Environmental performance of nano-structured Ca(OH)2/TiO2 photocatalytic coatings for buildings

This paper describes the environmental performance of a mixed phase coating (photocatalytic nanolime) manufactured from a colloidal nano-structured calcium hydroxide in alcohol (nanolime) combined with titanium dioxide. While nanolime is used as a consolidant in the field of cultural heritage and titanium dioxide is used as a photocatalytic material for self-cleaning coatings and environmental pollution control within the construction industry both materials are often used separately. We report in this paper an approach to combine both materials for the development of a photocatalytic nanolime coating. The photocatalytic effect of titanium dioxide on the carbonation of nanolime and its influence on the degradation processes in polluted environments is assessed. A suspension of 25 g/l of nanolime in ethanol and 7.4% wt/vol titanium dioxide was applied to specimens of Bath stone. For comparison, additional specimens were treated only with the nanolime. The specimens were exposed to oxides of nitrogen and sulphur under 30% relative humidity (RH) for 120 h. Exposure was carried out under both, UV and daylight. After exposure, the effect of titanium dioxide on the carbonation of nanolime and on the degradation processes was investigated using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and X-ray photo electron spectroscopy (XPS). Results were evaluated considering the dissolution processes of the two oxides in water and modelled using PHREEQC. Nanolime and the photocatalytic nanolime coatings were shown to promote the reaction of SO2. Results from the photocatalytic nanolime formulation suggest they could be used to reduce atmospheric pollutant levels through creation of a sacrificial layer. Such coatings have the potential to prevent further damage to limestone façades and could be applied in cities where pollution is detrimental to historic buildings.