Black crusts on ancient mortars

Pergamon

Vol. 32, No 2. pp. 215.-223, 1998 C 1997 Elsevm Saence Ltd All rights reserved. Prmted in Great Britain 1352-2310198 317.CO + 0.00

Armospherrc hnwonmmt

PII: S1352-2310(97)00259-8

BLACK CRUSTS ON ANCIENT

MORTARS

C. SABBIONIT,*, G. ZAPPIAt*$, N. GHEDINI§, G. GOBBIf, and 0. FAVONIS tlst. FISBAT- CNR, Via Gobetti 101,40129 Bologna, Italy; fDip. Scienze dei Materiali, Univ. di Ancona, 60131 Ancona, Italy; and §Dip. Scienze Farmaceutiche, Univ. di Bologna, 40126 Bologna, Italy (First received 22 October 1996 and in ,final form 18 May 1997. Published November 1997)

Abstract-Samples of mortar damage layers found on ancient masonry have been analysed. The paper presents the characterisation of the black crusts, which are the accumulation areas of material damage and atmospheric deposition, and includes a complete identification of the sulphur, nitrogen and carbon compounds. The presence of anthropic aerosol and its role in the process of damage is evidenced. c~ 1997 Elsevier Science Ltd. Kev word index: Damage, mortar, sulphur, aerosol, carbon compounds.

1. INTRODUCTION

Ancient masonries on historical buildings are composed of structural elements (stones and bricks) and binders (mortars) and, throughout Europe, general evidence exists that they are dramatically damaged due to attack of air pollutants present in today’s atmosphere (Baer et al., 1991). An increasing number of studies on the degradation of structural elements have been performed both in the laboratory (Gauri and Gwinn, 1982; Johansson et al., 1988), field exposure tests (Baedecker et al., 1992) and directly on historical buildings (Krumbein et al., 1992). However, only a limited amount of knowledge is available regarding the deterioration of the mortars in masonry, although these are, in fact, the binders experiencing the heaviest effects of degradation. Exposure tests performed in a laboratory chamber show that the reactivity of mortars to SO, is higher than that of stones (Zappia et al., 1994). Taking into account that the durability and weathering of historic stone and brick masonry constructions can only be understood if all components of the masonry and environmental parameters are considered, there exists a gap in the knowledge on mortar degradation. This paper aims to focus on the role of environmental pollution in the degradation of ancient mortars and to identify the different fractions due to atmospheric deposition.

2. EXPERIMENTAL

SECTION

In order to highlight both the products of the damage reactions and the components due to atmospheric depo-

*Author to whom correspondence

should be addressed.

sition, specimens of black crusts were sampled on the Zamboni Town Gate (Fig. l), a segment of Bologna’s outer city wall, which was built in the 14th centurv. This is a tvnical construction chosen as representative of fhe ancient brick masonry characteristic of Italy and Central Europe. Specimens of the damaged jointing mortars were collected as bulk fragments and, where the degradation patina was of sufficient thickness, the material was scraped away in three subsequent layers: the surface degradation layer (A), the underlying degradation layer (B) and the unaltered mortar layer (C). Once collected, the samples were dried and preserved at a temperature of 20°C in an inert environment (N,), until the time of their characterisation using a combination of physico-chemical analytical techniques. Observations by optical microscopy using a Zeiss-PO1 III were performed on transversal thin sections of black crusts and the surface features of the damage samples were analysed by scanning electron microscope (SEM) with a Philips XL20. X-ray diffraction (XRD) by Philips PW 1730 was employed for the identification of the main crystalline species; quantification of gypsum and carbonates was performed on both bulk samples and stratigraphic layers bv gravimetric and differentialthermal analyses @GA-DTA) by Netzsch STM 429. The anion concentrations of the black crusts samples were measured by ion chromatography (IC) using a Dionex 45OOi, through water dissolution following the methodology set up in a previous work (Gobbi et al., 1995). In order to quantify carbon, nitrogen and sulphur, elemental analyses by combustion (CHNSO) using a Carlo Erba analyser were performed, while the remaining elements were analysed by inductively coupled plasma emission spectrometry (ICP) with a Perkin Elmer 5500 device, through the digestion of samples in Teflon vessels with a HF-HNO, mixture at 120°C. The carbon compounds present in the alteration patinas on mortars may be of three different origins: (1) calcium carbonate deriving almost exclusively from the underlying materials (Pie, 1987); (2) deposition of atmospheric particles containing elemental carbon and primary and secondary organic compounds, (Turpin and Huntzicker, 1995); (3) biological weathering (Saiz-Jimenez, 1995). 215

216

C. SABBIONI

Total carbon (C,) can be considered two main fractions

c, = cc + where Cc is the carbonate ate carbon and

carbon

c”c =

as being composed

of

c”c

and Cmc is the noncarbon-

ce+ co.

C., is, in turn, composed of elemental carbon (C,), predominantly a product of combustion processes, and organic carbon (C,) of biological and anthropic origin. The measurement of noncarbonate carbon, the discrimination of elemental and organic carbon and the characterisation of the organic fraction are all essential for a complete identification of the major components constituting the damage layers on mortars. The noncarbonate carbon (C,,) content of our samples was measured according to a methodology developed and described in a previous work (Zappia et al., 1993). In addition, a new procedure was set up (Sabbioni et al., 1996b) in order to discriminate the elemental (C,) and organic (C,) fraction. The total carbon (C,) was obtained by combustion of the bulk samples, while C,, and C, were quantified after elimination of C, and C,, respectively. The carbonate carbon (C,) and the organic carbon fractions were then calculated. The identification of the solvent-extracted organic compounds was performed by gas chromatography-mass spectrosconv (GUMS) usine a HP 5890 GC interfaced with HP 5971 &ss selective deikctor, operating with an ionisation energy of 70 eV, a mass range of m/z 5@500 and a cycle time of 2s. Compound separation was achieved using a fusedsilica capillary column (30 m x 0.25 mm id.) coated with CP Sil5. The temperature programming consisted of the following steps: (a) injection at 65°C (b) isothermal hold at 65°C for 10 min, (c) temperature ramp of lO”C/min and(d) an isother-

Fig. 1. Zamboni

et u/

ma1 hold at 275’C for another 49 min. The major organic compounds present in the samples were identified by computer matches to standard reference mass fragmentograms in the NBS75K.L library. For solvent extraction the black crusts were ground and extracted in a Soxhlet apparatus using firstly n-hexane and then a methylene chloride/methanol 1 : 1 mixture. Inorganic materials were eliminated by treatment with bidistilled cold water. The extracts, mixed and centrifugated, were reduced to about 250 ~1 using rotary evaporation(below 40°C) under high-purity NZ stream evaporation. By adding freshly produced diazomethane, the organic acids and aromatic hydroxy compounds were converted into methyl esters. The extracts were injected into the GC/MS system.

RESULTS AND DISCUSSION

Optical analysis in thin transversal section show: (a) a thin surface layer embedding a high number of spherical black particles (soot) due to the atmospheric deposition of aerosol emitted by oil combustion and (b) the mineralogical transformation of the carbonate matrix and disaggregation of the inert component (sand) (Fig. 2). SEM micrograph (Fig. 3) reveals the presence of gypsum crystals with laminar structure, crystallising mainly within the pores of the mortar structure. X-ray diffractograms permit the identification of the main crystalline species constituting the black crusts, such as gypsum, quartz, calcite, albite, plagioclase and microcline.

Town Gate in Bologna showing the typical damage occurring the mortar results to be highly degraded.

on brick masonry,

where

Black crusts on ancient

211

mortars

Fig. 2. Optical micrograph of a thin transversal section showing spherical black particles (soot), due to atmospheric deposition from oil combustion emissions, embedded in the surface damage layer. The mineralogical transformation of the carbonate matrix with disaggregation of the inert component (sand) is also evident.

Fig. 3. Scanning electron micrograph laminar structure, mainly

of a black crystallised

DTA and TGA data indicate gypsum to range between 38.0 and 60.0% and carbonates from trace to 3.8%; low concentrations of clay minerals were also detected. Figure 4 shows the mean gypsum and carbonate concentrations found in the black surface

crust showing the presence of gypsum within the pores of the mortar structure.

crystals

with

crusts (A), intermediate damage layers (B) and unaltered mortars (C); gypsum presents the maximum concentration in layer A, values lower than 15% in B and is absent in C; conversely, the concentration of carbonates increases from 3.9% in A. to 18.8%

218

C. SABBIONI et al.

Fig. 4. Mean gypsum and carbonates concentrations measured in layers A (surface), B (intermediate) and C (undamaged) of the analysed mortar samples.

1000000

T

100000

10000

z% ‘:

1000

2 s 100

10

1

SO4=

N03-

Fig. 5. Mean anion concentrations

NOZ-

F-

Cl-

Br-

P04-

1

C204= HCOO-

measured in the black crusts (layer A) sampled on the mortar.

in layer C. Furthermore, the analyses of layer C permitted the identification of the mortar as lime mortar (i.e. prepared with lime and sand), which was extensively used as a jointing binder in ancient masonry throughout Europe from the Roman period up to the 19th century. The correlation of these data indicate that the main damage mechanism affecting lime mortars occurs with the formation of two different layers: a thin

surface black crust A composed of gypsum and carbonaceous particles, which is the accumulation area of atmospheric deposition and the products of interaction between the mortar and atmospheric gas and aerosol, and an inner layer B, where the partial dissolution and sulphation of the carbonate matrix produce the disaggregation of the sand grains. These results indicate that the damage layers analysed on lime mortars are different from the black crusts found

Black crusts on ancient

mortars

219

on marbles and limestones (Sabbioni and Zappia, 1992a), which are composed of a single layer similar to layer A on mortars, while the underlying rock is unaltered. The damage layers on mortars present a similar composition to those detected on sandstones (Sabbioni and Zappia, 1992b). The IC data reported in Fig. 5 reveal the presence of sulphates with the highest mean concentrations, followed by oxalates, nitrates, chlorides, phosphates, nitrites, formates, fluorides, and bromides. Table 1 summarises the data relative to sulphur, nitrogen, carbon and their main compounds obtained, respectively, by CHNSO and IC. A mean total sulphur (St) concentration of 9.4% is found, alongside nitrogen (N,), with a mean value of 2.1%, and total carbon (C,) with 1.9%. As can be seen, in the black crusts analysed, 98.7% of the total sulphur is present as sulphate and is due to dry and wet deposition of atmospheric SOz and aerosol on the mortar surface; its reaction with the calcium carbonate constituting the binding component of the lime mortar leads to the formation of gypsum (CaS04. 2HzO), as confirmed by the DTA-TGA data. Nitrate (N(N0; )) accounts for 57.9% of N,. In general, nitrogen compounds, such as nitrate and nitrite, which are very soluble and do not form stable compounds with the calcium of the mortar, present a remarkable variability in the samples analysed. The noncarbonate carbon (C,,), accounting for 1.5%, represents 80.3% of the total carbon (C,) and contains the fractions, 43.3% elemental carbon (C,) and 56.7% organic carbon (C,), with a C, /C, ratio of 1.3. The C, must be considered as being entirely due to the deposition of atmospheric particles and, as such, constitutes a quantitative index of the carbonaceous particles (Leysen et al., 1989) embedded in our crusts. A correlation between C, and gypsum is reported in Fig. 6. These data confirm the results achieved on different materials (stones and mortars) in exposure-chamber tests, indicating that the role played by carbonaceous particles in the heterogeneous oxidation of SO2 into SOi- is basically catalytic on account of their heavy metal content (Sabbioni et d., 1996a). Oxalates are anions present in the highest concentrations after sulphates (as shown in Fig. 5) and their carbon content represents 25.8% of the C,. Calcium oxalates have been widely found on stone monuments and their origin is reported in the literature as being due to: (a) the transformation of organic materials such as oils, waxes, aliphatic and aromatic carboxylic and dicarboxylic acids, etc., utilised in the past for protective treatments (Rossi Manaresi, 1996); (b) biological weathering, lichens, in fact, produce oxalic acid which reacts with the underlying carbonate material, leading to the formation of calcium oxalate monohydrate and dihydrate, weddellite and whewellite, respectively (Jones and Wilson, 1985; Sabbioni and Zappia. 1991); (c) deposition of primary pollutants due to the incomplete combustion of fossil fuels (Kawamura and

220

C. SABBIONI et al. 60 55

g

. .

T

50--

145-F it 6

.

.

40 --

..

.

. 35 -30 !

0

I 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Ce (%) Fig. 6. Elemental carbon (C,) as a function of gypsum measured in the black crust samples found on mortar.

Kaplan, 1987; Saiz-Jimenez, 1993), and secondary pollutants formed by photochemical oxidation of olefin compounds (Saiz-Jimenez, 1989). Formates are always present in considerable quantities in the samples analysed (their carbon content representing 1.2% of the C,); in this case, possible sources could include primary (Allen and Miguel, 1995) and secondary formation by photochemical reactions in the atmosphere (Harley and Cass, 1994) and/or after deposition. The results obtained by GC/MS reveal the presence of several organic compounds including hydrocarbons (alkanes, alkenes and PAH) and other organic classes (n-alkanoic acids, benzoic acids, benzaldehydes, azanaphthalenes, hopanes and sulphur-containing compounds). Among the main compounds identified in the TIC chromatogram from the extract of black crusts, Fig. 7 shows benzo[b]thiophene (15.14 min), acenaphthylene (19.59 min), dibenzofuran (20.49 min), dibenzothiophene (23.30 min), fluoranthene (26.52 min) and pyrene (26.99 min). Minor concentrations were found of fatty acids, such as methyl and ethyl esters. The PAH found are tracers of specific anthropic sources typical of urban areas, such as exhaust from noncatalyst vehicles, domestic heating systems and industrial fuel combustion, further confirming the presence of carbonaceous particles embedded within the black fatty acids, which are typical compounds emitted by catalyst-equipped vehicles (about 13 times the amount emitted by the noncatalyst) (Rogge et al., 1993), show as expected to be minor components in the formation of the damage layers, which can be considered the product of the integral deposition on the surface crusts on lime mortars. Finally, Fig. 8 shows the mean concentrations of the elements measured by means of inductively coupled plasma emission spectrometry (ICP); Rb and

Cs (due to the low sensitivity of ICP to these two elements), were determined by IC. They reveal the presence of a high number of elements with a wide range of variability, covering five orders of magnitude from a few ppm of certain metals to about 10% in the case of Si. The presence of a consistent number of heavy metals, especially iron with a mean percentage of 1.4, again confirms the catalytic action of the carbonaceous particles in the SO2 oxidation processes.

4. CONCLUDING

REMARKS

The use of this set of physico-chemical analyses allows the identification of the main damage processes affecting mortars. The results concerning the components due to atmospheric deposition, i.e. sulphur, nitrogen and carbon compounds, can be summarised as follows: (a) sulphur is almost entirely present as sulphate and gypsum is the main crystalline species present in the black crusts due to the transformation of calcium carbonate determined by atmospheric deposition; (b) nitrogen compounds, as nitrate and nitrite, which are very soluble and do not form stable compounds with the calcium of the mortar, present a remarkable variability in the samples analysed; (c) after sulphur, carbon is the main element due to atmospheric deposition present in the damage layers on mortars; 80.3% is present in the form of noncarbonate carbon, while 43.3% of C,, is present as elemental carbon. The remaining organic fraction is mostly composed of oxalates and formates; the presence of PAHs was also evidenced. Finally, the damaged layers found on lime mortars are different from the black crusts on marbles and

Black crusts on ancient mortars

221

Abundance 16OOOOC

15ooooc

140000C

13ooooc

12ooooc

1100000 1000000 900000 800000 700000

600000

500000

400000

300000

200000

100000 0

Time-->

1

16.00

18.00

2

k .OO

22.00

24.00

26.00

28.00

30.00

32.00

Fig. 7. Typical total ion current chromatogram from GC/MS analyses of the organic fraction extracted from the black crust. Peak identification is as follows: (1) benzo[b]thiophene; (2) acenaphthylene; (3) dibenzofuran; (4) fluorene; (5) dibenzothiophene; (6) phenanthrene; (7) n-hexadecanoic acid, methyl ester; (8) n-hexadecanoic acid, ethyl ester; (9) fluoranthene; (10) pyrene; (11) n-octadecanoic acid, ethyl ester; (12) benzo[k]fluoranthene.

C. SABBIONI

Si

Ca Al

Fe Na

K Mg Sr

Fig. 8. Mean elemental

Ti Mn Ba Zn Pb

concentrations

limestones and present a similar composition and structure to the damaged layers found on sandstones. Acknowledgements-This tional Research Council tegico Beni Culturali”.

et al.

research was supported by the Nawithin the Program “Progetto Stra-

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V

Cu Ni

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