calcareous stone

This paper looks at the changes in urban air quality and the effect on building materials with particular regard to their deterioration arising from pollution due to vehicles. The paper aims to review basic information on the likely Ž effect of NO and ozone on calcareous building materials limestone, marble, dolomite, calcareous cemented x . sandstone by examining experimental and fieldwork relating to the principle reaction of the oxidation of the sulphite to sulphate. NO is only one of many possible oxidants present in the urban and rural environment. A brief synopsis 2 of the effects of other oxidants and inhibitors is given along with a review of the processes involved in aqueous phase oxidation, which seems to be the phase where the reactions take place. Controlling factors in the reaction and oxidation of sulphite to sulphate are the type and porosity of the stone, the level of humidity, the nature of the oxidants and to a lesser extent the presence of other extraneous reactants, e.g. metal oxides, chlorine and ammonia salts. For freshly cut stone unpolished marble is much less reactive than porous limestone and the degree of activity has different relative humidity thresholds. Reaction of the stone with the gaseous compounds occurs during the process of absorption onto the surface. The oxidation is accelerated at the gas]water interface by the presence of NO , and seems to depend on the NO component but it is clear that NO is only one of many possible oxidants in x 2 x the urban and rural environments. Correlation studies of the damage to stone materials are unlikely to pick up the effects of NO due to the nature of their duration and the rather transient nature of the appearance and removal of x Ž 3q 2q. nitrates on the stone. Presence of other oxidants and catalysts such as H O , O catalysed by Fe and Mn , OH, 2 2 2 HSOy, SO2y, O , HO , NOy, NO , Oy and PAN, plus the fact that the reactions are pH and ionic strength 5 4 3 2 2 3 2 sensitive, add greatly to the complications of determining what affects the oxidation rate. These difficulties are further compounded by the presence of trace elements in the chemical reagents, particulate matter and the stone Ž 2q 2q. itself such as Mn or Vn . In conclusion there is sufficient evidence that NO has a role to play in the oxidation x of sulphite. However, the magnitude of influence NO and related compounds have in the environment is far from x U Tel: q44-1923-664297; fax: q44-1923-664786; e-mail: [email protected] 0048-9697r99r$ see front matter Q 1999 Building Research Establishment. Published by Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 8 9 6 9 7 9 8 0 0 4 0 9 4 ( ) S.W. Massey r The Science of the Total En ̈ironment 227 1999 109]121 110 Ž . clear due to the prevalence of other stronger oxidants ozone and hydrogen peroxide , the intricately linked NO and x ozone chemistry in relation to their diurnal cycle and the diurnal humidity variations, seasonal variations, the Ž . presence of metal oxide catalysts Fe and Mn in the stone and environment, and the possibility of organic oxidation inhibitors. Q 1999 Building Research Establishment. Published by Elsevier Science B.V. All rights reserved.