Influence of surface finish and composition on the deterioration of building stones exposed to acid atmospheres.

Six stone types with differences in composition and texture were exposed to four strong acid atmospheres formed from different acids: H 2 SO 3 , HNO 3 , and two mixed solution with different proportions of H 2 SO 3 and HNO 3 . The changes on the surface were assessed by weight, colour, roughness and microscopic observation. Exposure to the atmosphere formed by HNO 3 hardly affected the stone, whereas these formed from H 2 SO 3 produced evident alterations. Depending on the HNO 3 /H 2 SO 3 ratio, the nitrogen compounds may react with the stone and precipitate nitrates or nitrites or may only act as a catalyser of SO 2 and enhance the formation of gypsum. Colour and roughness are efficient non-destructive approaches to evaluating the damage produced by acid atmospheres. [ABSTRACT FROM AUTHOR]