Green multi-action products for the sustainable conservation of historic porous building stones

Different porous stones (PS) were used as building materials of historical and monumental constructions that nowadays are important memories of the past. However, several degradation phenomena are erasing the tangible testimonies of their artistic, historic, social and cultural value. Conservation of PS involves a panoply of actions and products with different functions such as consolidants, hydrophobic products, biocides, cleaning agents (e.g. solvent, water), among others. Therefore, current conservation practices have poor environmental sustainability due to the diversity of actions and products needed, because products often have toxic compounds and because their performance depends on PS chemical/mineralogical characteristics. Although environmental concerns have been addressed for a long time in other sectors of construction, green strategies or methodologies for assessing environmental impacts of practices and products are still limited in the conservation field. The project is aligned with UN Sustainable Development Goals(2030 Agenda) and offers a unique approach in the conservation field by proposing an innovative multi-action product (MaP) based in green chemistry and a thoughtful working plan involving its technical and environmental performance assessment. The objective is to develop and validate a MaP appropriate for both silicate and carbonate PS. MaP will have the ability to consolidate and protect against physical/chemical decay, biocolonization and graffitis by means of its hydro/oleophobic and long-term biocide properties. The concept enables the rationale use of resources and consumption of raw materials, minimizes the manufacturing processes footprint and reduces the amounts of hazardous substances, while making available more eco-friendly products and practices. This innovative concept will be achieved by a new strategy, combining green chemistry routes and nanotechnology, i.e., by using alkoxysilane-base formulations tuned with polymers and calcium-layered double hydroxides nanoparticles (CaXx-LDH). The pioneering integration of surface-modified CaXx-LDH loaded with biocides into eco-friendly siloxanes (having affinity with silicate PS) envisions multiple roles. CaXx-LDH are proposed to: -favor the physical/chemical compatibility of alkoxysilane also with carbonate PS by inducing discontinuities into the Si-O network for an overall increase of porosity and deformability; -impart hydro/oleophobic properties after being superficially modified with low-surface-energy species; -act as “smart” containers for the controlled release of biocides and for long-term action against biocolonization. Besides, to solve the conceptual problem between consolidation and protection treatments (one is supposed to act in-depth and the other superficially), the MaP is designed to allow a multifunctional gradient, i.e., molecules and components responsible for protection will concentrate at surface, while the ones responsible for consolidation will act along PS depth. An advanced testing strategy, covering efficacy, compatibility and durability of treatments on PS of both natures, is proposed for optimization and adequate technical performance. Consolidation efficacy will be assessed by mechanical tests, while microstructural and water transport studies, laboratory inoculation and cleanability tests are proposed to assess the protection efficacy against water-related degradation, biocolonization and graffiti, respectively. Compatibility of the treatments with the PS will be verified by searching unwanted modifications on specific characteristics (e.g. aesthetics or permeability) and its potential durability under artificial aging tests. The confirmed commitment of important stakeholders in built heritage conservation will narrow research-practice gaps by assuring the dissemination of the results and achievements through conservation practioneers and will allow to test the efficacy and compatibility of the final MaP into historic PS from Portuguese monuments. The concept validation in both, laboratory samples and in real historic PS materials, will enable a high technology readiness level at the end of the project and will guarantee a great impact on the field. Life cycle assessment of the new MaP and involved practices will be evaluated to compare the environmental performance with current single-action products and practices using repeated applications. The data will contribute for future LCA studies, which are a critical need in the conservation field. Even though the development of an eco-friendly MaP, effective, compatible and durable in different PS is a major challenge, the consortium is very well placed to successfully accomplish this objective. The project will be implemented by a team of recognized researchers from multidisciplinary fields with proven experience in the development of cuttingedge materials and eco-friendly solutions for built heritage conservation (see selected publications: [1-5].