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DEVELOPMENT OF SACRIFICIAL COATINGS FOR PROTECTING MARBLE FROM SULPHATION

ATZENI, CIRILLO; MAMELI, PIER LUCA; MASSIDDA, LUIGI and SANNA, ULRICO

Department of Chemical Engineering and Materials, University of Cagliari, Piazza d'Armi, 09123 CAGLIARI (Italy)

KEY WORDS: Conservation of cultural heritage, marble deterioration, sacrificial coating, gypsum formation, reversibility of conservation treatment.

SUMMARY A coating has been developed for the purpose of shielding the surfaces of calcareous materials used in monuments from the deleterious action of sulphur oxides. The material consists of a polymer admixed with suitable proportions of white, inorganic granular substances, that are capable of neutralizing the acid pollutants. In this way, the aggressive substances are prevented from depositing on the carbonate surface of the monuments, and attacking them (dissolution, sulphation, etc.). The results obtained have confirmed the feasibility of the principle of this type of neutralizing-sacrificial coating and some efficacious formulations have been developed.

1. INTRODUCTION

Few other building stones can equal the importance of marble in historical monuments. In the Mediterranean region in particular, marble began to be employed in statuary in the Bronze Age, but it was during the Classical Ages, the Renaissance, etc., that the use of this stone gained enormous importance. Countless works (statues and temples, facades and reliefs) universally considered symbols of our culture have been constructed in marble [1-3). The intense urbanization and industrialization that has been taking place all over the world since the Second Wor1d War has altered significantly the composition of the atmosphere. The acid constituents (C02, NOx. SOx and their derivatives) acting on marble surfaces have caused irremedial damage to the cultural heritage, often in the space of a few decades. In Athens, for instance, it became necessary to house the statues of the Acropolis in a museum and conserve them there "under glass" in nitrogen flow [31). Sulphur oxides have a particularly harmful effect. Though the chemical processes involved in the sulphation of marble and carbonate surfaces in general are not yet fully understood, these substances lead unequivocally to the formation of gypsum crystals often damaging directly the carbonate stone employed in the building [4-6). Legislation enforced to abate air pollution has meant that technologies for power generation, transport systems and the entire manufacturing industry have had to adapt. However, until such time as a general improvement in the quality of air and atmospheric precipitations materializes, the marble surfaces of historical monuments exposed to the open air need to be isolated from the outside

environment. The search for a miraculous polymer film able to prevent sulphation of marble surfaces, chemically stable and removable f'the philosopher's stone" of modern conservation alchemists) has so far

produced only mediocre results (7-12). Materials technology is now adopting a more complex approach to tackle this problem. Some workers (for example Skoulikidis (7, 13-14)) have developed a polymer film that acts simply as a binder of an inorganic phase that interferes in the chemical reactions involved in the sulphation process, halting its progress. Other researchers have experimented shelter coating methods based on inorganic substances. For example Ashurst (15) has used neutralizing limebased coatings, while Kozlowsky et al. [16] have tested the effectiveness of chemically passive silica layers. Conceptually, these approaches are ideally fairly satisfactory, but a lot of research work needs to be done before really effective solutions can be found. The work described here is intended as a contribution in this

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sense. The aim was to examine the possibility of adding inorganic substances to a polymer phase having proven chemico-physical stability and reversibility (possibility of being removed from the stone using a solvent, even after many years of exposure to atmospheric agents) to neutralize the acid

components present in the atmosphere, thus preventing them from reaching and deteriorating the

carbonate surface of the marble. The neutralizing additives tested were calcium carbonate, zinc oxide and a microporous silica-based calcium Ion exchanger. An acrylic resin with the trade mark Paraloid (Acryloid) 872 was used as

polymer binder. These coatings are designed to perform a sacrificial function, protecting the marble surfaces and should be removed once their neutralizing effect has been exhausted. Proper maintenance should guarantee, in this way, the conservation of the marble surface.

2. EXPERIMENTAL

Tests were conducted on 30x12x2 cm slabs of Carrara marble. The samples used have granuloblasts with maximum particle size of 150 µm, volume fraction porosity of 0.5% and compressive strengths ranging from 100 to 120 MPa. Paraloid 872 was used as the polymer binder. This binder has found wide use in a variety of applications in conservation over the last decades thanks to its excellent stability ("class A" durability, according to the standards proposed by Feller [17-18)). Para-xilene was used as a solvent. As neutralizing charges a calcite and a zinc oxide powders were tested, both in the form of analytically pure reagents with mean particle diameter of 2 µm, as well as an Ion exchanger composed of microporous silica, having mean particle size of 5 µm, capable of exchanging the calcium ions on its surface with the hydrogen ions present in the environmental fluids. Table 1 gives the denomination and composition of the coatings tested. The powders were dispersed in the polymer solution (30% by mass 872 in paraxilene) by means of a small high speed ball mill.

Table 1 - Composition of the coatings.

coating P1 C2 C3 Z4 Z5 $6 S7

Composition Paralold 872 at 30% in weight in para-xilene P1 + calcite 70 + 30 % in weight P1 + calcite 50 + 50 P1 + zinc oxide 70 + 30 P1 +zinc oxide 50 + 50 P1 + exchanger Ca/H 70 + 30 P1 + exchanger Ca/H 50 + 50

Coatings of P1 were prepared with wet film thickness of 50 and 100 µm. The systems containing additives had wet film thicknesses of 100, 200 µm (C2, Z4 and S6) and 500 µm (C3, ZS and S7). Coatings with wet film thickness of 50, 100 and 200 µm were applied by means of grooved rods, whereas for the 500 µm coating a roller was used, pressing the film as it was spread. Once they had aged, the coatings matched the original marble surface very closely. The dry film thicknesses measured after ageing are given in the next paragraph where the results are discussed. Though the process of transformation of sulphur dioxide into sulphate on the marble surface is not fully understood, the presence and action of sulphur as sulphate is unequivocal.

In line with the experimental approach adopted by other workers (for example [19)) an accelerated simulation of the sulphation process was performed in the laboratory. Samples with coatings aged for three months in air at 20-25°C and 70-80% relative humidity were sprayed with a 3% by weight sulphuric acid solution.

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Each treatment consisted in spraying the sample for 4' and then leaving it to stand in the treatment

chamber for 30'. This procedure was repeated ten times and the sample was then removed from the chamber and left in ambient conditions for 20 hat 20°C and 65% relative humidity.

The effects of the treatment were assessed by means of scanning electron microscope SEM + EDS observations on sections of fractures.

3. RESULTS

As can be seen in Fig. 1, the acid spray treatment simulates fairly closely the formation of gypsum

crystals on unprotected marble surfaces. In fact the morphology of the crystals is similar to that currently observed on black crusts.

At. ~ Fig. 1 - Formation of gypsum crystals on the marble surface (3 treatments).

Table 2 shows the dry film thickness and the number of acid sprays required before the first gypsum crystals begin to appear at the marble/film interface.

Figure 2 shows that after ust 5 treatments, gypsum crystals start to form beneath the 10-15 µm thick P1 film, though the film itself remains substantially unchanged.

The thicker P1 film (25-30 µm) is more effective in preventing the acid component from diffusing

towards the marble. In this case the first appreciable signs of gypsum formation begin to appear after

about 20 treatments. As the process advances, the newly formed crystals beneath the polymer film

tend to concentrate around the boundaries between the granuloblasts rather than "corrode" them

evenly. Clearly these represent weak points in the marble's crystalline structure, and they are also

widened by the mechanical stresses undergone during surface processing. Predictably, this

phenomenon, over the "long life" typical of objects of cultural interest, will eventually lead to the

pulverization of the uppermost layers of the marble.

Table 2 - Dry film thickness (gm) and number of treatments requ'red for the first gypsum to form at marble/film Interface.

Coating Wet film Dry film No. of treatments thickness, µm thickness, µm first gypsum

P1 50 10-15 5 100 25-35 20

C2 100 40-50 40

200 70-80 60

C3 500 180-220 80 Z4 100 20-25 60

200 60-70 > 80

Z5 500 100-120 > 80

S6 100 30-40 30

200 70-80 40

S7 500 180-200 > 80

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Fig. 2. Gypsum crystals at the marble/P1 film 10-15 µm) Interface (after 5 treatments).

The presence of calcite in the coatings C2 and C3 has a significant neutralizing effect on the acid spray. The reaction on the surface is rather rapid, but attack of the marble interface is delayed considerably: 40 and 60 treatments for C2 having dry film thickness of 40-50 and 70-80 µm respectively. Fig. 3 shows the onset of attack at the interface for C3 after 80 treatments.

Fig.3 - Gypsum crystals on the coating C3 after 80 treatments with acid spray.

The effectiveness of the chemical filter created by the calcite grains in the sacrificial coating clearly emerges.

Even better results were obtained for the systems containing zinc oxide. Spraying with the acid solution leads to the formayion of zinc sulphate crystals, as confirmed by the EDS analysis. The data of Table 2 show that the formation of crystals on the marble is delayed considerably. Figure 4 shows a section of the marble coated with ZS: after 80 treatments it remains practically unchanged.

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lOOµm

Fig. - 4. Marble/ZS interface after 80 treatments with acid spray.

In the films containing the Ion exchanger, which were applied with grooved rod, the acid manages to penetrate rapidly into the section and only reacts partially with the active substance. After 30

treatments on S6, see Fig. 5, gypsum crystals start to form and in some cases this actually resulted in the film coming away from the marble surface.

Fig. 5 - Gypsum crystals cause the S6 coating to detach from the marble surface.

By contrast, the results obtained with the coating S7 were highly satisfactory. After 80 treatments the interface with the marble is still intact (Fig. 6). Application of the coating using a roller enhanced compaction reducing permeability and hence prevented the direct percolation of the acid towards the

marble surface.

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:,: ., · oc~· :1 .100 µ~ I

~

Fig. 6 - After 80 treatments no gypsum formed at the marble/S7 Interface.

4. CONCLUSIONS

The experimental tests conducted here have shown that it is possible to protect marble from attack by sulphuric acid spray by coating the surface with a polymer film containing inorganic neutralizing substances (calcite, zinc oxide, a Ca/H ion exchanger). Once its neutralizing effect has been exhausted, the coating may be removed, on account of the molecular stability of the binding polymer and of the fact that it may be dissolved with different solvents (reversibility), and restored to its original condition in the course of normal maintenance work on the marble surfaces. Research is now in progress to attempt to correlate the number of accelerated ageing treatments performed in the laboratory with the useful life of sacrificial coatings in real conditions of exposure in urban-industrial environments with different pollution rates.

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