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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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DETERIORATION AND RESTORATION

OF PLASTER, CONCRETE AND MORTAR

EDWARD V. SAYRE

This paper discusses plaster, concrete and mortar in terms of their

chemical composition, reviews the deterioration of these materials

and suggests possible conservation treatments for them.

In terms of chemical components, plaster is considered in this

paper as being formed primarily of calcium carbonate or calcium sulfate dihydrate (or a mixture of the two); concrete as primarily a mixture of calcium silicates and aluminates; and most mortar as a mixture of calcium carbonate and concrete. It should be remembered that minor constituents and impurities, whether introduced in the original preparation of a material or added in some later treatment, can significantly affect the deterioration process. In addition, the physical structure of a material significantly influences its resistance to deterioration. A notable example of this type of influence is the effect of air-filled voids in concrete, which can either increase or decrease the concrete's resistance to frost damage, depending on their nature and distribution.

PLASTER

Plaster is used not only as a general wall covering, but also as a

support for frescoes. Often, it is molded into decorative stuccowork. Both lime pl~ster and gypsum plaster have been used extensively from the tin1e of the earliest civilizations. Often, as in the f:

Edward V. Sayre is adjunct professor of fine arts at the Conservation Center of the Institute of Fine Arts at New York University, New York, N.Y., and senior chemist at Brookhaven National Laboratory, Upton, N.Y.

case of some types of New Kingdom Egyptian plaster, analysis shows them to be mixtures containing major proportions of calcium carbonate and sulfate. 1 Such natural organic polymeric binding materials as egg albumin, keratin, casein or glue have occasionally been added to, or painted on, these plasters. In recent years, different types of plaster have been experimentally combined with various synthetic polymers (e.g., methacrylate, vinyl acetate, urea melamine and epoxy ester resins). In contrast to concrete and mortar, air is seldom deliberately entrained into plaster; however, such solid aggregates as sand, volcanic ash, loess and other soils and clays are often added. In principle, a mud or clay wall coating can be considered as plaster, but in this paper only man-made construction materials that become firm through chemical actions will be so considered.

Lime Plaster A predominant constituent of lime plaster is calcium carbonate.

From early historical periods, this type of plaster has been made by roasting limestone until it has largely decomposed into lime.

The lime is then 1nixed with water and an aggregate to achieve a proper state of plasticity. It is then molded or applied in position and allowed to harden. The cementing of the mixture is achieved by the slow regeneration of calcium carbonate through reaction of the mixture with carbon dioxide in the air. The solid thus formed is similar to a fine-grained and relatively nonporous limestone, and the problems of its deterioration and treatment are analogous to those of limestone and marble.

Almost every type of deterioration experienced by lime plaster, and for that matter by all of the materials discussed here, is strongly affected and accelerated by the presence of water. Pure liquid water alone should have relatively little effect on "pure" lime plaster. However, when water reaches plaster, it almost always contains some dissolved acidic gases, such as carbon, sulfur or nitrogen oxides, that cause chemical alteration and dissolution of the plaster. Also, water usually contains some dissolved salts that contribute to the development of destructive efflorescence.

Plaster itself frequently contains relatively water-soluble constituents, the solution of which weakens its structure and the redeposition of which leads to the development of destructive mechanical stresses. Hence, water percolating through plaster-coated walls leads to a breakdown of the plaster. Therefore, a frequently encountered problem in the conservation of plaster-coated walls and wall paintings done on plaster is the elimination of sources of water that are affecting the wall and its plaster. This is often accomplished by (1) installing an impervious barrier between the source of the water and the plaster to be protected, (2) interposing a void between the source and the inner wall or (3) waterproofing the outer surface of the building. Lead dykes are sometimes placed horizontally across walls near ground level to block the upward migration of groundwater through capillary action, and vertical impervious layers are often placed in walls to prevent water from migrating inward. Such 1neasures can be effective, although they are relatively expensive to carry out. In underground structures, where it would be necessary to excavate around the structure to insert any isolating layer, it is often more practical to leave the structure so exposed standing free of the surrounding stone or soil.

Waterproofing exterior walls with sealing coatings may produce some protection for a period of time, but such coatings have a limited lifetime. They may also have undesirable effects on the appearance and durability of the walls.





Possibly the most elaborate installation for isolating a lime plaster structure and protecting it from moisture is that supporting Leonardo da Vinci's Last Supper in the convent church of Santa Maria delle Grazie at Milan. The painting/ along with its plaster support, was detached fron1 the wall on which it was originally placed to prevent the moisture that percolates through the wall from reaching it, and it is also gently heated electrically to prevent moisture from condensing on its surface.

Corrosive acidic gases present in sizable concentrations in today's polluted atmospheres and in solution in water exposed to these atmospheres can deteriorate li1ne plaster in several ways.

These include (1) direct solubilization of the carbonate structure, causing leaching of the binding 1naterial and the formation of destructive efflorescent salt deposits; (2) pulverization of the carbonate structure through che1nical alteration, which includes among changes in crystal structure instances of expansions in molecular size that generate internal stresses; and (3) fonnation of hydrated crystals that undergo volu1ne changes with changing humidity and, hence, also generate internal stresses.

Carbon dioxide and water produce a te1nporary solubilization of calcium carbonate by converting it to bicarbonate. Although the bicarbonate usually reverts to calcium carbonate, the process results in a breakdown in structure and the development of efflorescence. In relatively unpolluted rural areas in Italy, fresco paintings have been found covered with an efflorescent layer of nearly pure calcium carbonate. The surfaces of such paintings are often covered with small pitlike cavities. Both effects were probably produced by the reaction of the plaster support with carbon dioxide and moisture.

Sulfur dioxide and water can undergo a series of distinctive reactions with calcite. One reaction of sulfurous acid with calcium carbonate is the formation of calciu1n bicarbonate and bisulfite, both of which are relatively water soluble. Hence, rainwater in which some sulfur dioxide is dissolved can wash away a carbonate surface on which it falls. Also, of course, the calcium bicarbonate and bisulfite can be carried in solution through the plaster and deposited beneath or on the surface. Oxidation and hydration eventually convert the bisulfite to calcium sulfate dihydrate; that is, mineral gypsun1. The complete conversion of the calcium carbonate into gypsum nearly doubles the volume. The efflorescence found on the surfaces of Italian frescoes in urban and industrial atmospheres is usually pure gypsum. Frescoes so exposed also tend to develop swollen bulges beneath their outer pigmented layers; these bulges contain unconsolidated, powdery material rich in gypsum. The bulges eventually swell to the extent that the surface containing the1n breaks open, and a bare area of plaster, which increases rapidly in size as 1nore and more of the surrounding surface falls off, is exposed. This swelling can result from the conversion of gypsum of {l) calcite in situ beneath the surface or {2) calcium bicarbonate and bisulfite deposits near the surface.

The gypsum itself is slightly water soluble, and the action of water might cause it to migrate to some extent.

Another aspect of the generation of sulfates within plasters is that they often contain magnesium carbonate that, in turn, is transfonned into a hydrated sulfate. The hydrated magnesium sulfates are all water-soluble and, therefore, migrate readily and change their states of hydration when humidity and temperature fluctuate greatly. The stress forces created by such changes of hydration can be extremely great. These forces are capable of literally pushing apart the structure enclosing the hydrate.

On reaction with calcite, nitrogen oxides form water-soluble nitrates and nitrites. Other water-soluble calcium compounds sometimes formed through the action of air pollutants include chlorides and salts of organic acids. Of course, a variety of watersoluble salts may also have been included in the original plaster and its support. An outstanding example of the destructive effect of migration and deposition of a simple soluble salt exists in the tomb of the New Kingdom Egyptian Queen N efertari at Luxor.

Sodium chloride has been deposited beneath the surface of the painted walls of this tomb, causing the outer painted surfaces to be compeltely pushed off the wall. In some parts of the wall, the sodium chloride layer has built up to a thickness as great as 2 centimeters. This deposition occurs beneath the surface rather than on it because evaporation of the water carrying the salt occurs at that depth. · As with stone, it is possible for plaster to be attacked by molds and microorganisn1s. Because plaster is usually used indoors, it is not likely to be subjected to penetration by roots and tendrils of larger plants. However, molds and bacteria can be found on plaster and are thought by son1e conservators to contribute significantly to deterioration on occasion.

Plaster is not often subjected to freeze-thaw cycles with their attendant trapped n1oisture which so often damages materials exposed to the weather. This type of attack is discussed in more detail in the section on concrete.

Treatment of damaged lime plaster usually requires both {a) consolidation and repair of the existing damage and (b) protection against continued attack. A large number of organic binding materials are used to fasten down plaster that has become loose and friable. Polyvinyl alcohol, polymethyl methacrylates and soluble nylon are among those most frequently employed by art conservators. These materials function reasonably well and, if the treated material is well protected, should last for some years. However, organic materials tend to have much shorter lifetimes than inorganic ones. They eventually break down, discolor and oxidize into acidic cmnpounds. Such oxidation of organic material within lime plaster can result in a slow internal corrosion of the plaster.

All of the inorganic materials used to consolidate and protect limestone can be used on lime plaster. Such methods include treatment with magnesium or zinc fluosilicate, fluosilicic acid, hydrofluoric acid or fluorides to create inert calcium fluoride and silica within the plaster. Reasonably s1nall objects may be exposed to gaseous silicon tetrafluoride. Silica may also be generated within deteriorated plaster by treatment with various silicate esters.

Solutions of sodium silicate have even been used, but they should be considered for treatment only if it is certain that the treated objects will be well protected from water.

Because of the chemical similarity of calcium, barium and strontium and because of the insolubility and inertness of many barium and strontium salts, there is a long history of treating calcium carbonate structures with solutions of barium or strontium compounds. Such treatment immediately converts any water-soluble sulfates present into inert barium or strontium sulfate. In the 19th century, it was observed that treatment with barium hydroxide tended to consolidate damaged calcium carbonate structures. In general, barium hydroxide treatments result i:t;1 the eventual formation of barium carbonate within the treated structure. Arthur H. Church treated stone surfaces repeatedly with barium hydroxide, allowing the hydroxide to react with carbon dioxide in the air between treatments. 2 Maxmilian Dennstedt increased the rate of carbonate formation by exposing the hydroxidetreated surface to an atmosphere of carbon dioxide. 3 Seymour Z.

Lewin promoted the conversion to carbonate more effectively by adding urea to the barium hydroxide solution. 4 The urea produces a slow, controlled precipitation of barium carbonate from the homogeneous solution, according to the reaction Ba(OH) 2 +(NH 2 ) 2 CO~BaC0 3 ~ +2NH 3 The slow growth of carbonate crystals from the homogeneous solution results in a particularly well crystallized cohesive structure. Although the barium carbonate formed this way is essentially as chemically reactive as calcite, the sulfur oxides in the air slowly convert it to barium sulfate, which is extremely insoluble and chemically inert. The barium sulfate tends to protect the carbonate over which it is formed from acidic attack. This type of treatment may be extended by providing for direct precipitation of the protective barium sulfate in a slow, controlled manner from the homogeneous solution. 5 This is accomplished by adding the water-soluble salt barium ethyl sulfate to the barium hydroxide solution. The deposition occurs through the reaction Ba(OH) 2 + Ba(C2 H 5 S0 4 ) 2 ~BaS0 4 + 2C2 H 5 0H ~ The second product of this reaction, ethyl alcohol, evaporates harmlessly.



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