---4
.*..*
Pollutant effects on stone monuments The outcome can be predicted with reasonable certainty K. La1 Cauri
C. C . Holdren, Jr. University of Louisoille Louisville, Ky. 40292
An oil refinery is being built nearly 30 km upwind from the Taj Mahal in Agra, India. This refinery is expected to emit 25-30 tons of sulfur dioxide daily, which is likely to travel towards the Taj Mahal from October to March due to the prevailing northwesterly winds (I). Such SO2 emissions are expected to corrode the marble at the Taj Mahal in the same fashion that air pollution has contributed to the corrosion of marble at the nearly 70year-old Field Museum of Natural History in Chicago. Fcorrrrc arrirlu.7 in ES&T hoc< hy-linrr. w p r c v m rhr c i u w offhe ovrhorr. ond ore vdirrd h? rhu Worhinpron .sra// If?.ou ore iiirmwed in w n r r i h u r i n ~m arricle. omrocI rhr nmnopinx
rdin,,.
380 Environmental Science 8 Technology
In December 1978, the senior (first) author of this paper collected a few marble samples at the Taj Mahal to compare their condition with the marbles exposed at the Field Museum of Natural History in Chicagoand the Erechtheion at the Acropolis in Athens. Knowledge of the mechanisms of marble decay enables the conclusion that the marble a t the Taj Mahal-in the wake of the effluents of industrial combustion expected to pervade the environment of Agra-shall meet the same fate as the monuments of antiquity in industrial Europe and North America. Of constituents produced by the combustion of fossil fuels, NO, and SO2 are the most potent for stone decay. During periods of dryness, they accumulate as particulate matter on stone surfaces and are activated by subsequent wetness. Dissolved in precipitation. they descend as acid solutions. In the eastern US.. the acidity of precipitation has achieved a regional pH value below four ( 2 ) . In the Los Angeles Basin, similar pH values exist
due to acid nitrates (3). Emissions rising from stationary sources are commonly deposited at considerable distances from their sources. The pollutants generated in the Ohio Valley ( 4 ) ,for instance. have significantly contributed to the acidity of precipitation in the northeastern US., just as Scandanavian precipitation has become contaminated with the emission effluents of Western Europe in the wake of prevailing westerly winds ( 5 ) . Precipitation in equilibrium with atmospheric COz, the natural cause of acidity, has a pH of 5.65.The weak carbonic acid (H2CO3) that forms by the dissolution of C 0 2 in water has been, until recently, the major cause of marble decay. NO, and SO2 emissions, however, have increased the acidity of precipitation. The black crusts and crumbling stone on marble structures in industrialized regions are composed of calcium sulfate, nitrates, and organic particles. Some representative reactions of these emissions with the marble’s calcite (CaCO3)
0013-936X/81/0915-0386$01.25/0 @ 1981 American Chemical Society
i
Figure 1B. The rap one-Jburrh o J r h i s scanning rlecrron m i c w graph shows rhr surfacr oJrhr crusr; brnrarh rhir is rhr rnrire rhickness offhe crusr.followd hy rhe calcirr grains ofmarble. which is unalrered but has gypsum in rhr inrergranular space
' :.. I
F i g u ~IA. The black crusr. made ofgypsunt andsoor. occur.s in a r t w prorecredfrom rain. A portion ofrhe crus! is exfoliored
a1 rlw .sire Offhe braids. oblireraring derails ofrhe sculprure (Field Mssuunr of Narriral Hisfor). Chicago)
may be written as follows: CaCO,
so2
--+ Hi0
+
CaSO,. I / 2 H 2 0 C 0 2 CaSO, 2 H 2 0 C 0 2
.
+
0 2
--+CaS04-2H20
(I)
Hz0
-++
CaCO3
-
+
+
Sod2- 2H+ H 2 0 C a S 0 4 . 2 H 2 0 COz (2)
+
+
CaCO, 2N03- 2H+ C a ( N 0 3 ) ~+ CO2 + H 2 0 (3) Some of the gypsum ( C a s 0 4 2H20) in solution is able to penetrate into the intergranular space (our methods of study-x-ray diffraction. and S E M and petrographic microscopy-have not enabled us to resolve Ca(N03)2 as yet) while the rest is either washed away or deposited in the form of a crust on the marble surface (Figure I ) . Such crusts are commonly found on stone underneath cornices and domes where the marble is protected from the direct impact of rain.
.
Continued weathering behind these crusts causes them to fall off in layers, seriously damaging the structures (Figure 2). I n unprotected areas of buildings (those washed by rain), crusts are unable to form. However, acidic solutions freely migrate around the grains. The dissolution of calcite results in the grain-by-grain dissociation of marble. It may seem paradoxical, but marble in sheltered regions suffers more serious damage than marble in unprotected regions (Figure 2). I n crusted marble, the thickness of the zone of weathering-the region including the surface crust as well as the depth to which the gypsum has intercalated the intergranular space-varies with different marbles. This is controlled by the porosity characteristics of the marbles. Georgia marble is highly massive and contains bands of fine-grained mineralsprobably clay minerals formed by the weathering of phlogopite, which occurs profusely in this marble. The zone of weathering of crusted Georgia marble
is nearly 0.5 mm thick. A carrara-type marble, however, used in the construction of certain belt courses at the Field Museum, is more porous and lacks the secondary Fine-grain minerals in the interstices. This permits a freer circulation of chemically active sohtions through the pore space. As a result, the zone of weathering in this marble is much thicker, approaching a thickness of nearly 4 mm. I n unprotected, naturally cleaned surfaces, the zone of weathering lacks a definite identity. However, the enlarged space between calcite grains is clearly visible (Figure 3). Here. in fact. the thin zone of weathering is the region with enlarged intergranular space. X-ray diffraction and optical observations do not reveal any gypsum, but its presence has been confirmed by atomic absorption spectrophotometric analysis (Table I ). To obtain correlations between different samples, the ionic composition data, generated as mg/L of water-soluble species, is converted to weight percent of dry stone. This table reveals the trend of a Volume 15, Number 4. April 1981
307
decreasing quantity of Ca2+, NO3-, and from crusted marble to naturally cleaned marble specimens. The black crust on the marble surface at Field Museum contains as much as 13.6% Ca2+ and 20.4% sod2-:however, the black soot on certain protected surfaces a t the Taj Mahal (Table 2) contains only 0.6% Ca2+ and indeterminably small traces of sod2-.X-ray diffraction reveals that gypsum and soot are the major constituents of the crust at the Field Museum while scat and quartz (SiO2) are the main constituents of the black coating at the Taj Mahal. The encrustation and marble specimens from the Taj Mahal for this study weighed a fraction of a gram. Although the general conclusions derived from them in terms of absence of sod2-are correct, the following explanations for the distribution of other constituents are only speculative. The ionic components of Specimens 1-3 (Table 2) seem to come from environmental dust, although some Ca2+ may be due to CO2 reaction on marble. Data on Specimen 4, which has only trace quantities of these elements, lend credence lo this supposition. This specimen is a clean marble chip devoid of any adhered environmental dust. The moral is that the Taj Mahal, presently in a healthy state, will be affected by SO2 emissions, over an extended period of time. in the same fashion as the Field Museum has been during its nearly 70 years of exposure. It is noteworthy that Chicago achieves I .4 ppm SO2 levels (6) while the air in Louisville, the site of the monuments in Figure 2, seldom exceeds 0.5 ppm SO>.Although it is difficult to project the future levels ofS02in Agra. it may
I I
4
3
Fipure 2. The anxei under rhr dome is .seriously damaged due to exjoiiarion of the rrusr: the angel ourside the dome is in much berrer condition hecause of .surface reduction of the marhie caused by grainby-grain dissociationofcalcire (Care Hili Cemetery. Louisaiiie)
Figure 3. Indioidual grains in this specimen o/ wearhered Georgia marble hare been dissociated due 10 enlargemenr of rhe infergranular space (Cace Hill Cemetery) 380
EnvironmentalScience 8 Technology
be assumed that the effects on this historic structure, envisaged to last for posterity, will be considerable. Air pollution affects marblein more ways than those described above. The increased ionic concentration in marble, produced by direct deposition and by reactivity of calcite with chemically active gases, enhances decay. In regards lo the Taj Mahal, this has twofold significance. Firstly, the anchoring iron bars, which have already fragmented the marble in various places (Figure 4). will, by accelerated rusting, cause further damage to the stone. Secondly, efflorescences in the sandstone used in construction around the Taj Mahal have already caused peeling of the surface layers of sandstone (Figure 5). and this will become more activated. These points are discussed in the following sections. Rustingofiron bars. Iron bars and dowels, respectively, were used to attach marble blocks to the structural framework and to fasten adjacent blocks to each other. The bars near the surface have rusted, producing hydrous iron oxides. The increased volume attending change of iron into hydrous oxides has mechanically disrupted the stone (Figure 4). This fragmented stone has been artfully replaced with dutchmen (a term used in architectural jargon for artificially replaced stone in masonry structure). The following equations give the chemical reactions (6) that iron undergoes due to SO2 pollution: 4Fe
+ 4H2S04 + 2 0 2
-
+ 4H20
(4)
O2 6H20 4 F e 0 . O H 4H2S04
(5)
-+
4FeS04
4FeS04
+
+
Although no soluble iron was detected in scrapings made for this study (Tables 2 and 3), these equations suggest the reactions will continue to change all the iron into hydrous oxides. It may thus be projected that the enhanced deterioration of iron bars and continued replacement of fragmented marble will eventually convert the entire facade of the Taj Mahal into a mosaic of dutchmen. Efflorescences. Efflorescences on the sandstone surfaces around the Taj Mahal consist of whitish, patchy en-
crustations of gypsum (Figure 5 ) . which was deposited contemporaneously with the formation of the sandstone. Initially, the gypsum was uniformly dispersed in the sandstone, but centuries of wetting followed by evaporation of water at the surface has concentrated these salts at the surface and in the subsurface regions. The efflorescences, being watersoluble, repeatedly dissolve and recrystallize in the alternating episodes of wetness and dryness of the stone. Thermodynamics suggest that enor-
mous pressures are generated in such processes. For instance, gypsum crystallizing from a IOX supersaturated solution at 50 OC generates a pressure of 334 atmospheres (7). While such supersaturatlon conditions do not exist in the pore space of sandstone, the repeated crystallization over long periods of time has resulted in failure of the stone'and creation of hollow patches. Table 3 shows the ionic composition of sandstone specimens with visible whitish efflorescence. Although x-ray diffraction analysis reveals only the
TABLE 1
ionic composition of weathered marble at the Field Museum of Natural History, Chicago (concentratlons of water soluble species, wt % of dry sample) speeim 0
N+ .
0.144 0.035 0.159 0.160 0.123 0.032 0.206 0.010
I(+
w+
0.213 0.005
0.062 3.010
n niu
n niu
3.003 0.022 0.001
0.001 0.033 0.003
cz+ 13.6 0.825 1.05 12.6 1.72 0.464 2.74 0.188
0.264 0.073 0.075 0.260 0.031 0.006 0.043 0.033
20.39 1.20 0.24 0.46 0.46 0.096 3.04 0.074
1-3-From a m i c e dnp made of c ~ m r marble: a (1) scraplng horn black cnrsled polecled surface. (2)Scraping hom p H i l l y protected brown surface, (3) saaping from unprdecled surface. 4-6-Frm a m i c e alp made of Georgia marble:(4) maping f r o m black crusted polectd surlace, (5) scraping horn partiallyprotected h - m surface. I61 m a i m horn unmmected surface I-Scraping horn &#ally protected ares under he putim at p u n d level --scraping h m unprotMted svface ai sound level.
4 0
0.30
0.04
. I
-
I-Sooi on marble surface but not directly in cMtact wtth marble 2 - h l in contact wlth marble 3--sWly marble SUTfaCe &Chip fragmented horn C m t d m n lower man
. ,
TABLE 3
I
ces on sanonone sirucxures a ionic composition 01 e (concentrations of water-soluble species, wt % of dry sample) 8p.clm.n
1 2
N.+
1.26 0.07
.+
0.75 0.07
w+ 0.002 0.025
' ~ - ~ f f ~ m e sfrom c ~ sacdsmne o inside mudyard. !--Efflw~8~-
from sandstone oulside courtyard.
Volume 15. Number 4. April 1981 389
presence of gypsum (besides the minera1 composition of the sandstone), atomic absorption data suggest that some complex sodium salts may also be present. The large quantity of NO3- in these specimens may be of cultural origin. The emission of particulate matter from refinery emissions will make these potentially hazardous salts even more dangerous by facilitating their migration and their con- 1 .e. e4 sequent accumulation near the stone surfaces. Existing technology lacks proven methods for preserving stone structures containing gypsum in the zone of . weathering (8). Because gypsum Fipure 4. 7 h e iron pin uisible in the cenrral commonly accumu~ateSon sculpture porrion ojrhefigure has rusred and caused Some works are during a Jragmenr of marble to dislodge. Note gypsum removal. At the hternational rhar rhe carvings around this area are in Meetinp. on the Restoration of the excellent condition due to the ahsence of SO2 i n the atmosphere ( T a j Mahaf. Erechthkion ( 9 ) . methods were suggested 10 convert gypsum, in Situ. to CaCO, by reaction with C 0 2 in an autoclave at high pressure and ternperature. It was also proposed that the gypsum be transformed lo B a s 0 4 by reaction with Ba(OH)2. Polymeric treatmentS were discussed, but the degree development these 4 s does not promise complete success and therefore further research is essential. Following decisions at this meeting, the caryatids have now been moved from the Acropolis into a muFigure 5 E/f7uorescence.v in rhr sand.rlonc seum. are risible here as white parches. The Cleaning the Taj Mahal is a relafiontalporrion is more sei3erely weathered tively simple matter. The brownish because a/ more frequenr episodes of discolorations under arches at the Taj welting and drying, causing repeated (Figure 6) are presumably only envicrysrallizarion of the gypsum ( T a j ronmental dust adhering with recrysMahal) tallized calcite formed by C 0 2 reaction with marble; these encrustations should be mechanically removed. Fine abrasives should be used to polish the marble surface. Such an operation should, in addition to removing dirt, give a polish to the marble. increasing its water repellency. The cleaned marble surfaces then should be washed regularly with water to prevent crusting. After cleaning, the marble may be given a surface treatment with polymeric materials. which act as semipermeable mem'branes (they do not absorb atmospheric gases) and are resistant to UV radiation (10). Calcite grains mixed in such polymers may be forced into cracks caused by the ex- . pansion of iron attachments to inhibit the movement of water into the I- .. e stone. The sandstone structures, however, need optimal removal of efflorescences Fipure 6. The sur/ace of the marhh. !!!tdcr and conso~idation improve strength rhc iln.he.7 is brownish due ro rhc dcpo.
