Raman spectroscopy after accelerated ageing tests to assess the origin of some decayed products found in real historical bricks affected by urban polluted atmospheres

Abstract

Bricks, together with stones and mortars, can be considered as one of the most important building materials that constitute our built heritage. Numerous factors which cause several decaying pathologies in bricks can be listed, but it should be emphasised that the most severe and damaging one is the wet and dry deposition of both combustion and greenhouse gases (CO₂, SO _x and NO _x mainly). For instance, after the impact of CO₂ and SO _x , the decayed products promoted in bricks are carbonates and sulphates. Once identified in all these kinds of salts in real samples, it is necessary to make sure that the aggressive atmospheric conditions are sufficient to promote the formation of these salts. Therefore, accelerated exposure test are a good alternative in order to simulate the formation of these decayed compounds and to predict the reactions that promote the decaying mechanism. In this work, brick samples manufactured at different firing temperatures following ancient methods were subjected to humidity/dryness, freeze/thaw, CO₂ and SO₂ (KESTERNICH DIN 50018) accelerated ageing tests followed by a Raman spectroscopy screening in order to verify the formation of sulphate and carbonate salts in bricks on accelerated conditions, simulating the damage caused by a polluted atmosphere throughout many years of exposure.