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Abstract

Abstract. Sulfur dioxide (SO 2 ), prevalent in the modern urban environment of industrial countries, attacks calcite (CaCO 3 ) in marble. As a result, a gypsum (CaSO 4 ·2H 2 O) crust is produced at rainsheltered surfaces while areas exposed to rain experience accelerated erosion. We have investigated the effect of certain surfactants as protective agents against SO 2 attack. We report that the anions oxalate (C 2 O −2 4 ) and oleate (C 17 H 33 COO − ) from solutions of their highly soluble alkali salt species are able to replace carbonate (CO −23 ) in calcite producing less reactive substrate of oxalate and oleate of calcium. Experiments to measure the protection obtained by these treatments were carried out in the laboratory and field conditions at nearly 1ppm and 10 ppb SO 2 concentrations, respectively. We found that these treatments provided significant protection to marble exposed in sheltered areas, up to 30% reduction of reaction rate by treatment with 2 × 10 −4 M sodium oleate and up to 14% by a 2 × 10 −3 M with potassium oxalate solutions, but become ineffective over long term exposure when applied to surfaces exposed to rain. Carrara marble was used in the reported study. Ion chromatography was the analytical tool, which allowed precise measurements of ionic concentrations of these salts, the amount of their uptake by marble, and the thickness of the gypsum crust. X-ray diffraction allowed determination of the new minerals formed at the marble surface by the treatment with surfactants.

Influence of chemical and structural factors on the calcite–calcium oxalate transformation

“…24 Moreover, this work represents a source of information that may help in the design of effective conservation treatments for ornamental and building calcitic stones based on the in situ replacement of the original calcitic substrate by an oxalate protective barrier that, because of its lower solubility and reactivity, may reduce pollution attack to these stones. 2,[25][26][27] Such a process would mimic the natural process of oxalate patina formation taking place when a calcareous stone is exposed to the environment. 28 Finally, calcium carbonate and other calcium-bearing phases (e.g.…”