Análise da influência da pozolana metacaulim no desempenho de argamassas de cal destinadas ao restauro de revestimentos externos de prédios históricos

Abstract

This study aims at highlighting the importance of knowing physical, chemical and mechanical characteristics of aerial lime-based mortars hydrated with pozzolan, as well as their compatibility with rehabilitation and preservation processes of historic buildings. The study of mortars for external covering of some historic buildings in Rio Grande, a city located in the south of Brazil, defined the selection of raw material in terms of characteristics of granulometry of the aggregate and mineralogical composition of these mortars. New mortars were evaluated in relation to the influence of partial replacement (10%) of aerial lime mass for two types of metakaolin with trace 1:4 (binder:aggregate). New mortars were submitted to a set of mechanical and physical assays so as to establish whether they were adequate for these buildings. Evolution of characteristics of mechanical strength and of behavior towards exposure to water, both in liquid and vapor forms, was analyzed on the 28th and 540th curing days, and assays were carried out at intermediate ages. The curing system aimed at achieving realistic conditions in which specimens – after having been unmolded – were kept in a climatized room at 20±2ºC and 80±5% humidity, up to 28 days. Afterwards, they were kept outdoors, with no direct exposure to rain. Metakaolin in the mortars exerted positive influence on the first curing days, due to the pozzolanic reaction, and led to higher compressive strength and easy handling of specimens. Crystalline phases observed by XRD throughout the study showed the prevalence of the carbonation reaction, while the pozzolanic reaction occurred on a small scale, as the result of the curing conditions. The microstructural analysis identified the morphologies of dolomitic lime mortars (hydromagnesite) and of metakaolin (hydrated calcium aluminate). At the end of the study, when the mortar of reference was hardened, results showed that its values of mechanical strength were higher than the ones of mortars with metakaolin. However, in the global analysis, values of mechanical strength shown by mortars with metakaolin were adequate for restoration of external coverings. These mortars also had better performance against salt spray, due to the refinement in their pore structure.