An experimental study on earth plasters for earthen building protection: the effect of different admixtures and surface treatments

In rainy climates, the external surfaces of earthen buildings suffer water erosion. In this paper, the properties of earth plasters have been investigated considering the specific relationship with the underlying substrate. Ten typologies of earth plasters containing different admixtures and surface treatments, a cob wall and a rammed earth wall were produced in laboratory. The aim is to evaluate the effectiveness of the coatings in protecting the earthen walls against weathering. An in situ procedure consisting of a shrinkage test followed by an adhesion strength test was performed in order to identify the earth/sand ratio optimal for the plaster manufacturing. Then, a series of tests was carried out both on the plasters and the two walls: compression, water vapor permeability, surface color, wettability, water absorption and erosion. The results demonstrate that all the plasters are physically and mechanically compatible with the earthen substrates and that the most important differences are in the specific relationship with water. The earth plaster treated with the silane–siloxane product was found to be the best one: it is fully compatible, water-repellent and highly resistant to water erosion. Hence, there is a potential for the use of earth plasters for the protection of earthen buildings against weathering.