Effect of Different Surface Treatments on the Performance of Earth Plasters

Earth plasters have several advantages. Nevertheless, they are vulnerable when in contact with liquid water. For that reason, they have low durability when applied as an outdoor coating or in indoor areas with potential contact with water. In this study, the influence of six different surface treatments (traditional and innovative, based on raw materials and on waste) applied on a pre-mixed earth plaster, applied by a roller (r) or as a spray (s), was assessed. The treatments were: limewash (L), beeswax (BW), linseed oil (LO), graphene oxide dispersion (GO), water from paper immersion (WP) and water from gypsum plasterboard paper immersion (WPG). The application of L, BW and LO, despite the color change, improved the water resistance and the surface performance of the earth plaster (less than 80%–86%, 93%–98% and 97%–99% of mass loss from surface cohesion, from water erosion by dripping action and from dry abrasion, respectively, compared to the reference untreated plaster). However, the application of BW and LO had a negative effect on the hygroscopic capacity of the plaster (less than 28%–38% of water vapor adsorbed after 24 h and the MBV decreased 29%–50% compared to the reference plaster). Finally, the application of the remaining surface treatments did not significantly improve the characteristics of the plaster, having even worsened it in certain cases (more than 42%–149% of mass loss from water erosion, compared to the reference plaster). These results demonstrated that, among the treatments analyzed, the L, BW and LO treatments are the best options to apply on an earth plaster. In particular, the application of BW and LO are recommended in situations where it is necessary to improve water resistance and surface performance, and the hygroscopic capacity is not a conditioning characteristic, such as outdoor applications.