Development of lime injection grouts for the stabilisation of detached lime plasters with wall paintings in the unique Romanesque round church in Slovenia

Abstract

This paper presents the design of lime grouts to stabilise wall paintings from the 13th and 14th centuries in the Romanesque church of St. Nicholas in Selo, Slovenia. Most of the paintings were executed using the secco technique on a single layer of lime plaster made of slaked lime and quartz sand, which was applied to a clay brick masonry base. The thickness of the detached pockets was generally 2 mm to 1 cm, in a few places up to 5 cm, especially in the vault area. Consolidation of detached plaster layers is an irreversible treatment to restore the adhesion between the detached plaster layers. It was therefore important that the grouts met certain criteria in terms of composition and properties in the fresh and hardened state. We developed three grouts that meet general criteria and two specific key requirements, such as low density and sufficient injectability without pre-wetting to prevent new damage and/or the plaster falling off the wall or ceiling during or after the consolidation treatment. The grouts consist of hydrated lime, fillers, polycarboxylate ether superplasticizer (PCE) and water. During the design process, various quartz fillers were tested in different proportions. The reduced density of the grout was achieved by using expanded glass granules as a partial filler substitute. The appropriate injectability of the grout was achieved through the combination of reduced water content and superplasticizer PCE.