Physical-chemical study, characterisation and use of image analysis to assess the durability of earthen plasters exposed to rain water and acid rain

This paper aims to determine the physical performance and rain resistance of earthen plasters made with a red clayey binder, rich in silico-aluminates and a yellow binder which is mainly calcareous. The nature of the binder had little influence on practical properties of plasters, such as density, consistency, shrinkage, consumption, thermal conductivity and capillary water absorption. However, important properties of the plaster as for instance, the cohesion of the surface were affected by the type of binder. In addition, the plasters durability was evaluated by rain erosion tests conducted with distilled water and acid rain simulated with hydrochloric and sulphuric acid solutions (5% w/w). It was found that earthen plasters showed a better resistance to sulphuric acid than to hydrochloric acid. Scanning electron microscopy and X-ray diffraction analyses confirmed the formation of gypsum coatings on the surface of plasters attacked by sulphuric acid. In this case, the erosion was smaller due to the passivating role of gypsum, which protects the underneath material from dissolution. The textural alterations assessment, evaluated by optical microscopy and image analysis software, showed that hydrochloric acid rain was the most aggressive attack, followed by sulphuric acid rain and rain water.