Sustainable lime-based nano-reinforced pastes for structural health monitoring of the restoration areas of Monuments of Cultural Heritage

The aim of the present article is to investigate the sustainability of the production of different carbon-nanostructures that can be used to reinforce lime-based mortars for structural health monitoring of the restoration areas in Monuments of Cultural Heritage. Aspects like manufacturing cost, environmental impact, mechanical properties, and piezo-resistive response were investigated to select a sustainable carbon nanostructure to reinforce lime-based mortars under different, multi-decision criteria. A quantitative cost assessment methodology for five (5) different carbon nanostructures (graphene - G and multi-wall carbon nanotubes - MWCNTs) in laboratory-scale was adopted. The results were interpreted against the respective enhancement on the mechanical properties resulted on the end-products, e.g., the nano-reinforced lime-based pastes. The mechanical tests results showed that the modified MWCNTs enhanced flexural strength by 109 % due to increased anchoring of the reinforcing nanostructures in the paste, while compressive strength was not essentially affected. On the contrary, the modified graphene nanostructures enhanced only the compressive strength of the paste by 29 % due to their platelet geometry and their respective load transfer capability. The high manufacturing cost of reduced graphene (rGO) 2.62 €/g is related to the high cost of raw materials utilized, while graphene oxide (GO) has a manufacturing cost of 1.37 €/g, mainly due to the balanced contribution of material and labour cost, respectively. These nanostructures were calculated to have the lowest environmental impact (0.10 kg CO_2eq/g), while the carboxylation process increased essentially the equivalent carbon footprint by approximately (8 x) eight times (0.83 kg CO_2eq/g). In most cases, GO excel among the different carbon nanostructures studied, due to its low values in several criteria, like mechanical properties per manufacturing cost or mechanical properties per carbon footprint, as well as showed the best piezo-resistive response to cyclic compressive mechanical loads. To this end, it proved to be the most sustainable carbon nanostructure to be exploited in relevant applications.