Restoration of ancient gypsum-based plasters: Design of compatible materials

Abstract In this paper, the process of design and evaluation of compatible materials to partially or completely substitute damaged interior walls and ceilings coatings made of gypsum-based plasters is presented and the results obtained are discussed. The methodology used comprised the definition of quantitative requirements to be accomplished by the new materials, based on previous works of the authors with characterization of an extensive set of samples of historic buildings. It was concluded that there is need to develop three different restoration materials, one for each family of plaster elements: thin-layer finishing plasters (L), elements moulded on site (M) and precast decorative elements (P). The physical and mechanical properties of seven mixes based on gypsum and lime, with addition of other components, were determined and allowed concluding that the binder proportions (gypsum: lime) is the most influential factor. Higher gypsum content leads to stiffer materials, less shrinkage and higher absorption coefficients than in lime-based mixes. The formation of micro cracks was pointed out as a possible cause for the decrease of flexural strength and water vapour permeability values of the M mixes. The comparison of the results obtained with the compatibility requirements allowed finding well-suited restoration materials for thin-layer coatings and precast elements. The mixes for moulded on site elements revealed to be the less balanced, and some adjustments are advised.