ORIGIN AND GROWTH MECHANISMS OF THE SULF ATED

CRUSTS ON URBAN LIMESTONE

D. CAMUFFO, M. DEL MONTE, and C. SABBIONI

Istituto di Chimica e Tecnologia dei Radioelementi-CNR. Padova, Italia Istituto di Geologia-Universitd di Bologna, Italia

Istituto FISBAT-CNR, Bologna, Italia

(Received February 5, 1982; Revised April 22, 1982)

Abstract. Building stone surfaces exposed to the urban environment are subjected to rapid and differentiated deterioration, due to various chemical and physical attacks. Apart from a chemical deterioration below the sufated crusts, which mainly grow outwards at the unaltered gypsum/carbonatic rock interface, another physical deterioration was observed. The latter is, at times, even more important than the rapid chemical attack typical of the zones which are subjected to heavy washing by rainwater. The formation and growth of three different kinds of deterioration layers are discussed with reference to the main kinds of deterioration characterized by white, grey and black external appearance.

Analyses of airborne particulate matter and of deterioration layers of urban monuments led to the conclusion that not all the gypsum of crusts which can be observed on marble monuments is derived from the transformation of the underlying rock, but it is also partially transported by aerosols.

1. Introduction

In recent years sulfation has been recognized to be one of the main causes responsible for carbonatic stone deterioration processes in urban environments (Winkler, 1966; Henley, 1967; Spedding, 1969; Braun and Wilson, 1970). The transformation of limestone into gypsum is not only to be associated with the presence of gaseous S02 alone, but also with the S contained in the residual carbonaceous particles of com­bustion from urban heating (Del Monte et al., 1981). It has also been observed that the degree of rock deterioration and the distribution of three main types of observable deterioration are related to different kinds of wetting and particularly to the exposure of the limestone surface to rain (Camuffo et al., 1982), namely: the zones chemically unaltered and covered with a thin layer of dust with a grey appearence are shielded from rainwater; the zones that are apparently white, covered with a thin layer of reprecipitated calcite, are subjected to intense washing-out; finally the zones frequently soaked by rainwater but not experiencing washing-out show a thicker black crust characterized by the presence of gypsum crystals and black carbonaceous particles.

The aim of this paper is to study the origin and growth mechanisms of the sulfated crusts on urban carbonatic stones and in particular to investigate:

Ca) if the black heavily sulfated surfaces (generally in relief with respect to the original surface) constitute a protection or a deterioration more intense than that of the white areas subjected to direct washing-out by rainwater;

(b) processes leading to the formation and growth of the deterioration layer; and (c) if all the gypsum forming the deterioration layer is completely determined by a local

Water, Air, and Soil Pollution 19 (1983) 351-359. 0049-6979/83/0194-0351$01.95. Copyright © 1983 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

352 D. CAMUFFO ET AL.

process of transformation of the underlying and surrounding rocks or is it also partially transported from other regions by aerosols.

2. Data Analyses and Discussion

By observing transversal thin sections of the deterioration layers much information can be obtained about the structure, formation and growth of the layers themselves, namely:

(a) the deterioration layer with a black appearance consists of gypsum crystals with radial structure, normal to the surface of underlying carbonatic rock (Figure 1). Black

Fig. I. Gypsum layer showing crystals normal to the carbonatic surface. A calcite fragment of the original rock is clearly embedded in the sulfated layer (Thin section: nicols X).

carbonaceous particles are uniformly distributed inside the layer. The presence of several metallic cations has been evidenced with atomic absorption spectrometry (AAS) (Table I);

(b) the boundary between gypsum and limestone is extremely irregular, showing chemical dissolution in progress (Figure 2A);

(c) within the deterioration layer one can at times observe fragments of limestone containing microfossils. As a consequence the fragments are certainly part of the original rock (Figure 2B); and

(d) from the carbonatic rock some less soluble bioclasts wedges protrude one or more millimeters into the deterioration layer (Figure 2C).

It can be noted that the sulfated black surface observed on marble monuments generally appears in relief compared to the original level and therefore could be con-

ORIGIN AND GROWTH MECHANISMS ON THE SULFATED CRUSTS ON URBAN LIMESTONE 353

TABLE I

Concentrations (ppm) of some elements in crusts sampled at Bologna (Bo) and Venice (Ve), tr means traces i.e. concentrations below 1 ppm.

sample V Mn Fe Cu Zn Pb

Bo 25 160 21000 260 360 60 Bo 25 120 13 800 180 320 19 Bo 35 667 53025 416 380 47 Bo 38 150 14851 129 312 51 Bo 48 157 20013 145 145 57 Bo 27 149 17744 150 210 65 Bo 29 261 23998 130 462 31 Bo 35 177 16307 129 151 25

Ve 10 80 1350 70 110 tr Ve 10 70 1300 60 120 tr Ve 7 54 5912 64 95 tr Ve 5 75 6644 53 175 tr

------- - - - - - - - - - - - a.s.

C

- - - - - - - - - - - - - - - - - - o. s.

C

G+Cp

- - - - - a.s.

Fig. 2. Sketches of typical sections normal to the surface as observed with the optical microscope. The less dotted area is a section of the carbonatic rock (C); the deterioration layer of gypsum and carbonaceous particles (G + Cp) is represented by needles and full dots; the bioclasts (B) is indicated with closely dotted area. The original surface (OS) is indicated with a broken line. The figures A, B, and C exemplify the

situations mentioned in the text in points a, band c.

354 D. CAMUFFO ET AL.

sidered as a deposit and growth zone, built up also with part of the original rock. As . a consequence the original carbonatic rock has undergone a transformation and often the actual surface level is underlying the original one. In attending to evaluate the entity of the transformation and hence the deterioration to which the carbonatic surface was subjected, the same ornamental motif in marble, but exposed to different environmental situations, was used to investigate the three typical situations shown in Figure 3, i.e.:

c

B

A

I

I I

I

o.s.

Fig. 3. Pattern of the same ornamental motif in limestone in three different situations: (A) protected from rainwater; (B) subject to heavy washingout by rainwater. The maximum chemical dissolution is observed in this case; (C) simply made wet but not washed out: the crust is sulfated, gypsum crystals and black particles are evidenced with needles and full dots. Chemical dissolution is less intense than in B. In

(D) are evidenced the effects of mechanical weathering following the microfractures indicated in C.

(a) In the zones where the protection from rainwater is sufficient to prevent chemical­physical transformations of the rock, as already discussed, the ornamental motif was a reference for the thickness of the original surface of the unaltered rock (Figure 3A), so that a comparison of the thinning of carbonatic rock with the identical motif in other zones, subjected to other expositions and therefore deteriorations, was possible.

(b) In the zone where the rock surface is white, i.e. subjected to washing out and partial dissolution, the thickness of the same ornamental motif has been observed to be

ORIGIN AND GROWTH MECHANISMS ON THE SULFATED CRUSTS ON URBAN LIMESTONE 355

reduced by several millimeters (Figure 3B) (4 to 5 mm for a one century old building at Bologna where the averaged yearly total of precipitation is 651 mm).

(c) In the zones directly or indirectly wetted by the rain, but prevented from washing out, the recrystallization on the spot of the products in solution and the capture of new airborne particles is possible. Therefore the actual thickness of the motif is greater than the original one, even if the original carbonatic rock after transformation is lowered by about 2 mm during the last century (Figure 3C). In addition several fractures and large fragments oflimestone breaking off from the edge of the motif have also been observed.

Finally, it should be mentioned that gypsum crystals with fibrous structure grow also over metallic surfaces, as already noted by Riederer (1972) and frequently observed by

(8)

(b)

Fig. 4. Gypsum crystals observed on a metallic surface with the mineralogical microscope (a: nicols X). Carbonaceous particles from fuel firing embedded in the sulfated layer are clearly evident (b: nicols /I). Sample from the bronze statue (by F. Mochi) located in the main square of Piacenza (Northern Italy) since

erection in 1625.

356 D. CAMUFFO ET AL.

ourselves e.g. on the Gianbologna's Nettuno (known also as 'the Colosso' at Bologna), or the Mochi's bronze horses at Piacenza, or iron gratings, railings, and so on. Also these cases the black carbonaceous particles are embedded into the external crust (Figure 4).

The experimental results suggest the following mechanism for the formation and growth of the deterioration layers. Inside the deterioration layer, the black carbonaceous particles, which are derived from combustion residues, contain a high level of S and metallic cations which are recognized as efficient catalysts leading to the oxidation of S02' These particles can deposit on all the surfaces (whether carbonatic or not), but especially on surfaces that are able to remain wet for a long time due to their porous structure such as those covered by gypsum crusts. The particles are then incorporated inside the crust during the crystallization phase associated with the evaporation, so that the growth of the porous surface layer is favored. In the meantime, the local trans­formation of calcium carbonate into gypsum occurs on the marble surfaces. Therefore, a feedback process occurs and the black crusts are caused to grow again and again. The growth of the deterioration layer of gypsum with particles inside may be sketched as follows. Rainwater which is generally acid (as shown in Figure 5 for Venice), is adsorbed into the porous deterioration layer and acidifies further on contact with the carbona­ceous particles. The rainwater arrives at the boundary between the gypsum and the limestone where the transformation of the calcite into gypsum occurs. The deterioration layer grows outwards starting from the boundary between the layer and the underlying rock as described for dolomite (Del Monte and Sabbioni, 1980) or similarly to the 'cave grass', well-known in a karst environment (Choppy, 1975; Hill, 1976). As a consequence, the greater the prorosity of the limestone or marble, the stronger is this process which depends on the ratio of the attackable surface to the volume of the attackable material.

pH

~ 7.0

'f-

f--6.5 JJ.

JJ. JJ.

6.0 JJ. JJ. JJ. JJ. JJ. JJ. JJ. JJ. JJ.

JJ.

5.5 1960 1965 1970

Fig. 5. Acidity of rainwater at Venice. Period from 1959 to 1972 (Source of the data: Sordelli and Zilio Grandi, 1972). At it can be noted, the pH level is above the usual acidic values. This is mainly due to the buffering action of agents like sand and calcium carbonate, which tend to neutralize the monthly totals of

precipitation. Depending on the kind of precipation, the pH at Venice is subjected to vary noticeably.

ORIGIN AND GROWTH MECHANISMS ON THE SULFATED CRUSTS ON URBAN LIMESTONE 357

The thickness of the crust does not increase indefinitly since the older (and thus more external) gypsum acicular crystals break off and are carried away by the wind.

Analyses of airborne particulate matter sampled in Venice, Bologna and Tel-Aviv were carried out by means of several experimental techniques, i.e. X-ray diffraction (XRD), scanning electron microscope (SEM) interfaced with the energy dispersive X-ray analyzer (EDAX). From the XRD analyses, quartz, calcite, feldspar, carbona­ceous material and considerable percentage of gypsum are generally observed. An example of XRD pattern is shown in Figure 6. The chemical data obtained at SEM­EDAX confirm this finding and in addition revealed the presence of a high concen­tration of S, Ca, Si and a smaller quantity K and Na, plus traces of various metallic elements namely Fe, Cu, Zn, V, Pb and Mn. Also many other authors (Charlson et al., 1978; Bigginns and Harrison, 1979; Bloch et al., 1979; Moharram and Sowelin, 1980; Davies, 1981) found gypsum by sampling atmospheric aerosol.

:,.

" v '" '" ~

r

l3 +

l3 a

"

1 '"

is " j '"

JL ~4 ~4

~

30 28 26 24 22 20 12

Fig. 6. X-ray Cu-Ka diffraction pattern of a sample of particulate matter of Tel-Aviv. The minerals observed are: graphite (Gr), gypsum (Gy), calcite (C) and quartz (Q). Gypsum is seen to be the most

abundant crystalline type.

It has been noted that the gypsum is present in aerosols even if in the neighbouring regions there are no natural deposits of this mineral. For this reason Davies (1981) suggested that the airborne CaS04 .2H20 may be the result of the chemical reaction

CaC03 + H2 S04 + H20:;;:::CaS04 2H20 + CO2

in the atmosphere. The CaC03 in the aerosol is of natural origin and observed from the surrounding areas. Furthermore, some authors emphasize that gypsum microcrystals are already present on the surface of carbonaceous particles emitted by fuel-oil and coal combustions (Fisher et al., 1976; Henry and Knapp, 1980). Therefore, the gypsum included in the deterioration layer is not always completely originated by the underlying rock.

358 D. CAMUFFO ET AL.

3. Conclusion

Building stone surfaces experiencing washing-out and frequent heavy rainwater show only a very thin layer of reprecipitated calcite and at times traces of gypsum giving an appearance of white. Deterioration layers characterized by deposits of particles were never observed over these surfaces. It is here that the maximal erosion of the rock causing a diminution in the thickness of the limestone can be found. The slow, constant attack is exclusively chemical, while the removal of the solutions formed thereon is physical. On the 'black' surfaces the underlying rock is subjected to chemical attack. The absence of washing-out makes the thinning of the original rock less intense, with reference to the observed one on the 'white' surfaces. However, a mechanical weathering happens on the black surfaces, but is generally not so effective as on the white surfaces.

We have seen that the weathering requires the presence of a liquid bed on the surface; however, the absorption of water and the condensation-evaporation cycles leave the rock apparently unaltered. It was well-known since Kaiser (1929) that the soluble salts are subject to outward migration due to moisture travelling through the complex pore and capillary system and finally to crystallization near the surface during the evapo­ration cycle. As a result, the distribution of soluble salts decreases exponentially from the surface toward the inside of the rock. Moreover, besides the effects due to the crystallization, the differences in thermal expansion of the salts entrapped in the pores of the surface layer of the rock, may favour the disruption of the face of the monuments where the thermal shock is maximal. The consequence of the differences in thermal expansion becomes dramatic in the case of gypsum wedges with respect to calcite, which may lead to pieces breaking off from the original stone. These pieces may also be quite large. If the geometry of the deposit zone allows it, this type of mechanical weathering can be more destructive than the slow, chemical one.

The above observations and particularly the presence of gypsum crystals over metallic surfaces give rise to the hypothesis that a fraction of the gypsum observed on carbonatic surfaces was originally airborne and therefore deposited on the actual surfaces when they were wet. Thus, besides the growth mechanism described before, there is also a less intense growth due to deposit of gypsum transported by the wind or by percolation from neighboring deterioration layers. The analysis of atmospheric aerosols led us to suppose that part of the gypsum observed in deterioration layers was deposited by some car­bonaceous particles on the wet surface and therefore recrystallized on it together with the local gypsum due to the transformation of the limestone. Further work is in progress in order to determine the ratio between the gypsum transported and that locally origi­nated by the transformation of CaC03 ·

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Symposium on Physico-Chemical Behavior of Atmospheric Pollutants', Ispra, October 16-18. Braun, R. C. and Wilson, M. J. c.: 1970, Atmospheric Environment 4, 371.

ORIGIN AND GROWTH MECHANISMS ON THE SULFATED CRUSTS ON URBAN LIMESTONE 359

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