Life cycle assessment of stone consolidants for conservation of cultural heritage

The field of the conservation and repair of built cultural heritage shows a growing attention towards the environmental sustainability of materials and treatments used for conservation projects. As a standardized methodology that is widely adopted for the comparative evaluation of construction and building materials, life cycle assessment (LCA) can be usefully applied to conservation materials, if specific field-related issues on the definition of functional units and reference flows are properly addressed. In the present study, LCA is used to quantify the environmental footprint of four alternative consolidants for restoration of weathered carbonate substrates. An innovative treatment based on an aqueous solution of diammonium hydrogen phosphate (DAP) is compared against three commercial alternatives (lime nanoparticles, ethyl silicate, and acrylic resin). The issue of the definition of the reference flow (i.e., the amount of each consolidant required for the restoration of a given surface) is addressed through experimental tests on three model substrates (marble, limestone, and lime mortar), following two alternative criteria: i) application until apparent refusal, as recommended by product manufacturers, and ii) application to obtain a certain degree of consolidation, in terms of increase in compressive strength. The novel DAP consolidant is shown to have lower impacts than the alternatives on several environmental impact categories, mainly thanks to the absence of organic solvents in its formulation. The evaporation of volatiles after application (volatile organic compounds for the commercial consolidants, ammonia for the DAP consolidant) is highlighted as an environmental hotspot for all the consolidants.