Graphene oxide as a surface consolidant for conservation-compatible earth- and lime-based plasters

Plasters are protective coatings for building walls and ceilings, but they suffer degradation over time, particularly in vernacular and historic buildings where compatible, low-embodied-energy mortars are required. In this study, the effect of a graphene oxide (GO) surface treatment was evaluated on five different plastering mortars representative of traditional and conservation-compatible formulations: earth-sand (E); air lime-sand (CL); air lime-sand with the addition of ceramic dust (CL+CD); air lime-ceramic waste (CL_CW); and natural hydraulic lime-sand (NHL). GO was applied by spraying under controlled conditions, and the specimens were characterized before and after treatment in terms of color, dry bulk density, thermal conductivity, surface cohesion, surface hardness and moisture buffering behaviour. Quantitative colorimetric analysis showed that GO promoted perceptible darkening in the lighter mortars (E: ΔE = 12.95; CL: ΔE = 9.14), while CL_CW exhibited negligible chromatic variation (ΔE = 0.57). GO application resulted in a slight increase in dry bulk density (+0.6% to +0.8%) and a reduction in thermal conductivity (up to 11.1% in CL_CW), depending on the mortar type. GO also tended to increase surface cohesion (up to a 56.6% reduction in cohesive mass loss) and surface hardness, with statistically significant improvements in NHL and CL_CW. Additionally, GO slightly enhanced moisture buffering capacity, although these changes were not statistically significant. This work provides the first systematic comparative assessment of GO as a surface treatment on multiple conservation-compatible earthen and lime-based plaster mortars, combining quantitative colorimetric, hygrothermal and surface-mechanical characterization to support its potential use in preventive conservation strategies for historic and vernacular architecture. • Graphene oxide was evaluated as a spray-applied surface treatment for earth- and lime-based plasters. • GO reduced thermal conductivity and slightly increased dry bulk density in all mortars. • Surface hardness significantly increased in hydraulic and ceramic-modified lime mortars. • GO reduced cohesive mass loss and improved surface performance. • Moisture buffering capacity was maintained or slightly enhanced after treatment.