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

• Acid atmospheres may cause damage to all types of stones.
• In calcitic stones, the effect of acid atmospheres is related to porosity.
• Intensity of colour change depends mainly on the inherent stone colour.
• Artificial finish influences the acid atmosphere’s attack.
• HNO3/H2SO3 ratio controls whether nitrogen compounds react with the stone or acts only as a catalyser.

Six stone types with differences in composition and texture were exposed to four strong acid atmospheres formed from different acids: H2SO3, HNO3, and two mixed solution with different proportions of H2SO3 and HNO3. The changes on the surface were assessed by weight, colour, roughness and microscopic observation. Exposure to the atmosphere formed by HNO3 hardly affected the stone, whereas these formed from H2SO3 produced evident alterations. Depending on the HNO3/H2SO3 ratio, the nitrogen compounds may react with the stone and precipitate nitrates or nitrites or may only act as a catalyser of SO2 and enhance the formation of gypsum. Colour and roughness are efficient non-destructive approaches to evaluating the damage produced by acid atmospheres.