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«EDWARD V. SAYRE This paper discusses plaster, concrete and mortar in terms of their chemical composition, reviews the deterioration of these ...»

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Barium treatments deposit the protective material as deeply within the treated structure as the aqueous solutions will carry the reactants. Also, if carried out correctly, none of the treatments will completely seal the surface and cut off the normal flow of water vapor into and out of the plaster. Instead, they tend to cover the reactive carbonates within the structure. In applying such deposition treatments to a structure whose surface must remain totally free of deposits, such as a fresco, the surface can be protected by first placing a layer of Japanese mulberry paper on it and then placing a thick layer of cellulose pulp thoroughly wetted with the treatment solution on the mulberry paper. Keeping the outer pulp layer wet with the solution during the period required for proper reaction will guarantee completion of the treatment. Deposition occurring exterior to the treated structure will occur within the paper and pulp layers, and the deposits will come off when the paper and pulp layers are removed. Frescoes in Florence have been successfully treated in this manner with barium hydroxide solu· tions. 6 In this instance, treatment was preceded by careful wetting of the walls with an aqueous ammonium carbonate solution.

Specimens taken from Florentine frescoes have also been successfully treated with a barium hydroxide-barium ethyl sulfate solution, and it was found that the deteriorated support structure was consolidated and that the appearance of the pigment layers was not appreciably altered in color or appearance. 7 Gypsum Plaster Gypsum plasters are those whose principal binding component after setting is calcium sulfate dihydrate, or mineral gypsum. For many centuries, these plasters have been formed by heating gypsum to various high temperatures, causing a partial or complete dehydration of the heated mineral. On the addition of water, the reaction of rehydration regenerates a crystalline gypsum that is firmly consolidated. A number of types of gypsum plaster, with greatly differing setting characteristics, are available. Plaster of paris, the most common of them, is prepared by heating gypsum to about 150°C. in open vessels. Its composition is largely calcium sulfate hemihydrate {that is, CaS0 4 • lhH20), and it sets rapidly.

The rate of setting can be retarded by adding 0.1 percent keratin or by forming the he1nihydrate through an autoclave treatment.

More slowly setting plasters are formed by heating gypsum totemperatures sufficiently high to dehydrate the calcium sulfate completely. Of these, anhydrous gypsum plaster is formed by heating gypsum to about 200°C, and Keene's cement and parian cement by heating to 600°C and adding slightly less than 1 percent potassiun1 alum or potassium sulfate as an accelerator. Estrich Gips, an extremely slow-setting plaster, can be formed by heating gypsum to 1,100°-l,200°C. It is co1nposed of anhydrous calcium sulfate and lime. The conservation problems associated with all of these types of gypsum plaster are basically the same, once the plaster is set. Moreover, with a few exceptions, the deterioration problen1s are similar to those encountered with lime plaster.

Gypsum plaster is inherently more water soluble than lime plaster and, hence, is more susceptible to damage by water; however, it is considerably more inert toward acidic substances and is correspondingly less sensitive to air pollutants. Another characteristic way in which gypsum plaster differs from lime plaster is that at sufficiently high temperature and low humidity, it will spontaneously decompose through dehydration. At temperatures of 75°-l00°F. (24°-38°C), relative humidity in excess of 35 to 45 percent must be maintained to prevent such spontaneous dehydration. Fortunately, the rate of such dehydration is slow, and occasional departures from stable ambient conditions do not result in immediate extensive damage.

Virtually all of the conservation treatments outlined for lime plaster can be applied to gypsum plaster, with the possible exception of those involving soluble barium or strontium salts. When barium or strontium solutions are applied to gypsum, they convert it to barium and strontium sulfates, which are considerably more insoluble than gypsun1. In some circumstances, a light treatment of this type might help bring about some consolidation of a broken-down structure and protect any calcium carbonate present in the plaster. Heavy treatments of this type, however, might cause deterioration through excessive conversion of the sulfate.

CONCRETE

The invention of hydraulic cement is attributed to Roman builders who discovered that if they mixed lime with pozzolana (pulvis puteolanus), a volcanic earth occurring in large deposits near Rome and Naples, the two formed a mixture that set strongly under water and was fire resistant. The quality of this early concrete is well demonstrated in the many sound Roman concrete structures still standing. Modern types of cement, such as portland cement, are more often manufactured by grinding, firing and regrinding together roughly two parts limestone or other calcareous material and one part clay, shale or other argillaceous materiaL Ground gypsum and other additives are added after firing. The resultant product is a complex mixture of calcium aluminosilicates that, in the case of portland cement, is roughly equivalent to a combination of 60 to 67 percent lime, 17 to 25 percent silica, 3 to 8 percent alu1nina, 0.5 to 6 percent iron oxide,





0.1 to 4 percent Inagnesia, 1 to 3 percent sulfur trioxide and 0.5 to 1.3 percent soda or potash.

A great variety of cement is manufactured today, including a number of variations of portland cement and modern equivalents of pozzolana cement in which burnt clays and pulverized fuel ash are substituted for volcanic material. Rather different in their basic composition are two types of modern cement developed to produce concrete that is highly resistant to sulfate and other chemical attack. These are the high-alumina and supersulfate cements. In high-alumina cement, aluminates with an alumina content as high as 45 percent replace silicates. Such cement is usually manufactured frmn bauxite and limestone. These raw materials tend to fuse completely during firing; hence, the resulting cement is frequently sold under such names as Chnent Fondu, Cementa Fuso or Schmelzzement. High-alumina cement is also desirable because of its rapid hardening and excellent refractory qualities. Su persulfate cement is manufactured fron1 blast furnace slag and gypsum, with the addition of a small amount of portland cement. Also available are special types of cement that expand rather than contract on setting and are useful for producing sharply defined castings. The choice of the right cement for the job is obviously a primary consideration in using cement for conservation purposes.

There is also great variety in the proportions in which cement, water, the aggregate, air, air-entraining agents and other additives are mixed to form the final concrete mixture. The chemical reactions of setting are extremely complex, as they form a variety of colloidal and crystalline products of a hydrated silicate and aluminate nature. Frequently cited as a typical reaction product is the mineral tobermorite, which is a long-chain inorganic polymer with the formula [Ca5 (Si 3 0 9H) 2 • 4H 2 0]x. Also, some free calcium hydroxide or carbonate is usually formed and significantly affects the susceptibility of the finished concrete to acidic attack.

Although most types of concrete are usually less affected by hydrolytic and acidic action than is lime plaster, they are not immune to such attack. When exposed to an aqueous solution with pH as low as 3.5 to 4, concrete often deteriorates rapidly. Concrete is also badly damaged by efflorescent and subsurface salt deposits. Because it is often used outdoors, it is frequently damaged by frost and by the growth of plants and microorganisms on it.

Many types of concrete deteriorate rapidly if exposed to solutions containing soluble sulfates. If mildly attacked by sulfates, the concrete will develop a whitish appearance. At a more advanced stage, it expands, cracks and spalls. It eventually becomes friable throughout and is finally reduced to a powder. The sulfate ions react with concrete components to form the mineral ettringite (that is, 3Ca0 • Al 2 0 3 • 3CaS0 4 • 32H2 0). Other products, which produce a considerable expansion in crystalline volume, are formed also. The susceptibility to attack seems to depend on the concentration of tricalcium aluminate (that is, 3Ca0 • Al2 0 3 ) in the original cement. Protection against sulfate attack is best provided by blocking the source of sulfate ions or by using sulfateresistant cement, such as high-alumina or supersulfate cement.

The freezing of water within pores of concrete can be destructive in two ways: (1) through the direct expansion that occurs on freezing and (2) through the growth of ice crystals previously formed by processes involving migration of moisture through the concrete to the frozen layer. This latter process can result in the breakdown of the concrete in layers parallel to the outer surface.

The presence, in the cement structure, of open pores leading to the suface promotes such frost attacki however, the presence of trapped air bubbles within the concrete structure tends to protect the concrete from frost damage, provided that the bubbles do not form continuous channels. Accordingly, in current practice up to 10 percent air or other gas is deliberately entrained within the concrete at the time of formation. This is accomplished by (1) adding materials such as hydrogen peroxide or powdered aluminum or zinc that react with the moist cement to generate gases, (2) reducing surface tension through the introduction of surface-active agents and (3) using dispersing agents that prevent coagulation without appreciably lowering surface tension.

Surface treatment by any of the impregnating agents discussed under lime plaster can be applied to concrete. Because of concrete's silicic nature, inorganic impregnants depositing silica would seem to be particularly appropriate. When concrete is exposed on external surfaces, organic treatments should be avoided unless only relatively temporary protection is desired. Also, if external exposure is involved, the concrete should not be given a treatment that attempts to seal the surface. Experience has shown that all such "impermeable" layers are eventually broken.

When this occurs, water and other destructive agents tend to accumulate beneath the "protective" surface, resulting in decay beneath it and eventually spalling off of the surface. Attempts to seal off a surface have frequently resulted in more damage than if the concrete had been left untreated.

MORTAR

Mortar ranges in composition from straight lime plaster to concrete, but most modern types of mortar are mixtures of cement, lime and sand. The lime content of mortar tends to be from one to three times that of cement so that mortar can be expected to have many properties similar to lime plaster. When mortar is intended for external use, air is usually deliberately entrained within it. For example, the American Society for Testing and Materials Standard 91-60 specifies that there should be not less than 12 percent air in mortar.

The deterioration and conservation problems of mortar are similar to those of plaster and concrete. Because of the high concentration of calcium carbonate in mortar, a barium inorganic impregnation treatment would be more appropriate than a silicon one.

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NOTES J. R. Harris, Ancient Egyptian Materials and Industries, 4th ed.

1. A. Lucas and (New York: St. Martins Press, Inc., 1962).

2. Arthur H. Church, "Treatment of Decayed Stonework in the Chapter House, Westminster Abbey," Journal of the Society of Chemical Industry 23, no. 824 (1904).

3. Maxmilian Dennstedt, stone colouring and preserving cements. British Patent 13,761 (1884).

4. Seymour Z. Lewin., composition for preserving limestone structures. U.S.

Patent 3,577,244 (1971).

5. Edward V. Sayre, "Direct Deposition of Barium Sulfate from Homogenous Solution within Porous Stone/' in 1970 New York Conference on Conservation of Stone and Wooden Obiects, ed. G. Thomson, 2 vols. (London: International Institute for Conservation of Historic and Artistic Works, 1971), 1:115-17

6. E. Ferroni, V. Malaguzzi-Valeri and G. Rovida, 11 Experimental Study by Diffraction of Heterogeneous Systems as a Preliminary to the Proposal of a Technique for the Restoration of Gypsum Polluted Murals" (Paper delivered at the International Council of Museums Conference, Amsterdam, September 1969).

7. Edward V. Sayre, "Investigation of Italian Frescos, Their Materials1 Deteriora· tion and Treatment," in Application of Science in Examination of Works of Art, ed. William J. Young (Boston: Museum of Fine Arts, 1973) 1 pp. 176-81.



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