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In this review, stone consolidating materials are divided into four main groups, according to their chemistry. These groups are inorganic materials, alkoxysilanes, synthetic organic polymers, and waxes. Considerations of their performance are based on the requirements described in Section 3.
Inorganic stone consolidants were extensively used during the l9th century and still are occasionally being used. Most inorganic consolidants produce a white insoluble phase within the voids and pores of a stone, either by precipitation of a salt or by chemical reactions with the stone. It has been rationalized that the development of a new phase similar in composition to the matrix of a stone will bind together the particles of deteriorated stone. For example, consolidants which result in the formation of a silica phase should be used to consolidate sandstone, and calcium carbonate or barium carbonate used to consolidate calcareous stones such as limestone. In practice, however, little concern is given to chemical compatibility between the consolidants and stone.
Little success has been achieved in consolidating stone with inorganic materials, and in some cases their use has greatly accelerated stone decay [16, 50, 78]. Some of the reasons given for the poor performance of inorganic consolidants are their tendencies to produce shallow and hard crusts [16, 60, 76], the formation of soluble salts as reaction by-products [1, 16, 60, 86, 87], growth of precipitated crystals [50], and the questionable ability of some of them to bind stone particles together [6, 85]. Of these, the most difficult problem to overcome is the formation of shallow hard surface layers by inorganic consolidants because of their poor penetration abilities. Precipitation processes are often so rapid that precipitates are formed before the inorganic chemicals can appreciably penetrate the stone. A method, referred to as precipitation from homogeneous solutions, has been developed to obtain deeper penetration of stone by some inorganic consolidants. This method is discussed in Section 4.1.2.
Siliceous consolidants are materials which have been used to consolidate sandstone and limestone through the formation of silica or insoluble silicates.
Both nonstoichiometric dispersions of silica in sodium hydroxide and soluble alkali silicates have been used to conserve and consolidate stone. When dispersions of silica in sodium hydroxide solutions are applied to a stone. silica is deposited [16, 88]. If sodium hydroxide is not removed by washing, it can react with carbon dioxide or sulfur trioxide to form sodium carbonate or sodium sulfate, respectively. These salts can cause unsightly efflorescence and salt crystallization damage. In addition, it seems that sodium hydroxide can react with the constituent of some stones, thereby accelerating stone deterioration [16].
Silica can be precipitated by the reaction between sodium silicate, as well as potassium silicate, and acids such as hydrochloric acid and arsenic acid [16, 88, 89]. However, these reactions result in the formation of soluble salts such as sodium chloride and sodium arsenate. If the sodium silicate-arsenic acid mixture is used to consolidate limestone, crystalline calcium arsenate can be produced by a reaction between calcium carbonate and arsenic acid. The crystalline calcium arsenate appears to damage limestone by anisotropic crystal growth [16].
Insoluble silicates have been precipitated in stone by alternate treatments of sodium silicate and a variety of salts such as calcium chloride [16, 85, 88, 91] and zinc carbonate [90]. Colloidal silicates are first produced which eventually become crystalline [16], while soluble salts are produced as by-products. Impervious surface layers are also produced which trap water beneath [92]. Apparently, the silicates precipitate relatively rapidly so that they are deposited near the surfaces of the treated stones.
Even with all the problems associated with the use of alkali silicates, they are still occasionally being applied [5]. Recently, the successful use of soluble silicates was reported [93]. However, the overwhelming evidence clearly indicates that alkali silicates should not be used for stone consolidating purposes.
Both hydrofluosilicic acid and soluble silicofluorides have been used to preserve and consolidate stone. Hydrofluorosilic acid should not be used on limestone as it reacts vigorously with calcium carbonate to form crystalline calcium silicofluoride, carbonic acid, and carbonate salts [91]. The reaction occurs upon contact of the acid with the limestone producing a shallow crust with little consolidating value. Hydrofluorosilic acid reacts more slowly with siliceous-based sandstones to form a cementitious material, but again only the surface is hardened. Hydrofluorosilic acid has a tendency to discolor both limestones and sandstones, especially if they contain iron [16].
Many soluble types of silicofluorides, such as magnesium, zinc, and aluminum, have been applied to limestone. Resulting products are silica, insoluble fluoride salts and carbon dioxide, which are formed near the surface of the limestone. Therefore, only the surface is hardened, which eventually exfoliates [52, 76, 94]. Soluble silicofluorides also react with calcareous sandstones and again only a hardened surface is obtained. Further, soluble salts are formed when both limestone and calcerous sandstone are treated with silicofluorides [60]. These soluble salts have caused damage through salt recrystallization processes [86]. Penkala [95] recently carried out a systematic study of several stone treatments and also found that fluorosilicates were not effective consolidators.
Aqueous solutions of calcium hydroxide (its saturated solution is often called limewater) have been used for many centuries to protect and consolidate limestone [96]. Calcium hydroxide itself does not appear to consolidate stone, but when in solution or a wet state it reacts with atmospheric carbon dioxide to form insoluble calcium carbonate, which may bind particles of calcareous stones together. The solubility of calcium hydroxide is only about 1 gram per liter at room temperature [97], therefore repeated applications are necessary to produce sufficient calcium carbonate to consolidate stone. Furthermore, unless very dilute solutions are used, only the calcium hydroxide deposited near the surface of a stone is carbonated. This happens if the dense calcium carbonate being formed at the surface fills the pores and voids in the stone. This severely impedes the migration of carbon dioxide through the treated surface to the interior of the stone.
The newly produced calcium carbonate is susceptible to the same deterioration processes as the calcareous stone. For example, it can react with sulfur trioxide to form calcium sulfate, which is relatively soluble compared to calcium carbonate. Therefore, the treated stone may not be protected against further weathering. However, the treated stone may eventually gain the authentic appearance of the weathering stone.
Conflicting opinions have been given of the effectiveness of the calcium hydroxide process. Some conservators [8, 16] have felt that while treatment with calcium hydroxide causes no harm, little permanent consolidation is obtained, while others [50, 96, 98, 99] have recommended the use of lime water to protect limestone from weathering and to consolidate them. The effectiveness of freshly prepared slaked lime (calcium oxide mixed with water) in consolidating statues at the Wells Cathedral in England is being investigated by Professor Baker [100]. He is applying 38 mm thick layers of slaked lime to statues, which are being removed several weeks later. Some consolidation appears to be occurring.
Apparently, repeated limewater and slaked lime treatment can gradually consolidate limestone, but such processes are only economically feasible for small objects.
Similar to calcium hydroxide, strontium and barium hydroxides will react with carbon dioxide to form insoluble carbonates. Again, only the hydroxide near surface of a stone is usually carbonated. However, unlike calcium sulfate, strontium and barium sulfates are insoluble and thus the application of strontium and barium hydroxides may reduce the weathering of stone exposed to polluted environments.
The early work on the use of barium hydroxide to conserve stone was performed by Church [101-103]. Initially, excellent results were obtained. However, only a surface hardening was being obtained and eventually the barium carbonate or barium sulfate layer exfoliated [16, 50, 76, 92]. The exfoliation problem has been attributed not only to the formation of a dense impervious surface layer, but also to anisotropic crystal growth of barium carbonate and barium sulfate [16, 50].
Lewin [104] and Sayre [105] have developed methods intended to precipitate barium carbonate and barium sulfate deeply within a stone. These methods are based on a process known as precipitation from homogeneous solution [106]. In this process the material to be precipitated and the precipitating chemicals are present in the same solution. For example, barium carbonate is precipitated from an aqueous solution of barium hydroxide and urea [104, 108]. The urea slowly undergoes hydrolysis producing ammonia and carbon dioxide. The liberated ammonia and carbon dioxide dissolves in the water forming ammonium carbonate which raises the pH of the solution. When a certain pH is reached, barium hydroxide reacts with the carbonate ion and barium carbonate is precipitated. The reaction rate can be controlled so the barium carbonate precipitate forms days after a stone is treated. The slow formation of barium carbonate is reported to give a crystalline solid solution with the calcite crystals of calcareous stone. Barium sulfate can be precipitated in a stone by an analogous method. An aqueous solution of a barium monoester of sulfuric acid hydrolyzes slowly when a base is added, releasing barium and sulfate ions [106].
The precipitation of barium carbonate and barium sulfate by homogeneous solution precipitation methods is clearly a promising approach. To date, however, only experimental testing has been carried out and little is known concerning the long-term consolidating effectiveness of this approach. Warnes [16] and Marsh [50] have both suggested that crystalline inorganic precipitates, such as barium carbonate and sulfate, do not have long term consolidating value. Also the precipitates of barium carbonate and barium sulfate have a larger molecular volume than calcite and appear to exhibit anisotropic crystal growth [16, 50]. It should not be assumed that deteriorated stone will have sufficient empty volume to accommodate these precipitates. Therefore, until more is known of the long-term effects of barium carbonate and barium sulfate on the durability of stone, they should be regarded as experimental materials which should not be applied to important historic structures.
Many other inorganic materials have been used in attempts to conserve and consolidate stone, including zinc and aluminum stearates [16, 28, 50, 95], aluminum sulfate [16, 50, 106], phosphoric acid [50], phosphates [50], and hydrofluoric acid [5]. Hydrofluoric acid appears to have a consolidating effect because it removes deteriorated stone, thereby leaving a sound surface. A saturated aqueous solution of calcium sulfate has been recently used to consolidate a stone consisting of a conglomerate of microfossils cemented by gypsum [107].
Alkoxysilanes are regarded by many stone conservators [8, 25, 59, 99, 109-114] as being among the most promising stone consolidating materials for siliceous sandstones. The feasibility of using alkoxysilanes to consolidate calcareous stone is also being studied [115-116]. The main reasons that alkoxysilanes are being highly regarded are their abilities to penetrate deeply into porous stone and the fact that their polymerization can be delayed until deep penetration has been achieved [25, 58, 99, 109, 110, 112]. In addition, they polymerize to produce materials similar to the binder in siliceous sandstone.
The use of alkoxysilanes for consolidating stone is not a recent development. For example, Laurie [117] received a patent in 1925 for producing such a material to be used for stone consolidation. Other early researchers on the use of alkoxysilanes to consolidate stone are Cogan and Setterstrom [118-119]. Alkoxysilanes have been commonly used since around 1960 in Germany [5]. Recently, a promising alkoxysilane consolidating material was developed at the UK Building Research Establishment, called "Brethane" [112].
Alkoxysilanes are a family of monomeric molecules which react with water to form either silica or an alkylpolysiloxane. Three alkoxysilanes are commonly used to consolidate stone. They are tetraethoxysilane, triethoxymethylsilane and trimethoxymethylsilane [109]. Tetraethoxysilane is an example of a silicic acid ester [110]. Their polymerization is initiated by a hydrolysis reaction,
| catalyst | (2)
--Si-OR + H20 --Si - OH + ROH.
| |
Then polymerization commences,
| | | |
--Si -- OH + Si-OR --Si-O-Si-+ R' OH- (3)
| |
where R = CH3 (methyl), C2H5 (ethyl)
and R' = H, CH3, C2H5
Polymerization continues until all the alkoxy groups have been liberated and either an alkylpolysiloxane or silica is produced. Silica is produced by the polymerization of a silicic acid ester. An alkylpolysiloxane is formed by the polymerization of other types of alkoxysilanes. An acidic catalyst, e.g., hydrochloric acid, is used to increase the rate of hydrolysis (equation 2). The alkoxysilanes are diluted with solvents to reduce their viscosities. Thus, their reaction rate and depth of penetration into stone can be controlled. It is claimed that their consolidating ability can be increased by using a mixture of alkoxysilanes [110].
Some confusion appears in the literature regarding silicon esters, silicones and alkoxysilanes. Silicon esters are partially polymerized alkoxysilanes which still have ester groups attached to silicon. Silicones are polymerized alkoxysilanes which are dissolved in organic solvents, and used as water repellents [110].
Price [58] and Weber [111] have observed that alkoxysilanes can penetrate porous stones to a depth of between 20 to 25 mm. The newly developed Brethane has been reported [112] to penetrate as deeply as 50 mm. No noticeable polymerization occurs with Brethane for at least 3 hours after it is mixed with a solvent and catalysis [58, 109]. The large sizes of unpolymerized alkoxysilane molecules, no doubt, will prevent them from entering the smaller pores of a stone.
Marschner reported [120] that alkoxysilanes improved the resistance of sandstone to sodium sulfate crystallization. However, she also observed that their performance varied from sandstone to sandstone and also depended on the compatibility between the solvent and the specific stone being treated. Similar findings were reported by Moncrieff [115] who studied the consolidation of marble. Snethlage and Klemm [121] observed in a scanning electron microscope analysis of impregnated sandstone that a polymerized alkoxysilane appeared to fill the space between sandstone grains and form a continuous coating. However, polymerized alkoxysilanes are reported [25, 58,110, 115] to have little effect on moisture passage in stone and the frost resistance of stone. Some slight changes in the color of treated stone have been observed [122, 123]. For example, statutes on the Wells Cathedral have become more dull grey following treatment with an alkoxysilane [123]. Further, a treated stone panel on the Cathedral has acquired a slightly more orange tone than adjacent untreated panels.
Once a section of stone is treated with alkoxysilane, it will probably weather differently than the untreated stone. Thus, unless most of the visible parts of a structure are similarly treated, the contrast between the treated and untreated stone could become very noticeable.
Strength improvements of around 20 percent have been reported [25, 110] when sandstone specimens were impregnated with alkoxysilanes. The ability of alkoxysilanes to consolidate deteriorated stone in the field, however, has not been unequivocally demonstrated. Further, it appears that the performance of alkoxysilanes varies from stone to stone.
Even if alkoxysilanes are found to be effective consolidants, their high cost [28, 112] will probably limit their use to statues and smaller-sized stone objects.
Two general types of synthetic organic polymer systems are used to consolidate stone. In the first, monomeric organic molecules are first polymerized, dissolved in appropriate solvents, and then applied to stone. They are deposited within the voids and pores of the stone as the solvent evaporates. The second type are monomeric organic molecules, either pure or dissolved in a solvent, which are polymerized within the voids and pores of a stone. Viscous monomers are diluted with solvents so that deep penetration can be achieved [57]. Solvents which evaporate rapidly (most common organic solvents), however, have been found to draw organic consolidants back to the surface of a stone, resulting in the formation of impervious hard surface crusts [57, 61]. Munnikendam [59] has recommended that organic consolidants should be selected whose solidification does not depend on solvent evaporation.
Both thermoplastics and thermosets have been used to consolidate stone. A thermoplastic is a material which can be reformed by the application of heat without significant changes in properties. Examples of thermoplastics are poly(vinylchloride), poly(ethylene), nylon, poly(styrene) and poly(methylmethacrylate). A thermoset is a material which is formed into a permanent shape by the application of heat, and once formed, cannot be remelted or reformed. Polyester, epoxy, and polyurethane are examples of thermosets. Methylmethacrylate can be converted into a thermoset by copolymerization with a three dimensional cross-linking material.
The use of synthetic organic polymer systems to consolidate stone is a recent development, dating back to around the early 1960's. Therefore, little is known regarding the long-term performance of these materials. Some organic consolidants have been found to improve significantly the mechanical properties of deteriorated stone. Many organic polymers are susceptible to degradation by oxygen and ultraviolet radiation, but this would only affect the materials on the surface of a treated stone [1]. Riederer reported [5] that the surfaces of some stone structures in Germany which had been consolidated with organic polymers in 1965 had exhibited deep channel erosion by 1975. Apparently, water gradually eroded the consolidated surface and once the surface was pierced, erosion proceeded rapidly into the untreated stone.
Methylmethacrylate and to a lesser extent butylmethacrylate have been used to consolidate concrete [64, 65] and stone [58]. These monomers can be applied solvent-free to porous solids and can be polymerized in situ. An excellent source for information on their polymerization as well as on polymer-impregnated concrete is the report by Kukacha et al [64]. Methylmethacrylate has been polymerized into poly(methylmethacrylate) by heating with an initiator, by gamma radiation, and at ambient temperature by combination of promoters and initiators [64, 124]. For thermal polymerization, the chemical initiator (catalyst) azobis(isobutyronitrile) has been found to be effective [125]. Heating blankets could be used to thermally polymerize methylmethacrylate or other monomerics applied to a stone structure. Polymerization by radiation is only feasible if carried out in special chambers because of the radiation hazards. Chemical promoters convert initiators into free radicals at ambient temperatures. Then these free radicals induce the polymerization of methylmethacrylate. Munnikendam [61] used N,N-dimethyl-P-toluidine to decompose benzoyl peroxide into free radicals. He found, however, that oxygen inhibited the subsequent polymerization reaction of methylmethacrylate. Better success probably could be achieved by using azobis(isobutyronitrile) as the initiator [125].
Where deep or complete impregnation and complete polymerization was achieved, methylmethacrylate and other acrylates have been shown to improve substantially the mechanical properties and durability of porous materials such as concrete [64]. However, incomplete impregnation, with acrylates may result in the formation of a distinct interface between treated and untreated stone [120].
Polymer-impregnated concretes based on acrylics are classified as brittle materials based on their stress-strain curves [64, 125, 126]. Stone consolidated with methylmethacrylate and other acrylics can be expected to exhibit a similar brittle behavior.
Methylmethacrylate, no doubt, can harden the surface of a stone and effectively consolidate the stone if both deep penetration and complete polymerization are achieved. Similar to the case with alkoxysilanes, however, stone impregnated with methylmethacrylate will probably weather differently than untreated stone. In addition, erosion through the treated stone [5] could contribute to the development of an unsightly appearance.
Copolymers are produced by the joining of two or more different monomers in a polymer chain [127]. A commercially available acrylic copolymer used for stone consolidation is copolymerized from ethylmethacrylate and methylacrylate [38, 121]. Other acrylic copolymers which have been studied for stone conservation include copolymers between acrylics and fluorocarbons [128, 129] and between acrylics and silicon esters [59, 121].
The acrylic copolymers are dissolved in organic solvents then applied to stone. As discussed earlier, unless very dilute solutions are applied to a stone solvent evaporation will tend to draw the acrylic copolymers back to the surface. Then, even if diluted to the lowest concentration that will give some consolidation, their solutions still may have viscosities which impede their penetration into stone.
Several vinyl polymers have been studied or used for conservation and consolidating of stone including poly(vinylchloride) [54, 130], chlorinated-poly(vinylchloride) [130], and poly(vinylacetate) [38, 54, 130, 131]. These polymers are dissolved in organic solvents and then applied to stone. Photochemical processes could release chlorine from the chloride polymers, which could damage stone [130]. Poly(vinylacetate) has been found to produce a glossy stone surface [130]. If not carefully applied and if not sufficiently diluted, use of the vinyl polymers undoubtedly will result in the formation of impervious layers which entrap moisture and salts underneath [38].
The feasibility of using epoxies to consolidate stone is addressed by first briefly discussing their chemistry and then applications.
An epoxy consists of an epoxy resin and a curing agent which is actually a polymerization agent. Cure, i.e., polymerization, of an epoxy is initiated by mixing the epoxy resin with the curing agent. The epoxy resin is then converted into a hard thermosetting cross-linked polymer. The most commonly used epoxy resins are monomers of diphenylolpropane, called bisphenol A, and epichlorohydrin. Resins produced from these reactants are liquids, but are too viscous to penetrate stone deeply. Therefore, they are diluted with organic solvents. These epoxy resins are often cured using an amine curing agent. Their cure time can be adjusted by selecting a slowly or rapidly reacting curing agent and by controlling the curing temperature. The resulting cross-linked polymers have excellent adhesion to stone and concrete, and excellent chemical resistances. Two recommended sources for information on epoxies, such as their chemistry, curing, and applications are reference Nos. 132 and 133.
Gauri [134-135] developed a method to achieve deep penetration with viscous epoxy resins and at the same time avoid the formation of a sharp interface between the consolidated and untreated stone. First, specimens are soaked in acetone, then in a dilute solution of epoxy resin in acetone, followed by soaking in increasingly concentrated solutions. This method is feasible for small stone objects such as tombstones and statues, but would be too time consuming and expensive for stone structures.
Less viscous epoxy resins are available including diepoxybutane diglycidyl ether and butanediol diglycidyl ether [58]. Munnikendam [61] cured butanediol diglycidyl ether with alicyclic polyamines such as menthane diamine. However, the viscosity was still too high and he diluted the mixture with tetraethoxysilane and tetramethoxysilane. A complex reaction took place involving the epoxy resin, curing agent and solvent to produce a tough, glassy material. A white efflorescence also developed due to a reaction between the polyamine and carbon dioxide to Form amine-carbonates [61, 137]. Formation of the aminecarbonates can be avoided by preventing carbon dioxide from coming in contact with the solution. Gauri [128, 136] observed that when low viscosity aliphatic epoxy resins were applied to calcareous stones, the reaction rates between the stones and carbon dioxide and sulfur dioxide were increased compared to the rates with untreated stones. He suggested that the increased reactivity could be caused by absorption of the gases by the epoxy polymer or by the polymer acting as a semipermeable film to the gases. In contrast, bisphenol A-based epoxy polymers were found to protect the stone from both carbon dioxide and sulfur dioxide.
The use of epoxies has been suggested for consolidating limestone [14, 128, 129], marble [134-139], and sandstone [61, 121] as well as for re-adhering large stone fragments to mass stone [1]. Moncrieff and Hempel [138] found that certain epoxies could encapsulate salts in marble, thereby preventing them from re-crystallizing. A large restoration project using epoxies for masonry consolidation is that of the Santa Maria Maggiore Church in Venice [140].
Similar to poly(methylmethacrylate), epoxies have produced brittle epoxy-impregnated concretes with high mechanical properties [65, 141,142]. The long-term effect on incorporating a brittle material in stone is not known, but could render a structure vulnerable to seismic shock, vibrations and thermal-dimensional effects.
Many types of epoxies have a tendency to chalk, i.e., to form a white powdery surface, when exposed to sunlight [132]. Therefore, epoxy should be removed from the surface of a treated stone before it cures.
Other synthetic organic polymers studied as possible stone consolidants include polyester [38, 143], polyurethane [121], and nylon [77]. Polyester has been shown to decrease the porosity of stone substantially [143] and, therefore, may form an impervious layer which prevents the passage of entrapped moisture or salts [38]. Manaresi [121] and Steen [144] observed that polyurethanes were poor cementing agents. Steen [145] also found that polyurethane film gradually became brittle when exposed to sunlight. Similarly, DeWhite (77) found that nylon can produce a brittle film on the surface of stone.
Waxes have been applied to stone for over 2,000 years. Vitruvius [146] described the impregnation of stone with wax in the first century B.C. A wax dissolved in turpentine was one of several materials applied to the decaying stone of Westminster Abbey between 1857 to 1859 [147]. Cleopatra's Needle (London, England) was treated with wax first in 1879 and several times since [148]. Kessler [149] found that paraffin waxes were effective in increasing the water repellency of stone. Waxes have also been found to be effective consolidants [25, 53, 54, 58]. For example, a paraffin wax increased the tensile strength of a porous stone from 1.06 MN/m2 to 4.12 MN/m2, while triethoxymethylsilane only increased it to 1.88 MN/m2 [25, 53]. In addition, paraffin waxes are among the most durable stone conservation materials [16, 54] and can immobilize soluble salts [58].
Waxes have been applied to stone by applying the wax dissolved in organic solvents [16, 78, 148], by immersing a stone object in molten wax [58], or by applying molten wax to preheated stone [150]. If deep penetration is not achieved a nonporous surface layer may be formed causing the eventual spalling of the treated stone surface [78].
Major problems encountered in using waxes to conserve stone include their tendency to soften at high ambient temperatures [76], and to entrap dust and grime [2, 54, 58]. Wax applied to Cleopatra's Needle has gradually converted to a tarry substance which cannot be removed by ordinary washing methods. For example, a mixture of carbon tetrachloride, benzene and detergent was needed in 1947 to clean the Needle [148].
| James. R. Clifton. Stone Consolidating Materials: A Status Report | ||||||||
| Contents | Intro | Deterioration | Performance | Stone consolidants | Comments on consolidants | Conclusions | References | Notes on electronic version |