Mechanism of Sulphation by Atmospheric SO 2 of the Limestones and Marbles of the Ancient Monuments and Statues: I. Observations in situ (Acropolis) and laboratory measurements

Abstract

This work was done in order to clarify the mechanism of sulphation of limestone and marbles by atmospheric SO₂, This knowledge is indispensable if the marbles are to be satisfactorily protected. The reaction products from SO₂ attack consist of 85–97% gypsum, with the film thickness rangingfrom 0·1 to 1·5 cm. The deterioration has been accelerated during the last 20–25 years by reason of the intense industrialisation in the Athens area. The details of the statues are preserved on the surface of the gypsum, but have disappeared from the marble/gypsum interface; thus the gypsum films must not be washed off. Neither organic nor inorganic coatings must be applied as accelerated tests have shown that gypsum will still form in or on the coatings. Measurements of weight gain using a quartz spiral balance have confirmed that the gypsum forms in two consecutive stages; a rectilinear stage up to about 11% gypsum formation, followed by a parabolic one. Activation energies corresponding to the two stages were determined. In Part II of this work a new hypothesis concerning the rate-determining step of sulphation will be reported.