Surface change and decay of sandstone samples exposed to a polluted urban atmosphere over a six-year period: Belfast, Northern Ireland

Test blocks of a siliceous sandstone (Dunhouse sandstone) and a calcareous sandstone (Baumberger sandstone) were mounted in vertical aluminium racks, both sheltered and exposed to rainwash, in a range of locations within the Belfast urban area, Northern Ireland, UK. Blocks were retrieved after 3 months, 2 years, 4 years and 6 years of exposure. Visual and chemical analyses of the sandstone samples revealed a variable response to polluted atmospheric conditions, dependent on stone type and exposure conditions. An exponential increase in salt content was recorded in several sheltered samples; exposed samples displayed an inconsistent change in salt content over time. Visual analysis by scanning electron microscopy demonstrated variable amounts of gypsum crust development, biological growth and depositional material. Results indicate that stone surface alteration in a polluted atmosphere may be significant during initial exposure of fresh or cleaned stone.