Developing a lime-based injection grout with no additives for very thin delamination : the role of aggregates and particle size/morphology

Abstract

A thin grout was designed and tested for the stabilisation of the 16th c. Perez d’Aleccio’s wall painting cycle of the Great Siege, in the Grandmaster’s Palace in Valletta, Malta. The painting, on a single layer of lime-based plaster applied on Globigerina Limestone, presented delaminated pockets up to 5 mm thick, with narrow access points 1.5 mm wide (cracks) to inject the grout. For this reason, the grout could just be injected through very fine needles, a 18G (0.84 mm inner diameter) and/or 19G needle (0.69 mm inner diameter). Considering the porous original plaster, to avoid a reduction in porosity of the grout observed in previous studies, the site-specific grout was designed without the use of additives such as superplasticisers, and included just binder (slaked lime putty), aggregates, water. Different aggregates were selected and tested in different proportion, the variables for their selection being: particle shape, particle size and water absorption. Testing of the grouts included: injectability, flow on plastered tile, bleeding, shrinkage and adhesion in Globigerina Limestone cups, cohesion. A very thin grout, passing through a needle 0.69 mm wide, was obtained, without the use of superplasticisers, relying only on the role of aggregates and their particle size and morphology.