Assessment of compatibility between historic mortars and lime-METAKAOLIN restoration mortars made from amazon industrial waste

Abstract

In the field of conservation and restoration, incompatibility between historic materials and restoration materials should be avoided, and, therefore, a lot of studies are aimed at assessing compatibility in restoration projects. Thus, this work aims to evaluate the mineralogical, physical, and mechanical compatibility between historic mortars and lime-metakaolin (MK) restoration mortars, using MK derived from the industrial waste from the Amazon region. It is worth mentioning that this is the first attempt to use this kind of residue for the restoration of cultural heritage. Historical samples used in this work are from the 18th and 19th centuries, from Belém do Pará (northern Brazil). The composition of the restoration mortars was determined using a complete factorial duplicated in 22 with three central points. The evaluation of mineralogical compatibility was performed by XRD and DSC-TG, and the evaluation of physical and mechanical compatibility occurred by determining the compressive strength, open porosity, capillarity coefficient, and drying rate. As expected, lime mortars were the most compatible with historic mortars in terms of mineralogical composition. The mineralogy of lime-MK mortars resulted in the immediate formation of hydraulic products in the first days of curing, indicating rapid hardening and high initial mechanical strength. In general, the physical and mechanical properties of restoration mortars were compatible with historic mortars, which was influenced by the adopted features and mineralogy. The results indicate restoration mortars with lower mechanical resistance and greater open porosity, capillarity coefficient, and drying rate. This indicates that some of the restoration materials would not cause excessive stress on the old structure, favor gas exchange, and reduce the prolonged presence of moisture. The results also indicate that restoration mortars with MK are probably more resistant to weathering than restoration mortars without this additive, especially the prolonged presence of water, a common situation in the humid tropics. Thus, the conclusions of this work can be useful both for the field of conservation and restoration and for the field of applied clay science, as it directs towards the sustainable use of clays from industrial wastes as an additive in restoration mortars.