Reactions of gases on calcareous stones under dry conditions in field and laboratory studies

Dry deposition of gases plays an important role for the deterioration of stone materials and a better understanding of the processes involved will improve our ability to maintain stone monuments and buildings. As a part of an EU-project an investigation with four calcareous stone types have been exposed outdoor at two test sites in Norway for two years. The exposure has been carried out in sheltered position and the amount of reaction products and the penetration depth of SO₂ into the stones was determined as soluble sulphate after half a year and after one and two years. Even if most of the sulphate was found in the upper 0.3 mm of the stone, there was an increase in the sulphate content in stone even down to the center of the stone sample. In laboratory tests with SO₂, NO₂ and changing relative humidity the synergistic effect of NO₂ and the importance of the relative humidity was investigated. The uptake rates were calculated from the laboratory studies by analyzing the gas concentrations before and after the exposure chamber. By calculating the deposition velocity from the field study by using the amount of sulphate found in the stones together with the average outdoor concentration of SO₂ at the test sites, the values were a magnitude higher than in the laboratory test, highest at the industrial paper mill sites with high concentrations both of SO₂ and some hypochlorite and lower in urban atmosphere with fairly low values of SO₂ and high values of NO₂.