestone after chemical treatment with Na2SO4 of (a) gypsum, thenardite and Na–Ca sulphate—grey limestone,

b detail of Na–Ca sulphates—grey limestone, c, d Na–Ca sulphate—porous limestone

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limestone because the formed phases contained calcium.

Glauberite is only stable anhydrous phase in nature, but

several phases contain hydroxyl groups or molecules of

water: hydroglauberite (Na4Ca(SO4)3�2(H2O)), eugsterite

(Na4Ca(SO4)3�2(H2O)), cesanite (Ca2Na3[(OH)(SO4)3])

and omongwaite (Na2Ca5(SO4)6�3H2O) (Mees et al. 2008;

Cavarretta et al. 1981). The identified sodium–calcium

sulphate was probably glauberite or hydroglauberite/

eugsterite. Hydroglauberite and eugsterite are rhombic and

monoclinic polymorphic modifications with the same

chemical composition, but omongwaite and cesanite have a

higher calcium content compared to their sodium content,

which is not the case with the identified Na–Ca sulphate

(Table 3).

The chemical treatment of sandstone with the sodium

sulphate solution did not cause chemical reactions between

the solution and substrate because only thenardite was

formed.

Mechanism of Ca and double Na–Ca salt formation

The majority of sodium salts occurred due to the utilisation

of cement mortar and the influence of atmospheric water.

The permanent influence of atmospheric water (rain,

moisture and snow) led to the disintegration of the cement

mortar and its partial dissolution and enriched it with

various ions, such as Ca2?, Na?, K?, Mg2? OH-, HCO3-,

SO42-, etc., enabling the precipitation of diverse salts

under certain temperature conditions and saturations. Pro-

cesses of salt formation, their repeated dissolution and re-

precipitation were common. The occurrence of salt on the

stone blocks was largely the result of the transport of

solutions formed in the cement mortar.

The role of the rock type was also important to salt

formation (chemical composition and solubility of mineral

phases, such as constituents, porosity, etc.). Chemical

treatments of stone block samples were performed to

examine the possibility that part of the calcium ions

required for salt formation originated from the substrate.

The simulated conditions of salt formation included solu-

tion enriched by sodium and sulphate ions (related to the

cement solution, but without calcium and carbonate ions).

According to the results of the conducted experiments, the

influence of chemical composition and porosity of the

substrate on the type of salt formation were considered.

The influence of 0.1 M sodium sulphate on limestone was

seen in the release of Ca2? ions from the substrate and the

formation of small quantities of gypsum and Na–Ca sul-

phate (Fig. 6a, b). The sodium–calcium sulphate (such as

glauberite) that occurred with gypsum under this chemical

treatment could have been formed by:

(a) The direct influence of the sodium sulphate solution

on limestone:

CaCO3ðsÞ þ 2Na2SO4ðaqÞ

! Na2Ca SO4ð Þ2ðsÞþ Na2CO3ðaqÞð1Þ

(b) The influence of sodium sulphate solution on already-

formed gypsum (glauberite developed at the expense

of gypsum as a precursor under the influence of

sodium-rich solutions (Orti et al. 2002):

CaSO4 � 2H2OðsÞ þ Na2SO4ðaqÞ! Na2Ca SO4ð Þ2ðsÞþ 2H2O ð2Þ

and/or

(c) The synchronous formation of gypsum and glauberite:

2CaCO3ðsÞ þ 3Na2SO4ðaqÞ þ 2H2O

! 2Ca2þ þ 3SO2�4 þ 6Naþ þ 2CO2�

3 þ 2Hþ þ 2OH�

! CaSO4 � 2H2OðsÞ þ Na2Ca SO4ð Þ2ðsÞþ 2Na2CO3ðaqÞ

ð3Þ

In a similar way hydroglauberite or eugsterite could be

formed. On the other hand, under natural conditions, there

are a number of other factors (amount of water and its

circulation, changing the pH of the microenvironment, etc.)

that can significantly influence the mechanism of formation

of these salts.

According to a comparison of the experimentally

obtained results with those obtained through the study

of natural samples, certain facts should not be excluded.

At first, the experiments were partially controlled (without

the additional dilution that always occurs in natural con-

ditions under precipitation, and without rapid changes

in temperature and humidity) and significantly invasive

(a high concentration of sodium sulphate similar to its

Table 3 EDS analyses of salts formed in laboratory (Fig. 6)

Weight %

1 2 3 4 5 6 7 8 9

Na – 18.56 34.25 17.55 19.22 33.28 32.54 20.05 19.86

Ca 25.02 9.42 – 8.91 8.75 – 0.05 9.12 8.94

S 20.30 20.19 21.30 21.34 21.26 23.47 22.38 19.53 20.39

O 54.68 51.83 44.45 52.20 50.77 43.25 45.03 51.30 50.81

Environ Earth Sci

123

concentration in natural conditions). Second, the source of

calcium was not only limestone itself. Certainly, Ca2? ions

originated partly from the cement mortar. The sulphates

were mostly related to the cement mortar, and to a lesser

extent to the sulphuric acid that formed due to the oxida-

tion and dissolution of SO2 present in the air. Sulphur oxide

(SO2) from the atmosphere can be oxidised to sulphate ions

on the surfaces of stone blocks, which is one of the reasons

for the occurrence of gypsum on limestone monuments

(without the use of cement mortar) in urban areas (Camuffo

et al. 1983; Garcia-Valles et al. 1998; Siegesmund et al.

2007). According to data from the Agency for Environ-

mental Protection, the average annual content of SO2 in the

air in this rural environment has been low in the last few

years (\30 lg/m3). However, these values were estimated

in a city (Paracin) approximately 20 km from the Monas-

tery. The heating of the Monastery for hundreds of years by

wood and coal have significantly contributed to the amount

of smoke, soot and SO2 in close vicinity to the Monastery

complex.

The formation of glauberite as Na–Ca sulphate in

experimental conditions was compared with the occurrence

of eugsterite (also Na–Ca sulphate) in natural conditions

(the salts on porous limestone in samples 1 and 4).

According to past studies, eugsterite has been determined

as a meta-stabile phase in the system CaSO4–Na2SO4–H2O

(Vergouwen 1981; Freyer et al. 1997). The precipitation of

eugsterite is possible if the Na/Ca ratio exceeds four

(Vergouwen 1981). Nonetheless, Li et al. (2010) have

studied mineral assemblages in evaporates such as halite–

gypsum–eugsterite, halite–gypsum–eugsterite–glauberite

and gypsum–thenardite–eugsterite, and concluded that

eugsterite may develop either under the influence of

sodium and sulphate ion- enriched solutions on gypsum:

CaSO4 � 2H2OðsÞ þ 4Naþaqð Þ þ 2SO2�4 aqð Þ

! Na4Ca SO4ð Þ3�2H2OðsÞ ð4Þ

or under the influence of calcium and sulphate ion-enriched

solutions on thenardite:

2Na2SO4ðsÞ þ Ca2þaqð Þ þ SO2�

4 aqð Þ ! Na4Ca SO4ð Þ3�2H2OðsÞ

ð5Þ

The sandstone does not have a direct influence on the

formation of calcium and sodium-calcium salts because

only thenardite was formed after chemical treatment.

It should be noted that the formation of distinctive salts

and the deterioration of stone depends on the porosity, and

the distribution of pores, such as the content of pores with

diameters ranging from 0.1–10 lm (Benavente 2011).

Although the total porosity (which includes closed pores) is

significant, the open porosity is of greater importance, as it

has a direct impact on the likelihood that undesired fluids

will penetrate the stone. The flow mechanism is quantified

by the capillary absorption coefficient because it is linked

to the characteristics of the pore interconnectivity, the

migration of fluid and the stone permeability (Benavente

et al. 2002). The porous limestone has a homogeneous pore

system with the domination of capillary pores that are

responsible for a high value of capillary coefficient and

water uptake. The porosity in this stone is characterised by

microcracks and ink-bottle pores (pores with diameter

below 10 lm). Similar values of total and open porosity

accompanied with the high capillary coefficient, suggest on

interconnected pores that enable a high absorption, but

slow evaporation of solution. The porous structure is

responsible for the easy entrance of salt solution in porous

limestone, where they remain long enough to enable the

dissolution of substrates and the interaction and subsequent

deposition of glauberite, eugsterite and other salts in a form

of subflorescence. The pore spaces in grey limestone allow

only for the efflorescence of thenardite, gypsum crystals

and double Na–Ca salts, but in smaller amounts than in

porous limestone. The grey limestone is very resistant to

weathering processes related to fluid circulation into and

within a stone. It can be explain by the characteristics of its

porous system. The pore space in grey limestone includes

only open pores, which not allow retain of solution in the

stone, so subflorescence cannot develop. The low value of

open porosity and water uptake correspond to very low

capillary absorption coefficient. However, the rapid

chemical reaction solution in grey limestone occurred only

at the stone surface, probably due to the dominant presence

of micrite carbonate particles. The composition of sand-

stone does not have a direct influence on the formation of

calcium and sodium–calcium salts, but the porosity and

hydric properties of this stone enables solution to be

retained inside it. The sandstone shows a homogeneous

pore system (more than 90 % of pore radius [ 0.1 lm)

with high pore connectivity that allows internal movement

of salt solution through stone. This statement is in agree-

ment with its high absorption coefficient and very fast

capillary water uptake (Fig. 3d). The deposition of salts

beneath the stone surface occurs during periods when

evaporation is slower than capillary movement (Benavente

2011). Such a process leads to thenardite deposition, whose

crystallisation pressure is responsible for sandstone scaling

and spalling.

Conclusion

The main salts contributing to efflorescence and subflo-

rescence that occurred on limestone in the Manasija

Monastery complex were thenardite, thermonatrite, trona,

eugsterite, darapskite and gypsum, while only thenardite

Environ Earth Sci

123

was detected on sandstones in the complex. The source of

Na and Ca for the formation of these salts was definitely

the cement mortar used during repairs made to the Mon-

astery. However, the concentration of Ca ions was con-

siderably increased by dissolving substrate such as

limestone, thereby increasing the possibility of the forma-

tion of Ca and Na–Ca double salts. This phenomenon was

confirmed by the interaction between the limestone and

sandstone in the Manasija Monastery (substrates) and a

solution rich in sodium (but without calcium) in laboratory

conditions. The presence of small amounts of gypsum and

sodium–calcium sulphate as experimental products indi-

cated that, in addition to a solution of cement–mortar,

limestone could be an important source of calcium for the

formation of these salts. Sodium–calcium sulphate could

have been formed by the direct influence of a solution rich

in sodium sulphate on limestone or already-formed gyp-

sum. The simultaneous formation of gypsum and Na–Ca

sulphate was also possible.

Acknowledgments The authors would like to thank Jugoslav Krstic

for performing the mercury porosimetry measurements. The Serbian

Ministry of Science has financially supported this work under contract

No. 176016. This work is partly supported from EGIDE Pavle Savic

cooperation also. We are also very grateful to architects from the

Republic Institute of Heritage Protection who helped us during

observation and recognition of the stone conditions within the

Monastery.

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The influence of building materials on salt formation in rural environments

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